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...

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For some patients, administration of SQV 600 mg three times daily results in very low SQV plasma levels and possibly little antiviral effect. Combination of SQV with RIT results in a significant drug interaction mediated by enzyme inhibition which exposes patients to very high SQV concentrations and potential toxicity. If combination therapy with SQV plus RIT is considered then the dose of SQV should be greatly reduced.

Patients taking oral contraceptive steroids (OCS) are known to suffer contraceptive failure while taking anticonvulsants such as phenobarbitone, phenytoin and carbamazepine. We have studied the single dose kinetics of ethinyloestradiol (EE2); 50 ,ug, and levonorgestrel (Ng); 250 p,g in groups of women before and 8-12 weeks after starting therapy with phenytoin (n = 6) and carbamazepine (n = 4). The area under the plasma concentration-time curve (AUC) was measured over a 24 h period for each steroid and significant reductions were seen with both anticonvulsants. Phenytoin reduced the AUC for EE2 from 806 ± 50 (mean ± s.d.) to 411 ± 132 pg ml-' h (P < 0.05) and for Ng from 33.6 ± 7.8 to 19.5 ± 3.8 ng ml-' h (P < 0.05). Carbamazepine reduced the AUC for EE2 from 1163 ± 466 to 672 ± 211 pg ml-' h (P < 0.05) and for Ng from 22.9 ± 9.4 to 13.8 ± 5.8 ng ml-' h (P < 0.05). These changes are compatible with the known enzyme inducing effects of phenytoin and carbamazepine. Patients taking these anticonvulsants will need to be given increased doses of OCS (equivalent to 50-100 ,ug EE2 daily) to achieve adequate contraceptive effects.

Aims Methadone is predominantly metabolized by cytochrome P450 3A4 and the non nucleoside reverse transcriptase inhibitor (NNRTI) efavirenz is a recognized inducer of this enzyme. We evaluated the pharmacokinetics of methadone in the presence and absence of efavirenz when administered to HIV infected patients with a history of injection drug use (IDU). Methods Eleven patients on stable methadone maintenance therapy, due to commence antiretroviral therapy (ART), participated in this study. Steady state methadone kinetic pro®les were obtained on two occasions 0, 1, 2, 3, 4, 5, 6, 7, 8 and 24 h post dosing. Following centrifugation, separated plasma was heated at 58uC for 30 min to inactivate HIV and stored at x80uC until methadone analysis by high performance liquid chromatography. Results When combined with efavirenz there was a marked decrease in the maximum plasma concentration (C max ) of methadone from 689 (range 212±1568) to 358 (range 205±706) ng ml x1 , P=0.007 : 95% con®dence interval (CI) 112±549. The mean area under the methadone concentration curve 0±24 h (AUC(0,24 h)) was also signi®cantly reduced from 12341 (range 3682±34147) to 5309 (range 2430±10349) ng ml x1 h in the presence of efavirenz, P=0.012 : 95% CI 1921±12143. Nine patients described symptoms of methadone withdrawal and received a dose increase. Although methadone AUC(0,24 h) was reduced by over 50% following efavirenz the mean increase in methadone dose required was 22% (range 15±30 mg). Conclusion The inclusion of the NNRTI efavirenz in once daily ART for HIV patients with a history of IDU receiving methadone maintenance results in a signi®cant reduction in methadone plasma concentrations mediated by enzyme induction. It is important to distinguish efavirenz neurological effects which were observed between days 1±5 of therapy from symptoms of methadone withdrawal which occurred from day 8 onwards. The dose of methadone was adjusted by increments of 10 mg to counteract the efavirenz inducing effect. Keywords: efavirenz, HIV, methadone IntroductionInjection drug use is the second most common risk factor for the acquisition of Human Immunode®ciency Virus (HIV) [1]. Many patients are reluctant to accept treatment for their HIV, and physicians are generally unwilling to prescribe antiretroviral therapy (ART) until their drug habit has stabilized [2,3]. Linking antiretroviral therapy with daily directly observed methadone maintenance therapy is an attractive approach in an attempt to resolve this problem. The current standard of antiretroviral therapy includes a three drug combination consisting of two nucleoside analogues in addition to a protease inhibitor or a non-nucleoside reverse transcriptase inhibitor (NNRTIs) [4]. The convenient once daily dosing of the NNRTIs makes them an attractive option for directly administered therapy. The potential for an interaction between NNRTIs and methadone has been described with reports of methadone withdrawal symptoms in patients receiving nevirapine (NVP) [5,6]. To date there have been no de®nitive to...