Inclusion Criteria:
* Have histologically confirmed adenocarcinoma of the breast that is either TNBC or HR positive/HER-2 negative. TNBC is defined as: ER/PR \<1% and HER-2 negative disease (IHC 0-1+ or 2+ with HER2/17 ratio on FISH ≤1.8) according to ASCO/CAP guidelines11,67. HR positive is defined as: ER/PR \>= 1% and HER-2 negative as per ASCO/CAP guidelines.
* Have received 0-2 lines of cytotoxic chemotherapy for metastatic breast cancer endocrine therapy and/or targeted therapy is allowed.
* Be willing and able to provide written informed consent/assent for the trial
* Be 18 years of age on day of signing informed consent.
* Have measurable disease based on RECIST 1.1.
* 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. 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 PI or designee.
* Have a performance status of 0 or 1 on the ECOG Performance Scale. Be willing to undergo tissue biopsies as mandatory as per protocol for patients with biopsy accessible disease.
* Must have \</= Grade 1 pre-existing peripheral neuropathy (as per CTCAE).
* Demonstrate adequate organ function as defined in all screening labs should be performed within 10 days of treatment initiation.
* Female subject of childbearing potential should have a negative urine or serum pregnancy within 72 hours prior to receiving the first dose of study medication. This applies even if the subject practices true abstinence\* from heterosexual contact. If the urine test is positive or cannot be confirmed as negative, a serum pregnancy test will be required.
* A female subject of childbearing potential is a sexually mature women who (1) has not undergone hysterectomy \[the surgical removal of the uterus\] or bilateral oophorectomy \[the surgical removal of both ovaries\] or (2) has not been naturally postmenopausal for at least 24 consecutive months \[i.e., has had menses at any time during the preceding 24 consecutive months\]. The female subject must: either commit to true abstinence\* from heterosexual contact (which must be reviewed on a monthly basis), or agree to use, and be able to comply with, effective contraception without interruption (2 methods of birth control), 28 days prior to starting IP therapy (including dose interruptions), and while on study medication or for a longer period if required by local regulations following the last dose of IP.
* Male subjects must practice true abstinence\* or agree to use a condom during sexual contact with a pregnant female or female of childbearing potential starting with the first dose of study therapy, during dose interruptions, and for up to 6 months following last dose of study therapy, even if he has undergone a successful vasectomy.
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.
* Taxane therapy within the past 3 months (90 days) prior to study Day 1.
* Has had prior chemotherapy, targeted small molecule therapy, or radiation 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.
* Note: 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 within the last five 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 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 pneumonitis requiring treatment with steroids or 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.
* Has a known history of Human Immunodeficiency Virus (HIV) (HIV 1/2 antibodies, testing not mandatory).
* 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
