Inclusion Criteria

Age: From 18 Years To N/A

Gender: Both

Inclusion Criteria

• Centrally confirmed 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

• Documented disease progression on 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

• 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 Merck pembrolizumab (MK-3475) clinical studies