Does Lymphocyte Count Impact Dosing of Anti-Thymocyte Globulin in Unrelated Donor Stem Cell Transplantation?

Anti-thymocyte globulin (ATG) is used to reduce the incidence and severity of graft-versus-host disease (GVHD) with hematopoietic cell transplantation, yet optimum dosing has yet to be determined. We have previously demonstrated that 2.5 mg/kg ATG in conditioning can reduce the incidence of GVHD in unrelated donor transplants. Recent literature has suggested that ATG dosing based on absolute lymphocyte count (ALC) could lead to more optimum exposure of the drug. We sought to determine if ALC at the time of transplant could impact clinical outcomes. We conducted a retrospective single-center study analyzing all consecutive patients at The Ottawa Hospital who received a matched unrelated donor stem cell transplant with ATG between 2009 and 2014. Patients received rabbit ATG (thymoglobulin) at 0.5 mg/kg on day -2 and 2.0 mg/kg on day -1. Univariate and multivariate analyses were used to determine if any patient- or transplant-related factors, including weight, ALC, and total ATG dose given, impacted GVHD, relapse, or mortality. In total, 111 patients met inclusion, with a median age of 50 years (range, 19 to 70). The most common diagnoses were acute myelogenous leukemia (43%), Myelodysplasia/myeloproliferative neoplasms (13%), and lymphoma (12%). The median weight at time of conditioning was 80.3 kg (range, 45 to 216). The median ALC on the first day of ATG administration was 0.1 × 10⁹/L (range, 0 to 190). The median total dose of ATG received was 201 mg (range, 112 to 540 mg). The incidence of acute and chronic GVHD was 35.1% and 21.6%, respectively. In the multivariate model, the actual dose of ATG given to patients was not associated with GVHD (hazard ratio [HR], 1.11; 95% confidence interval [CI], 0.99 to 1.25; P = .07), relapse (HR, 1.13; 95% CI, 0.98 to 1.30; P = .1), or mortality (HR, 1.09; 95% CI, 0.92 to 1.28; P = .32). Similarly, the pretransplant ALC was not associated with GVHD (HR, 1; P = .82), relapse (HR, 1; P = .90), or mortality (HR, 1; P = .39). If patients had received ALC-based dosing according to previously published work (Admiraal et al., Lancet Haematol 2017), the mean total dose of ATG received would have been 1205 mg, more than 5 times the mean dose that was actually given based on weight. With GVHD outcomes being similar to that published by Admiraal et al. and ALC not independently associated with outcomes in our study, further studies are still needed to compare standard weight-based dosing to ALC-based dosing of ATG in matched unrelated donor stem cell transplant.