Pembrolizumab in Metastatic Castration Resistant Prostate Cancer (mCRPC) With or Without DNA Damage Repair Defects

All subjects will receive pembrolizumab 200mg IV every 3 weeks until disease progression or unacceptable toxicity.

At time of progression, all subjects will also have the option of receiving taxane-based chemotherapy followed by repeat pembrolizumab for those who have a clinical response to chemotherapy. Chemotherapy regimen will be at the discretion of the treating physician, and may consist of docetaxel or cabazitaxel with or without a platinum agent (e.g. carboplatin). A minimum of 4 cycles and a maximum of 8 cycles of chemotherapy will be given.