Circulating tumor cells as a marker of response: implications for determining treatment efficacy and evaluating new agents

Purpose of review To assess the current and future potential of using circulating tumor cells (CTC) as a biomarker to assess staging/prognosis and treatment efficacy. Recent findings The shedding of prostate cancer cells by the primary tumor into the circulation can occur very early in the disease process but the detection of CTC at the time of initial presentation is not necessarily a poor prognostic. Furthermore, some patients who have undergone a radical prostatectomy and have no evidence of disease for 5 years still have detectable tumor cells in their bone marrow. In some cases these dormant tumor cells can eventually be activated and form a metastasis. In other situations the shed cells might contain aggressive stem-like cells. Overall, a pattern of an evolving genomic and molecular profile appears to be apparent over the course of initial dissemination to development of overt metastases. Clinically, several studies suggest that the enumeration of CTC prior to and during chemotherapy is predictive of the overall therapeutic response. Additionally, the absolute count or CTC threshold could be patient specific and not universal, suggesting the change in CTC count on a case-by-case basis may be more significant for patient management. Summary CTC are clinically significant in the management of prostate cancer. However, to determine the true efficacy of CTC detection in the active clinical arena, coordinated multi-institutional studies with a standardized detection methodology need to be undertaken.