Vibostolimab Coformulated with Pembrolizumab in Participants with Docetaxel-pretreated Metastatic Castration-resistant Prostate Cancer: KEYNOTE-365 Cohort G.

Novel therapeutic options are needed to extend disease control and survival for patients with metastatic castration-resistant prostate cancer (mCRPC) after docetaxel. Cohort G of the phase 1b/2 KEYNOTE-365 study evaluated the efficacy and safety of vibostolimab coformulated with pembrolizumab in participants with docetaxel-pretreated mCRPC. Eligible participants with mCRPC whose disease progressed following docetaxel received vibostolimab 200 mg coformulated with pembrolizumab 200 mg intravenously every 3 wk for up to 35 cycles. Primary endpoints were the prostate-specific antigen (PSA) response rate, the objective response rate (ORR) according to Response Evaluation Criteria in Solid Tumors (RECIST) v1.1 by blinded independent central review, and safety. Forty participants received the study treatment. The median follow-up was 10.2 mo (interquartile range 9.5-12.2). The confirmed PSA response rate in the overall cohort was 18% (95% confidence interval [CI] 7.3-33%). The ORR in the subgroup with RECIST-measurable disease (n = 29) was 6.9% (95% CI 0.80-23%). Treatment-related adverse events (TRAEs) occurred in 25 participants (63%); pruritus (25%), nausea (10%), and rash (7.5%) were the most common. Grade 3-5 TRAEs occurred in nine participants (23%). One treatment-related death (due to pulmonary embolism) occurred. Vibostolimab coformulated with pembrolizumab had limited antitumor activity in participants with mCRPC previously treated with docetaxel. The AE profile was manageable and consistent with the profiles of each agent. PATIENT SUMMARY: There is a need for new treatments for patients with metastatic prostate cancer that no longer responds to hormone therapy. We looked at whether a combination of two immunotherapy drugs called vibostolimab and pembrolizumab has antitumor activity with manageable side effects in a group of these patients who had already received chemotherapy. We found that this combination had limited antitumor activity, with side effects as expected for the two drugs individually. The KEYNOTE-365 trial is registered on ClinicalTrials.gov as NCT02861573.