DIFFERENTIAL SELECTIVITY OF CYTOCHROME-P450 INHIBITORS AGAINST PROBE SUBSTRATES IN HUMAN AND RAT-LIVER MICROSOMES

Aims Chemical inhibitors of cytochrome P450 (CYP) are a useful tool in defining the role of individual CYPs involved in drug metabolism. The aim of the present study was to evaluate the selectivity and rank the order of potency of a range of isoform-selective CYP inhibitors and t o compare directly the effects of these inhibitors in human and rat he patic microsomes. Methods Four chemical inhibitors of human cytochrome P450 isoforms, furafylline (CYP1A2), sulphaphenazole (CYP2C9), diethy ldithiocarbamate (CYP2E1), and ketoconazole (CYP3A4) were screened for their inhibitory specificity towards CYP-mediated reactions in both h uman and rat liver microsomal preparations. Phenacetin O-deethylation, tolbutamide 4-hydroxylation, chlorzoxazone 6-hydroxylation and testos terone 6 beta-hydroxylation were monitored for enzyme activity. Result s Furafylline was a potent, selective inhibitor of phenacetin O-deethy lation (CYP1A2-mediated) in human liver microsomes (IC50=0.48 mu M), b ut inhibited both phenacetin O-deethylation and tolbutamide 4-hydroxyl ation (CYP2C9-mediated) at equimolar concentrations in rat liver micro somes (IC50=20.8 and 24.0 mu M respectively). Sulphaphenazole demonstr ated selective inhibition of tolbutamide hydroxylation in human liver microsomes but failed to inhibit this reaction in rat liver microsomes . DDC demonstrated a low level of selectivity as an inhibitory probe f or chlorzoxazone 6-hydroxylation (CYP2E1-mediated). DDC also inhibited testosterone 6 beta-hydroxylation (CYP3A-mediated) in man and rat, an d tolbutamide 4-hydroxylase activity in rat. Ketoconazole was a very p otent, selective inhibitor of CYP3A4 activity in human liver (IC50=0.0 4 mu M). Although inhibiting CYP3A in rat liver it also inhibited all other reactions at concentrations less than or equal to 5 mu M. Conclu sions It is evident that CYP inhibitors do not exhibit the same select ivity in human and rat liver microsomes. This is due to differential s electivity of the inhibitors and/or differences in the CYP isoform res ponsible for metabolism in the different species.