Background: The KEYNOTE-199 multicohort phase 2 study (NCT02787005) showed that pembro monotherapy has antitumor activity and acceptable safety in patients (pts) with mCRPC previously treated with a next-generation hormonal agent (NHA) and docetaxel in cohort 1 (C1) (RECIST-measurable, PD-L1⁺ disease), C2 (RECIST-measurable, PD-L1⁻ disease), and C3 (bone-predominant disease, irrespective of PD-L1). Updated results with additional follow-up for C1-3 are presented. Methods: Pts previously received ≥1 NHAs and 1 or 2 chemotherapies, including docetaxel. Pts received pembro 200 mg Q3W for 35 cycles or until progression or intolerable toxicity. Primary end point was ORR. Key secondary end points were DCR, DOR, PSA (≥50%) response rate, rPFS, OS, and safety. Results: Of 258 pts enrolled (C1=133; C2=67; C3=58), 6 completed (C1=4; C3=2) and 252 discontinued (C1=129; C2=67; C3=56) therapy, primarily due to progression (C1=106; C2=61; C3=45). Median follow-up was 9.6 mo (C1, 9.5; C2, 7.9; C3, 14.2). ORR (95% CI) for pts with measurable disease was 6% (2.6-11.5) in C1 and 3% (0.4-10.4) in C2 (Table; includes other efficacy results). Treatment-related AEs of any grade/grade 3-5 occurred in 57%/16% in C1, 60%/15% in C2, and 71%/17% in C3. 1 pt in each cohort died of a treatment-related AE (C1, sepsis; C2, unknown; C3, immune-related pneumonitis). Conclusions: With additional follow-up, pembro monotherapy continued to show antitumor activity and disease control in pts with RECIST-measurable and bone-predominant mCRPC previously treated with both NHA and docetaxel. Pts experienced durable responses. Safety was consistent with the known safety profile of pembro. Clinical trial information: NCT02787005

^aProduct-limit (Kaplan-Meier) method for censored data