Form 424B5 Aclaris Therapeutics,

Article Stock Quotes (1)

October 27, 2026 5:16 PM EDT

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Filed Pursuant to Rule 424(b)(5)

Registration No. 333-298084

PROSPECTUS SUPPLEMENT
(To Prospectus Dated August 6, 2026)

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7,785,714 Shares of Common Stock

Pre-funded Warrants to Purchase 6,500,000 Shares of Common Stock

We are offering 7,785,714 shares of our common stock, par value $0.00001 per share, and in lieu of common stock to certain investors that so choose, pre-funded warrants to purchase 6,500,000 shares of our common stock. The purchase price of each pre-funded warrant equals the price per share at which shares of our common stock are being sold in this offering, minus $0.00001, and the exercise price of each pre-funded warrant is $0.00001 per share. We are also offering the shares of our common stock that are issuable from time to time upon the exercise of the pre-funded warrants.

There is no established public trading market for the pre-funded warrants, and we do not expect a market to develop. We do not intend to apply for listing of the pre-funded warrants on the Nasdaq Global Select Market or other securities exchange or nationally recognized trading system. Without an active trading market, the liquidity of the pre-funded warrants will be limited.

Our common stock is listed on The Nasdaq Global Select Market under the symbol “ACRS.” The last reported sale price of our common stock on The Nasdaq Global Select Market on October 26, 2026 was $3.79 per share.

Investing in our common stock involves a high degree of risk. See “Risk Factors” beginning on page S-10 of this prospectus supplement, page 5 of the accompanying prospectus and under similar headings in the documents incorporated by reference into this prospectus supplement and the accompanying prospectus.

We are a “smaller reporting company” under applicable Securities and Exchange Commission rules and are subject to reduced public company reporting requirements. See “Prospectus Supplement Summary — Implications of Being a Smaller Reporting Company.”

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined if this prospectus supplement or the accompanying prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

	PER SHARE			PER PRE-FUNDED WARRANT			TOTAL
Offering Price		$	3.50		$	3.49999		$	49,999,934
Underwriting Discounts and Commissions(1)		$	0.21		$	0.21000		$	3,000,000
Proceeds to Aclaris Therapeutics, Inc. before expenses		$	3.29		$	3.28999		$	46,999,934

(1)

We have agreed to reimburse the underwriters for certain expenses. See “Underwriting” beginning on page S-25 of this prospectus supplement for additional information regarding underwriter compensation.

Delivery of the shares of common stock and pre-funded warrants is expected to be made on or about October 28, 2026.

Joint Bookrunning Managers

Leerink Partners Piper Sandler Cantor LifeSci Capital

Prospectus Supplement dated October 27, 2026

Table of Contents

Table of Contents
Prospectus Supplement
	Page
About This Prospectus Supplement		S-1
Special Note Regarding Forward-Looking Statements		S-2
Prospectus Supplement Summary		S-4
The Offering		S-8
Risk Factors		S-10
Use of Proceeds		S-14
Dividend Policy		S-15
Dilution		S-16
Description of Securities We are Offering		S-18
Material U.S. Federal Income Tax Consequences		S-20
Underwriting		S-25
Legal Matters		S-34
Experts		S-34
Where You Can Find More Information		S-34
Incorporation of Certain Information by Reference		S-35
Prospectus
About This Prospectus		ii
Prospectus Summary		1
Risk Factors		5
Special Note Regarding Forward-Looking Statements		6
Use of Proceeds		8
Description of Capital Stock		9
Description of Debt Securities		14
Description of Warrants		21
Legal Ownership of Securities		23
Selling Securityholders		26
Plan of Distribution		27
Legal Matters		29
Experts		29
Where You Can Find More Information		29
Incorporation of Certain Information by Reference		30

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ABOUT THIS PROSPECTUS SUPPLEMENT

This document is part of an automatic “shelf” registration statement on Form S-3 (File No. 333-298084) that we filed with the Securities and Exchange Commission, or the SEC, on August 6, 2026, and which became automatically effective upon filing, as a “well-known seasoned issuer” as defined in Rule 405 under the Securities Act of 1933, as amended, or the Securities Act, using an automatic “shelf” registration process. This document is in two parts. The first part is this prospectus supplement, which describes the specific terms of this offering of common stock and pre-funded warrants and also adds to and updates information contained in the accompanying prospectus and the documents incorporated by reference herein. The second part, the accompanying prospectus, provides more general information. Generally, when we refer to this prospectus, we are referring to both parts of this document combined. To the extent there is a conflict between the information contained in this prospectus supplement and the information contained in the accompanying prospectus or any document incorporated by reference therein filed prior to the date of this prospectus supplement, you should rely on the information in this prospectus supplement; provided that if any statement in one of these documents is inconsistent with a statement in another document having a later date — for example, a document incorporated by reference in the accompanying prospectus — the statement in the document having the later date modifies or supersedes the earlier statement.

We further note that the representations, warranties and covenants made by us in any agreement that is filed as an exhibit to any document that is incorporated by reference herein were made solely for the benefit of the parties to such agreement, including, in some cases, for the purpose of allocating risk among the parties to such agreements, and should not be deemed to be a representation, warranty or covenant to you. Moreover, such representations, warranties or covenants were accurate only as of the date when made. Accordingly, such representations, warranties and covenants should not be relied on as accurately representing the current state of our affairs.

We have not authorized anyone to provide any information other than that contained or incorporated by reference in this prospectus supplement, the accompanying prospectus or in any free writing prospectus prepared by or on behalf of us or to which we have referred you. We take no responsibility for, and can provide no assurance as to the reliability of, any other information that others may give you. This prospectus supplement and the accompanying prospectus do not constitute an offer to sell, or a solicitation of an offer to purchase, the securities offered by this prospectus supplement and the accompanying prospectus in any jurisdiction to or from any person to whom or from whom it is unlawful to make such offer or solicitation of an offer in such jurisdiction. The information contained in this prospectus supplement or the accompanying prospectus, or incorporated by reference herein or therein is accurate only as of the respective dates thereof, regardless of the time of delivery of this prospectus supplement and the accompanying prospectus or of any sale of our securities. It is important for you to read and consider all information contained in this prospectus supplement and the accompanying prospectus, including the documents incorporated by reference herein and therein, in making your investment decision. You should also read and consider the information in the documents to which we have referred you in the sections entitled “Where You Can Find More Information” and “Incorporation of Certain Information by Reference” in this prospectus supplement and in the accompanying prospectus.

We are offering to sell, and seeking offers to buy, shares of our common stock and pre-funded warrants only in jurisdictions where offers and sales are permitted. The distribution of this prospectus supplement and the accompanying prospectus and the offering of shares of our common stock and pre-funded warrants in certain jurisdictions may be restricted by law. Persons outside the United States who come into possession of this prospectus supplement and the accompanying prospectus must inform themselves about, and observe any restrictions relating to, the offering of shares of our common stock and pre-funded warrants and the distribution of this prospectus supplement and the accompanying prospectus outside the United States. This prospectus supplement and the accompanying prospectus do not constitute, and may not be used in connection with, an offer to sell, or a solicitation of an offer to buy, any securities offered by this prospectus supplement and the accompanying prospectus by any person in any jurisdiction in which it is unlawful for such person to make such an offer or solicitation.

Unless otherwise stated, all references in this prospectus supplement and the accompanying prospectus to “we,” “us,” “our,” “Aclaris,” “company” and similar designations refer, collectively, to Aclaris Therapeutics, Inc. and, where appropriate, our subsidiaries.

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SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS

This prospectus supplement, the accompanying prospectus and the information incorporated by reference herein and therein contain forward-looking statements. These are based on our management’s current beliefs, expectations and assumptions about future events, conditions and results and on information currently available to us. Discussion containing these forward-looking statements may be found, among other places, in the Sections entitled “Business,” “Risk Factors” and “Management’s Discussion and Analysis of Financial Condition and Results of Operations” contained in the documents incorporated herein and therein.

Any statements in this prospectus supplement, the accompanying prospectus or incorporated herein or therein about our expectations, beliefs, plans, objectives, assumptions or future events or performance are not historical facts and are forward-looking statements within the meaning of Section 27A of the Securities Act and Section 21E of the Securities Exchange Act of 1934, as amended, or the Exchange Act. These forward-looking statements include statements regarding:

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our plans to develop our product candidates;

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the timing of our planned clinical trials of our product candidates and the reporting of the results from these trials;

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the clinical utility of our product candidates and our expectations about their properties and therapeutic effects;

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our plans and expectations related to manufacturing capabilities and strategy;

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our expectations regarding coverage and reimbursement of our product candidates, if approved;

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the timing of our regulatory filings and approvals for our product candidates;

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our intellectual property position;

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our plans to identify and consummate transactions with third-party partners to further develop, obtain marketing approval for and/or commercialize our product candidates, and earn revenue from such arrangements;

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our expectations regarding competition;

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the size and growth potential of the markets for our product candidates;

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our expectations regarding our continued reliance on third parties;

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the impacts of macroeconomic conditions on our business;

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our expectations regarding our use of capital;

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our estimates regarding future revenue, expenses and needs for additional financing;

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other risks and uncertainties, including those listed under the caption “Risk Factors” in our most recent Annual Report on Form 10-K and other filings we make with the SEC; and

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our intended use of proceeds from this offering.

We caution you that the foregoing list may not contain all of the forward-looking statements made in this prospectus supplement, the accompanying prospectus and the documents incorporated by reference herein and therein.

In some cases, you can identify forward-looking statements by the words “may,” “might,” “can,” “will,” “to be,” “could,” “would,” “should,” “expect,” “intend,” “plan,” “objective,” “anticipate,” “believe,” “estimate,” “predict,” “project,” “potential,” “likely,” “continue” and “ongoing,” or the negative of these terms, or other comparable terminology intended to identify statements about the future, although not all forward-looking statements contain these words. These statements involve known and unknown risks, uncertainties and other factors that may cause our actual results, levels of activity, performance or achievements to be materially different from the information expressed or implied by these forward-looking statements.

We have included important factors in the cautionary statements included in this prospectus supplement, particularly in the “Risk Factors” section, that we believe could cause actual results or events to differ materially from the forward-looking statements that we make. Our forward-looking statements do not reflect the potential impact of any future acquisitions, mergers, dispositions, joint ventures or investments we may make.

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You should read this prospectus supplement, the accompanying prospectus and the information incorporated by reference herein and therein completely and with the understanding that our actual future results may be materially different from what we expect. Except as required by law, we assume no obligation to update these forward-looking statements publicly, or to revise any forward-looking statements to reflect events or developments occurring after the date of this prospectus supplement, even if new information becomes available in the future.

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PROSPECTUS SUPPLEMENT SUMMARY

This summary highlights selected information contained elsewhere in this prospectus supplement and the accompanying prospectus and in the documents we incorporate by reference. This summary does not contain all of the information you should consider before investing in our securities. You should read this entire prospectus supplement and the accompanying prospectus carefully, especially the risks of investing in our securities discussed under “Risk Factors” beginning on page S-10 of this prospectus supplement and under a similar heading in our Annual Report on Form 10-K for the year ended December 31, 2025, which is incorporated by reference in this prospectus supplement, along with our consolidated financial statements and notes to those consolidated financial statements and the other information incorporated by reference in this prospectus supplement and the accompanying prospectus, before making an investment decision.

Company Overview

We are a clinical-stage biopharmaceutical company focused on developing novel small and large molecule product candidates for immuno-inflammatory diseases. Our proprietary KINect drug discovery platform coupled with our integrated discovery approach to small and large molecules enables us to identify and advance product candidates designed to have superior target affinity, specificity and potency. We are seeking to identify and consummate transactions with third-party partners to further develop, obtain marketing approval for and/or commercialize our novel product candidates.

Recent Developments

ATI-052 Phase 1b Primary Analysis Results

On October 27, 2026, we announced primary analysis results from our Phase 1b proof-of-concept trials of our anti-TSLP/IL-4Rα bispecific antibody ATI-052 in atopic dermatitis, or AD, and asthma. The results are summarized below.

Phase 1b Proof-of-Concept Trial in AD

The randomized, double-blind, placebo-controlled Phase 1b trial evaluated the safety and tolerability of ATI-052 compared with placebo in 14 patients with severe AD. Patients were randomized 3:1 to receive five weekly doses of 480 mg of ATI-052 or placebo from weeks 0 through 4. Endpoints for the primary analysis were assessed at day 57.

Four patients were excluded from the prespecified per-protocol analysis for reasons defined before unblinding. Two patients in the ATI-052 arm were excluded because one received only two doses and one did not meet the eligibility criteria. Two patients in the placebo arm were excluded because they took rescue medication early in the study. The resulting per-protocol population consisted of nine patients in the ATI-052 arm and one patient in the placebo arm. Among per-protocol patients in the ATI-052 arm, 78% had severe disease at baseline, as defined by a score of 4 on the Validated Investigator’s Global Assessment, or vIGA.

Primary analysis results from the AD trial included:

Eczema Area and Severity Index, or EASI, Score:

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Patients treated with ATI-052 experienced a 45% placebo-adjusted mean improvement in EASI score from baseline at day 57, reflecting a 58% mean improvement in the ATI-052 arm compared with a 13% mean improvement in the placebo arm. Patients treated with ATI-052 experienced a 62% mean improvement from baseline in EASI score at week 10 and EASI improvement was maintained through week 16, for the subset of patients with available data.

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In post hoc analyses of patients treated with ATI-052, mean improvements from baseline in EASI score at week 10 were 82% among patients with a baseline vIGA of 3 and 56% among those with a baseline vIGA of 4.

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Patients treated with ATI-052 experienced a 78% placebo-adjusted EASI-50 at day 57. Of those, in a post-hoc analysis, 100% of patients with a baseline severity of vIGA 3 achieved placebo-adjusted EASI-50 at day 57 compared to 71% of patients with a baseline severity of vIGA 4.

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In a post-hoc analysis, 89% of subjects treated with ATI-052 achieved EASI-50 during the study.

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Peak Pruritus Numerical Rating Scale, or PP-NRS:

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Improvement in itch was observed in the first week and continued to deepen through week 10 in patients with available data.

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At week 4, patients that received ATI-052 experienced a 40% placebo-adjusted mean improvement from baseline and absolute mean improvement from baseline in PP-NRS.

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At day 57, patients that received ATI-052 experienced a 43% placebo-adjusted mean improvement in PP-NRS, reflecting a 38% mean absolute improvement in the ATI-052 arm compared with a 5% mean worsening in the placebo arm.

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At day 57, patients treated with ATI-052 achieved a placebo-adjusted response rate of 33% for a ≥4-point improvement from baseline in PP-NRS.

Pharmacodynamic Results:

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At day 57, patients treated with ATI-052 experienced a 95% overall improvement in the difference between lesional and non-lesional skin proteome.

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ATI-052 broadly downregulated inflammatory pathways, with modulation across multiple inflammatory mediators in AD, including Th1, Th2 and Th17/Th22, fibrosis/extracellular matrix remodeling and itch-associated molecular pathways.

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ATI-052 produced a pronounced reduction of established dupilumab-associated biomarkers and additionally suppressed Th1, Th17 innate and myeloid pathways. Endogenous serum biomarkers, including CCL17, IL-5, and blood eosinophils, were also reduced.

Phase 1b Proof-of-Concept Trial in Asthma

The randomized, double-blind, placebo-controlled Phase 1b trial evaluated the safety and tolerability of a single 480 mg dose of ATI-052 compared with placebo in 16 patients with mild-to-moderate asthma and baseline fractional exhaled nitric oxide, or FeNO, of at least 35 parts per billion. Patients were randomized 3:1 to receive a single 480 mg dose of ATI-052 or placebo. Endpoints for the primary analysis were assessed through day 29. The analysis population consisted of 12 patients in the ATI-052 arm and four patients in the placebo arm.

Primary analysis results from the asthma trial included:

FeNO:

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At day 15, patients that received ATI-052 experienced a 59% mean reduction from baseline in FeNO, representing a mean absolute reduction of 45 parts per billion, compared with a 15% mean reduction in the placebo arm.

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A 40% mean reduction from baseline in FeNO was observed at day 29 in patients that received ATI-052.

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At day 15, 58% of patients that received ATI-052 achieved FeNO normalization, defined as FeNO of 25 parts per billion or less.

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In a post-hoc analysis, at day 15, patients treated with ATI-052 experienced FeNO reductions of 60% in the eosinophil ≥300 subgroup (T2-high) and 58% in the eosinophil <300 subgroup (T2-low).

Forced Expiratory Volume in One Second, or FEV1:

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At day 15, patients that received ATI-052 experienced a 200 mL placebo-adjusted and absolute mean improvement in FEV1.

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In a post hoc subgroup analysis of patients with baseline pre-bronchodilator FEV1 of less than 80% predicted and qualifying reversibility, patients that received ATI-052 experienced a 350 mL mean improvement from baseline in FEV1 at day 15, compared with a 140 mL mean decrease from baseline in the placebo arm.

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In a post-hoc analysis, FEV1 improvement was also observed in T2-high and T2-low subgroup populations.

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Area Under Reactance Curve, or AX:

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In a post-hoc analysis, at day 15, patients treated with ATI-052 experienced a placebo-adjusted AX change of −28 cmH2O·L-1 in patients with baseline FEV1 <80% predicted and qualifying reversibility.

Safety and Tolerability

ATI-052 was well tolerated across both trials and demonstrated a favorable safety profile. Treatment-emergent adverse events (“TEAEs”) were predominantly Grade 1. There were no Grade 3 or Grade 4 TEAEs, and no adverse events led to study discontinuation. Injection-site reactions were transient, almost exclusively Grade 1 and observed at a low frequency. No conjunctivitis was observed.

Pipeline Update

We intend to prioritize the clinical development of ATI-052 initially in eosinophilic esophagitis, or EoE. We intend to initiate a Phase 2b trial of ATI-052 in EoE in early 2027 and expect to provide an update regarding plans for the potential development of ATI-052 in an additional indication in the near term.

Certain Preliminary Financial Information (Unaudited)

As of September 30, 2026, we had approximately $204.6 million of cash, cash equivalents and marketable securities. This amount is an unaudited and preliminary estimate that (i) represents the most current information available to management as of the date of this prospectus supplement, (ii) is subject to completion of financial closing and review procedures that could result in significant changes to the estimated amounts, and (iii) does not present all information necessary for an understanding of our financial condition as of, and our results of operations for the quarter ended, September 30, 2026. Accordingly, you should not place undue reliance on this preliminary estimate.

The preliminary financial data included in this prospectus supplement has been prepared by, and is the responsibility of, our management. PricewaterhouseCoopers LLP has not audited, reviewed, examined, compiled, nor applied agreed-upon procedures with respect to the preliminary financial data. Accordingly, PricewaterhouseCoopers LLP does not express an opinion or any other form of assurance with respect thereto.

It is possible that we may identify items that require us to make adjustments to the financial information set forth above. This preliminary estimate should not be viewed as a substitute for financial statements prepared in accordance with generally accepted accounting principles in the United States and it is not necessarily indicative of the balance to be achieved in any future period. Additional information and disclosure would be required for a more complete understanding of our financial position and results of operations as of September 30, 2026. Accordingly, you should not place undue reliance on this preliminary estimate.

Risks Associated With Our Business

Our business is subject to a number of risks of which you should be aware before making an investment decision. These risks are discussed more fully in the “Risk Factors” section of this prospectus supplement immediately following this prospectus supplement summary and in the “Risk Factors” section in our Annual Report on Form 10-K for the year ended December 31, 2025, which is incorporated by reference in this prospectus supplement. These risks include the following:

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We have incurred significant losses since our inception. We expect to incur losses over the next several years and may never achieve or maintain profitability.

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We will need substantial additional funding to meet our financial obligations and to pursue our business objectives. If we are unable to raise capital when needed, we could be forced to curtail our planned operations.

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We have a limited history as a clinical-stage biopharmaceutical company developing and partnering our product candidates, which may make it difficult to evaluate the success of our business to date and to assess our future viability.

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If we are unable to successfully develop our product candidates and identify and consummate transactions with third-party partners to further develop, obtain marketing approval for and/or commercialize our product candidates, or experience significant delays in doing so, our business will be harmed.

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Our future success depends on our ability to retain key executives and to attract, retain and motivate qualified personnel.

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We rely heavily on third parties for clinical trials, manufacturing, and development support. Their performance impacts our timelines and success.

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If we are unable to obtain and maintain patent protection for our product candidates, or if the scope of the patent protection obtained is not sufficiently broad, our competitors could develop and commercialize technology and drugs similar or identical to ours, and our ability to successfully identify and consummate transactions with potential third-party partners to commercialize our technology and product candidates may be impaired.

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We face substantial competition, which may result in others discovering, developing or commercializing drugs before or more successfully than we do.

