Reduced Intensity Vs. Non-Myeloablative Conditioning Regimens for Haploidentical Transplantation in Complete Remission Acute Myeloid Leukemia: A Study from the ALWP of the EBMT

Background: In the context of a haploidentical stem cell transplantation (Haplo-SCT) platform with post transplantation cyclophosphamide (PT-Cy) for acute myeloid leukemia (AML) patients, the optimal conditioning regimen remains unknown. A non-myeloablative conditioning (NMAC) regimen (cyclophosphamide + fludarabine + 2Gy TBI [CyFluTBI]) was initially reported by the Johns Hopkins group as a safe approach in this setting, notably to treat patients of advanced age and/or with comorbid conditions. However, relapse incidence after NMAC Haplo-SCT remains high in AML where it can reach 45%. Alternatively, a reduced intensity conditioning (RIC) regimen containing an antileukemic drug combination like thiotepa and reduced-dose busulfan in addition to fludarabine (TBF) may decrease AML relapse. However, this anticipated benefit may be counterbalanced by a higher incidence of toxicity, graft-versus-host disease (GVHD) and non-relapse mortality (NRM). To date, no study comparing TBF vs. CyFluTBI has been published in complete remission (CR) AML. We performed this retrospective comparison on behalf of the Acute Leukemia Working Party (ALWP) of the European Society for Blood and Marrow Transplantation (EBMT).

Methods: We retrospectively analyzed 398 patients from the EBMT registry database with the following inclusion criteria: 1) adult patient in CR1 or CR2 AML; 2) T-replete Haplo-SCT with PT-Cy; 3) no in vivo depletion using antithymocyte globulin or alemtuzumab; and 4) receiving either TBF RIC (equivalent of 2-day iv busulfan dose) or CyFluTBI NMAC regimen. We compared separately TBF vs. CyFluTBI in patients younger (n=170, 82 TBF vs. 88 CyFluTBI) and older (n=228, 141 TBF vs. 87 CyFluTBI) than 60 years.

Results: In patients younger than 60 years, the 2-year cumulative incidence of relapse (CIR) was significantly lower in the TBF group compared with the CyFluTBI group (TBF vs. CyFluTBI: 14% vs. 43%, p<0.01). No significant increase in 2-year NRM was observed (TBF vs. CyFluTBI: 22% vs. 16%, p=0.15). This led to a significantly higher 2-year leukemia-free survival (LFS) probability in the TBF group (64% vs. 41%, p=0.03). After adjustment in multivariate analysis, CyFluTBI was associated with a higher risk of relapse (hazard ratio [HR] 3.4, 95%CI [1.4-6.9], p<0.01), lower LFS (HR 1.8, 95%CI 1.1-3.0, p=0.03) and lower overall survival (OS) (HR 1.8, 95%CI 1.1-3.1, p=0.02), without significant impact of conditioning regimen on incidence of GVHD and NRM.

In patients older than 60 years, univariate analysis did not show any significant difference in outcome according to the type of conditioning regimen (2-year NRM: TBF vs. CyFluTBI: 33% vs. 25%, p=0.23; 2-year CIR: TBF vs. CyFluTBI: 23% vs. 28%, p=0.20; 2-year LFS: TBF vs. CyFluTBI: 44% vs. 47%, p=0.96). Multivariate analysis showed a significant reduction in the risk of NRM after CyFluTBI (HR 0.5, 95%CI [0.2-0.9], p=0.04), while a non-significant increase in the risk of relapse was observed (HR 1.9, 95%CI [0.8-4.2], p=0.13). Finally, there was no significant difference in LFS (HR 0.9, 95%CI [0.5-1.5], p=0.67) and OS (HR 0.9, 95%CI [0.5-1.5], p=0.67).

Conclusion: Our study suggests that in CR AML patients aged younger than 60 years, the use of TBF RIC provides better outcomes than NMAC CyFluTBI due to lower incidence of relapse, without significant increase in the risk of NRM. Conversely, it seems that older patients do not benefit from such conditioning intensification, due to a significantly higher risk of NRM after TBF RIC. Thus, in CR AML patients who will not receive a truly myeloablative regimen prior to PT-Cy Haplo-SCT, age could be used for determining the conditioning intensity from the wide variety of reduced toxicity conditioning regimens. Beyond the patient age, further prospective trials should assess patient-based parameters that may be useful for a fine tuning of conditioning intensity in a more individualized approach.