Epidermal Growth Factor Receptor Status in Circulating Tumor Cells as a Predictive Biomarker of Sensitivity in Castration-Resistant Prostate Cancer Patients Treated with Docetaxel Chemotherapy

Objective: We examined whether epidermal growth factor receptor (EGFR) expression in circulating tumor cells (CTCs) can be used to predict survival in a population of bone-metastatic castration-resistant prostate cancer (mCRPC) patients treated with docetaxel chemotherapy. Methods: All patients with mCRPC who had experienced treatment failure with androgen-deprivation therapy and had received docetaxel chemotherapy were eligible. CTCs and EGFR expression in CTCs were enumerated with the CellSearch System in whole blood. This system is a semi-automated system that detects and enriches epithelial cells from whole blood using an EpCAM antibody-based immunomagnetic capture. In addition, the EGFR-positive CTCs were assessed using CellSearch(®) Tumor Phenotyping Reagent EGFR. Results: The median CTC count at baseline before starting trial treatment was eight CTCs per 7.5 mL of blood (range 0–184). There were 37 patients (61.7%) who had ≥5 CTCs, with median overall survival of 11.5 months compared with 20.0 months for 23 patients (38.3%) with <5 CTCs (p < 0.001). A total of 15 patients (40.5%, 15/37) with five or more CTCs were subjected to automated immunofluorescence staining and cell sorting for EGFR protein. Patients with EGFR-positive CTCs had a shorter overall survival (OS) (5.5 months) than patients with EGFR-negative CTCs (20.0 months). CTCs, EGFR-positive CTCs, and alkaline phosphatase (ALP) were independent predictors of overall survival time (p = 0.002, p < 0.001, and p = 0.017, respectively). Conclusion: CTCs may be an independent predictor of OS in CRPC treated with docetaxel chemotherapy. The EGFR expression detected in CTCs was important for assessing the response to chemotherapy and predicting disease outcome.