Gemcitabine at a dose of 1000 mg/m2 (iv, 30 minutes) will be given weekly, for 3 weeks, followed by one week without treatment. Erlotinib will be administered at a daily dose of 100 mg at least one hour before or 2 hours after the ingestion of food. Metformin/ placebo will be administered at a dose of 500 mg twice daily. If well tolerated the dose will be increased to 1000 mg twice daily in the second week.