Challenges in HER2-low breast cancer identification, detection, and treatment

Recently, human epidermal growth factor receptor 2 (HER2)-low breast cancer (BC) has emerged as a novel subset within the category of HER2-negative BC, prompting a reassessment of the immunohistochemical negative scores of 0, 1+, and the 2+/in situ hybridization (ISH) negative phenotype. Recent clinical trials have provided compelling evidence of the substantial clinical advantages offered by novel antibody-drug conjugates (ADCs) that target HER2 in the treatment of these specific tumor cohorts. Notably, trastuzumab deruxtecan (T-Dxd), an ADC that specifically targets HER2, has recently received approval from the US Food and Drug Administration as the inaugural targeted therapeutic option for HER2-low BC. However, the classification of HER2-low BC as a distinct subtype, the methods for detecting HER2-low BC, and the optimal treatment approach for HER2-low BC remain subjects of ongoing debate and lack consensus. This comprehensive review aims to address these pertinent concerns, offering insights into the nuanced tumor biology underlying HER2-low BC and critically analyzing the current treatment pathways available. By synthesizing available evidence, the objective is to contribute to an enhanced understanding of HER2-low BC, providing a foundation for more informed clinical decisions and further advancements in tailored therapeutic approaches. As the medical community navigates these uncertainties, this review seeks to consolidate existing knowledge, fostering a collective effort toward establishing consensus in the diagnosis and treatment of HER2-low BC.