HER2-low Breast Cancer: Now and in the Future

Breast cancer is a heterogeneous disease, and its subtypes are characterized by hormone receptor and human epidermal growth factor receptor 2 (HER2) expression status."HER2-low" tumors, which exhibit a low level of HER2 expression (immunohistochemistry 1+ or 2+ without gene amplification), were conventionally considered not amenable to anti-HER2 targeting agents based on the results of a phase III trial of trastuzumab.However, this perspective is being challenged by the emergence of novel anti-HER2 antibody-drug conjugates, such as trastuzumab-deruxtecan.These innovative therapies have demonstrated remarkable efficacy against HER2-low breast cancer, shedding new light on a previously overlooked category of breast cancer.Such promising results highlight the need for in-depth investigations of the biology and prognostic implications of HER2-low tumors.In this review, we comprehensively summarize the current evidence surrounding this topic and highlight areas that warrant further exploration and research in the future.