Clinical Trial to Evaluate the Safety, Tolerability, Pharmacokinetics and Pharmacodynamics of Tuspetinib (HM43239) in Patients With Relapsed or Refractory Acute Myeloid Leukemia

Inclusion Criteria for Parts A/B/C: * Study participant is defined as having morphologically documented primary or secondary AML, MDS-IB2 (≥ 10% bone marrow blasts), or CMML by the World Health Organization (WHO) criteria (2016), and fulfills one of the following: 1. Refractory to at least 1 cycle of prior therapy 2. Relapsed after achieving remission with a prior therapy * Study participant has an Eastern Cooperative Oncology Group (ECOG) performance status ≤ 2. * Study participant's interval from prior treatment to time of study drug administration is at least 2 weeks for cytotoxic agents (except hydroxyurea given for controlling blast cells), at 4 weeks for biologic or cellular immunotherapies, or least 5 half-lives for prior experimental agents or noncytotoxic agents, including immunosuppressive therapy post hematopoietic stem cell transplantation (HSCT). Upon discussion with the Medical Monitor, a shorter than stated washout period may be considered provided that the study participant has recovered from any clinically relevant safety issue and recovered to Grade ≤ 1 toxicity from prior therapies. * Study participant must meet the following criteria as indicated on the clinical laboratory tests. 1. Serum aspartate aminotransferase (AST) and alanine aminotransferase (ALT) ≤ 3× institutional upper limit normal (ULN) 2. Total serum bilirubin ≤ 1.5× institutional ULN 3. Creatinine clearance as calculated by the Cockcroft-Gault formula or measured by 24-h urine collection of \> 45 mL/min. * Study participant is suitable for oral administration of study drug and has minimum life expectancy (≥ 3 months) * Female study participants must be either: * Of non-childbearing potential 1. Post-menopausal (defined as at least 1 year without any menses) prior to screening, or 2. Documented surgically sterile or status post hysterectomy (at least 1 month prior to screening) * Or, if of childbearing potential, 1. Must have a negative serum or urine pregnancy test at screening (within 72 hours prior to start of treatment), and 2. Must use an acceptable highly effective method of birth control starting at screening and throughout the study period and for 90 days after the final study drug administration. * Female study participants must not be breastfeeding at screening and during the study period, and for 90 days after the final study drug administration * Female study participants must not donate ova starting at screening and throughout the study period, and for 90 days after the final study drug administration. * Male study participants and their female spouse/partners who are of childbearing potential must be using highly effective contraception starting at screening and continue throughout the study period and for 90 days after the final study drug administration. * Male study participants must not donate sperm starting at screening and throughout the study period and for 90 days after the final study drug administration. * Study participant agrees not to participate in another interventional study while on treatment. Exclusion Criteria for Parts A/B/C: Study participants must not enter the study if any of the following exclusion criteria are met: * Study participant was diagnosed with acute promyelocytic leukemia (APL). * Study participant has known BCR-ABL-positive leukemia. * Study participant has an active malignancy other than AML, MDS-IB2, or CMML. * Study participant has persistent non-hematological toxicities of ≥ Grade 2 (CTCAE v4.03), with symptoms and objective findings, from prior AML, MDS-IB2, or CMML treatment (including chemotherapy, kinase inhibitors, immunotherapy, experimental agents, radiation, or surgery) * Study participant has had hematopoietic stem cell transplant (HSCT) and meets any of the following criteria: 1. Has undergone HSCT within the 2-month period prior to the first study dose 2. Has clinically significant graft-versus-host-disease (GVHD) requiring treatment 3. Has ≥ Grade 2 persistent non-hematological toxicity related to the transplant 4. Had a donor lymphocyte infusion (DLI) ≤ 30 days prior to the first study dose. * Study participant has meningeal or central nervous system (CNS) involvement with leukemia or other CNS disease related to underlying and secondary effects of malignancy. * Study participant has disseminated intravascular coagulation abnormality (DIC). * Study participant has had major surgery within 4 weeks prior to the first study dose. * Study participant has had radiation therapy within 4 weeks prior to the first study dose. * Study participant has congestive heart failure New York Heart Association (NYHA) class 3 or 4, or study participant with a history of congestive heart failure NYHA class 3 or 4 in the past, unless a screening echocardiogram or multigated acquisition (MUGA) scan performed within 3 months prior to study entry results in a left ventricular ejection fraction (LVEF) that is ≥ 45%. * Study participant has any of the following cardiac abnormalities of history: 1. Study participant has any clinically important abnormalities in rhythm, conduction or morphology of resting ECG, e.g., complete left bundle branch block, third-degree heart block, second-degree heart block, or PR interval \> 250 milliseconds (ms). 2. Study participant has a mean QT interval (QTc) by Friderica's method (QTcF) \> 450 ms in three successive screening measurements. 3. Study participant has any factors that increase the risk of QTc prolongation or risk of arrhythmic events, such as congenital long QT, syndrome, family history of long QT syndrome. * Study participant is known to have active infection including any identified active COVID-19 infection. * Study participant is known to have human immunodeficiency virus infection. * Study participant has known active hepatitis B or C, or other active hepatic disorder. * Study participant has any condition which, in the Investigator's opinion, makes the study participant unsu