The Phase 1 portion of this study is a single-arm, open-label, multicenter, non-randomized interventional study to evaluate the pharmacokinetic (PK) interaction, safety, and efficacy of ASTX727 when given in combination with venetoclax for the treatment of newly diagnosed acute myeloid leukemia (AML) in adults who are age 75 years or older, or who have comorbidities that preclude use of intensive induction chemotherapy. The primary purpose of the study is to rule out drug-drug interactions between ASTX727 and venetoclax combination therapy by evaluating area under the curve (AUC) and maximum plasma concentration (Cmax) exposure. The Phase 2 portion of the study is to assess the efficacy of ASTX727 and venetoclax when given in combination and to evaluate potential PK interactions. Phase 2 will follow the same overall study design as Phase 1 and has two parts, Part A and Part B. Following study completion, participants who get clinical benefit without unnecessary risks may continue to receive study treatment in the study extension phase.