Corporate Information

We were incorporated under the laws of the State of Delaware in July 2012. Our principal executive offices are located at 701 Lee Road, Suite 103, Wayne, PA 19087. Our telephone number is (484) 324-7933. Our common stock is listed on The Nasdaq Global Select Market under the symbol “ACRS.”

Our internet website address is www.aclaristx.com. The information contained on our website is not incorporated by reference into this prospectus supplement, and you should not consider any information contained on, or that can be accessed through, our website as part of this prospectus supplement or in deciding whether to purchase our securities.

“Aclaris”, the Aclaris logo, KINect and our other trademarks or service marks appearing in this prospectus supplement or incorporated herein by reference are our property. This prospectus supplement and the information incorporated herein by reference contains additional trade names, trademarks and service marks of others, which are the property of their respective owners.

Implications of Being a Smaller Reporting Company

We are a “smaller reporting company” as defined in Item 10(f)(1) of Regulation S-K. Smaller reporting companies may take advantage of certain reduced disclosure obligations, including, among other things, providing only two years of audited financial statements and reduced disclosure obligations regarding executive compensation. We will remain a smaller reporting company until the last day of any fiscal year for so long as either (1) the market value of our shares of common stock held by non-affiliates does not equal or exceed $250.0 million as of June 30th of such fiscal year, or (2) our annual revenues did not equal or exceed $100.0 million during the most recently completed fiscal year for which audited financial statements are available and the market value of our shares of common stock held by non-affiliates did not equal or exceed $700.0 million as of June 30th of such fiscal year. To the extent we take advantage of any reduced disclosure obligations, it may make the comparison of our financial statements with other public companies difficult or impossible.

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THE OFFERING

Common stock offered by Aclaris

7,785,714 shares

Pre-funded warrants offered by Aclaris

We are also offering, in lieu of common stock to certain investors that so choose, pre-funded warrants to purchase 6,500,000 shares of our common stock. The purchase price of each pre-funded warrant equals the price per share at which the shares of our common stock are being sold in this offering, minus $0.00001, which is the exercise price of each pre-funded warrant per share. The pre-funded warrants do not expire and each pre-funded warrant will be exercisable at any time after the date of issuance of such pre-funded warrant, subject to an ownership limitation. See “Description of Securities We Are Offering — Pre-funded Warrants” on page S-18 of this prospectus supplement. This prospectus supplement also relates to the offering of the shares of our common stock issuable upon exercise of the pre-funded warrants.

Common stock to be outstanding after this offering

147,609,987 shares, assuming no exercise of any pre-funded warrants issued in this offering.

Use of proceeds

We estimate that the net proceeds to us from this offering, after deducting underwriting discounts and commissions and estimated offering expenses payable by us, will be approximately $46.7 million. We will receive only nominal additional proceeds from any exercise of pre-funded warrants. We intend to use the net proceeds from this offering for the research and development of our product candidates in our pipeline and for working capital and general corporate purposes. See “Use of Proceeds” on page S-14 of this prospectus supplement.

Risk factors

You should read the “Risk Factors” section of this prospectus supplement beginning on page S-10, page 5 of the accompanying prospectus and the “Risk Factors” section in our Annual Report on Form 10-K for the year ended December 31, 2025, which is incorporated by reference, for a discussion of factors to consider carefully before deciding to invest in our securities.

Nasdaq Global Select Market symbol

Our common stock is listed on the Nasdaq Global Select Market under the symbol “ACRS.” We do not intend to apply for listing of the pre-funded warrants on the Nasdaq Global Select Market or other securities exchange or nationally recognized trading system.

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The number of shares of our common stock to be outstanding after this offering is based on 139,824,273 shares of our common stock outstanding as of June 30, 2026 and excludes:

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15,392,992 shares of our common stock issuable upon the exercise of stock options outstanding as of June 30, 2026, at a weighted average exercise price of $5.93 per share;

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3,441,263 shares of our common stock issuable upon the vesting or other settlement of restricted stock units outstanding as of June 30, 2026;

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3,000,000 shares of common stock issuable upon the exercise of warrants outstanding as of June 30, 2026 at an exercise price of $0.00001 per share;

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6,754,427 shares of our common stock reserved for future issuance under our 2025 Equity Incentive Plan and 502,830 shares of common stock reserved for future issuance under our 2024 Inducement Plan, in each case as of June 30, 2026; and

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10,827,017 shares of our common stock issued and sold under our at-the-market facility subsequent to June 30, 2026.

Unless otherwise indicated, all information in this prospectus supplement assumes no exercise or settlement, as applicable, of the outstanding options, restricted stock units and warrants described above, and no exercise of pre-funded warrants offered and sold in this offering.

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RISK FACTORS

Investing in our securities involves a high degree of risk. Before you decide to invest in our securities, you should carefully consider the risks and uncertainties described below together with all other information contained in this prospectus supplement, the accompanying prospectus and in our filings with the SEC that we have incorporated by reference in this prospectus supplement and the accompanying prospectus. The risk factors included in this prospectus supplement update the corresponding risk factors incorporated by reference in this prospectus supplement from our Annual Report on Form 10-K for the year ended December 31, 2025. If any of the following risks or any of the risks incorporated herein by reference actually occurs, our business, prospects, operating results and financial condition could suffer materially. In such event, the trading price of our common stock could decline and you might lose all or part of your investment.

Risks Related to this Offering

Our management might apply the net proceeds from this offering in ways with which you do not agree and in ways that may impair the value of your investment.

We currently intend to use the net proceeds from this offering for the research and development of our product candidates in our pipeline and for working capital and general corporate purposes. We may also use a portion of the net proceeds to invest in or acquire additional businesses or technologies that we believe are complementary to our own, although we have no current plans, commitments or agreements with respect to any future acquisitions as of the date of this prospectus supplement. Pending these uses, we expect to invest the net proceeds in short-term, interest-bearing obligations, certificates of deposit or direct or guaranteed obligations of the United States or foreign government agencies. Our management has broad discretion as to the use of these proceeds and you will be relying on the judgment of our management regarding the application of these proceeds. We might apply these proceeds in ways with which you do not agree, or in ways that do not yield a favorable return. If our management applies these proceeds in a manner that does not yield a significant return, if any, on our investment of these net proceeds, it could compromise our ability to pursue our growth strategy and adversely affect the market price of our common stock.

The pre-funded warrants are not listed on any exchange, and we do not intend to list the pre-funded warrants on any exchange. There is no public market for the pre-funded warrants being offered in this offering.

There is no established public trading market for the pre-funded warrants being offered in this offering, and we do not expect a market to develop. In addition, we do not intend to apply to list these pre-funded warrants on any securities exchange or nationally recognized trading system, including the Nasdaq Global Select Market. Without an active market, the liquidity of these pre-funded warrants will be limited. See “Description of Securities We are Offering — Pre-funded Warrants” on page S-18.

We will not receive any meaningful amount of additional funds upon the exercise of the pre-funded warrants.

Each pre-funded warrant has an exercise price of $0.00001 per share of common stock. Each pre-funded warrant will be exercisable until it is fully exercised by means of payment of this nominal purchase price. Accordingly, we will not receive any meaningful additional funds upon the exercise of the pre-funded warrants.

Holders of pre-funded warrants purchased in this offering will have no rights as common stockholders until such holders exercise their pre-funded warrants and acquire our common stock.

Until holders of the pre-funded warrants acquire shares of our common stock upon exercise of such pre-funded warrants, the holders will have no rights with respect to the shares of our common stock underlying such pre-funded warrants, such as voting rights or the rights to receive dividends. Upon exercise of the pre-funded warrants, the holders will be entitled to exercise the rights of a common stockholder only as to matters for which the record date occurs after the exercise.

Significant holders or beneficial owners of our common stock may not be permitted to exercise pre-funded warrants that they hold.

Holders of the pre-funded warrants will not be entitled to exercise any portion of any pre-funded warrant which, upon giving effect to such exercise, would cause the aggregate number of shares of our common stock beneficially owned by the holder (together with its affiliates) to exceed a specified percentage of the number of shares of our common stock outstanding immediately after giving effect to the exercise, as such percentage ownership is determined in accordance with the terms of the pre-funded warrants. See “Description of

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Securities We Are Offering — Pre-funded Warrants.” As a result, you may not be able to exercise your pre-funded warrants for shares of our common stock at a time when it would be financially beneficial for you to do so. In such circumstance you could seek to sell your pre-funded warrants to realize value, but you may be unable to do so in the absence of an established trading market for the pre-funded warrants.

If you purchase shares of common stock in this offering, you will suffer immediate and substantial dilution in the book value of your shares.

The offering price per share of our common stock and per pre-funded warrant in this offering each exceeds the pro forma net tangible book value per share of our common stock outstanding prior to this offering. As a result, if you purchase shares of our common stock in this offering, you will experience immediate dilution of $2.06 per share, based on the offering price of $3.50 per share of common stock and $3.49999 per pre-funded warrant and our pro forma net tangible book value per share as of June 30, 2026. The exercise or vesting and settlement, as applicable, of outstanding stock options, restricted stock units or warrants may result in further dilution of your investment. See the section titled “Dilution” below for a more detailed illustration of the dilution you may incur if you participate in this offering.

You may experience future dilution as a result of future equity offerings or the issuance of equity as consideration for complementary businesses or assets we may acquire, and the perception that any such offerings or issuances could occur may depress our stock price and our ability to raise funds in any new stock offering.

In order to raise additional capital, we may in the future offer additional shares of our common stock or other securities convertible into or exchangeable for our common stock. We cannot assure you that we will be able to sell shares or other securities in any other offering at a price per share that is equal to or greater than the price per share paid by investors in this offering, and investors purchasing shares or other securities in the future could have rights superior to existing stockholders. The price per share at which we sell additional shares of our common stock or other securities convertible into or exchangeable for our common stock in future transactions may be higher or lower than the price per share in this offering. We also may issue additional shares of our common stock or other securities convertible into or exchangeable for our common stock as consideration for acquisitions of businesses or products, strategic alliances or joint ventures with third parties that we believe will complement or augment our existing business, which may result in significant dilution to our stockholders, including the investors in this offering.

We do not currently intend to pay dividends on our common stock, and any return to investors is expected to come, if at all, only from potential increases in the price of our common stock.

We have never declared or paid any cash dividends on our capital stock, and you should not rely on an investment in our common stock to provide dividend income. We currently intend to retain all of our future earnings, if any, to finance the growth and development of our business and do not anticipate declaring or paying any cash dividends for the foreseeable future. As a result, capital appreciation, if any, of our common stock will be your sole source of gain for the foreseeable future.

An active trading market for our common stock may not be sustained following this offering.

Although our common stock is listed on the Nasdaq Global Select Market, an active trading market for our shares may not be sustained. If an active market for our common stock does not continue, it may be difficult for you to sell your shares, including shares you may purchase in this offering, without depressing the market price for the shares or sell your shares at all. Any inactive trading market for our common stock may also impair our ability to raise capital to continue to fund our operations by selling shares and may impair our ability to acquire other companies or technologies by using our shares as consideration.

Risks Related to our Business

Clinical development involves a lengthy and expensive process with an uncertain outcome, and results of earlier studies and trials may not be predictive of future trial results.

All of our product candidates are in preclinical or early clinical development and their risk of failure is high. It is impossible to predict when or if any of our product candidates will prove effective and safe in humans or will receive regulatory approval. Before obtaining marketing approval from regulatory authorities for the sale of any product candidate, we must complete preclinical studies and then conduct extensive clinical trials to demonstrate the safety and efficacy of our product candidates in humans. Clinical testing is expensive and can take many years to complete, and its outcome is inherently uncertain. Failure can occur at any time during the development process. The results of preclinical studies and early clinical trials of our product candidates,

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including ATI-052, may not be predictive of the results of later-stage clinical trials. Product candidates in later stages of clinical trials may fail to show the desired safety and efficacy traits despite having progressed through preclinical studies and initial clinical trials. A number of companies in the pharmaceutical industry have suffered significant setbacks in advanced clinical trials due to lack of efficacy or safety profiles, notwithstanding promising results in earlier trials.

Further, others, including regulatory agencies, may not accept or agree with our assumptions, estimates, calculations, conclusions or analyses or may interpret or weigh the importance of data differently, which could impact the potential of the particular program, the likelihood of marketing approval or commercialization of the particular product candidate, any approved product, and our company in general. In addition, the information we choose to publicly disclose regarding a particular study or clinical trial is derived from information that is typically extensive, and you or others may not agree with what we determine is material or otherwise appropriate information to include in our disclosure, and any information we determine not to disclose may ultimately be deemed significant with respect to future decisions, conclusions, views, activities or otherwise regarding a particular program, product candidate or our business.

If the results of later stage clinical trials differ from the data that we report from preclinical or early clinical trials, or otherwise fail to show the desired safety and efficacy traits, or if others, including regulatory authorities, disagree with the conclusions reached, our ability to identify and consummate transactions with third-party partners to further develop, obtain marketing approval for and/or commercialize our product candidates may be harmed, which could harm our business, operating results, prospects or financial condition.

Interim, topline and preliminary data from our clinical trials that we announce or publish from time to time may change as more subject data become available and are subject to audit and verification procedures that could result in material changes in the final data.

From time to time, we may publicly disclose interim, topline or preliminary data from our clinical trials, which are based on a preliminary analysis of then-available data, and the results and related findings and conclusions are subject to change following a full analysis of all data related to the particular trial. For example, we recently announced Phase 1b results for ATI-052 based on the primary analysis, rather than the complete trial data set. These Phase 1b trials enrolled a small number of patients, and in the AD trial, the per-protocol population included only one patient in the placebo arm. In addition, certain of the reported results were based on post hoc or subgroup analyses, which can be subject to variability and may not be predictive of results in larger, later-stage trials. We also make assumptions, estimations, calculations and conclusions as part of our analyses of data, and we may not have received or had the opportunity to fully and carefully evaluate all data. In addition, we may report preliminary analyses of only certain endpoints rather than all endpoints. As a result, the interim, topline or preliminary results that we report may differ from future results of the same trials, or different conclusions or considerations may qualify such results, once additional data have been received and fully evaluated. Topline data also remain subject to audit and verification procedures that may result in the final data being materially different from the preliminary data we previously published. As a result, interim, topline and preliminary data should be viewed with caution until the final data are available. Interim data from clinical trials that we may complete are subject to the risk that one or more of the clinical outcomes may materially change as subject enrollment continues and more subject data become available. Adverse differences between interim, topline or preliminary data and final data could significantly harm our reputation and business prospects. Further, disclosure of interim, topline or preliminary data by us or by our competitors could result in volatility in the price of our common stock.

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If the interim, topline or preliminary data that we report differ from actual results, or if others, including regulatory authorities, disagree with the conclusions reached, our ability to identify and consummate transactions with third-party partners to further develop, obtain marketing approval for and/or commercialize our product candidates may be harmed, which could harm our business, operating results, prospects or financial condition.

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USE OF PROCEEDS

We estimate that the net proceeds to us from our issuance and sale of our securities in this offering will be approximately $46.7 million, after deducting underwriting discounts and commissions and estimated offering expenses payable by us. We will receive only nominal additional proceeds from any exercise of pre-funded warrants.

We intend to use the net proceeds from this offering for the research and development of our product candidates in our pipeline and for working capital and general corporate purposes. We may also use a portion of the net proceeds to invest in or acquire additional businesses or technologies that we believe are complementary to our own, although we have no current plans, commitments or agreements with respect to any future acquisitions as of the date of this prospectus supplement.

Our expected use of the net proceeds from this offering represents our intentions based upon our current plans and business conditions, which could change in the future as our plans and business conditions evolve. The amounts and timing of our actual expenditures may vary significantly depending on numerous factors, including the progress of our product candidate development, the status of and results from clinical trials, as well as any partnerships that we may enter into with third parties for our product candidates, and any unforeseen cash needs.

As a result, our management will retain broad discretion over the allocation of the net proceeds from this offering, and investors will be relying on the judgment of our management regarding the application of the net proceeds from this offering. The timing and amount of our actual expenditures will be based on many factors, including cash flows from operations and the anticipated growth of our business. Pending these uses, we expect to invest the net proceeds in short-term, interest bearing obligations, certificates of deposit or direct or guaranteed obligations of the United States or foreign government agencies. The goal with respect to the investment of these net proceeds is capital preservation and liquidity so that such funds are readily available to fund our operations.

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DIVIDEND POLICY

We have never declared or paid any dividends on our common stock. We anticipate that we will retain all of our future earnings, if any, for use in the operation and expansion of our business and do not anticipate paying cash dividends in the foreseeable future.

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DILUTION

If you invest in our common stock or pre-funded warrants in this offering, your interest will be diluted immediately to the extent of the difference between the offering price per share or pre-funded warrant that you will pay in this offering and the as adjusted net tangible book value per share of our common stock after this offering. Our historical net tangible book value as of June 30, 2026 was $122.5 million, or $0.88 per share of common stock. Historical net tangible book value per share represents the amount of our total tangible assets less total liabilities, divided by the number of shares of our common stock outstanding on June 30, 2026.

On a pro forma basis, after giving effect to the issuance and sale of 10,827,017 shares of our common stock under our at-the-market facility subsequent to June 30, 2026, or the Pro Forma Adjustment, our pro forma net tangible book value would have been approximately $180.9 million, or $1.20 per share.

After giving effect to our issuance and sale of 7,785,714 shares of common stock and pre-funded warrants to purchase 6,500,000 shares of common stock in this offering at an offering price of $3.50 per share or $3.49999 per pre-funded warrant (and excluding the shares of common stock to be issued and the proceeds received, if any, from exercises of the pre-funded warrants), and after deducting the underwriting discounts and commissions and estimated offering expenses payable by us, our pro forma as adjusted net tangible book value as of June 30, 2026 would have been $227.6 million, or $1.44 per share. This represents an immediate increase in pro forma net tangible book value per share of $0.24 to existing stockholders and immediate dilution of $2.06 in pro forma net tangible book value per share to new investors purchasing common stock in this offering. Dilution per share to new investors is determined by subtracting pro forma as adjusted net tangible book value per share after this offering from the offering price per share paid by new investors. The following table illustrates this per share dilution to the new investors purchasing shares of common stock in this offering:

Offering price per share			$	3.50
Net tangible book value per share as of June 30, 2026	$	0.88
Increase per share attributable to the Pro Forma Adjustment		0.32
Pro forma net tangible book value per share as of June 30, 2026	$	1.20
Increase in pro forma net tangible book value per share attributable to investors purchasing our common stock and pre-funded warrants in this offering		0.24
Pro forma as adjusted net tangible book value per share after this offering				1.44
Dilution per share to new investors			$	2.06

If holders of pre-funded warrants exercise the pre-funded warrants offered hereby in full for cash, without regard to any limitations on beneficial ownership contained in the pre-funded warrants, the pro forma as adjusted net tangible book value per share of our common stock after giving effect to this offering would be $1.38 per share, and the dilution per share to investors purchasing common stock in this offering would be $2.12 per share.

The number of shares of our common stock to be outstanding after this offering is based on 139,824,273 shares of our common stock outstanding as of June 30, 2026 and excludes:

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15,392,992 shares of our common stock issuable upon the exercise of stock options outstanding as of June 30, 2026, at a weighted average exercise price of $5.93 per share;

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3,441,263 shares of our common stock issuable upon the vesting or other settlement of restricted stock units outstanding as of June 30, 2026;

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3,000,000 shares of common stock issuable upon the exercise of warrants outstanding as of June 30, 2026 at an exercise price of $0.00001 per share;

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10,827,017 shares of our common stock issued and sold under our at-the-market facility subsequent to June 30, 2026.

New investors will experience further dilution if any of our outstanding options, restricted stock units or warrants are exercised or vest and settle, as applicable, or new options or restricted stock units are issued and exercised or vest and settle, as applicable, under our equity incentive plans. In addition, we may choose to raise additional capital due to market conditions or strategic considerations even if we believe that we have sufficient funds for our current or future operating plans. To the extent that additional capital is raised through the sale of equity or convertible debt securities, the issuance of these securities could result in further dilution to our stockholders. We also may issue additional shares of our common stock or other securities convertible into or exchangeable for our common stock as consideration for acquisitions of businesses or products, strategic alliances or joint ventures with third parties that we believe will complement or augment our existing business, which may result in significant dilution to our stockholders, including the investors in this offering.

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DESCRIPTION OF SECURITIES WE ARE OFFERING

Common Stock

We are offering shares of our common stock in this offering. See “Description of Capital Stock” in the accompanying prospectus for more information regarding our shares of common stock.

