An antibody-drug conjugate (ADC) composed of a humanized monoclonal antibody directed against tumor-associated mucin-1 (TA-MUC1) conjugated to the cytotoxic DNA topoisomerase I inhibitor and exatecan derivative DXd (MAAA-1181a; MAAA-1181) via an enzymatically cleavable tetrapeptide-based linker, with potential antineoplastic activity. Upon administration of anti-TA-MUC1/DXd ADC DS-3939a, the anti-TA-MUC1 antibody targets and binds to TA-MUC1-expressing tumor cells. Upon cellular uptake and lysosomal degradation of the linker, DXd 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 TA-MUC1-expressing tumor cells. MUC1, a glycoprotein normally expressed on epithelial cells and overexpressed on the surface of a variety of cancer cells, plays a key role in tumor cell survival and proliferation. TA-MUC1 is designated to MUC1 epitopes with carbohydrate-induced conformational structures that are tumor-specific.