FDA approves Ultragenyx's Fayuvi as first Sanfilippo Type A treatment

The Food and Drug Administration on Thursday approved the first treatment for Sanfilippo syndrome Type A, a rare, fatal neurodegenerative disease of childhood that until now had none. Ultragenyx Pharmaceutical, the rare-disease company behind Fayuvi (rebisufligene etisparvovec-hopf), received a standard full approval for a one-time intravenous gene therapy. It is the company's second gene-therapy clearance and its sixth FDA approval overall.

Sanfilippo Type A and Fayuvi's narrow label

Fayuvi uses a modified, non-infectious adeno-associated virus (AAV9) to deliver a working copy of the SGSH gene, whose deficiency causes heparan sulfate to build up in cells and damage the brain. The approved use is treatment of the neurologic manifestations of mucopolysaccharidosis type IIIA, also called Sanfilippo syndrome Type A, in pediatric patients with preserved neurodevelopmental function. Acting FDA Commissioner Kyle Diamantas called the decision "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."

The agency said effectiveness was evaluated in an open-label, single-arm, multicenter study. The cognitive-score analysis covered children ages 2 to 5 years; treated patients maintained or improved cognitive function against an untreated historical-control group, a path the FDA contrasted with the usual plateau and decline in that window. The most common adverse reactions, reported in more than 5% of patients, included elevated liver enzymes, nausea and vomiting, fever, decreased appetite, lower white-blood-cell and platelet counts, and increased amylase. Important warnings include thrombotic microangiopathy, a serious clotting and blood-vessel injury, and a potential long-term risk that the inserted genetic material could integrate into the genome and contribute to tumor development.

The product had Orphan Drug, Fast Track, and Breakthrough Therapy designations. This was not the first filing. On July 11, 2025, Ultragenyx said the FDA had issued a complete response letter citing chemistry, manufacturing, and controls observations from facility inspections; the company said the letter did not note review issues related to the clinical data package. Ultragenyx resubmitted early this year. The agency accepted that file in April with a September 19 action date. Thursday's clearance came two days early. The resubmitted application had sought accelerated approval; Ultragenyx described the outcome as standard full approval.

Peak sales, cash, and two gene therapies

The company estimates MPS IIIA affects about 3,000 to 5,000 patients in commercially accessible geographies, with a median life expectancy of 15 years. That is not a U.S.-only count, and the labeled group, pediatric patients with preserved neurodevelopmental function, is narrower still.

Leerink Partners, in an April 2 note after the resubmission was accepted, projected $150 million of peak sales for the therapy. That is an analyst estimate, not reported revenue. It sits against a commercial base that already runs on other products: second-quarter 2026 revenue was $214 million, including $156 million from Crysvita, and 2026 guidance of $730 million to $760 million explicitly excludes revenue from new launches.

Commercial product is expected to ship to U.S. Qualified Treatment Centers within 30 to 60 days, Ultragenyx said, manufactured entirely in the United States at the company's gene-therapy plant in Bedford, Massachusetts, and at Andelyn Biosciences in Columbus, Ohio. The Thursday release named no wholesale acquisition cost, reimbursement figure, or company sales forecast. The closest in-house comparison is Genglycos, Ultragenyx's first gene therapy, approved on August 19 for glycogen storage disease type Ia and priced at a $2.7 million wholesale acquisition cost. Ultragenyx is now launching two gene therapies at once, into specialized centers, off $436 million of cash, cash equivalents, and marketable securities as of June 30, against $97 million of net cash used in operations in the second quarter. The net loss that quarter was $92 million, or $0.90 a share. Those figures predate Genglycos product sales.

The medical first is larger than the commercial one the Street has modeled.

Shares of Ultragenyx closed at $14.50, up 12.6%, two weeks after the stock fell 44% on September 3 when its Phase 3 Aspire study in Angelman syndrome missed its primary and key secondary endpoints.

For a holder of a biotech index fund, Thursday means little, yet. An equal-weight U.S. biotech fund, AXBI, holds the stock at 0.47% of a 154-name book; a concentrated U.S. small-cap growth fund, FTMFS, holds it at 1.64% of 35 holdings. The stock itself is the exposure.

The priority review voucher

Approval also brought a rare-pediatric-disease Priority Review Voucher, a transferable right to a faster FDA review of a different product. Ultragenyx said it received the voucher. On the company's second-quarter call, Chief Executive Emil Kakkis told analysts that upon approval the intent would be to monetize both the Genglycos voucher and this one. The company sold an earlier voucher to Novartis for $130 million in December 2017. Genglycos came with a voucher of its own last month. Recent disclosed sales give a range, not a quote: Rocket Pharmaceuticals closed a $180 million sale on June 12, 2026; Abeona Therapeutics closed a $155 million sale on June 27, 2025. After Congress reauthorized the program in February 2026, the FDA says it may not award vouchers under it after September 30, 2029.

The cash nearest to hand is the voucher Kakkis already said the company intends to sell. The product still needs a price and a child who still has the function the label requires.