Inclusion Criteria:

          -  Signed, informed consent must be obtained prior to any study specific procedures

          -  Subjects with a histologically confirmed diagnosis of MDS, including both MDS and
             refractory anemia with excess blasts in transformation (RAEB-T) (acute myeloid
             leukemia [AML] with 20-30% blasts and multilineage dysplasia by
             French-American-British [FAB] criteria) by World Health Organization (WHO), and
             chronic myelomonocytic leukemia (CMML) are eligible

          -  Subjects must have an IDH2 gene mutation (IDH2-R140 or R172) as determined by local
             laboratory result

          -  (Arm A only): Subject must be hypomethylating agent naive (i.e. prior azacitidine,
             decitabine, SGI-110 is exclusionary). Receipt of other MDS-directed therapy such as
             lenalidomide is allowed

          -  (Arm A only): Subjects with high-risk MDS (i.e. International Prostate Symptom Score
             [IPSS] intermediate-2 or high-risk; or revised [R]-IPSS high or very-high risk).
             Patients with intermediate-1 risk by IPSS or intermediate risk by R-IPSS with
             high-risk molecular features including TP53, ASXL1, EZH2, and/or RUNX1 mutations are
             also eligible

          -  (Arm B only): Subject must be relapsed or refractory to prior hypomethylating agent
             therapy, defined as prior receipt of 6 cycles of HMA therapy with failure to attain a
             response, or relapse after prior response to HMA therapy

          -  Eastern Cooperative Oncology Group (ECOG) performance status of 0-2

          -  Serum bilirubin =< 2 x the upper limit of normal (ULN) (except for patients with
             Gilbert's disease)

          -  Alanine aminotransferase (ALT) and/or aspartate aminotransferase (AST) =< 3 x the
             laboratory ULN

          -  Serum creatinine =< 2 x the ULN

          -  Able to understand and voluntarily sign a written informed consent, and willing and
             able to comply with protocol requirements

          -  Resolution of all clinically significant treatment-related, non-hematological
             toxicities, except alopecia, from any previous cancer therapy to =< grade 1 prior to
             the first dose of study treatment

          -  Female patients of childbearing potential must have a negative serum or urine
             pregnancy test within 7 days of the first dose of study drug and agree to use dual
             methods of contraception during the study and for a minimum of 3 months following the
             last dose of study drug. Post-menopausal females (> 45 years old and without menses
             for > 1 year) and surgically sterilized females are exempt from these requirements.
             Male patients must use an effective barrier method of contraception during the study
             and for a minimum of 3 months following the last dose of study drug if sexually active
             with a female of childbearing potential

        Exclusion Criteria:

          -  Any prior or coexisting medical condition that in the investigator's judgment will
             substantially increase the risk associated with the subject's participation in the
             study

          -  Subject has received a prior targeted IDH2 inhibitor

          -  Psychiatric disorders or altered mental status precluding understanding of the
             informed consent process and/or completion of the necessary study procedures

          -  Active uncontrolled infection at study enrollment including known diagnosis of human
             immunodeficiency virus or chronic active hepatitis B or C infection

          -  Clinically significant gastrointestinal conditions or disorders that may interfere
             with study drug absorption, including prior gastrectomy

          -  Patients with known active central nervous system (CNS) disease, including
             leptomeningeal involvement

          -  Impaired cardiac function, uncontrolled cardiac arrhythmia, or clinically significant
             cardiac disease including the following: a) New York Heart Association grade III or IV
             congestive heart failure, b) myocardial infarction within the last 6 months

          -  Subjects with a corrected QT (QTc) > 480 ms (QTc > 510 msec for subjects with a bundle
             branch block at baseline

          -  Nursing or pregnant women

          -  Subjects with known hypersensitivity to study drugs or their excipients