Study to Assess the Efficacy of Pembrolizumab Plus Radiotherapy in Metastatic Triple Negative Breast Cancer Patients

Inclusion Criteria

• Be willing and able to provide written informed consent/assent for the trial.
• Be female ≥ 18 years of age on day of signing informed consent.
• Histologically or cytologically-confirmed TNBC (defined as ER 1.5 X institutional ULN Hepatic
• Serum total bilirubin ≤ 1.5 X ULN OR Direct bilirubin ≤ ULN for subjects with total bilirubin levels > 1.5 ULN
• AST (SGOT) and ALT (SGPT) ≤ 2.5 X ULN OR ≤ 5 X ULN for subjects with liver metastases
• Albumin ≥2.5 mg/dL Coagulation
• International Normalized Ratio (INR) or Prothrombin Time (PT) ≤1.5 X ULN unless subject is receiving anticoagulant therapy as long as PT or aPTT is within therapeutic range of intended use of anticoagulants ≤1.5 X ULN unless subject is receiving anticoagulant therapy
• Activated Partial Thromboplastin Time (aPTT) ≤1.5 X ULN unless subject is receiving anticoagulant therapy as long as PT or aPTT is within therapeutic range of intended use of anticoagulants
• Female subject of childbearing potential should have a negative pregnancy test or documentation of absence of pregnancy by a gynecologist within 2 weeks of initiating radiation therapy.
• Female subjects 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 medication. Subjects of childbearing potential are those who have not been surgically sterilized or have not been free from menses for > 1 year.

Note: ^aCreatinine clearance should be calculated per institutional standard.

Exclusion Criteria

• 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 treatment.
• 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 trial treatment.
• Has a known history of active TB (Bacillus Tuberculosis)
• Hypersensitivity to pembrolizumab or any of its excipients.
• Has had a prior anti-cancer monoclonal antibody (mAb) within 4 weeks prior to study Day 1 or who has not recovered (i.e., ≥ Grade 1 or at baseline) from adverse events due to agents administered more than 4 weeks earlier.
• Has had prior chemotherapy or targeted small molecule therapy within 2 weeks prior to study Day 1 or who has not recovered (i.e., ≥ Grade 1 or at baseline) from adverse events due to a previously administered agent.

Note: Subjects with ≤ Grade 2 neuropathy are an exception to this criterion and may qualify for the study.

• If subject received major surgery, they must have recovered adequately from the toxicity and/or complications from the intervention prior to starting therapy.
• Has a known additional malignancy that progressed or required treatment in the last 5 years. Exceptions include basal cell carcinoma of the skin or squamous cell carcinoma of the skin that has undergone potentially curative therapy or in situ cervical cancer.
• Has known active central nervous system (CNS) metastases and/or carcinomatous meningitis. Subjects with previously treated brain parenchymal metastases may participate provided they are stable (without evidence of progression by imaging for at least four weeks prior to the first dose of trial treatment and any neurologic symptoms have returned to baseline), have no evidence of new or enlarging brain metastases, and are not using steroids for at least 7 days prior to trial treatment. This exception does not include carcinomatous meningitis, which is excluded regardless of clinical stability.
• Has 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). Replacement therapy (eg., thyroxine, insulin, or physiologic corticosteroid replacement ther