An antibody-drug conjugate (ADC) composed of patritumab, a monoclonal antibody directed against the human epidermal growth factor receptor HER3 (ErbB3),linked to the topoisomerase I inhibitor DX 8951, a semisynthetic, water-soluble derivative of camptothecin, with potential antineoplastic activity. Upon administration of patritumab deruxtecan, the patritumab moiety targets and binds to HER3. After internalization, DX 8951 inhibits topoisomerase I activity by stabilizing the complex between topoisomerase I and DNA and inhibiting religation of DNA breaks, thereby inhibiting DNA replication and triggering apoptotic cell death. HER3, a member of the epidermal growth factor receptor (EGFR) family of receptor tyrosine kinases, is frequently overexpressed in tumors.