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

Participants who discontinue study treatment after 24 months of therapy for reasons other
than disease progression or intolerability or who discontinue treatment after attaining a
Complete Response (CR) may be eligible for up to one year of retreatment after they have
experienced disease progression by tumor imaging.

Inclusion Criteria:

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

For Cohorts A and C:

- At least one systemic treatment for metastatic breast cancer

- 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 Cohort B:

- No prior systemic treatment for metastatic breast cancer

- Programmed cell death-ligand 1 (PD-L1)-positive mTNBC.

For Cohort C:

- PD-L1 strong positive mTNBC

For all cohorts:

- 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)

- Measurable metastatic disease

- 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

- Adequate organ function

Exclusion Criteria:

- 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

- Prior anti-cancer monoclonal antibody (mAb) therapy for direct anti-neoplastic
treatment within 4 weeks prior to study Day 1

- Prior chemotherapy, targeted small molecule therapy, or radiation therapy within at
least 2 weeks prior to study Day 1

- 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

- Active autoimmune disease requiring systemic treatment in past 2 years

- 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

- 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

- Radiographically-detectable central nervous system (CNS) metastases and/or
carcinomatous meningitis

- History of (non-infectious) pneumonitis that required steroids or current pneumonitis
or a history of interstitial lung disease

- Active infection requiring systemic therapy

- Known psychiatric or substance abuse disorders that would interfere with cooperation
with the requirements of the study

- 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

- 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

- Known history of human immunodeficiency virus (HIV)

- Known active Hepatitis B or C

- Received a live vaccine within 30 days of planned start of study treatment