Relationship Between Antithymocyte Globulin Concentrations and Lymphocyte Sub-Populations in Kidney Transplant Patients.

BACKGROUND Rabbit antithymocyte globulins (rATGs) are polyclonal antibodies used to prevent acute cellular rejection in kidney transplantation. Their dosing remains largely empirical and the question of an individualized dose is still unresolved. METHODS Data from a prospective study in 17 kidney transplant patients were used to develop a model describing the dose-concentration-response relationship of rATG with T-lymphocyte subpopulation counts over time. The model was validated using an independent cohort of kidney transplant patients treated by rATG in the same center. RESULTS Pharmacokinetics of rATG was described using a two-compartment model integrating a third compartment and a target-mediated elimination for active rATG. The kinetics of CD3+, CD4+, CD8+, and CD3-CD56+ cell counts over time were described by a pharmacokinetic-pharmacodynamic model with transit compartments, integrating both CD3-CD56+-independent and CD3-CD56+-dependent rATG-mediated lymphocyte depletion, and a positive feedback. Elimination of rATG was influenced by age and body surface area, while its distribution was also influenced by body surface area. CD3+ proliferation rate decreased with age and CD3-CD56+-mediated elimination was influenced by the V158F-FCGR3A polymorphism. Binary efficacy and tolerance endpoints were defined as a CD3+ count 200 mm-3 at 1 year, respectively. Simulations showed that increasing or decreasing the standard 6-mg/kg dose will impact both tolerance and efficacy, while a dose decrease may be beneficial in elderly patients. CONCLUSIONS Our results can be used to design prospective clinical trials testing dose individualization based on patients' characteristics. CLINICAL TRIAL REGISTRATION Eudract No. 2009-012673-35.