FDA Extends Review Period of Hunter Syndrome Gene Therapy

The Food and Drug Administration (FDA) has extended the review for the Biologics License Application (BLA) for clemidsogene lanparvovec for the treatment of mucopolysaccharidosis II (MPS II), also known as Hunter Syndrome. 

MPS II is a rare, X-linked recessive disorder caused by lysosomal iduronate-2-sulfatase (I2S) enzyme deficiency, which results in glycosaminoglycan accumulation and organ dysfunction. Clemidsogene lanparvovec, previously known as RGX-121, is a one-time adeno-associated virus therapy designed to address the neurological manifestations of MPS II by delivering the iduronate-2-sulfatase gene, which encodes the I2S enzyme, directly to the central nervous system. 

The original BLA submission was supported by data from the CAMPSIITE trial (ClinicalTrials.gov Identifier: NCT03566043), which evaluated clemidsogene lanparvovec in children aged 4 months to 5 years old with MPS II. Findings showed treatment with clemidsogene lanparvovec reduced cerebral spinal fluid (CSF) D2S6 levels, a key biomarker of brain disease activity, and improved skill acquisition. 

The extension was needed to review newly submitted long-term clinical data for all patients in the pivotal study following an FDA information request. The updated 12-month clinical data were consistent with data previously submitted in the original BLA. 

Notably in August 2025, the FDA also completed a pre-license inspection and bioresearch monitoring information inspection for the clemidsogene lanparvovec BLA; no safety issues were raised during the review. 

“Boys with this rare, devastating disease have no treatment options to address neurodevelopmental decline, and the Hunter syndrome community is in urgent need for a therapeutic option with the potential to improve these patients’ lives,” said Curran M. Simpson, President and Chief Executive Officer of REGENXBIO. “We promptly provided the FDA with the information requested and expect the commercial launch plans to remain on track.”

A new Prescription Drug User Fee Act target date of February 8, 2026 has been set for the application.  

This article originally appeared on MPR