Material Event (Form 8-K)

On October 30, 2025, Ultragenyx Pharmaceutical Inc. (the "Company") issued a press release announcing that the first patient has been dosed in the Aurorastudy (NCT07157254) evaluating the efficacy and safety of GTX-102 (apazunersen), an investigational antisense oligonucleotide ("ASO") for Angelman syndrome ("AS"). Data from the Aurorastudy will expand the population of Angelman patients treated to include both younger and older patients and those with other non-deletion AS genotypes not included in the Company's Phase 3 Aspirestudy. The fully enrolled Phase 3 Aspirestudy (NCT06617429) of GTX-102 is focused on patients aged four to 17 years with a genetically confirmed diagnosis of full maternal UBE3A gene deletion.

The global Aurora study will enroll approximately 60 participants ages one to less than 65 with Angelman syndrome across all genotypes and will include some additional countries or regions that could not be included in the AspirePhase 3 study. Participants will be assigned to one of four cohorts with corresponding subprotocols:

Cohorts A, B and C are single-arm only and in Cohort D participants will be randomized 2:1 to a GTX-102 group or a No Treatment group. All cohorts have a 48-week primary efficacy period; the No Treatment group will cross over to treatment receive their first loading dose of GTX-102 at Week 24. Participants from all cohorts have the option to continue treatment in a long-term extension study following their Aurora end of study visit.

In July 2025, enrollment for the global Phase 3 Aspirestudy was completed ahead of plan due to patient and investigator interest, with 129 participants ages four to 17 with a genetically confirmed diagnosis of full maternal UBE3Agene deletion randomized across 28 global sites. Data from the Aspire study are expected in the second half of 2026.