The objective of this study is to establish the efficacy, safety, and tolerability of S243249 and to establish the recommended phase 2 dose (RP2D) of S243249 monotherapy in participants with relapsed/refractory (R/R) acute leukemia with select mutations. Phase 1 dose optimization will determine the RP2D to be used in Phase 2 dose expansion. The study will include a screening period, a treatment period consisting of continuous 28-day cycles of treatment, a safety follow-up period and a long-term follow-up period. Participants may undergo blood tests, electrocardiogram (ECG), bone marrow aspirations, vital sign checks, questionnaires, and physical exams.