Material Event (Form 8-K)

Item 8.01 Other Events.

On March 30, 2026, Ultragenyx Pharmaceutical Inc. (the "Company") issued a press release announcing that the U.S. Food and Drug Administration (the "FDA") has cleared the Investigational New Drug ("IND") application for UX016, an investigational small molecule prodrug of sialic acid ("SA") being evaluated as a substrate replacement therapy for GNE myopathy ("GNEM"). GNEM is a rare, severely debilitating, inherited neuromuscular disorder caused by mutations in the GNE gene that lead to deficient SA production. The UX016 program is externally funded by a patient group through clinical proof-of-concept, including a Phase 1/2 study expected to begin in the second half of 2026.

UX016 is a prodrug composed of SA and a hydrophobic fatty acid tail that enhances efficient delivery to muscle as compared to naturally occurring SA. UX016 aims to address challenges that have historically limited the efficacy of prior substrate replacement therapies. Based on preclinical data, the fatty acid tail improves UX016 distribution to muscle and other tissues and supports more efficient uptake and release of SA within muscle cells.

The planned first-in-human, Phase 1/2 study will enroll approximately 24 adults ages 18 to 55 years with GNEM in the United States. The study, expected to begin in the second half of 2026, will evaluate the safety and efficacy of UX016. The study will also evaluate the pharmacokinetics and delivery to muscle at two doses 3:1 relative to placebo over the first 12 weeks and then will evaluate upper and lower muscle strength, patient reported outcomes relevant to GNEM, and other functional measures through Week 48 of treatment.