RARE: Other company developments

Item 8.01 Other Events. On September 2, 2026, Ultragenyx Pharmaceutical Inc. (“Ultragenyx” or the “Company”) announced results from the Phase 3 Aspire study for apazunersen (GTX-102) in Angelman syndrome. The study did not achieve the primary endpoint of change from Baseline in Bayley-4 cognitive raw score nor the key secondary endpoint of net response in Multidomain Responder Index (“MDRI”). The safety profile observed in Aspire was consistent with Phase 1/2. In Aspire , the randomized groups were comparable at baseline and consistent with the patients studied in Phase 2. There were no differences between the treated and control groups that could support efficacy in the Bayley Cognition raw scores nor in the MDRI when looking at net response or mean changes of the individual five endpoints included in the MDRI. The Company will evaluate the apazunersen program in light of this outcome and make a decision on its disposition. The Company will also assess its planned operations to define and implement significant expense reductions, while supporting its growing commercial business. SIGNATURES Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized. Ultragenyx Pharmaceutical Inc. Date: September 2, 2026 By: /s/ Howard Horn Howard Horn Executive Vice President, Chief Financial Officer, Corporate Strategy