Two late‑stage Phase III trials of setrusumab failed to meet primary endpoints, forcing Ultragenyx and Mereo into a reassessment of their brittle bone disease programs. The readouts triggered steep equity declines as investors digested the clinical miss and its implications for near‑term revenue potential. Both companies said they will review the datasets to determine next steps, but the immediate market reaction underscores the sensitivity of small biotech valuations to single pivotal outcomes.