Trial Targeting Gut Bacterial Androgen Production to Reverse Therapeutic Resistance to Abiraterone in Patients With Metastatic Prostate Cancer

Inclusion Criteria: * Males aged 18 years of age and above. * Prostate adenocarcinoma * Absolute PSA ≥ 2.0 ng/mL at screening. * PSA (+/- radiographic) progression after having been on abiraterone and prednisone for at least 12 weeks. * Must be maintained on a GnRH analogue or have undergone orchiectomy. * Participants must have a life expectancy ≥ 6 months * Ability to swallow study medication tablets * Willing to abstain from alcohol during and for 14 days after treatment with metronidazole * Willing and able to collect urine and stool samples per protocol Exclusion Criteria: * Active infection or other medical condition that would make dexamethasone use contraindicated * Any chronic medical condition requiring a higher systemic dose of corticosteroid * Pathological finding consistent with small cell carcinoma of the prostate * Has imminent or established spinal cord compression based on clinical findings and/or MRI. * Chronic liver disease with Child-Pugh class C cirrhosis (see calculator in protocol) * Bilirubin \>3x ULN or AST and ALT \>5x ULN * Congenital prolonged QTc syndrome or QTc \> 500 msec (non-paced rhythm) * History of pituitary or adrenal dysfunction * Uncontrolled diabetes (Hemoglobin A1c \> 10%) or increasing doses of insulin within the past 4 weeks due to poorly controlled glucoses. * Administration of an investigational therapeutic or invasive surgical procedure (not including surgical castration) within 30 days of Cycle 1 Day 1 or currently enrolled in an investigational drug study * Any other serious illness or medical condition that would, in the opinion of the investigator, make this protocol unreasonably hazardous, including, but not limited to: * Any uncontrolled major infection. * Crohn's disease or ulcerative colitis. * Known or suspected toxic megacolon and/or known small bowel ileus. * Known allergy to any of the compounds under investigation. * On antibacterial therapy within 30 days prior to administration of study treatment. * Any condition or situation which, in the opinion of the investigator, would put the subject at risk, or interfere with the subject's participation in this study.