The use of liquid biopsy could be the key for precision medicine. Circulating tumor cells (CTCs) are the most studied circulating biomarkers used to assess the metastatic process and they have shown their clinical validity and utility in cancer patients. However, their detection and capture are still a challenge as they are very rare in the bloodstream. In this clinical trial named EPIDROP, the investigators will use an innovative device called EPIDROP (for EPIspot in a DROP) in metastatic prostate cancer. EPIDROP is a completely new technology answering to biological and clinical questions by proposing a procedure detecting the functional subset of prostatic CTCs at the single cell level. The investigators will stain cells in the sample for EpCAM, PSMA, CD45 before to encapsulate them one by one in microdroplets and measure the PSA secretion by only the viable CTCs. This study aims to demonstrate the non-inferiority of the EPIDROP compared to the CellSearch system.