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.

Inclusion Criteria:

1. Adult (≥ 18 year-old) male and female patients

2. Recipient of a single kidney or liver allograft

3. Patient transplanted for more than 2 weeks and less than 1 year at enrolment

4. Patient with stable Prograf® dose, defined by the following criteria:

   • Criterion 1: unchanged Prograf® dose for at least one week; if not, apply criterion #2
   • Criterion 2: unchanged Prograf® dose since the last two therapeutic drug monitorings (TDM)

5. Patient for whom the decision is made to switch from Prograf® to Envarsus®

6. Written informed consent obtained prior to any study-related procedure

7. Patient with tacrolimus C0 between 4 and 12 µg/L at V1

8. Patient with hematocrit > 27% at V1

Exclusion Criteria:

1. Patient presenting any contra-indication to tacrolimus according to the summary of product characteristics (SmPC) of Envarsus®
2. Recipient of any transplanted organ other than kidney or liver
3. Patient treated by Prograf® for less than 7 days at enrolment
4. Patient previously treated by any other investigational agent if it is not stopped at least 7 days prior to enrolment
5. Pregnant or lactating woman (based on declaration)
6. Patient under judicial protection
7. Patient incapable of understanding the purposes and risks of the study, who cannot give written informed consent, or who are unwilling to comply with the study protocol.