Pre-funded Warrants

The following is a summary of the material attributes and characteristics of the pre-funded warrants. The form of pre-funded warrants will be provided to the pre-funded warrant purchasers in this offering and will be filed as an exhibit to a Current Report on Form 8-K in connection with this offering and incorporated by reference into the registration statement of which this prospectus supplement forms a part. The following summary is subject in all respects to the provisions contained in the pre-funded warrants. Pre-funded warrants will be issued in certificated form only.

Duration and Exercise Price

Each pre-funded warrant entitles the holder thereof to purchase shares of our common stock at an exercise price equal to $0.00001 per share. Each pre-funded warrant will be exercisable at any time beginning on the date of issuance. The number of shares of our common stock issuable upon exercise of each pre-funded warrant is subject to appropriate adjustment in the event of certain stock dividends, stock splits, recapitalizations or similar events affecting our common stock and the exercise price. The pre-funded warrants do not expire.

Exercisability

The holder of the pre-funded warrant may exercise the pre-funded warrant by delivering an exercise notice, completed and duly signed, and payment in cash of the exercise price for the number of shares of our common stock for which the pre-funded warrant is being exercised. The holder of the pre-funded warrant may also satisfy its obligation to pay the exercise price through a “cashless exercise,” in which the holder receives the net value of the pre-funded warrant in shares of common stock determined according to the formula set forth in the pre-funded warrant. Upon a holder’s exercise of the pre-funded warrant, we will issue the shares of common stock to which the holder is entitled pursuant to such exercise within one trading day after the exercise date. If we do not issue the warrant shares within such time period, and the holder is required to purchase shares of common stock to deliver in satisfaction of a sale by the holder of the warrant shares, we will pay the holder the amount, if any, by which the holder’s purchase price (including brokerage commissions, if any) for the shares of common stock so purchased exceeds the price at which the sell order giving rise to such purchase obligation was executed, or in the alternative and at the option of the holder, either reinstate the exercised portion of the warrant (in which case such exercise shall be deemed rescinded) or deliver to the holder the number of shares of common stock that were required to be issued. A holder of the pre-funded warrant will not be entitled to exercise any portion of such pre-funded warrant that, upon giving effect to such exercise, would cause the aggregate number of shares of our common stock beneficially owned by such holder (together with its affiliates and any other persons whose beneficial ownership of common stock would be aggregated with the holder for purposes of Section 13(d) or Section 16 of the Exchange Act) to exceed 9.99% of the total number of then issued and outstanding shares of common stock, as such percentage ownership is determined in accordance with the terms of the pre-funded warrant and subject to such holder’s rights under the pre-funded warrant to increase or decrease such percentage to any other percentage not in excess of 19.99% upon, with respect to an increase, at least 61 days’ prior notice from such holder to us.

Fundamental Transaction

In the event of any fundamental transaction, as described in the pre-funded warrants and generally including the sale of all or a majority of our properties or assets or the consummation of any merger with or into another entity, tender offer or exchange offer, or stock purchase agreement or business combination resulting in another party acquiring a majority of our shares of common stock, then the pre-funded warrant, unless earlier exercised, will be deemed to have been fully exercised immediately prior to the consummation of the fundamental transaction, without regard to any limitations on exercise.

Transferability

Subject to applicable laws, a pre-funded warrant may be transferred at the option of the holder upon surrender of the pre-funded warrant to us together with the appropriate instruments of transfer and payment of funds sufficient to pay any transfer taxes (if applicable).

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Exchange Listing

There is no trading market available for the pre-funded warrants on any securities exchange or nationally recognized trading system. We do not intend to list the pre-funded warrants on the Nasdaq Global Select Market or any securities exchange or nationally recognized trading system. We will initially serve as the warrant agent under the pre-funded warrants.

Rights as a Stockholder

Except as otherwise provided in the pre-funded warrants or by virtue of such holder’s ownership of shares of our common stock, the holders of the pre-funded warrants do not have the rights or privileges of holders of our common stock, including any voting rights, until they exercise their pre-funded warrants.

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MATERIAL U.S. FEDERAL INCOME TAX CONSEQUENCES

The following is a summary of the material U.S. federal income tax consequences to U.S. holders and non-U.S. holders (each, as defined below) of the acquisition, ownership and disposition of our common stock and pre-funded warrants, which we collectively refer to as our securities. This discussion is not a complete analysis of all potential tax considerations relating thereto and does not address the potential application of other U.S. federal tax laws, such estate or gift tax considerations, or any tax considerations arising under any state, local or non-U.S. tax laws. This discussion is based on the Internal Revenue Code of 1986, as amended, or the Code, Treasury Regulations promulgated thereunder, judicial decisions and published rulings and administrative pronouncements of the U.S. Internal Revenue Service, or the IRS, all as in effect as of the date of this prospectus supplement. These authorities are subject to change and to differing interpretations, possibly retroactively, which may result in U.S. federal income tax consequences different from those discussed below. We have not requested a ruling from the IRS with respect to the statements made and the conclusions reached in the following summary, and there can be no assurances that the IRS or a court will agree with such statements and conclusions.

This discussion is limited to holders who purchase our securities pursuant to this offering and who hold our securities as a “capital asset” within the meaning of Section 1221 of the Code (generally, property held for investment). This discussion does not address all of the U.S. federal income tax consequences that may be relevant to a particular holder in light of such holder’s particular circumstances, including the Medicare contribution tax on net investment income, the alternative minimum tax provisions of the Code, and the impact of special tax accounting rules under Section 451(b) of the Code. This discussion also does not consider any specific facts or circumstances that may be relevant to holders subject to special rules under the U.S. federal income tax laws, including:

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certain former citizens or long-term residents of the United States;

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S-corporations, partnerships or other entities or arrangements treated as pass-throughs for U.S. federal income tax purposes (and investors therein);

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controlled foreign corporations, passive foreign investment companies, and corporations that accumulate earnings to avoid U.S. federal income tax;

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banks, insurance companies, real estate investment trusts, regulated investment companies, investment funds, and other financial institutions;

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brokers, dealers, or traders in securities or currencies;

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tax-exempt organizations and governmental organizations;

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pension plans (including “qualified foreign pension funds” as defined in Section 897(l)(2) of the Code) and tax-qualified retirement plans;

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persons who hold or receive our securities through the exercise of an option or otherwise as compensation;

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persons that own, or have owned, actually or constructively, more than 5% of our securities;

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persons holding our securities as part of a hedge, straddle or other risk reduction strategy, or as part of a conversion transaction or other integrated investment; and

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persons deemed to sell our securities under the constructive sale provisions of the Code.

If an entity or arrangement that is classified as a partnership or other pass-through entity for U.S. federal income tax purposes holds our securities, the U.S. federal income tax treatment of a partner (or owner of such pass-through entity) generally will depend on the status of the partner (or owner) and the activities of the partnership or such other entity. Partnerships and other pass-through entities holding our securities and the partners or other owners thereof are urged to consult their tax advisors about the particular U.S. federal income tax consequences to them of acquiring, holding, and disposing of our securities.

THIS DISCUSSION IS FOR INFORMATIONAL PURPOSES ONLY AND IS NOT TAX ADVICE. PROSPECTIVE INVESTORS SHOULD CONSULT THEIR TAX ADVISORS REGARDING THE PARTICULAR U.S. FEDERAL INCOME TAX CONSEQUENCES TO THEM OF ACQUIRING, OWNING AND DISPOSING OF OUR SECURITIES, AS WELL AS ANY TAX CONSEQUENCES ARISING UNDER ANY STATE, LOCAL OR NON-U.S. TAX LAWS AND ANY OTHER U.S. FEDERAL TAX LAWS.

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Definition of U.S. Holder and Non-U.S. Holder

For purposes of this discussion, a U.S. holder is any U.S. person that is a beneficial owner of our securities. A U.S. person is any person that, for U.S. federal income tax purposes, is or is treated as any of the following:

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an individual citizen or resident of the United States;

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a corporation or other entity taxable as a corporation for U.S. federal income tax purposes created or organized under the laws of the United States, any state thereof, or the District of Columbia;

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an estate, the income of which is subject to U.S. federal income tax regardless of its source; or

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a trust (i) whose administration is subject to the primary supervision of a U.S. court and which has one or more U.S. persons who have the authority to control all substantial decisions of the trust, or (ii) that has a valid election in effect under applicable Treasury Regulations to be treated as a U.S. person.

For purposes of this discussion, a non-U.S. holder is any person who is a beneficial owner of our securities that is not a “U.S. person” or a partnership or other pass-through entity for U.S. federal income tax purposes (or an investor therein).

Treatment of Pre-funded Warrants

Although it is not entirely free from doubt, the pre-funded warrants are generally expected to be treated as our common stock for U.S. federal income tax purposes and a holder of a pre-funded warrant should generally be taxed in the same manner as a holder of such common stock, as described below. Accordingly, no gain or loss should be recognized (other than with respect to cash paid in lieu of a fractional share of common stock) upon exercise of a pre-funded warrant (however, in the case of a cashless exercise, the treatment for U.S. federal income tax purposes is not clear), and the holding period of a pre-funded warrant should carry over to the common stock received upon exercise. Similarly, the tax basis of the pre-funded warrant should carry over to the common stock received upon exercise, increased by the exercise price.

The remainder of this discussion assumes the characterization described above is respected for U.S. federal income tax purposes. However, our characterization is not binding on the IRS and the IRS may treat a pre-funded warrant as a warrant to acquire our common stock, in which case the amount and character of any gain or loss with respect to an investment in our pre-funded warrants could be materially different than the discussion set forth below. Each U.S. holder and non-U.S. holder is urged to consult its tax advisor regarding the tax considerations and risks associated with the acquisition, ownership, and disposition of pre-funded warrants pursuant to this offering (including potential recharacterizations).

Constructive Distributions on Pre-funded Warrants

The terms of each pre-funded warrant provide for adjustments to the terms of the pre-funded warrants upon certain events. An adjustment which has the effect of preventing dilution generally is not a taxable event. Nevertheless, a U.S. holder or non-U.S. holder of pre-funded warrants may be treated as receiving a constructive distribution from us if, for example, an adjustment (or the non-occurrence of an adjustment) increases the holder’s proportionate interest in our assets or earnings and profits (e.g., through an increase in the number of shares of common stock that would be obtained upon exercise). Such constructive distribution would be subject to tax in the same manner as if such U.S. holder or non-U.S. holder received a cash distribution from us, equal in amount to the fair market value of such increased interest, as described below. Each U.S. holder and non-U.S. holder is urged to consult its tax advisor regarding the U.S. federal income tax considerations of constructive distributions, if any, on the pre-funded warrants.

Tax Considerations for U.S. Holders

Distributions

It is unlikely that we will pay any dividends on our securities in the foreseeable future. If we make cash or other property distributions on our securities (or if we are deemed to have made a constructive distribution as described above), such distributions will constitute dividends for U.S. federal income tax purposes to the extent paid from our current or accumulated earnings and profits, as determined under U.S. federal income tax principles. Distributions constituting dividend income to certain non-corporate U.S. holders (including individuals) may qualify for reduced rates applicable to qualified dividend income, provided that certain holding

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period and other requirements are satisfied. Distributions constituting dividend income to corporate U.S. holders may qualify for the dividends received deduction, subject to applicable limitations.

Sale or Other Taxable Disposition

A U.S. holder generally will recognize capital gain or loss on a sale, exchange, redemption (other than a redemption that is treated as a distribution) or other taxable disposition of our securities equal to the difference between the amount realized upon the disposition and the U.S. holder’s adjusted tax basis in the securities so disposed. Such capital gain or loss will be a long-term capital gain or loss if the U.S. holder’s holding period for the securities disposed of exceeds one year at the time of disposition. Long-term capital gains of certain non-corporate taxpayers (including individuals) generally will be subject to reduced rates of U.S. federal income tax. The deductibility of net capital losses is subject to limitations.

Information Reporting and Backup Withholding

A U.S. holder may be subject to information reporting and backup withholding if such holder receives distributions (including constructive distributions) on our securities or proceeds from the sale or other taxable disposition of our securities. Certain U.S. holders are exempt from backup withholding. Backup withholding will apply to payments to a U.S. holder unless such holder provides proof of an applicable exemption or a correct taxpayer identification number and otherwise complies with applicable requirements of the backup withholding rules.

Backup withholding is not an additional tax. Any amount withheld under the backup withholding rules from a payment to a U.S. holder is allowable as a credit against such U.S. holder’s U.S. federal income tax, which may entitle the U.S. holder to a refund, provided that the U.S. holder timely provides the required information to the IRS. Moreover, certain penalties may be imposed by the IRS on a U.S. holder who is required to furnish information but does not do so in the proper manner.

Tax Considerations for Non-U.S. Holders

Distributions

Amounts not treated as dividends for U.S. federal income tax purposes will constitute a return of capital and will first be applied against and reduce a holder’s tax basis in our securities, but not below zero. Any excess will be treated as gain realized on the sale or other disposition of our securities and will be treated as described under “— Sale or Other Taxable Disposition” below. We may not know the extent to which a distribution is a dividend for U.S. federal income tax purposes at the time it is made. Therefore, for purposes of the withholding rules discussed below, we or the applicable withholding agent may treat the entire distribution as a dividend.

Subject to the discussion below regarding effectively connected income, any dividend income paid to a non-U.S. holder of our securities generally will be subject to U.S. federal withholding tax at a rate of 30% of the gross amount of the dividends, or such lower rate specified by an applicable income tax treaty. To receive the benefit of a reduced treaty rate, a non-U.S. holder must furnish a valid IRS Form W-8BEN or IRS Form W-8BEN-E (or applicable successor form) certifying such holder’s qualification for the reduced rate. This certification must be provided to the applicable withholding agent before the payment of dividends and must be updated periodically. Non-U.S. holders that do not provide the required certification on a timely basis, but that qualify for a reduced treaty rate, may obtain a refund of any excess amounts withheld by timely filing an appropriate claim for refund with the IRS.

If a non-U.S. holder holds our securities in connection with the conduct of a trade or business in the United States, and dividends paid on our securities are effectively connected with such holder’s U.S. trade or business

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(and are attributable to such holder’s permanent establishment in the United States if required by an applicable tax treaty), the non-U.S. holder will be exempt from U.S. federal withholding tax. To claim the exemption, the non-U.S. holder generally must furnish a valid IRS Form W-8ECI (or applicable successor form) to the applicable withholding agent.

However, any such effectively connected dividends paid on our securities generally will be subject to U.S. federal income tax on a net income basis at the regular U.S. federal income tax rates in the same manner as if such holder were a resident of the United States. A non-U.S. holder that is a foreign corporation also may be subject to an additional branch profits tax equal to 30% (or such lower rate specified by an applicable income tax treaty) of its effectively connected earnings and profits for the taxable year, as adjusted for certain items. Non-U.S. holders should consult their tax advisors regarding these rules and any applicable income tax treaties that may provide for different rules.

Sale or Other Taxable Disposition

Subject to the discussion below regarding backup withholding and FATCA, a non-U.S. holder generally will not be subject to U.S. federal income tax on any gain realized on the sale or other taxable disposition of our securities, unless:

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the gain is effectively connected with the non-U.S. holder’s conduct of a trade or business in the United States, and if required by an applicable income tax treaty, is attributable to a permanent establishment maintained by the non-U.S. holder in the United States;

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the non-U.S. holder is a nonresident alien individual present in the United States for 183 days or more during the taxable year of the disposition, and certain other requirements are met; or

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our securities constitute a “United States real property interest” by reason of our status as a United States real property holding corporation, for U.S. federal income tax purposes at any time within the shorter of the five-year period preceding the disposition or the non-U.S. holder’s holding period for our securities, and our securities are not regularly traded on an established securities market during the calendar year in which the sale or other disposition occurs.

Gain described in the first bullet point above generally will be subject to U.S. federal income tax on a net income basis at the regular U.S. federal income tax rates in the same manner as if such holder were a resident of the United States. A non-U.S. holder that is a foreign corporation also may be subject to an additional branch profits tax equal to 30% (or such lower rate specified by an applicable income tax treaty) of its effectively connected earnings and profits for the taxable year, as adjusted for certain items.

A non-U.S. holder described in the second bullet point above will be subject to U.S. federal income tax at a flat 30% rate (or such lower rate specified by an applicable income tax treaty) on any gain realized on the sale or other taxable disposition of our securities, but any such gain may be offset by certain U.S.-source capital losses (even though the individual is not considered a resident of the United States), provided that the non-U.S. holder has timely filed U.S. federal income tax returns with respect to such losses. Non-U.S. holders should consult their tax advisors regarding any applicable income tax treaties that may provide for different rules.

Determining whether we are a United States real property holding corporation as described in the third bullet point above depends on the fair market value of our U.S. real property interests relative to the fair market value of our other trade or business assets and our foreign real property interests. We believe that we are not currently, and we do not anticipate becoming, a United States real property holding corporation for U.S. federal income tax purposes but cannot give assurances that we are not, or will not become, a United States real property holding corporation. Even if we are, or were to become, a United States real property holding corporation, gain arising from the sale or other taxable disposition by a non-U.S. holder of our securities will not be subject to U.S. federal income tax on transfers of United States real property holding corporation shares if our securities are “regularly traded,” as defined by applicable Treasury Regulations, on an established securities market, and such non-U.S. holder owned, actually and constructively, 5% or less of our securities throughout the shorter of the five-year period ending on the date of the sale or other taxable disposition or the non-U.S. holder’s holding period. We expect that our common stock but not our pre-funded warrants will be treated as regularly traded on an established securities market, but there can be no assurance that any of our securities will qualify or continue to qualify as regularly traded on an established securities market. If any gain on a non-U.S. holder’s disposition is taxable because we are a United States real property holding corporation and our securities are not treated as regularly traded on an established securities market, the

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non-U.S. holder will be taxed on such disposition generally in the same manner as gain that is effectively connected with the conduct of a U.S. trade or business (subject to the provisions under an applicable income tax treaty), except that the branch profits tax generally will not apply.

Information Reporting and Backup Withholding

Annual reports are required to be filed with the IRS and provided to each non-U.S. holder indicating the amount of dividends on our securities (including dividends deemed paid with respect to the pre-funded warrants, if any, as described above) paid to such holder and the amount of any tax withheld with respect to those dividends. These information reporting requirements apply even if no withholding was required because the dividends were effectively connected with the holder’s conduct of a U.S. trade or business, or withholding was reduced or eliminated by an applicable income tax treaty. This information also may be made available under a specific treaty or agreement with the tax authorities in the country in which the non-U.S. holder resides or is established.

Backup withholding generally will not apply to payments to a non-U.S. holder of dividends on, or the gross proceeds of a disposition of, our securities, provided the non-U.S. holder furnishes the required certification for its non-U.S. status, such as by providing a valid IRS Form W-8BEN, IRS Form W-8BEN-E or IRS Form W-8ECI, or certain other requirements are met. Backup withholding may apply if the payor has actual knowledge, or reason to know, that the holder is a U.S. person who is not an exempt recipient.

Information reporting and backup withholding generally will apply to the proceeds of a disposition of our securities by a non-U.S. holder effected by or through the U.S. office of any broker, U.S. or foreign, unless the holder certifies its status as a non-U.S. holder and satisfies certain other requirements, or otherwise establishes an exemption. Generally, information reporting and backup withholding will not apply to a payment of disposition proceeds to a non-U.S. holder where the transaction is effected outside the United States through a non-U.S. office of a non-U.S. broker. However, for information reporting purposes, dispositions effected through a non-U.S. office of a broker with substantial U.S. ownership or operations generally will be treated in a manner similar to dispositions effected through a U.S. office of a broker.

Backup withholding is not an additional tax. If any amount is withheld under the backup withholding rules, the non-U.S. holder should consult with a U.S. tax advisor regarding the possibility of and procedure for obtaining a refund or a credit against the non-U.S. holder’s U.S. federal income tax liability, if any.

Withholding on Foreign Entities or Accounts

Sections 1471 to 1474 of the Code, or FATCA, imposes a U.S. federal withholding tax of 30% on certain payments made to a “foreign financial institution” (as specially defined under these rules) unless such institution enters into an agreement with the U.S. government to withhold on certain payments and to collect and provide to the U.S. tax authorities substantial information regarding certain U.S. account holders of such institution (which includes certain equity and debt holders of such institution, as well as certain account holders that are foreign entities with U.S. owners) or an exemption applies. FATCA also generally will impose a U.S. federal withholding tax of 30% on certain payments made to a non-financial foreign entity unless such entity provides the withholding agent a certification identifying certain direct and indirect U.S. owners of the entity or an exemption applies. An intergovernmental agreement between the United States and an applicable foreign country may modify these requirements. Under certain circumstances, a non-U.S. holder might be eligible for refunds or credits of such taxes.

