FDA Approves First Gene Therapy for Sanfilippo Syndrome Type A

FDA has approved Fayuvi, Ultragenyx’s gene therapy for Sanfilippo syndrome type A, making it the first approved treatment of any kind for a fatal pediatric neurodegenerative disease that previously had none. The approval, announced 17 September 2026, follows a July 2025 complete response letter over manufacturing and inspection findings — a corrected chemistry, manufacturing and controls package and an added year of follow-up data cleared the path to full approval, two days ahead of FDA’s own target action date.

A rejection over manufacturing, not efficacy

FDA’s July 2025 Complete Response Letter did not question whether Fayuvi worked. It turned on chemistry, manufacturing and controls: questions about the CMC package and observations from inspections of the facilities producing the AAV9 vector. That is a common but underreported failure mode for gene-therapy applications, where the clinical case can be strong while the manufacturing record is not yet ready to support a commercial-scale license — and it is a distinct problem from a clinical hold or an efficacy shortfall, with a different fix.

Ultragenyx resubmitted the Biologics License Application on 30 January 2026, addressing the CMC questions and adding a full additional year of long-term follow-up data across the trial’s biomarker and clinical measures. FDA accepted the resubmission for review on 2 April 2026 and set a new PDUFA target action date of 19 September 2026. The agency approved Fayuvi on 17 September — two days ahead of that date.

What the pivotal trial showed

The application rests on the Transpher A trial (NCT02716246), a Phase 1/2/3 study of intravenous UX111 in children with MPS IIIA. The efficacy set comprised 27 patients dosed at 3x10^13 vg/kg; a broader safety set of 33 patients was followed for 0.6 to 8.5 years, a median of 4.5 years — a long observation window for a rare pediatric disease trial, and one that let FDA weigh durability alongside initial response.

• Cognitive outcome: treated patients scored 23.5 points higher on the Bayley-III Cognitive raw score, assessed between 24 and 60 months of age, than an external natural-history cohort of 27 untreated patients (p<0.0001).
• Biomarker response: cerebrospinal-fluid heparan sulfate, the accumulating substrate behind MPS IIIA’s neurodegeneration, fell by a median of 63.98% within the first month of treatment.
• Safety follow-up: the 33-patient safety set was followed a median of 4.5 years, up to a maximum of 8.5 years, giving FDA a longer durability record than most single-administration gene-therapy applications carry at filing.

A one-time therapy for a disease with no other option

Sanfilippo syndrome type A is caused by a deficiency in the enzyme that breaks down heparan sulfate, a complex sugar that accumulates in cells — especially neurons — when the pathway is missing. The buildup drives progressive, fatal neurodegeneration in early childhood. Before Fayuvi, families had supportive care and nothing that changed the disease’s course. Fayuvi is designed as a single intravenous infusion, delivering a functional copy of the deficient gene via an AAV9 vector, rather than a repeated-dosing therapy.

Ultragenyx said commercial product is expected to ship to Qualified Treatment Centers within 30 to 60 days of approval — a practical detail that matters for a rare-disease launch, where the number of centers able to administer and monitor a gene therapy is itself a gating factor on how quickly newly diagnosed patients can be treated.

Frequently asked questions

What did FDA approve on 17 September 2026?

Fayuvi (rebisufligene etisparvovec-hopf), an AAV9 gene therapy from Ultragenyx, for pediatric patients with mucopolysaccharidosis type IIIA (MPS IIIA), also called Sanfilippo syndrome type A. It is the first FDA-approved treatment of any kind for the disease, which previously had no disease-modifying therapy.

Why did FDA reject the application in 2025?

FDA issued a Complete Response Letter on 11 July 2025 tied to chemistry, manufacturing and controls (CMC) questions and observations from inspections of manufacturing facilities — not to the clinical efficacy data. Ultragenyx resubmitted the BLA on 30 January 2026 with a corrected CMC package and an additional year of long-term follow-up data; FDA accepted the resubmission on 2 April 2026.

What did the pivotal trial show?

The Transpher A trial (NCT02716246) enrolled 27 patients in its efficacy set, dosed at 3x10^13 vg/kg, and a 33-patient safety set followed for 0.6 to 8.5 years (median 4.5). Treated patients scored 23.5 points higher on the Bayley-III cognitive assessment than an external natural-history cohort of untreated patients (p<0.0001), and cerebrospinal-fluid heparan sulfate fell by a median of 63.98% within the first month post-treatment.

How soon will Fayuvi be available?

Ultragenyx said commercial product is expected to ship to Qualified Treatment Centers within 30 to 60 days of approval. Fayuvi is administered as a single one-time intravenous infusion.