The U.S. Food and Drug Administration (FDA) on Monday extended its review timeline for REGENXBIO Inc.’s Biologics License Application (BLA) for clemidsogene lanparvovec (RGX-121) for Mucopolysaccharidosis II (MPS II), also known as Hunter syndrome.

The Prescription Drug User Fee Act (PDUFA) goal date has been extended from November 9, 2025, to February 8, 2026.

The extension follows the company’s submission of longer-term clinical data for all patients in the pivotal study of RGX-121 (n=13) in response to an FDA information request. 

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These positive 12-month clinical data are consistent with biomarker and neurodevelopmental data previously submitted on the same patients in the BLA and will be presented during the International Congress of Inborn Errors of Metabolism (ICIEM) in September 2025.

In August 2025, the FDA completed a pre-license and bioresearch monitoring information inspection for the RGX-121 BLA with no observations. The FDA has raised no safety-related concerns during the BLA review.

RGX-121 is a potential one-time AAV therapeutic for boys with MPS II, designed to deliver the iduronate-2-sulfatase (IDS) gene to the central nervous system (CNS).

Delivery of the IDS gene within cells in the CNS could provide a permanent source of secreted iduronate-2-sulfatase (I2S) protein beyond the blood-brain barrier, allowing for long-term cross-correction of cells throughout the CNS. The RGX-121 expressed protein is structurally identical to normal I2S.

MPS II is a rare, X-linked recessive disease caused by a deficiency in the lysosomal enzyme I2S, leading to an accumulation of glycosaminoglycans (GAGs), including heparan sulfate (HS), in tissues, which ultimately results in cell, tissue, and organ dysfunction.

In 2024, REGENXBIO ( RGNX ) announced topline results from the Phase 1/2/3 CAMPSIITE trial of RGX-121 in patients up to 5 years old. The trial met its primary endpoint with statistical significance.

MPS II patients treated with RGX-121 achieved decreased cerebrospinal fluid levels of D2S6, a key biomarker of brain disease activity, below maximum attenuated disease levels at 16 weeks.

Patients receiving RGX-121 demonstrated an 86% median reduction in D2S6, approaching normal levels.

New long-term follow-up of patients treated with RGX-121 in the dose-finding phase also showed a high rate of patients for whom trial investigators chose to discontinue standard-of-care intravenous enzyme replacement therapy (ERT) or were allowed to remain ERT-naïve.

Price Action: RGNX stock is down 7.46% at $8.06 at the last check on Tuesday.

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