FATCA applies to dividends paid on our securities. Proposed regulations issued by the Treasury Department (on which taxpayers are entitled to rely until final regulations are issued) eliminate the federal withholding tax of 30% imposed by FATCA to gross proceeds of a sale or other disposition of our securities. Prospective investors are encouraged to consult with their own tax advisors regarding the possible implications of this FATCA on their investment in our securities.

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UNDERWRITING

Subject to the terms and conditions set forth in the underwriting agreement, dated October 27, 2026, between us and Leerink Partners LLC, Piper Sandler & Co. and Cantor Fitzgerald & Co., as the representatives of the underwriters named below and joint bookrunning managers of this offering, we have agreed to sell to the underwriters, and each of the underwriters has agreed, severally and not jointly, to purchase from us, the respective number of shares of common stock and pre-funded warrants shown opposite its name below.

Underwriter	Number of Shares		Number of Pre-funded Warrants
Leerink Partners LLC		2,958,571		2,470,000
Piper Sandler & Co.		2,102,143		1,755,000
Cantor Fitzgerald & Co.		2,024,286		1,690,000
LifeSci Capital LLC		700,714		585,000
Total		7,785,714		6,500,000

The underwriting agreement provides that the obligations of the several underwriters are subject to certain conditions precedent such as the receipt by the underwriters of officers’ certificates and legal opinions and approval of certain legal matters by their counsel. The underwriting agreement provides that the underwriters will purchase all of the shares of common stock and pre-funded warrants if any of them are purchased. If an underwriter defaults, the underwriting agreement provides that the purchase commitments of the non-defaulting underwriters may be increased or the underwriting agreement may be terminated.

We have agreed to indemnify the underwriters and certain of their controlling persons against certain liabilities, including liabilities under the Securities Act, and to contribute to payments that the underwriters may be required to make in respect of those liabilities.

The underwriters have advised us that, following the completion of this offering, they currently intend to make a market in the common stock as permitted by applicable laws and regulations. However, the underwriters are not obligated to do so, and the underwriters may discontinue any market-making activities at any time without notice in their sole discretion. Accordingly, no assurance can be given as to the liquidity of the trading market for the common stock, that you will be able to sell any of the common stock held by you at a particular time or that the prices that you receive when you sell will be favorable. There is currently no trading market for the pre-funded warrants and no such trading market is expected to develop.

The underwriters are offering the shares of common stock and pre-funded warrants, subject to their acceptance of the shares of common stock and pre-funded warrants from us and subject to prior sale. The underwriters reserve the right to withdraw, cancel or modify offers and to reject orders in whole or in part.

Commission and Expenses

The underwriters have advised us that they propose to offer the shares of common stock and pre-funded warrants at the offering prices set forth on the cover page of this prospectus supplement and to certain dealers, which may include the underwriters, at that price less a concession not in excess of $0.126 per share of common stock or pre-funded warrant. After the offering, the offering price and concession may be reduced by the representatives. No such reduction will change the amount of proceeds to be received by us as set forth on the cover page of this prospectus supplement.

The following table shows the offering price, the underwriting discounts and commissions that we are to pay the underwriters and the proceeds, before expenses, to us in connection with this offering.

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	Per Share of Common Stock			Per Pre-funded Warrant
Offering price		$	3.50		$	3.49999
Underwriting discounts and commissions paid by us		$	0.21		$	0.21000
Proceeds to us, before expenses		$	3.29		$	3.28999

We estimate expenses payable by us in connection with this offering, other than the underwriting discounts and commissions referred to above, will be approximately $300,000. We also have agreed to reimburse the underwriters for up to $20,000 relating to determining this offering’s compliance with the rules of the Financial Industry Regulatory Authority, Inc., or FINRA, and will pay $211,910.52 in advisory fees to an affiliate of an underwriter. In accordance with FINRA Rule 5110, the underwriters’ reimbursed fee and the advisory fees are deemed underwriting compensation in connection with this offering.

Listing

Our common stock is listed on the Nasdaq Global Select Market under the trading symbol “ACRS.” The pre-funded warrants are not and will not be listed on a national securities exchange.

No Sales of Similar Securities

We, our officers and our directors have agreed, subject to specified exceptions, without the prior written consent of the representatives, not to directly or indirectly:

▪

sell, offer to sell, contract to sell or lend, pledge, hypothecate or grant any security interest, transfer, effect any short sale or establish or increase a “Put Equivalent Position” or liquidate or decrease any “Call Equivalent Position” within the meaning of Rules 16a-1(b) and 16a-1(h) under the Exchange Act, with respect to our common stock, options or warrants or other rights to acquire shares of common stock, or securities exchangeable or exercisable for or convertible into shares of common stock (collectively with the common stock, “lock-up securities”), or

▪

enter into any swap, hedge or similar arrangement or agreement that transfers, in whole or in part, the economic risk of ownership of lock-up securities, or

▪

otherwise dispose of any lock-up securities, or

▪

solely as it applies to our directors and officers, make any demand for, or exercise any right with respect to, the registration under the Securities Act of the offer and sale of any lock-up securities, or cause to be filed a registration statement, prospectus or prospectus supplement (or an amendment or supplement thereto) with respect to any such registration, or

▪

solely as it applies to us, announce the offering of any lock-up securities, submit or file any registration statement under the Securities Act in respect of any lock-up securities (other than with respect to this offering or filings of Form S-8 registration statements relating to our equity incentive plans existing as of the date of this prospectus supplement), or effect a reverse stock split, recapitalization, share consolidation, reclassification or similar transaction affecting the outstanding shares of common stock, or

▪

publicly announce an intention to do any of the foregoing.

This restriction terminates after the close of trading of the common stock on and including the 60th day after the date of this prospectus supplement.

With respect to us, this restriction does not apply to (i) the transactions contemplated by this offering, (ii) the issuance by us of common stock or options to purchase common stock, the issuance of restricted stock units, or the issuance of common stock upon exercise of options or vesting of restricted stock units, (iii) the issuance and sale of common stock pursuant to the conversion or exchange of convertible or exchangeable securities or the exercise of options or warrants, (iv) the issuance of common stock in connection with any joint venture, commercial or collaborative relationship or the acquisition or license by us of the securities, businesses, property or other assets of another person or entity or pursuant to any employee benefit plan as assumed by us in connection with any such acquisition, provided, however, such shares of common stock shall not in the aggregate exceed five percent of our outstanding shares of common stock immediately following the

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consummation of this offering and that each recipient executes and delivers to the representatives a lock-up agreement in the form entered into by our directors and officers, or (v) the issuance and sale, from time to time, of common stock pursuant to the Second Amended and Restated Sales Agreement, dated August 6, 2026, or the Sales Agreement, by and among us, Leerink Partners LLC and Cantor Fitzgerald & Co., provided that no sales shall be made under the Sales Agreement until the 30th day following the date of the underwriting agreement.

With respect to our directors and officers, subject to certain conditions, this restriction does not apply to (i) the transfer of lock-up securities by gift or by will or intestate succession to the legal representative, heir, beneficiary or any family member or to a trust whose beneficiaries consist exclusively of one or more of the director or officer and/or family member(s), (ii) transfers or dispositions of the director’s or officer’s lock-up securities to any corporation, partnership, limited liability company or other entity all of the beneficial ownership interests of which are held by such director or officer or a family member, (iii) distributions of the director’s or officer’s lock-up securities to partners, members, stockholders or other equityholders of such director or officer, provided that any such transfer or distribution shall not involve a disposition of value, (iv) the transfer of lock-up securities by operation of law, including pursuant to a domestic order or negotiated divorce settlement, (v) the exercise of options to purchase shares of common stock granted under any equity incentive plan or stock purchase plan, existing as of the date of the lock-up agreement, (vi) the establishment of a trading plan pursuant to Rule 10b5-1 under the Exchange Act for the transfer of shares of common stock, provided that such plan does not provide for any transfers of lock-up securities during the 60-day lock-up period, (vii) the transfer or disposal of shares of common stock acquired in the open market following this offering, and (viii) the transfer of shares of common stock (A) to us as forfeitures to satisfy tax withholding obligations of the director or officer in connection with the vesting or exercise of equity awards by such director or officer pursuant to our equity incentive plans existing as of the date of the lock-up agreement, (B) pursuant to a net exercise or cashless exercise by the director or officer of outstanding equity awards pursuant to our equity incentive plans existing as of the date of the lock-up agreement, including any transfer of common stock to us for the payment of tax withholdings or remittance payments due as a result of the vesting, settlement or exercise of such equity awards, or sales of common stock pursuant to a “sell-to-cover” arrangement upon the vesting, settlement or exercise of such equity awards solely to cover tax withholding obligations in connection with such transaction and any transfer of common stock to us for the payment of taxes as a result of such transaction, or (C) pursuant to a bona fide third-party tender offer for all of our outstanding shares, merger, consolidation or other similar transaction made to all holders of our securities involving a change of control (including, without limitation, the entering into any lock-up, voting or similar agreement pursuant to which the director or officer may agree to transfer, sell, tender or otherwise dispose of common stock or other such securities in connection with such transaction, or vote any common stock or other such securities in favor of any such transaction); provided, however, that (A) in any such case, it shall be a condition to a transfer pursuant to subparagraphs (i), (ii), (iii) and (iv) that each transferee executes and delivers to the representatives an agreement in form and substance satisfactory to the representatives stating that such transferee is receiving and holding such lock-up securities subject to the provisions of the lock-up agreement and agrees not to sell or offer to sell such lock-up securities, engage in any swap or engage in any other activities restricted under the lock-up agreement except in accordance with the lock-up agreement, and (B) in any such case it shall be a condition to a transfer that prior to the expiration of the 60-day lock-up period, no public disclosure or filing under the Exchange Act by any party to the transfer (donor, donee, transferor or transferee) shall be required, or made voluntarily, reporting a reduction in beneficial ownership of common stock in connection with such transfer, except as provided in subparagraph (viii) above.

The representatives may, in their sole discretion and at any time or from time to time before the termination of the 60-day period release all or any portion of the securities subject to lock-up agreements. There are no existing agreements between the underwriters and any of our directors and officers who will execute a lock-up agreement, providing consent to the sale of shares prior to the expiration of the lock-up period.

Stabilization

The underwriters have advised us that they, pursuant to Regulation M under the Exchange Act, may engage in short sale transactions, stabilizing transactions, syndicate covering transactions or the imposition of penalty bids in connection with this offering. These activities may have the effect of stabilizing or maintaining the market price of the common stock at a level above that which might otherwise prevail in the open market.

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Short sales involve the sale by the underwriters of a greater number of shares of common stock than they are required to purchase in this offering. Because the underwriters do not have an option to purchase additional shares from us, the underwriters must close out any short position by purchasing shares of our common stock in the open market. A short position is more likely to be created if the underwriters are concerned that there may be downward pressure on the price of the shares of our common stock in the open market after pricing that could adversely affect investors who purchase in this offering.

A stabilizing bid is a bid for the purchase of shares of common stock on behalf of the underwriters for the purpose of fixing or maintaining the price of the common stock. A syndicate covering transaction is the bid for or the purchase of shares of common stock on behalf of the underwriters to reduce a short position incurred by the underwriters in connection with the offering. Similar to other purchase transactions, the underwriter’s purchases to cover the syndicate short sales may have the effect of raising or maintaining the market price of our common stock or preventing or retarding a decline in the market price of our common stock. As a result, the price of our common stock may be higher than the price that might otherwise exist in the open market. A penalty bid is an arrangement permitting the underwriters to reclaim the selling concession otherwise accruing to a syndicate member in connection with the offering if the common stock originally sold by such syndicate member are purchased in a syndicate covering transaction and therefore have not been effectively placed by such syndicate member.

Neither we nor any of the underwriters make any representation or prediction as to the direction or magnitude of any effect that the transactions described above may have on the price of our common stock. The underwriters are not obligated to engage in these activities and, if commenced, any of the activities may be discontinued at any time.

The underwriters may also engage in passive market making transactions in our common stock on the Nasdaq Global Select Market in accordance with Rule 103 of Regulation M under the Exchange Act during a period before the commencement of offers or sales of shares of our common stock in this offering and extending through the completion of distribution. A passive market maker must display its bid at a price not in excess of the highest independent bid of that security. However, if all independent bids are lowered below the passive market maker’s bid, that bid must then be lowered when specified purchase limits are exceeded.

Electronic Distribution

A prospectus in electronic format may be made available by e-mail or on the web sites or through online services maintained by one or more of the underwriters or their affiliates. In those cases, prospective investors may view offering terms online and may be allowed to place orders online. The underwriters may agree with us to allocate a specific number of shares of common stock for sale to online brokerage account holders. Any such allocation for online distributions will be made by the underwriters on the same basis as other allocations. Other than the prospectus in electronic format, the information on the underwriters’ web sites and any information contained in any other web site maintained by any of the underwriters is not part of this prospectus supplement and the accompanying prospectus, has not been approved and/or endorsed by us or the underwriters and should not be relied upon by investors.

Other Activities and Relationships

The underwriters and certain of their respective affiliates are full service financial institutions engaged in various activities, which may include securities trading, commercial and investment banking, financial advisory, investment management, investment research, principal investment, hedging, financing and brokerage activities. The underwriters and certain of their respective affiliates have, from time to time, performed, and may in the future perform, various commercial and investment banking and financial advisory services for us and our affiliates, for which they received or will receive customary fees and expenses. For example, Leerink Partners LLC and Cantor Fitzgerald & Co. are sales agents under the Sales Agreement.

In the ordinary course of their various business activities, the underwriters and certain of their affiliates may make or hold a broad array of investments and actively trade debt and equity securities (or related derivative securities) and financial instruments (including bank loans) for their own account and for the accounts of their customers, and such investments and securities activities may involve securities and/or instruments of ours or our affiliates. If the underwriters or their respective affiliates have a lending relationship with us, they routinely hedge their credit exposure to us consistent with their customary risk management policies. The

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underwriters and their respective affiliates may hedge such exposure by entering into transactions which consist of either the purchase of credit default swaps or the creation of short positions in our securities or the securities of our affiliates, including potentially the common stock offered hereby. Any such short positions could adversely affect future trading prices of the common stock offered hereby. The underwriters and certain of their respective affiliates may also communicate independent investment recommendations, market color or trading ideas and/or publish or express independent research views in respect of such securities or instruments and may at any time hold, or recommend to clients that they acquire, long and/or short positions in such securities and instruments.

Notice to Prospective Investors in the European Economic Area

In relation to each Member State of the European Economic Area, or a Relevant State, no securities have been offered or will be offered pursuant to the offering to the public in that Relevant State prior to the publication of a prospectus in relation to the securities which has been approved by the competent authority in that Relevant State or, where appropriate, approved in another Relevant State and notified to the competent authority in that Relevant State, all in accordance with the Prospectus Regulation, except that offers of securities may be made to the public in that Relevant State at any time under the following exemptions under the Prospectus Regulation:

to any legal entity which is a qualified investor as defined under the Prospectus Regulation;

to fewer than 150 natural or legal persons (other than qualified investors as defined under the Prospectus Regulation), subject to obtaining the prior consent of representatives for any such offer; or

in any other circumstances falling within Article 1(4) of the Prospectus Regulation, provided that no such offer of securities shall require us or any representative to publish a prospectus pursuant to Article 3 of the Prospectus Regulation or supplement a prospectus pursuant to Article 23 of the Prospectus Regulation.

Each person in a Relevant State who initially acquires any securities or to whom any offer is made will be deemed to have represented, acknowledged and agreed to and with us and the representatives that it is a qualified investor within the meaning of the Prospectus Regulation.

In the case of any securities being offered to a financial intermediary as that term is used in Article 5(1) of the Prospectus Regulation, each such financial intermediary will be deemed to have represented, acknowledged and agreed that the securities acquired by it in the offer have not been acquired on a non-discretionary basis on behalf of, nor have they been acquired with a view to their offer or resale to, persons in circumstances which may give rise to an offer to the public other than their offer or resale in a Relevant State to qualified investors, in circumstances in which the prior consent of the representatives has been obtained to each such proposed offer or resale.

We, the underwriters and their affiliates will rely upon the truth and accuracy of the foregoing representations, acknowledgements and agreements.

For the purposes of this provision, the expression an “offer to the public” in relation to any securities in any Relevant State means the communication in any form and by any means of sufficient information on the terms of the offer and any securities to be offered so as to enable an investor to decide to purchase or subscribe for any securities, and the expression “Prospectus Regulation” means Regulation (EU) 2017/1129.

The above selling restriction is in addition to any other selling restrictions set out below.

In connection with the offering, the underwriters are not acting for anyone other than us and will not be responsible to anyone other than us for providing the protections afforded to their clients nor for providing advice in relation to the offering.

Notice to Prospective Investors in the United Kingdom

In relation to the United Kingdom, or UK, no securities have been offered or will be offered pursuant to the offering to the public in the UK prior to the publication of a prospectus in relation to the securities which has been approved by the Financial Conduct Authority in the UK in accordance with the UK Prospectus Regulation

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and the FSMA, except that offers of securities may be made to the public in the UK at any time under the following exemptions under the UK Prospectus Regulation and the FSMA:

to any legal entity which is a qualified investor as defined under the UK Prospectus Regulation;

to fewer than 150 natural or legal persons (other than qualified investors as defined under the UK Prospectus Regulation), subject to obtaining the prior consent of representatives for any such offer; or

at any time in other circumstances falling within section 86 of the FSMA, provided that no such offer of securities shall require us or any representative to publish a prospectus pursuant to Section 85 of the FSMA or Article 3 of the UK Prospectus Regulation or supplement a prospectus pursuant to Article 23 of the UK Prospectus Regulation.

Each person in the UK who initially acquires any securities or to whom any offer is made will be deemed to have represented, acknowledged and agreed to and with us and the representatives that it is a qualified investor within the meaning of the UK Prospectus Regulation.

In the case of any securities being offered to a financial intermediary as that term is used in Article 5(1) of the UK Prospectus Regulation, each such financial intermediary will be deemed to have represented, acknowledged and agreed that the securities acquired by it in the offer have not been acquired on a non-discretionary basis on behalf of, nor have they been acquired with a view to their offer or resale to, persons in circumstances which may give rise to an offer to the public other than their offer or resale in the UK to qualified investors, in circumstances in which the prior consent of the representatives has been obtained to each such proposed offer or resale.

We, the underwriters and their affiliates will rely upon the truth and accuracy of the foregoing representations, acknowledgements and agreements.

For the purposes of this provision, the expression an “offer to the public” in relation to any securities in the UK means the communication in any form and by any means of sufficient information on the terms of the offer and any securities to be offered so as to enable an investor to decide to purchase or subscribe for any securities, the expression “UK Prospectus Regulation” means Regulation (EU) 2017/1129 as it forms part of domestic law by virtue of the European Union (Withdrawal) Act 2018, and the expression “FSMA” means the Financial Services and Markets Act 2000.

In connection with the offering, the underwriters are not acting for anyone other than the issuer and will not be responsible to anyone other than the issuer for providing the protections afforded to their clients nor for providing advice in relation to the offering.

This document is for distribution only to persons who (i) have professional experience in matters relating to investments and who qualify as investment professionals within the meaning of Article 19(5) of the Financial Services and Markets Act 2000 (Financial Promotion) Order 2005, as amended, or the Financial Promotion Order, (ii) are persons falling within Article 49(2)(a) to (d) (“high net worth companies, unincorporated associations etc.”) of the Financial Promotion Order, (iii) are outside the United Kingdom, or (iv) are persons to whom an invitation or inducement to engage in investment activity (within the meaning of Section 21 of the Financial Services and Markets Act 2000, as amended, or FSMA) in connection with the issue or sale of any securities may otherwise lawfully be communicated or caused to be communicated (all such persons together being referred to as “relevant persons”). This document is directed only at relevant persons and must not be acted on or relied on by persons who are not relevant persons. Any investment or investment activity to which this document relates is available only to relevant persons and will be engaged in only with relevant persons.

Notice to Prospective Investors in Switzerland

The securities may not be publicly offered in Switzerland and will not be listed on the SIX Swiss Exchange, or SIX, or on any other stock exchange or regulated trading facility in Switzerland. This document has been prepared without regard to the disclosure standards for issuance prospectuses under art. 652a or art. 1156 of the Swiss Code of Obligations or the disclosure standards for listing prospectuses under art. 27 ff. of the SIX Listing Rules or the listing rules of any other stock exchange or regulated trading facility in Switzerland. Neither

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this document nor any other offering or marketing material relating to the securities or the offering may be publicly distributed or otherwise made publicly available in Switzerland.

Neither this document nor any other offering or marketing material relating to the offering, our company or the securities have been or will be filed with or approved by any Swiss regulatory authority. In particular, this document will not be filed with, and the offer of securities will not be supervised by, the Swiss Financial Market Supervisory Authority FINMA, or FINMA, and the offer of securities has not been and will not be authorized under the Swiss Federal Act on Collective Investment Schemes, or CISA. The investor protection afforded to acquirers of interests in collective investment schemes under the CISA does not extend to acquirers of securities.

