REGENXBIO plans to resubmit the BLA for NAVSUNLI in Hunter syndrome after the FDA confirmed no additional studies are required and agreed to review existing long-term data under the accelerated approval pathway.
Written By: Shaik Yasmeen, PharmD
Reviewed By: Pharmacally Editorial Team
REGENXBIO has secured a significant regulatory breakthrough for NAVSUNLI™ (clemidsogene lanparvovec-sngl, RGX-121), its investigational one-time gene therapy for Mucopolysaccharidosis Type II (MPS II), also known as Hunter syndrome.
Following discussions with the U.S. Food and Drug Administration (FDA), the agency agreed that the existing clinical dataset may support accelerated approval and confirmed that additional studies are not required before resubmission.
The decision follows REGENXBIO’s appeal of a Complete Response Letter (CRL) issued in February 2026. At that time, the FDA had requested further evidence, including consideration of an untreated control arm. The agency has now acknowledged that the current clinical evidence is sufficient for review under the accelerated approval framework and has asked the company to submit existing longer-term biomarker and clinical data for evaluation.
A Potential One-Time Gene Therapy for Hunter Syndrome
Hunter syndrome is a rare X-linked lysosomal storage disorder caused by deficiency of the enzyme iduronate-2-sulfatase (I2S). The condition leads to accumulation of glycosaminoglycans (GAGs), including heparan sulfate, resulting in progressive damage to multiple organs and the central nervous system (CNS).
The disease affects approximately 2,000 diagnosed patients worldwide, with more than 500 new cases reported annually. Severe forms of the disease often become apparent during early childhood and are associated with progressive neurocognitive decline.
NAVSUNLI delivers a functional IDS gene directly to the CNS through gene therapy. By enabling long-term production of I2S beyond the blood-brain barrier, the therapy aims to address neurological manifestations that remain difficult to treat with currently available enzyme replacement therapies.
FDA to Review Existing Long-Term Data
As part of the next regulatory steps, the FDA requested a Type A meeting to review longer-term biomarker and clinical findings from the ongoing CAMPSIITE® study (NCT03566043). The agency indicated that once the BLA is resubmitted, it intends to conduct an expedited review and begin labeling discussions shortly afterward.
A key biomarker under review is cerebrospinal fluid (CSF) heparan sulfate, particularly the D2S6 disaccharide, which has shown correlation with neurocognitive disease burden in patients with MPS II. The biomarker is considered clinically relevant for assessing disease progression and treatment response.
Rare Disease Regulatory Momentum
According to REGENXBIO President and CEO Curran Simpson, the FDA’s latest feedback reflects a growing willingness to use accelerated approval pathways for serious rare diseases with substantial unmet need. The company plans to hold the Type A meeting in July 2026 and rapidly resubmit the BLA during the third quarter.
Patient advocacy groups also welcomed the development. The National MPS Society highlighted the urgent need for new therapeutic options for Hunter syndrome and noted that accelerated review could help bring a potentially transformative treatment to affected families sooner.
Regulatory Path Forward
If approved, NAVSUNLI would become the first one-time gene therapy for Hunter syndrome and could offer a new treatment approach for the disease’s neurological manifestations. Under REGENXBIO’s partnership with Nippon Shinyaku’s U.S. subsidiary NS Pharma, the therapy will be commercialized in the United States following approval.
The program has received FDA Orphan Drug, Fast Track, Rare Pediatric Disease, and Regenerative Medicine Advanced Therapy (RMAT) designations. Approval may also qualify the company for a Priority Review Voucher, providing an additional strategic asset as NAVSUNLI moves closer to a potential market launch.
What This Means for Patients
For families affected by Hunter syndrome, this update means NAVSUNLI is still moving toward a possible FDA approval. Earlier this year, the FDA asked for more information before approving the treatment, creating uncertainty about its future. After further discussions, the agency has now confirmed that REGENXBIO does not need to run new studies or enroll more patients. Instead, the FDA will review the long-term results already collected from treated patients. If the upcoming review goes as planned, NAVSUNLI could move closer to becoming the first one-time gene therapy approved for Hunter syndrome, offering hope for a treatment that may help address the disease’s effects on the brain and nervous system.
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About the Writer
Shaik Yasmeen (LinkedIn) is a Pharm.D graduate with interests in clinical pharmacy, pharmacovigilance, and medical writing. She has gained experience through hospital clinical postings, patient case reviews, case presentations, and literature evaluation. Passionate about evidence-based healthcare, she is committed to creating accurate and engaging medical content while continuously expanding her professional knowledge.