| Description |
The characteristics of inhibition and induction by imidazoles, and their possible relationship, were investigated. The N-substituted imidazole clotrimazole (CloTZ) was a strong inhibitor of in vitro cytochrome P-450 associated monooxygenase activity. So potent was its inhibition of monooxygenase activity that the concentration required for 50% inhibition (IC50) was similar to half the concentration of cytochrome P-450 used in the assay. CloTZ was also a strong inducer of drug metabolizing enzymes. Investigation of the dose dependence of induction by CloTZ revealed that its inducing properties changed as the dose was increased. Microsomes from rats induced with low doses of CloTZ (10-25 mg/kg) were associated with a different set of induced monooxygenase activities than those from rats induced with higher doses (45-75 mg/kg). These data suggest the existence of multiple and dose-differentiated mechanisms of cytochrome P-450 induction. Like CloTZ, other N-substituted imidazoles including miconazole (MCZ), tioconazole (TCZ), and ketoconazole (KCZ) bound to and inhibited cytochrome P-450 in a highly potent manner. In contrast, imidazole and cimetidine were 1000 fold less potent. These data were paralleled by those of in vivo induction studies, where the strongly inhibiting N-substituted imidazoles induced enzymes involved in oxidative and conjugative drug metabolism, and the weakly inhibiting cimetidine and imidazole lacked detectable inducing ability. Among N-substituted imidazoles, MCZ, TCZ, and KCZ induced cytochrome P-450 to an extent much less than CloTZ. Based on SDS-PAGE profiles of microsomal protein and on ratios of induced monooxygenase activities, the selectivity of induction by MCZ and TCZ also differed from that of CloTZ. Differences in the extent and selectivity of drug metabolizing enzyme induction by imidazoles with similar inhibitory properties therefore do not support a relationship between inhibition and induction. N-Substituted imidazoles also induced enzymes involved in conjugations, including glutathione S- and UDP-glucuronosyl-transferases, but not sulfotransferase. Marked differences in the specificity of induction of UDP-glucuronosyltransferase activity by these agents toward a series of aglycones were not predictable from their inductive effects on cytochrome P-450. Together with Phase I induction data, these findings indicate that structually related N-substituted imidazoles cannot be considered as a single class of inducing agents. |
