Ultragenyx Pharmaceutical Inc. (NASDAQ: RARE) announced Thursday September 17, 2026 that the FDA granted standard full approval to FAYUVI (rebisufligene etisparvovec-hopf, previously UX111) — a one-time intravenous AAV9-delivered gene therapy carrying a functional copy of the SGSH (N-sulfoglucosamine sulfohydrolase) gene — for the treatment of pediatric patients with mucopolysaccharidosis type IIIA (MPS IIIA, Sanfilippo syndrome Type A). MPS IIIA is a progressive, ultimately fatal autosomal-recessive lysosomal storage disorder that causes rapid neurodegeneration in early childhood; before Thursday, no disease-modifying therapy existed. The approval landed two days ahead of the September 19 PDUFA action date, followed the July 2025 complete response letter that had cited chemistry-manufacturing-controls issues plus manufacturing-site observations. The clinical package supporting standard full approval (not accelerated): up to 8 years of follow-up in treated children, sustained cerebrospinal fluid heparan sulfate reduction (the accumulating substrate), and a 23.5-point cognitive-score advantage over natural-history controls. Ultragenyx received a Rare Pediatric Disease Priority Review Voucher — historically valued $150-350 million on the secondary market — alongside the approval, plus U.S. per-patient wholesale acquisition cost set at $3.95 million. RARE shares closed the day up 13% and extended gains after hours. FAYUVI is manufactured domestically at Andelyn Biosciences in Columbus, Ohio and Ultragenyx's own Bedford, Massachusetts facility. Second gene therapy approval for the company.
The Ultragenyx (NASDAQ: RARE) UX111 (rebisufligene etisparvovec, AAV9-delivered gene therapy for the SGSH gene) FDA PDUFA action date for Sanfilippo Syndrome Type A (mucopolysaccharidosis type IIIA, or MPS IIIA — a rare autosomal recessive lysosomal storage disorder causing progressive irreversible neurodegeneration in young children with a median life expectancy in the mid-teens) is set for Saturday September 19, 2026 (weekend PDUFA date; the FDA typically issues weekend action-date decisions on the adjacent Friday or the following Monday), five days from the September 14 opening of Morgan Stanley conference week. The resubmitted Biologics License Application under Accelerated Approval was accepted by the FDA in April 2026 following a July 2025 complete response letter that identified chemistry, manufacturing, and controls (CMC) plus manufacturing-site inspection issues. The clinical package includes up to 8 years of follow-up in treated children, with sustained cerebrospinal fluid heparan sulfate (the accumulated substrate) reduction plus preserved developmental trajectories relative to untreated natural history. If approved, UX111 would become the first-ever therapy for MPS IIIA anywhere in the world; the drug would also confer a Rare Pediatric Disease Priority Review Voucher on Ultragenyx (historically valued $150-350 million on the secondary market). Manufacturing runs domestically at Andelyn Biosciences in Columbus, Ohio and Ultragenyx's own Bedford, Massachusetts facility.
Ultragenyx Pharmaceutical (NASDAQ: RARE) awaits its September 19, 2026 PDUFA target action date for UX111 (rebisufligene etisparvovec, an AAV9 gene therapy delivering the SGSH gene) for the treatment of Sanfilippo syndrome type A (mucopolysaccharidosis type IIIA, MPS IIIA, a rare autosomal recessive lysosomal storage disorder caused by SGSH gene mutations that results in progressive cognitive decline in early childhood and death typically by the second decade). If approved, UX111 would become the first FDA-approved therapy for MPS IIIA, where the current standard of care remains supportive symptom management. The AAV9-based intravenous delivery approach mirrors the Novartis Zolgensma (onasemnogene abeparvovec) commercial template for spinal muscular atrophy and continues the neurometabolic gene therapy pipeline. Not a peptide, but the approval would extend the AAV gene therapy commercial category that has continued to reshape the rare-disease landscape alongside enzyme replacement therapies and the emerging small-molecule chaperone class.