Expression Of Immune Checkpoints (Ics) On Circulating Tumor Cells (Ctcs) In Men With Metastatic Prostate Cancer (Mpc).

5059Background: Most immune checkpoint inhibitors have shown limited efficacy in unselected men with mPC, and there is limited understanding about which ICs are relevant in mPC. We evaluated ICs on the cell surface of CTCs in patients (pts) with mPC. Methods: Pts were enrolled prospectively at the Duke Cancer Center in three cohorts: A) metastatic castration resistant prostate cancer (mCRPC) starting abiraterone acetate (AA) or enzalutamide (enza), B) mCRPC after AA or enza, and C) metastatic hormone sensitive PC (mHSPC). The Cellsearch platform was used to capture EpCAM- and CK-expressing CTCs and analyzed for PD-L1, PD-L2, B7-H3, and CTLA-4 expression at baseline, 12 weeks, and disease progression. Results: Through December 2017, we enrolled 18 pts (6 in cohort A, 8 in cohort B, and 4 in cohort C). CTCs were detectable in every cohort. At baseline, B7-H3 was the most prevalent IC while the other ICs were detected less frequently (Table 1). PD-L1 expression on CTCs was heterogeneous between and within pt...