FDA Accepts Priority Review of REGENXBIO's BLA for RGX-121 in MPS II

REGENXBIO Inc., a leading biotechnology firm, has recently announced a significant advancement in the treatment of Mucopolysaccharidosis II (MPS II), commonly known as Hunter Syndrome. The U.S. Food and Drug Administration (FDA) has accepted for review the Biologics License Application (BLA) for clemidsogene lanparvovec, also referred to as RGX-121. This development marks a prospective breakthrough as RGX-121 is set to become the first gene therapy and one-time treatment for MPS II, a rare and debilitating genetic disorder.

The FDA has granted Priority Review status to the BLA, with a target action date of November 9, 2025, under the Prescription Drug User Fee Act (PDUFA). This expedited review process underscores the urgent need for innovative treatments for MPS II, a condition characterized by a deficiency in the enzyme iduronate-2-sulfatase (I2S), leading to harmful accumulations in the body that cause progressive damage to various organs, including the central nervous system (CNS).

Curran M. Simpson, President and Chief Executive Officer of REGENXBIO, highlighted the importance of this development, noting that the acceptance of the BLA for RGX-121 signifies a critical step towards delivering a transformative treatment to the MPS II community. Currently, patients and their families face the considerable burden of weekly enzyme replacement therapy, and RGX-121 offers the potential to significantly alleviate this through a one-time gene therapy approach. The therapy aims to address both neurodevelopmental and systemic symptoms associated with Hunter Syndrome.

RGX-121 has garnered multiple designations from the FDA, including Orphan Drug Product, Rare Pediatric Disease, Fast Track, and Regenerative Medicine Advanced Therapy (RMAT) status. In addition, it has received the classification of advanced therapy medicinal products (ATMP) from the European Medicines Agency, reflecting its potential as a pioneering treatment option.

To facilitate the commercialization of RGX-121, REGENXBIO has formed a strategic partnership with NS Pharma, Inc., a subsidiary of Nippon Shinyaku. Upon potential FDA approval, NS Pharma will oversee the commercialization efforts within the United States. REGENXBIO will manage the commercial manufacturing and supply chain aspects. A successful approval could also yield a Priority Review Voucher (PRV), allowing REGENXBIO the opportunity to retain and potentially sell the PRV for financial gain.

RGX-121 is designed as a one-time adeno-associated virus (AAV) therapeutic to deliver the missing IDS gene directly to the CNS. By doing so, it aims to provide a permanent source of the I2S protein, crossing the blood-brain barrier to enable long-term correction of affected cells throughout the CNS. The expressed protein is identical to the naturally occurring I2S, offering hope for sustainable symptom management and improvement in patients’ quality of life.

Hunter Syndrome primarily affects males and is a challenging condition with no current cure. It manifests early in life with developmental delays becoming apparent between 18 to 24 months. The condition remains a significant unmet medical need, particularly concerning its neurological aspects. Key biomarkers for the disease include heparan sulfate (HS) D2S6, which correlates with the neurocognitive symptoms MPS II patients experience.

REGENXBIO, founded in 2009, is a pioneer in AAV gene therapy, focusing on developing one-time treatments for rare and retinal diseases. Among their pipeline are late-stage therapies for various conditions, including Duchenne muscular dystrophy and other mucopolysaccharidoses. With a commitment to improving lives through gene therapy's potential, REGENXBIO continues to advance its innovative treatment approaches to meet the needs of patients with rare disorders.