Busulfan, Cyclophosphamide, and Melphalan or Busulfan and Fludarabine Phosphate Before Donor Hematopoietic Cell Transplant in Treating Younger Patients With Juvenile Myelomonocytic Leukemia

PRIMARY OBJECTIVES: I. To compare ? in a randomized fashion ? the day 100 treatment related mortality (TRM) incidence for two myeloablative conditioning regimens, busulfan-fludarabine (fludarabine phosphate) (BU-FLU) and busulfan-cyclophosphamide-melphalan (BU-CY-MEL), prior to hematopoietic cell transplant (HCT) for children with juvenile myelomonocytic leukemia (JMML), in order to determine the preferred regimen for future trials. II. To compare ? in a randomized fashion ? the 18-month event-free survival (EFS) following two different myeloablative conditioning regimens (BU-FLU vs. BU-CY-MEL) prior to HCT for children with JMML, in order to determine the preferred regimen for future trials. SECONDARY OBJECTIVES: I. To determine the 18-month relapse incidence (RI) following two different myeloablative conditioning regimens (BU-FLU vs. BU-CY-MEL) prior to HCT for children with JMML. II. To determine the graft failure rates following two different myeloablative conditioning regimens (BU-FLU vs. BU-CY-MEL) prior to HCT for children with JMML. TERTIARY OBJECTIVES: I. To determine the rates of severe toxicities (grade 3/4) at day 100 post-HCT between the two myeloablative conditioning regimens (BU-FLU vs. BU-CY-MEL). II. To determine the rates of acute and chronic (at 18 months post-HCT) graft-versus-host disease (GVHD) following HCT using two different conditioning regimens (BU-FLU vs. BU-CY-MEL) in children with JMML. III. To create a JMML-specific pre-HCT index to allow better risk-stratification of future patients. IV. To determine the feasibility of assessing post-transplant disease burden by donor chimerism measurements and allele-specific polymerase chain reaction (PCR) in mononuclear and sorted cell subsets. V. To validate gene expression and methylation classifiers predictive of relapse in patients with JMML. VI. To comprehensively assess genetic and biochemical alterations amongst patients with JMML who are treated on this transplant protocol. OUTLINE: Patients are randomized to 1 of 2 treatment arms. ARM I: CONDITIONING REGIMEN: Patients receive busulfan intravenously (IV) over 2-3 hours once daily (QD), every 12 hours, or every 6 hours on days -8 to -5, cyclophosphamide IV over 60 minutes QD on days -4 and -3, and melphalan IV over 15-30 minutes on day -1. TRANSPLANT: Patients undergo allogeneic HCT on day 0. Patients receive tacrolimus IV or orally (PO) on days -1 to 98 (related donor) or 180 (unrelated donor) and mycophenolate mofetil IV over 2 hours or PO every 8 hours on days 1-30 (related donor) or 45 (unrelated donor). ARM II: CONDITIONING REGIMEN: Patients receive busulfan as in Arm I and fludarabine phosphate IV over 30-60 minutes on days -5 to -2. TRANSPLANT: Patients undergo allogeneic HCT as in Arm I. Patients receive tacrolimus IV or PO on days -1 to 98 (related donor) or 180 (unrelated donor) and mycophenolate mofetil IV over 2 hours or PO every 8 hours on days 1-30 (related donor) or 45 (unrelated donor). After completion of study treatment, patients are followed up for 5 years.