1Division of Hemato-oncology, Department of Internal Medicine, Chungnam National University Hospital, Daejeon, Korea 2Department of Oncology, Asan Medical Center, University of Ulsan College of Medicine, Seoul, Korea
Received: October 17, 2023; Accepted: January 28, 2024. Published online: January 30, 2024.
ABSTRACT
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.
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