HER2 expression on tumor-derived extracellular vesicles and circulating tumor cells in metastatic breast cancer

Tumor-derived extracellular vesicles (tdEVs) and circulating tumor cells (CTCs) in blood of metastatic cancer patients associate with poor outcome. In this study, we explored the human epidermal growth factor receptor 2 (HER2) expression on CTCs and tdEVs of metastatic breast cancer patients.

Blood samples from 98 patients (IC 2006-04 study) were originally processed with the CellSearch^® system using the CTC kit and anti-HER2 as an additional marker in the staining cocktail. CTCs and tdEVs were automatically enumerated from the generated CellSearch image datasets using the open-source ACCEPT software. The HER2 status of the tissue was assessed by fluorescence in situ hybridization (FISH).

The inclusion of anti-HER2 increased the percentage of informative samples with ≥1 detectable CTC and tdEV from 89% and 99% to 95 and 100%, respectively. CTCs and tdEVs were subdivided based on their cytokeratin (CK) and HER2 phenotype into CK+HER2-, CK-HER2+ and CK+HER2+. Inter- and intra- patient heterogeneity was found regarding the phenotype of CTCs and tdEVs with the majority of patients having all different subclasses present. CK- CTCs and tdEVs correlated equally well with clinical outcome as CK+ CTCs and tdEVs. Use of ≥7% HER2+CK+ tdEVs can predict HER2 expression of the tissue with 74% sensitivity and specificity, whereas the respective use of ≥23% HER2+CK+ CTCs leads to 65% sensitivity and 66% specificity.

HER2 can be detected on CTCs and tdEVs not expressing CK, which have similar clinical relevance to CTCs and tdEVs expressing CK. tdEVs perform better than CTCs in predicting the HER2 status of the tissue.