An antibody-drug conjugate (ADC) composed of trastuzumab, a humanized immunoglobulin G1-kappa (IgG1k) monoclonal antibody directed against the tumor-associated antigen (TAA) human epidermal growth factor receptor 2 (EGFR2; HER2; ErbB2) conjugated to rezetecan, which is composed of a cleavable linker and a camptothecin derivative, with potential antineoplastic activity. Upon administration of trastuzumab rezetecan, the trastuzumab moiety targets and binds to HER2 expressed on tumor cells. Upon cellular uptake and linker cleavage, the camptothecin derivative stabilizes covalent topoisomerase I-DNA complexes, and results in single-stranded and double-stranded DNA breaks, the inhibition of DNA replication, the induction of apoptosis and the inhibition of tumor cell proliferation in HER2-expressing tumor cells. HER2, a tyrosine kinase receptor, is overexpressed by many cancer cell types.