A Study of BTX-A51 in People With Relapsed or Refractory Acute Myeloid Leukemia or High-Risk Myelodysplastic Syndrome

Inclusion Criteria: * Demonstration of understanding and voluntarily signing of an informed consent form * Age ≥ 18 years * Diagnosis of acute myeloid leukemia (AML) or myelodysplastic syndrome (MDS) according to the World Health Organization classification and, with respect to MDS, that is high risk; participants must have refractory or relapsed disease and be ineligible for or have exhausted standard therapeutic options that would otherwise be likely to provide clinical benefit * Eastern Cooperative Oncology Group performance status ≤ 2 and life expectancy of ≥ 6 weeks * Adequate organ function (Grade 1 serum creatinine; Grade 1 total bilirubin; aspartate aminotransferase and/or alanine transaminase ≤ 2 × ULN) * Females of childbearing age must not be pregnant at time of Screening/beginning of treatment and agree to either abstain from sexual intercourse or use highly effective methods of contraception (for up to 3 months after last dose of study drug) * Males sexually active with a woman of childbearing age must agree to use barrier method of birth control during and after the study (up to 3 months after last dose of study drug) Exclusion Criteria: * Diagnosis of acute promyelocytic leukemia * White blood cell count \> 20 x 10\^9/L * Receipt of cancer chemotherapy (other than hydroxyurea) within 2 weeks prior to the start of study drug * In participants who have undergone autologous or allogeneic stem cell transplantation: transplantation within the 3 months prior to Screening; active graft-versus-host disease requiring anything other than topical corticosteroids and budesonide; treatment with systemic immunosuppressive medications including high-dose steroids (≥ 20 mg prednisolone or equivalent per day), or calcineurin inhibitors (e.g., cyclosporine, tacrolimus) for at least 1 week prior to Screening, and sirolimus, mycophenylate mofetil, azathioprine, or ruxolitinib for at least 2 weeks prior to Screening * Immediate life-threatening severe complications of leukemia such as uncontrolled bleeding, pneumonia with hypoxia or shock, and/or disseminated intravascular coagulation * Persistent toxicities from prior treatment of Grade 2 or higher * Active uncontrolled systemic fungal, bacterial, mycobacterial, or viral infection * Clinically significant cardiac disease * Known dysphagia, short-gut syndrome, gastroparesis, or other conditions that limit the ingestion or gastrointestinal absorption of drugs administered orally * Any other concurrent medical condition or disease that is likely to interfere with study procedures or results, or that, in the opinion of the Investigator, would constitute a hazard for participating in this study * If female, pregnant or breastfeeding