Metformin in Castration-Resistant Prostate Cancer

Metformin is a medication that is prescribed for people with diabetes to help the body respond better to its own insulin and decrease sugar production by the liver. This helps control the body's blood sugar level and is approved by the Food and Drug Administration (FDA) for the treatment of diabetes. Participant's in this research study will already be receiving androgen deprivation therapy (ADT) for prostate cancer. ADT is considered standard of care for prostate cancer. Changes in the participant's metabolism, including changes in insulin and blood sugar levels, are often seen as a result of this type of hormone therapy. Some studies have shown a relationship between insulin and prostate cancer. These studies have suggested that insulin may signal tumor cells to grow. Other studies suggest that people receiving metformin treatment for diabetes may enjoy better outcomes from their prostate cancer then other similar patients who are not treated with metformin.

* Each treatment cycle lasts 4 weeks (28 days). Participants will take metformin twice a day for all 28 days (except for the first 7 days of the first cycle when they will take it just once daily.)
* The following procedures will be done on Day 1 of treatment cycles 1, 2, 4, 7 and 10: medical history review, performance status, physical exam, blood tests.
* If the participant's first computed tomography (CT) or bone scan at the time of screening showed evidence of cancer, CT and bone scans will be repeated on Day 1 of treatment cycles 4, 7 and 10.
* Participants will be in this research study for about 12 months.

Eligibility Criteria:
Inclusion Criteria:

* Histologically or cytologically confirmed adenocarcinoma of the prostate
* History of bilateral orchiectomies or ongoing treatment with a GnRH agonist for GnRH antagonist
* Disease progression according to PSA Working Group 2
* Minimum starting PSA (Prostate Specific Antigen) level of 2.0 ng/mL
* Eastern Cooperative Oncology Group (ECOG) Performance Status 0 or 1

Exclusion Criteria:

* Symptomatic metastases
* Receiving any other agents for the treatment of prostate cancer except gonadotropin releasing hormone (GnRH) agonist or antagonist within the last 30 days
* Received any investigational cancer treatment agents within the last 30 days
* Prior treatment with docetaxel
* History of diabetes requiring drug therapy
* Current treatment with metformin or metformin treatment within the last year
* History of allergic reaction to metformin
* Have uncontrolled intercurrent illness including, but not limited to ongoing or unstable angina pectoris, cardiac arrhythmia, or psychiatric illness/social situations that would limit compliance with study requirements
* Serum creatinine 1.5mg/dL or greater
* Hepatic impairment
* Need for ongoing treatment with cimetidine
* History of a different malignancy except for the following circumstances: Individuals with a history of other malignancies are eligible of they have been disease-free for at least 5 years and are deemed by the investigator to be at low risk for recurrence of that malignancy. Individuals with the following cancers are eligible if diagnosed and treated within the past 5 years: basal cell or squamous cell carcinoma of the skin