A Study of Crenolanib With Fludarabine and Cytarabine in Pediatric Patients With Relapsed/Refractory FLT3-Mutated Acute Myeloid Leukemia

Inclusion Criteria: 1. Age ≥ 1 years and ≤ 21 years 2. Confirmed diagnosis of AML according to World Health Organization (WHO) 2016 classification 3. Definitive evidence of a FLT3-ITD and/or FLT3-TKD (D835/I836) mutation at the time of enrollment 4. Patients must have histologically or molecularly confirmed relapsed or refractory AML 5. Karnofsky or Lansky performance score ≥ 50. Use Karnofsky for patients \> 16 years old and Lansky for patients ≤ 16 years of age. 6. Adequate renal function, defined as: * Creatinine clearance or radioisotope GFR ≥ 70 mL/min/1.73 m2 or * Normal serum creatinine based on age/gender 7. Adequate liver function, defined as: * Serum total bilirubin ≤ 1.5x ULN for age, * Serum aspartate aminotransferase (AST) ≤ 3.0x ULN for age, and * Serum alanine aminotransferase (ALT) ≤ 3.0x ULN for age. Exclusion Criteria: 1. Patients with any of the following current or previous diagnoses: * Acute promyelocytic leukemia (APL) * Down syndrome * DNA fragility or bone marrow failure syndromes (such as Fanconi anemia, Bloom syndrome, Kostmann syndrome, or Shwachman syndrome) * AML secondary to prior MDS/MPN, including chronic myelomonocytic leukemia and juvenile myelomonocytic leukemia * Blastic plasmacytoid dendritic cell neoplasm * Acute leukemia of ambiguous lineage * B-lymphoblastic leukemia/lymphoma * T-lymphoblastic leukemia/lymphoma, including early T-cell precursor lymphoblastic leukemia (ETP-ALL) 2. Patients who are refractory to first line (induction and re-induction) and a second line (1st salvage) treatment for AML. 3. Patients who have received more than 1 prior allogeneic HSCT 4. Patients will be excluded if they have a systemic fungal, bacterial, viral or other infection of which they exhibit ongoing signs/symptoms related to the infection without improvement despite appropriate antibiotics or other treatment. 5. Patients will be excluded if there is a plan to administer non-protocol chemotherapy, radiation therapy, or immunotherapy during the study period. 6. Known severe liver disease (e.g. cirrhosis, non-alcoholic steatohepatitis, sclerosing cholangitis or hyperbilirubinemia) 7. Known, active infection with hepatitis B virus (HBV) or hepatitis C virus (HCV) 8. Currently receiving prophylactic treatment of hepatitis B with anti-viral therapy 9. Known infection with human immunodeficiency virus (HIV)