Impact of CYP3A5*3 on plasma exposure and urinary excretion of fentanyl and norfentanyl in the early postsurgical period.

BACKGROUND: The pharmacokinetic characteristics of intravenous fentanyl have not been fully clarified in the early postsurgical period. The aim of this study was to evaluate the plasma exposure and urinary excretion of fentanyl and norfentanyl according to cytochrome P450 (CYP) 3A5 genetic polymorphism.
METHODS: Fifty-two adult Japanese postoperative patients receiving a continuous intravenous fentanyl infusion were enrolled. Plasma concentrations of fentanyl and norfentanyl were determined at 24 hours after the operation, and their urinary excretion from 12 to 36 hours was evaluated.
RESULTS: Plasma concentrations of fentanyl normalized for infusion rate were significantly higher in the *3/*3 group than in the *1 carrier group. The plasma concentration ratio of norfentanyl to fentanyl was significantly lower in the *3/*3 group than in the *1 carrier group. Urinary excretion rates of fentanyl and norfentanyl were 4.4% and 71%, respectively, and no significant differences were observed between the CYP3A5 genotypes. Renal clearance ratios of fentanyl and norfentanyl to creatinine were 0.34 and 3.4, respectively. There were no significant differences in the renal clearance ratios between the genotypes. Free fractions of fentanyl and norfentanyl in human plasma were 4.9% and 95%, respectively. Total and nonrenal clearance of fentanyl were significantly lower in the *3/*3 group than in the *1 carrier group.
CONCLUSIONS: CYP3A5*3 affected the plasma exposure of fentanyl but not urinary excretion in our postoperative patients. The renal clearance ratios of fentanyl and norfentanyl to creatinine were much higher than their free fractions in plasma. These findings suggest a slight contribution of renal tubular secretion of fentanyl and norfentanyl to their plasma exposures.