HER2 evaluation from tissue to liquid biopsy: improving CTC HER2 characterization for clinical implementation in breast cancer

Breast cancer (BC) is a highly heterogeneous and complex disease, defined by a multitude of etiologic and pathologic features. It is the most frequently diagnosed cancer in women worldwide. Subtype classification is therefore of paramount importance in enabling oncologists to make well-informed treatment decisions. The assessment and monitoring of HER2 status in breast cancer is of critical importance for the selection of appropriate therapies and the evaluation of treatment response, with the current benchmark method for its determination being based on tissue biopsy. As a matter of fact, tissue biopsies are limited in their ability to capture tumor heterogeneity and provide real-time monitoring, while circulating tumor cells (CTCs) offer a less invasive, real-time alternative. However, challenges remain regarding the validation and standardization of this approach, to ensure the consistency of results. The present study investigates the correlation between HER2 status on CTCs and tissue samples by developing a quantification evaluation that considers the ratio of HER2positive CTCs (CTC-HER2ratio). The study demonstrated a significant correlation between HER2 status on tissue samples and on CTCs. These findings indicate that an evaluation of HER2 expression based solely on positivity or negativity may not be an accurate approach. In contrast, a ratio-based assessment may provide a more clinically relevant evaluation of HER2 expression on CTCs. The change in the CTC-HER2ratio in a small cohort of patients was examined, and finally, a pipeline optimized for the identification of CTCs and automated quantification of HER2 expression on CTCs was developed and compared with the gold standard CellSearch® system. The novel pipeline demonstrated superior detection of CTCs compared to the CellSearch® system, including a higher number of HER2positive CTCs. Furthermore, it identified a potential novel subpopulation with low or negative epithelial cell adhesion molecule (EPCAM)/cytokeratin (CK) expression, indicating the necessity for a more comprehensive method for CTC characterization.