Quantitative HER2 profiling on circulating tumor cells using an EpCAM-independent platform in metastatic breast cancer

Accurate human epidermal growth factor 2 (HER2) assessment is critical for guiding breast cancer (BC) treatment. Circulating tumor cells (CTCs), which detach from the primary or secondary tumors and circulate through the bloodstream, can colonize distant sites to form metastases. Previous studies have demonstrated the clinical validity of CTCs as prognostic biomarkers in metastatic breast cancer (mBC). Liquid biopsy testing offers a non-invasive alternative for real-time evaluation of HER2 expression on CTCs, overcoming limitations of repeated histological sampling. This study employed a pipeline combining EpCAM-independent enrichment with Parsortix® and immunofluorescence (IF)-based HER2 characterization to assess HER2 expression in patient-derived CTCs from sixteen patients with mBC and compared it to the FDA-approved CellSearch® system and to the HER2 tissue status. CTC detection rates, median CTC counts per 7.5 mL blood, and the fraction of HER2-positive CTCs per sample were determined, and CTC versus tissue status was compared to assess concordance. The two methods had a strong positive correlation in CTC counts, with the label-free workflow showing a higher CTC detection (100% versus 77% samples with ≥ 1 CTC). Median total CTC counts per 7.5 mL blood were 16.5 (range 3–65) for Parsortix® and 3 (range 1–245) for CellSearch®. Despite a different HER2-positive CTC detection rate (38% versus 11% using Parsortix® and CellSearch®, respectively), both methods showed concordance with the tissue, with the distribution of HER2-positive CTCs reflecting the HER2 status of the biopsy. Patients with HER2-positive mBC showed a higher proportion of HER2-positive CTCs compared to patients with HER2-negative tissue both with Parsortix (55% versus 33%) and with CellSearch (61% versus 1%). No differences in HER2-positive CTC distribution were observed between patients with HER2-low and HER2-zero tumors. These results support the clinical utility of HER2 assessment on CTCs with both workflows and highlight the potential diagnostic value of label-free CTC enrichment combined with HER2 quantification. Further studies in larger cohorts should be conducted to validate our findings and investigate the clinical relevance of HER2-positive CTCs detected with the developed pipeline, particularly in the context of anti-HER2 therapies.