An antibody-drug conjugate (ADC) composed of a monoclonal antibody directed against the G protein-coupled receptor 20 (GPR20) conjugated to the cytotoxic DNA topoisomerase I inhibitor and exatecan derivative DXd (MAAA-1181a; MAAA-1181), with potential antineoplastic activity. Upon administration of the anti-GPR20/DXd ADC DS-6157a, the anti-GPR20 antibody targets and binds to GPR20-expressing tumor cells. Upon cellular uptake, the DXd moiety targets and binds to DNA topoisomerase I, thereby stabilizing the cleavable complex between topoisomerase I and DNA, resulting in DNA breaks, inhibition of DNA replication and apoptosis. This inhibits the proliferation of GPR20-expressing tumor cells. GPR20 is overexpressed on certain tumor cell types.