Comparative effects of antithrombitic and antimycotic N-substituted imidazoles on rat hepatic microsomal steroid and xenobiotic hydroxylases in vitro.

N-Substituted imidazoles have been shown to be potent inhibitors of microsomal mixed-function oxidase activities in vitro and in vivo. In the present study the effects of two antithrombitic (dazmegrel and dazoxiben) and four antimycotic (ketoconazole, econazole, miconazole and clotrimazole) imidazoles on microsomal cytochrome P-450-mediated steroid and xenobiotic hydroxylases were studied in vitro. Despite the presence of the N-substituted imidazole moiety, the antithrombitic agents were essentially non-potent as inhibitors of all of the oxidase activities evaluated. In contrast, the antimycotic drugs were potent inhibitory compounds. Binding studies revealed that all six imidazoles elicited type II optical difference spectra and exhibited relatively high affinity for ferricytochrome P-450 in microsomal suspensions (Ks range 0.26-0.73 microM for the antimycotic agents and 6.5 microM and 21 microM for dazmegrel and dazoxiben, respectively). The structural feature that the antithrombitic compounds share is a carboxylate function so that, at physiological pH, less than 1% of the drug would be present in the unionised form. This functionality is absent from the structures of the antimycotic agents which possess much greater hydrophobic character. Even though the antithrombitic imidazoles elicit type II binding interactions of quite high affinity it would appear from this study that significant inhibition potency does not necessarily follow. The present findings also suggest that interesting differences exist between the active site binding regions in the cytochrome P-450 that catalyse thromboxane synthetase activity and those involved in microsomal drug oxidation. Inhibitor hydrophobicity is clearly an important factor in the inhibition of microsomal cytochromes P-450 whereas effective thromboxane synthetase inhibitors may be quite hydrophilic at physiological pH.