FDA Reverses Course on Regenxbio’s MPS II Gene Therapy, Clearing Path to Possible Accelerated Approval

Rare Daily Staff

Regenxbio said it has reached agreement with the U.S. Food and Drug Administration on a path to resubmit its application for marketing approval of Navsunli, clearing a key regulatory hurdle just months after the gene therapy was rejected earlier this year.

The FDA indicated that no new clinical trials will be required to support a resubmission, allowing the company to rely on existing data from its Campsiite study. The agency will instead review longer-term biomarker and clinical data under the accelerated approval pathway and has committed to an expedited review timeline, with labeling discussions expected to begin shortly after filing.

The update marks a reversal from the agency’s February 2026 complete response letter, which had called for additional evidence, including the potential incorporation of an untreated control arm. According to Regenxbio, those requirements were lifted following a formal appeal and subsequent discussions with the FDA, which concluded that the existing dataset is sufficient for consideration under accelerated approval.

The outcome of the Navsunli review could also serve as a bellwether for how the FDA applies accelerated approval standards to central nervous system–directed gene therapies in ultra-rare pediatric conditions.

The company plans to hold a type A meeting with the FDA in July to align on the resubmission package and expects to refile the application in the third quarter.

Hunter syndrome is characterized by progressive, multisystem disease, with severe forms leading to early-onset neurodegeneration. Most patients begin to show developmental delays between 18 and 24 months of age.

Navsunli is an experimental one-time AAV gene therapy designed to deliver the iduronate-2-sulfatase gene directly to the central nervous system. By enabling sustained enzyme production within the brain, the therapy seeks to address the neurological manifestations of mucopolysaccharidosis type II, or Hunter syndrome—an area where current enzyme replacement therapies have limited impact because the infused enzyme does not effectively cross the blood-brain barrier.

The regulatory shift highlights increasing FDA flexibility in ultra-rare diseases, particularly where traditional randomized controlled trials are difficult to execute. In this case, biomarkers such as cerebrospinal fluid heparan sulfate, especially the D2S6 disaccharide linked to neurocognitive outcomes, are expected to play a central role in the agency’s review.

“We are encouraged by recent signals from the new FDA leadership reinforcing a commitment to address the unique nature of rare diseases and use the accelerated approval pathway to bring transformative therapies to patients with serious, unmet medical needs,” said Curran Simpson, president and CEO of Regenxbio.

Photo: Curran Simpson, president and CEO of Regenxbio