Effect of Pembrolizumab (Keytruda®) on Biomarkers in Early Breast Cancer.

Inclusion Criteria: 1. Be willing and able to provide written informed consent/assent for the trial. 2. Be 18 years of age on day of signing informed consent. 3. Be willing to provide tissue from a newly obtained core or excisional biopsy of a tumor lesion if needed. Newly obtained is defined as a specimen obtained up to 6 weeks (42 days) prior to initiation of treatment on Day 0. 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. 4. Have a performance status of 0 or 1 on the ECOG Performance Scale. 5. Have non-metastatic operable newly diagnosed primary invasive carcinoma of the breast that is: 1. Histologically confirmed 2. ER/PR negative or ER positive. ER/PR status will be evaluated with Allred score (semi-quantitative measurement) following ASCO CAP guidelines 2009. 3. HER2 negative of positive. HER2 status will be evaluated using IHC followed by FISH with dual probe (ASCO CAP guidelines 2013). 4. Primary tumor size greater than 1 cm, measured by any of clinical examination, mammography, ultrasound or magnetic resonance imaging 5. Any clinical nodal status 6. Have evaluable core biopsy for IHC 7. Be willing to provide plasma/blood samples 8. After neo-adjuvant chemotherapy (cohort B1 and B2) patients must have residual tumor \>1cm and must be willing to provide evaluable new tumor biopsy for IHC 9. Demonstrate adequate organ function, all screening labs should be performed within 14 days of treatment initiation. 10. Female subject of childbearing potential should have a negative urine or serum pregnancy test within 72 hours prior to receiving the study medication. If the urine test is positive or cannot be confirmed as negative, a serum pregnancy test will be required. 11. Female subjects of childbearing potential (Section 5.7.2) must be willing to use an adequate method of contraception as outlined in Section 5.7.2 - Contraception, for the course of the study through 120 days after receiving the study medication. Note: Abstinence is acceptable if this is the usual lifestyle and preferred contraception for the subject. 12. Male subjects of childbearing potential (Section 5.7.1) must agree to use an adequate method of contraception as outlined in Section 5.7.1- Contraception, until 120 after receiving the study therapy. Note: Abstinence is acceptable if this is the usual lifestyle and preferred contraception for the subject. Exclusion Criteria: 1. 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 receiving the treatment dose. 2. Has a diagnosis of immunodeficiency or is receiving systemic steroid therapy or any other form of immunosuppressive therapy within 7 days prior to receiving the trial treatment. 3. Has a known history of active TB (Bacillus Tuberculosis) 4. Hypersensitivity to pembrolizumab or any of its excipients. 5. 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. 6. Has had prior chemotherapy, targeted small molecule therapy, or radiation therapy within 2 weeks prior to study Day 0 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. 7. 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. 8. 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. 9. Has known history of (non-infectious) pneumonitis that required steroids or current pneumonitis. 10. Has an active infection requiring systemic therapy. 11. 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. 12. Has known psychiatric or substance abuse disorders that would interfere with cooperation with the requirements of the trial. 13. 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. 14. Has received prior therapy with an anti-PD-1, anti-PD-L1, anti-PD-L2 agent or co-inhibitory T-cell receptor therapy (e.g. OX40-CD137, CTLA-4) 15. Has a known history of Human Immunodeficiency Virus (HIV) (HIV 1/2 antibodies). 16. Has known active Hepatitis B (e.g., HBsAg reactive) or Hepatitis C (e.g., HCV RNA \[qualitative\] is detected). 17. 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.