5 minOn June 22, 2026, REGENXBIO announced that it has reached an agreement with the U.S. Food and Drug Administration (FDA) regarding the regulatory pathway for NAVSUNLI™ (RGX-121), its gene therapy candidate for Hunter syndrome (Mucopolysaccharidosis II, or MPS II). The FDA confirmed that existing clinical data are sufficient to support a resubmission under the accelerated approval pathway. Consequently, the company is not required to conduct new clinical trials, nor is it required to include the previously requested untreated control arm. Based on this outcome, REGENXBIO plans to resubmit its Biologics License Application (BLA) in the third quarter of 2026, with the FDA committing to an expedited review process.
Regulatory Breakthrough: No Additional Clinical Studies Required
This progress follows the company’s successful appeal of the Complete Response Letter (CRL) received in February 2026. The FDA acknowledged that the long-term data accumulated from the CAMPSIITE® study is sufficient to support consideration for accelerated approval.
This regulatory pivot means that REGENXBIO is exempt from the demanding requirement to enroll additional patients or incorporate an untreated control arm. The FDA has directed the company to request a Type A meeting to review existing biomarker and clinical data, which will serve as the prelude to the BLA resubmission. The agency has pledged an expedited review of the resubmitted application and indicated that labeling discussions will commence shortly thereafter. This development reflects both a deeper regulatory appreciation for the unique clinical evidence challenges in rare disease drug development and a clear commitment to accelerating access to transformative therapies.
Hunter Syndrome: A Severe Therapeutic Challenge
Hunter syndrome is a rare, X-linked recessive disorder caused by a deficiency of the lysosomal enzyme iduronate-2-sulfatase (I2S). This deficiency leads to the accumulation of glycosaminoglycans (GAGs), particularly heparan sulfate (HS), in tissues, causing progressive cell, tissue, and organ dysfunction—with particularly devastating effects on the central nervous system (CNS).
Globally, there are approximately 2,000 patients diagnosed with MPS II, with over 500 new cases identified annually. Most patients suffer from a severe form of the disease; while early developmental milestones may be met, developmental delays typically become readily apparent between 18 and 24 months of age. Research has shown that HS in cerebrospinal fluid (CSF) and its quantified disaccharide, D2S6, correlate closely with neurocognitive manifestations, serving as critical biomarkers for disease severity and therapeutic response.
NAVSUNLI™: A Transformative One-Time Gene Therapy
NAVSUNLI™ is designed as a one-time treatment. Utilizing REGENXBIO’s proprietary AAV gene delivery platform, it delivers the functional IDS gene directly to the CNS. This enables CNS cells to become a persistent source of secreted I2S protein, allowing it to cross the blood-brain barrier and provide long-term "cross-correction" for cells throughout the CNS.
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Technical Advantage: The expressed protein is structurally identical to the natural, healthy I2S enzyme.
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Regulatory Designations: NAVSUNLI™ has received numerous key designations from the FDA, including Orphan Drug, Rare Pediatric Disease, Fast Track, and Regenerative Medicine Advanced Therapy (RMAT). It has also been classified as an Advanced Therapy Medicinal Product (ATMP) by the European Medicines Agency (EMA).
Next Steps: Target for Q3 2026
REGENXBIO expects to hold a Type A meeting with the FDA in July, with a plan to resubmit the BLA rapidly in the third quarter of 2026. If approved, commercialization in the U.S. will be handled by NS Pharma, Inc., a subsidiary of Nippon Shinyaku, per the strategic partnership announced previously. Additionally, the approval of NAVSUNLI™ may qualify the company for a Priority Review Voucher (PRV), for which REGENXBIO retains all rights and proceeds.
Curran Simpson, President and CEO of REGENXBIO, stated: "We are encouraged by the flexibility shown by the new FDA leadership regarding rare diseases. They are committed to utilizing the accelerated approval pathway to bring transformative therapies to patients with serious, unmet medical needs. We are dedicated to working closely with the agency to ensure this important therapy reaches boys living with Hunter syndrome as quickly as possible."
