The FDA has again halted testing of a Regenxbio gene therapy — this time RGX-121 for Hunter syndrome (MPS II) — after safety concerns emerged from patient spinal scans.
The agency placed a clinical hold after spine MRI abnormalities were found in trial participants. According to the company, masses described as “likely” benign were identified on the spines of five patients treated three to six years ago. Those patients remained asymptomatic, with stable or improved neurocognitive assessments, and no brain masses were detected. Regenxbio emphasized there is “no clinical or pathological evidence to confirm the nature or causation of the spine MRI findings.”
A second setback
This is the second clinical hold on a Regenxbio program in 2026. The first, in January, followed a brain-cancer case in a patient receiving a similar therapy for a different rare disease; the FDA rejected the related application in February. Regenxbio said it no longer plans to file a resubmission “in the near term,” and its shares fell more than 25% on the news. The company is working with partner NS Pharma to evaluate the data and incorporate FDA feedback.
Why it matters
Gene therapies for rare diseases can be transformative, but they also raise long-term safety questions that can take years to surface. Repeated holds are a reminder of how carefully regulators and developers must weigh durable benefit against uncertain, delayed risks — especially in children with few other options.