Metformin as Maintenance Therapy in Patients With Bone Sarcoma and High Risk of Relapse

Inclusion Criteria: * Non metastatic osteosarcoma patients with necrosis ≤ 60 % in the end of post-operatory chemotherapy treatment (within 45 days.from the end of chemotherapy treatment). * Osteosarcoma or Ewing sarcoma patients free from the disease after first relapse (within 45 days.from the local surgery or 45 days from the end of chemotherapy treatment). * ≥ 14 years. * Patient should not be participating in other clinical study. * Patients should be able to swallow. * Screening up to 30 days from the end of chemotherapy treatment or relapse. * Start therapy up to 30 days from the screening. * Normal renal function (creatinine \<1,3 mg/L, clearance creatinine ≥ 70 ml/min) end liver function (total serum bilirubina \<1,2 mg/dL, except Gilbert syndrome patients), AST e ALT \<1,8 times the normal range value Exclusion Criteria: * Type B diabetic patient * Metastatic patients * Patients with acute metabolic acidosis (lactic acidosis, diabetic ketoacidosis). * Patients with renal insufficiency (GFR \< 70 ml/min). * Patients with acute conditions that could cause kidney alterations as dehydration, severe infection and shock. * Patients with hepatic insufficiency as acute alcohol intoxication and alcoholism. * Patients with medical conditions that could cause tissue hypoxia: decompensated heart failure, respiratory failure, recent myocardial infarction, shock. * Patient who does not meet the inclusion criteria * Pregnant and breastfeeding patients, or young women at childbearing age who cannot exclude the state of pregnancy (possibly excluded by serum pregnancy test (BhCG test), in accordance with the recommendations of the CTFG.