HER2-targeted antibody–drug conjugate composed of trastuzumab linked via a cleavable linker to deruxtecan (DXd), a membrane-permeable topoisomerase I inhibitor. Binding to HER2 drives internalization and lysosomal cleavage to release DXd, causing DNA damage (single-strand breaks) and apoptosis. The trastuzumab component also inhibits HER2 signaling and can mediate ADCC. High drug-to-antibody ratio and diffusible payload enable a bystander killing effect of neighboring HER2-low tumor cells.