Herein we report the use of pyridazinediones to functionalise the native solvent accessible interstrand disulfide bonds in trastuzumab with monomethyl auristatin E (MMAE). This method of conjugation delivers serum stable antibody–drug conjugates (ADCs) with a controlled drug loading of 4. Moreover, we demonstrate that the MMAE-bearing ADCs are potent, selective and efficacious against cancer cell lines in both in vitro and in vivo models.