FDA approves Fayuvi gene therapy, first treatment for Sanfilippo syndrome type A

FDA has approved Fayuvi (rebisufligene etisparvovec-hopf) for pediatric patients with mucopolysaccharidosis type IIIA (MPS IIIA), also known as Sanfilippo syndrome type A, who have preserved neurodevelopmental function.

MPS IIIA is a rare lysosomal storage disease caused by deficiency of the sulfamidase (SGSH) enzyme, leading to accumulation of heparan sulfate and progressive damage to the central nervous system. Children typically experience developmental delay followed by loss of cognitive, language, and motor function. Treatment has historically been limited to supportive care.

Fayuvi is an intravenous adeno-associated virus serotype 9 (AAV9) gene therapy designed to deliver a functional copy of the SGSH gene, enabling production of the missing enzyme and reducing harmful substrate accumulation in the brain and other tissues.

Approval was supported by the Transpher A study and long-term follow-up data. Children treated with Fayuvi gained or maintained cognitive skills from 24 to 60 months of age, while children in a comparable untreated natural-history cohort lost skills as the disease progressed. Clinical follow-up extended to nearly eight years, with durable effects across clinical and biomarker measures. Reductions in cerebrospinal fluid heparan sulfate provided evidence of sustained biologic activity.

The most common adverse reactions included elevated liver enzymes, vomiting, fever, abnormal behavior, diarrhea, decreased white blood cell counts, decreased appetite, thrombocytopenia, anemia, nausea, and increased amylase levels. FDA labeling includes warnings regarding hepatotoxicity, thrombocytopenia, thrombotic microangiopathy, hypersensitivity and infusion reactions, and potential malignancy risk associated with AAV gene therapies. Patients require corticosteroid treatment before and after infusion, along with ongoing laboratory monitoring.

Fayuvi will be available through a network of specialized US Qualified Treatment Centers with expertise in gene therapy administration. Ultragenyx plans to support treatment access through its UltraCare® program.

“This gene therapy addresses a pressing unmet clinical need and offers families a promising therapeutic option,” said Kevin M. Flanigan, M.D., director of the Center for Gene Therapy at Nationwide Children's Hospital and principal investigator on the study that led to its approval.

Sources: FDA. (2026 Sep 17). FDA approves first gene therapy for pediatric patients with Sanfilippo syndrome type A; Ultragenyx. (2026 Sep 17). Ultragenyx announces approval of FAYUVI™ gene therapy, the first-ever FDA-approved treatment for Sanfilippo syndrome Type A