Humanized anti-HER2 monoclonal antibody (trastuzumab) linked via a cleavable linker to the topoisomerase I inhibitor deruxtecan (DXd). After binding HER2 on tumor cells, the ADC is internalized and the linker is cleaved to release DXd, which inhibits topoisomerase I, causing DNA damage and cell death. The membrane-permeable payload produces a bystander effect that can kill adjacent HER2-expressing and HER2-low tumor cells.