Antonarakis ES, Piulats JM, Gross-Goupil M, et al. - In three parallel cohorts of a larger metastatic castration-resistant prostate cancer (mCRPC) population, researchers evaluated pembrolizumab for its antitumor activity as well as safety in this phase II KEYNOTE-199 study. The participants received docetaxel and one or more targeted endocrine therapies. Patients with RECIST-measurable PD-L1(programmed death ligand 1)–positive and PD-L1–negative disease were included in the cohorts 1 and 2, respectively. Cases with bone-predominant disease, irrespective of PD-L1 expression, were included in cohort 3. Pembrolizumab 200 mg was administered to participants every 3 weeks for up to 35 cycles. In a subset of patients having RECIST-measurable and bone-predominant mCRPC earlier managed with docetaxel and targeted endocrine therapy, findings revealed the antitumor activity as well as the acceptable safety profile of pembrolizumab monotherapy. In cohort 1 and in cohort 2, the objective response rate was estimated to be 5% and 3%, respectively. In cohort 1, cohort 2, and, cohort 3, the disease control rate was reported to be 10%, 9%, and 22%, respectively, and the median overall survival (OS) was estimated to be 9.5 months, 7.9 months, and 14.1 months, respectively. The documented responses appeared to be durable, and OS estimates were encouraging.