Aims To compare the inhibitory potential of the HIV protease inhibitors saquinavir, ritonavir and indinavir against CYP1A2, CYP2C9, CYP2E1 and CYP3A4 catalysed metabolic reactions in human liver microsomes in vitro. Methods Microsomes from six human livers were utilized in this study. The probe substrates were phenacetin (CYP1A2), tolbutamide (CYP2C9), chlorzoxazone (CYP2E1) and testosterone (CYP3A4). Metabolites were analysed by high performance liquid chromatography. IC 50 (concentration of inhibitor giving 50% decrease in enzyme activity) and, where appropriate, K i values were calculated. Results Ritonavir was a very potent inhibitor of CYP3A4 mediated testosterone 6b-hydroxylation (mean K i =0.019±0.004 mm, mean±s.d.; n=6) and also inhibited tolbutamide hydroxylation (IC 50 =4.2±1.3 mm, mean±s.d.; n=6). Inhibition of phenacetin O-deethylation and chlorzoxazone 6-hydroxylation was negligible. Indinavir was an order-of-magnitude less potent in inhibiting CYP3A4 (K i = 0.17±0.01 mm) and did not produce appreciable inhibition of the CYP1A2, CYP2C9 or CYP2E1 catalysed reactions. Saquinavir was the least potent CYP3A4 inhibitor (K i =2.99±0.87 mm) and produced some inhibition of CYP2C9 (approximately 50% at 50 mm). Conclusions The HIV protease inhibitors have differential effects on CYP isozymes. There is obvious potential for clinically significant drug interactions particularly with ritonavir. Pharmacokinetic drug interaction studies are crucial to gain an overall understanding of the beneficial and potentially harmful effects of this important group of drugs.

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 to compare directly the effects of these inhibitors in human and rat hepatic microsomes. Methods Four chemical inhibitors of human cytochrome P450 isoforms, furafylline (CYP1A2), sulphaphenazole (CYP2C9), diethyldithiocarbamate (CYP2E1), and ketoconazole (CYP3A4) were screened for their inhibitory specificity towards CYPmediated reactions in both human and rat liver microsomal preparations. Phenacetin O-deethylation, tolbutamide 4-hydroxylation, chlorzoxazone 6-hydroxylation and testosterone 6b-hydroxylation were monitored for enzyme activity. Results Furafylline was a potent, selective inhibitor of phenacetin O-deethylation (CYP1A2-mediated) in human liver microsomes ( IC 50 =0.48 mm), but inhibited both phenacetin O-deethylation and tolbutamide 4-hydroxylation (CYP2C9-mediated) at equimolar concentrations in rat liver microsomes ( IC 50 =20.8 and 24.0 mm respectively). Sulphaphenazole demonstrated 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 for chlorzoxazone 6-hydroxylation (CYP2E1-mediated). DDC also inhibited testosterone 6b-hydroxylation (CYP3A-mediated) in man and rat, and tolbutamide 4-hydroxylase activity in rat. Ketoconazole was a very potent, selective inhibitor of CYP3A4 activity in human liver ( IC 50 =0.04 mm). Although inhibiting CYP3A in rat liver it also inhibited all other reactions at concentrations ≤5 mm. Conclusions It is evident that CYP inhibitors do not exhibit the same selectivity in human and rat liver microsomes. This is due to differential selectivity of the inhibitors and/or differences in the CYP isoform responsible for metabolism in the different species.Keywords: cytochrome P450, cytochrome P450 inhibition, differential selectivity other species such as the rat are used and predictions made Introduction to man. Currently gene families 1, 2 and 3 are thought to be The correct assignment of individual cytochrome P450 (CYP) isoforms to specific metabolic pathways is an area of involved in the biotransformation of xenobiotics in both humans and rodents. However, isoforms are not conserved considerable importance; in particular in the rational prediction of drug-drug interactions. Many different between species and differences are known to occur in catalytic and regulatory specificities between human CYP strategies are currently employed in the unambiguous identification of CYP isoforms responsible for the biotransisoforms and their rat orthologues, although the CYP1A and CYP2E subfamilies show remarkable conservation formation of therapeutic agents. These include the use of selective chemical inhibitors of CYP isoforms, inhibitory between human and rat [3...

We have demonstrated that saquinavir is a substrate for P-gp and that ritonavir, nelfinavir and indinavir modulate P-gp function in both human lymphocytes and Caco-2 cells.

Co-administration of sildenafil 25 mg did not significantly alter the plasma indinavir levels. However, plasma sildenafil AUC was markedly increased in the presence of indinavir compared with historical controls. From the in vitro data, the mechanism of increase is indinavir inhibition of the hepatic metabolism of sildenafil. The magnitude of this interaction suggests a lower starting dose of sildenafil may be more appropriate in this clinical setting.