Regenxbio Inc. (Nasdaq: RGNX) said the U.S. Food and Drug Administration has placed a clinical hold on its investigational gene therapy RGX-121 (clemidsogene lanparvovec) for Mucopolysaccharidosis type II (MPS II), also known as Hunter Syndrome, after asymptomatic spine MRI findings were discovered in five participants in the company's CAMPSIITE study.

The company said it does not expect to resubmit the RGX-121 Biologics License Application in the near term. Regenxbio and its partner NS Pharma said they will evaluate additional patient imaging and longer-term follow-up data, and will incorporate FDA feedback into next steps for the program.

The spine MRI findings — either a small nodule or a small cystic mass — were identified in five participants who received intracisternal or intraventricular RGX-121 approximately three to six years ago. The findings were detected through an expanded MRI monitoring plan that Regenxbio implemented after a separate clinical hold related to RGX-111. Investigators deemed the findings nonserious, and radiologists believe they are likely benign. No brain nodules or masses were identified. All five participants continue to show overall stability to improvement on neurocognitive and neurobehavioral assessments.

There is no clinical or pathological evidence to confirm the nature or causation of the spine MRI findings. Because spine MRI is not routinely conducted for MPS in clinical practice, the underlying prevalence and clinical significance of such asymptomatic findings in this patient population is unknown.

Curran Simpson, President and CEO of Regenxbio, said the company believes the findings are unique and limited to the Hunter Syndrome program and require longer-term follow-up and additional data analysis. He added that the company's Duchenne and retinal disease candidates, which use a different capsid and routes of administration, remain on track, including a planned Duchenne BLA submission in the current quarter and wet AMD topline pivotal data expected in the fourth quarter.