A high antithymocyte globulin dose increases the risk of relapse after reduced intensity conditioning HSCT with unrelated donors

The study included 110 consecutive patients with hematological malignancies receiving fludarabine‐based reduced intensity conditioning (RIC) and hematopoietic stem cell transplantation (HSCT) from matched unrelated donors. The median age was 55 yr (range 11–68) and all but 15 patients received peripheral blood stem cell grafts. Antithymocyte globulin (ATG) (Thymoglobulin, Genzyme) at a total dose of 6 mg/kg (n = 66) or 8 mg/kg (n = 44) was given to all patients according to protocol. The ATG dose did not affect time‐to‐neutrophil or platelet engraftment. The incidences of acute GVHD grades II–IV were 34% and 18% (p = 0.11) and of chronic GVHD were 40% and 26% (p = 0.46) in patients receiving 6 and 8 mg/kg of ATG, respectively. The five‐yr relapse‐free survival (RFS) was 61% and 36% (p = 0.14) in patients, given low and high ATG dose, respectively. In patients given low‐dose ATG, the incidence of relapse was lower compared to those given high‐dose ATG, 19% vs. 41% (p = 0.04). In multivariate analysis, age >50 yr (p < 0.001), absence of acute (p < 0.001) and chronic GVHD (p = 0.001) were correlated to relapse, and low‐dose ATG was associated with improved RFS (p < 0.05). A high dose (8 mg/kg) of ATG in RIC HSCT with unrelated donors increased the risk for relapse and reduced the RFS.