 For cohorts 0 and 1, gemcitabine is administered at the dose of 1000mgsqm from week 1 of treatment, 4 weeks in a row for the first month, then for 3 consecutive weeks followed by a week of pause. For all other cohorts, gemcitabine will be administered starting from week 2 of treatment at the dose of 1000mgsqm, for 3 consecutive weeks followed by a week of pause. 