A Feasibility Window Study of Pembrolizumab Prior to Second Evacuation for Post-molar Gestational Trophoblastic Neoplasia

Inclusion Criteria: 1. Written informed consent prior to initiation of any study procedures and willingness and ability to comply with the study schedule. 2. Age ≥18yrs 3. Postmolar GTN defined as recurrence or persistence of histologically confirmed CHM after primary surgical evacuation with no intervening treatment. 4. Postmolar GTN defined as plateau or rising human chorionic gonadotropin (hCG). Plateaued hCG is defined as four or more equivalent values of hCG over at least 3 weeks. Rising hCG is defined as two consecutive rises in hCG of 10% or greater over at least 2 weeks. 5. hCG under 20,000 IU/L 6. Low risk disease as defined by the Federation of Obstetrics and Gynecology (FIGO) 2000 risk scoring criteria (score of 6 or less) 7. No metastatic disease on chest X-ray. 8. Eastern Cooperative Oncology Group (ECOG) performance status of 0-1 9. Disease present within the uterine cavity not within 5mm of the serosal surface. 10. Adequate bone marrow reserve or organ function as defined by any one of the following parameters: * Absolute neutrophil count ≥ 1.5 x 10\^9 /L; * Platelet count ≥ 100 x 10\^9 /L; * Haemoglobin ≥ 9.0 g/dL (may have been blood transfused) * Creatinine clearance ≥ 30 ml/min (Cockcroft-Gault formula) * Serum bilirubin ≤ 1.5 x ULN * AST/ALT ≤ 2.5 X ULN 11. All patients must agree to a highly effective method of contraception, or to complete abstinence\* for 1 year following second evacuation. This is standard practice following second evacuation of GTN because hCG levels rise in pregnancy thus masking a potential cancer recurrence Exclusion Criteria: 1. Patients with other invasive malignancies, with the exception of non-melanoma skin cancer, patients who have had any evidence of the other cancer present within the last 2 years or patients whose previous cancer treatment contraindicates this protocol therapy. 2. Patients with histologically confirmed choriocarcinoma, placental site trophoblastic tumor (PSTT) or epithelioid trophoblastic tumor (ETT) on the first curettage. 3. Pregnant women. 4. Uncontrolled vaginal bleeding. 5. Administration of live vaccine within 30 days prior to the first dose of study drug. Examples of live vaccines include, but are not limited to, the following: measles, mumps, rubella, varicella/zoster (chicken pox), yellow fever, rabies, Bacillus Calmette-Guérin (BCG), and typhoid vaccine. Seasonal influenza vaccines for injection are generally killed virus vaccines and are allowed; however, intranasal influenza vaccines (eg, FluMist®) are live attenuated vaccines and are not allowed. 6. History of immunodeficiency or receiving chronic systemic steroid therapy (in dosing exceeding 10 mg daily of prednisone equivalent) or any other form of immunosuppressive therapy within 7 days prior to the first dose of study drug. 7. 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 and is allowed. 8. History of (non-infectious) pneumonitis that required steroids or has current pneumonitis. 9. History of Human Immunodeficiency Virus (HIV) infection. 10. Has a known history of Hepatitis B (defined as Hepatitis B surface antigen \[HBsAg\] reactive) or known active Hepatitis C virus (defined as HCV RNA \[qualitative\] is detected) infection. Note: no testing for Hepatitis B and Hepatitis C is required unless mandated by local health authority. 11. History of active TB (Bacillus Tuberculosis). 12. History or current evidence of any condition, therapy, or laboratory abnormality that might confound the results of the study, interfere with the subject's participation for the full duration of the study, or is not in the best interest of the subject to participate, in the opinion of the treating investigator. 13. History of allogenic tissue/solid organ transplant.