REGENXBIO’s BLA for MPS II Gene Therapy RGX-121 Accepted for Priority Review by the FDA

The FDA has accepted REGENXBIO’s biologics license application (BLA) clemidsogene lanparvovec (also known as RGX-121), an investigational adeno-associated virus vector-based gene therapy intended to treat mucopolysaccharidosis type 2 (MPSII, also known as Hunter syndrome).¹ The BLA was accepted for priority review and its Prescription Drug User Fee Act (PDUFA) action date has been set as November 9, 2025.

"Acceptance of the RGX-121 BLA marks an exciting milestone on our path to bring the MPS II patient community a one-time treatment with the potential to address both the neurodevelopmental and systemic effects of Hunter syndrome," Curran M. Simpson, the president and chief executive officer of REGENXBIO, said in a statement.¹ "Supported by positive biomarker data and long-term outcomes, RGX-121 has the potential to be a first-in-class gene therapy that could dramatically transform the MPS II treatment landscape and reduce the significant burden patients and families currently face with weekly enzyme replacement therapy."

The FDA has previously granted RGX-121 orphan drug, rare pediatric disease, fast track, and regenerative medicine advanced therapy designations. The European Medicines Agency has granted it advanced therapy medicinal products classification.

Notably, REGENXBIO has stated that if RGX-121 is approved by the FDA, its commercialization will be carried out byNippon Shinyaku subsidiary NS Pharma in accordance with a strategic partnership agreement that the company revealed in January of this year. NS Pharma will also carry out potential commercialization activities for RGX-111, another of REGENXBIO’s investigational gene therapy products, which is in development for MPS I (also known as Hurler syndrome).

"This partnership with Nippon Shinyaku is exciting in that it maximizes our collective strengths and enables access of 2 potentially transformational medicines to key markets," Simpson said in a January 2025 statement.² "The structure of the agreement allows us to leverage our expertise in gene therapy manufacturing while also capturing milestones and a meaningful share of future product revenues. RGX-121 is poised to be the first gene therapy for MPS II with potential FDA approval as early as late 2025, and RGX-111 has demonstrated very promising results in phase 1/2 study. With Nippon Shinyaku's expertise in rare disease and strong commercial capabilities, we look forward to working together to get both of these promising candidates across the finish line for patients."

Results from the pivotal phase 1/2/3 CAMPSIITE clinical trial evaluating RGX-121 were recently presented at the Society for the Study of Inborn Errors of Metabolism (SSIEM) Annual Symposium 2024, held September 3-6, in Porto, Portugal.³ REGENXBIO reported that among patients who received the pivotal dose level (dose level 3, DL3) in the study, a median reduction of 85% in heparan sulfate D2S6 levels was seen in the cerebrospinal fluid (CSF). The company noted that these levels of the biomarker approach normal levels and were sustained for up to 2 years.

"A potential one-time treatment that can allow these boys to exceed the natural history of this disease in their neurocognitive development, as well as the ability to remain off enzyme replacement therapy for multiple years represents a meaningful option for patients and their families," Roberto Giugliani, MD, PhD, a professor in the Department of Genetics, UFRGS, Medical Genetics Service, HCPA, Porto Alegre, Brazil, said in a September 2024 statement.³ "I continue to be very encouraged by the data supporting RGX-121 and look forward to the seeing this program advance towards potential approval for this community."

REFERENCES
1. REGENXBIO announces FDA acceptance and priority review of the BLA for RGX-121 for MPS II. News release. REGENXBIO Inc. May 13, 2025. Accessed May 15, 2025. https://ir.regenxbio.com/news-releases/news-release-details/regenxbio-announces-fda-acceptance-and-priority-review-bla-rgx
2. REGENXBIO and Nippon Shinyaku announce exclusive partnership to develop and commercialize RGX-121 and RGX-111 for MPS diseases. News release. REGENXBIO Inc. January 14, 2025. Accessed May 15, 2025. https://regenxbio.gcs-web.com/news-releases/news-release-details/regenxbio-and-nippon-shinyaku-announce-exclusive-partnership/
3. REGENXBIO announces positive data from pivotal dose level of RGX-121 demonstrating long-term systemic effect. News release. REGENXBIO Inc. September 3, 2024. Accessed May 15, 2025. https://ir.regenxbio.com/news-releases/news-release-details/regenxbio-announces-positive-data-pivotal-dose-level-rgx-121