Andrew Armstrong, MD, Duke Cancer Institute, Durham, NC, discusses the prognostic role of circulating tumor cell (CTC) counts in patients with metastatic castration-resistant prostate cancer (mCRPC) treated with androgen receptor signalling inhibitors, and the development of the Epic Sciences platform. Pre-treatment blood samples were collected from men with progressing mCRPC due to receive abiraterone or enzalutamide therapy during the PROPHECY trial (NCT02269982). Enumeration was performed using the Epic Sciences platform. Overall, pre-treatment CTC counts determined using the Epic Sciences platform were validated as having prognostic significance in predicting overall survival (OS) in patients with progressing mCRPC starting androgen receptor signalling inhibitor therapy. This interview took place during the 2021 Genitourinary Cancers Symposium.