Augmentation of the Graft vs. Leukemia Effect Via Checkpoint Blockade With Pembrolizumab

Inclusion Criteria:

Acute Myeloid Leukemia (AML), Acute Lymphoblastic Leukemia (ALL) or Myelodysplastic Syndrome (MDS) in confirmed relapse
Confirmation of 'measurable disease'
Patient may not have received definitive salvage chemotherapy for their post-transplant relapse within the past 21 days.
Be willing and able to provide written informed consent/assent for the trial
Be ≥ 18 years of age on day of signing informed consent
Be willing to provide tissue from bone marrow biopsies
Have a performance status of 0, to 1 on the ECOG Performance Scale. Eastern Cooperative Oncology Group Performance Status: an attempt to quantify cancer patients' general well-being and activities of daily life. The score ranges from 0 to 5 where 0 is asymptomatic and 5 is death.
Demonstrate adequate organ function
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.
Female subjects of childbearing potential must be willing to use an adequate method of contraception
Male subjects of childbearing potential must agree to use an adequate method of contraception

Exclusion Criteria:

Has had relapse prior to primary neutrophil engraftment or ≤21 days post HCT.
Has received >1 line of chemotherapy or other treatment directed towards post-transplant relapse prior to study entry
Rapidly progressive relapse requiring urgent chemotherapy as determined by treating physician
Is currently participating and receiving study therapy of an investigational agent and received study therapy within 2 weeks of the first dose of treatment.
Has a diagnosis of active GvHD (≥ Grade I)
Receiving systemic steroid therapy of > 10mg prednisone daily or equivalent*
Has received GM-CSF within 14 days of first dose of pembrolizumab
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 from adverse events
Has had prior chemotherapy within 21 days or radiation therapy within 14 days prior to study Day 1 or who has not recovered from adverse events
Has a known additional (secondary) malignancy that is progressing or requires active treatment
Has known or suspected active central nervous system (CNS) metastases and/or carcinomatous meningitis
Has a history of (non-infectious) pneumonitis that required steroids, or current pneumonitis
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
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)
Has known active Hepatitis B or Hepatitis C
Has received a live vaccine within 30 days of planned start of study therapy