Study Of Single-dose Cyclophosphamide +Pembrolizumab In Patients With Metastatic Triple Negative Breast Cancer

This phase II, single-arm, multicenter study will evaluate efficacy and toxicity of administration of pembrolizumab following cyclophosphamide therapy, in advanced stage triple-negative breast cancer. Duration of Therapy Treatment may continue until one of the following occurs: * Disease progression * Inter-current illness that prevents further administration of treatment * Unacceptable adverse event(s) * Pregnancy * Patient decides to withdraw from study treatment, * General or specific changes in the patient's condition render the patient unacceptable for further treatment in the judgment of the investigator * Completed 24 months of uninterrupted treatment with pembrolizumab or 35 administrations of study medication, whichever is later Duration of Follow Up * Subjects will be followed for up to 3 years after removal from study treatment or until death, whichever occurs first. * Patients removed from study for unacceptable adverse events (AEs) will be followed until resolution or stabilization of the event(s).