Inclusion Criteria

DISEASE CHARACTERISTICS:

- Histologically confirmed adenocarcinoma of the prostate

- Locally advanced or metastatic disease with no curative therapy possible

- PSA progression defined as the following:

- Increase in PSA of ≥ 25% (and an absolute increase of ≥ 2 ng/mL) over nadir
value on hormonal therapy measured on 3 successive occasions at least 1 week
apart

- If the third measurement is not higher than the second, a fourth
measurement will be taken and only if the fourth measurement is higher than
the second, the patient may be enrolled

- PSA doubling time ≥ 55 days (if used to define progression, must not be older than 6
months)

- PSA < 114 ng/mL

- Testosterone level ≤ 1.7 nmol/L (≤ 50 ng/dL) after at least 1 hormonal treatment
(orchiectomy or luteinizing hormone-releasing hormone [LHRH] agonist)

- Patients who have not undergone surgical castration must continue LHRH agonist
therapy during study treatment

- Oligosymptomatic or asymptomatic in relation to disease

- No known or suspected CNS metastases

PATIENT CHARACTERISTICS:

- WHO performance status 0-1

- Hemoglobin ≥ 90 g/L

- Neutrophil count ≥ 1.5 x 10^9/L

- Platelet count ≥ 100 x 10^9/L

- AST ≤ 2.5 times upper limit of normal (ULN)

- Bilirubin ≤ 1.5 times ULN

- Creatinine clearance ≥ 60 mL/min

- Compliant and geographically proximal for proper staging and follow-up

- No previous malignancy within the past 2 years except for localized nonmelanoma skin
cancer or Ta or Tis bladder cancer

- No history of diabetic ketoacidosis, diabetic coma, or pre-coma

- No known history of HIV, hepatitis B, or hepatitis C positivity

- No known hypersensitivity to the trial drug or any of its components

- No serious underlying medical condition that, in the judgment of the investigator,
would impair the ability of the patient to participate in the trial (e.g.,
uncontrolled or acute severe infection, uncontrolled diabetes, advanced chronic
obstructive pulmonary disease [COPD], or heart failure)

- No psychiatric disorder precluding understanding of information on trial related
topics, giving informed consent, or interfering with compliance for oral drug intake

- No known alcohol abuse

PRIOR CONCURRENT THERAPY:

- See Disease Characteristics

- At least 6 weeks since prior antiandrogen therapy and without withdrawal response

- At least 30 days since prior treatment in another clinical trial

- At least 4 weeks since prior major surgery

- At least 4 weeks since prior products known to affect PSA levels

- At least 2 weeks since prior local radiation

- No prior chemotherapy, radioisotopes, small molecules, or immunotherapy for prostate
cancer

- No prior metformin hydrochloride

- No concurrent pharmacotherapy for diabetes mellitus

- No concurrent finasteride, dutasteride, ketoconazole, or abiraterone acetate

- No concurrent corticosteroids with an equivalent dose of > 7.5 mg of prednisolone

- No concurrent radiotherapy

- No bisphosphonates started after registration

- No concurrent drugs contraindicated for use with the trial drug according to the
Swissmedic approved product information

- No other concurrent anticancer drugs

- No other concurrent experimental or investigational drugs