Population Pharmacokinetic Modeling of Thymoglobulin (R) in Children Receiving Allogeneic-Hematopoietic Cell Transplantation: Towards Improved Survival Through Individualized Dosing

Background and Objectives To prevent graft-versus-host disease and rejection in hematopoietic cell transplantation (HCT), children receive Thymoglobulin (R), a polyclonal antibody acting mainly by depleting T cells. The therapeutic window is critical as over-exposure may result in delayed immune reconstitution of donor T cells. In this study, we describe the population pharmacokinetics of Thymoglobulin (R) as a first step towards an evidence-based dosing regimen of Thymoglobulin (R) in pediatric HCT. Methods Serum active Thymoglobulin (R) concentrations were measured in all pediatric HCTs performed between 2004 and 2012 in two pediatric HCT centers in The Netherlands. Population pharmacokinetic analysis was performed using NONMEM (R) version 7.2. Results A total of 3,113 concentration samples from 280 pediatric HCTs were analyzed, with age ranging from 3 months to 23 years old. The cumulative Thymoglobulin (R) dose was 10 mg/kg in 94 % of the patients given in 4 consecutive days. A model incorporating parallel linear and concentration-dependent clearance of Thymoglobulin (R) was identified. Body weight [for linear clearance (CL) and central volume of distribution] as well as lymphocyte count pre-Thymoglobulin (R) infusion (for CL) were important covariates. As such, the current dosing regimen results in higher exposure in children with a higher bodyweight and/ or a lower lymphocyte count pre-Thymoglobulin (R) infusion. Conclusion This model can be used to develop an individual dosing regimen for Thymoglobulin (R), based on both body weight and lymphocyte counts, once the therapeutic window has been determined. This individualized regimen may contribute to a better immune reconstitution and thus outcome of allogeneic HCT.