Ketoconazole and sulphaphenazole as the respective selective inhibitors of P4503A and 2C9

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Abstract
1. The potential of ketoconazole and sulphaphenazole to inhibit specific P450 enzyme activities (1A2, 2A6, 2B6, 2C9/8, 2C19, 2D6, 2E1, 3A and 4A) was investigated using human liver microsomes. 2. Ketoconazole demonstrated an inhibitory effect on cyclosporine oxidase and testosterone 6β-hydroxylase activities, with mean IC50's of 0.19 and 0.22 μM respectively. Ketoconazole inhibition of the other P450 activities investigated was significantly less, as illustrated by IC50's of at least a magnitude higher. 3. Sulphaphenazole was shown to have an inhibitory effect on tolbutamide hydroxylase activity, with a mean IC50 of 0.8μM in incubations containing 100μ) tolbutamide. Sulphaphenazole (at concentrations of up to 100μ) did not exhibit any significant inhibition of the other enzyme activities investigated. 4. Ketoconazole and sulphaphenazole are the respective selective inhibitors of P4503 A and 2C9. Ketoconazole at 1 μM and sulphaphenazole at 10μM can be used to establish the involvement of P4503A and 2C9 respectively in oxidative reactions in human liver microsomes.