Patients eligible for this protocol will be randomized to paclitaxel 175mg/m2 plus metformin or placebo until disease progression or unacceptable toxicity. The primary end-point is disease control (complete response, partial response or stable disease, according to RECIST 1.1) in the 12th week. Secondary end-points are PFS, OS, response rate and safety. Molecular markers in samples collected before and under treatment will be tested for correlation with clinical outcomes.