FDA Approves Fayuvi — First-Ever Treatment for Sanfilippo Syndrome Type A, and Ultragenyx's Second Gene Therapy Approval in a Month
On September 17, 2026, the FDA approved Fayuvi (rebisufligene etisparvovec-hopf), a one-time intravenous AAV9 gene therapy from Ultragenyx Pharmaceutical that delivers a working copy of the SGSH gene, letting patients' cells produce sulfamidase — the enzyme missing in mucopolysaccharidosis type IIIA (MPS IIIA, Sanfilippo syndrome type A). It is the first approved therapy for the disease, which until now was managed symptomatically; children with MPS IIIA face relentless neurological regression with a median life expectancy of about 15 years.
Notably, Ultragenyx sought accelerated approval but the FDA granted full approval — unusual for a gene therapy filed on an open-label, single-arm study, and a signal about how the agency weighed the natural-history contrast. In treated children ages 2–5, cognitive scores were maintained or improved versus an untreated historical control cohort that normally plateaus or declines in that window. Acting FDA Commissioner Kyle Diamantas: "The approval of Fayuvi marks a historic moment for children and families living with MPS IIIA, which is a disease that has, until now, offered no approved treatment to alter its devastating course." (FDA press announcement)
Commercial and strategic context
- Price: $3.95 million wholesale before discounts — against Ultragenyx's estimate that lifetime care for a Sanfilippo patient can top $8 million. Addressable population ~3,000–5,000 patients; first shipments expected in 30–60 days. The label indicates "children with preserved neurodevelopmental function" rather than formal infant/toddler assessment cutoffs, which CEO Emil Kakkis said leaves the door open broadly: "I think we'd all agree that someone who is at end-stage disease is not a good candidate for gene therapy and the goal is to treat as young as possible… We should be newborn screening and treating kids before they're age 2 or even age 1." (MedCity News)
- This is Ultragenyx's second gene therapy approval in a month (after Genglycos for glycogen storage disease type Ia in August), and each carried a rare pediatric disease priority review voucher that the company plans to sell — recent PRV sales have run around $200 million each, a meaningful cash bridge.
- The approval follows a July 2025 Complete Response Letter over manufacturing issues, and lands two weeks after the Phase 3 failure of apazunersen in Angelman syndrome — the pipeline asset that carried most of the company's financial expectations — which cut the stock ~45% and triggered announced cost cuts. Leerink's Joseph Schwartz: "the positive after-hours reaction to a full approval, real pricing, and a leveraged launch says the platform discount applied over the past two weeks was too indiscriminate."
- Key safety warnings: thrombotic microangiopathy (TMA) risk, and the class-wide theoretical risk that AAV genome integration could lead to tumor development; patients receive corticosteroids from one day before infusion for at least eight weeks.