The FDA placed a partial clinical hold on Daiichi Sankyo and Merck & Co.’s global Phase 3 antibody‑drug conjugate (ADC) program after the sponsors paused enrollment citing a "higher than anticipated incidence" of patient deaths. The agencies flagged safety signals in ongoing lung cancer trials and requested additional analyses and safety data before enrollment can resume. Daiichi Sankyo and Merck paused enrollment across the program, prompting immediate regulatory review and trial amendments. The partial hold affects global registrational studies and will delay near‑term readouts and regulatory timelines. Companies must submit investigator‑level adverse event narratives, root‑cause analyses, and updated risk‑mitigation plans to satisfy FDA concerns. For drug developers and investors, the action underscores the narrow safety margins for ADC payloads and the agency’s low tolerance for unexpected mortality in oncology registrational studies. Sponsors running ADC or cytotoxic‑payload programs should expect heightened FDA scrutiny and should prepare detailed safety monitoring and external adjudication strategies.