Effects of single and repeated treatment with itraconazole on the pharmacokinetics of midazolam in rats.

To estimate the influence of repeated administration of drug metabolism inhibitors on the extent of drug interaction, we investigated the effects of single intravenous or repeated oral administration of itraconazole on the pharmacokinetics of midazolam in rats. In the single administration study, the plasma concentration of itraconazole was maintained by intravenous infusion, and midazolam was administered into the portal vein to investigate its kinetics. In the repeated administration study, the kinetics of midazolam was investigated after seven-day oral treatment with itraconazole. The in vitro metabolism of midazolam and the contents of cytochrome P450 were investigated using liver microsomes from the itraconazole-treated rats. The area under the curve (AUC) of midazolam was increased by 1.45- or 1.44-fold after single or repeated itraconazole treatment, respectively. Meanwhile, the liver concentrations of itraconazole after single administration and repeated administration were 38.2 and 20.3 (nmol/g), respectively. In vitro maximum metabolic reaction velocity (V(max)) and Michaelis-Menten constant (K(m)) of midazolam were increased from 2.26 to 3.84 (nmol/min/mg protein) and from 8.28 to 13.0 (microM) by single itraconazole treatment, respectively, and decreased from 2.23 to 1.17 (nmol/min/mg protein) and from 7.86 to 4.47 (microM) by repeated treatment, respectively. Correspondingly, the content of CYP3A2 was significantly altered by single or repeated itraconazole administration. The increases in AUC could be predicted only when the changes in V(max) and K(m) were taken into consideration, in addition to the hepatic unbound concentration of itraconazole. In conclusion, changes in enzyme kinetics should be taken into account to predict the extent of drug interaction after repeated treatment with inhibitors.