Circulating Tumor Cells with Stemness and Epithelial-to-Mesenchymal Transition Features Are Chemoresistant and Predictive of Poor Outcome in Metastatic Breast Cancer.

Circulating tumor cells (CTCs) bearing phenotypes related to cancer stem cells (CSCs) and epithelial-to-mesenchymal transition (EMT) have been identified in breast cancer; however, their clinical significance is not clear. In the current study, we investigated the prognostic relevance of single CSC/partial-EMT CTCs in patients with metastatic breast cancer and the effect of first-line chemotherapy on their incidence. For this purpose, triple immunofluorescence against cytokeratin, ALDH1, and TWIST1 was performed in peripheral blood mononuclear cell (PBMC) cytospins from 130 patients before and after first-line chemotherapy. CSC/partial-EMT CTCs were characterized as cells co-expressing cytokeratin, high levels of ALDH1, and nuclear TWIST1. CSC/partial-EMT CTCs were evident in 27.7% of patients at baseline and were correlated to lung metastases ( = 0.010) and decreased progression-free survival [PFS; median 10.2 (8.9-11.6) vs. 13.5 (11.3-15.7) months; = 0.024]. Their detection was an independent factor predicting for increased risk of relapse [multivariate analysis; HR (95% confidence interval (CI)): 1.785 (1.171-2.720); = 0.007]. In HER-2-negative patients, CSC/partial-EMT CTCs were additionally associated with reduced overall survival (OS) [median 39 (26.2-51.9) vs. 51 (15.7-86.4) months; = 0.020] and increased risk of death [multivariate analysis; HR (95% CI): 2.228 (1.066-4.655); = 0.033]. Chemotherapy resulted in a significant increase in the incidence of CSC/partial-EMT CTCs (mean CTC% per patient: 59.4% post vs. 39.5% pre; = 0.018), which was subsequently confirmed only in HER2-negative patients ( = 0.040) and in non-responders at the end of treatment ( = 0.020). In conclusion, CSC/partial-EMT CTCs represent a chemoresistant subpopulation, which independently predicts for unfavorable outcome in metastatic breast cancer. Efficient targeting of these CTCs could potentially increase patient survival.