Update: Ultragenyx Rare Neurogenetic Disorder Phase 3 Study Misses Endpoints; Shares Down Pre-Bell

05:38 AM EDT, 09/03/2026 (MT Newswires) -- (Updates with the latest stock price movement in the headline and the first paragraph.) Ultragenyx Pharmaceutical (RARE) shares were down 44% in premarket activity Thursday after the company overnight said its phase 3 Aspire study of apazunersen in Angelman syndrome -- a rare, neurogenetic disorder -- failed to achieve the primary endpoint or a key secondary endpoint. The late-stage study did not reach 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. The safety profile for the phase 3 trial remained consistent with phase 1 and phase 2, the company said. Ultragenyx will assess the apazunersen program following this outcome and then decide on its disposition, it said.