Effect of quinidine on the dextromethorphan O-demethylase activity of microsomal fractions from human liver

In vitro inhibitory effects of glucosamine, chondroitin and diacerein on human hepatic CYP2D6

OBJECTIVES

Glucosamine, chondroitin and diacerein are natural compounds commonly used in treating osteoarthritis. Their concomitant intake may trigger drug-natural product interactions. Cytochrome P450 (CYP) has been implicated in such interactions. Cytochrome P450 2D6 (CYP2D6) is a major hepatic CYP involved in metabolism of 25% of the clinical drugs. This study aimed to investigate the inhibitory effect of these antiarthritic compounds on CYP2D6.

METHODS

CYP2D6 was heterologously expressed in Escherichia coli. CYP2D6-antiarthritic compound interactions were studied using in vitro enzyme kinetics assay and molecular docking.

RESULTS

The high-performance liquid chromatography (HPLC)-based dextromethorphan O-demethylase assay was established as CYP2D6 marker. All glucosamines and chondroitins weakly inhibited CYP2D6 (IC₅₀ values >300 µM). Diacerein exhibited moderate inhibition with IC₅₀ and K _i values of 34.99 and 38.27 µM, respectively. Its major metabolite, rhein displayed stronger inhibition potencies (IC₅₀=26.22 μM and K _i =32.27 μM). Both compounds exhibited mixed-mode of inhibition. In silico molecular dockings further supported data from the in vitro study. From in vitro-in vivo extrapolation, rhein presented an area under the plasma concentration-time curve (AUC) ratio of 1.5, indicating low potential to cause in vivo inhibition.

CONCLUSIONS

Glucosamine, chondroitin and diacerein unlikely cause clinical interaction with the drug substrates of CYP2D6. Rhein, exhibits only low potential to cause in vivo inhibition.

In vitro inhibitory effects of glucosamine, chondroitin and diacerein on human hepatic CYP2D6

OBJECTIVES

Glucosamine, chondroitin and diacerein are natural compounds commonly used in treating osteoarthritis. Their concomitant intake may trigger drug-natural product interactions. Cytochrome P450 (CYP) has been implicated in such interactions. Cytochrome P450 2D6 (CYP2D6) is a major hepatic CYP involved in metabolism of 25% of the clinical drugs. This study aimed to investigate the inhibitory effect of these antiarthritic compounds on CYP2D6.

METHODS

CYP2D6 was heterologously expressed in Escherichia coli. CYP2D6-antiarthritic compound interactions were studied using in vitro enzyme kinetics assay and molecular docking.

RESULTS

The high-performance liquid chromatography (HPLC)-based dextromethorphan O-demethylase assay was established as CYP2D6 marker. All glucosamines and chondroitins weakly inhibited CYP2D6 (IC50 values >300 µM). Diacerein exhibited moderate inhibition with IC50 and K i values of 34.99 and 38.27 µM, respectively. Its major metabolite, rhein displayed stronger inhibition potencies (IC50=26.22 μM and K i =32.27 μM). Both compounds exhibited mixed-mode of inhibition. In silico molecular dockings further supported data from the in vitro study. From in vitro-in vivo extrapolation, rhein presented an area under the plasma concentration-time curve (AUC) ratio of 1.5, indicating low potential to cause in vivo inhibition.

CONCLUSIONS

Glucosamine, chondroitin and diacerein unlikely cause clinical interaction with the drug substrates of CYP2D6. Rhein, exhibits only low potential to cause in vivo inhibition.

Molecular Docking as a Promising Predictive Model for Silver Nanoparticle-Mediated Inhibition of Cytochrome P450 Enzymes

Cytochrome P450 (CYP) enzymes are responsible for oxidative metabolisms of a large number of xenobiotics. In this study, we investigated interactions of silver nanoparticles (AgNPs) and silver ions (Ag⁺) with six CYP isoforms, namely, CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4, within CYP-specific inhibitor-binding pockets by molecular docking and quantum mechanical (QM) calculations. The docking results revealed that the Ag₃ cluster, not Ag⁺, interacted with key amino acids of CYP2C9, CYP2C19, and CYP2D6 within a distance of about 3 Å. Moreover, the QM analysis confirmed that the amino acid residues of these CYP enzymes strongly interacted with the Ag₃ cluster, providing more insight into the mechanism of the potential inhibition of CYP enzyme activities. Interestingly, these results are consistent with previous in vitro data indicating that AgNPs inhibited activities of CYP2C and CYP2D in rat liver microsomes. It is suggested that the Ag₃ cluster is a minimal unit of AgNPs for in silico modeling. In summary, we demonstrated that molecular docking, together with QM analysis, is a promising tool to predict AgNP-mediated CYP inhibition. These methods are useful for deeper understanding of reaction mechanisms and could be used for other nanomaterials.

Characterization of Porcine Hepatic and Intestinal Drug Metabolizing CYP450: Comparison with Human Orthologues from A Quantitative, Activity and Selectivity Perspective

Over the past two decades, the pig has gained attention as a potential model for human drug metabolism. Cytochrome P450 enzymes (CYP450), a superfamily of biotransformation enzymes, are pivotal in drug metabolism. Porcine CYP450 has been demonstrated to convert typical substrates of human CYP450. Nevertheless, knowledge and insight into porcine CYP450 quantity and substrate selectivity is scant, especially regarding intestinal CYP450. The current study aimed to map the quantities of hepatic and intestinal CYP450 in the conventional pig by using a proteomic approach. Moreover, the selectivity of the six most common used probe substrates (phenacetin, coumarin, midazolam, tolbutamide, dextromethorphan, and chlorzoxazone) for drug metabolizing enzyme subfamilies (CYP1A, CYP2A, CYP3A, CYP2C, CYP2D and CYP2E respectively), was investigated. Hepatic relative quantities were 4% (CYP1A), 31% (CYP2A), 14% (CYP3A), 10% (CYP2C), 28% (CYP2D) and 13% (CYP2E), whereas for the intestine only duodenal CYP450 could be determined with 88% for CYP3A and 12% for CYP2C. Furthermore, the results indicate that coumarin (CYP2A), midazolam (CYP3A), tolbutamide (CYP2C), and dextromethorphan (CYP2D) are as selective for porcine as for human CYP450. However, phenacetin (CYP1A2) and chlorzoxazone (CYP2E1) are less selective for the specific enzyme, despite similarities in selectivity towards the different enzymes involved compared to humans.

