High-Exposure, Targeted Daily Busulfan and Fludarabine-Based Conditioning for Children Undergoing Hematopoietic Stem Cell Transplantation for Inherited Metabolic Disorders: Outcomes at a Single Center

Allogeneic hematopoietic stem cell transplantation (HSCT) has emerged as standard therapy for children suffering from various life-threatening inherited metabolic disorders (IMD). Historically, successful outcomes have been limited by high rates of both graft failure and regimen-related toxicity. Real-time busulfan pharmacokinetic monitoring, combined with once daily dosing, has allowed for more precise delivery of targeted exposures during transplant conditioning. Recently, the Utrecht group reported highly favorable outcomes following a high-dose busulfan and fludarabine-based preparative regimen in patients with a variety of diagnoses, particularly in comparison to historical regimens that employ two or more alkylating agents. We report our experience using the same regimen specifically in patients with IMD. Since July 2014, 24 children undergoing first HSCT for IMD were conditioned with once daily IV busulfan (4 days; total regimen exposure of 90 mg x h/L) and once daily IV fludarabine (40 mg/m²/day x 4 days). Busulfan pharmacokinetic monitoring was performed on each of the first 3 doses (days); subsequent doses were altered to achieve the desired total regimen exposure. Serotherapy use was donor-dependent: HLA-matched related donor (MRD, n = 5), none; unrelated UCB (n = 16), thymoglobulin (2.5 mg/kg/day, days - 8 to -5); HLA-matched unrelated donor (MUD, n = 1), thymoglobulin (2.5 mg/kg/day, days - 5 -to -2); and, related HLA haploidentical marrow (n = 2), alemtuzumab (0.5 mg/kg divided days -11 and -10), +/- rituximab (375 mg/m², day -12). All patients received CSA for GvHD prophylaxis. Additional GvH prophylaxis depended upon the allograft source: UCB, prednisolone; MRD/MUD, MMF; related haploidentical, MMF and post-transplant cyclophosphamide. Growth factor support was administered to recipients of UCB or haploidentical grafts. IMD diagnoses were as follows: Hurler syndrome (n = 10); cerebral adrenoleukodystrophy (n = 9); metachromatic leukodystrophy (n = 1); osteopetrosis (n = 1); fucosidosis (n = 1); alpha-mannosidosis (n = 1); and, beta-mannosidosis (n = 1). At a median follow-up of 229 days (range, 15 - 743), 22 of 24 patients survive, with a Kaplan Meier estimate of survival at one year of 86%. Causes of death were pulmonary bronchiolitis obliterans (BOS, n = 1) and graft failure (n = 1). Twenty-two patients had initial donor neutrophil recovery at a median day +14 (range, +11 to +21); for these patients, the median duration of severe neutropenia (ANC < 500/μL) was 7.5 days (range, 4 to 16 days). Of 20 patients who at the time of analysis are ≥ 30 days from HSCT, 18 had platelet engraftment (≥20,000/μL, transfusion unsupported) at a median day +22.5 (range, +10 to +94). One patient developed Grade II aGvHD (haploidentical marrow graft) which was successfully treated; no patients developed Grade III-IV aGvHD. One patient developed chronic GvHD with fatal BOS (UCB). Regimen-related complications were as follows: veno-occlusive disease, n = 0; hemorrhagic cystitis, n = 1; diffuse alveolar hemorrhage n = 1; idiopathic pneumonia syndrome, n = 1; immune-mediated cytopenias, n = 2; CMV viremia, n = 3; and, adenovirus infection, n = 1. Twenty patients (83%) have demonstrated durable, complete donor myeloid engraftment following the regimen. Of these, all have demonstrated mixed donor-recipient T-cell chimerism. Four patients, (all UCB grafts), have experienced graft failure: aplastic graft failure, n = 3, and autologous hematopoietic recovery, n = 1. Three of these patients underwent second HSCT and 2 are alive and engrafted while the other died of complications following second transplant. One patient awaits second transplant. In summary, this regimen of high-exposure, targeted daily busulfan and fludarabine in children with IMD provides favorable engrafted survival with minimal toxicities.