The Optimal Dose of Anti-Thymocyte Globulin in Beta-Thalassemia Major Following Matched Sibling Donor Hematopoietic Stem Cell Transplantation: A Single-Center, Open-Label, Randomized, Controlled Clinical Study

Background: Matched sibling donor hematopoietic stem cell transplantation (MSD-HSCT) has been a frontline option for beta-thalassemia major (beta-TM). Our previous single-center study showed that the 3-year overall survival (OS), thalassemia-free survival (TFS), and GVHD-free and relapse-free survival (GRFS) were 97.8%, 97.3%, 89.5% in MSD-HSCT for beta-TM, when the graft type was G-CSF-mobilized BM and G-CSF-mobilized PBSCs (G-BM&PBSCs). However, we noted some limitations in our previous study, including the relatively high incidence of reactivation of cytomegalovirus (CMV) and hemorrhagic cystitis (HC). Moreover, half of the death cases died of severe infections. Anti-thymocyte globulin (ATG) as a regimen for in vivo T-cell depletion reduces the incidence of graft-versus-host disease (GvHD) and graft failure (GF), but also increases the risk of severe infections. The optimal dose of ATG to preserve the satisfactory OS and prevent severe infections following MSD-HSCT is unknown. We initiated a study to evaluate the effect of the 8mg/kg of ATG as the experimental group (ATG-8 group) vs 10mg/kg of ATG as the control group (ATG-10 group) on OS and TFS for G-BM&PBSCs graft transplantation and BM&CB graft transplantation. The secondary endpoints were GvHD, GF, transplantation-related mortality (TRM), infection-related mortality (IRM), and post-transplantation lymphoproliferative disorder (PTLD).

Methods: Patients received 20–30 mg/kg hydroxyurea orally once daily for 2–3 months before transplantation. The conditioning regimen included busulfan (Bu, BUSULFEX, 1mg/kg i.v. 4x daily on days -9 to -6), cyclophosphamide (Cy, 50mg/kg i.v. once daily, days -5 to -2), fludarabine (Flu, 50mg/m² i.v. once daily from days -12 to -10), and anti-thymocyte globulin (ATG, Thymoglobulin™, 2 or 2.5mg/kg i.v. once daily, days -4 to -1). Patients were randomly assigned in a 1:1 radio to either the ATG-8 group or the ATG-10 group. Subgroup analysis was performed between G-BM&PBSCs graft transplantation and BM & CB graft transplantation. The prophylaxis regimen included cyclosporine (CsA, 1.5-3mg/kg/d), methotrexate (MTX, 15mg/m2×1d + 10mg/m2 x3d), and mycophenolate mofetil (MMF, 250mgx30d). The study was registered at www.chictr.org.cn (ChiCTR-IPR-15005779).

Results: A total of 295 patients (147 in the ATG-8 group and 148 in the ATG-10 group) from The First Affiliated Hospital of Guangxi Medical University were enrolled in this study with a median age of 7 years old (IQR 5-9, range 2-17) between July 2015 to December 2023. The baselines were matched between the ATG-8 group and the ATG-10 group.

All patients achieved primary engraftment without thalassemia relapse. The 3-year OS was comparable between the ATG-8 group and the ATG-10 group (98.6% [95% confidence interval 96.8%-100%] for ATG-8 and 95.3% [91.9%-98.7%] for ATG-10, p=0.093). The 3-year TFS was consistent with the 3-year OS. The cumulative incidence of grades 3-4 aGvHD and moderate-to-severe cGVHD were also comparable between the ATG-8 group and the ATG-10 group (grades 3-4 aGvHD: 7.2% [3.7%-12.4%] for ATG-8 and 4.2% [1.7%-8.5%] for ATG-10 [p=0.364]; moderate-to-severe cGVHD: 5.0% [2.2%- 9.6%] for ATG-8 and 4.2% [1.7%-8.4%] for ATG-10, p=0.812). The incidence of reactivation of CMV and Epstein-Barr virus (EBV) were comparable between the ATG-8 group and the ATG-10 group (CMV: 33.3% [25.8%-41.6%] for ATG-8 and 35.8% [28.1%-44.1%] for ATG-10 [p=0.655]; EBV: 12.9% [8%-19.4%] for ATG-8 and 14.9% [9.6%-21.6%] for ATG-10 [ p=0.630]). Besides, the incidence of HC was lower in the ATG-8 group compared to the ATG-10 group (15.0% [9.6%-21.8%] for ATG-8 and 25.7% [18.9%-33.5%] for ATG-10, p=0.022).

Nevertheless, in the BM&CB subgroup, the 3-year OS in the ATG-8 group was higher compared with the ATG-10 group (100% [100%-100%] for ATG-8 and 86.5% [75.5%-98.2%] for ATG-10; p=0.007). In the BM&CB subgroup, the TRM was the only cause of death and all occurred in the ATG-10 group, including 3 who died of IRM and 2 who died of PTLD.

Conclusions: It was our first attempt to explore the optimal dose of ATG in MSD-HSCT for beta-TM. Considering the outcomes of the 3-year OS, TFS, TRM, the cumulative incidence of GVHD, and the incidence of HC, an 8mg/kg dose of ATG was recommended in MSD-HSCT for beta-TM.