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%, PR <1%, her-2-neu
0-1+ by IHC or FISH-negative or as per MD discretion) at each enrolling institution.

- Metastatic or locally recurrent TNBC.

- Subjects who are resistant to conventional chemotherapy or have declined conventional
therapy for TNBC. Patients having received any prior line of systemic therapy for
inoperable/recurrent or metastatic disease are eligible.

- At least one tumor for which palliative RT is considered appropriate standard therapy.

- At least one index lesion that will not undergo RT and which is measurable based on
RECIST 1.1.

- If an archived tumor tissue is available, be willing to provide tissue from a newly
obtained core or excisional biopsy of a tumor lesion. Newly-obtained is defined as a
specimen obtained up to 6 weeks (42 days) prior to initiation of treatment on Day 1 of
RT.

- Repeat palliative RT will be permitted for the treatment of isolated, non-target
lesions.

- Consent for blood draws for research purposes.

- Consent for use of available archived tissue for research purposes.

- Have a performance status of 0 or 1 on the ECOG Performance Scale.

- Demonstrate adequate organ function; all screening labs should be performed within 28
days (+7 days) of treatment initiation, unless otherwise indicated.

Hematological

- Absolute neutrophil count (ANC) ≥1.5, k/mcL

- Platelets ≥100 k,/ mcL

- Hemoglobin ≥9 g/dL or ≥5.6 mmol/L without transfusion or EPO dependency (within 7 days
of assessment) Renal

- Serum creatinine OR Measured or calculated^a creatinine clearance (GFR can also be
used in place of creatinine or CrCl) ≤1.5 X upper limit of normal (ULN) OR ≥60 mL/min
for subject with creatinine levels > 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 therapy for adrenal or pituitary insufficiency, etc.) is not considered a
form of systemic treatment.

- Has known history of/active, non-infectious pneumonitis requiring treatment with
steroids or has history of/active interstitial lung disease.

- Has an active infection requiring systemic therapy.

- Has a history or current evidence of any condition, therapy, or laboratory abnormality
that might confound the results of the trial, interfere with the subject's
participation for the full duration of the trial, or is not in the best interest of
the subject to participate, in the opinion of the treating investigator.

- Has known psychiatric or substance abuse disorders that would interfere with
cooperation with the requirements of the trial.

- Is pregnant or breastfeeding, or expecting to conceive or father children within the
projected duration of the trial, starting with the pre-screening or screening visit
through 120 days after the last dose of trial treatment.

- Has received prior therapy with an anti-PD-1, anti-PD-L1, or anti-PD-L2 agent or has
participated in another Merck pembrolizumab clinical trial.

- Has a known history of Human Immunodeficiency Virus (HIV) (HIV 1/2 antibodies).

- Has known active Hepatitis B (e.g., HBsAg reactive) or Hepatitis C (e.g., HCV RNA
[qualitative] is detected).

- Has received a live vaccine within 30 days of planned start of study therapy.

Note: Seasonal influenza vaccines for injection are generally inactivated flu vaccines and
are allowed; however intranasal influenza vaccines (e.g., Flu-Mist®) are live attenuated
vaccines, and are not allowed.