Validity of HER2-amplified Circulating Tumor Cells to Select Metastatic Breast Cancer Considered HER2-negative for Trastuzumab-emtansine (T-DM1) Treatment.

Inclusion Criteria: Inclusion criteria for screening: * Breast adenocarcinoma considered HER2-negative on the primary tumour or unknown status HER2 * A least one metastatic site and/or inoperable loco-regional relapse * Measurable disease (RECIST v1.1) * Age from 18 to 75 years * Performance status of 0-2 * Efficient contraceptive in non-menopause women Inclusion criteria for treatment : * At least 1 (Cohort " L ") or 3 (cohort " H ") HER2 amplified CTC * Performance status of 0-2 * Adequate cardiac function * Adequate hematological and biochemical blood tests Exclusion Criteria: * Life expectancy of less than 3 months * Previous history of any other stage III or IV invasive cancer * Male breast cancer * Uncontrolled brain metastases * Significant cumulated exposure to anthracyclines * Current or previous significant history of cardio-vascular/pulmonary disease * Previous use of trastuzumab