Fayuvi (rebisufligene etisparvovec)

The first treatment ever approved for Sanfilippo syndrome, and the first for any form of MPS III. Fayuvi is a single intravenous infusion given once, approved on September 17, 2026 for children with type A whose neurodevelopmental function is still preserved.

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.

The label gives 5 warnings their own sections. 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. There is also 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 single intravenous infusion of 3.0 × 10¹³ vector genomes per kilogram of body weight, given over approximately 1 hour in a healthcare setting equipped to manage infusion reactions. Because the dose scales with weight, Fayuvi ships as a kit of 5 to 105 vials matched to the child, stored frozen at −60°C or below. Before infusion, a child needs baseline liver function tests, a platelet count, and clotting tests, and must have anti-AAV9 antibody titers below 1:100, since the therapy was not studied in children with higher titers. Corticosteroids begin one day before the infusion and continue for a minimum of 8 weeks. Monitoring of liver enzymes and platelets continues for months afterward.

Yes, Rebisufligene etisparvovec (Fayuvi) is FDA approved (2026) for the treatment of Sanfilippo Syndrome.

Fayuvi (rebisufligene etisparvovec) is approved to treat the neurologic manifestations of mucopolysaccharidosis type IIIA, Sanfilippo syndrome type A, in pediatric patients with preserved neurodevelopmental function. The FDA approved it on September 17, 2026. It is the first treatment ever approved for any form of Sanfilippo syndrome.

No. It gives the body a working SGSH gene so cells can make the missing enzyme going forward, and in the trial children who were treated early gained developmental skills where untreated children lost them. It is not described as reversing damage that has already occurred, which is why the approval covers children whose neurodevelopmental function is still preserved.

The 17 children in the efficacy analysis were treated at a mean age of 21.8 months, with a range of 3 months to 33 months. Every one of them was either 2 years old or younger at treatment, or older than 2 with a developmental quotient of at least 60. The label sets no age limit, so the direct evidence is concentrated in children under 3 even though older children with preserved function are inside the indication.

Between the ages of 24 and 60 months, treated children gained a mean of 16.0 points on the Bayley-III Cognitive raw score while 27 untreated children from a natural history cohort lost 7.6 points, a difference of 23.5 points (95% CI 17.2 to 29.9). Children were followed for a median of 4.2 years. That is a gain of skills where the disease normally takes them away.

No. The label and approval letter contain no accelerated approval language and no confirmatory trial requirement. The FDA granted traditional approval based on a clinical cognitive outcome, even though Ultragenyx had applied for accelerated approval.

No. Fayuvi delivers the SGSH gene, which is the gene broken only in type A. Types B, C and D involve different genes and enzymes and would each need their own therapy. Families with those subtypes should look at recruiting trials rather than this drug.

The label warns about liver toxicity, low platelets, thrombotic microangiopathy, hypersensitivity and infusion reactions, and the theoretical cancer risk shared by AAV gene therapies. Every child receives corticosteroids from the day before the infusion for at least 8 weeks, and liver enzymes and platelets are monitored for months afterward. The most common side effects were raised liver enzymes, vomiting, behavior changes, diarrhea and fever, followed by low white cells, steroid-related Cushingoid features, reduced appetite, low platelets and anemia.

As a single intravenous infusion over about 1 hour, dosed by body weight at 3.0 × 10¹³ vector genomes per kilogram, in a healthcare setting able to manage infusion reactions. It is given once. Before treatment a child needs liver, platelet and clotting tests and an anti-AAV9 antibody titer below 1:100.