Notice to Prospective Investors in the Dubai International Financial Centre

This prospectus supplement relates to an Exempt Offer in accordance with the Offered Securities Rules of the Dubai Financial Services Authority, or DFSA. This prospectus supplement is intended for distribution only to persons of a type specified in the Offered Securities Rules of the DFSA. It must not be delivered to, or relied on by, any other person. The DFSA has no responsibility for reviewing or verifying any documents in connection with Exempt Offers. The DFSA has not approved this prospectus supplement nor taken steps to verify the information set forth herein and has no responsibility for the prospectus supplement. The securities to which this prospectus supplement relates may be illiquid and/or subject to restrictions on their resale. Prospective purchasers of the securities offered should conduct their own due diligence on the securities. If you do not understand the contents of this prospectus supplement you should consult an authorized financial advisor.

Notice to Prospective Investors in Australia

No placement document, prospectus, product disclosure statement or other disclosure document has been lodged with the Australian Securities and Investments Commission, or ASIC, in relation to the offering. This prospectus supplement does not constitute a prospectus, product disclosure statement or other disclosure document under the Corporations Act 2001, or the Corporations Act, and does not purport to include the information required for a prospectus, product disclosure statement or other disclosure document under the Corporations Act.

Any offer in Australia of the securities may only be made to persons, or the Exempt Investors, who are “sophisticated investors” (within the meaning of section 708(8) of the Corporations Act), “professional investors” (within the meaning of section 708(11) of the Corporations Act) or otherwise pursuant to one or more exemptions contained in section 708 of the Corporations Act so that it is lawful to offer the securities without disclosure to investors under Chapter 6D of the Corporations Act.

The securities applied for by Exempt Investors in Australia must not be offered for sale in Australia in the period of 12 months after the date of allotment under the offering, except in circumstances where disclosure to investors under Chapter 6D of the Corporations Act would not be required pursuant to an exemption under section 708 of the Corporations Act or otherwise or where the offer is pursuant to a disclosure document which complies with Chapter 6D of the Corporations Act. Any person acquiring securities must observe such Australian on-sale restrictions.

This prospectus supplement contains general information only and does not take account of the investment objectives, financial situation or particular needs of any particular person. It does not contain any securities recommendations or financial product advice. Before making an investment decision, investors need to consider whether the information in this prospectus supplement is appropriate to their needs, objectives and circumstances, and, if necessary, seek expert advice on those matters.

Notice to Prospective Investors in Hong Kong

The securities have not been offered or sold and will not be offered or sold in Hong Kong, by means of any document, other than (a) to “professional investors” as defined in the Securities and Futures Ordinance (Cap. 571) of Hong Kong and any rules made under that Ordinance; or (b) in other circumstances which do not result in the document being a “prospectus” as defined in the Companies Ordinance (Cap. 32) of Hong Kong or which do not constitute an offer to the public within the meaning of that Ordinance. No advertisement, invitation or document relating to the securities has been or may be issued or has been or may be in the possession of any person for the purposes of issue, whether in Hong Kong or elsewhere, which is directed at, or the contents of which are likely to be accessed or read by, the public of Hong Kong (except if permitted to do

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so under the securities laws of Hong Kong) other than with respect to securities which are or are intended to be disposed of only to persons outside Hong Kong or only to “professional investors” as defined in the Securities and Futures Ordinance and any rules made under that Ordinance.

Notice to Prospective Investors in Japan

The securities have not been and will not be registered under the Financial Instruments and Exchange Law of Japan (Law No. 25 of 1948, as amended) and, accordingly, will not be offered or sold, directly or indirectly, in Japan, or for the benefit of any Japanese Person or to others for re-offering or resale, directly or indirectly, in Japan or to any Japanese Person, except in compliance with all applicable laws, regulations and ministerial guidelines promulgated by relevant Japanese governmental or regulatory authorities in effect at the relevant time. For the purposes of this paragraph, “Japanese Person” shall mean any person resident in Japan, including any corporation or other entity organized under the laws of Japan.

Notice to Prospective Investors in Singapore

This prospectus supplement has not been registered as a prospectus with the Monetary Authority of Singapore. Accordingly, the securities were not offered or sold or caused to be made the subject of an invitation for subscription or purchase and will not be offered or sold or caused to be made the subject of an invitation for subscription or purchase, and this prospectus supplement or any other document or material in connection with the offer or sale, or invitation for subscription or purchase, of the securities, has not been circulated or distributed, nor will it be circulated or distributed, whether directly or indirectly, to any person in Singapore other than (i) to an institutional investor (as defined in Section 4A of the Securities and Futures Act (Chapter 289) of Singapore, as modified or amended from time to time, or the SFA) pursuant to Section 274 of the SFA, (ii) to a relevant person (as defined in Section 275(2) of the SFA) pursuant to Section 275(1) of the SFA, or any person pursuant to Section 275(1A) of the SFA, and in accordance with the conditions specified in Section 275 of the SFA, or (iii) otherwise pursuant to, and in accordance with the conditions of, any other applicable provision of the SFA.

Where the securities are subscribed or purchased under Section 275 of the SFA by a relevant person which is:

(a)

a corporation (which is not an accredited investor (as defined in Section 4A of the SFA)) the sole business of which is to hold investments and the entire share capital of which is owned by one or more individuals, each of whom is an accredited investor; or

(b)

a trust (where the trustee is not an accredited investor) whose sole purpose is to hold investments and each beneficiary of the trust is an individual who is an accredited investor,

securities or securities-based derivatives contracts (each term as defined in Section 2(1) of the SFA) of that corporation or the beneficiaries’ rights and interest (howsoever described) in that trust shall not be transferred within six months after that corporation or that trust has acquired the securities pursuant to an offer made under Section 275 of the SFA except:

(a)

to an institutional investor or to a relevant person, or to any person arising from an offer referred to in Section 275(1A) or Section 276(4)(i)(B) of the SFA;

(b)

where no consideration is or will be given for the transfer;

(c)

where the transfer is by operation of law; or

(d)

as specified in Section 276(7) of the SFA.

Notice to Prospective Investors in Canada

The securities may be sold only to purchasers purchasing, or deemed to be purchasing, as principal that are accredited investors, as defined in National Instrument 45-106 Prospectus Exemptions or subsection 73.3(1) of the Securities Act (Ontario), and are permitted clients, as defined in National Instrument 31-103 Registration Requirements, Exemptions and Ongoing Registrant Obligations. Any resale of the securities must be made in accordance with an exemption from, or in a transaction not subject to, the prospectus requirements of applicable securities laws.

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Securities legislation in certain provinces or territories of Canada may provide a purchaser with remedies for rescission or damages if this prospectus supplement (including any amendment thereto) contains a misrepresentation, provided that the remedies for rescission or damages are exercised by the purchaser within the time limit prescribed by the securities legislation of the purchaser’s province or territory. The purchaser should refer to any applicable provisions of the securities legislation of the purchaser’s province or territory for particulars of these rights or consult with a legal advisor.

Pursuant to section 3A.3 (or, in the case of securities issued or guaranteed by the government of a non-Canadian jurisdiction, section 3A.4) of National Instrument 33-105 Underwriting Conflicts (NI 33-105), the underwriters are not required to comply with the disclosure requirements of NI 33-105 regarding underwriter conflicts of interest in connection with this offering.

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LEGAL MATTERS

Cooley LLP will pass upon the validity of the issuance of the securities offered in this prospectus supplement. Covington & Burling LLP, New York, New York is acting as counsel for the underwriters in connection with this offering.

EXPERTS

The financial statements incorporated in this prospectus supplement by reference to the Annual Report on Form 10-K for the year ended December 31, 2025 have been so incorporated in reliance on the report of PricewaterhouseCoopers LLP, an independent registered public accounting firm, given on the authority of said firm as experts in auditing and accounting.

WHERE YOU CAN FIND MORE INFORMATION

This prospectus supplement is part of a registration statement on Form S-3 we filed with the SEC under the Securities Act and does not contain all the information set forth or incorporated by reference in the registration statement. Whenever a reference is made in this prospectus supplement to any of our contracts, agreements or other documents, the reference may not be complete and you should refer to the exhibits that are a part of the registration statement or the exhibits to the reports or other documents incorporated by reference into this prospectus supplement for a copy of such contract, agreement or other document.

We file annual, quarterly and current reports, proxy statements and other information with the SEC. The SEC maintains a website that contains reports, proxy statements and other information regarding issuers that file electronically with the SEC, including Aclaris. The address of the SEC website is www.sec.gov.

We maintain a website at www.aclaristx.com. Information contained in or accessible through our website does not constitute a part of this prospectus supplement.

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INCORPORATION OF CERTAIN INFORMATION BY REFERENCE

The SEC allows us to “incorporate by reference” information into this prospectus supplement and the accompanying prospectus, which means that we can disclose important information to you by referring you to another document filed separately with the SEC. The SEC file number for the documents incorporated by reference in this prospectus supplement and accompanying prospectus is 001-37581. The documents incorporated by reference into this prospectus supplement and the accompanying prospectus contain important information that you should read about us.

The following documents are incorporated by reference into this document:

▪

our Annual Report on Form 10-K for the year ended December 31, 2025, filed with the SEC on February 26, 2026;

▪

our Quarterly Reports on Form 10-Q for the fiscal quarters ended March 31, 2026 and June 30, 2026, filed with the SEC on May 7, 2026 and August 6, 2026, respectively;

▪

our Current Reports on Form 8-K filed with the SEC on January 6, 2026, April 28, 2026, June 4, 2026, August 6, 2026, and October 27, 2026, to the extent the information in such reports is filed and not furnished; and

▪

the description of our common stock contained in Exhibit 4.1 to our Annual Report on Form 10-K for the fiscal year ended December 31, 2025, filed with the SEC on February 26, 2026, and any amendment or report filed with the SEC for the purpose of updating such description.

We also incorporate by reference into this prospectus supplement all documents (other than current reports furnished under Item 2.02 or Item 7.01 of Form 8-K and exhibits filed on such form that are related to such items) that are filed by us with the SEC pursuant to Sections 13(a), 13(c), 14 or 15(d) of the Exchange Act after the date of this prospectus supplement but prior to the termination of the offering. These documents include periodic reports, such as Annual Reports on Form 10-K, Quarterly Reports on Form 10-Q and Current Reports on Form 8-K, as well as proxy statements.

We will provide to each person, including any beneficial owner, to whom a prospectus is delivered, without charge upon written or oral request, a copy of any or all of the documents that are incorporated by reference into this prospectus but not delivered with the prospectus, including exhibits that are specifically incorporated by reference into such documents. You should direct any requests for documents to Aclaris Therapeutics, Inc., Attn: Matthew Rothman, General Counsel and Corporate Secretary, 701 Lee Road, Suite 103, Wayne, Pennsylvania 19087; telephone: (484) 324-7933.

Any statement contained herein or in a document incorporated or deemed to be incorporated by reference into this prospectus supplement will be deemed to be modified or superseded for purposes of the document to the extent that a statement contained in this document or any other subsequently filed document that is deemed to be incorporated by reference into this prospectus supplement modifies or supersedes the statement.

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PROSPECTUS

Common Stock
Preferred Stock
Debt Securities
Warrants

From time to time, we or selling securityholders may offer and sell any combination of shares of our common stock, shares of our preferred stock, debt securities and/or warrants to purchase any of such securities described in this prospectus in one or more offerings as described in this prospectus. We or selling securityholders may also offer securities as may be issuable upon conversion, redemption, repurchase, exchange or exercise of any securities registered hereunder, including any applicable anti-dilution provisions.

This prospectus provides a general description of the securities we may offer. Each time we offer securities, we will specify in an accompanying prospectus supplement the terms of the securities being offered. We may also authorize one or more free writing prospectuses to be provided to you in connection with these offerings.

The prospectus supplement and any related free writing prospectus may also add, update or change information contained in this prospectus. You should carefully read this prospectus, the applicable prospectus supplement and any related free writing prospectus, as well as any documents incorporated by reference, before you invest in any of the securities being offered.

This prospectus may not be used to consummate a sale of any securities unless accompanied by a prospectus supplement.

Our common stock is traded on the Nasdaq Global Select Market under the symbol “ACRS.” On August 5, 2026, the last reported sale price of our common stock was $6.36 per share. The applicable prospectus supplement will contain information, where applicable, as to any other listing on the Nasdaq Global Select Market or any securities market or other exchange of the securities, if any, covered by the prospectus supplement.

We or selling securityholders may sell these securities directly to investors, through agents designated from time to time or to or through underwriters or dealers, on a continuous or delayed basis. If any agents or underwriters are involved in the sale of any securities with respect to which this prospectus is being delivered, the names of such agents or underwriters and any applicable fees, commissions, discounts or over-allotment options will be set forth in a prospectus supplement. The price to the public of such securities and the net proceeds we expect to receive from such sale will also be set forth in a prospectus supplement. Unless the applicable prospectus supplement provides otherwise, we will not receive any proceeds from the sale of securities by selling securityholders. For additional information on the methods of sale, you should refer to the section titled “Plan of Distribution” in this prospectus.

We are a “smaller reporting company” under the federal securities laws and, as such, are subject to reduced public company reporting requirements. See “Prospectus Summary — Implications of Being a Smaller Reporting Company.”

Investing in our securities involves a high degree of risk. You should review carefully the risks and uncertainties described under the heading “Risk Factors” on page 5 of this prospectus and in any applicable prospectus supplement and related free writing prospectus, and under similar headings in the other documents that are incorporated by reference into this prospectus.

NEITHER THE SECURITIES AND EXCHANGE COMMISSION NOR ANY STATE SECURITIES COMMISSION HAS APPROVED OR DISAPPROVED OF THESE SECURITIES OR DETERMINED IF THIS PROSPECTUS IS TRUTHFUL OR COMPLETE. ANY REPRESENTATION TO THE CONTRARY IS A CRIMINAL OFFENSE.

The date of this prospectus is August 6, 2026.

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	Page
ABOUT THIS PROSPECTUS		ii
PROSPECTUS SUMMARY		1
RISK FACTORS		5
SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS		6
USE OF PROCEEDS		8
DESCRIPTION OF CAPITAL STOCK		9
DESCRIPTION OF DEBT SECURITIES		14
DESCRIPTION OF WARRANTS		21
LEGAL OWNERSHIP OF SECURITIES		23
SELLING SECURITYHOLDERS		26
PLAN OF DISTRIBUTION		27
LEGAL MATTERS		29
EXPERTS		29
WHERE YOU CAN FIND MORE INFORMATION		29
INCORPORATION OF CERTAIN INFORMATION BY REFERENCE		30

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ABOUT THIS PROSPECTUS

This prospectus is a part of an automatic registration statement on Form S-3 that we filed with the U.S. Securities and Exchange Commission, or the SEC, utilizing a “shelf” registration process as a “well-known seasoned issuer,” as defined in Rule 405 under the Securities Act of 1933, as amended, or the Securities Act. Under this shelf registration process, we or selling securityholders may offer and sell, either individually or in combination, from time to time in one or more offerings, any combination of the securities described in this prospectus. No limit exists on the aggregate number or amount of securities we or selling securityholders may sell pursuant to the registration statement of which this prospectus is a part.

This prospectus provides you with a general description of the securities we or selling securityholders may offer. Each time we or selling securityholders sell securities under this prospectus, we will provide a prospectus supplement that will contain specific information about the terms of that offering. We may also authorize one or more free writing prospectuses to be provided to you that may contain material information relating to these offerings. The prospectus supplement and any related free writing prospectus that we may authorize to be provided to you may also add, update or change information contained in this prospectus or in any documents that we have incorporated by reference into this prospectus.

You should read this prospectus, any applicable prospectus supplement and any related free writing prospectus, together with the information incorporated herein by reference as described under the heading “Incorporation of Certain Information by Reference,” before investing in any of the securities offered.

THIS PROSPECTUS MAY NOT BE USED TO CONSUMMATE A SALE OF SECURITIES UNLESS IT IS ACCOMPANIED BY A PROSPECTUS SUPPLEMENT.

Neither we, nor any selling securityholder, agent, underwriter or dealer has authorized any person to give any information or to make any representation other than those contained or incorporated by reference in this prospectus, any applicable prospectus supplement or any related free writing prospectus prepared by or on behalf of us or to which we have referred you. This prospectus, any applicable supplement to this prospectus or any related free writing prospectus do not constitute an offer to sell or the solicitation of an offer to buy any securities other than the registered securities to which they relate, nor do this prospectus, any applicable supplement to this prospectus or any related free writing prospectus constitute an offer to sell or the solicitation of an offer to buy securities in any jurisdiction to any person to whom it is unlawful to make such offer or solicitation in such jurisdiction.

You should not assume that the information contained in this prospectus, any applicable prospectus supplement or any related free writing prospectus is accurate on any date subsequent to the date set forth on the front of the document or that any information we have incorporated by reference is correct on any date subsequent to the date of the document incorporated by reference, even though this prospectus, any applicable prospectus supplement or any related free writing prospectus is delivered, or securities are sold, on a later date.

This prospectus and the information incorporated herein by reference contain summaries of certain provisions contained in some of the documents described herein, but reference is made to the actual documents for complete information. All of the summaries are qualified in their entirety by the actual documents. Copies of some of the documents referred to herein have been filed, will be filed or will be incorporated by reference as exhibits to the registration statement of which this prospectus is a part, and you may obtain copies of those documents as described below under the heading “Where You Can Find More Information.”

Unless otherwise mentioned or unless the context requires otherwise, all references in this prospectus to “Aclaris,” “company,” “we,” “us” and “our” or similar references refer to Aclaris Therapeutics, Inc. and its consolidated subsidiaries.

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PROSPECTUS SUMMARY

This summary highlights selected information from this prospectus and does not contain all of the information that you need to consider in making your investment decision. You should carefully read the entire prospectus, the applicable prospectus supplement and any related free writing prospectus, including the risks of investing in our securities discussed under the heading “Risk Factors” contained in the applicable prospectus supplement and any related free writing prospectus, and under similar headings in the other documents that are incorporated by reference into this prospectus. You should also carefully read the information incorporated by reference into this prospectus, including our financial statements and related notes, and the exhibits to the registration statement of which this prospectus is a part, before making your investment decision.

Company Overview

Risks Associated with our Business

Our business is subject to numerous risks and uncertainties, including those highlighted in the section titled “Risk Factors” immediately following this prospectus summary and those described under similar headings in the documents incorporated by reference into this prospectus. These risks include:

•

We have incurred significant losses since our inception. We expect to incur losses over the next several years and may never achieve or maintain profitability.

•

Our future success depends on our ability to retain key executives and to attract, retain and motivate qualified personnel.

•

We rely heavily on third parties for clinical trials, manufacturing, and development support. Their performance impacts our timelines and success.

•

We face substantial competition, which may result in others discovering, developing or commercializing drugs before or more successfully than we do.

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Corporate Information

We were incorporated under the laws of the State of Delaware in July 2012. Our principal executive offices are located at 701 Lee Road, Suite 103, Wayne, Pennsylvania 19087. Our telephone number is (484) 324-7933. Our common stock is listed on the Nasdaq Global Select Market under the symbol “ACRS.”

Our internet website address is www.aclaristx.com. We do not incorporate the information on or accessible through our website into this prospectus. The information contained on, or that can be accessed through, our website is not a part of this prospectus. We have included our website address in this prospectus solely as an inactive textual reference.

“Aclaris,” the Aclaris logo, KINect and our other trademarks or service marks appearing in this prospectus or incorporated herein by reference are our property. This prospectus and the information incorporated herein by reference contains additional trade names, trademarks and service marks of others, which are the property of their respective owners.

Implications of Being a Smaller Reporting Company

Securities That May Be Offered

We or selling securityholders may offer shares of our common stock and preferred stock, various series of debt securities and/or warrants to purchase any of such securities, from time to time in one or more offerings under this prospectus, together with any applicable prospectus supplement and any related free writing prospectus, at prices and on terms to be determined by market conditions at the time of the relevant offering. We or selling securityholders may also offer common stock, preferred stock and/or debt securities upon the exercise of warrants. This prospectus provides you with a general description of the securities we or selling securityholders may offer pursuant to the registration statement of which this prospectus is a part. Each time we or selling securityholders offer a type or series of securities under this prospectus, we will provide a prospectus supplement that will describe the specific amounts, prices and other important terms of the securities, including, to the extent applicable:

•

designation or classification;

•

aggregate principal amount or aggregate offering price;

•

maturity;

•

original issue discount;

•

rates and times of payment of interest or dividends;

•

redemption, conversion, exchange or sinking fund terms;

•

conversion or exchange prices or rates and any provisions for changes to or adjustments in the conversion or exchange prices or rates and in the securities or other property receivable upon conversion or exchange;

•

ranking;

•

restrictive covenants;

•

voting or other rights; and

•

material U.S. federal income tax considerations.

