Inclusion Criteria:  Adults ≥18 years of age with acute myeloid leukemia (AML) and at least 1 prior line of therapy FMS-like tyrosine kinase 3 (FLT3)-mutated disease documented in a local reference laboratory Have the ability to understand the requirements and procedures of the study and sign a written informed consent form Eastern Cooperative Oncology Group (ECOG) performance status (PS) of 0, 1 or 2 Adequate hepatic and renal function Prothrombin time (PT), activated partial thromboplastin time (aPTT) and international normalized ratio (INR) ≤1.5x upper limit of normal (ULN) unless receiving therapeutic anticoagulation Negative serum ß-human chorionic gonadotropin (HCG) test in women of child-bearing potential (WOCBP) Left ventricular ejection fraction ≥50% confirmed by echocardiogram (ECHO) or multi-gated acquisition (MUGA) scan  Exclusion Criteria:  Known central nervous system (CNS) involvement with leukemia Failure to achieve at least a partial response (PR) or have relapsed following prior exposure to gilteritinib or other next-generation FLT3 inhibitor monotherapy (eg. quizartinib, crenolanib) Clinical signs/symptoms of leukostasis that have failed therapy including hydroxyurea and/or leukapheresis of at least 3 days duration Pregnant or breastfeeding women Active infection with hepatitis B, C or known human immunodeficiency virus (HIV) infection Disseminated intravascular coagulation with active bleeding or signs of thrombosis Known active coronavirus disease 2019 (COVID-19) Administration of a live attenuated virus vaccine within 35 days before Cycle 1 Day 1 (C1D1) History of non-myeloid malignancy except for the following: adequately treated localized basal cell or squamous cell carcinoma of the skin; cervical carcinoma in situ; superficial bladder cancer; asymptomatic prostate cancer without known metastatic disease, with no requirement for therapy or requiring only hormonal therapy and with normal prostate specific antigen for > 1 year prior to start of study therapy; or any other cancer that has been in complete remission without treatment for ≥3 years prior to enrollment Clinically significant heart disease Prolongation of the congenital long measure between Q wave and T wave in the electrocardiogram (QT) interval at baseline Evidence of ongoing uncontrolled systemic bacterial, fungal, or viral infection at the time of study treatment initiation Current (within 30 days of study enrollment) drug-induced liver injury, chronic active hepatitis, alcoholic liver disease, nonalcoholic steatohepatitis, primary biliary cirrhosis, extrahepatic obstruction caused by cholelithiasis, cirrhosis of the liver, orportal hypertension Ongoing (within 6 weeks of study enrollment) hepatic encephalopathy Ongoing immunosuppressive therapy, including systemic chemotherapy for treatment of leukemia