In vivo prediction of CYP-mediated metabolic interaction potential of formononetin and biochanin A using in vitro human and rat CYP450 inhibition data

Formononetin (FMN) and Biochanin A (BCA) are the principal isoflavones present in commercially available extracts of red clover that are widely been consumed for various health benefits. We investigated the in vitro effects of FMN and BCA on catalytic activity of human/rat cytochrome P450 enzymes to assess the drug interaction potential of red clover. IC50 and Ki values of FMN and BCA for CYPs were determined in human/rat liver microsomes. FMN and BCA showed concentration-dependent inhibition of CYP1A2 activity with IC50 values of 13.42 and 24.98μM in human liver microsomes and 38.57 and 11.86μM in rat liver microsomes, respectively. The mode of inhibition of human CYP1A2 by FMN was found to be competitive with apparent Ki value of 10.13±1.96μM. FMN also inhibited human CYP2D6. BCA exerted moderately inhibitory effects on human CYP2C9. The predicted in vivo inhibition for CYP1A2 was insignificant (R value <1.1) at hepatic level while at intestinal level, it was significant (R value >11). The inhibitory effects on other CYPs were found to be minimal. Red clover may be considered safe to be consumed along with co-prescribed medications; however, precaution must be taken while co-administering it with CYP1A2 substrates.

Comprehensive in vitro analysis of voriconazole inhibition of eight cytochrome P450 (CYP) enzymes: major effect on CYPs 2B6, 2C9, 2C19, and 3A

Voriconazole is an effective antifungal drug, but adverse drug-drug interactions associated with its use are of major clinical concern. To identify the mechanisms of these interactions, we tested the inhibitory potency of voriconazole with eight human cytochrome P450 (CYP) enzymes. Isoform-specific probes were incubated with human liver microsomes (HLMs) (or expressed CYPs) and cofactors in the absence and the presence of voriconazole. Preincubation experiments were performed to test mechanism-based inactivation. In pilot experiments, voriconazole showed inhibition of CYP2B6, CYP2C9, CYP2C19, and CYP3A (half-maximal [50%] inhibitory concentrations, <6 microM); its effect on CYP1A2, CYP2A6, CYP2C8, and CYP2D6 was marginal (<25% inhibition at 100 microM voriconazole). Further detailed experiments with HLMs showed that voriconazole is a potent competitive inhibitor of CYP2B6 (K(i) < 0.5), CYP2C9 (K(i) = 2.79 microM), and CYP2C19 (K(i) = 5.1 microM). The inhibition of CYP3A by voriconazole was explained by noncompetitive (K(i) = 2.97 microM) and competitive (K(i) = 0.66 microM) modes of inhibition. Prediction of the in vivo interaction of voriconazole from these in vitro data suggests that voriconazole would substantially increase the exposure of drugs metabolized by CYP2B6, CYP2C9, CYP2C19, and CYP3A. Clinicians should be aware of these interactions and monitor patients for adverse effects or failure of therapy.

Identification and relative contributions of human cytochrome P450 isoforms involved in the metabolism of glibenclamide and lansoprazole: evaluation of an approach based on thein vitrosubstrate disappearance rate

1. The identification and relative contributions of human cytochrome P450 (CYP) enzymes involved in the metabolism of glibenclamide and lansoprazole in human liver microsomes were investigated using an approach based on the in vitro disappearance rate of unchanged drug. 2. Recombinant CYP2C19 and CYP3A4 catalysed a significant disappearance of both drugs. When the contribution of CYPs to the intrinsic clearance (CL(int)) of drugs in pooled human microsomes was estimated by relative activity factors, contributions of CYP2C19 and CYP3A4 were determined to be 4.6 and 96.4% for glibenclamide, and 75.1 and 35.6% for lansoprazole, respectively. 3. CL(int) of glibenclamide correlated very well with CYP3A4 marker activity, whereas the CL(int) of lansoprazole significantly correlated with CYP2C19 and CYP3A4 marker activities in human liver microsomes from 12 separate individuals. Effects of CYP-specific inhibitors and anti-CYP3A serum on the CL(int) of drugs in pooled human liver microsomes reflected the relative contributions of CYP2C19 and CYP3A4. 4. The results suggest that glibenclamide is mainly metabolized by CYP3A4, whereas lansoprazole is metabolized by both CYP2C19 and CYP3A4 in human liver microsomes. This approach, based on the in vitro drug disappearance rate, is useful for estimating CYP identification and their contribution to drug discovery.

Potential of pranlukast and zafirlukast in the inhibition of human liver cytochrome P450 enzymes

1. The potential of zafirlukast to inhibit several human cytochrome P450 enzymes is well known. However, pranlukast, a structural analogue of zafirlukast, has not been studied. Accordingly, the inhibitory potential of pranlukast was evaluated and compared with that of zafirlukast, a known CYP2C9 inhibitor, in in vitro microsomal incubation studies. 2. Both pranlukast and zafirlukast showed moderate inhibition of CYP2C9-catalysed tolbutamide 4-methylhydroxylation, competitively inhibiting tolbutamide 4-methylhydroxylation with estimated mean K(i) values of 3.82 +/- 0.50 and 5.86 +/- 0.08 microM, respectively. 3. Pranlukast had no effect on CYP2C19-catalysed S-mephenytoin 4'-hydroxylation or CYP3A4-catalysed midazolam 1-hydroxylation. However, zafirlukast showed minor inhibition of these reactions. Neither pranlukast nor zafirlukast inhibited CYP1A2-catalysed phenacetin O-deethylation, CYP2D6-catalysed dextromethorphan O-demethylation or CYP2E1-catalysed chlorzoxazone 6-hydroxylation. 4. The results suggest that like zafirlukast, pranlukast also has the potential moderately to inhibit CYP2C9-catalysed tolbutamide 4-methylhydroxylation. Therefore, the inhibitory potential of pranlukast should be considered when it is co-administered with CYP2C9 substrates with narrow therapeutic ranges (e.g. S-warfarin, phenytoin).

Inhibition and induction of human cytochrome P450 enzymes: current status

Variability of drug metabolism, especially that of the most important phase I enzymes or cytochrome P450 (CYP) enzymes, is an important complicating factor in many areas of pharmacology and toxicology, in drug development, preclinical toxicity studies, clinical trials, drug therapy, environmental exposures and risk assessment. These frequently enormous consequences in mind, predictive and pre-emptying measures have been a top priority in both pharmacology and toxicology. This means the development of predictive in vitro approaches. The sound prediction is always based on the firm background of basic research on the phenomena of inhibition and induction and their underlying mechanisms; consequently the description of these aspects is the purpose of this review. We cover both inhibition and induction of CYP enzymes, always keeping in mind the basic mechanisms on which to build predictive and preventive in vitro approaches. Just because validation is an essential part of any in vitro-in vivo extrapolation scenario, we cover also necessary in vivo research and findings in order to provide a proper view to justify in vitro approaches and observations.

