Fecal Microbiota Transplant and Pembrolizumab for Men With Metastatic Castration Resistant Prostate Cancer.

All patients will be required to have a biopsy of a metastatic tumor deposit at study entry. Pembrolizumab will be administered at a dose of 200 mg as a 30 minute IV infusion every 3 weeks. Enzalutamide will be continued at dose of 160 mg orally every day. Patients who have neither rapid disease progression or disease response will undergo a fecal microbiota transplant, have a second biopsy (if medically feasible), and be re-treated with pembrolizumab.