Study of Single Agent Pembrolizumab (MK-3475) Versus Single Agent Chemotherapy for Metastatic Triple Negative Breast Cancer (MK-3475-119/KEYNOTE-119)

Inclusion Criteria

• Centrally confirmed Stage IV/M1 mTNBC
• Newly obtained tumor biopsy from metastatic site
• Central determination of programmed cell death ligand 1 (PD-L1) tumor status
• Received either one or two prior systemic treatments for metastatic breast cancer and have documented disease progression on or after the most recent therapy
• Previously treated with an anthracycline and/or taxane in the neoadjuvant/adjuvant or metastatic setting
• Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 within 10 days prior to study start
• Adequate organ function

Exclusion Criteria

• Participation in another clinical trial within 4 weeks
• Monoclonal antibody (mAb) for direct anti-neoplastic treatment within 4 weeks
• Chemotherapy, targeted small molecule therapy, or radiation therapy within at least 2 weeks
• Active autoimmune disease that required systemic treatment in the past 2 years
• Diagnosed with immunodeficiency or receiving systemic steroid therapy or another form of immunosuppressive therapy within 7 days
• Known additional malignancy that required treatment or progressed in last 5 years
• Known active brain metastases and/or carcinomatous meningitis
• Prior therapy with an anti-programmed cell death (PD)-1, anti-PD-ligand-1 (anti-PD-L1), anti-PD-L2 agent or with an agent directed to another co-inhibitory T-cell receptor (e.g. cytotoxic T-lymphocyte associated protein 4 [CTLA-4], OX-40, CD137) or previously participated in any pembrolizumab (MK-3475) clinical studies