Detection and Characterization of Estrogen Receptor α Expression of Circulating Tumor Cells as a Prognostic Marker

SIMPLE SUMMARY: CTCs are considered the seeds of metastases and their presence in the blood is related to survival in breast cancer. Estrogen receptor (ER) is considered a direct target of hormonal therapy. Evaluating the expression of ER in CTCs to characterize the tumor and assess therapy efficacy might lead to individualized treatment. In ER-positive metastatic breast cancer patients, CTCs were detected in half of the patients. Most of these patients carry ER-negative CTCs only, whereas a third shows a mixture of ER-positive and -negative CTCs. Shorter relapse-free survival was associated with CTC-positivity. The CTC-ER expression was intra- and inter-patient heterogeneous, highlighting potential endocrine therapy resistance. Therefore, monitoring ER-CTC status in advanced breast cancer could add a prognostic value to CTC enumeration and may serve as a predictive marker for therapeutic resistance, which may need to be addressed on a larger scale in future studies. ABSTRACT: CTCs have increasingly been used as a liquid biopsy analyte to obtain real-time information on the tumor through minimally invasive blood analyses. CTCs allow for the identification of proteins relevant for targeted therapies. Here, we evaluated the expression of estrogen receptors (ER) in CTCs of patients with metastatic breast cancer. From sixty metastatic breast cancer patients who had ER-positive primary tumors (range of 1–70% immunostaining) at initial cancer diagnosis, 109 longitudinal blood samples were prospectively collected and analyzed using the CellSearch System in combination with the ERα monoclonal murine ER-119.3 antibody. Prolonged cell permeabilization was found to be required for proper staining of nuclear ER in vitro. Thirty-one cases were found to be CTC-positive; an increased number of CTCs during endocrine and chemotherapy was correlated with disease progression, whereas a decrease or stable amount of CTC number (<5) during treatment was correlated with a better clinical outcome. Survival analyses further indicate a positive association of CTC-status with progression-free survival (HR, 66.17; 95%CI, 3.66–195.96; p = 0.0045) and overall survival (HR, 6.21; 95%CI, 2.66–14.47; p < 0.0001). Only one-third of CTC-positive breast cancer patients, who were initially diagnosed with ER-positive primary tumors, harbored ER-positive CTCs at the time of metastasis, and even in those patients, both ER-positive and ER-negative CTCs were found. CTC-positivity was correlated with a shorter relapse-free survival. Remarkably, ER-negative CTCs were frequent despite initial ER-positive status of the primary tumor, suggesting a switch of ER phenotype or selection of minor ER-negative clones as a potential mechanism of escape from ER-targeting therapy.