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

Abstract

1. Cytochrome P450 (CYP) 3A catalysis of testosterone 6β-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 μM, 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 μM, 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.