Allogeneic Hematopoietic Cell Transplantation for Juvenile Myelomonocytic Leukemia with a Busulfan, Fludarabine, and Melphalan Regimen: JPLSG JMML-11.

Juvenile myelomonocytic leukemia (JMML), which is classified as a myelodysplastic/myeloproliferative neoplasm, is a rare hematologic malignancy of childhood. Most patients with JMML require allogeneic hematopoietic cell transplantation (HCT) as a curative therapy. A Japanese retrospective analysis demonstrated favorable outcomes for a busulfan (BU) + fludarabine (FLU) + melphalan (MEL) regimen, with an overall survival (OS) of 72% and an event-free survival (EFS) of 53%. To further validate the efficacy and safety of this regimen, the Japan Pediatric Leukemia/Lymphoma Study Group (JPLSG) conducted a nationwide prospective study, JMML-11. Between July 2011 and June 2017, 28 patients with newly diagnosed JMML were enrolled in JMML11. Low-dose chemotherapy for tumor control before HCT was recommended, and patients treated with AML-type chemotherapy and azacitidine were excluded. The conditioning regimen comprised i.v. BU, 16 doses administered every 6 h, with dose adjustment based on pharmacokinetic (PK) studies on days -11 to -8; FLU, 30 mg/m/day or 1 mg/kg/day for patients <10 kg or age <1 year on days -7 to -4; and MEL, 90 mg/m/day or 3 mg/kg/day for patients <10 kg or <1 year on days -3 to -2. The donor was selected by the physician in charge. A family donor was available for 7 patients (3 HLA-matched siblings, 3 HLA-1-antigen mismatched parents, and 1 haploidentical father). Overall, 21 patients received grafts from unrelated donors, including 8 HLA-matched donors and 13 HLA-mismatched donors. The graft source was related bone marrow (BM) for 7 patients, unrelated BM for 14 patients, and unrelated cord blood for 7 patients. Neutrophil engraftment was achieved in 21 of 28 patients (75%), with a median of 20.5 days (range, 11 to 39 days) after transplantation. The 3-year OS, 3-year EFS, 3-year relapse rate, and 3-year transplantation-related mortality were 63% (95% confidence interval [CI], 42% to 78%), 52% (95% CI, 32% to 69%), 18% (95% CI, 6% to 34%), and 21% (95% CI, 9% to 38%), respectively. WBC count before the conditioning regimen (≥7.0 × 10/L) was significantly associated with inferior EFS and OS. Body surface area ≥.5 m, spleen size <4 cm before conditioning, and HLA-matched unrelated BM donors were significantly associated with better OS. Adverse effects related to the conditioning regimen included febrile neutropenia (86%), diarrhea (39%), hypoxemia (21%), and mucositis (18%). BU-associated toxicity, including sinusoidal obstruction syndrome (SOS) and thrombotic microangiopathy (TMA), occurred in 7 patients (25%; SOS, n = 6; TMA, n = 2). Retrospective analysis of PK data after the first BU dose in 23 patients, including 6 with SOS and 17 without SOS, did not show significant differences between groups. The JMML-11 study confirms the positive results of previous retrospective analyses. BU+FLU+MEL might become a standard conditioning regimen for patients with JMML.