No accrual in initial period, PI decided to close study.

This study is being conducted to determine the effectiveness of standard 3 weekly treatments using docetaxel and prednisone with metformin in patients with castration-resistant metastatic prostate cancer.

It is also being conducted to determine the levels of toxicity of metformin when added to every 3 weekly docetaxel treatments in patients with endocrine resistent, metastatic prostate cancer.

This is a single-arm, non-randomized Phase II trial of docetaxel given every 3 weeks with metformin twice daily and Prednisone twice daily. The study duration of this trial is open ended. The patient would take the metformin only for as long as they are treated with docetaxel.

Inclusion Criteria:

• All patients, 18 of age or older, with hormone refractory prostate cancer are eligible.
• Patients must have a life expectancy of at least 12 weeks.
• Patients must have a Zubrod performance status of 0-2.
• Patients must sign an informed consent.
• Patients should have adequate bone marrow function defined by an absolute peripheral granulocyte count of > 1,500 or cells/mm3 and platelet count >100,000/mm3 and absence of a regular red blood cell transfusion requirement.
• Patients should have adequate hepatic function with a total bilirubin < 2 mg/dl and SGOT or SGPT < two times the upper limit of normal, and adequate renal function as defined by a serum creatinine < 1.5 x upper limit of normal.
• Patients must have pathologically confirmed prostate cancer with PSA increasing despite androgen deprivation therapy.
• Diabetic patients on therapy with Metformin are eligible provided that their Metformin dose is at least 850mg twice daily.

Exclusion Criteria:

• Patients with symptomatic brain metastases are excluded from this study.
• Patients on insulin are not eligible.
• Patients may receive no other concurrent chemotherapy or radiation therapy during this trial.
• Patients with severe medical problems such as uncontrolled diabetes mellitus, cardiovascular disease, and congestive heart failure defined as New York Heart Association Class III or IV, hepatic dysfunction, or active infections are not eligible for this trial.