Two late‑stage Phase III programs of the monoclonal antibody setrusumab failed to meet primary endpoints, leaving partners Mereo Biopharma and Ultragenyx reeling. Both companies reported negative top‑line outcomes for the Orbit and Cosmic studies in brittle bone disease, triggering dramatic share price declines — Mereo plunged and Ultragenyx shares tumbled. The setbacks force both firms to reassess R&D priorities and demonstrate the market sensitivity to single‑asset late‑stage failures in rare‑disease biotech.