An antibody-drug conjugate (ADC) composed of trastuzumab, a humanized monoclonal antibody targeting human epidermal growth factor receptor 2 (EGFR2; HER2; ErbB2), conjugated, via a cleavable linker, to the microtubule inhibitor and auristatin derivative duostatin-5 (Duo-5), with potential antineoplastic activity. Upon administration of trastuzumab botidotin, the trastuzumab moiety targets and binds to HER2 on tumor cells. Upon antibody/antigen binding and internalization, duo-5 binds to tubulin and inhibits its polymerization, which results in cell cycle arrest and induces tumor cell apoptosis. HER2, a tyrosine kinase receptor, is overexpressed by many cancer cell types.