This study aims to see if metformin can delay the time to progression in men with low risk prostate cancer when compared to a placebo.

Inclusion Criteria:

1. Must be male > 18 and < 80 years of age
2. Have biopsy proven, low-risk, localized prostate cancer choosing expectant management as primary treatment ≤ 1year. [For the purposes of assessing subject eligibility a diagnostic biopsy must have included at least 10 cores, ≤1/3 of total number of cores sampled and < 50% of any one core positive) and must have been obtained within 6 months of screening]. Initial diagnosis of T1a/T1b obtained during a TURP is not allowed
3. Gleason score ≤ 6 [Gleason pattern 4 or above must not be present on any biopsy (initial or entry)]
4. Clinical stage T1c-T2a
5. Serum PSA ≤10 ng/mL (prior to biopsy)
6. Life expectancy greater than 5 years, as judged by the treating clinician/urologist
7. Able to swallow and retain oral medication
8. Hemoglobin A1c < 6.5%
9. Able and willing to participate in the full 3 years of the study
10. Able to understand instructions related to study procedures
11. Able to read and write (health outcome questionnaires are self-administered), understand instructions related to study procedures and give written informed consent

Exclusion Criteria:

1. Subject that has ever been treated for prostate cancer with any of the following:

   • Radiotherapy (external beam or brachytherapy)
   • Chemotherapy
   • Hormonal therapy (e.g., megestrol, medoxyprogesterone, cyproterone)
   • Oral glucocorticoids
   • GnRH analogues (e.g., leuprolide, goserelin, degarelix)

2. Current and/or previous use of the following medications:

   • Use of 5α-reductase inhibitors (eg. Finasteride, Dutasteride) within the past 6 months of screening
   • Drugs with antiandrogenic properties (e.g., flutamide, bicalutamide, ketoconazole, progestational agents) within 6 months prior to screening

3. Previous or current diagnosis of type 1 or type 2 diabetes

4. Exposure to metformin within 12 months of screening

5. Planned or concurrent use of metformin hydrochloride, sulfonylureas, thiazolidinediones, or insulin for any reason

6. Known hypersensitivity or intolerance to metformin hydrochloride

7. Any condition associated with increased risk of metformin hydrochloride-associated lactic acidosis (e.g. congestive heart failure defines as NYHA class III or IV, history of any type of acidosis, habitual intake of ≥ 4 alcoholic beverages per day)

8. Subject has had prior prostatic surgery including TUNA, TURP, TUIP, laser treatment, thermotherapy, balloon dilatation, prosthesis, and ultrasound ablation within 3 months of screening

9. Participation in any investigational or marketed drug trial within 30 days prior to screening or anytime during the study period. This includes any interventional or exercise trials

10. Any unstable serious co-existing medical condition(s) 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 visit

11. Abnormal liver function test:

    • Total bilirubin > 1.8 X institutional upper limit of normal (ULN)
    • Aspartate aminotransferase (AST) > 1.8 X institutional ULN
    • Alanine aminotransferase (ALT) > 1.8 X institutional ULN
    • Alkaline phosphatase (ALP) > 1.8 X institutional ULN

12. Serum creatinine > 1.8 X ULN

13. History of other malignancies, with the exception of adequately treated nonmelanoma skin cancer, stage I melanoma, NMIBC or other solid tumors curatively treated with no evidence of disease for at least 5 years

14. History or current evidence of substance abuse, as defined in DSM-IV, within 12 months of screening

15. History of any illness (including psychiatric) that, in the opinion of the investigator, might confound the results of the study or pose additional risk to the subject

16. No other concurrent metformin hydrochloride, sulfonylureas, thiazolidinediones, or insulin for any reason