Inclusion Criteria:

          -  Written informed consent and HIPAA authorization for release of personal health
             information. NOTE: HIPAA authorization may be included in the informed consent or
             obtained separately.

          -  Men and women ≥ 18 years of age at the time of informed consent.

          -  ECOG Performance Status of 0 or 1 within 28 days prior to registration.

          -  Histologic or cytologic diagnosis of metastatic breast cancer (unless metastatic
             disease is definitive from most recent scans)

          -  Has received no more than two lines of prior hormonal therapy for advanced
             non-resectable/metastatic disease or no more than two lines of prior chemotherapy for
             advanced non-resectable/metastatic disease. Prior fulvestrant is allowed. Combination
             therapy is considered as one regimen.

          -  Tumor is estrogen receptor positive (ER+) and/or (PR+), HER-2 negative (HER2-). ER and
             PR positivity is defined as >1%. HER-2 negative is defined as by IHC (0, 1+) or FISH.
             HER2 positive test result includes: Single-probe average HER2 copy number ≥6.0
             signals/cell; Dual-probe HER2/CEP17 ratio ≥2.0 with an average HER2; copy number ≥4.0
             signals/cell; Dual-probe HER2/CEP17 ratio ≥2.0 with an average HER2copy number <4.0
             signals/cell; or Dual-probe HER2/CEP17 ratio < 2.0 with an average HER2 copy number
             ≥6.0 signals/cell. Equivocal findings for IHC as 2+ should be reflexed to FISH.
             Equivocal results by FISH may be considered with approval from the
             Sponsor-Investigator.

          -  Measurable disease based on RECIST 1.1 within 28 days prior to registration. NOTE:
             Bone-only disease is allowed and biopsy is required. To meet RECIST 1.1 criteria,
             bone-only disease must have soft tissue component.

          -  Be willing to provide tissue from a newly obtained core or excisional biopsy of a
             metastatic tumor lesion. NOTE: Newly-obtained is defined as a specimen obtained up to
             6 weeks (42 days) prior to study registration. Subjects for whom newly-obtained
             samples cannot be provided (e.g. inaccessible or subject safety concern) may submit an
             archived specimen only upon agreement from the Sponsor-Investigator.

          -  Normal cardiac function as determined by treating physician per institutional
             standards via Multiple Gated acquisition (MUGA) scan or echocardiogram (ECHO)
             performed within 28 days prior to registration.

          -  Prior chemotherapy or targeted therapy, no more than one line, must be completed at
             least 28 days prior to registration and the subject must have recovered from all
             reversible acute toxic effects of the regimen (other than alopecia) to ≤Grade 1 or
             baseline.

          -  No more than one prior hormonal therapy (only aromatase inhibitors with or without
             ovarian suppression or tamoxifen allowed) or radiation therapy must be completed at
             least 14 days prior to registration and the subject must have recovered from all
             reversible acute toxic effects of the regimen to ≤Grade 1 or baseline.

          -  Absolute Neutrophil Count (ANC) ≥ 1500/mm3

          -  Platelets ≥100,000 / mcL

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

          -  Serum creatinine OR Measured or calculateda creatinine clearance (GFR can also be used
             in place of creatinine or CrCl) ≤1.5 X upper limit of normal (ULN) OR

             ≥30 mL/min for subjects with creatinine levels > 1.5 X institutional ULN

          -  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

          -  International Normalized Ratio (INR) or Prothrombin Time (PT) ≤1.5 X ULN unless
             subject is receiving anticoagulant therapy as long as PT or PTT is within therapeutic
             range of intended use of anticoagulants

          -  Activated Partial Thromboplastin Time (aPTT) International Normalized Ratio (INR) or
             Prothrombin Time (PT)

          -  Activated Partial Thromboplastin Time (aPTT) ≤1.5 X ULN unless subject is receiving
             anticoagulant therapy as long as PT or PTT is within therapeutic range of intended use
             of anticoagulants

             ≤1.5 X ULN unless subject is receiving anticoagulant therapy as long as PT or PTT is
             within therapeutic range of intended use of anticoagulants

          -  Females of childbearing potential must have a negative urine or serum pregnancy test
             within 72 hours prior to study registration. If the urine test is positive or cannot
             be confirmed as negative, a serum pregnancy test will be required.NOTE: Females are
             considered of child bearing potential unless: they are postmenopausal; are surgically
             sterile; or they have a congenital or acquired condition that prevents childbearing.
             NOTE: Abstinence is acceptable if this is the usual lifestyle and preferred
             contraception for the subject.

          -  Females and males of reproductive potential must be willing to abstain from
             heterosexual activity or agree to use an adequate method of contraception as outlined
             in the protocol. Hormonal contraceptives are contraindicated in this population and
             are not allowed. Contraception will begin from the time of informed consent through
             120 days after the last dose of study drug(s).

        Exclusion Criteria:

          -  Is currently receiving an investigational agent or has received an investigational
             agent or used an investigational device within 28 days of study registration.

          -  Has a diagnosis of immunodeficiency or is receiving systemic steroid therapy or any
             other form of immunosuppressive therapy within 7 days prior to study registration.

        Subjects are permitted to enroll if they have vitiligo, type I diabetes mellitus, residual
        hypothyroidism due to autoimmune condition only requiring hormone replacement, psoriasis
        not requiring systemic treatment, or conditions not expected to recur in the absence of an
        external trigger.

        NOTE: Replacement therapy (e.g., thyroxine, insulin, or physiologic corticosteroid
        replacement therapy for adrenal or pituitary insufficiency, etc.) is not considered a form
        of systemic treatment.

          -  Has a known history of active TB (Bacillus Tuberculosis). NOTE: TB testing is not
             required.

          -  Has a known history of Human Immunodeficiency Virus (HIV) (HIV 1/2 antibodies). NOTE:
             HIV testing is not required.

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

        NOTE: Hepatitis B and Hepatitis C testing is not required.

          -  Hypersensitivity to pembrolizumab or any of its excipients.

          -  Has received prior chemotherapy within 28 days prior to study registration or has
             received prior hormonal/targeted therapy within 14 days prior to study registration

          -  More than two lines of chemotherapy or more than two lines of hormonal therapy
             excludes participation.

          -  Has a known additional malignancy that is progressing or requires active treatment.
             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 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 known history of, or any evidence of active, non-infectious pneumonitis.

          -  Has known history of, or any evidence of active interstitial lung disease, Class II-IV
             congestive heart failure, or myocardial infarction within 6 months from randomization.

          -  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.

          -  Breastfeeding during the projected duration of the trial, starting with the screening
             visit through 120 days after the last dose of trial treatment.

        NOTE: breast milk cannot be stored for future use while the mother is being treated on
        study.

          -  Has received prior therapy with an anti-PD-1, anti-PD-L1, or anti-PD-L2 agent

          -  Has received a live vaccine within 30 days of study registration. 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.