An antibody-drug conjugate (ADC) composed of trastuzumab, a monoclonal antibody targeting human epidermal growth factor receptor 2 (ERBB2; EGFR2; HER2) conjugated to deruxtecan, a derivative of the camptothecin analog exatecan (DXd; DX-8951 derivative), a DNA topoisomerase 1 (topoisomerase I; Top1) inhibitor, with antineoplastic activity. Upon administration of trastuzumab deruxtecan, trastuzumab targets and binds to HER2 on tumor cells. Upon antibody/antigen binding and internalization, deruxtecan binds to and inhibits Top1-DNA complexes, which results in an inhibition of DNA replication, cell cycle arrest and tumor cell apoptosis. HER2, a tyrosine kinase receptor, is overexpressed by many cancer cell types. In addition, trastuzumab deruxtecan induces antibody-dependent cell-mediated cytotoxicity (ADCC) and causes a bystander killing effect.