Anti-Thymocyte Globulin for Graft-Versus-Host Disease Prophylaxis in Patients with Acute Myeloid Leukemia Undergoing Reduced Intensity Conditioning Allogeneic Stem Cell Transplantation in First Complete Remission: A Survey on Behalf of the EBMT Acute Leukemia Working Party

Introduction: Anti-thymocyte globulin (ATG) prophylaxis was shown to be effective in reducing both acute and chronic graft-versus-host disease (GVHD) following allogeneic stem cell transplantation (allo-SCT). In previously published studies, the addition of ATG to the pre-allo-SCT conditioning regimen resulted in substantial reduction in GVHD incidence, but theoretically ATG could be associated with increased relapse rate, especially if administered at high doses (> 6 mg/kg of ATG Thymoglobulin or >15mg/kg of ATG Fresenius) to patients with high-risk acute myeloid leukemia (AML). To further address this question, we assessed transplantation outcome in adult AML patients who underwent reduced intensity conditioning (RIC) allo-SCT from a matched sibling donor (MSD) while in complete remission (CR1), with or without ATG. We used the registry data of the Acute Leukemia Working Party of the European Society forBlood and Marrow Transplantation (EBMT).

Patients and Methods: Inclusion criteria were adult patients with AML (de novo or secondary) who underwent RIC allo-SCT from an MSD in CR1 between the years 2000 and 2014. RIC conditioning was defined as per the EBMT criteria. Only patients presenting with intermediate- or poor-risk cytogenetics or secondary AML were included in the analysis. Patients who received any kind of T cell depletion including Campath were excluded. Patients were considered to receive high-dose ATG if they received > 6mg/kg of ATG-Thymoglobulin or >15mg/kg of ATG-Fresenius. All other patients were considered as a control.

Results: Two hundred and five of the 1750 patients, included in the analysis, received high-dose ATG. The median patient age at diagnosis was 56 years and the median donor age was 55 years for both groups. Fifty four percent of the patients were males. The graft properties were similar in the high-dose ATG and control groups, with female-to-male transplants in 48/205 (23.4%) and 378/1545 (24.6%) patients, respectively. Peripheral blood was the source of grafts in 193/205 (94.1%) and 1413/1545 (91.5%) patients, respectively (p=NS). Poor cytogenetics was reported in 28/205 (13.7%) patients who were given high-dose ATG and in 247/1545 (16%) patients from the control group. Secondary AML was more prevalent among patients from the high-dose ATG group compared to the control group [84/205 (41%) vs 425/1545 (27.5%; p=0.0003]. The median follow-up was 44.6 (0-177) months.

The 3-year overall survival (OS) and leukemia-free survival (LFS) were almost identical in both groups, amounting to 54.9 and 46.3 months, respectively for patients who received high-dose ATG and to 54 and 49.5 months, respectively for patients in the control group (p=0.978 and 0.376 ). Differences in relapse incidence (RI) (40.9% vs 34.4%; p=0.1) and non-relapse mortality (NRM) (11.7% vs 16%; p=0.152) were insignificant. The composite endpoint of GVHD-free/relapse-free survival (GRFS) was similar in the groups after one and three years of follow-up (42.9 vs 49.7 and 31.1 vs 36 months, respectively; p=0.152 and p=0.124). In multivariate analysis, the use of ATG prophylaxis was not found to be associated with any of the evaluated parameters. In contrast, OS, LFS, RI, NRM and GRFS were all affected by patient age and presence of secondary AML. Poor cytogenetics correlated with inferior OS, LFS, RI and GRFS. Donor CMV positivity was associated with lower OS, LFS and GRFS and a higher rate of NRM.

Conclusions: In AML patients presenting with intermediate- or poor-risk cytogenetics who are referred to RIC allo-SCT from an MSD in CR1, the addition of ATG to conditioning regimens does not influence the outcome. The composite GRFS endpoint is mainly affected by AML risk factors (hazard ratio of 1.43 for secondary AML and 1.37 for poor-risk cytogenetics; p=0.0001). The role of ATG prophylaxis in AML patients undergoing RIC allo-SCT should be prospectively evaluated.