IDH2 Inhibition Using Enasidenib as Maintenance Therapy for IDH2-mutant Myeloid Neoplasms Following Allogeneic Stem Cell Transplantation

Inclusion Criteria: * Pathologically confirmed diagnosis of IDH2-mutant acute myeloid leukemia (AML), myelodysplastic syndrome (MDS) or chronic myelomonocytic leukemia (CMML). * IDH2 mutations will include any IDH2 R140 or R172 alterations * Eligibility and enrollment will be based on local IDH2 mutational testing performed at any center. The presence of an IDH2 mutation at the time of initial diagnosis or any other time thereafter is necessary and sufficient. The presence of an IDH2 mutation at time of enrollment is not necessary for the purposes of eligibility. * Between the ages of 18 and 75 years * Will undergo first allogeneic hematopoietic stem cell transplantation (HSCT) for their malignancy. Conditioning may have been either conventional myeloablative (MAC) or reduced intensity conditioning (RIC). * HSCT Donor will be one of the following: * 5/6 or 6/6 (HLA-A, B, DR) matched related donor * 7/8 or 8/8 (HLA-A, B, DR, C) matched unrelated donor. Matching in the unrelated setting must be at the allele level. * Haploidentical related donor, defined as ≥ 3/6 (HLA-A, B, DR) matched --≥ 4/6 (HLA-A, B, DR) umbilical cord blood (UCB). Matching in the UCB setting is at the antigen level. Recipients may receive either one or two UCB units. In the case of 2 UCB units, both units must have been at least 4/6 matched with the recipient. * ECOG performance status ≤ 2 * Participants must have normal organ and marrow function as defined below: * Absolute neutrophil count ≥ 1000/µL without growth factor support (e.g. GCSF) in the previous 7 days * Platelet count ≥ 50,000/µL without transfusional support in the previous 7 days * AST (SGOT), ALT (SGPT) and Alkaline phosphatase \< 3x institutional upper limit of normal (ULN) * Direct bilirubin \< 2.0 mg/dL * Calculated creatinine clearance ≥ 40 mL/min (Cockcroft-Gault formula) * LVEF must be equal to or greater than 50%, as measured by MUGA scan or echocardiogram * Female patients of childbearing potential must have a negative pregnancy test, as measured by serum or urine testing * The effects of enasidenib on the developing human fetus are unknown. For this reason women of child-bearing potential and men must agree to use adequate contraception (hormonal or barrier method of birth control; abstinence) during the entire study treatment period and through 6 months after the last dose of treatment * Ability to understand and the willingness to sign a written informed consent document Exclusion Criteria: * Prior allogeneic hematopoietic stem cell transplants. * Evidence of relapsed/recurrent/residual disease as assessed by bone marrow aspirate and biopsy performed within 42 days prior to study entry. * History of other malignancy(ies) unless * the participant has been disease-free for at least 5 years and is deemed by the investigator to be at low risk of recurrence of that malignancy, or * the only prior malignancy was cervical cancer in situ and/or basal cell or squamous cell carcinoma of the skin * Known diagnosis of active hepatitis B or hepatitis C * Current or history of congestive heart failure New York Heart Association (NHYA) class 3 or 4, or any history of documented diastolic or systolic dysfunction (LVEF \< 50%, as measured by MUGA scan or echocardiogram) * Current or history of ventricular or life-threatening arrhythmias or diagnosis of long-QT syndrome * Systemic infection requiring IV antibiotic or antifungal or antiviral therapy within 7 days preceding the first dose of study drug, or other severe infection * Known dysphagia, short-gut syndrome, gastroparesis, or other condition(s) that limits the ingestion or gastrointestinal absorption of drugs administered orally * Uncontrolled hypertension (systolic blood pressure \[BP\] \> 180 mmHg or diastolic BP \> 100 mmHg) * QTc interval (i.e., Friderica's correction \[QTcF\]) ≥ 450 ms or other factors that increase the risk of QT prolongation or arrhythmic events (e.g., heart failure, hypokalemia, family history of long QT interval syndrome) at screening * Concomitant receipt of the following sensitive CYP substrate medications that have a narrow therapeutic range (unless the participant can be transferred to other medications at least 5 half-lives prior to the start of study treatment): paclitaxel and docetaxel (CYP2C8), phenytoin (CYP2C9), S-mephenytoin (CYP2C19), thioridazine (CYP2D6), theophylline, and tizanidine (CYP1A2) * Concomitant receipt of the breast cancer resistance protein (BCRP) transporter-sensitive substrate rosuvastain (unless the participant can be transferred to another medication at least 5 half-lives prior to the start of study treatment) * Uncontrolled intercurrent illness that would limit compliance with study requirements. * HIV-positive participants on combination antiretroviral therapy are ineligible because of the potential for pharmacokinetic interactions with study drug. In addition, these participants are at increased risk of lethal infections when treated with marrow-suppressive therapy.