FDA Approves Ultragenyx Fayuvi for Sanfilippo Syndrome

The US Food and Drug Administration (FDA) has approved Ultragenyx’s Fayuvi (rebisufligene etisparvovec-hopf), the first gene therapy for Sanfilippo syndrome Type A, a rare inherited neurodegenerative disorder in children. Announced on 18 September 2026, the one-time gene therapy carries a US list price of $3.95 million, making it one of the world’s most expensive drugs and marking Ultragenyx’s second gene therapy approval in as many months.

Sanfilippo syndrome Type A, also known as mucopolysaccharidosis Type IIIA (MPS IIIA), is a rare inherited disease that progressively damages the brain and nervous system, causing children to lose cognitive, language and developmental abilities. Patients have an average lifespan of around 15 years, and the syndrome affects an estimated 3,000 to 5,000 patients across commercially accessible geographies. Fayuvi uses an AAV9 vector to deliver a working copy of the SGSH gene, enabling production of the enzyme sulfamidase, which is missing or deficient in MPS IIIA. A clinical trial demonstrated one-time intravenous administration maintained or improved cognitive function versus an untreated historical control cohort.

Ultragenyx justified the price by pointing to the lifetime cost of caring for a Sanfilippo Type A patient, which can exceed $8 million. Fayuvi will be available through a network of qualified treatment centres. As approval covers a rare paediatric disease, Ultragenyx also secured a priority review voucher (PRV), which can be redeemed for accelerated FDA review of another asset or sold for cash (PRVs currently trade in the $150-200 million range). Fayuvi is Ultragenyx’s second gene therapy approval after Genglycos (for glycogen storage disease Type Ia), approved in August 2026 with a $2.7 million list price.

The commercial signal is a rapidly maturing rare disease gene therapy category where sky-high pricing, PRV monetisation and QTC infrastructure are strategic financial levers for gene therapy sponsors. Ultragenyx’s back-to-back approvals validate its rare-disease portfolio strategy. Expect BioMarin, Sanofi, Takeda, Orchard Therapeutics, Amicus Therapeutics, Sarepta and Rocket Pharmaceuticals to sharpen their gene therapy positioning through 2027 as the category matures.

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