June 22, 2026 —
REGENXBIO announced that it has reached alignment with the U.S. FDA on next steps for potential accelerated approval of NAVSUNLI™, also known as clemidsogene lanparvovec-sngl or RGX-121, an investigational one-time gene therapy for Mucopolysaccharidosis II, or MPS II, also known as Hunter syndrome.
The update follows REGENXBIO’s appeal of the FDA’s February 2026 Complete Response Letter for NAVSUNLI. According to the company, the FDA acknowledged during a recent discussion that existing clinical data from the CAMPSIITE® study are sufficient to be considered under the accelerated approval pathway. The agency also confirmed that REGENXBIO does not need to enroll additional patients or conduct additional studies, including the previously recommended addition of an untreated control arm.
The FDA requested that REGENXBIO hold a Type A meeting to review existing longer-term biomarker and clinical data before resubmitting the Biologics License Application. The company expects the meeting to take place in July and plans to resubmit the BLA rapidly afterward in the third quarter of 2026. REGENXBIO said the FDA would review the resubmission on an expedited basis, with labeling discussions expected to begin shortly after resubmission.
NAVSUNLI is designed as a one-time gene therapy for boys with MPS II. The therapy is intended to deliver the IDS gene to the central nervous system, enabling cells in the CNS to produce iduronate-2-sulfatase, or I2S, the enzyme deficient in Hunter syndrome. By providing a source of secreted I2S protein beyond the blood-brain barrier, the therapy is designed to support long-term cross-correction of cells throughout the CNS.
MPS II is a rare, X-linked recessive lysosomal storage disorder caused by I2S deficiency. The resulting accumulation of glycosaminoglycans, including heparan sulfate, can lead to progressive tissue and organ dysfunction, including central nervous system involvement. Severe forms of the disease are often associated with developmental delay beginning in early childhood and progressive neurodegeneration.
Cerebrospinal fluid heparan sulfate is a key disease biomarker in MPS II, and the D2S6 disaccharide has been associated with neurocognitive manifestations, making it a clinically relevant biomarker of disease severity and therapeutic response. The FDA’s willingness to review existing longer-term biomarker and clinical data under the accelerated approval pathway may be important for rare neurodegenerative diseases where conventional trial designs can be difficult.
NAVSUNLI has received Orphan Drug, Rare Pediatric Disease, Fast Track, and Regenerative Medicine Advanced Therapy designations from the FDA. It has also received advanced therapy medicinal product classification from the European Medicines Agency.
If approved, NAVSUNLI would be commercialized in the United States by NS Pharma, a wholly owned subsidiary of Nippon Shinyaku, under a strategic partnership announced in January 2025. Approval could also result in a Priority Review Voucher, with REGENXBIO retaining full rights to any proceeds from a potential sale of the voucher.
The latest FDA alignment gives REGENXBIO a clearer path to resubmission and potential accelerated approval for NAVSUNLI. If approved, the therapy could become the first one-time gene therapy option for boys with Hunter syndrome and a potentially important treatment for addressing the neurological manifestations of this ultra-rare disease.
