Horse versus rabbit antithymocyte globulin in acquired aplastic anemia

Abstract

Background: In severe acquired aplastic anemia, hematopoietic failure is the result of immune-mediated destruction of bone marrow stem and progenitor cells. Immunosuppressive therapy with antithymocyte globulin (ATG) plus cyclosporine is an effective alternative to stem-cell transplantation and improves blood counts and survival. Although horse ATG is the standard therapy, rabbit ATG is more potent in depleting peripheral-blood lymphocytes and is preferred in other clinical circumstances.

Methods: From December 2005 through July 2010, we performed a randomized trial comparing these two ATG formulations in conventional regimens. Patients were treated at a single facility. The primary outcome was hematologic response at 6 months, as determined by blood counts. The study was designed to enroll 60 patients each for the rabbit-ATG and horse-ATG groups and was powered to detect a difference of 25 percentage points in the response rate.

Results: A large, unexpected difference was observed in the rate of hematologic response at 6 months in favor of horse ATG (68%; 95% confidence interval [CI], 56 to 80) as compared with rabbit ATG (37%; 95% CI, 24 to 49; P<0.001). Overall survival at 3 years also differed, with a survival rate of 96% (95% CI, 90 to 100) in the horse-ATG group as compared with 76% (95% CI, 61 to 95) in the rabbit-ATG group (P=0.04) when data were censored at the time of stem-cell transplantation, and 94% (95% CI, 88 to 100) as compared with 70% (95% CI, 56 to 86; P=0.008) in the respective groups when stem-cell-transplantation events were not censored.

Conclusions: In a randomized study, rabbit ATG was inferior to horse ATG as a first treatment for severe aplastic anemia, as indicated by hematologic response and survival. (Funded by the Intramural Research Program of the National Institutes of Health; ClinicalTrials.gov number, NCT00260689.).

Figure 1. Increase in blood counts in patients with hematologic improvement after ATG

Among responders to immunosuppression (n=41 for horse ATG and n=22 for rabbit ATG), there were comparable increments in blood counts at 3 and 6 months between the two groups. The mean ± standard error of mean is shown.

Figure 2. Kaplan-Meier curves of overall survival

Patients were censored at time of stem cell transplantation in Panel A while stem cell transplantation events were ignored in Panel B. Numbers at the bottom of the graph indicate patients at risk for each time point.

Figure 3. Lymphodepletion after ATG administration

(A) The initial decrease in total absolute lymphocyte count was similar between the two ATGs, but lymphocyte counts remained lower longer after rabbit ATG. (B) T cells (CD3⁺CD45⁺) decreased rapidly with both ATGs with reconstitution in subsequent weeks. (C, D) There was a large difference between the kinetics of CD4⁺ T cell depletion after horse and rabbit ATG, with a much lower frequency and absolute numbers after rabbit ATG. (E, F) The frequency of regulatory T cells (Tregs; defined as CD4⁺CD25⁺CD127⁻ for this analysis) was higher after rabbit ATG, but absolute numbers were markedly lower due to more potent depletion of CD4⁺ T cells, as compared to horse ATG. (G, H) The difference in depletion kinetics of CD8⁺ T cells was less striking between the two ATGs when compared to CD4⁺ T cells. The mean ± standard error of mean is shown. All 120 patients are depicted in panel A. Fourteen patients are depicted in panels B–H (7 from each ATG group). Differences for each time point that are statistically significant (p<0.05) are denoted by an asterisk (paired t-test). For details on flow cytometric analysis, see Methods section in Supplementary Appendix.