Expression of immune checkpoints on circulating tumor cells in men with metastatic prostate cancer (mPC).

Abstract

191 Background: Most immune checkpoint inhibitors have shown limited efficacy in unselected men with mPC, and there is limited understanding about which immune checkpoints (ICs) are relevant in mPC. We evaluated ICs on the cell surface of circulating tumor cells (CTCs) in patients with mPC. Methods: Patients were enrolled prospectively at the Duke Cancer Center in three cohorts: A) mCRPC starting the novel androgen receptor inhibitors abiraterone acetate (AA) or enza with or without sipuleucel-T, B) mCRPC with disease progression after AA or enza, and C) metastatic castration sensitive PC. 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 at further disease progression. Results: Through August 2017, we enrolled 4 subjects in cohort A, 7 in cohort B, and 3 in cohort C. CTCs were detectable in every cohort. At baseline, B7-H3 was the most prevalent IC on a per-patient basis (mean 96% in cohort A, and 100% in cohort B, and 88% in cohort C). PD-L1 was detected less frequently (cohort A mean 8%, cohort B mean 42%, and cohort C mean 67%), with evidence of heterogeneity of CTC PD-L1 expression between and within patients. PD-L1 was expressed on 8% of CTCs in cohort C and cohort A, as compared to 28% of CTCs in cohort B, while CTLA-4 expression was not identified in cohort C, 3% of cohort A, and 15% of cohort B. B7H3 was expressed on 68% of cohort C, 91% of cohort A, and 98% of cohort B. PD-L2 was expressed on 0-7% of CTCs overall. Conclusions: Patients with mPC can have detectable and heterogeneous ICs on CTCs, particularly PD-L1 and B7-H3, and these characteristics can be monitored over time. B7-H3 was the most prevalent IC on CTCs, regardless of disease state. Our preliminary data suggests that men with mCRPC post-enza/AA have higher levels of PD-L1 and CTLA-4 expression on their CTCs as compared with men with mHSPC and mCRPC prior to enza/AA. Patient enrollment and analysis are ongoing.