Pharmacokinetics and pharmacodynamics of tacrolimus in liver transplant recipients: inside the white blood cells

Objectives: Despite improvements in patient management and extensive use of therapeutic drug monitoring (TDM), the rate of acute cellular rejection (ACR) remains high in patients treated with tacrolimus (TAC). Moreover, some patients experienced ACR while their whole-blood (WB) concentrations were maintained within the therapeutic range meaning that TDM in WB misrepresents the drug effect. Thus, monitoring TAC directly inside of its effect compartment (intracellular concentrations) or monitoring directly the inhibitory effect on the target protein (calcineurin activity) could be more relevant. The aim of the present study was to explore, in 10 de novo liver transplant recipients, the relationship between TAC whole-blood concentrations, TAC intracellular concentrations and TAC-induced intracellular calcineurin inhibition at day 1 and day 7 after treatment initiation. Design and methods: Prospective monocentric observational pharmacokinetic (WB and intracellular concentrations)-pharmacodynamic (calcineurin activity) study. Results: Full intracellular TAC pharmacokinetic as well as calcineurin activity steady-state profiles is presented in the study. The main result of this study is the lack of relationship between TAC pharmacokinetics (WB and leukocytes) and calcineurin activity in leukocytes at day 1 and day 7 after the graft implantation. Conclusions: Drug monitoring of TAC intracellular concentrations and determination of the calcineurin activity are among future potential biomarkers of acute rejection in transplant recipients. A better knowledge of the relationship between TAC whole blood and intracellular concentrations and calcineurin activity appears necessary before planning clinical trials to evaluate their potential interest as predictive biomarkers. (C) 2014 The Canadian Society of Clinical Chemists. Published by Elsevier Inc. All rights reserved.