FDA signals reversal on Regenxbio rare-disease gene therapy approval path

The FDA has signaled a sharp reversal on Regenxbio’s rare-disease gene therapy, telling the company that existing data could support an accelerated approval filing for Navsunli after rejecting the program only four months earlier. The change revived investor confidence in a treatment for Hunter syndrome, a severe ultra-rare disease that can drive progressive cognitive decline and organ damage in boys.

Regenxbio said the agency asked it to request a Type A meeting in July 2026 to review longer-term biomarker and clinical data, and the company expects to resubmit its biologics license application in the third quarter of 2026. The FDA said it would review that resubmission on an expedited basis and begin labeling discussions shortly afterward. In a major departure from its earlier stance, the agency indicated that no additional patients, no new clinical studies and no untreated control arm would be required.

The therapy, known as Navsunli, clemidsogene lanparvovec-sngl and RGX-121, is being developed as a one-time gene therapy for mucopolysaccharidosis type II, also called MPS II. Hunter syndrome is caused by a deficiency of iduronate-2-sulfatase, or IDS, and often erodes both physical function and brain development over time. Regenxbio describes Navsunli as the first potential gene therapy for the disorder, a claim that underscores how much hangs on the FDA’s willingness to accept biomarker-based evidence in a brain disease where direct clinical proof is hard to assemble quickly.

The data package centers on the CAMPSIITE study, a phase 1/2/3 open-label trial in boys with neuronopathic MPS II. The study follows patients over 24 months after dosing, with an optional three-year long-term follow-up. Regenxbio said in September 2025 that it had submitted longer-term pivotal results to the FDA after presenting 12-month data showing more than an 80% reduction in the CSF biomarker HS D2S6 and continued skill acquisition or stability through one year. Public reports on the pivotal phase said 13 patients had an 82% median reduction in CSF HS D2S6 through one year.

The turnaround from the February 2026 complete response letter to renewed FDA support was about four months, a striking shift for a company whose valuation is tightly tied to regulatory momentum. Regenxbio shares rose 16% on the news. The move also fits a broader pattern of recent reversals in rare-disease gene therapy, including cases involving uniQure and Replimune, raising the possibility that the agency under current leadership is becoming more willing to use accelerated approval when confirmatory data are difficult to generate in tiny patient populations.