Regenxbio aligns with FDA on accelerated approval of MPS II gene therapy

Regenxbio says it has aligned with the U.S. FDA regarding the next steps needed for a potential accelerated approval of Navsunli, a one-time gene therapy for Mucopolysaccharidosis II, an ultra-rare neurodegenerative disease also known as Hunter syndrome.

Hunter syndrome is caused by a variation in the IDS gene, which encodes the enzyme iduronate-2-sulfatase (I2S). Without it, the body cannot break down certain sugar molecules, leading to progressive neurodegeneration. Navsunli is an AAV-based therapy designed to deliver a functional copy of the IDS gene directly to the central nervous system and, if approved, would be the first gene therapy and one-time treatment for the disease.

The FDA accepted Regenxbio BLA for Navsunli in May 2025, granting it priority review with a PDUFA action date of November 9, 2025. In August 2025, the agency extended its review to February 8, 2026 after the company submitted longer-term clinical data for all pivotal study patients. Then in January 2026, the FDA placed both Navsunli and RGX-111 on clinical hold after a brain tumor was found in a 5-year-old participant in the RGX-111 trial.

The agency issued a CRL for Navsunli in February 2026, citing concerns about study eligibility criteria, the comparability of the natural history external control to the study population, and the appropriateness of CSF HS D2S6 as a surrogate endpoint reasonably likely to predict clinical benefit. Regenxbio appealed the CRL, arguing it had addressed those concerns through additional data and responses to FDA information requests. The CRL had outlined several potential remedies — a new study, additional patients with longer follow-up, or an untreated control arm — but the company said each would be difficult to execute given the ultra-rare patient population.

Now, as a result of the appeal, the FDA acknowledged that the existing clinical data is sufficient to support consideration under the accelerated approval pathway and that no additional patients or studies are required, including the previously suggested untreated control arm. The agency asked Regenxbio to request a Type A meeting to review existing longer-term biomarker and clinical data, and to resubmit the BLA following that meeting — committing to an expedited review and labeling discussions shortly after resubmission.

Regenxbio expects the Type A meeting in July and a BLA resubmission in Q3 2026.

Last week, uniQure found itself in a similar situation, with the agency seemingly reversing course and deciding that the 3-year analysis from the company’s phase 1/2 study would be acceptable as the primary basis of a BLA for the accelerated approval of the company’s Huntington’s disease gene therapy. The minutes from uniQure’s Type A meeting in March had strongly recommended that the company conduct a randomized, double-blind, sham surgery-controlled study —a point of conflict between uniQure and the FDA, given that AMT-130 is administered as a one-time gene therapy directly into the brain via a single, complex neurosurgical procedure.