Therapeutic use of dextromethorphan: Key learnings from treatment of pseudobulbar affect

A variety of neurological conditions and disease states are accompanied by pseudobulbar affect (PBA), an emotional disorder characterized by uncontrollable outbursts of laughing and crying. The causes of PBA are unclear but may involve lesions in neural circuits regulating the motor output of emotional expression. Several agents used in treating other psychiatric disorders have been applied in the treatment of PBA with some success but data are limited and these agents are associated with unpleasant side effects due to nonspecific activity in diffuse neural networks. Dextromethorphan (DM), a widely used cough suppressant, acts at receptors in the brainstem and cerebellum, brain regions implicated in the regulation of emotional output. The combination of DM and quinidine (Q), an enzyme inhibitor that blocks DM metabolism, has recently been tested in phase III clinical trials in patients with multiple sclerosis and amyotrophic lateral sclerosis and was both safe and effective in palliating PBA symptoms. In addition, clinical studies pertaining to the safety and efficacy of DM/Q in a variety of neurological disease states are ongoing.

OPC-67683, a nitro-dihydro-imidazooxazole derivative with promising action against tuberculosis in vitro and in mice

BACKGROUND

Tuberculosis (TB) is still a leading cause of death worldwide. Almost a third of the world's population is infected with TB bacilli, and each year approximately 8 million people develop active TB and 2 million die as a result. Today's TB treatment, which dates back to the 1970s, is long and burdensome, requiring at least 6 mo of multidrug chemotherapy. The situation is further compounded by the emergence of multidrug-resistant TB (MDR-TB) and by the infection's lethal synergy with HIV/AIDS. Global health and philanthropic organizations are now pleading for new drug interventions that can address these unmet needs in TB treatment.

METHODS AND FINDINGS

Here we report OPC-67683, a nitro-dihydro-imidazooxazole derivative that was screened to help combat the unmet needs in TB treatment. The compound is a mycolic acid biosynthesis inhibitor found to be free of mutagenicity and to possess highly potent activity against TB, including MDR-TB, as shown by its exceptionally low minimum inhibitory concentration (MIC) range of 0.006-0.024 microg/ml in vitro and highly effective therapeutic activity at low doses in vivo. Additionally, the results of the post-antibiotic effect of OPC-67683 on intracellular Mycobacterium tuberculosis showed the agent to be highly and dose-dependently active also against intracellular M. tuberculosis H37Rv after a 4-h pulsed exposure, and this activity at a concentration of 0.1 microg/ml was similar to that of the first-line drug rifampicin (RFP) at a concentration of 3 microg/ml. The combination of OPC-67683 with RFP and pyrazinamide (PZA) exhibited a remarkably quicker eradication (by at least 2 mo) of viable TB bacilli in the lung in comparison with the standard regimen consisting of RFP, isoniazid (INH), ethambutol (EB), and PZA. Furthermore, OPC-67683 was not affected by nor did it affect the activity of liver microsome enzymes, suggesting the possibility for OPC-67683 to be used in combination with drugs, including anti-retrovirals, that induce or are metabolized by cytochrome P450 enzymes.

CONCLUSIONS

We concluded that based on these properties OPC-67683 has the potential to be used as a TB drug to help combat the unmet needs in TB treatment.

Minimizing Polymorphic Metabolism in Drug Discovery: Evaluation of the Utility of in Vitro Methods for Predicting Pharmacokinetic Consequences Associated with CYP2D6 Metabolism

Minimizing interindividual variability in drug exposure is an important goal for drug discovery. The reliability of the selective CYP2D6 inhibitor quinidine was evaluated in a retrospective analysis using a standardized approach that avoids laboratory-to-laboratory variation. The goal was to evaluate the reliability of in vitro metabolism studies for predicting extensive metabolizer (EM)/poor metabolizer (PM) exposure differences. Using available literature, 18 CYP2D6 substrates were selected for further analysis. In vitro microsomal studies were conducted at 1 microM substrate and 0.5 microM P450 to monitor substrate depletion. An estimate of the fraction metabolized by CYP2D6 in microsomes was derived from the rate constant determined with and without 1 microM quinidine for 11 substrates. Clearance in EM and PM subjects and fractional recovery of metabolites were taken from the literature. A nonlinear relationship between the contribution of CYP2D6 and decreased oral clearance for PMs relative to EMs was evident. For drugs having <60% CYP2D6 involvement in vivo, a modest difference between EM and PM exposure was observed (<2.5-fold). For major CYP2D6 substrates (>60%), more dramatic exposure differences were observed (3.5- to 53-fold). For compounds primarily eliminated by hepatic P450 and with sufficient turnover to be evaluated in vitro, the fraction metabolized by CYP2D6 in vitro compared favorably with the in vivo data. The in vitro estimation of fraction metabolized using quinidine as a specific inhibitor provided an excellent predictive tool. Results from microsomal substrate depletion experiments can be used with confidence to select compounds in drug discovery using a cutoff of >60% metabolism by CYP2D6.

Chlorpropamide 2-hydroxylation is catalysed by CYP2C9 and CYP2C19 in vitro: chlorpropamide disposition is influenced by CYP2C9, but not by CYP2C19 genetic polymorphism

AIMS

We evaluated the involvement of cytochrome P450 (CYP) isoforms 2C9 and 2C19 in chlorpropamide 2-hydroxylation in vitro and in chlorpropamide disposition in vivo.

METHODS

To identify CYP isoforms(s) that catalyse 2-hydroxylation of chlorpropamide, the incubation studies were conducted using human liver microsomes and recombinant CYP isoforms. To evaluate whether genetic polymorphisms of CYP2C9 and/or CYP2C19 influence the disposition of chlorpropamide, a single oral dose of 250 mg chlorpropamide was administered to 21 healthy subjects pregenotyped for CYP2C9 and CYP2C19.

