An antibody-drug conjugate (ADC) composed of a humanized antibody targeting the cell surface antigen cluster of differentiation (CD228; melanotransferrin; MFI2; MELTF) that is conjugated, via a beta-glucuronidase-cleavable linker, to the microtubule-disrupting cytotoxic agent monomethyl auristatin E (MMAE), with potential antineoplastic activity. Following administration, the antibody moiety of anti-CD228/MMAE ADC SGN-CD228A targets and binds to CD228 on the surface of tumor cells. Following internalization of SGN-CD228A and release of MMAE, MMAE targets and binds to tubulin, and inhibits microtubule polymerization. This results in G2/M phase cell cycle arrest and apoptosis in CD228-expressing tumor cells. CD228, a cell-surfaced, glycosylphosphatidylinoitol (GPI)-anchored glycoprotein, belongs to the transferrin family of iron-binding proteins.