Background: Pembro + enza (cohort C) has shown antitumor activity and acceptable safety in abi-pretreated pts with mCRPC in the phase I/II KEYNOTE-365 study (NCT02861573). Updated results with new biomarker data from cohort C are reported. Methods: Pts who became intolerant to or for whom ≥4 weeks of abi failed in the prechemotherapy mCRPC state and who progressed within 6 mo of screening were enrolled. Pts received pembro 200 mg IV Q3W + enza 160 mg/day orally. Primary end points were PSA response rate (PSA decrease ≥50%; confirmed by a second value ≥3 weeks later), ORR per RECIST v1.1 by blinded independent central review, and safety. Key secondary end points were DCR per RECIST v1.1 (CR+PR+SD or non-CR/non-PD ≥6 mo), DOR per RECIST v1.1, radiographic PFS (rPFS) per PCWG-modified RECIST v1.1, and OS. Biospecimens (eg, blood, tissue) were collected at baseline and during the study for biomarker analysis, including tissue PD-L1 expression, androgen receptor variant 7 (AR-v7) expression in circulating tumor cells (CTCs), and a T-cell-inflamed gene expression profile (GEP). Results: Of 103 enrolled pts, 102 were treated; 39% of treated pts had measurable disease. Median (range) time from enrollment to data cutoff was 19.1 mo (1.1-28.8) for all pts and 21.4 mo (15.1-28.8) for pts with ≥27 wks’ follow-up (n=69). Confirmed PSA response rate was 22% in 101 pts with a baseline PSA assessment. Median time to PSA progression was 3.5 mo (95% CI, 2.9-4.0). In pts with measurable disease and ≥27 wks’ follow-up (n=25), confirmed ORR was 12% (2 CRs, 1 PR) and DCR was 32%. Median DOR was not reached (range, 0.0+ to 24.4+ mo); 2 pts had a response for ≥6 mo. In all pts, median (95% CI) rPFS was 6.1 mo (4.4-6.5) and median OS was 20.4 mo (15.5-NR). At 6 mo, rPFS rate was 55.1% and OS rate was 88.2%. Treatment-related AEs occurred in 92 pts (90%); most frequent (≥20%) were fatigue (38%), nausea (22%), and rash (20%). Grade 3-5 treatment-related AEs occurred in 40 pts (39%). Three pts died of AEs (1 AE was treatment related [cause unknown]). Of all pts, 29% had PD-L1⁺ tumors (combined positive score ≥1). Of 51 pts with AR-v7 data, 13.7% were AR-v7⁺ and 86.3% were AR-v7⁻. GEP was not significantly associated with ORR or PSA response. Conclusions: Pembro + enza continued to show activity in pts with abi-pretreated mCRPC. Safety of the combination was consistent with the known profiles of pembro and enza. A phase III study of this combination is ongoing (KEYNOTE-641, NCT03834493). Clinical trial information: NCT02861573.