Ultragenyx Stock Plunges 22.8% Amid Phase 3 Study Delay

Ultragenyx Pharmaceutical Inc. (RARE) stock experienced a significant drop of 22.8% in pre-market trading on July 10, 2025.

Ultragenyx Pharmaceutical Inc. and Mereo BioPharma Group plc have announced that their Phase 3 Orbit study, evaluating UX143 (setrusumab) for osteogenesis imperfecta (OI), will proceed to final analysis with results expected by the end of 2025. The Data Monitoring Committee has confirmed UX143’s safety profile as acceptable, allowing the study to continue as initially planned.

The Phase 3 Orbit study, conducted in pediatric and young adult patients aged 5 to 25 years, focuses on assessing the safety and efficacy of setrusumab in reducing fracture rates. Patients will undergo at least 18 months of therapy before final analyses, with a significance threshold set at p<0.04 for the Orbit study.

Parallelly, the Cosmic study involving younger patients, aged 2 to less than 7 years, is also progressing as expected without any significant safety concerns. The significance threshold for this cohort is set at p<0.05. Final analyses for both studies will conclude after the patients complete the treatment duration, ensuring comprehensive results.

Setrusumab, a fully human monoclonal antibody, works by inhibiting sclerostin, a negative regulator of bone formation, hence potentially increasing bone density and strength. The ongoing collaboration between Ultragenyx and Mereo BioPharma is essential for advancing treatment options for OI, a genetic disorder affecting approximately 60,000 individuals in commercially accessible areas.