REGENXBIO Releases Positive 12-Month Data for Hunter Syndrome AAV Gene Therapy

ROCKVILLE, Md., Sept. 5, 2025 — REGENXBIO Inc. (Nasdaq: RGNX) announced new, positive data for its AAV gene therapy, RGX-121, for patients with Hunter syndrome, also known as MPS II. The company hopes the new 12-month results will secure accelerated approval from the U.S. Food and Drug Administration (FDA), despite a recent regulatory delay in the review process for the AAV therapeutic.

The company’s one-time AAV therapeutic treatment resulted in a median 82% reduction of a key biomarker, heparan sulfate (HS) D2S6, in the cerebrospinal fluid of 13 patients after one year. This reduction was sustained and was strongly correlated with continued neurodevelopmental skill acquisition or stability, supporting the use of the biomarker as a surrogate endpoint for accelerated approval of the AAV gene therapy. The primary endpoint of CSF HS D2S6 reduction at week 16 was met with statistical significance.

REGENXBIO submitted these longer-term pivotal results to the FDA in response to an information request. The submission led to the agency extending the decision date for the Biologics License Application (BLA) from November 9 to February 8, 2026. The company stated that the AAV therapy has been well-tolerated with no safety concerns raised by the FDA during the review so far.

An Urgent Need for New Therapies

Hunter syndrome is a devastating, degenerative disease that affects an estimated 2,000 people worldwide. Patients lack a crucial enzyme that breaks down complex sugars, leading to a decline in health as early as 18 months of age.

Dr. Roberto Giugliani, a professor at UFRGS in Brazil, highlighted the urgent need for new therapies. “The vast majority of Hunter syndrome patients have no current treatment options to address the neurodevelopmental decline of this disease and are in urgent need of new AAV therapies,” he said.

RGX-121 is a potential one-time AAV-based treatment designed to directly address the underlying genetic cause of Hunter syndrome by delivering a therapeutic gene to the central nervous system. If approved, it would be the first and only one-time, commercially available AAV gene therapy of its kind.