Phase I Evaluating the Combination of Ribociclib+Capecitabine in Locally Advanced/Metastatic Breast Cancer HER2 Negative

The proposed study is a multicenter, open-label phase I trial, conducted in locally advanced or metastatic breast cancer HER2 negative patients and divided into 2 parts: * STEP 1: a dose escalation part (n= up to 30) to evaluate the safety profile and pharmacokinetics and to define the MTD and RP2D to recommend in a phase II. * STEP 2: an expansion cohort part to confirm the safety and tolerability of ribociclib and capecitabine association on a longer follow-up, and to obtain preliminary evidence of anti-tumor activity on two expanded cohorts of HR positive and HR negative patients. Up to 14 patients in each cohort, taking into account patients already included in step one at this DL, may be enrolled, for a total of 28 at the RP2D.