PK Assessment of Tacrolimus Exposure Before and After a Switch From Twice Daily Immediate-release (Prograf®) to Once-daily Prolonged Release Tacrolimus (Envarsus®)

Tools have been developed in our unit to calculate the inter-dose AUC (Area Under Curve) of immunosuppressive drugs (ISD) based on a limited number of blood concentrations (i.e., blood samples) using Bayesian methods. Since 2005, we have implemented these tools in an expert system and made them available to the transplant community through our very successful ISBA (Immunosuppressive drugs Bayesian dose Adjustment) website. Briefly, we first need to develop a population pharmacokinetic model using rich pharmacokinetic (PK) profiles (about 10 samples per patient over the dosing interval). The model developed can then be used for inference of ISD PK parameters in new patients using Bayesian estimation. Bayes' theorem is based on conditional probability: individual PK parameters are estimated based on the known PK parameters in the population (mean and distribution), given the dose and concentrations observed in a patient. Our previous studies have shown that a limited sampling strategy (LSS) based on 3 samples collected within the first 3 hours after drug intake can estimate adequately the interdose AUC of ISD. In the present study, the AUC0-24h and the recommended dose will be calculated using Bayesian estimators previously developed using PK data from the clinical trials run by Veloxis, and proposed to the clinicians via a dedicated website comparable with ISBA.