RESULTS

In human liver microsomal incubation studies, the formation of 2-hydroxychlorpropamide (2-OH-chlorpropamide), a major chlorpropamide metabolite in human, has been best described by a one-enzyme model with estimated K(m) and V(max) of 121.7 +/- 19.9 microm and 16.1 +/- 5.0 pmol min(-1) mg(-1) protein, respectively. In incubation studies using human recombinant CYP isoforms, however, 2-OH-chlorpropamide was formed by both CYP2C9 and CYP2C19 with similar intrinsic clearances (CYP2C9 vs. CYP2C19: 0.26 vs. 0.22 microl min(-1) nmol(-1) protein). Formation of 2-OH-chlorpropamide in human liver microsomes was significantly inhibited by sulfaphenazole, but not by S-mephenytoin, ketoconazole, quinidine, or furafylline. In in vivo clinical trials, eight subjects with the CYP2C9*1/*3 genotype exhibited significantly lower nonrenal clearance [*1/*3 vs.*1/*1: 1.8 +/- 0.2 vs. 2.4 +/- 0.1 ml h(-1) kg(-1), P < 0.05; 95% confidence interval (CI) on the difference 0.2, 1.0] and higher metabolic ratios (of chlorpropamide/2-OH-chlorpropamide in urine: *1/*3 vs.*1/*1: 1.01 +/- 0.19 vs. 0.56 +/- 0.08, P < 0.05; 95% CI on the difference - 0.9, - 0.1) than did 13 subjects with CYP2C9*1/*1 genotype. In contrast, no differences in chlorpropamide pharmacokinetics were observed for subjects with the CYP2C19 extensive metabolizer vs. poor metabolizer genotypes.

CONCLUSIONS

These results suggest that chlorpropamide disposition is principally determined by CYP2C9 activity in vivo, although both CYP2C9 and CYP2C19 have a catalysing activity of chlorpropamide 2-hydroxylation pathway.

Multiple P450 substrates in a single run: rapid and comprehensive in vitro interaction assay

The dramatically increased number of new chemical entities (NCE) used in drug discovery has raised a demand for efficient and rapid drug metabolism screening techniques. The aim of this study was to develop a global in vitro metabolic interaction screening test utilising the N-in-1 approach. A cocktail consisting of 10 CYP-selective probes with known kinetic, metabolic and interaction properties in vivo was incubated in a pool of human liver microsomes, and metabolites of melatonin (CYP1A2), coumarin (CYP2A6), bupropion (CYP2B6), amodiaquine (CYP2C8), tolbutamide (CYP2C9), omeprazole (CYP2C19 and CYP3A4), dextromethorphan (CYP2D6), chlorzoxazone (CYP2E1), midazolam (CYP3A4) and testosterone (CYP3A4) were analysed simultaneously using LC/TOF-MS. Performance of the method was assessed with cDNA expressed P450s and diagnostic CYP-specific inhibitors. The results were in good accordance with literature and our previous studies. The cocktail developed is suitable for fast and reliable in vitro screening of the interaction potential and characteristics of NCEs.

Inhibition of Human Cytochrome P450 2E1 by Halogenated Anilines, Phenols, and Thiophenols

A total of 44 variously halogenated derivatives of aniline, phenol, and thiophenol were subjected to analysis of their inhibitory effect on human cytochrome P450 (CYP) 2E1 to investigate the structure-activity relationships in halogenated phenyl derivatives. The activity of human CYP2E1 of the microsomes from baculovirus-transfected insect cells expressing recombinant human CYP2E1 was determined by measuring quinoline 3-hydroxylation, which was detectable by fluorescence monitoring (Ex=355 nm and Em=460 nm). Diethyldithiocarbamate (DDTC), a specific inhibitor of CYP2E1, potently inhibited quinoline 3-hydroxylation (IC50=8.9 microM). The effects of halogen-substitution in 32 aniline derivatives on the CYP2E1 inhibition can be summarized as follows: more enhancement by chlorine- and bromine-substitution than by fluorine-substitution, more enhancement by para- and metha-halogen-substitution than by ortho-halogen-substitution, and more enhancement by dihalogen-substitution than by mono- and trihalogen-substitution except for trifluorine-substitution. The greatest enhancement of the inhibitory activity was observed in 3,4-dichloroaniline (IC50=8.0 microM) and 3,5-dichloroaniline (IC50=9.2 microM), and their inhibitory activities were very close to that of DDTC. All of the dichlorophenols and dichlorothiophenols were compared with dichloroanilines for CYP2E1 inhibition. Although dichlorothiophenols showed similar or more potent inhibitory activities than dichloroanilines, dichlorophenols showed less inhibitory activities. 3,4-Dichlorothiophenol and 3,5-dichlorothiophenol showed very potent inhibition and their IC50 values were 5.3 and 5.2 microM, respectively. These results suggest that 3,4- and 3,5-dichlorophenyl derivatives may be useful as potent CYP2E1 inhibitors.

Predicting drug clearance from recombinantly expressed CYPs: intersystem extrapolation factors

1. Recombinantly expressed human cytochromes P450 (rhCYPs) have been underused for the prediction of human drug clearance (CL). 2. Differences in intrinsic activity (per unit CYP) between rhCYP and human liver enzymes complicate the issue and these discrepancies have not been investigated systematically. We define intersystem extrapolation factors (ISEFs) that allow the use of rhCYP data for the in vitro-in vivo extrapolation of human drug CL and the variance that is associated with interindividual variation of CYP abundance due to genetic and environmental effects. 3. A large database (n = 451) of metabolic stability data has been compiled and used to derive ISEFs for the most commonly used expression systems and CYP enzymes. 4. Statistical models were constructed for the ISEFs to determine major covariates in order to optimize experimental design to increase prediction accuracy. 5. Suggestions have been made for the conduct of future studies using rhCYP to predict human drug clearance.

VALIDATED ASSAYS FOR HUMAN CYTOCHROME P450 ACTIVITIES

The measurement of the effect of new chemical entities on human cytochrome P450 marker activities using in vitro experimentation represents an important experimental approach in drug development. In vitro drug interaction data can be used in guiding the design of clinical drug interaction studies, or, when no effect is observed in vitro, the data can be used in place of an in vivo study to claim that no interaction will occur in vivo. To make such a claim, it must be assured that the in vitro experiments are performed with absolute confidence in the methods used and data obtained. To meet this need, 12 semiautomated assays for human P450 marker substrate activities have been developed and validated using approaches described in the GLP (good laboratory practices) as per the code of U.S. Federal Regulations. The assays that were validated are: phenacetin O-deethylase (CYP1A2), coumarin 7-hydroxylase (CYP2A6), bupropion hydroxylase (CYP2B6), amodiaquine N-deethylase (CYP2C8), diclofenac 4'-hydroxylase and tolbutamide methylhydroxylase (CYP2C9), (S)-mephenytoin 4'-hydroxylase (CYP2C19), dextromethorphan O-demethylase (CYP2D6), chlorzoxazone 6-hydroxylase (CYP2E1), felodipine dehydrogenase, testosterone 6 beta-hydroxylase, and midazolam 1'-hydroxylase (CYP3A4 and CYP3A5). High-pressure liquid chromatography-tandem mass spectrometry, using stable isotope-labeled internal standards, has been applied as the analytical method. This analytical approach, through its high sensitivity and selectivity, has permitted the use of very low incubation concentrations of microsomal protein (0.01-0.2 mg/ml). Analytical assay accuracy and precision values were excellent. Enzyme kinetic and inhibition parameters obtained using these methods demonstrated high precision and were within the range of values previously reported in the scientific literature. These methods should prove useful in the routine assessments of the potential for new drug candidates to elicit pharmacokinetic drug interactions via inhibition of cytochrome P450 activities.

