• Patients with resected pathologic stage IIB, IIC and III dMMR colon cancer (American Joint Committee on Cancer \[AJCC\] 8)

• Deficient mismatch repair (MMR) by immunohistochemistry or microsatellite instability (MSI-H) by polymerase chain reaction (PCR) or next generation sequencing (NGS)

• Complete (R0) resection of pathologic stage IIB, IIC and III dMMR colon cancer 4 to 12 weeks prior to first dose of study drug

• Available tissue sample from surgical specimen

• Eastern Cooperative Oncology Group (ECOG) performance status 0, 1, or 2

• Absolute neutrophil count (ANC) ≥ 1,500 /mcL

• Platelets ≥ 100,000 / mcL

• Hemoglobin ≥ 9 g/dL or ≥ 5.0 mmol/L

⁃ Transfusion is allowed to obtain an adequate hemoglobin level

• Creatinine ≤ 1.5 x upper limit of normal (ULN) or measured or calculated creatinine clearance ≥ 40 mL/min for patient with creatinine levels \> 1.5 x institutional ULN (glomerular filtration rate \[GFR\] can also be used in place of creatinine or creatinine clearance \[CrCl\])

⁃ Creatinine clearance should be calculated per institutional standard

• Total bilirubin ≤ 1.5 x ULN or direct bilirubin ≤ ULN for patients with total bilirubin levels \> 1.5 x ULN

⁃ Patients with previously diagnosed Gilbert syndrome can have total bilirubin \< 3.0 mg/dL

• Aspartate aminotransferase (AST) (serum glutamic-oxaloacetic transaminase \[SGOT\]) and alanine aminotransferase (ALT) (serum glutamate pyruvate transaminase \[SGPT\]) ≤ 2.5 x ULN

• Alkaline phosphatase ≤ 2.5 x ULN

• Signed informed consent

• Patients at least 18 years of age

• Must have had a full colonoscopy prior to enrollment. If synchronous colon cancers are present, both must have deficient MMR and both must have undergone complete resection for patient to be eligible

• Women of child bearing potential (WOCBP) must have a negative serum (or urine) pregnancy test within 72 hours prior to the first dose of study treatment. WOCBP are women younger than 55 years (yrs) of age excluding those who are surgically unable to get pregnant due to prior hysterectomy and or bilateral salpingo-oophorectomy

• Patients of childbearing / reproductive potential should use adequate birth control methods, as defined by the investigator, during the study treatment period and for a period of 90 days after the last dose of study drug. A highly effective method of birth control is defined as those which result in low failure rate (i.e. less than 1% per year) when used consistently and correctly. Abstinence is acceptable if this is established and preferred contraception for the patient and is accepted as a local standard

• Female patients who are breast feeding should discontinue nursing prior to the first dose of study treatment and until 90 days after the last dose of study drug