Comparative Results of Pediatric Patients with ALL Receiving Either of Two Different Types of Human Anti-T Lymphocyte Globulin Prior to Hematopoietic Stem Cell Transplantation: Findings from the International Prospective Forum Study

Introduction Anti T lymphocyte globulins (ATG) are effective in preventing both severe acute and chronic Graft-versus-Host Disease (GVHD) and rejection after unrelated donor (UD) hematopoietic stem cell transplantation. The two most commonly used types of ATG are rabbit polyclonal antibodies. Grafalon® (Neovii) is produced by rabbit immunization with the Jurkat T-cell line, while Thymoglobulin® (Sanofi) is made after the animal immunization with human thymocytes. Although both agents are lymphodepleting, the different formulations vary in antibody quantity, antigen recognition, and ability to eliminate or modulate the function of T, B, NK, and dendritic cells. Few studies have directly compared the efficacy and outcomes of Grafalon and Thymoglobulin. Methods Here we report the outcomes of a homogenous cohort of pediatric acute lymphoblastic leukemia (ALL) patients who received over 3 days before the allograft either Grafalon (45 mg/kg total dose (TD)) or Thymoglobulin (7.5 mg/kg, TD) in the international prospective ALL SCTped 2012 FORUM trial (NCT01949129). Patients, aged 4-18 years, were transplanted from a 9/10 or 10/10 HLA matched UD after uniform conditioning with 12 Gy total body irradiation and etoposide (60 mg/kg TD, max 3600 mg). Results From 2012 until 2023, 400 patients from 24 countries were enrolled. 303 received Grafalon (17 countries) and 97 received Thymoglobulin (15 countries). The two cohorts were comparable in terms of gender, age, ALL immunophenotype, remission status, minimal residual disease (MRD) level at transplant, donor/recipient- HLA matching (HLA 10/10 68% vs 58%, and HLA 9/10 32% vs 42%, p=0.13), stem cell source (63% vs 54% bone marrow, p=0.25) and CMV/EBV- donor/recipient IgG results. The 3-yr overall survival (OS) and event-free survival (EFS) rates were similar in both groups with 84% (+/-2) vs 77% (+/-5), (p=0.398), and 73% (+/-3) vs 74% (+/-5) (p=0.708), for Grafalon vs Thymoglobulin, respectively. The 3-yr cumulative incidence of relapse (CIR) was 17% (+/-2) for Grafalon vs 13% (+/-4) for Thymoglobulin (p=0.375). We observed no differences in acute GVHD incidence, with grade III-IV of 6% vs 10%, (p=0.176) for Grafalon and Thymoglobulin. Grafalon patients had a lower 3-year cumulative incidence of any chronic GVHD, estimated at 7% (+/-2) vs 16% (+/-4) (p=0.013). This was not associated with a significant difference in non-relapse mortality (3-yr NRM of 7% (+/-1) for Grafalon vs 13% (+/-4) for Thymoglobulin (p=0.097)). There was a trend towards a higher incidence of mild chronic GVHD in the Thymoglobulin group ((7% +/- 3) vs 3% (+/-1) p=0.05), but no difference was seen in severe chronic GVHD (3% +/-2 vs 2% +/- 1 (p=0.338)). Early grade III-IV toxicity varied between groups, with more frequent disseminated intravascular coagulation (3% vs 0%, p=0.002) in the Thymoglobulin group, but higher incidence of stomatitis, nausea, and elevated liver enzymes after Grafalon treatment. Grafalon patients had higher frequencies of hematological grade III-IV adverse events (cytopenia and hemolysis). Leukocyte engraftment was slightly delayed, with Grafalon patients reaching a WBC count of >1G/l at a median of day 22, vs day 20 for Thymoglobulin (p=0.036), while neutrophil engraftment was similar at day 28. Compared to Grafalon, more patients developed CMV reactivation/disease after Thymoglobulin exposure (14% vs 24% at day 100, p=0.031) and more Thymoglobulin-patients developed EBV reactivation/infection and post-transplant lymphoproliferative disease (PTLD) (Grafalon 17%, Thymoglobulin 38%, p <0.001; PTLD n= 1 vs 5). Subgroup analyses (CR1 and ≥ CR2) showed comparable outcomes (OS, EFS, CIR and NRM). In ≥ CR2 patients, the 3-year cumulative incidence of any chronic GVHD was lower after Grafalon with 6% (+/-2) vs 17% (+/-5) for Thymoglobulin (p=0.032). Conclusion We observed excellent outcome results following serotherapy with both Grafalon and Thymoglobulin in ALL pediatric patients transplanted from 9/10 or 10/10 allelic matched UD conditioned with TBI/etoposide. The incidence of acute GVHD did not differ significantly. Patients treated with Grafalon had lower rates of any chronic GVHD. The frequency of CMV disease after reactivation and EBV reactivation, disease and PTLD was significantly higher after Thymoglobulin. The main outcome measures - OS, EFS, and NRM did not show significant differences between the two treatment groups.