This study was terminated due to slow accrual.

This research study tests an investigational drug called DS-3201b. An investigational drug is a medication that is still being studied and has not yet been approved by the United States Food and Drug Administration (FDA). The FDA allows DS-3201b to be used only in research. It is not known if DS-3201b will work or not.

This study consists of two parts. The first part (Part 1) is a dose escalation that will enroll subjects with AML or ALL that did not respond or no longer respond to previous standard therapy. The purpose of Part 1 of this research study is to determine the highest dose a patient can tolerate or recommended dose of DS-3201b that can be given to subjects with AML or ALL. Once the highest tolerable dose is determined, additional subjects will be enrolled at that dose into Part 2 of the study.

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

De-identified individual participant data (IPD) and applicable supporting clinical trial documents may be available upon request at https://vivli.org/. In cases where clinical trial data and supporting documents are provided pursuant to our company policies and procedures, Daiichi Sankyo will continue to protect the privacy of our clinical trial participants. Details on data sharing criteria and the procedure for requesting access can be found at this web address: https://vivli.org/ourmember/daiichi-sankyo/

Studies for which the medicine and indication have received European Union (EU) and United States (US), and/or Japan (JP) marketing approval on or after 01 January 2014 or by the US or EU or JP Health Authorities when regulatory submissions in all regions are not planned and after the primary study results have been accepted for publication.

Formal request from qualified scientific and medical researchers on IPD and clinical study documents from clinical trials supporting products submitted and licensed in the United States, the European Union and/or Japan from 01 January 2014 and beyond for the purpose of conducting legitimate research. This must be consistent with the principle of safeguarding study participants' privacy and consistent with provision of informed consent.

Demographic and baseline characteristics were assessed in the Safety Analysis Set.

Treatment-emergent adverse events were collected from baseline up to 30 days after last study dose, up to approximately 4 years.

A treatment-emergent adverse event (TEAE) is defined as an adverse event (AE) that occurs, having been absent before the first dose of study drug or has worsened after initiating the study drug until 30 days after the last dose of the study drug.

Open label