REGENXBIO Receives FDA PDUFA Date Extension for Hunter Syndrome Gene Therapy

ROCKVILLE, Maryland, August 19, 2025 – REGENXBIO Inc. announced today that the U.S. Food and Drug Administration (FDA) has extended the review period for its investigational AAV-based gene therapy, clemidsogene lanparvovec (RGX-121), a potential treatment for Hunter syndrome. The new Prescription Drug User Fee Act (PDUFA) target action date is February 8, 2026, a delay from the previous date of November 9, 2025.

The extension follows the FDA’s request for additional data, to which REGENXBIO responded with 12-month data from the pivotal trial. The company stated that the data are consistent with previously submitted biomarker and neurodevelopmental information. The FDA has not raised any safety concerns about the candidate and has completed its inspections with no objections.

RGX-121 is a one-time gene therapy designed to use an AAV vector to deliver a functional copy of the iduronate-2-sulfatase (IDS) gene to the central nervous system. Hunter syndrome is a rare, severe disorder caused by the lack of the IDS enzyme, which is critical for breaking down certain sugars. The resulting buildup of these sugars can lead to irreversible brain damage.

While enzyme replacement therapies exist, they cannot cross the blood-brain barrier to address the neurological symptoms of the disease. The company noted that the FDA’s request for additional data on a biomarker-based accelerated approval may reflect a preference for clinical outcomes, though experts argue that such trials are difficult to conduct ethically for ultrarare diseases.