Material Event (Form 8-K)

Item 8.01 Other Events.

On January 30, 2026, Ultragenyx Pharmaceutical Inc. (the "Company") issued a press release announcing that that it has resubmitted its Biologics License Application ("BLA") seeking accelerated approval for UX111 (rebisufligene etisparvovec) AAV9 gene therapy as a treatment for patients with Sanfilippo syndrome type A (MPS IIIA) to the U.S. Food and Drug Administration (the "FDA" or the "Agency"). The submission contains substantial longer-term data on multiple measures of neurologic benefit to support an intermediate clinical endpoint for accelerated approval supported further by CSF heparan sulfate and other biomarker data, as agreed with the FDA during the last clinical review.

The resubmitted BLA includes comprehensive responses to chemistry, manufacturing, and controls (CMC)-related observations outlined in a Complete Response Letter (the "CRL") issued in July 2025, as well as additional long-term clinical data from current patients as requested by the Agency in the CRL.

During its prior review, the FDA acknowledged that the neurodevelopmental outcome data are robust and that the biomarker data provide additional supportive evidence; updated clinical data included in the BLA representing an additional year of follow-up continue to show a durable treatment effect across multiple biomarkers and further clinical separation from natural history, while maintaining an acceptable safety profile. Detailed updates will be presented next week at the WORLDSymposium™ 2026 in San Diego.

In February 2025, the FDA granted the UX111 BLA Priority Review. A Prescription Drug User Fee Act (PDUFA) action date is expected to be assigned within a month of resubmission. The Company anticipates up to a six-month review period from the date of resubmission per FDA regulations, with a PDUFA date expected in the third quarter of 2026. If approved, UX111 will be the first approved therapy for Sanfilippo syndrome type A.