Study of Pembrolizumab Following Surgery in Patients With Microsatellite Instability High (MSI-H) Solid Tumors

Inclusion Criteria: * Age 18 years or older * ECOG performance status 0-1 * Any solid tumor with MSI or MRD by IHC, PCR or NGS testing. MSKCC confirmation of MSI-H/MRD status is not mandatory prior to enrollment and treatment on the study. For patients with outside testing, if sufficient tissue is available NGS will be repeated at MSKCC and will not impact the patient's eligibility. * Must have genetic testing of DNA from primary tumor for somatic genomic alterations across a minimum of 50 genes. * Must have undergone a complete curative surgical resection (R0) * Must have completed standard of care (SOC) surgery, neoadjuvant or adjuvant therapy * To be eligible for pembrolizumab or placebo therapy, the patients must have positive ctDNA (as defined in section 7.0) within 4 months after completion of appropriate standard of care therapy (surgery, chemotherapy, radiation as appropriate). Note, if ctDNA has a negative result, ctDNA can be re-tested up to 3 months later, within 9 months of completion of standard therapy. * Patients must sign informed consent within 4 weeks of positive ctDNA result. The 4 weeks is considered from the date that the ctDNA is resulted, and not the date it is drawn. * Demonstrate adequate organ function: * Absolute neutrophil count (ANC) ≥1,500 /mcL * Platelets ≥100,000 / mcL * Hemoglobin ≥9 g/dL * Serum creatinine ≤1.5 X upper limit of normal (ULN) * Serum total bilirubin ≤ 1.5 X ULN OR Direct bilirubin ≤ ULN for subjects with total bilir…