Antirelapse effect of pretransplant exposure to rabbit antithymocyte globulin

It remains unknown why rabbit antithymocyte globulin (ATG; Thymoglobulin) has not affected relapse after hematopoietic cell transplantation (HCT) in randomized studies. We hypothesized that high pre-HCT ATG area under the curve (AUC) would be associated with a low incidence of relapse, whereas high post-HCT AUC would be associated with a high incidence of relapse. We measured serum levels of ATG capable of binding to mononuclear cells (MNCs), lymphocytes, T cells, CD4 T cells, or CD33 cells. We estimated pre- and post-HCT AUCs in 152 adult recipients of myeloablative conditioning and blood stem cells. High pre-HCT AUCs of MNC- and CD33 cell–binding ATG were associated with a low incidence of relapse and high relapse-free survival (RFS). There was a trend toward an association of high post-HCT AUC of lymphocyte-binding ATG with a high incidence of relapse and low RFS. High pre-HCT AUCs were also associated with faster engraftment and had no impact on graft-versus-host disease (GVHD) or fatal infections. High post-HCT AUCs were associated with a low risk of GVHD, seemed associated with an increased risk of fatal infections, and had no impact on engraftment. In conclusion, pre-HCT AUC seems to have a positive, whereas post-HCT AUC seems to have a negative, impact on relapse.