Resistance mechanisms and post-trastuzumab deruxtecan (T-DXd) strategies in HER2+ and HER2-low breast cancer: From biology to clinical practice.

Trastuzumab deruxtecan (T-DXd), a HER2-directed antibody-drug conjugate (ADC), has revolutionized the treatment landscape for HER2-positive and HER2-low breast cancer, offering unprecedented improvements in progression-free survival (PFS) and overall survival (OS). However, the emergence of resistance limits its long-term efficacy and highlights the need for rational post-progression strategies. This review summarizes current understanding of resistance mechanisms to T-DXd, including dynamic HER2 expression loss, dysregulation of DNA damage repair, tumor microenvironment remodeling, and immune evasion. Building upon these mechanistic insights, we explore therapeutic strategies following T-DXd resistance, including sequential ADCs with alternative targets or payloads, combinations with HER2-targeted tyrosine kinase inhibitors (TKIs), CDK4/6 or PARP inhibitors, endocrine therapy, and immunotherapy. Novel agents such as ARX788, patritumab deruxtecan, and Dato-DXd demonstrate promise in overcoming resistance through diversified mechanisms. Real-world data further support a "three-step strategy" involving ADC rechallenge after interim non-ADC therapies, offering a practical, mechanism-informed treatment model. This review provides a framework for precision-guided care in patients with advanced breast cancer and supports ongoing efforts to extend the therapeutic window beyond initial T-DXd benefit.