A bispecific antibody-drug conjugate (ADC) that targets two different non-overlapping epitopes of the human tumor-associated antigen (TAA) epidermal growth factor receptor 2 (HER2), ECD2 and ECD4, that is conjugated to an as of yet undisclosed cytotoxic payload, with potential antineoplastic activity. Upon administration, anti-HER2/HER2 bispecific ADC KM501 selectively and simultaneously targets, binds to and blocks the two distinct HER2 domains on the tumor cell surface, thereby enhancing the binding between KM501 and HER2. Upon binding and internalization, the cytotoxic payload kills, through an as of yet undisclosed mechanism of action (MoA), tumor cells that overexpress HER2. HER2, a tyrosine kinase receptor, is overexpressed by many cancer cell types. The HER2 epitopes ECD4 and ECD2 are the targeting and binding domains for trastuzumab and pertuzumab, respectively.