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

PRIMARY OBJECTIVE: To determine the anticancer effect of fecal microbiota transplant from participants who respond to pembrolizumab into those who have not responded in metastatic castration resistant prostate cancer. OUTLINE: The investigators propose to study the effects of fecal microbiota transplant (FMT) in patients whose disease does not respond to treatment with the combination of pembrolizumab and enzalutamide. Patients will remain on enzalutamide throughout the study and be treated with pembrolizumab for 4 cycles. Their disease will be assessed by tumor imaging. Patients whose disease responds to treatment will become stool donors to non-responders. Non-responders will undergo a second biopsy (if medically feasible) and be re-treated with pembrolizumab.