FDA Extends Review Period for REGENXBIO Hunter Syndrome Treatment

Author(s):

Key Takeaways

Clemidsogene lanparvovec targets Hunter syndrome, a rare X-linked recessive disease, by delivering the iduronate-2-sulfatase gene to the CNS for long-term cross-correction.
The FDA extended the BLA review timeline to February 2026, with no safety concerns reported during the review process.
REGENXBIO submitted positive long-term clinical data, consistent with previous biomarker and neurodevelopmental data, to the FDA.
The treatment has received orphan drug, rare pediatric disease, fast track, and regenerative medicine advanced therapy designations from the FDA, and advanced therapy medicinal products classification from the EMA.

The agency has completed both pre-license and bioresearch monitoring inspections with no observations, and no safety-related concerns have been raised to date.

The front of the REGENXBIO facility in Rockville, Montgomery County, Maryland. REGENXBIO is a biotechnology company researching Adeno-Associate Virus (AAV) gene therapy | Image Credit: © MJ Kerr - stock.adobe.com

The Rockville, Md.-based clinical-stage biotechnology company, REGENXBIO, announced on Aug. 18, 2025 that its adeno-associated viral therapeutic, clemidsogene lanparvovec (RGX-121), will have the timeline for its biologics license application (BLA) review extended by FDA, with the medicine’s Prescription Drug User Fee Act goal date being moved from Nov. 9, 2025 to Feb. 8, 2026 (1).

Hunter syndrome’s rarity

Clemidsogene lanparvovec is being developed as a potential one-time treatment for juvenile males with mucopolysaccharidosis II (MPS II), which is also known as Hunter syndrome (1). Hunter syndrome is a rare, X-linked recessive disease caused by a deficiency in the lysosomal enzyme I2S, or secreted iduronate-2-sulfatase. This deficiency leads to an accumulation of glycosaminoglycans, including heparan sulfate, in tissues—ultimately resulting in cell, tissue, or organ dysfunction, including in the central nervous system.

The disease may initially present as hydrocephalus, often followed by behavioral changes and, eventually, central nervous system involvement (2). While early developmental milestones may be met by boys with Hunter syndrome, severe disease makes developmental delays apparent by 18 to 24 months of age, according to REGENXBIO (1).

"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 CEO of REGENXBIO, in a company press release (1). "We promptly provided [FDA] with the information requested and expect the commercial launch plans remain on track."

No red flags reported

Clemidsogene lanparvovec is designed to deliver the iduronate-2-sulfatase gene to the central nervous system, which REGENXBIO says could provide a permanent source of the I2S protein beyond the blood-brain barrier, allowing for long-term cross-correction of cells (1).

As of August 2025, REGENXBIO said, FDA had completed both pre-license and bioresearch monitoring inspections for clemidsogene lanparvovec’s BLA with no observations, and the company said no safety-related concerns have been raised by FDA during BLA review to date (1).

REGENXBIO has submitted longer-term clinical data for all patients in a pivotal study, in response to an FDA request, and reports that the positive 12-month data are consistent with biomarker and neurodevelopmental data previously submitted, concerning the same patients, in the BLA (1). The data will be presented at the International Congress of Inborn Errors of Metabolism, to be held in Kyoto, Japan, Sept. 2–6, 2025.

Data previously presented

Results from late-stage clinical trials of RGX-121 had been presented at the American Society of Gene & Cell Therapy’s 28th annual meeting, in New Orleans from May 13–17, 2025 (2).

At that conference, Olivier Danos, PhD, REGENXBIO executive vice president and chief scientific officer, made the oral presentation “RGX-121 (clemidsogene lanparvovec): An Investigational AAV [adeno-associated virus] Gene Therapy for the Treatment of Neuronopathic Mucopolysaccharidosis Type II” during a session on “CNS Gene Delivery for Metabolic Diseases: History and Challenges.”

REGENXBIO’s treatment for Hunter syndrome has received orphan drug product, rare pediatric disease, fast track, and regenerative medicine advanced therapy designations from FDA, and advanced therapy medicinal products classification from the European Medicines Agency (1).

References

1. REGENXBIO. REGENXBIO Announces FDA Review Extension of BLA for RGX-121 to Treat Patients with MPS II. Press Release. Aug. 18, 2025.
2. Lavery, P. REGENXBIO Releases Schedule of Its ASGCT Oral Sessions and Poster Exhibits. BioPharmInternational.com, May 8, 2025.

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