The applicable prospectus supplement and any related free writing prospectus that we may authorize to be provided to you may also add, update or change information contained in this prospectus or in documents

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we have incorporated by reference. However, no prospectus supplement or free writing prospectus will offer a security that is not registered and described in this prospectus at the time of the effectiveness of the registration statement of which this prospectus is a part.

This prospectus may not be used to consummate a sale of securities unless it is accompanied by a prospectus supplement.

We or selling securityholders may sell the securities directly to investors or through underwriters, dealers or agents. We, any selling securityholders and our and their underwriters or agents reserve the right to accept or reject all or part of any proposed purchase of securities. If we or selling securityholders do offer securities through underwriters or agents, we will include in the applicable prospectus supplement:

•

the names of those underwriters or agents;

•

applicable fees, discounts and commissions to be paid to them;

•

details regarding over-allotment options, if any; and

•

the estimated net proceeds to us.

Common Stock. We may issue shares of our common stock from time to time. The holders of our common stock are entitled to one vote for each share held of record on all matters submitted to a vote of stockholders and do not have cumulative voting rights. Subject to preferences that may be applicable to any outstanding shares of preferred stock, the holders of our common stock are entitled to receive ratably such dividends as may be declared by our board of directors out of legally available funds. Upon our liquidation, dissolution or winding up, holders of our common stock are entitled to share ratably in all assets remaining after payment of liabilities and the liquidation preferences of any then outstanding shares of preferred stock. Our common stock does not carry any preemptive rights enabling a holder to subscribe for, or receive shares of, any class of our common stock or any other securities convertible into shares of any class of our common stock, or any redemption rights.

Preferred Stock. We may issue shares of our preferred stock from time to time, in one or more series. Under our certificate of incorporation, our board of directors has the authority, without further action by the stockholders (unless such stockholder action is required by applicable law or the rules of any stock exchange or market on which our securities are then traded), to designate up to 10,000,000 shares of preferred stock in one or more series and to determine the designations, voting powers, preferences and rights of each series of the preferred stock, as well as the qualifications, limitations or restrictions thereof, including dividend rights, conversion rights, preemptive rights, terms of redemption or repurchase, liquidation preferences, sinking fund terms and the number of shares constituting any series or the designation of any series, any or all of which may be greater than the rights of the common stock. Any convertible preferred stock we may issue will be convertible into our common stock or exchangeable for our other securities. Conversion may be mandatory or at the holder’s option and would be at prescribed conversion rates.

If we sell any series of preferred stock under this prospectus, we will fix the designations, voting powers, preferences and rights of such series of preferred stock, as well as the qualifications, limitations or restrictions thereof, in the certificate of designation relating to that series. We will file as an exhibit to the registration statement of which this prospectus is a part, or will incorporate by reference from reports that we file with the SEC, the form of any certificate of designation that describes the terms of the series of preferred stock that we are offering before the issuance of the related series of preferred stock. We urge you to read the applicable prospectus supplement (and any free writing prospectus that we may authorize to be provided to you) related to the series of preferred stock being offered, as well as the complete certificate of designation that contains the terms of the applicable series of preferred stock.

Debt Securities. We may issue debt securities from time to time, in one or more series, as either senior or subordinated debt or as senior or subordinated convertible debt. The senior debt securities will rank equally with any other unsecured and unsubordinated debt. The subordinated debt securities will be subordinate and junior in right of payment, to the extent and in the manner described in the instrument governing the debt, to all of our senior indebtedness. Convertible debt securities will be convertible into or exchangeable for our common stock or other securities. Conversion may be mandatory or at the holder’s option and would be at prescribed conversion rates.

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The debt securities issued under this prospectus will be issued under one or more documents called indentures, which are contracts between us and a national banking association or other eligible party, as trustee. In this prospectus, we have summarized certain general features of the debt securities. We urge you, however, to read the applicable prospectus supplement (and any free writing prospectus that we may authorize to be provided to you) related to the series of debt securities being offered, as well as the complete indentures that contain the terms of the debt securities. A form of indenture has been filed as an exhibit to the registration statement of which this prospectus is a part, and supplemental indentures and forms of debt securities containing the terms of the debt securities being offered will be filed as exhibits to the registration statement of which this prospectus is a part or will be incorporated by reference from reports that we file with the SEC.

Warrants. We may issue warrants for the purchase of common stock, preferred stock and/or debt securities in one or more series. We may issue warrants independently or together with common stock, preferred stock and/or debt securities, and the warrants may be attached to or separate from these securities. In this prospectus, we have summarized certain general features of the warrants. We urge you, however, to read the applicable prospectus supplement (and any free writing prospectus that we may authorize to be provided to you) related to the particular series of warrants being offered, as well as the complete warrant agreements and warrant certificates that contain the terms of the warrants. Forms of the warrant agreements and forms of warrant certificates containing the terms of the warrants being offered have been filed as exhibits to the registration statement of which this prospectus is a part, and supplemental warrant agreements and forms of warrant certificates will be filed as exhibits to the registration statement of which this prospectus is a part or will be incorporated by reference from reports that we file with the SEC.

We will evidence each series of warrants by warrant certificates that we will issue. Warrants may be issued under an applicable warrant agreement that we enter into with a warrant agent. We will indicate the name and address of the warrant agent, if applicable, in the prospectus supplement relating to the particular series of warrants being offered.

Selling Securityholders

Selling securityholders are persons or entities that, directly or indirectly, have acquired or will from time to time acquire from us, our securities. Information about selling securityholders, if any, will be set forth in a prospectus supplement. See the section titled “Selling Securityholders” in this prospectus.

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RISK FACTORS

Investing in our securities involves a high degree of risk. You should carefully review the risks and uncertainties described under the heading “Risk Factors” contained in the applicable prospectus supplement and any related free writing prospectus, and under similar headings in our most recent Annual Report on Form 10-K, as updated by our subsequent quarterly and other reports and documents that are incorporated by reference into this prospectus and the applicable prospectus supplement, before deciding whether to purchase any of the securities being registered pursuant to the registration statement of which this prospectus is a part. Each of the risk factors described in the documents referenced above could adversely affect our business, operating results and financial condition, as well as adversely affect the value of an investment in our securities, and the occurrence of any of these risks might cause you to lose all or part of your investment. Additional risks not presently known to us or that we currently believe are immaterial may also significantly impair our business operations.

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SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS

This prospectus and the documents incorporated by reference herein contain forward-looking statements. These are based on our management’s current beliefs, expectations and assumptions about future events, conditions and results and on information currently available to us. Discussions containing these forward-looking statements may be found, among other places, in the Sections entitled “Business,” “Risk Factors” and “Management’s Discussion and Analysis of Financial Condition and Results of Operations” contained in the documents incorporated herein.

Any statements in this prospectus, or incorporated herein, about our expectations, beliefs, plans, objectives, assumptions or future events or performance are not historical facts and are forward-looking statements within the meaning of Section 27A of the Securities Act of 1933, as amended, or the Securities Act, and Section 21E of the Securities Exchange Act of 1934, as amended, or the Exchange Act. Forward-looking statements in this prospectus include statements about:

•

our plans to develop our product candidates;

•

the timing of our planned clinical trials of our product candidates and the reporting of the results from these trials;

•

the clinical utility of our product candidates and our expectations about their properties and therapeutic effects;

•

our plans and expectations related to manufacturing capabilities and strategy;

•

our expectations regarding coverage and reimbursement of our product candidates, if approved;

•

the timing of our regulatory filings and approvals for our product candidates;

•

our intellectual property position;

•

our expectations regarding competition;

•

the size and growth potential of the markets for our product candidates;

•

our expectations regarding our continued reliance on third parties;

•

the impacts of macroeconomic conditions on our business;

•

our expectations regarding our use of capital;

•

our estimates regarding future revenue, expenses and needs for additional financing;

•

our use of cash and other resources, including our expected use of proceeds from any offering under this prospectus, if any; and

•

other risks and uncertainties, including those listed under the caption “Risk Factors” in our most recent Annual Report on Form 10-K and other filings we make with the SEC.

We caution you that the foregoing list may not contain all of the forward-looking statements made in this prospectus and the documents incorporated by reference herein.

In some cases, you can identify forward-looking statements by the words “may,” “might,” “can,” “will,” “to be,” “could,” “would,” “should,” “expect,” “intend,” “plan,” “objective,” “anticipate,” “believe,” “estimate,” “predict,” “project,” “potential,” “likely,” “continue” and “ongoing,” or the negative of these terms, or other comparable terminology intended to identify statements about the future, although not all forward-looking statements contain these words. These forward-looking statements reflect our management’s beliefs and views with respect to future events and are based on estimates and assumptions as of the date the statement is made and are subject to risks and uncertainties. In addition, statements that “we believe” and similar statements reflect our beliefs and opinions on the relevant subject. These statements are based upon information available to us as of the date the statement is made, and while we believe such information

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forms a reasonable basis for such statements, such information may be limited or incomplete, and our statements should not be read to indicate that we have conducted an exhaustive inquiry into, or review of, all potentially available relevant information. These statements involve known and unknown risks, uncertainties and other factors that may cause our actual results, levels of activity, performance or achievements to be materially different from the information expressed or implied by these forward-looking statements.

You should refer to the “Risk Factors” section contained in the applicable prospectus supplement and any related free writing prospectus, and under similar headings in the other documents that are incorporated by reference into this prospectus, for a discussion of important factors that may cause our actual results to differ materially from those expressed or implied by our forward-looking statements. Given these risks, uncertainties and other factors, many of which are beyond our control, we cannot assure you that the forward-looking statements in this prospectus will prove to be accurate, and you should not place undue reliance on these forward-looking statements. Furthermore, if our forward-looking statements prove to be inaccurate, the inaccuracy may be material. In light of the significant uncertainties in these forward-looking statements, you should not regard these statements as a representation or warranty by us or any other person that we will achieve our objectives and plans in any specified time frame, or at all.

Except as required by law, we assume no obligation to update these forward-looking statements publicly, or to revise any forward-looking statements to reflect events or developments occurring after the date of this prospectus, even if new information becomes available in the future.

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USE OF PROCEEDS

We will retain broad discretion over the use of the net proceeds from the sale of the securities offered hereby. Except as described in any applicable prospectus supplement or in any free writing prospectuses that we may authorize to be provided to you in connection with a specific offering, we currently intend to use the net proceeds from the sale of the securities offered hereby, if any, primarily to fund the research and development of the product candidates in our pipeline and for working capital and general corporate purposes. We may also use a portion of the net proceeds to invest in or acquire businesses or technologies that we believe are complementary to our own, although we have no current plans, commitments or agreements with respect to any acquisitions as of the date of this prospectus. We will set forth in the applicable prospectus supplement or free writing prospectus our intended use for the net proceeds received from the sale of any securities sold pursuant to the prospectus supplement or free writing prospectus. Pending these uses, we plan to invest these net proceeds in short-term, interest-bearing investments, investment-grade instruments, certificates of deposit or direct or guaranteed obligations of the United States.

Unless the applicable prospectus supplement provides otherwise, we will not receive any proceeds from the sale of securities by selling securityholders.

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DESCRIPTION OF CAPITAL STOCK

The following description of our capital stock and provisions of our certificate of incorporation and bylaws and certain provisions of Delaware law are summaries. References to our certificate of incorporation herein refer to our certificate of incorporation as amended to the date of this prospectus. You should also refer to the certificate of incorporation (including the certificate of amendment thereto) and the bylaws, which are filed as exhibits to our most recent Annual Report on Form 10-K and are incorporated by reference herein.

Common Stock

Under our certificate of incorporation, we are authorized to issue up to 400,000,000 shares of common stock, $0.00001 par value per share, and 10,000,000 shares of preferred stock, $0.00001 par value per share, all of which shares of preferred stock are undesignated. Our board of directors may establish the rights and preferences of the preferred stock from time to time. As of June 30, 2026, we had outstanding 139,824,273 shares of common stock and no shares of preferred stock outstanding.

Voting Rights

Each holder of common stock is entitled to one vote for each share on all matters submitted to a vote of the stockholders, including the election of directors. Under our certificate of incorporation and the bylaws, common stockholders do not have cumulative voting rights. Because of this, the holders of a majority of the shares of common stock entitled to vote in any election of directors can elect all of the directors standing for election, if they should so choose.

Dividends

Subject to preferences that may be applicable to any then-outstanding preferred stock, holders of common stock are entitled to receive ratably those dividends, if any, as may be declared from time to time by the board of directors out of legally available funds.

Liquidation

In the event of our liquidation, dissolution or winding up, holders of common stock will be entitled to share ratably in the net assets legally available for distribution to stockholders after the payment of all debts and other liabilities and the satisfaction of any liquidation preference granted to the holders of any then-outstanding shares of preferred stock.

Rights and Preferences

Holders of common stock have no preemptive, conversion or subscription rights and there are no redemption or sinking fund provisions applicable to the common stock. The rights, preferences and privileges of the holders of common stock are subject to, and may be adversely affected by, the rights of the holders of shares of any series of preferred stock that we may designate in the future.

Preferred Stock

Pursuant to our certificate of incorporation, our board of directors has the authority, without further action by the stockholders (unless such stockholder action is required by applicable law or stock exchange listing rules), to designate and issue up to 10,000,000 shares of preferred stock in one or more series, to establish from time to time the number of shares to be included in each such series, to fix the designations, powers, preferences, privileges and relative participating, optional or special rights and the qualifications, limitations or restrictions thereof, including dividend rights, conversion rights, voting rights, terms of redemption and liquidation preferences, any or all of which may be greater than the rights of the common stock, and to increase or decrease the number of shares of any such series, but not below the number of shares of such series then outstanding.

Our board of directors, without stockholder approval, can issue preferred stock with voting, conversion or other rights that could adversely affect the voting power and other rights of the holders of common stock. Preferred stock could be issued quickly with terms designed to delay or prevent a change in control of our

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company or make removal of management more difficult. Additionally, the issuance of preferred stock may have the effect of decreasing the market price of the common stock and may adversely affect the voting power of holders of common stock and reduce the likelihood that common stockholders will receive dividend payments and payments upon liquidation.

Our board of directors will fix the designations, voting powers, preferences and rights of each series, as well as the qualifications, limitations or restrictions thereof, of the preferred stock of each series that we offer under this prospectus and applicable prospectus supplements in the certificate of designation relating to that series. We will file as an exhibit to the registration statement of which this prospectus is a part, or will incorporate by reference from reports that we file with the SEC, the form of any certificate of designation that describes the terms of the series of preferred stock we are offering before the issuance of that series of preferred stock. This description will include:

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the title and stated value;

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the number of shares we are offering;

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the liquidation preference per share;

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the purchase price per share;

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the dividend rate per share, dividend period and payment dates and method of calculation for dividends;

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whether dividends will be cumulative or non-cumulative and, if cumulative, the date from which dividends will accumulate;

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our right, if any, to defer payment of dividends and the maximum length of any such deferral period;

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the procedures for any auction and remarketing, if any;

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the provisions for a sinking fund, if any;

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the provisions for redemption or repurchase, if applicable, and any restrictions on our ability to exercise those redemption and repurchase rights;

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any listing of the preferred stock on any securities exchange or market;

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whether the preferred stock will be convertible into our common stock or other securities of ours, including depositary shares and warrants, and, if applicable, the conversion period, the conversion price, or how it will be calculated, and under what circumstances it may be adjusted;

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whether the preferred stock will be exchangeable into debt securities, and, if applicable, the exchange period, the exchange price, or how it will be calculated, and under what circumstances it may be adjusted;

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voting rights, if any, of the preferred stock;

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preemption rights, if any;

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restrictions on transfer, sale or other assignments, if any;

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whether interests in the preferred stock will be represented by depositary shares;

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a discussion of any material or special U.S. federal income tax considerations applicable to the preferred stock;

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the relative ranking and preferences of the preferred stock as to dividend rights and rights if we liquidate, dissolve or wind up our affairs;

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any limitations on issuances of any class or series of preferred stock ranking senior to or on a parity with the series of preferred stock being issued as to dividend rights and rights if we liquidate, dissolve or wind up our affairs; and

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any other specific terms, rights, preferences, privileges, qualifications or restrictions of the preferred stock.

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The General Corporation Law of the State of Delaware, the state of our incorporation, provides that the holders of preferred stock will have the right to vote separately as a class (or, in some cases, as a series) on an amendment to our certificate of incorporation if the amendment would change the par value or, unless the certificate of incorporation provided otherwise, the number of authorized shares of the class or change the powers, preferences or special rights of the class or series so as to adversely affect the class or series, as the case may be. This right is in addition to any voting rights that may be provided for in the applicable certificate of designation.

Anti-Takeover Provisions

Section 203 of the DGCL

We are subject to Section 203 of the General Corporation Law of the State of Delaware, or the DGCL, which prohibits a Delaware corporation from engaging in any business combination with any interested stockholder for a period of three years after the date that such stockholder became an interested stockholder, with the following exceptions:

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before such date, the board of directors of the corporation approved either the business combination or the transaction that resulted in the stockholder becoming an interested stockholder;

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upon completion of the transaction that resulted in the stockholder becoming an interested stockholder, the interested stockholder owned at least 85% of the voting stock of the corporation outstanding at the time the transaction began, excluding for purposes of determining the voting stock outstanding, but not the outstanding voting stock owned by the interested stockholder, those shares owned (i) by persons who are directors and also officers and (ii) employee stock plans in which employee participants do not have the right to determine confidentially whether shares held subject to the plan will be tendered in a tender or exchange offer; or

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on or after such date, the business combination is approved by the board of directors and authorized at an annual or special meeting of the stockholders, and not by written consent, by the affirmative vote of at least 662∕3% of the outstanding voting stock that is not owned by the interested stockholder.

In general, Section 203 defines a “business combination” to include the following:

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any merger or consolidation involving the corporation and the interested stockholder;

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any sale, transfer, pledge or other disposition of 10% or more of the assets of the corporation involving the interested stockholder;

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subject to certain exceptions, any transaction that results in the issuance or transfer by the corporation of any stock of the corporation to the interested stockholder;

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any transaction involving the corporation that has the effect of increasing the proportionate share of the stock or any class or series of the corporation beneficially owned by the interested stockholder; or

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the receipt by the interested stockholder of the benefit of any loans, advances, guarantees, pledges or other financial benefits by or through the corporation.

In general, Section 203 defines an “interested stockholder” as an entity or person who, together with the person’s affiliates and associates, beneficially owns, or within three years prior to the time of determination of interested stockholder status did own, 15% or more of the outstanding voting stock of the corporation

Certificate of Incorporation and Bylaws

Our certificate of incorporation provides for our board of directors to be divided into three classes with staggered three-year terms. Only one class of directors is elected at each annual meeting of stockholders, with the other classes continuing for the remainder of their respective three-year terms. Because our stockholders do not have cumulative voting rights, stockholders holding a majority of the shares of common stock outstanding are able to elect all of our directors. The certificate of incorporation and bylaws also provide that directors may be removed by the stockholders only for cause upon the vote of 662∕3% or more

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of outstanding common stock. Furthermore, the authorized number of directors may be changed only by resolution of the board of directors, and vacancies and newly created directorships on the board of directors may, except as otherwise required by law or determined by the board, only be filled by a majority vote of the directors then serving on the board, even though less than a quorum.

The certificate of incorporation and bylaws also provide that all stockholder actions must be effected at a duly called meeting of stockholders and stockholders cannot act by written consent without a meeting. The bylaws also provide that only the chairman of the board, chief executive officer or the board of directors pursuant to a resolution adopted by a majority of the total number of authorized directors may call a special meeting of stockholders.

The bylaws also provide that stockholders seeking to present proposals before a meeting of stockholders or to nominate candidates for election as directors at a meeting of stockholders must provide timely advance notice in writing, and specify requirements as to the form and content of a stockholder’s notice.

The certificate of incorporation and bylaws provide that the stockholders cannot amend many of the provisions described above except by a vote of 662∕3% or more of outstanding common stock.

The combination of these provisions make it more difficult for our existing stockholders to replace the board of directors as well as for another party to obtain control of us by replacing our board of directors. Since our board of directors has the power to retain and discharge our officers, these provisions also make it more difficult for existing stockholders or another party to effect a change in management. In addition, the authorization of undesignated preferred stock makes it possible for our board of directors to issue preferred stock with voting or other rights or preferences that could impede the success of any attempt to change our control.

