Two late-stage trials of setrusumab missed primary endpoints and triggered severe market reactions for their developers. Ultragenyx Pharmaceutical and Mereo Biopharma reported Phase III failures in programs targeting brittle-bone disease, leaving partners to reassess clinical and commercial strategies. BioWorld and other outlets reported major share declines on the news, with Mereo’s stock plunging at the open. The readouts came from the Orbit and Cosmic Phase III programs and, while some secondary endpoints showed activity, neither trial achieved its prespecified primary outcomes. Company statements indicate teams are still dissecting the data to determine whether any subgroups or secondary signals justify continued investment or regulatory dialogue. Investors reacted immediately: trading volatility and sharp write-downs followed the announcements, forcing both firms to weigh corporate options including restructuring, asset sales, or program termination. The failures underscore the clinical risk and market sensitivity surrounding monoclonal antibody programs for rare skeletal disorders.