Phase II Trial of Pembrolizumab (MK-3475) in Subjects With Metastatic Castration-Resistant Prostate Cancer (mCRPC)  (KEYNOTE-199)

• Adenocarcinoma of prostate with no small-cell histology
• RECIST 1.1 Measurable disease on CT or MRI. Bone met only disease is not currently eligible.
• Metastatic or locally confined inoperable disease that cannot be treated with curative intent
• Failing or showing signs of failure on current pre-chemotherapy enzalutamide as defined by PCWG3 guidelines. May have failed prior abiraterone treatment before current enzalutamide treatment.
• Must have had a clinically meaningful response to enzalutamide therapy
• Has progression within 6 months prior to screening determined as either of the following:
  • PSA progression (3 rising PSA levels with an interval of at least 1 week between each assessment with screening level at 2 ng/mL or higher) OR
  • Radiographic disease progression in soft tissue or bone
• Testosterone level <50 ng/dL
• Must have been on stable doses of bone resorptive agents (if on them at all) for at least 4 weeks
• ECOG PS 0-2
• Adequate organ function
• No other malignancy within the previous 3 years
• No active autoimmune disease that required systemic treatment in past 2 years
• No known active CNS disease
• No prior receipt of immunotherapy
• No live vaccine within 30 days
• Measurable disease likely required (one cohort allows bone-only disease but may not be available)

Primary- ORR: Secondary- DCR, PSA response, Safety