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 therapy for their post-transplant
relapse.

- 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.

- Must have been off immunosuppressive therapy for ≥ 7 days before receiving first dose
of pembrolizumab

- 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 cytotoxic chemotherapy for 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 been on immunosuppressant therapy within 7 days prior to the first dose of
pembrolizumab

- 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, targeted small molecule therapy, or radiation therapy
within 2 weeks 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