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 55yr (range 11-68) and all but 15 patients received peripheral blood stem cell grafts. Antithymocyte globulin (ATG) (Thymoglobulin, Genzyme) at a total dose of 6mg/kg (n=66) or 8mg/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 8mg/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 >50yr (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 (8mg/kg) of ATG in RIC HSCT with unrelated donors increased the risk for relapse and reduced the RFS.