Inclusion Criteria:

          -  Subject is considered an adult according to local regulation at the time of signing
             informed consent form (ICF).

          -  Subject has defined AML by the World Health Organization (WHO) criteria (2017) and
             fulfills one of the following:

               -  Refractory to at least 1 cycle of induction chemotherapy

               -  Relapsed after achieving remission with a prior therapy

          -  Subject is positive for FLT3 mutation in bone marrow or blood after completion of the
             subject's last interventional treatment.

          -  Subject has an Eastern Cooperative Oncology Group (ECOG) performance status ≤ 2 at
             screening.

          -  Subject must meet the following criteria as indicated on the clinical laboratory
             tests:

               -  Serum Aspartate aminotransferase (AST) and Alanine Aminotransferease (ALT) ≤ 2.5
                  x upper limit of normal (ULN)

               -  Serum total bilirubin (TBL) ≤ 1.5 x ULN

               -  Serum creatinine ≤ 1.5 x ULN or an estimated glomerular filtration rate of > 50
                  mL/min as calculated by the Modification of Diet in Renal Disease equation.

          -  Subject is suitable for oral administration of study drug.

          -  A female subject is eligible to participate if she is not pregnant and at least one of
             the following conditions applies:

               -  Not a woman of childbearing potential (WOCBP) OR

               -  WOCBP who agrees to follow the contraceptive guidance throughout the treatment
                  period and for at least 180 days after the final study drug administration.

          -  Female subject must agree not to breastfeed starting at screening and throughout the
             study period, and for at least 180 days after the final study drug administration.

          -  Female subject must not donate ova starting at screening and throughout the study
             period, and for at least 180 days after the final study drug administration.

          -  A male subject must not donate sperm starting at screening and throughout the
             treatment period, and for at least 120 days after the final study drug administration.

          -  A male subject with female partner(s) of child-bearing potential must agree to use
             contraception during the treatment period, and for at least 120 days after the final
             study drug administration.

          -  Male subject with a pregnant or breastfeeding partner(s) must agree to remain
             abstinent or use a condom for the duration of the pregnancy or time partner is
             breastfeeding throughout the treatment period, and for 120 days after the final study
             drug administration.

          -  Subject agrees not to participate in another investigational study while on treatment.

        Exclusion Criteria:

          -  Subject was diagnosed as acute promyelocytic leukemia.

          -  Subject has BCR-ABL-positive leukemia (chronic myelogenous leukemia in blast crisis).

          -  Subject has AML secondary to prior chemotherapy for other neoplasms (except for
             myelodysplastic syndrome).

          -  Subject has clinically active central nervous system leukemia.

          -  Subject has uncontrolled or significant cardiovascular disease, including:

               -  A myocardial infarction within 12 months

               -  Uncontrolled angina within 6 months

               -  History of clinically significant ventricular arrhythmias (such as ventricular
                  tachycardia, ventricular fibrillation, torsades de pointes) or any history of
                  arrhythmia

               -  Uncontrolled hypertension

          -  Subject has baseline left ventricular ejection fraction that is ≥ 45%.

          -  Subject has mean triplicate Fridericia-corrected QT interval (QTcF) > 450 ms at
             Screening based on central reading.

          -  Subject has congenital or acquired Long QT Syndrome at screening.

          -  Subject has hypokalemia and/or hypomagnesemia at screening.

          -  Subject has been diagnosed with another malignancy that requires concurrent treatment
             or hepatic malignancy regardless of the need for treatment.

          -  Subject has clinically significant coagulation abnormality unless secondary to AML.

          -  Subject is receiving or plans to receive concomitant chemotherapy or immunotherapy.

          -  Subject has had major surgery within 4 weeks prior to the first study dose.

          -  Subject has radiation therapy within 4 weeks prior to the first study dose.

          -  Subject requires treatment with concomitant drugs that are strong inducers of
             Cytochrome P450 (CYP3A).

          -  Subject has known pulmonary disease with diffusion capacity of lung for carbon
             monoxide ≤ 65%, forced expiratory volume in the first second (FEV1) ≤ 65%, dyspnea at
             rest or requiring oxygen or any pleural neoplasm.

          -  Subject with systemic fungal, bacterial, viral or other uncontrolled infection that is
             exhibiting ongoing signs/symptoms related to the infection without improvement despite
             appropriate antibiotics or other treatment. Subject needs to be off pressors and have
             negative blood cultures for 48 hours.

          -  Subject has not recovered from any prior therapy related toxicities.

          -  Subject is known to have human immunodeficiency virus infection.

          -  Subject has active hepatitis B or C or other active hepatic disorder.

          -  Subject has previously been treated with gilteritinib, quizartinib or crenolanib (will
             only apply to subjects enrolled in the phase 2 portion of the study).

          -  Subject has active clinically significant graft-versus-host disease (GVHD) or is on
             treatment with systemic corticosteroids for GVHD.

          -  Subject has relapsed after allogeneic hematopoietic stem cell transplant (HCST).

          -  Subject has an active autoimmune disorder that makes the subject unsuitable for study
             treatment or participation.

          -  Subject has any condition that makes the subject unsuitable for study participation.