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