Ultragenyx has secured Food and Drug Administration approval for rebisufligene etisparvovec, which will be sold as Fayuvi, for Sanfilippo syndrome type A. The clearance makes Fayuvi the first drug specifically approved for the fatal disorder, an ultra-rare pediatric disease that progressively damages the brain and nervous system.
The company said the therapy is indicated for patients with preserved neurodevelopmental function. Cara O’Neill, chief science officer of the Cure Sanfilippo Foundation, told STAT earlier this week that approval would change the conversation for families receiving the diagnosis by replacing a message of limited options with “hope and an action plan for treatment.”
The commercial picture
Jefferies said in a Thursday note that Fayuvi will carry a wholesale acquisition cost of $3.95 million and that Ultragenyx expects the product to be available to ship within 30 to 60 days. At that price, the one-time treatment ranks among the most expensive gene therapies on the market, trailing Orchard Therapeutics’ Lenmeldy at $4.25 million, according to BioSpace.
Ultragenyx estimates there are around 3,000 to 5,000 treatable patients in “commercially accessible geographies.” Jefferies expects the disease’s high unmet need to support uptake and said Ultragenyx was already receiving requests for the drug, including from outside the U.S. Jefferies projects peak sales of around $250 million.
The approval also gives Ultragenyx a second priority review voucher, following the one it received for Genglycos last month. Ultragenyx plans to sell the vouchers, and William Blair said the going market rate is about $200 million each, which the firm said should significantly bolster the company’s balance sheet. Ultragenyx reported $436 million in cash, equivalents and marketable securities as of June 30.
The data and the road here
Fayuvi is an adeno-associated virus gene therapy that delivers a functional copy of the SGSH gene. The gene encodes an enzyme that is deficient in Sanfilippo syndrome type A, also called mucopolysaccharidosis type IIIA, and that deficiency leads to buildup of a molecule that progressively harms the nervous system.
Approval was supported by results from the Phase 1/2/3 Transpher A trial. According to BioSpace, the study showed biomarker and cognitive improvements compared with untreated patients from an external national history control group.
The decision marks a turnaround for a program that the FDA initially rejected in July 2025 because of manufacturing issues. Fayuvi was originally developed at Abeona Therapeutics as ABO-102, and Ultragenyx acquired exclusive licensing rights in 2022. Under that agreement, Abeona is eligible for up to $30 million in commercial milestones plus tiered royalties.
For Ultragenyx, the approval is strategically important beyond the Sanfilippo market itself. It establishes the company’s second gene therapy approval in quick succession after Genglycos and arrives after a Phase 3 failure in Angelman syndrome earlier this month led management to assess planned operations and begin significant expense reductions. Thursday’s approval helped shift investor focus back toward the company’s commercial gene therapy base, with Ultragenyx shares rising 13% by the end of Thursday trading to $14.50.
