Background: For men with mCRPC, systemic therapies such as docetaxel and cabazitaxel improve survival, but more effective treatments are needed. KEYNOTE-365 (NCT02861573) is a phase 1b/2 study to examine the safety and efficacy of pembro in combination with 4 different study medications (cohorts A, B, C, D) in mCRPC. Previous data from cohort B with a median of 20 months of follow-up showed that pembro + docetaxel and prednisone was well tolerated and had antitumor activity in patients (pts) with mCRPC previously treated with abi or enza. New efficacy and safety data after an additional year of follow-up are presented. Methods: Cohort B enrolled pts who did not respond to or were intolerant to ≥4 weeks of abi or enza in the prechemotherapy mCRPC state and whose disease progressed within 6 months of screening (determined by PSA progression or radiologic bone/soft tissue progression). Pts received pembro 200 mg IV every 3 weeks (Q3W), docetaxel 75 mg/m² IV Q3W, and oral prednisone 5 mg twice daily. Primary end points were safety, PSA response rate (PSA decrease >50% from baseline), and ORR per RECIST v1.1 by blinded independent central review. Efficacy and safety were assessed in all pts as treated. Results: Of the 104 treated pts, median age was 68.0 years (range, 50-86), 23.1% had PD-L1–positive tumors (combined positive score ≥1), 25.0% had visceral disease, and 50.0% had measurable disease. Median time from enrollment to data cutoff was 32.4 months (range 13.9-40.3); 101 pts discontinued, primarily because of disease progression (77.9%). Efficacy outcomes are reported in the table below. Treatment-related adverse events (TRAEs) occurred in 100 pts (96.2%); the most frequent (≥30%) were diarrhea (41.3%), fatigue (41.3%), and alopecia (40.4%). Grade 3-5 TRAEs occurred in 46 pts (44.2%). Five pts (4.8%) died of AEs; 2 were treatment-related pneumonitis. Conclusions: After another year of follow-up, pembro + docetaxel and prednisone showed improved ORR and PSA response rates compared to the prior dataset in pts with mCRPC previously treated with abi or enza. Safety was consistent with known profiles of each agent and will be further evaluated in a phase 3 study (KEYNOTE-921). Clinical trial information: NCT02861573

^aDefined as CR+PR+SD+non-CR/non-PD ≥6 months.

^b+ indicates ongoing responder.