Evaluation of Pembrolizumab in Lymphopenic Metastatic Breast Cancer Patients Treated With Metronomic Cyclophosphamide

The investigators hypothesize that the administration of pembrolizumab combined to metronomic cyclophosphamide may be an interesting therapeutic option for female patients breast cancer with lymphopenia. Include only patients with breast cancer with lymphopenia is based on the particularly poor prognosis of these patients. The approach suggested here is to deplete and active the immune response of these immunosuppressed patients. The combination of pembrolizumab and cyclophosphamide would provide a higher gain in anti-tumor response in these patients than in those without lymphopenia and in chemotherapy alone. The investigators proposal is to conduct a multicentric, non-comparative, single arm, two-stage, Phase II trial in lymphopenic patients with metastatic breast cancer. The study is divided in 2 parts: * a safety run-in Phase aiming to evaluate the safety of the combination therapy pembrolizumab + metronomic cyclophosphamide based on the occurrence of severe toxicities. * a two-stage Phase II aiming to evaluate the clinical activity of the combination therapy pembrolizumab + metronomic cyclophosphamide. In both parts of the study, patients will receive cyclophosphamide (50 mg/day, daily, per os) and pembrolizumab (200 mg every 3 weeks, intravenously \[IV\]). The adverse events of the two drugs are well known.