FDA places clinical hold on REGENXBIO's RGX-121 for Hunter Syndrome

REGENXBIO Inc. disclosed on August 24, 2026 that the FDA has placed a clinical hold on RGX-121 (clemidsogene lanparvovec), its investigational gene therapy for Mucopolysaccharidosis type II (MPS II, or Hunter Syndrome). The hold followed the discovery of asymptomatic spine MRI findings in five participants in the CAMPSIITE study, and REGENXBIO does not expect to resubmit the RGX-121 Biologics License Application in the near term.¹

The company said the affected patients remain clinically stable. All five participants continue to do well and have shown overall stability or improvement on neurocognitive and neurobehavioral assessments, REGENXBIO said.¹ The findings emerged from enhanced screening put in place after an earlier hold tied to a related program. The findings were identified through an expanded MRI monitoring plan that REGENXBIO began a few months earlier, following the clinical hold related to RGX-111, and the monitoring found either a small nodule or a small cystic mass on spine imaging in five patients who had received intracisternal or intraventricular RGX-121 between roughly three and six years earlier.¹

Investigators do not consider the findings dangerous. Investigators deemed the findings nonserious and radiologists believe they are likely benign, though there is no clinical or pathological evidence confirming their nature or cause, and no nodules or masses were seen on brain MRIs.¹ Because spine MRI is not standard practice in MPS trials, the underlying prevalence of such findings in this population is unknown, and investigators plan to keep monitoring these patients with periodic imaging only.¹

REGENXBIO and its partner NS Pharma are reviewing further imaging and longer-term data and plan to factor in the FDA's full hold letter, once received, in determining the program's path forward, the company said. REGENXBIO's CEO Curran Simpson said the company remains focused on its Duchenne and retinal disease programs, which use a different capsid and delivery routes, including a planned Duchenne BLA submission this quarter and topline wet AMD pivotal data expected in the fourth quarter.¹