Study of CFI-400945 Fumarate in Patients With Relapsed or Refractory AML or MDS

Inclusion Criteria: * 18 years of age or older * Relapsed and/or refractory myeloid leukemia (AML) or myelodysplastic syndrome (MDS) that meets certain criteria for prior treatments * Have acceptable circulating blasts count * Have clinically acceptable laboratory blood and urine test results * Have an Eastern Cooperative Oncology Group (ECOG) performance status of 0, 1 or 2 * Be able to swallow oral medications * Have a life expectancy of 3 months or more * Agree to use highly effective means of contraception during a defined period * Negative serum pregnancy test before the start of the study drugs * Have the ability to understand the requirements of the study, provide written informed consent which includes authorization for release of protected health information, abide by the study restrictions, provide a blood and bone marrow sample for genetic testing and agree to return for the required assessments Exclusion Criteria: * Have received cancer therapies within 14 days or 5 half-lives (whichever is shorter) prior to first dose of study drug or have not recovered from toxicities from prior treatments * Not recovered from toxicities related to allogeneic transplant * Known active extramedullary central nervous system (CNS) AML * Secondary cancer needing therapy with exceptions * Known active human immunodeficiency virus (HIV), hepatitis B virus (HBV), or hepatitis C virus (HCV) infection * Known significant mental illness or other condition that may affect the ability to follow the requirements of the study * Have a chronic infection * Have uncontrolled severe hypertension * Have symptomatic congestive heart failure * Have active angina pectoris or recent myocardial infarction * Have chronic atrial fibrillation or unacceptable QTc * Have had major surgery within 21 days of starting therapy * Have additional uncontrolled serious medical or psychiatric illness * Have any medical condition that would affect taking the study drug * Receiving treatment with full dose warfarin * Receiving treatment with drugs not allowed in this study * Women who are pregnant or lactating.