In-vivo kinetics of the interaction between midazolam and erythromycin in rats, taking account of metabolic intermediate complex formation

Effect of the Treatment Period With Erythromycin on Cytochrome P450 3A Activity in Humans

The aim of the present study was to estimate the time course change in cytochrome P450 3A (CYP3A) activity during repeated doses of erythromycin. Twelve healthy male volunteers participated in this randomized, 4 x 4 Latin square design study. The pharmacokinetics of a single oral dose of midazolam, a probe for CYP3A activity, were assessed in 4 conditions: (1) midazolam (5 mg) without erythromycin (EM0), (2) erythromycin 2 days + midazolam (2.5 mg) (EM2), (3) erythromycin 4 days + midazolam (2.5 mg) (EM4), and (4) erythromycin 7 days + midazolam (2.5 mg) (EM7). The dose of erythromycin was 800 mg/d. Erythromycin produced a 2.3-, 3.4-, and 3.4-fold increase in dose-corrected area under the curve of midazolam for EM2, EM4, and EM7, respectively, as compared with EM0 (P <.05/6). A significant prolongation of terminal half-life was observed in EM4 and EM7. The relationship between the duration of erythromycin treatment and total clearance of midazolam indicated that a plateau level of CYP3A inhibition can be achieved by 4 days or more of erythromycin treatment. The repeated treatment with erythromycin yields CYP3A inhibition in a duration-dependent manner. A 4-day course of erythromycin treatment produces 90% or more of the maximal inhibition of CYP3A in humans.

Model for the drug–drug interaction responsible for CYP3A enzyme inhibition. II: establishment and evaluation of dexamethasone-pretreated female rats

1. Cytochrome P450 (CYP) 3A catalysis of testosterone 6beta-hydroxylation in female rat liver microsomes was significantly induced, then reached a plateau level after pretreatment with 80 mg kg(-1) day(-1) dexamethasone (DEX) for 3 days. 2. Midazolam was mainly metabolized by CYP3A in DEX-treated female rat liver microsomes from an immuno-inhibition study, and the apparent K(m) was 1.8 microM, similar to that in human microsomes. 3. Ketoconazole and erythromycin, typical CYP3A inhibitors, demonstrated extensive inhibition of midazolam metabolism in DEX-treated female rat liver microsomes, and the apparent K(i) values were 0.088 and 91.2 microM, respectively. The values were similar to those in humans, suggesting that DEX-treated female rat liver microsomes have properties similar to those of humans. 4. After oral administration of midazolam, the plasma midazolam concentration in DEX-treated female rats significantly decreased compared with control female rats. The area under the plasma concentration curve (AUC) and elimination half-life were one-11th and one-20th of those of control female rats, respectively. 5. Using DEX-treated female rats, the effect of CYP3A inhibitors on midazolam pharmacokinetics was evaluated. The AUC and maximum concentration in plasma (C(max)) increased when ketoconazole was co-administered with midazolam. 6. It was shown that the drug-drug interaction that occurs in vitro is also observed in vivo after oral administration of midazolam. In conclusion, the DEX-treated female rat could be a useful model for evaluating drug-drug interactions based on CYP3A enzyme inhibition.

The recovery time-course of CYP3A after induction by St John's wort administration

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

St John's wort causes the induction of CYP3A. Little is known about how long the effect remains after cessation of St John's wort.

WHAT THIS STUDY ADDS

The in vivo CYP3A activity returns progressively to the basal level approximately 1 week after cessation of St John's wort administration

AIMS

To examine the recovery time course of CYP3A after enzyme induction by St John's wort administration.

METHODS

The subjects were 12 healthy men, aged 20-33 years. On the first day, they received an oral dose of midazolam 5 mg without St John's wort (day -14). From the next day, they took St John's wort for 14 days. On the last day of St John's wort treatment (day 0) and 3 and 7 days after completion of St John's wort treatment (days 3 and 7), they received the same dose of midazolam. On each day, blood samples were obtained until 8 h after midazolam administration. Plasma concentrations of midazolam were measured by HPLC. Pharmacokinetic parameters of midazolam were determined using noncompartmental analysis.

RESULTS

Apparent oral clearance of midazolam was significantly increased after St John's wort administration from 65.3 +/- 8.4 l h(-1) (day -14) to 86.8 +/- 17.3 l h(-1) (day 0). It returned to the control level 7 days after the completion of St John's wort (day 7, 59.7 +/- 3.8 l h(-1)). No significant difference in the elimination half-life between the four periods of the study was observed. The changes in apparent oral clearance after St John's wort discontinuation indicated that CYP3A activity recovers from enzyme induction with an estimated half-life of 46.2 h.

CONCLUSIONS

CYP3A activity induced by St John's wort administration progressively returns to the basal level after approximately 1 week. This finding may provide useful information to avoid clinically significant interactions of St John's wort with CYP3A substrates.

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.