On Sept. 17, 2026, Ultragenyx Pharmaceutical announced that the U.S. Food and Drug Administration (FDA) granted standard full approval of FAYUVI™ (rebisufligene etisparvovec-hopf), also known as UX111, for the treatment of pediatric patients with mucopolysaccharidosis type IIIA (MPS IIIA, Sanfilippo syndrome Type A). FAYUVI is the first-ever FDA-approved treatment for Sanfilippo syndrome Type A, a progressive and fatal neurodegenerative disease, and the second gene therapy approval for Ultragenyx. The Company received a Priority Review Voucher upon this approval.

Sanfilippo syndrome Type A is an ultra-rare, fatal lysosomal storage disease that primarily affects the brain and is marked by rapid, progressive neurodegeneration beginning in early childhood. Children with Sanfilippo syndrome Type A typically experience progressive global developmental delay, followed by the loss of cognitive, language, and motor function, ultimately leading to early death. Sanfilippo syndrome Type A is estimated to affect approximately 3,000 to 5,000 patients in commercially accessible geographies, with a median life expectancy of 15 years. The disease is caused by a deficiency of the sulfamidase (SGSH) enzyme, which results in the accumulation of heparan sulfate substrate in cells and progressive damage to the central nervous system. FAYUVI is a single-dose intravenous AAV9 gene therapy designed to deliver a functional copy of the deficient enzyme gene that can express and replace the SGSH enzyme.

The final delivery of this therapy did not come without tremendous difficulties during its development, and the Company hopes this approval will revitalize the investment in other ultra-rare gene therapies. The therapy was developed by Haiyan Fu, PhD, and Doug McCarty, PhD, during their tenures at Ohio State University/Nationwide Children’s Hospital and was licensed to Abeona. When funding constraints arose despite positive clinical data, Abeona made the pivotal decision to out-license the asset to Ultragenyx, ensuring this vital treatment reached the finish line for patients. Ultragenyx thanks the researchers, the development and leadership team at Abeona, and so many families, patient advocacy groups, and investigators who worked tirelessly through so many obstacles over the many years to lead the Company to this moment of shared success.

The approval of FAYUVI is supported by data from the pivotal Transpher A trial and long-term follow-up studies, which demonstrated clinical benefit relative to the decline observed in natural history, along with durable treatment effect across clinical assessments and multiple biomarkers while maintaining an acceptable safety profile. Clinical data now extend to up to nearly 8 years of follow-up.

Biochemical efficacy in replacing the missing enzyme was demonstrated by a reduction in accumulated cerebral spinal fluid (CSF) heparan sulfate (HS) levels throughout the study and across all age groups. Clinical efficacy was assessed based on patients’ mean change in Bayley-III Cognitive raw score from 24 to 60 months of age. FAYUVI-treated patients from the modified intention-to-treat (mITT) population (N=17) were compared to untreated patients with Sanfilippo syndrome Type A from an external, comparable natural history cohort (N=27). FAYUVI-treated patients (mITT) demonstrated a 23.5 point higher (p<0.0001) cognitive score over natural history during the period of study, providing the efficacy basis for standard full approval.

FAYUVI is manufactured entirely within the U.S., at Ultragenyx’s Gene Therapy Manufacturing Facility in Bedford, Massachusetts, and Andelyn Biosciences in Columbus, Ohio.