An antibody-drug conjugate (ADC) composed of a monoclonal antibody targeting human epidermal growth factor receptor 2 (EGFR2; HER2; ErbB2) and conjugated to an as of yet undisclosed cytotoxic agent, with potential antineoplastic activity. Upon administration of anti-HER2 ADC A166, the antibody moiety targets and binds to HER2 on tumor cells. Upon antibody/antigen binding and internalization, the cytotoxic agent induces tumor cell apoptosis, through an as of yet not publicly known mechanism. HER2, a tyrosine kinase receptor, is overexpressed by many cancer cell types.