DS-3201b for Acute Myelogenous Leukemia (AML) or Acute Lymphocytic Leukemia (ALL)

Inclusion Criteria: 1. Has AML or ALL and failed any prior induction therapy regimen or have relapsed after prior therapy 2. Has Eastern Cooperative Oncology Group (ECOG) performance status 0-2 3. Has adequate renal and hepatic function 4. Had at least 14 days for prior treatment to clear the body before initiation of DS-3201b administration (except for hydroxyurea that needs only 2 days for clearance) 5. Able to provide written informed consent, comply with protocol visits and procedures, be able to take oral medication, and not have any active infection or comorbidity that would interfere with therapy. 6. Agrees to use an adequate method of contraception during the study and until 3 months after the last treatment. 7. Is willing to provide bone marrow biopsies and comply with protocol-defined evaluations 8. Has a life expectancy of at least 3 months Exclusion Criteria: 1. Has presence of central nervous system (CNS) involvement of leukemia or a history of CNS leukemia 2. Has a second concurrent active primary malignancy such as solid tumor or lymphoma under active treatment 3. Has refractory nausea and vomiting, malabsorption, biliary shunt, significant bowel resection, graft versus- host disease (GVHD) significantly affecting gut motility or absorption, or any other condition that would preclude adequate absorption of DS- 3201b in the opinion of the treating physician and/or principal investigator (PI) 4. Has an uncontrolled infection requiring intravenous antibiotics, antivirals, or antifungals, known human immunodeficiency virus infection, or tested positive for active hepatitis B or C infection 5. Has a concomitant medical condition that would increase the risk of toxicity, in the opinion of the Investigator or Sponsor 6. Has unresolved toxicities from previous anticancer therapy 7. Has received hematopoietic stem cell transplantation (HSCT) within 60 days of the first dose of DS-3201b 8. Has received concomitant treatment with a strong inhibitor or inducer of cytochrome P450 (CYP)3A4/5 within 7 days of first receipt of DS-3201b 9. Has consumed herbs/fruits that may have an influence on pharmacokinetics (PK) of DS-3201b from 3 days (14 days for St. John's wort) prior to the start of the study and throughout the entire study 10. Had major surgery within 4 weeks before study drug treatment 11. Has prolonged corrected QT interval by Fridericia's method (QTcF) at rest, where the mean QTcF interval is \> 450 milliseconds (ms) based on triplicate electrocardiograms (ECGs) 12. Is pregnant or breastfeeding 13. Has substance abuse or medical, psychological, or social conditions that, in the opinion of the Investigator, may interfere with the subject's participation in the clinical study or evaluation of the clinical study results 14. Has received prior treatment with enhancer of zeste homolog (EZH) inhibitor