MK-3475-756/KEYNOTE-756

• Has a history of non-infectious pneumonitis that required treatment with steroids or has current pneumonitis.
• Has breast cancer with lobular histology.
• Has bilateral invasive breast cancer.
• Has metastatic (Stage IV) breast cancer.
• Has multi-centric breast cancer (presence of more than 1 tumor in different quadrants of the breast).
• Has any of the following clinical lymph node staging per current American Joint Committee on Cancer (AJCC) staging criteria for breast cancer staging based on radiological and/or clinical assessment: cN3, cN3a, cN3b, or cN3c.
• Has ER-, progesterone receptor positive breast cancer.
• Has undergone excisional biopsy of the primary tumor and/or axillary lymph nodes or has undergone sentinel lymph node biopsy prior to study treatment.
• Has a known additional, invasive, malignancy that is progressing or required active treatment in the last 5 years.

Note: Participants with basal cell carcinoma of the skin, squamous cell carcinoma of the skin, ductal breast carcinoma in situ, or cervical carcinoma in situ that has undergone potentially curative therapy are not excluded.

• Has a diagnosis of immunodeficiency or is receiving chronic systemic steroid therapy (dosing exceeding 10 mg daily of prednisone equivalent) or any other form of immunosuppressive therapy within 7 days prior to the first dose of study treatment.
• Has an active autoimmune disease that has required systemic treatment in the past 2 years (i.e., with use of disease modifying agents, corticosteroids, or immunosuppressive drugs) Note: Replacement therapy (e.g. thyroxine, insulin, or physiologic corticosteroid replacement therapy for adrenal or pituitary insufficiency) is not considered a form of systemic treatment.
• Has a known history of active tuberculosis (Bacillus tuberculosis).
• Has an active infection requiring systemic therapy.
• Has left ventricular ejection fraction (LVEF) of <50% or below the institution limit of normal, as assessed by echocardiogram (ECHO) or multigated acquisition (MUGA) scan performed at screening.
• Has other significant cardiac disease, such as: 1) History of myocardial infarction, acute coronary syndrome, or coronary angioplasty/stenting/bypass within the last 6 months. or 2) Congestive heart failure (CHF) New York Heart Association (NYHA) Class II-IV or history of CHF NYHA Class III or IV.
• Has a known history of human immunodeficiency virus (HIV) infection.
• Has a known history of hepatitis B or known active hepatitis C virus infection.
• Has received prior treatment for breast cancer.
• Has received prior therapy with an anti-programmed cell death protein 1 (anti-PD-1), anti-programmed cell death-ligand 1 (anti-PD-L1), or anti-PD-L2 agent or with an agent directed to another stimulatory or co-inhibitory T-cell receptor (e.g. cytotoxic T-lymphocyte-associated protein 4 [CTLA-4], OX 40, CD137).
• Has received a live vaccine within 30 days prior to the first dose of study treatment.
• Has severe hypersensitivity (≥Grade 3) to any of the components or excipients used in the study treatments.
• Is/was enrolled in a study of an investigational agent and received study therapy, or used an investigational device within 4 weeks (12 months for an investigational agent or device with anticancer or antiproliferative properties) prior to the first dose of study treatment.
• Is pregnant, breastfeeding, or expecting to conceive or father children within the projected duration of the study, starting with the screening visit through 12 months (for participants who received cyclophosphamide) or 6 months (for participants who did not receive cyclophosphamide) after the last dose of study treatment.