Inhibition of cytochrome P450 activities by oleanolic acid and ursolic acid in human liver microsomes

Oleanolic acid (OA) and ursolic acid (UA), triterpene acids having numerous pharmacological activities including anti-inflammatory, anti-cancer, and hepato-protective effects, were tested for their ability to modulate the activities of several cytochrome P450 (CYP) enzymes using human liver microsomes. OA competitively inhibited CYP1A2-catalyzed phenacetin O-deethylation and CYP3A4-catalyzed midazolam 1-hydroxylation, the major human drug metabolizing CYPs, with IC50 (Ki) values of 143.5 (74.2) microM and 78.9 (41.0) microM, respectively. UA competitively inhibited CYP2C19-catalyzed S-mephenytoin 4'-hydroxylation with an IC50 (Ki) value of 119.7 (80.3) microM. However, other CYPs tested showed no or weak inhibition by both OA and UA. The present study demonstrates that OA and UA have inhibitory effects on CYP isoforms using human liver microsomes. It is thus likely that consumption of herbal medicines containing OA or UA, or administration of OA or UA, can cause drug interactions in humans when used concomitantly with drugs that are metabolized primarily by CYP isoforms. In addition, it appears that the inhibitory effect of OA on CYP1A2 is, in part, related to its anti-inflammatory and anticancer activities.

Effect of .GAMMA.-Oryzanol on Cytochrome P450 Activities in Human Liver Microsomes

The effects of gamma-oryzanol, a drug mainly used for the treatment of hyperlipidaemia, on several cytochrome P450 (CYP) specific reactions in human liver microsomes were investigated to predict drug interactions with gamma-oryzanol in vivo from in vitro data. The following eight CYP catalytic reactions were used in this study: CYP1A1/2-mediated 7-ethoxyresorufin O-deethylation, CYP2A6-mediated coumarin 7-hydroxylation, CYP2B6-mediated 7-benzyloxyresorufin O-debenzylation, CYP2C8/9-mediated tolbutamide methylhydroxylation, CYP2C19-mediated S-mephenytoin 4'-hydroxylation, CYP2D6-mediated bufuralol 1'-hydroxylation, CYP2E1-mediated chlorzoxazone 6-hydroxylation, and CYP3A4-mediated testosterone 6beta-hydroxylation. gamma-Oryzanol had little inhibitory effects on CYP activities, indicating that this compound would not be expected to cause clinically significant interactions with other CYP-metabolized drugs at expected therapeutic concentrations.

Inhibitory effect of 5-fluorouracil on human cytochrome P(450) isoforms in human liver microsomes

OBJECTIVE

A number of case reports have been described regarding drug interactions with 5-fluorouracil (5-FU) and co-administered drugs. However, little is known regarding the inhibitory potential of 5-FU on the metabolism of co-administered drugs by cytochrome P(450) (CYP). The aim of the present study was to elucidate the inhibitory effect of 5-FU on CYP isoforms using human liver microsomes.

METHODS

The inhibitory effect of 5-FU on CYP1A2, CYP2C9, CYP2C19, CYP2C8, CYP2E1, CYP2D6, and CYP3A4 activities was examined with specific probe drugs in human liver microsomes.

RESULTS

5-FU showed little or no inhibitory effect on CYP-catalyzed reactions in human liver microsomal preparations.

CONCLUSION

5-FU has no inhibitory effect on CYP isoforms or drug metabolism causing drug interaction with 5-FU. The mechanism that causes drug interaction between co-administered drugs and 5-FU may not be related to direct CYP inhibition by 5-FU.

Inhibitory effect of glyburide on human cytochrome p450 isoforms in human liver microsomes

The inhibitory effect of glyburide [International Nonproprietary Name (INN), glibenclamide] on CYP1A2, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4 activities was evaluated using pooled human liver microsomes. Glyburide strongly inhibited CYP2C9-catalyzed S-warfarin and phenytoin metabolism in a competitive manner, with Ki (IC50) values of 2.4 (11.3) microM and 3.1 (9.4) microM, respectively. CYP3A4-catalyzed midazolam 1-hydroxylation was inhibited by glyburide with a Ki (IC50) value of 42.5 (90.0) microM. However, glyburide showed no appreciable inhibitory effect on CYP1A2, CYP2C8, CYP2C19, CYP2E1, or CYP2D6. In summary, glyburide showed potent inhibition on CYP2C9 and weak inhibition on CYP3A4, whereas it had minimal or no inhibitory effect on the other cytochromes p450 examined. It is anticipated that clinically significant drug-drug interactions will ensue when glyburide is coadministered with agents that are cleared primarily by the CYP2C9-mediated pathway and those with narrow therapeutic ranges.

Cytochrome P450 2C8 and CYP3A4/5 are involved in chloroquine metabolism in human liver microsomes

Chloroquine has been used for many decades in the prophylaxis and treatment of malaria. It is metabolized in humans through the N-dealkylation pathway, to desethylchloroquine (DCQ) and bisdesethylchloroquine (BDCQ), by cytochrome P450 (CYP). However, until recently, no data are available on the metabolic pathway of chloroquine. Therefore, the metabolic pathway of chloroquine was evaluated using human liver microsomes and cDNA-expressed CYPs. Chloroquine is mainly metabolized to DCQ, and its Eadie-Hofstee plots were biphasic, indicating the involvement of multiple enzymes, with apparent Km and Vmax values of 0.21 mM and 1.02 nmol/min/mg protein 3.43 mM and 10.47 nmol/min/mg protein for high and low affinity components, respectively. Of the cDNA-expressing CYPs examined, CYP1A2, 2C8, 2C19, 2D6 and 3A4/5 exhibited significant DCQ formation. A study using chemical inhibitors showed only quercetin (a CYP2C8 inhibitor) and ketoconazole (a CYP3A4/5 inhibitor) inhibited the DCQ formation. In addition, the DCQ formation significantly correlated with the CYP3A4/5-catalyzed midazolam 1-hydroxylation (r = 0.868) and CYP2C8-catalyzed paclitaxel 6alpha-hydroxylation (r = 0.900). In conclusion, the results of the present study demonstrated that CYP2C8 and CYP3A4/5 are the major enzymes responsible for the chloroquine N-deethylation to DCQ in human liver microsomes.

