Study of Pembrolizumab (MK-3475) Monotherapy for Metastatic Triple-Negative Breast Cancer (MK-3475-086/KEYNOTE-086)

Inclusion Criteria:

For the purposes of this study, neoadjuvant and/or adjuvant chemotherapy regimens do not count as a prior line of therapy.

For second line plus monotherapy (Parts 1 and 2):

* Has received at least one systemic treatment for metastatic breast cancer
* Has documented disease progression on or after the most recent therapy
* Prior treatment must include an anthracycline and a taxane in the neoadjuvant, adjuvant, or metastatic setting

For first line monotherapy (Part 1):

* Has received no prior systemic treatment for metastatic breast cancer
* Has PD-L1-positive mTNBC.

For second line plus monotherapy (Part 2):

\- Has PD-L1 strong positive mTNBC

For all parts:

* Has mTNBC confirmed by a central laboratory
* For biomarker analysis, adequate newly obtained core or excisional biopsy of a not-previously-irradiated metastatic tumor lesion (mandatory)
* Has measurable metastatic disease
* Has Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1
* Female participants of childbearing potential should be willing to use 2 methods of birth control or be surgically sterile, or abstain from heterosexual activity for the course of the study through 120 days after the last dose of study treatment
* Male participants should agree to use an adequate method of contraception starting with the first dose of study treatment through 120 days after the last dose of study treatment
* Has adequate organ function

Exclusion Criteria:

* Is currently participating and receiving study treatment, or has participated in a study of an investigational agent and received study therapy or used an investigational device within 4 weeks prior to study Day 1
* Has received prior anti-cancer monoclonal antibody (mAb) therapy for direct anti-neoplastic treatment within 4 weeks prior to study Day 1
* Has received prior chemotherapy, targeted small molecule therapy, or radiation therapy within at least 2 weeks prior to study Day 1
* Has not recovered (i.e., ≤ Grade 1 or at baseline) from adverse events due to agents administered within at least 2 weeks prior to study Day 1
* Has an active autoimmune disease requiring systemic treatment in past 2 years
* Has a diagnosis of immunodeficiency or receiving systemic steroid therapy or any other form of immunosuppressive therapy within 7 days prior to the first dose of study treatment
* Has known additional malignancy that progressed or required active treatment within the last 5 years. Exceptions include basal cell carcinoma of the skin, squamous cell carcinoma of the skin that has undergone potentially curative therapy, or in situ cervical cancer
* Has radiographically-detectable central nervous system (CNS) metastases and/or carcinomatous meningitis
* Has a history of (non-infectious) pneumonitis that required steroids or current pneumonitis or a history of interstitial lung disease
* Has an active infection requiring systemic therapy
* Has known psychiatric or substance abuse disorders that would interfere with cooperation with the requirements of the study
* Is pregnant, breastfeeding, or expecting to conceive or father children within the projected duration of the study, starting with the screening visit through 120 days after the last dose of study treatment
* Has received prior therapy with an anti-programmed cell death protein-1 (anti-PD-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 has participated in Merck MK-3475 (pembrolizumab) study
* Has a known history of human immunodeficiency virus (HIV)
* Has known active Hepatitis B or C
* Has received a live vaccine within 30 days of planned start of study treatment