Ultragenyx Pharmaceutical Inc. (Nasdaq: RARE) has received standard full approval from the U.S. Food and Drug Administration for Fayuvi, making it the first approved treatment for children with Sanfilippo syndrome type A.
The approval covers Fayuvi, formerly known as UX111, for treating the neurological manifestations of mucopolysaccharidosis type IIIA in pediatric patients with preserved neurodevelopmental function. The progressive genetic disorder damages the brain and nervous system, causing children to lose cognitive, language and motor abilities.
Sanfilippo syndrome type A results from a deficiency of the sulfamidase enzyme. That deficiency causes heparan sulfate to accumulate in cells, progressively damaging the central nervous system. Ultragenyx estimates that the disease affects approximately 3,000 to 5,000 patients across commercially accessible markets.
Fayuvi is a one-time intravenous AAV9 gene therapy designed to deliver a functional copy of the SGSH gene. The treatment enables cells to produce sulfamidase and reduce the accumulation of heparan sulfate throughout the body and brain.
The FDA’s approval was supported by an open-label, single-arm study rather than a randomized controlled trial. The efficacy analysis compared 17 treated patients with 27 untreated patients from an external natural-history cohort.
Ultragenyx reported that treated patients produced a 23.5-point higher cognitive score than the natural-history group between 24 and 60 months of age, with a p-value below 0.0001. The company said clinical follow-up now extends to nearly eight years for some patients.
The evidence supporting approval remains limited by the small treated population and reliance on an external comparison group. The approved indication is also restricted to pediatric patients who retain neurodevelopmental function, making early diagnosis and treatment an important commercial consideration.
Fayuvi carries warnings involving liver toxicity, reduced platelet counts, infusion and hypersensitivity reactions, thrombotic microangiopathy and a potential long-term malignancy risk associated with AAV vector integration. Patients require corticosteroid treatment beginning before the infusion and continuing for at least eight weeks afterward.
Ultragenyx expects to begin shipping Fayuvi to qualified U.S. treatment centers within 30 to 60 days. The company’s announcement did not disclose a U.S. list price.
“We recognize the profound urgency of making this therapy available to families,” President and CEO Emil Kakkis said.
The approval follows a July 2025 FDA rejection related to manufacturing processes and facilities rather than the submitted clinical evidence. Ultragenyx resubmitted its application in January 2026 after addressing the agency’s questions.
Fayuvi is Ultragenyx’s second gene-therapy approval and sixth FDA approval overall. The company also received a rare pediatric disease Priority Review Voucher, which may become an additional financial asset.
Ultragenyx reported second-quarter revenue of $214 million, a net loss of $92 million and $436 million in cash, cash equivalents and marketable securities as of June 30, 2026. Its previously issued 2026 revenue guidance of $730 million to $760 million excluded potential revenue from new product launches.
Upcoming Catalysts
- Initial Fayuvi shipments to qualified treatment centers
- Disclosure of the treatment’s U.S. price
- Insurance-coverage and reimbursement decisions
- Early patient uptake and launch revenue
- Expansion of the qualified-treatment-center network
- Possible sale or other use of the Priority Review Voucher
- International regulatory submissions and approvals
- Long-term safety and clinical follow-up data
Sources
- FDA approves the first gene therapy for pediatric Sanfilippo syndrome type A — September 17, 2026
- Ultragenyx announces Fayuvi approval — September 17, 2026
- Ultragenyx reports second-quarter 2026 results — August 4, 2026
- Ultragenyx resubmits the UX111 application — January 30, 2026
- Reuters coverage of the Fayuvi approval — September 17, 2026
Editorial Disclosure
This article is based entirely on publicly available information, including FDA materials, company announcements, SEC filings and publicly available news sources. Securities discussed or referenced include Ultragenyx Pharmaceutical Inc. (Nasdaq: RARE). Biotech Stocks Daily has not received compensation from Ultragenyx, its management, investor relations representatives or any third party for this specific article. Biotech Stocks Daily may have current or past paid business relationships with other companies, which do not influence the content or conclusions of this article. No staff member or principal of Biotech Stocks Daily holds a position in any security mentioned at the time of publication.
Sources include the FDA’s September 17, 2026 approval announcement, Ultragenyx’s September 17 approval release, its August 4 financial results, its January 2026 regulatory update and Reuters coverage.
Fayuvi received standard full FDA approval, but the efficacy analysis involved 17 treated patients compared with an external natural-history cohort of 27 untreated patients. The supporting study was open-label and did not include a randomized concurrent control group. The approved indication is limited to pediatric patients with preserved neurodevelopmental function.
Fayuvi is associated with potentially serious risks, including liver toxicity, thrombocytopenia, infusion reactions, thrombotic microangiopathy and a potential long-term risk of malignancy. FDA approval does not guarantee commercial adoption, insurance coverage, favorable reimbursement, manufacturing reliability or international approval.
Financial figures are reported in U.S. dollars and reflect Ultragenyx’s position as of June 30, 2026. The company reported substantial operating expenses, a quarterly net loss and continued cash use. Its revenue guidance and profitability objectives are forward-looking, while the commercial contribution from Fayuvi remains uncertain.
Biotechnology securities are speculative investments carrying significant risk, including the potential total loss of capital. Coverage on Biotech Stocks Daily is provided for informational and educational purposes only. Biotech Stocks Daily is not a registered investment advisor. Nothing in this article constitutes financial, investment or professional advice. Readers are encouraged to conduct their own due diligence and consult a qualified financial advisor before making investment decisions. For more information, please see our full DISCLAIMER.
