Pharmacokinetics of tacrolimus in adult renal transplant recipients.

BACKGROUND: The success of an immunosuppressive drug therapy depends on the extent of exposure to the drugs (the blood levels and duration), which is measured as the area under the curve (AUC). Tacrolimus shows considerable variability in its pharmacokinetics, with poor correlation between the tacrolimus trough level and systemic exposure, as measured by the AUC of concentration time. Monitoring trough levels helps not only in reducing nephrotoxiicity but also in reducing the chances of acute rejection; although there is no international consensus, the trough concentration is used to determine dosing and the AUC for calculating the exposure of the patient to the drug. The major objective of this study was to find the best sampling time for an abbreviated AUC0-6 (area under the concentration time curve) to predict the total body exposure to tacrolimus in adult renal transplantation recipients. METHODS: The study involved retrospective analysis of 14 renal transplant patients (2 female and 12 male) that were on triple immunosuppressive therapy, methyl prednisolone, mycophenolate mofetil and tacrolimus. To determine trough concentrations, blood samples were collected before administration of tacrolimus (0 h) and at fixed time points of 2 h, 4 h and 6 h after administration of oral tacrolimus and analyzed in duplicate by microparticle enzyme immunoassay. AUC0-6 was determined using the linear trapezoidal rule. The association between the blood concentration and AUC6 were evaluated by the Pearson correlation coefficient. All statistical analyses were performed using the SPSS software (IBM Corp., NY, USA) program. RESULTS: Trough levels were fairly consistent at 7.9-18 ng·h/mL in all the patients included in this study, and this did not show variation with age or sex. The AUC0-6 was higher [202-290 ng/mL at 3-8 mg bis-daily (b.d.) dosage] in patients who received kidneys from cadavers compared to recipients from live donors (60.5-171 ng/mL at 3-8 mg b.d. dosage), but the clinical significance of this is not known. The highest AUC0-6 was 246 ng/mL, observed at 4.5 mg b.d. dosage. Dosages higher than 2 mg b.d. did not result in a noticeable increase in AUC0-6. Peak blood levels of tacrolimus were obtained 4 h after administration. CONCLUSIONS: Trough level determination and a C2, C4 two-point limited sampling strategy may be useful to plan the dosing strategy and estimate the exposure of renal transplant patients to tacrolimus.