REGENXBIO Announces FDA Acceptance and Priority Review of BLA for RGX-121 Gene Therapy in Hunter Syndrome

ROCKVILLE, Md. – May 13, 2025 – REGENXBIO Inc. (Nasdaq: RGNX) today announced that the U.S. Food and Drug Administration (FDA) has accepted for review its Biologics License Application (BLA) seeking accelerated approval for clemidsogene lanparvovec (RGX-121) for the treatment of Mucopolysaccharidosis II (MPS II), also known as Hunter syndrome.

The FDA has granted the BLA Priority Review with a Prescription Drug User Fee Act (PDUFA) target action date of November 9, 2025.

Curran M. Simpson, President and Chief Executive Officer of REGENXBIO, stated that the BLA acceptance marks an exciting step towards providing a potential one-time treatment for MPS II that could address both the neurodevelopmental and systemic effects of the disease. He highlighted the potential of RGX-121, supported by positive biomarker data and long-term outcomes, to transform the MPS II treatment landscape and reduce the burden associated with weekly enzyme replacement therapy.

RGX-121 has received several designations from the FDA, including Orphan Drug Product, Rare Pediatric Disease, Fast Track, and Regenerative Medicine Advanced Therapy (RMAT). It has also received advanced therapy medicinal products (ATMP) classification from the European Medicines Agency.

Under a strategic partnership with NS Pharma, Inc. (a subsidiary of Nippon Shinyaku), NS Pharma will commercialize RGX-121 in the U.S. following potential FDA approval. REGENXBIO retains all rights to and proceeds from any potential sale of a Priority Review Voucher (PRV) that may be granted upon approval of RGX-121.

About RGX-121 (clemidsogene lanparvovec): RGX-121 is an investigational one-time AAV gene therapy designed to deliver the iduronate-2-sulfatase (IDS) gene to the central nervous system (CNS) of boys with MPS II. This delivery aims to provide a continuous source of secreted IDS protein beyond the blood-brain barrier, enabling long-term cross-correction of cells throughout the CNS. The RGX-121 expressed protein is structurally identical to the normal IDS enzyme.

About Mucopolysaccharidosis Type II (MPS II): MPS II, or Hunter syndrome, is a rare, X-linked recessive disease caused by a deficiency in the IDS enzyme, leading to the accumulation of glycosaminoglycans (GAGs) in tissues, resulting in cell, tissue, and organ dysfunction, including in the CNS. Neurological manifestations remain a significant unmet medical need in MPS II. Heparan sulfate (HS) D2S6 is a key biomarker that correlates with the neurocognitive aspects of the disorder.

This FDA acceptance and Priority Review represent a crucial step forward in the development of a potentially transformative gene therapy for individuals living with Hunter syndrome.