Fayuvi (rebisufligene etisparvovec)

The only approved disease-modifying treatment for any form of Sanfilippo syndrome. Before September 2026, care was entirely supportive: managing sleep, behavior, seizures and feeding as the disease progressed. Fayuvi applies to type A only and is aimed at children diagnosed early enough that neurodevelopmental function is still preserved, which makes speed of diagnosis the practical determinant of who can benefit.

Sanfilippo syndrome type A is caused by a broken SGSH gene. Without it, cells cannot make sulfamidase, the enzyme that breaks down a sugar chain called heparan sulfate, so heparan sulfate builds up inside cells, most damagingly in the brain. Fayuvi packages a working SGSH gene inside a modified, non-infectious adeno-associated virus of serotype 9, chosen because AAV9 can cross from the bloodstream into the central nervous system. A single infusion delivers that gene to cells throughout the body and brain, which then begin producing their own sulfamidase. The label describes it as treating the neurologic manifestations of the disease; it does not undo damage already done, which is why the approved population is children whose neurodevelopmental function is still preserved.

Five warnings carry their own sections in the label. Liver toxicity: liver enzymes and bilirubin are monitored until 2 weeks after the corticosteroid taper ends, and longer if they rise. Low platelets: platelet counts are checked weekly for the first 4 weeks, then monthly for 6 months. Thrombotic microangiopathy, a rare condition in which small clots damage the kidneys and destroy red blood cells; any sign of it triggers immediate testing. Hypersensitivity and infusion reactions, monitored during and after the infusion, with the infusion paused and restarted more slowly if they occur. And the risk of malignancy shared by all AAV gene therapies, because the delivered DNA could in theory integrate into the genome; families are asked to report any cancer to Ultragenyx at 1-888-756-8657. The most common reactions in the 27 children who got the approved dose included increased liver enzymes (85%), vomiting (67%), behavior changes such as irritability, aggression or trouble sleeping (56%), diarrhea (48%), fever (41%), decreased white blood cell count (30%), steroid-related Cushingoid features (30%), decreased appetite (22%), decreased platelet count (19%), anemia (19%), constipation (15%), and nausea, increased amylase, increased alkaline phosphatase, seizures and an enlarged liver (11% each). Every child receives corticosteroids starting the day before the infusion and continuing for at least 8 weeks, which may require adjusting the vaccination schedule. The label notes that no animal studies assessed effects on fertility or cancer risk.

A one-time AAV gene therapy for an ultra-rare fatal childhood disease, manufactured as a patient-specific kit of up to 105 vials. Gene therapies in this category have listed at several million dollars, and Ultragenyx's approval announcement did not include a price or outcomes-based agreements. Its UltraCare program (1-888-756-8657) supports access, with Gene Therapy Guides who help with insurance coverage, and product was expected to ship to Qualified Treatment Centers within 30 to 60 days of approval. The cost lands once, which changes how insurers evaluate it against a lifetime of supportive care.

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The label covers pediatric patients with MPS IIIA whose neurodevelopmental function is still preserved. Two things sit inside that sentence. First, it is type A only: Fayuvi delivers the SGSH gene, which is not the gene broken in types B, C or D. Second, the evidence behind the approval comes from children treated at a mean age of 22 months and none older than 33 months, all of whom were either under 2 or had a developmental quotient of at least 60. The label sets no age ceiling, so an older child with preserved function is inside the indication but outside the studied population, and that is a specific conversation to have with a metabolic specialist. Children with anti-AAV9 antibody titers of 1:100 or higher were not studied. Because the therapy treats the neurologic disease going forward rather than reversing existing damage, timing matters, and the label's own population is the argument for treating as early as a diagnosis allows.

Source: FDA product page for Fayuvi

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Approval rested on Study 1, the open-label, single-arm, dose-escalation Transpher A trial (NCT02716246), with long-term follow-up in Study 2 (NCT04360265). There was no placebo group. The efficacy population was 17 children who received the recommended dose and were either 2 years old or younger at treatment, or older than 2 with a cognitive developmental quotient of at least 60. They were treated at a mean age of 21.8 months, ranging from 3 months to 33 months, and followed for a median of 4.2 years. The primary measure compared their change in Bayley-III Cognitive raw score between ages 24 and 60 months against 27 untreated children from an external natural history cohort. Treated children gained a mean of 16.0 points over that window while untreated children lost 7.6, a 23.5-point difference (95% CI 17.2 to 29.9, p<0.0001). The FDA treated this as a demonstration of clinical benefit, granting traditional approval rather than the accelerated, biomarker-based approval that had been widely expected.

The therapy began at Nationwide Children's Hospital and Abeona Therapeutics as ABO-102, entering the clinic in April 2016 with the Transpher A trial. Ultragenyx took over the program under an exclusive worldwide license in May 2022 and renamed it UX111. The FDA rejected the original application on July 11, 2025 with a complete response letter citing manufacturing and facility inspection issues rather than the efficacy or safety data. Ultragenyx resubmitted in early 2026, the FDA accepted the resubmission on April 2 with a September 19 target date, and approved it on September 17, 2026 as Fayuvi, 2 days early. It is the first approved treatment for any form of Sanfilippo syndrome, and Ultragenyx received a Rare Pediatric Disease Priority Review Voucher with the approval. It carried Regenerative Medicine Advanced Therapy (RMAT), Fast Track, Rare Pediatric Disease and Orphan Drug designations.