Physiologically based modelling of inhibition of metabolism and assessment of the relative potency of drug and metabolite: dextromethorphan vs. dextrorphan using quinidine inhibition

AIMS

To define the relative antitussive effect of dextromethorphan (DEX) and its primary metabolite dextrorphan (DOR) after administration of DEX.

METHODS

Data were analysed from a double-blind, randomized cross-over study in which 22 subjects received the following oral treatments: (i) placebo; (ii) 30 mg DEX hydro-bromide; (iii) 60 mg DEX hydro-bromide; and (iv) 30 mg DEX hydro-bromide preceded at 1 h by quinidine HCl (50 mg). Cough was elicited using citric acid challenge. Pharmacokinetic data from all non-placebo arms of the study were fitted simultaneously. The parameters were then used as covariates in a link PK-PD model of cough suppression using data from all treatment arms.

RESULTS

The best-fit PK model assumed two- and one-compartment PK models for DEX and DOR, respectively, and competitive inhibition of DEX metabolism by quinidine. The intrinsic clearance of DEX estimated from the model ranged from 59 to 1536 l x h(-1), which overlapped with that extrapolated from in vitro data (12-261 l x h(-1)) and showed similar variation (26- vs. 21-fold, respectively). The inhibitory effect of quinidine ([I]/Ki) was 19 (95% confidence interval of mean: 18-20) with an estimated average Ki of 0.017 microM. Although DEX and DOR were both active, the potency of the antitussive effect of DOR was 38% that of DEX. A sustained antitussive effect was related to slow removal of DEX/DOR from the effect site (ke0 = 0.07 h(-1)).

CONCLUSIONS

Physiologically based PK modelling with perturbation of metabolism using an inhibitor allowed evaluation of the antitussive potency of DOR without the need for separate administration of DOR.

Inhibitory effects of tricyclic antidepressants (TCAs) on human cytochrome P450 enzymes in vitro: mechanism of drug interaction between TCAs and phenytoin

The ability of tricyclic antidepressants (TCAs) to inhibit phenytoin p-hydroxylation was evaluated in vitro by incubation studies of human liver microsomes and cDNA-expressed cytochrome p450s (p450s). The TCAs tested were amitriptyline, imipramine, nortriptyline, and desipramine. Amitriptyline and imipramine strongly and competitively inhibited phenytoin p-hydroxylation in microsomal incubations (estimated K(i) values of 5.2 and 15.5 micro M, respectively). In contrast, nortriptyline and desipramine produced only weak inhibition. In the incubation study using cDNA-expressed P450s, both CYP2C9 and CYP2C19 catalyzed phenytoin p-hydroxylation, whereas TCAs inhibited only the CYP2C19 pathway. All of the TCAs tested inhibited CYP2D6-catalyzed dextromethorphan-O-demethylation competitively, with estimated K(i) values of 31.0, 28.6, 7.9, and 12.5 micro M, respectively. The tertiary amine TCAs, amitriptyline and imipramine, also inhibited CYP2C19-catalyzed S-mephenytoin 4'-hydroxylation (estimated K(i) of 37.7 and 56.8 micro M, respectively). The secondary amine TCAs, nortriptyline and desipramine, however, showed minimal inhibition of CYP2C19 (estimated IC(50) of 600 and 685 micro M, respectively). None of the TCAs tested produced remarkable inhibition of any other p450 isoforms. These results suggest that TCAs inhibit both CYP2D6 and CYP2C19 and that the interaction between TCAs and phenytoin involves inhibition of CYP2C19-catalyzed phenytoin p-hydroxylation.

Effect of probucol on cytochrome P450 activities in human liver microsomes

The effects of probucol, a cholesterol-lowering agent, on several cytochrome P450 (CYP) isoform-specific reactions in human liver microsomes were investigated to predict drug interactions with probucol in vivo from in vitro data. The following eight CYP catalytic reactions were used in this study: CYP1A1/2-mediated 7-ethoxyresorufin O-deethylation, CYP2A6-mediated coumarin 7-hydroxylation, CYP2B6-mediated 7-benzyloxyresorufin O-debenzylation, CYP2C8/9-mediated tolbutamide methylhydroxylation, CYP2C19-mediated S-mephenytoin 4'-hydroxylation, CYP2D6-mediated bufuralol 1'-hydroxylation, CYP2E1-mediated chlorzoxazone 6-hydroxylation, and CYP3A4-mediated testosterone 6beta-hydroxylation. Probucol had neither stimulatory nor inhibitory effects on CYP1Al/2, 2A6, 2B6, 2C8/9, 2C19, 2D6, 2E1, and 3A4 activities at concentrations up to 300 microM, indicating that probucol, at the expected therapeutic concentrations, would not be predicted to cause clinically significant interactions with other CYP-metabolized drugs.

The roles of CYP2D6 and stereoselectivity in the clinical pharmacokinetics of chlorpheniramine

AIMS

To examine the stereoselective disposition of chlorpheniramine and to evaluate the role of CYP2D6 in chlorpheniramine pharmacokinetics in humans.

METHODS

Eight healthy volunteers (six extensive metabolizers with respect to CYP2D6 and two poor metabolizers) received a single 8 mg oral dose of rac-chlorpheniramine either given alone or following administration of quinidine 50 mg every 6 h for 2 days prior to the study day and every 6 h thereafter until the end of the study. Plasma concentrations of (S)-(+)- and (R)-(-)-enantiomers of chlorpheniramine were determined using liquid chromatography/mass spectrometry.

RESULTS

In extensive metabolizers, mean Cmax was greater (12.55+/-1.51 ng ml-1vs 5.38+/-0.44 ng ml-1) and CLoral was lower (0.49+/-0.08 l h-1 kg-1vs 1.07+/-0.15 l h-1 kg-1) for (S)-(+)- than for (R)-(-)-chlorpheniramine (P<0.005). For (S)-(+)-chlorpheniramine, administration of quinidine, an inhibitor of CYP2D6, resulted in an increase in Cmax to 13.94+/-1.51 (P<0.01), a reduction in CLoral to 0.22+/-0.03 l h-1 kg-1 (P<0.01), and a prolongation of elimination half-life from 18.0+/-2.0 h to 29.3+/-2.0 h (P<0.001). Administration of quinidine decreased CLoral for (R)-(-)-chlorpheniramine to 0.60+/-0.10 l h-1 kg-1 (P<0.005). In CYP2D6 poor metabolizers, systemic exposure was greater after chlorpheniramine alone than in extensive metabolizers, and administration of quinidine resulted in a slight increase in CLoral.

CONCLUSIONS

Stereoselective elimination of chlorpheniramine occurs in humans, with the most pharmacologically active (S)-(+)-enantiomer cleared more slowly than the (R)-(-)-enantiomer. CYP2D6 plays a role in the metabolism of chlorpheniramine in humans.

