Antibody-drug conjugates in breast cancer: redefining targeted therapy.

Antibody-drug conjugates (ADCs) have transformed the treatment landscape of breast cancer and redefined the conceptual distinction between targeted therapy and conventional chemotherapy. Originally conceived as "magic bullets" that selectively deliver cytotoxic warheads to antigen-expressing tumor cells, clinical and mechanistic evidence indicates that ADC activity depends on a broader interplay of target-dependent and target-independent mechanisms, including extracellular payload release, bystander killing, off-tumor uptake, and immune modulation. Here, we examine ADCs in breast cancer as a distinct therapeutic paradigm. We discuss how antigen biology, linker chemistry, payload features, and drug-to-antibody ratio collectively determine efficacy, toxicity, and therapeutic index. We then compare currently approved and emerging HER2- and TROP2-directed ADCs, highlighting how differences in linker stability, payload pharmacology, and bystander capacity can affect clinical outcomes in ADCs sharing the same target. We further discuss the biological basis and translational challenges of de novo and acquired resistance related to targets, payloads, and tumor microenvironmental constraints, as well as the implications of these mechanisms for biomarker development, sequencing rationales, and combination strategies with immune checkpoint inhibitors and DNA repair-targeting therapies. Finally, we outline future directions of ADC development, including expansion of the target space, novel payload modalities, and next-generation antibody and conjugation engineering.