These provisions are intended to enhance the likelihood of continued stability in the composition of our board of directors and its policies and to discourage coercive takeover practices and inadequate takeover bids. These provisions are also designed to reduce our vulnerability to hostile takeovers and to discourage certain tactics that may be used in proxy fights. However, such provisions could have the effect of discouraging others from making tender offers for our shares and may have the effect of delaying changes in control of the company or to our management. As a consequence, these provisions may also inhibit fluctuations in the market price of our stock that could result from actual or rumored takeover attempts. We believe that the benefits of these provisions, including increased protection of our potential ability to negotiate with the proponent of an unfriendly or unsolicited proposal to acquire or restructure the company, outweigh the disadvantages of discouraging takeover proposals, because negotiation of takeover proposals could result in an improvement of their terms.

Choice of Forum

Our certificate of incorporation provides that the Court of Chancery of the State of Delaware is the exclusive forum for the following types of actions or proceedings under Delaware statutory or common law: (i) any derivative action or proceeding brought on our behalf, (ii) any action asserting a claim of breach of a fiduciary duty owed by any of our directors, officers or other employees to us or our stockholders, (iii) any action asserting a claim arising pursuant to any provision of the DGCL, our certificate of incorporation or bylaws or (iv) any action asserting a claim governed by the internal affairs doctrine. However, this exclusive forum provision would not apply to suits brought to enforce a duty or liability created by the Securities Act or the Exchange Act.

In addition, our bylaws provide that unless we consent in writing to the selection of an alternative forum, the federal district courts of the United States of America shall, to the fullest extent permitted by law, be the sole and exclusive forum for the resolution of any complaint asserting a cause of action arising under the Securities Act, and that any person or entity purchasing or otherwise acquiring or holding any interest in shares of our capital stock shall be deemed to have notice of and consented to the federal forum selection provision.

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Transfer Agent and Registrar

The transfer agent and registrar for our common stock is Broadridge Corporate Issuer Solutions, Inc. The transfer agent’s address is 1717 Arch Street, Suite 1300, Philadelphia, Pennsylvania 19103.

Listing on the Nasdaq Global Select Market

Our common stock is listed on the Nasdaq Global Select Market under the symbol “ACRS.”

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DESCRIPTION OF DEBT SECURITIES

We may issue debt securities from time to time, in one or more series, as either senior or subordinated debt or as senior or subordinated convertible debt. While the terms we have summarized below will apply generally to any debt securities that we may offer under this prospectus, we will describe the particular terms of any debt securities that we may offer in more detail in the applicable prospectus supplement. The terms of any debt securities offered under a prospectus supplement may differ from the terms described below. Unless the context requires otherwise, whenever we refer to the indenture, we also are referring to any supplemental indentures that specify the terms of a particular series of debt securities.

We will issue the debt securities under the indenture that we will enter into with the trustee named in the indenture. The indenture will be qualified under the Trust Indenture Act of 1939, as amended, or the Trust Indenture Act. We have filed the form of indenture as an exhibit to the registration statement of which this prospectus is a part, and supplemental indentures and forms of debt securities containing the terms of the debt securities being offered will be filed as exhibits to the registration statement of which this prospectus is a part or will be incorporated by reference from reports that we file with the SEC.

The following summary of material provisions of the debt securities and the indenture is subject to, and qualified in its entirety by reference to, all of the provisions of the indenture applicable to a particular series of debt securities. We urge you to read the applicable prospectus supplements and any related free writing prospectuses related to the debt securities that we may offer under this prospectus, as well as the complete indenture that contains the terms of the debt securities.

General

The indenture does not limit the amount of debt securities that we may issue. It provides that we may issue debt securities up to the principal amount that we may authorize and may be in any currency or currency unit that we may designate. Except for the limitations on consolidation, merger and sale of all or substantially all of our assets contained in the indenture, the terms of the indenture do not contain any covenants or other provisions designed to give holders of any debt securities protection against changes in our operations, financial condition or transactions involving us.

We may issue the debt securities issued under the indenture as “discount securities,” which means they may be sold at a discount below their stated principal amount. These debt securities, as well as other debt securities that are not issued at a discount, may be issued with “original issue discount,” or OID, for U.S. federal income tax purposes because of interest payment and other characteristics or terms of the debt securities. Material U.S. federal income tax considerations applicable to debt securities issued with OID will be described in more detail in any applicable prospectus supplement.

We will describe in the applicable prospectus supplement the terms of the series of debt securities being offered, including:

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the title of the series of debt securities;

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any limit upon the aggregate principal amount that may be issued;

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the maturity date or dates;

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the form of the debt securities of the series;

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the applicability of any guarantees;

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whether or not the debt securities will be secured or unsecured, and the terms of any secured debt;

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whether the debt securities rank as senior debt, senior subordinated debt, subordinated debt or any combination thereof, and the terms of any subordination;

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if the price (expressed as a percentage of the aggregate principal amount thereof) at which such debt securities will be issued is a price other than the principal amount thereof, the portion of the principal amount thereof payable upon declaration of acceleration of the maturity thereof, or if applicable, the portion of the principal amount of such debt securities that is convertible into another security or the method by which any such portion shall be determined;

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•

the interest rate or rates, which may be fixed or variable, or the method for determining the rate and the date interest will begin to accrue, the dates interest will be payable and the regular record dates for interest payment dates or the method for determining such dates;

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our right, if any, to defer payment of interest and the maximum length of any such deferral period;

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if applicable, the date or dates after which, or the period or periods during which, and the price or prices at which, we may, at our option, redeem the series of debt securities pursuant to any optional or provisional redemption provisions and the terms of those redemption provisions;

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the date or dates, if any, on which, and the price or prices at which we are obligated, pursuant to any mandatory sinking fund or analogous fund provisions or otherwise, to redeem, or at the holder’s option to purchase, the series of debt securities and the currency or currency unit in which the debt securities are payable;

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the denominations in which we will issue the series of debt securities, if other than denominations of $1,000 and any integral multiple thereof;

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any and all terms, if applicable, relating to any auction or remarketing of the debt securities of that series and any security for our obligations with respect to such debt securities and any other terms which may be advisable in connection with the marketing of debt securities of that series;

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whether the debt securities of the series shall be issued in whole or in part in the form of a global security or securities; the terms and conditions, if any, upon which such global security or securities may be exchanged in whole or in part for other individual securities; and the depositary for such global security or securities;

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if applicable, the provisions relating to conversion or exchange of any debt securities of the series and the terms and conditions upon which such debt securities will be so convertible or exchangeable, including the conversion or exchange price, as applicable, or how it will be calculated and may be adjusted, any mandatory or optional (at our option or the holders’ option) conversion or exchange features, the applicable conversion or exchange period and the manner of settlement for any conversion or exchange;

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if other than the full principal amount thereof, the portion of the principal amount of debt securities of the series which shall be payable upon declaration of acceleration of the maturity thereof;

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additions to or changes in the covenants applicable to the particular debt securities being issued, including, among others, the consolidation, merger or sale covenant;

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additions to or changes in the events of default with respect to the securities and any change in the right of the trustee or the holders to declare the principal, premium, if any, and interest, if any, with respect to such securities to be due and payable;

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additions to or changes in or deletions of the provisions relating to covenant defeasance and legal defeasance;

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additions to or changes in the provisions relating to satisfaction and discharge of the indenture;

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additions to or changes in the provisions relating to the modification of the indenture both with and without the consent of holders of debt securities issued under the indenture;

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the currency of payment of debt securities if other than U.S. dollars and the manner of determining the equivalent amount in U.S. dollars;

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whether interest will be payable in cash or additional debt securities at our or the holders’ option and the terms and conditions upon which the election may be made;

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the terms and conditions, if any, upon which we will pay amounts in addition to the stated interest, premium, if any and principal amounts of the debt securities of the series to any holder that is not a “United States person” for federal tax purposes;

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any restrictions on transfer, sale or assignment of the debt securities of the series; and

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any other specific terms, preferences, rights or limitations of, or restrictions on, the debt securities, any other additions or changes in the provisions of the indenture, and any terms that may be required by us or advisable under applicable laws or regulations.

Conversion or Exchange Rights

We will set forth in the applicable prospectus supplement the terms on which a series of debt securities may be convertible into or exchangeable for our common stock or our other securities. We will include provisions as to settlement upon conversion or exchange and whether conversion or exchange is mandatory, at the option of the holder or at our option. We may include provisions pursuant to which the number of shares of our common stock or our other securities that the holders of the series of debt securities receive would be subject to adjustment.

Consolidation, Merger or Sale

Unless we provide otherwise in the prospectus supplement applicable to a particular series of debt securities, the indenture will not contain any covenant that restricts our ability to merge or consolidate, or sell, convey, transfer or otherwise dispose of our assets as an entirety or substantially as an entirety. However, any successor to or acquirer of such assets (other than a subsidiary of ours) must assume all of our obligations under the indenture or the debt securities, as appropriate.

Events of Default under the Indenture

Unless we provide otherwise in the prospectus supplement applicable to a particular series of debt securities, the following are events of default under the indenture with respect to any series of debt securities that we may issue:

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if we fail to pay any installment of interest on any series of debt securities, as and when the same shall become due and payable, and such default continues for a period of 90 days; provided, however, that a valid extension of an interest payment period by us in accordance with the terms of any indenture supplemental thereto shall not constitute a default in the payment of interest for this purpose;

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if we fail to pay the principal of, or premium, if any, on any series of debt securities as and when the same shall become due and payable whether at maturity, upon redemption, by declaration or otherwise, or in any payment required by any sinking or analogous fund established with respect to such series; provided, however, that a valid extension of the maturity of such debt securities in accordance with the terms of any indenture supplemental thereto shall not constitute a default in the payment of principal or premium, if any;

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if we fail to observe or perform any other covenant or agreement contained in the debt securities or the indenture, other than a covenant specifically relating to another series of debt securities, and our failure continues for 90 days after we receive written notice of such failure, requiring the same to be remedied and stating that such is a notice of default thereunder, from the trustee or holders of at least 25% in aggregate principal amount of the outstanding debt securities of the applicable series; and

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if specified events of bankruptcy, insolvency or reorganization occur.

If an event of default with respect to debt securities of any series occurs and is continuing, other than an event of default specified in the last bullet point above, the trustee or the holders of at least 25% in aggregate principal amount of the outstanding debt securities of that series, by notice to us in writing, and to the trustee if notice is given by such holders, may declare the unpaid principal of, premium, if any, and accrued interest, if any, due and payable immediately. If an event of default specified in the last bullet point above occurs with respect to us, the principal amount of and accrued interest, if any, of each issue of debt securities then outstanding shall be due and payable without any notice or other action on the part of the trustee or any holder.

The holders of a majority in principal amount of the outstanding debt securities of an affected series may waive any default or event of default with respect to the series and its consequences, except defaults or

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events of default regarding payment of principal, premium, if any, or interest, unless we have cured the default or event of default in accordance with the indenture. Any waiver shall cure the default or event of default.

Subject to the terms of the indenture, if an event of default under an indenture shall occur and be continuing, the trustee will be under no obligation to exercise any of its rights or powers under such indenture at the request or direction of any of the holders of the applicable series of debt securities, unless such holders have offered the trustee reasonable indemnity. The holders of a majority in principal amount of the outstanding debt securities of any series will have the right to direct the time, method and place of conducting any proceeding for any remedy available to the trustee, or exercising any trust or power conferred on the trustee, with respect to the debt securities of that series, provided that:

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the direction so given by the holder is not in conflict with any law or the applicable indenture; and

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subject to its duties under the Trust Indenture Act, the trustee need not take any action that might involve it in personal liability or might be unduly prejudicial to the holders not involved in the proceeding.

A holder of the debt securities of any series will have the right to institute a proceeding under the indenture or to appoint a receiver or trustee, or to seek other remedies only if:

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the holder has given written notice to the trustee of a continuing event of default with respect to that series;

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the holders of at least 25% in aggregate principal amount of the outstanding debt securities of that series have made written request,

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such holders have offered to the trustee indemnity satisfactory to it against the costs, expenses and liabilities to be incurred by the trustee in compliance with the request; and

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the trustee does not institute the proceeding, and does not receive from the holders of a majority in aggregate principal amount of the outstanding debt securities of that series other conflicting directions within 90 days after the notice, request and offer.

These limitations do not apply to a suit instituted by a holder of debt securities if we default in the payment of the principal, premium, if any, or interest on, the debt securities.

We will periodically file statements with the trustee regarding our compliance with specified covenants in the indenture.

Modification of Indenture; Waiver

We and the trustee may change an indenture without the consent of any holders with respect to specific matters:

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to cure any ambiguity, defect or inconsistency in the indenture or in the debt securities of any series;

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to comply with the provisions described above under “Description of Debt Securities — Consolidation, Merger or Sale;”

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to provide for uncertificated debt securities in addition to or in place of certificated debt securities;

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to add to our covenants, restrictions, conditions or provisions such new covenants, restrictions, conditions or provisions for the benefit of the holders of all or any series of debt securities, to make the occurrence, or the occurrence and the continuance, of a default in any such additional covenants, restrictions, conditions or provisions an event of default or to surrender any right or power conferred upon us in the indenture;

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to add to, delete from or revise the conditions, limitations, and restrictions on the authorized amount, terms, or purposes of issue, authentication and delivery of debt securities, as set forth in the indenture;

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to make any change that does not adversely affect the interests of any holder of debt securities of any series in any material respect;

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to provide for the issuance of and establish the form and terms and conditions of the debt securities of any series as provided above under “Description of Debt Securities — General” to establish the form of any certifications required to be furnished pursuant to the terms of the indenture or any series of debt securities, or to add to the rights of the holders of any series of debt securities;

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to evidence and provide for the acceptance of appointment under any indenture by a successor trustee; or

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to comply with any requirements of the SEC in connection with the qualification of any indenture under the Trust Indenture Act.

In addition, under the indenture, the rights of holders of a series of debt securities may be changed by us and the trustee with the written consent of the holders of at least a majority in aggregate principal amount of the outstanding debt securities of each series that is affected. However, unless we provide otherwise in the prospectus supplement applicable to a particular series of debt securities, we and the trustee may make the following changes only with the consent of each holder of any outstanding debt securities affected:

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extending the fixed maturity of any debt securities of any series;

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reducing the principal amount, reducing the rate of or extending the time of payment of interest, or reducing any premium payable upon the redemption of any series of any debt securities; or

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reducing the percentage of debt securities, the holders of which are required to consent to any amendment, supplement, modification or waiver.

Discharge

Each indenture provides that we can elect to be discharged from our obligations with respect to one or more series of debt securities, except for specified obligations, including obligations to:

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provide for payment;

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replace stolen, lost or mutilated debt securities of the series;

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pay principal of and premium and interest on any debt securities of the series;

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maintain paying agencies;

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hold monies for payment in trust;

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recover excess money held by the trustee;

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compensate and indemnify the trustee; and

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appoint any successor trustee.

In order to exercise our rights to be discharged, we must deposit with the trustee money or government obligations sufficient to pay all the principal of, any premium, if any, and interest on, the debt securities of the series on the dates payments are due.

Form, Exchange and Transfer

We will issue the debt securities of each series only in fully registered form without coupons and, unless we provide otherwise in the applicable prospectus supplement, in denominations of $1,000 and any integral multiple thereof. The indenture provides that we may issue debt securities of a series in temporary or permanent global form and as book-entry securities that will be deposited with, or on behalf of, The Depository Trust Company, or DTC, or another depositary named by us and identified in the applicable prospectus supplement with respect to that series. To the extent the debt securities of a series are issued in global form and as book-entry, a description of terms relating to any book-entry securities will be set forth in the applicable prospectus supplement.

At the option of the holder, subject to the terms of the indenture and the limitations applicable to global securities described in the applicable prospectus supplement, the holder of the debt securities of any

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series can exchange the debt securities for other debt securities of the same series, in any authorized denomination and of like tenor and aggregate principal amount.

Subject to the terms of the indenture and the limitations applicable to global securities set forth in the applicable prospectus supplement, holders of the debt securities may present the debt securities for exchange or for registration of transfer, duly endorsed or with the form of transfer endorsed thereon duly executed if so required by us or the security registrar, at the office of the security registrar or at the office of any transfer agent designated by us for this purpose. Unless otherwise provided in the debt securities that the holder presents for transfer or exchange, we will impose no service charge for any registration of transfer or exchange, but we may require payment of any taxes or other governmental charges.

We will name in the applicable prospectus supplement the security registrar, and any transfer agent in addition to the security registrar, that we initially designate for any debt securities. We may at any time designate additional transfer agents or rescind the designation of any transfer agent or approve a change in the office through which any transfer agent acts, except that we will be required to maintain a transfer agent in each place of payment for the debt securities of each series.

If we elect to redeem the debt securities of any series, we will not be required to:

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issue, register the transfer of, or exchange any debt securities of that series during a period beginning at the opening of business 15 days before the day of mailing of a notice of redemption of any debt securities that may be selected for redemption and ending at the close of business on the day of the mailing; or

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register the transfer of or exchange any debt securities so selected for redemption, in whole or in part, except the unredeemed portion of any debt securities we are redeeming in part.

Information Concerning the Trustee

The trustee, other than during the occurrence and continuance of an event of default under an indenture, undertakes to perform only those duties as are specifically set forth in the applicable indenture. Upon an event of default under an indenture, the trustee must use the same degree of care as a prudent person would exercise or use in the conduct of his or her own affairs. Subject to this provision, the trustee is under no obligation to exercise any of the powers given it by the indenture at the request of any holder of debt securities unless it is offered reasonable security and indemnity against the costs, expenses and liabilities that it might incur.

Payment and Paying Agents

Unless we otherwise indicate in the applicable prospectus supplement, we will make payment of the interest on any debt securities on any interest payment date to the person in whose name the debt securities, or one or more predecessor securities, are registered at the close of business on the regular record date for the interest.

We will pay principal of and any premium and interest on the debt securities of a particular series at the office of the paying agents designated by us, except that unless we otherwise indicate in the applicable prospectus supplement, we will make interest payments by check that we will mail to the holder or by wire transfer to certain holders. Unless we otherwise indicate in the applicable prospectus supplement, we will designate the corporate trust office of the trustee as our sole paying agent for payments with respect to debt securities of each series. We will name in the applicable prospectus supplement any other paying agents that we initially designate for the debt securities of a particular series. We will maintain a paying agent in each place of payment for the debt securities of a particular series.

All money we pay to a paying agent or the trustee for the payment of the principal of or any premium or interest on any debt securities that remains unclaimed at the end of two years after such principal, premium or interest has become due and payable will be repaid to us, and the holder of the debt security thereafter may look only to us for payment thereof.

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Governing Law

The indenture and the debt securities will be governed by and construed in accordance with the internal laws of the State of New York, except to the extent that the Trust Indenture Act of 1939 is applicable.

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DESCRIPTION OF WARRANTS

The following description, together with the additional information we may include in any applicable prospectus supplement and free writing prospectus, summarizes the material terms and provisions of the warrants that we may offer under this prospectus, which may consist of warrants to purchase common stock, preferred stock or debt securities and may be issued in one or more series. Warrants may be offered independently or in combination with common stock, preferred stock or debt securities offered by any prospectus supplement. While the terms we have summarized below will apply generally to any warrants that we may offer under this prospectus, we will describe the particular terms of any series of warrants in more detail in the applicable prospectus supplement. The following description of warrants will apply to the warrants offered by this prospectus unless we provide otherwise in the applicable prospectus supplement. The applicable prospectus supplement for a particular series of warrants may specify different or additional terms.

We have filed forms of the warrant agreements and forms of warrant certificates containing the terms of the warrants that may be offered as exhibits to the registration statement of which this prospectus is a part. We will file as exhibits to the registration statement of which this prospectus is a part, or will incorporate by reference from reports that we file with the SEC, the form of warrant and/or the warrant agreement and warrant certificate, as applicable, that contain the terms of the particular series of warrants we are offering, and any supplemental agreements, before the issuance of such warrants. The following summaries of material terms and provisions of the warrants are subject to, and qualified in their entirety by reference to, all the provisions of the form of warrant and/or the warrant agreement and warrant certificate, as applicable, and any supplemental agreements applicable to a particular series of warrants that we may offer under this prospectus. We urge you to read the applicable prospectus supplement related to the particular series of warrants that we may offer under this prospectus, as well as any related free writing prospectus, and the complete form of warrant and/or the warrant agreement and warrant certificate, as applicable, and any supplemental agreements, that contain the terms of the warrants.

