Prediction of the Intra‐T Lymphocyte Tacrolimus Concentration after Kidney Transplantation with Population Pharmacokinetic Modeling

The intracellular tacrolimus concentration in CD3 + T lymphocytes is proposed to be a better representative of the active component of tacrolimus than the whole blood concentration. However, intracellular measurements are complicated. Therefore, the aim of this study was to describe the relationship between intracellular and whole blood tacrolimus concentrations in a population pharmacokinetic model. Twenty‐eight de novo kidney transplant recipients, treated with a once‐daily oral extended‐release tacrolimus formulation, were followed during the first‐month post‐transplantation. Additional whole blood and intracellular tacrolimus concentrations were measured at day 6 ± 1 (pre‐dose, 4 and 8 hours post‐dose) and day 14 ± 3 (pre‐dose) post‐transplantation. Pharmacokinetic analysis was performed using nonlinear mixed effects modeling software (NONMEM). The ratio between intracellular ( n = 109) and whole blood ( n = 248) concentrations was best described by a two‐compartment whole blood model with an additional intracellular compartment without mass transfer from the central compartment. The ratio remained stable over time. Prednisolone dose influenced the absorption rate of tacrolimus, while hemoglobin, CYP3A4*22 allele carrier, and CYP3A5 expresser status were associated with the oral clearance of tacrolimus ( P ‐value < 0.001). Furthermore, the intracellular tacrolimus concentrations were correlated with the intracellular production of interleukin‐2 ( P ‐value 0.015). The intracellular tacrolimus concentration can be predicted from a measured whole blood concentration using this model, without the need for repeated intracellular measurements. This knowledge is particularly important when the intracellular concentration is ready to be implemented into clinical practice, to overcome the complexities of cell isolation and analytical methods.