Feasibility study of busulfan, fludarabine, and thiotepa conditioning regimen for allogeneic hematopoietic stemcell transplantationfor children and young adults with nonmalignant disorders

Background: Hematopoietic stem cell transplant (HSCT) is the only curative treatment for several pediatric non-malignant disorders. A widely used conditioning backbone is busulfan, fludarabine, and rabbit anti-thymocyte globulin (rATG). Thiotepa has improved engraftment when added to this regimen, however the minimum effective dose (MED) of thiotepa to achieve engraftment while minimizing toxicities has not been well established.Objectives: The primary objective of this prospective feasibility study was to determine the MED of thiotepa (5mg/kg) in combination with reduced-dose busulfan, fludarabine or cyclophosphamide, and rATG required to achieve engraftment in >90% of HSCT recipients for non-malignant disorders with acceptable toxicity.Results: Six patients who received fully matched HSCT were enrolled. Patient diagnoses included Wiskott-Aldrich syndrome (n = 1), CD40L deficiency (n = 1), sickle cell disease (n = 2), autoinflammatory syndrome (n = 1), and paroxysmal nocturnal hemoglobinuria (n = 1). All six patients achieved engraftment prior to Day +42 and five patients had stable full donor engraftment. Two of the six patients (33%) developed acute GVHD and/or chronic GHVD, both of whom had sickle cell disease. At a median follow-up of 2.25 years post-transplant, all patients were alive without evidence of disease recurrence. None of the patients experienced grade 4 or 5 toxicities. Three out of six patients (50%) developed grade 3 adverse events. Neurocognitive functioning of children under 10 years of age was not adversely affected by this regimen.Conclusion: This approach shows acceptable toxicity and reliable engraftment in children with non-malignant disorders receiving related or unrelated HLA-matched transplants.