Inhibition of human drug metabolizing cytochrome P450 by buprenorphine

The effects of buprenorphine, a powerful mixed agonist/antagonist analgesic, on several cytochrome P450 (CYP) isoform specific reactions in human liver microsomes were investigated to predict drug interaction of buprenorphine in vivo from in vitro data. The following eight CYP-catalytic reactions were used in this study: CYPlA1/2-mediated 7-ethoxyresorufin O-deethylation, CYP2A6-mediated coumarin 7-hydroxylation, CYP2B6-mediated 7-benzyloxyresorufin O-debenzylation, CYP2C8/9-mediated tolbutamide methylhydroxylation, CYP2C19-mediated S-mephenytoin 4-hydroxylation, CYP2D6-mediated bufuralol 1'-hydroxylation, CYP2E1-mediated chlorzoxazone 6-hydroxylation, and CYP3A4-mediated testosterone 6beta-hydroxylation. Buprenorphine strongly inhibited the CYP3A4- and CYP2D6-catalyzed reactions with Ki values of 14.7 microM and 21.4 microM, respectively. The analgesic also weakly inhibited specific reactions catalyzed by CYP1A1/2 (Ki=132 microM), CYP2B6 (Ki=133 microM), CYP2C19 (Ki=146 microM), CYP2C8/9 (IC50>300 microM), and CYP2E1 (IC50>300 microM), but not CYP2A6 mediated pathway. In consideration of the Ki values obtained in this study and the therapeutic concentration of buprenorphine in human plasma, buprenorphine would not be predicted to cause clinically significant interactions with other CYP-metabolized drugs.

Stereoselective disposition of talinolol in man

The disposition of the beta-blocking drug talinolol is controlled by P-glycoprotein in man. Because talinolol is marketed as a racemate, we reevaluated the serum-concentration time profiles of talinolol of a previously published study with single intravenous (30 mg) and repeated oral talinolol (100 mg for 14 days) before and after comedication of rifampicin (600 mg per day for 9 days) in eight male healthy volunteers (age 22-26 years, body weight 67-84 kg) with respect to differences in the kinetic profiles of the two enantiomers S(-) talinolol and R(+) talinolol. Additionally, the metabolism of talinolol in human liver microsomes was examined. After oral administration, S(-) talinolol was slightly less absorbed and faster eliminated than R(+) talinolol. The absolute bioavailabilty of the R(+) enantiomer of talinolol was slightly but significantly higher than of its S(-) enantiomer. Coadministration of rifampicin further intensified this difference in the disposition of R(+) and S(-) talinolol (p < 0.05). Formation of 4-trans hydroxytalinolol was the major metabolic pathway in human liver microsomes. All Cl(int) values of S(-) were higher than of R(+) talinolol; 0.1 microM ketoconazole inhibited the formation of all metabolites. In conclusion, the stereoselectivity of talinolol disposition is of minor importance, and most likely caused by presystemic biotransformation via CYP3A4. The less active R(+) talinolol might be suitable for phenotyping P-glycoprotein expression in man.

High-throughput cytochrome P450 (CYP) inhibition screening via cassette probe-dosing strategy. II. Validation of a direct injection/on-line guard cartridge extraction-tandem mass spectrometry method for CYP2D6 inhibition assessment

A highly efficient direct injection/on-line guard cartridge extraction-tandem mass spectrometry (DI/GCE-MS-MS) method has been validated for high-throughput evaluation of cytochrome P450 (CYP) 2D6 inhibition potential using human hepatic microsomes and 96-well microtiter plates. Microsomal incubations were terminated with formic acid, centrifuged, and the resulting supernatants were injected for DI/GCE-MS-MS analysis. Due to the novel use of an extremely short C18 guard cartridge, this method exhibits several advantages, such as no sample preparation, excellent on-line extraction, short run time (2.5 min), and minimized source contamination and performance deterioration. The DI/GCE-MS-MS method demonstrates acceptable accuracy and precision for the quantification of dextrorphan, a marker metabolite of dextromethorphan mediated by CYP2D6, in microsomal incubations. The CYP2D6 inhibition assay has been validated using quinidine as a known selective inhibitor of the isoform. The IC50 value (0.20 microM) measured by the new method is in good agreement with the literature value (0.22 microM).

Inhibition of cytochrome P450 2C9 activity in vitro by 5-hydroxytryptamine and adrenaline

In the present study, the occurrence of a modulatory effect of 14 neurotransmitters, precursors and metabolites on the cytochrome P450 2C9 (CYP2C9) enzyme activity, as determined by diclofenac 4-hydroxylation, was studied in human liver microsomes. Two indoleamines, 5-hydroxytryptamine (5-HT) and adrenaline, showed a non-competitive-type inhibitory effect of approximately 90% of the diclofenac 4-hydroxylase activity, with Ki values of 63.5 (0.7 and 156 (89.3 microM, respectively. The rest of substances analysed were weak inhibitors or had no inhibitory effect. CYP2C subfamily is present in human brain, although CYP2C9 isozyme has not yet been identified in this tissue, and CYP2C9 is involved in the metabolism of psychoactive drugs. Therefore, the fact that endogenous compounds could modulate the CYP2C9 activity, suggests that an hypothetical local activity of brain CYP2C9 might be susceptible to regulatory mechanisms. The possible clinical implications of this modulation are discussed.

Inhibition of cytochrome P450 (CYP450) isoforms by isoniazid: potent inhibition of CYP2C19 and CYP3A

