Pembrolizumab in Treating Patients With Metastatic Castration Resistant Prostate Cancer Previously Treated With Enzalutamide

This phase II trial studies how well pembrolizumab works in treating patients with prostate cancer that has spread to other places in the body and keeps growing even when the amount of testosterone in the body is reduced to very low levels despite previous treatment with enzalutamide. Monoclonal antibodies, such as pembrolizumab, may block tumor growth in different ways by targeting certain cells.

PRIMARY OBJECTIVES: I. Measure the anti-cancer activity of pembrolizumab in men with metastatic, castration resistant prostate cancer. SECONDARY OBJECTIVES: I. To investigate immunological and genetic parameters to evaluate for possible markers and functional changes that are predictive of a clinical response or linked to response or resistance to PD-1 inhibition. II. To collect circulating tumor cells (CTCs) and determine the degree to which tumor characteristics are shared by the CTCs. III. Changes in T cell numbers, activation, and phenotype as measured in whole blood at diagnosis and throughout therapy. IV. Systemic inflammatory markers: serum interleukin (IL)-8, IL-6, IL-1, tumor necrosis factor (TNF) and transforming growth factor (TGF)-beta. V. Objective disease response by radiographs. VI. Prostate-specific antigen (PSA) progression free survival. VII. Overall survival. VIII. Microbiome and correlation with response. TERTIARY OBJECTIVES: I. Additional genetic (deoxyribonucleic acid \[DNA\], ribonucleic acid \[RNA\]) and protein analyses will be conducted to further evaluate immunotherapy and profile/characterize disease. OUTLINE: INITIAL TREATMENT PHASE: Patients who are progressing on enzalutamide will receive pembrolizumab intravenously (IV) over 30 minutes on day 1. Treatment repeats every 3 weeks for 4 courses in the absence of disease progression or unacceptable toxicity. Patients continue to receive standard of care enzalutamide orally (PO) daily. MONITORING PHASE: After completion of the initial treatment phase, patients continue to receive standard of care enzalutamide PO daily for the duration of the trial. RETREATMENT PHASE: Patients with disease response or stability after the initial treatment phase will receive pembrolizumab IV over 30 minutes on day 1. Treatment repeats every 3 weeks for an additional 4 courses in the absence of disease progression or unacceptable toxicity. Patients continue to receive standard of care enzalutamide PO daily for the duration of the trial. After completion of study treatment, patients are followed up at 30 days, and then every 12 weeks for 2.5 years.