Comparative Effectiveness of Busulfan/Cyclophosphamide Versus Busulfan/Fludarabine Myeloablative Conditioning for Allogeneic Hematopoietic Cell Transplantation in Acute Myeloid Leukemia

Background Myeloablative conditioning (MAC) is a standard approach for allogeneic hematopoietic cell transplantation (alloHCT), but is associated with risks of morbidity and mortality. As regimen intensity affects post-transplant outcomes, assessment of pre-transplant cytogenetics and somatic mutations may refine which acute myeloid leukemia (AML) patients benefit the most. We compared the effectiveness of two approaches: busulfan/cyclophosphamide (Bu/Cy) and the MAC, but reduced toxicity regimen busulfan/fludarabine (Bu/Flu). Moreover, it is unclear whether somatic mutations in AML may differentially affect post-transplant outcomes between these regimens. We hypothesized that despite relative differences with these regimens, they may result in comparable outcomes. Methods We conducted a single center, retrospective analysis of adult AML patients in CR1 or CR2 who underwent a first T-cell replete HLA-8/8 matched related or unrelated donor alloHCT. Patients received either parenteral Bu (12.8 mg/kg total over 4 days) with Cy (120 mg/kg total over 2 days) or parenteral Bu (400 mg/m2 total over 4 days) with Flu (160 mg/m2 total over 4 days). Bu dose adjustment was not used in either cohort. In addition to cytogenetic risk group stratification by European LeukemiaNet criteria (Dohner H, et al, Blood 2016), a subset of patients had a 36-gene somatic mutation panel assessed prior to alloHCT by next-generation sequencing. Pre-transplant characteristics were compared between regimens with Chi-square, Fisher9s exact, or Wilcoxon rank sum tests. Differing characteristics were included in a multivariable Fine and Gray regression model with results as hazard ratios (HR) and 95% confidence intervals (CI). Results From 2008 - 2017, 76 AML patients receiving Bu/Cy and 50 receiving Bu/Flu were identified meeting inclusion criteria (Bu/Flu used starting in 2010). Median age at transplant was 51 (21-61) years for Bu/Cy vs. 64 (34-74) for Bu/Flu (P Conclusion Bu/Cy and Bu/Flu in alloHCT for AML results in comparable incidences of infection, GvHD, RM, NRM, RFS, and OS. This was despite Bu/Flu patients being older and more likely to have adverse cytogenetics and an unrelated donor. Bu/Flu is better tolerated with less toxicity. Faster hematopoietic recovery and shorter LOS with Bu/Flu reflects PBSC graft use and has implications for health care resource utilization. Future prospective studies with larger cohorts and cost-effectiveness analyses comparing these conditioning strategies are warranted. In this analysis, no mutations appeared to be sensitive to use of the more intensive regimen, Bu/Cy. Further investigation of pre-transplant or post-transplant persistence of somatic mutations may risk stratify those who may benefit from more intensive or innovative approaches to prevent relapse after transplant. Disclosures Gerds:Apexx Oncology: Consultancy; Celgene: Consultancy; CTI Biopharma: Consultancy; Incyte: Consultancy. Majhail:Incyte: Honoraria; Atara: Honoraria; Anthem, Inc.: Consultancy.