Regenxbio gene therapy refiling plan derailed by FDA hold over spine MRI findings

Brought to you by: Biotech Regenxbio gene therapy refiling plan derailed by FDA hold over spine MRI findings By Nick Paul Taylor Aug 24, 2026 7:51am REGENXBIO clinical hold Food and Drug Administration (FDA) Hunter syndrome The FDA has put a clinical hold on Regenxbio’s RGX-121 over asymptomatic spine MRI findings, derailing the biotech’s plans to refile for approval of the once-rejected gene therapy candidate. Regenxbio’s hopes of bringing RGX-121 to market have yo-yoed this year. The FDA rejected a request for approval of the asset in mucopolysaccharidosis type II (MPS II), also called Hunter syndrome, in February.

At the time, the agency told Regenxbio to enroll more patients and incorporate an untreated control arm. By June, the FDA had dropped those demands and agreed to review a filing based on existing studies. Yet the biotech, which had planned to refile for approval in the third quarter, now no longer expects to resubmit its application in the near term.

The latest swing in the prospects of RGX-121 follows concerns identified after Regenxbio implemented expanded monitoring in response to an earlier clinical hold. The FDA put both RGX-121 and RGX-111, a gene therapy in development for MPS I, on clinical hold in January. The regulatory action followed the discovery of a central nervous system tumor in an RGX-111 patient.

Similarities between the gene therapies, the study populations and the risks led the FDA to put both programs on hold despite only seeing the safety signal in an RGX-111 trial. In response, Regenxbio began enhanced monitoring of RGX-121 recipients, including brain and spine MRI scans. Images captured under the enhanced monitoring program showed asymptomatic small nodules or cystic masses in the spines of five participants in Regenxbio’s Campsiite trial.

The patients received intracisternal or intraventricular RGX-121 about three to six years ago. Investigators deemed the findings nonserious and radiologists believe they are likely benign, Regenxbio said. No clinical or pathological evidence confirms the nature or causation of the spine MRI findings, and no brain nodules or masses were identified.

The lack of routine spine MRI of MPS patients means the underlying prevalence and clinical significance of asymptomatic findings are unclear. “Boys with neuronopathic MPS II experience a multitude of neurodevelopmental and systemic effects,” Roberto Giugliani, M.D., Ph.D., a professor at the Federal University of Rio Grande do Sul in Brazil, said in a statement. “While imaging natural history is limited for this ultra-rare disease, I believe that asymptomatic, likely benign findings like these may be inherent to the impact of Hunter syndrome throughout the body.” Investigators plan to keep periodically imaging the five patients.

Regenxbio and its partner NS Pharma are evaluating additional patient imaging and longer-term follow-up data. The evidence, plus the clinical hold letter and other FDA feedback, will inform the next steps for RGX-121. While RGX-121’s future is uncertain, Regenxbio continues to advance RGX-202 and surabgene lomparvovec toward filings for approval in Duchenne muscular dystrophy and wet age-related macular degeneration, respectively.

The other gene therapies use a different capsid and routes of administration than RGX-121, informing Regenxbio’s belief that the spine findings are limited to RGX-121. Regenxbio’s share price fell 24% to $8.17 in premarket trading Monday from a Friday closing price of $10.72. REGENXBIO clinical hold Food and Drug Administration (FDA) Hunter syndrome Cell & Gene Therapy Biotech