Mechanisms of Resistance and Synergy: The Role of Tumor Microenvironment in HER2-Low Breast Cancer Therapy

HER2-low breast cancer, also known as IHC 1+ or IHC 2+ without ERBB2 amplification, is a new concept in the biology of breast cancer that has removed the binary classification of HER2-positive or HER2-negative breast cancer. The recent introduction of antibody-drug conjugates (ADCs), such as trastuzumab deruxtecan (T-DXd), has improved therapeutic outcomes for HER2-low breast cancer by demonstrating high efficacy in HER2-low tumors through efficient payload delivery. However, differences in ADC efficacy exist among HER2-low breast cancer patients, with tumor cells showing resistance to ADCs. Recent research indicates that the tumor microenvironment (TME) plays a critical role in determining the efficacy of ADCs against tumor cells. TME creates a barrier to the delivery of ADCs to tumor cells that show resistance to ADCs. This review article aims to highlight the current understanding of the biology of HER2-low breast cancer and its response to ADCs with reference to the tumor microenvironment.