Safety and Efficacy Study of Pembrolizumab (MK-3475) in Combination With Chemotherapy as Neoadjuvant Treatment for Participants With Triple Negative Breast Cancer (TNBC) (MK-3475-173/KEYNOTE-173)

Inclusion Criteria:

* Has previously untreated, locally advanced TNBC.
* Is able to provide 2 core needle biopsies from the primary tumor at screening to the central laboratory and agrees to have a core needle biopsy after single dose pembrolizumab treatment if tumor biopsy is feasible as judged by the investigator.
* Has Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1.
* Has adequate organ function.
* Females of childbearing potential must be willing to use adequate contraception for the course of the study through 12 months after the last dose of study drug for participants receiving cyclophosphamide and through 6 months after the last dose of study drug for participants who do not receive cyclophosphamide.

Exclusion Criteria:

* Has evidence of metastatic breast cancer, concurrent bilateral invasive breast cancer, or inflammatory breast cancer.
* Has another malignancy within the last 5 years. Exceptions include basal cell carcinoma of the skin, squamous cell carcinoma of the skin that has undergone potentially curative surgery, or in situ cervical cancer.
* Has received prior chemotherapy, targeted therapy, radiation therapy, immunotherapy that targets immune checkpoints, co-stimulatory or co-inhibitory pathways for T cell receptors within the past 12 months.
* Is currently participating and receiving study therapy, or has participated in a study of an investigational agent and received study therapy or used an investigational device within 4 weeks of the first dose of study drug.
* Has received a live vaccine within 30 days of the first dose of study drug.
* Has an active autoimmune disease that has required systemic treatment in past 2 years.
* Has a diagnosis of immunodeficiency or is receiving systemic steroid therapy or any other form of immunosuppressive therapy within 7 days prior to the first dose of study drug.
* Has a known history of Human Immunodeficiency Virus (HIV).
* Has known active Hepatitis B or Hepatitis C.
* Has evidence of current pneumonitis.
* Has a history of non-infectious pneumonitis requiring treatment with steroids or a history of interstitial lung disease.
* Has an active infection requiring systemic therapy.
* Has significant cardiovascular disease, such as: History of myocardial infarction, acute coronary syndrome or coronary angioplasty/stenting/bypass grafting within the last 6 months; Congestive heart failure (CHF) New York Heart Association (NYHA) Class II-IV or history of CHF NYHA class III or IV
* Has known psychiatric or substance abuse disorders that would interfere with cooperation with the requirements of the study.
* Is pregnant or breastfeeding, or expecting to conceive children within the projected duration of the study, starting with the screening visit through 12 months after the last dose of trial treatment for participants who have received cyclophosphamide, and for six months after the last dose of study medication for participants who have not.
* Has a known hypersensitivity to the components of the study drug or its analogs.