This is a Phase II clinical study to determine if Metformin can increase Prostate Specific Antigen (PSA) doubling time for patients with Prostate Cancer who have failed primary treatment with radiation, or surgical patients that are at high risk for recurrence based on surgical pathology. Men with confirmed prostate cancer and rising serum PSA levels will receive Metformin and will be monitored for PSA response and disease progression.

Prostate cancer patients who have received treatment with radiation therapy or surgery, who have indicators of high-risk disease will be administered 750mg Metformin Extended Release twice per day for a period of 9 months. Metformin is an FDA-approved drug that is prescribed to treat high blood sugar levels in patients with Type 2 Diabetes. To track prostate cancer response in study participants, investigators will obtain prostate specific antigen (PSA) levels every three months for the duration of the trial.

Inclusion Criteria:

• Male > 18 years of age
• Biopsy confirmed adenocarcinoma of the prostate
• Able to swallow and retain oral medication
• Hemoglobin A1C <7.0%
• Able and willing to participate in the full 12 months of the study
• Able to understand instructions related to study procedures
• Able to read and write English

Exclusion Criteria:

• Metastatic Prostate Cancer
• Prostate Specific Antigen Doubling Time < 6 months
• Prior Chemotherapy, hormonal therapy, oral glucocorticoid therapy, Gonadotropin-releasing hormone analogue therapy
• Current or previous use of 5α-reductase inhibitors, antiandrogen drugs, metformin, oral or injectable diabetes drug
• Diagnosis of Type 1 Diabetes Mellitus
• Known hypersensitivity to metformin
• any condition associated with increased risk of metformin associated lactic acidosis
• participation in any investigational or marketed drug trial within 30 days prior to screening or during study period
• any unstable, serious co-existing medical conditions including but not limited to myocardial infarction, coronary bypass surgery, unstable angina, cardiac arrhythmias, clinically evident congestive heart failure, or cerebrovascular accident within 6 months prior to screening
• history of megaloblastic anemia
• abnormal liver function test (total bilirubin, aspartate aminotransferase, alanine aminotransferase, alkaline phosphatase or serum creatinine above upper limit of normal)
• history of other malignancies, with the exception of adequately treated non-melanoma skin cancer, stage 1 melanoma, non-muscle invasive bladder cancer, or other solid tumors curatively treated with no evidence of disease for at least 5 years
• history or current evidence of substance abuse within 12 months of screening
• history of any illness (including psychiatric) that, in the opinion of the investigator, might confound the results of the study or post an additional risk to the subject

9 months

Adverse events were monitored on a monthly basis