An antibody-drug conjugate (ADC) composed of trastuzumab, a humanized immunoglobulin G1 (IgG1) a monoclonal antibody targeting human epidermal growth factor receptor 2 (EGFR2; HER2; ErbB2) and site-specifically conjugated, via a cleavable linker, to the microtubule-disrupting cytotoxic agent monomethyl auristatin E (MMAE), with potential antineoplastic activity. Upon administration of trastuzumab envedotin, the trastuzumab moiety targets and binds to HER2 on the surface of tumor cells. Following internalization, MMAE targets and binds to tubulin, and inhibits microtubule polymerization. This results in G2/M phase cell cycle arrest and apoptosis in HER2-expressing tumor cells. HER2, a tyrosine kinase receptor, is overexpressed by many cancer cell types. Trastuzumab envedotin has an average drug-to-antibody (DAR) ratio of 2.0; the two MMAE molecules are attached site-specifically through transamidation to residue Q295 in the antibody heavy chains constant region.