Post-Transplant Cyclophosphamide Versus Antithymocyte Globulin in Patients with Acute Myeloid Leukemia Undergoing Allogeneic Stem Cell Transplantation from HLA-Identical Sibling Donors: A Retrospective Analysis from the Acute Leukemia Working Party of the EBMT

Introduction. In patients undergoing allogeneic hematopoietic stem cell transplantation (allo-HSCT) from an HLA-identical matched sibling donor (MSD), use of cyclosporine A in association with methotrexate has been the main graft-versus-host disease (GVHD) prophylaxis strategy. Addition of antithymocyte globulin (ATG), especially when using peripheral blood stem cells (PBSC) as source, has also been shown to favor lower rates of chronic GVHD. More recently, use of post-transplant cyclophosphamide (PTCY) in the haploidentical setting resulted in low incidences of both acute and chronic GVHD. Its use has therefore been subsequently reported in other donor settings, including MSD. The aim of our study was to compare GVHD prophylaxis containing either PTCY or ATG in patients diagnosed with acute myeloid leukemia and undergoing allo-HSCT from a MSD.

Methods. This was a retrospective study from the EBMT registry. Included were adult patients undergoing their first allo-HSCT during the period 2008-2018 and who were in complete remission at time of allo-HSCT.

Results. Globally, 197 and 1,913 patients receiving either PTCY or ATG, respectively, were identified who fulfilled inclusion criteria. Patients in the PTCY group were younger (median age 47 versus 54 years, p<0.01) and had undergone allo-HSCT more recently (median year of allo-HSCT: 2015 versus 2014, p<0.01). Peripheral blood was the more frequently used stem cell source, this being significantly more frequent in the ATG group (95% versus 70%) than in the PTCY group (p<0.01). The conditioning regimen was more frequently myeloablative in the PTCY group (59% versus 48% in the ATG group, p<0.01). Cyclosporine-A alone was the most used systemic immunosuppressive agent associated with either PTCY (22%) or ATG (28%), while 34% and 29% of patients in the ATG group and 7% and 12% in the PTCY group also received either methotrexate or mycophenolate mofetil along with cyclosporine A. No statistical differences were observed for incidence of grade II-IV acute GVHD at 100 days after allo-HSCT, this being 19% versus 17% in the PTCY and ATG groups, respectively (p=0.81). On the other hand, a significantly higher incidence of chronic GVHD at 2 years was observed with the use of PTCY (37% versus 30%, p<0.02) and for extensive chronic GVHD (16% versus 12%, p<0.01) as compared to ATG. There was no impact of conditioning intensity on GVHD incidence. No statistical differences were observed on univariate analysis for all other transplant outcomes, with a leukemia-free survival (LFS) of 55% versus 58%(p=0.75), overall survival (OS) of 64% versus 65% (p=0.61), GVHD-free, relapse-free survival (GRFS) of 44% versus 49% (p=0.19), relapse incidence (RI) of 36% versus 32% (p=0.56) and non-relapse mortality (NRM) of 8% versus 10% (p=0.78). Of note, the cumulative incidences of death due to GVHD were 4.4% and 4% in the PTCY and ATG groups, respectively (p=0.53). Also, there were no differences between the 2 groups for death due to infections: 6% in the PTCY group and 6.3% in the ATG group, respectively (p=0.49). On multivariate analysis, these results were confirmed, with a higher risk of chronic GVHD of any grade (HR=1.41, 95%CI 1.03-1.92; p<0.01) and extensive chronic GVHD (HR=1.68, 95%CI, 1.07-2.62; p<0.01) with PTCY, and no differences with respect to the other outcomes. We also observed that, regardless of the use of PTCY or ATG, use of PBSC was associated with lower RI (HR=0.64, 95% CI 0.46-0.89; p<0.01), higher LFS (HR=0.71, 95% CI 0.53-0.95; p<0.03), and OS (HR=0.72, 95% CI 0.52-0.99; p<0.05). On the other hand, we found no statistical differences in terms of both acute GVHD (HR=0.73, 95%CI, 0.48-1.12; p=0.15) and chronic GVHD (HR=0.83, 95% CI 0.56-1.21; p=0.33) according to stem cell source. These results were also confirmed in a matched-pair analysis.

Conclusion. In conclusion, our results highlight that in the HLA-identical sibling setting, the use of ATG provides similar outcomes to those seen with PTCY except for chronic GVHD for which a protective effect of ATG is confirmed as previously reported by different other studies.