General

We will describe in the applicable prospectus supplement the terms of the series of warrants being offered, including:

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the offering price and aggregate number of warrants offered;

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the currency for which the warrants may be purchased;

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if applicable, the designation and terms of the securities with which the warrants are issued and the number of warrants issued with each such security or each principal amount of such security;

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in the case of warrants to purchase debt securities, the principal amount of debt securities purchasable upon exercise of one warrant and the price at, and currency in which, this principal amount of debt securities may be purchased upon such exercise;

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in the case of warrants to purchase common stock or preferred stock, the number of shares of common stock or preferred stock, as the case may be, purchasable upon the exercise of one warrant and the price at which these shares may be purchased upon such exercise;

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the effect of any merger, consolidation, sale or other disposition of our business on the warrant agreements and the warrants;

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the terms of any rights to redeem or call the warrants;

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any provisions for changes to or adjustments in the exercise price or number of securities issuable upon exercise of the warrants;

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the dates on which the right to exercise the warrants will commence and expire;

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the manner in which the warrant agreements and warrants may be modified;

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a discussion of any material or special U.S. federal income tax considerations of holding or exercising the warrants;

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the terms of the securities issuable upon exercise of the warrants; and

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any other specific terms, preferences, rights or limitations of or restrictions on the warrants.

Before exercising their warrants, holders of warrants will not have any of the rights of holders of the securities purchasable upon such exercise, including:

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in the case of warrants to purchase debt securities, the right to receive payments of principal of, or premium, if any, or interest on, the debt securities purchasable upon exercise or to enforce covenants in the applicable indenture; or

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in the case of warrants to purchase common stock or preferred stock, the right to receive dividends, if any, or, payments upon our liquidation, dissolution or winding up or to exercise voting rights, if any.

Exercise of Warrants

Each warrant will entitle the holder to purchase the securities that we specify in the applicable prospectus supplement at the exercise price that we describe in the applicable prospectus supplement. The warrants may be exercised as set forth in the prospectus supplement relating to the warrants offered. Unless we otherwise specify in the applicable prospectus supplement, warrants may be exercised at any time up to the close of business on the expiration date set forth in the prospectus supplement relating to the warrants offered thereby. After the close of business on the expiration date, unexercised warrants will become void.

Upon receipt of payment and the warrant or warrant certificate, as applicable, properly completed and duly executed at the corporate trust office of the warrant agent, if any, or any other office, including ours, indicated in the prospectus supplement, we will, as soon as practicable, issue and deliver the securities purchasable upon such exercise. If less than all of the warrants (or the warrants represented by such warrant certificate) are exercised, a new warrant or a new warrant certificate, as applicable, will be issued for the remaining warrants.

Governing Law

Unless we otherwise specify in the applicable prospectus supplement, the warrants and any warrant agreements will be governed by and construed in accordance with the laws of the State of New York.

Enforceability of Rights by Holders of Warrants

Each warrant agent, if any, will act solely as our agent under the applicable warrant agreement and will not assume any obligation or relationship of agency or trust with any holder of any warrant. A single bank or trust company may act as warrant agent for more than one issue of warrants. A warrant agent will have no duty or responsibility in case of any default by us under the applicable warrant agreement or warrant, including any duty or responsibility to initiate any proceedings at law or otherwise, or to make any demand upon us. Any holder of a warrant may, without the consent of the related warrant agent or the holder of any other warrant, enforce by appropriate legal action its right to exercise, and receive the securities purchasable upon exercise of, its warrants.

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LEGAL OWNERSHIP OF SECURITIES

We can issue securities in registered form or in the form of one or more global securities. We describe global securities in greater detail below. We refer to those persons who have securities registered in their own names on the books that we or any applicable trustee, depositary or warrant agent maintain for this purpose as the “holders” of those securities. These persons are the legal holders of the securities. We refer to those persons who, indirectly through others, own beneficial interests in securities that are not registered in their own names, as “indirect holders” of those securities. As we discuss below, indirect holders are not legal holders, and investors in securities issued in book-entry form or in street name will be indirect holders.

Book-Entry Holders

We may issue securities in book-entry form only, as we will specify in the applicable prospectus supplement. This means securities may be represented by one or more global securities registered in the name of a financial institution that holds them as depositary on behalf of other financial institutions that participate in the depositary’s book-entry system. These participating institutions, which are referred to as participants, in turn, hold beneficial interests in the securities on behalf of themselves or their customers.

Only the person in whose name a security is registered is recognized as the holder of that security. Global securities will be registered in the name of the depositary or its participants. Consequently, for global securities, we will recognize only the depositary as the holder of the securities, and we will make all payments on the securities to the depositary. The depositary passes along the payments it receives to its participants, which in turn pass the payments along to their customers who are the beneficial owners. The depositary and its participants do so under agreements they have made with one another or with their customers; they are not obligated to do so under the terms of the securities.

As a result, investors in a global security will not own securities directly. Instead, they will own beneficial interests in a global security, through a bank, broker or other financial institution that participates in the depositary’s book-entry system or holds an interest through a participant. As long as the securities are issued in global form, investors will be indirect holders, and not legal holders, of the securities.

Street Name Holders

We may terminate a global security or issue securities that are not issued in global form. In these cases, investors may choose to hold their securities in their own names or in “street name.” Securities held by an investor in street name would be registered in the name of a bank, broker or other financial institution that the investor chooses, and the investor would hold only a beneficial interest in those securities through an account he or she maintains at that institution.

For securities held in street name, we or any applicable trustee or depositary will recognize only the intermediary banks, brokers and other financial institutions in whose names the securities are registered as the holders of those securities, and we or any such trustee or depositary will make all payments on those securities to them. These institutions pass along the payments they receive to their customers who are the beneficial owners, but only because they agree to do so in their customer agreements or because they are legally required to do so. Investors who hold securities in street name will be indirect holders, not legal holders, of those securities.

Legal Holders

Our obligations, as well as the obligations of any applicable trustee or third party employed by us or a trustee, run only to the legal holders of the securities. We do not have obligations to investors who hold beneficial interests in global securities, in street name or by any other indirect means. This will be the case whether an investor chooses to be an indirect holder of a security or has no choice because we are issuing the securities only in global form.

For example, once we make a payment or give a notice to the legal holder, we have no further responsibility for the payment or notice even if that legal holder is required, under agreements with its participants or customers or by law, to pass it along to the indirect holders but does not do so. Similarly, we may want to obtain the approval of the holders to amend an indenture, to relieve us of the consequences

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of a default or of our obligation to comply with a particular provision of an indenture, or for other purposes. In such an event, we would seek approval only from the legal holders, and not the indirect holders, of the securities. Whether and how the legal holders contact the indirect holders is up to the legal holders.

Special Considerations for Indirect Holders

If you hold securities through a bank, broker or other financial institution, either in book-entry form because the securities are represented by one or more global securities or in street name, you should check with your own institution to find out:

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how it handles securities payments and notices;

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whether it imposes fees or charges;

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how it would handle a request for the holders’ consent, if ever required;

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whether and how you can instruct it to send you securities registered in your own name so you can be a holder, if that is permitted in the future;

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how it would exercise rights under the securities if there were a default or other event triggering the need for holders to act to protect their interests; and

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if the securities are in book-entry form, how the depositary’s rules and procedures will affect these matters.

Global Securities

A global security is a security that represents one or any other number of individual securities held by a depositary. Generally, all securities represented by the same global security will have the same terms.

Each security issued in book-entry form will be represented by a global security that we issue to, deposit with and register in the name of a financial institution or its nominee that we select. The financial institution that we select for this purpose is called the depositary. Unless we specify otherwise in the applicable prospectus supplement, DTC will be the depositary for all securities issued in book-entry form.

A global security may not be transferred to or registered in the name of anyone other than the depositary, its nominee or a successor depositary, unless special termination situations arise. We describe those situations below under “— Special Situations When a Global Security Will Be Terminated.” As a result of these arrangements, the depositary, or its nominee, will be the sole registered owner and legal holder of all securities represented by a global security, and investors will be permitted to own only beneficial interests in a global security. Beneficial interests must be held by means of an account with a broker, bank or other financial institution that in turn has an account with the depositary or with another institution that does. Thus, an investor whose security is represented by a global security will not be a legal holder of the security, but only an indirect holder of a beneficial interest in the global security.

If the prospectus supplement for a particular security indicates that the security will be issued as a global security, then the security will be represented by a global security at all times unless and until the global security is terminated. If termination occurs, we may issue the securities through another book-entry clearing system or decide that the securities may no longer be held through any book-entry clearing system.

Special Considerations for Global Securities

As an indirect holder, an investor’s rights relating to a global security will be governed by the account rules of the investor’s financial institution and of the depositary, as well as general laws relating to securities transfers. We do not recognize an indirect holder as a holder of securities and instead deal only with the depositary that holds the global security.

If securities are issued only as global securities, an investor should be aware of the following:

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an investor cannot cause the securities to be registered in his or her name, and cannot obtain non-global certificates for his or her interest in the securities, except in the special situations we describe below;

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an investor will be an indirect holder and must look to his or her own bank or broker for payments on the securities and protection of his or her legal rights relating to the securities, as we describe above;

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an investor may not be able to sell interests in the securities to some insurance companies and to other institutions that are required by law to own their securities in non-book-entry form;

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an investor may not be able to pledge his or her interest in the global security in circumstances where certificates representing the securities must be delivered to the lender or other beneficiary of the pledge in order for the pledge to be effective;

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the depositary’s policies, which may change from time to time, will govern payments, transfers, exchanges and other matters relating to an investor’s interest in the global security;

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we and any applicable trustee have no responsibility for any aspect of the depositary’s actions or for its records of ownership interests in the global security, nor will we or any applicable trustee supervise the depositary in any way;

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the depositary may, and we understand that DTC will, require that those who purchase and sell interests in the global security within its book-entry system use immediately available funds, and your broker or bank may require you to do so as well; and

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financial institutions that participate in the depositary’s book-entry system, and through which an investor holds its interest in the global security, may also have their own policies affecting payments, notices and other matters relating to the securities.

There may be more than one financial intermediary in the chain of ownership for an investor. We do not monitor and are not responsible for the actions of any of those intermediaries.

Special Situations When a Global Security Will Be Terminated

In a few special situations described below, a global security will terminate and interests in it will be exchanged for physical certificates representing those interests. After that exchange, the choice of whether to hold securities directly or in street name will be up to the investor. Investors must consult their own banks or brokers to find out how to have their interests in securities transferred to their own names, so that they will be direct holders. We have described the rights of holders and street name investors above.

Unless we provide otherwise in the applicable prospectus supplement, a global security will terminate when the following special situations occur:

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if the depositary notifies us that it is unwilling, unable or no longer qualified to continue as depositary for that global security and we do not appoint another institution to act as depositary within 90 days;

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if we notify any applicable trustee that we wish to terminate that global security; or

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if an event of default has occurred with regard to securities represented by that global security and has not been cured or waived.

The applicable prospectus supplement may also list additional situations for terminating a global security that would apply only to the particular series of securities covered by the prospectus supplement. When a global security terminates, the depositary, and neither we nor any applicable trustee, is responsible for deciding the names of the institutions that will be the initial direct holders.

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SELLING SECURITYHOLDERS

Selling securityholders are persons or entities that, directly or indirectly, have acquired or will from time to time acquire from us, our securities. If the registration statement of which this prospectus forms a part is used by selling securityholders for the resale of any securities registered thereunder pursuant to a registration rights agreement between us and such selling securityholders or otherwise, information about such selling securityholders, their beneficial ownership of our securities and their relationship with us will be set forth in a prospectus supplement, in a post-effective amendment or in filings we make with the SEC under the Exchange Act that are incorporated by reference.

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PLAN OF DISTRIBUTION

We or selling securityholders may sell the securities covered hereby from time to time pursuant to underwritten public offerings, direct sales to the public, negotiated transactions, block trades or a combination of these methods. A distribution of these securities offered by this prospectus may also be effected through the issuance of derivative securities, including without limitation, warrants. We or selling securityholders may sell the securities to or through underwriters or dealers, through agents, or directly to one or more purchasers. We or selling securityholders may distribute securities from time to time in one or more transactions:

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at a fixed price or prices, which may be changed;

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at market prices prevailing at the time of sale;

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at prices related to such prevailing market prices; or

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at negotiated prices.

We may also sell equity securities covered by this registration statement in an “at the market offering” as defined in Rule 415(a)(4) under the Securities Act. Such offering may be made into an existing trading market for such securities in transactions at other than a fixed price on or through the facilities of the Nasdaq Global Select Market or any other securities exchange or quotation or trading service on which such securities may be listed, quoted or traded at the time of sale. Such at the market offerings, if any, may be conducted by underwriters acting as principal or agent.

A prospectus supplement or supplements (and any related free writing prospectus that we may authorize to be provided to you) will describe the terms of the offering of the securities, including, to the extent applicable:

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the name or names of any underwriters, dealers or agents;

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the name or names of any managing underwriter or underwriters;

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the amounts of securities underwritten or purchased by each of them;

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the name or names of any selling securityholders;

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the purchase price of the securities and the proceeds we will receive from the sale;

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any over-allotment options under which underwriters may purchase additional securities from us;

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any agency fees or underwriting discounts and other items constituting agents’ or underwriters’ compensation;

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any public offering price;

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any discounts or concessions allowed or reallowed or paid to dealers; and

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any securities exchange or market on which the securities may be listed.

Only underwriters named in the prospectus supplement are underwriters of the securities offered by the prospectus supplement.

If underwriters are used in the sale, they will acquire the securities for their own account and may resell the securities from time to time in one or more transactions at a fixed public offering price or at varying prices determined at the time of sale. The obligations of the underwriters to purchase the securities will be subject to the conditions set forth in the applicable underwriting agreement. We or selling securityholders may offer the securities to the public through underwriting syndicates represented by managing underwriters or by underwriters without a syndicate. Subject to certain conditions, the underwriters will be obligated to purchase all of the securities offered by the prospectus supplement, other than securities covered by any overallotment or other option. Any public offering price and any discounts or concessions allowed or reallowed or paid to dealers may change from time to time. We or selling securityholders may use underwriters with whom we have a material relationship. We will describe in the prospectus supplement, naming the underwriter, the nature of any such relationship.

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We or selling securityholders may sell securities directly or through agents we designate from time to time. We will name any agent involved in the offering and sale of securities, and we will describe any commissions and other compensation we will pay the agent in the prospectus supplement. Unless the prospectus supplement states otherwise, our agent will act on a best-efforts basis for the period of its appointment.

We may authorize agents or underwriters to solicit offers by certain types of institutional investors to purchase securities from us at the public offering price set forth in the prospectus supplement pursuant to delayed delivery contracts providing for payment and delivery on a specified date in the future. We will describe the conditions to these contracts and the commissions we must pay for solicitation of these contracts in the prospectus supplement.

We may provide agents and underwriters with indemnification against civil liabilities related to this offering, including liabilities under the Securities Act, or contribution with respect to payments that the agents or underwriters may make with respect to these liabilities. Agents and underwriters may engage in transactions with, or perform services for, us in the ordinary course of business.

All securities we may offer, other than common stock, will be new issues of securities with no established trading market. Any underwriters may make a market in these securities, but will not be obligated to do so and may discontinue any market making at any time without notice. We cannot guarantee the liquidity of the trading markets for any securities.

Any underwriter may engage in overallotment, stabilizing transactions, short covering transactions and penalty bids in accordance with Rule 103 of Regulation M under the Exchange Act. Overallotment involves sales in excess of the offering size, which create a short position. Stabilizing transactions permit bids to purchase the underlying security so long as the stabilizing bids do not exceed a specified maximum price. Short covering transactions involve purchases of the securities in the open market after the distribution is completed to cover short positions. Penalty bids permit the underwriters to reclaim a selling concession from a dealer when the securities originally sold by the dealer are purchased in a stabilizing or covering transaction to cover short positions. Those activities may cause the price of the securities to be higher than it would otherwise be. If commenced, the underwriters may discontinue any of the activities at any time. These transactions may be effected on any exchange or over-the-counter market or otherwise.

Any underwriters or agents who are qualified market makers on the Nasdaq Global Select Market may engage in passive market making transactions in the securities on the Nasdaq Global Select Market in accordance with Rule 103 of Regulation M under the Exchange Act, during the business day prior to the pricing of the offering, before the commencement of offers or sales of the securities. Passive market makers must comply with applicable volume and price limitations and must be identified as passive market makers. In general, a passive market maker must display its bid at a price not in excess of the highest independent bid for such security; if all independent bids are lowered below the passive market maker’s bid, however, the passive market maker’s bid must then be lowered when certain purchase limits are exceeded. Passive market making may stabilize the market price of the securities at a level above that which might otherwise prevail in the open market and, if commenced, may be discontinued at any time.

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LEGAL MATTERS

Unless otherwise indicated in the applicable prospectus supplement, certain legal matters in connection with the offering and the validity of the securities offered by this prospectus, and any supplement thereto, will be passed upon by Cooley LLP, Reston, Virginia. Additional legal matters may be passed upon for us or any underwriters, dealers or agents, by counsel that we will name in the applicable prospectus supplement.

EXPERTS

The financial statements incorporated in this prospectus by reference to the Annual Report on Form 10-K for the year ended December 31, 2025 have been so incorporated in reliance on the report of PricewaterhouseCoopers LLP, an independent registered public accounting firm, given on the authority of said firm as experts in auditing and accounting.

WHERE YOU CAN FIND MORE INFORMATION

This prospectus is part of a registration statement we filed with the SEC. This prospectus does not contain all of the information set forth in the registration statement and the exhibits to the registration statement. For further information with respect to us and the securities we are offering under this prospectus, we refer you to the registration statement and the exhibits and schedules filed as a part of the registration statement. Neither we nor any selling securityholder, agent, underwriter or dealer has authorized any person to provide you with different information than the information contained in or incorporated by reference in this prospectus. We are not making an offer of these securities in any state where the offer is not permitted. You should not assume that the information in this prospectus is accurate as of any date other than the date on the front page of this prospectus, regardless of the time of delivery of this prospectus or any sale of the securities offered by this prospectus.

We file annual, quarterly and current reports, proxy statements and other information with the SEC. The SEC maintains a website at www.sec.gov that contains reports, proxy and information statements, and other information regarding issuers, such as us, that file electronically with the SEC.

Copies of certain information filed by us with the SEC are also available on our website at www.aclaristx.com. Information contained in or accessible through our website does not constitute a part of this prospectus and is not incorporated by reference in this prospectus. We have included our website address as an inactive textual reference only.

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INCORPORATION OF CERTAIN INFORMATION BY REFERENCE

The SEC allows us to “incorporate by reference” information into this prospectus, which means that we can disclose important information to you by referring you to another document filed separately with the SEC. The SEC file number for the documents incorporated by reference in this prospectus is 001-37581. The documents incorporated by reference into this prospectus contain important information that you should read about us.

The following documents are incorporated by reference into this document:

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our Annual Report on Form 10-K for the fiscal year ended December 31, 2025, filed with the SEC on February 26, 2026;

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our Quarterly Reports on Form 10-Q for the fiscal quarters ended March 31, 2026 and June 30, 2026 filed with the SEC on May 7, 2026 and August 6, 2026, respectively;

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our Current Reports on Form 8-K filed with the SEC on January 6, 2026, April 28, 2026, and June 4, 2026, to the extent the information in such reports is filed and not furnished; and

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We also incorporate by reference into this prospectus all documents (other than current reports furnished under Item 2.02 or Item 7.01 of Form 8-K and exhibits filed on such form that are related to such items) that are filed by us with the SEC pursuant to Sections 13(a), 13(c), 14 or 15(d) of the Exchange Act after the date of this prospectus but prior to the termination of the offering. These documents include periodic reports, such as Annual Reports on Form 10-K, Quarterly Reports on Form 10-Q and Current Reports on Form 8-K, as well as proxy statements.

We will provide to each person, including any beneficial owner, to whom a prospectus is delivered, without charge upon written or oral request, a copy of any or all of the documents that are incorporated by reference into this prospectus but not delivered with the prospectus, including exhibits that are specifically incorporated by reference into such documents. You should direct any requests for documents to Aclaris Therapeutics Inc., Attn: Matthew Rothman, General Counsel and Corporate Secretary, 701 Lee Road, Suite 103, Wayne, Pennsylvania 19087; telephone: (484) 324-7933.

Any statement contained in this prospectus or contained in a document incorporated or deemed to be incorporated by reference into this prospectus will be deemed to be modified or superseded to the extent that a statement contained in this prospectus or any subsequently filed supplement to this prospectus, or document deemed to be incorporated by reference into this prospectus, modifies or supersedes such statement.

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7,785,714 Shares of Common Stock

Pre-funded Warrants to Purchase 6,500,000 Shares of Common Stock

PROSPECTUS SUPPLEMENT

Joint Bookrunning Managers

Leerink Partners

Piper Sandler

Cantor

LifeSci Capital

October 27, 2026

ATTACHMENTS / EXHIBITS

EX-FILING FEES