Background: KEYNOTE-365 (NCT02861573) is a phase 1b/2 study evaluating pembro in combination with other agents in mCRPC. An earlier report of cohort C showed activity and acceptable safety with pembro + enza. Updated results from cohort C are reported. Methods: Pts who failed or became intolerant to ≥4 wks of abi in prechemotherapy mCRPC state and whose disease progressed within 6 mo of screening per PSA progression or radiologic bone or soft tissue progression enrolled. Pts received pembro 200 mg IV Q3W with enza 160 mg/day PO. Primary end points: safety, PSA response rate (confirmed PSA decline ≥50%), and objective response rate (ORR) per blinded independent central review. Key secondary end points: disease control rate (DCR), duration of response (DOR), time to PSA progression, rPFS, and OS. Results: Of 102 treated pts, 73 discontinued, primarily due to progression (60%). Median age was 70 y (range, 43-87), 29% were PD-L1+, 17% had visceral disease, and 39% had measurable disease. Median follow up was 13 mo for all patients (n=102) and 17 mo for patients with ≥27 wks’ follow-up (n=69). See Table for efficacy outcomes. 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]). Conclusions: With additional follow-up, pembro + enza continued to show activity in pts with abi-pretreated mCRPC. Safety of the combination was consistent with known profiles of pembro and enza. Clinical trial information: NCT02861573

^aWith ≥27 wks’ follow-up

^b‘+’ indicates ongoing responder