Isoniazid (INH) remains the most safe and cost-effective drug for the treatment and prophylaxis of tuberculosis. The use of INH has increased over the past years, largely as a result of the coepidemic of human immunodeficiency virus infection. It is frequently given chronically to critically ill patients who are coprescribed multiple medications. The ability of INH to elevate the concentrations in plasma and/or toxicity of coadministered drugs, including those of narrow therapeutic range (e.g., phenytoin), has been documented in humans, but the mechanisms involved are not well understood. Using human liver microsomes (HLMs), we tested the inhibitory effect of INH on the activity of common drug-metabolizing human cytochrome P450 (CYP450) isoforms using isoform-specific substrate probe reactions. Incubation experiments were performed at a single concentration of each substrate probe at its K(m) value with a range of INH concentrations. CYP2C19 and CYP3A were inhibited potently by INH in a concentration-dependent manner. At 50 microM INH (approximately 6.86 microg/ml), the activities of these isoforms decreased by approximately 40%. INH did not show significant inhibition (<10% at 50 microM) of other isoforms (CYP2C9, CYP1A2, and CYP2D6). To accurately estimate the inhibition constants (K(i) values) for each isoform, four concentrations of INH were incubated across a range of five concentrations of specific substrate probes. The mean K(i) values (+/- standard deviation) for the inhibition of CYP2C19 by INH in HLMs and recombinant human CYP2C19 were 25.4 +/- 6.2 and 13 +/- 2.4 microM, respectively. INH showed potent noncompetitive inhibition of CYP3A (K(i) = 51.8 +/- 2.5 to 75.9 +/- 7.8 microM, depending on the substrate used). INH was a weak noncompetitive inhibitor of CYP2E1 (K(i) = 110 +/- 33 microM) and a competitive inhibitor of CYP2D6 (K(i) = 126 +/- 23 microM), but the mean K(i) values for the inhibition of CYP2C9 and CYP1A2 were above 500 microM. Inhibition of one or both CYP2C19 and CYP3A isoforms is the likely mechanism by which INH slows the elimination of coadministered drugs, including phenytoin, carbamazepine, diazepam, triazolam, and primidone. Slow acetylators of INH may be at greater risk for adverse drug interactions, as the degree of inhibition was concentration dependent. These data provide a rational basis for understanding drug interaction with INH and predict that other drugs metabolized by these two enzymes may also interact.

Potent inhibition of CYP2D6 by haloperidol metabolites: stereoselective inhibition by reduced haloperidol

AIMS

We evaluated the inhibitory effect of haloperidol and its metabolites on CYP2D6 activity in order to better understand the potential role of these metabolites in drug interactions involving haloperidol.

METHODS

The inhibitory effects of haloperidol and five of its metabolites on dextrorphan formation from dextromethorphan, a marker probe of CYP2D6 activity, were measured in human liver microsomal preparations. Apparent kinetic parameters for enzyme inhibition were determined by nonlinear regression analysis of the data.

RESULTS

Racemic reduced haloperidol and its metabolite, RHPTP competitively inhibited dextromethorphan O-demethylation with estimated Ki values (0.24 microM and 0.09 microM, respectively) that were substantially lower than that of haloperidol (0.89 microM). The inhibitory effect of S(-)-reduced haloperidol was more potent than the R(+)-enantiomer, with estimated Ki values of 0.11 microM and 1.1 microM, respectively. The pyridinium metabolite of haloperidol, HPP+ inhibited the enzyme activity noncompetitively with a Ki value of 0.79 microM. The N-dealkylated metabolites of haloperidol (FBPA and CPHP) had a diminished inhibitory potency. While FBPA showed no notable inhibitory effect on dextrorphan formation, CPHP showed moderate competitive inhibition with a Ki value of 20.9 microM.

CONCLUSIONS

The principal metabolites of haloperidol inhibit CYP2D6, suggesting that they might contribute to the inhibitory effects of the drug. Reduced haloperidol seems to inhibit CYP2D6 activity in an enantioselective manner with the physiologically occurring S(-) enantiomer being more potent.

In vitro inhibition of the cytochrome P450 (CYP450) system by the antiplatelet drug ticlopidine: potent effect on CYP2C19 and CYP2D6

AIMS

To examine the potency of ticlopidine (TCL) as an inhibitor of cytochrome P450s (CYP450s) in vitro using human liver microsomes (HLMs) and recombinant human CYP450s.

METHODS

Isoform-specific substrate probes of CYP1A2, 2C19, 2C9, 2D6, 2E1 and 3A4 were incubated in HLMs or recombinant CYPs with or without TCL. Preliminary data were generated to simulate an appropriate range of substrate and inhibitor concentrations to construct Dixon plots. In order to estimate accurately inhibition constants (Ki values) of TCL and determine the type of inhibition, data from experiments with three different HLMs for each isoform were fitted to relevant nonlinear regression enzyme inhibition models by WinNonlin.

RESULTS

TCL was a potent, competitive inhibitor of CYP2C19 (Ki = 1.2 +/- 0.5 microM) and of CYP2D6 (Ki = 3.4 +/- 0.3 microM). These Ki values fell within the therapeutic steady-state plasma concentrations of TCL (1-3 microM). TCL was also a moderate inhibitor of CYP1A2 (Ki = 49 +/- 19 microM) and a weak inhibitor of CYP2C9 (Ki > 75 microM), but its effect on the activities of CYP2E1 (Ki = 584 +/- 48 microM) and CYP3A (> 1000 microM) was marginal.

CONCLUSIONS

TCL appears to be a broad-spectrum inhibitor of the CYP isoforms, but clinically significant adverse drug interactions are most likely with drugs that are substrates of CYP2C19 or CYP2D6.

Intestinal cytochrome P450 and response to rifampicin in rabbits

Properties of cytochrome P450 (P450) in rabbit intestines have been investigated to assess the possibility of an experimental model for human intestinal oxidation of drugs. Significant amounts of P450 and cytochrome b5 and activities of NADPH-cytochrome P450 reductase were detected in microsomes from rabbit duodenal, jejunal, ileac and colon mucosa. All the small intestinal fractions mediated phenytoin, dextromethorphan and testosterone oxidations. Several P450 forms belonging to the CYP1A, CYP2C, CYP2D and CYP3A, but not CYP2B and CYP2E, subfamilies were detected in these tissues by Western blotting. A good correlation was observed between immunodetectable levels of CYP3A and activities of testosterone 6beta-hydroxylation. Small intestine, but not colon, CYP3A levels were increased by the pretreatment of rabbits with rifampicin (50 mg/kg for 4 days, p.o.). The extent of the increase was similar between duodena and livers. These properties of rabbit intestinal P450s were comparable to those of human intestine. These phenomena suggest the possibility that the rabbit is a beneficial in vivo model for the assessment of drug interaction occurring at the first pass of drugs ingested.

Expression and characterization of canine cytochrome P450 2D15

CYP2D15 is the canine ortholog of human CYP2D6, the human CYP2D isoform involved in the metabolism of drugs such as antiarhythmics, adrenoceptor antagonists, and tricyclic antidepressants. Similar to human, canine CYP2D15 is expressed in the liver, with detectable levels in several other tissues. Three different CYP2D15 cDNA clones were obtained by RT-PCR from dog liver RNA. Two clones corresponded to variant full-length CYP2D15 cDNAs (termed CYP2D15 WT2 and CYP2D15 V1); the third was identified as a splicing variant missing exon 3 (termed CYP2D15 V2). Recombinant baculoviruses were constructed containing full-length cDNAs and used to express CYP2D15 WT2 and CYP2D15 V1 in Spodoptera frugiperda (Sf9) cells with expression levels of up to 0.14 nmol/mg cell protein. As with human CYP2D6, the recombinant CYP2D15 enzymes exhibited bufuralol 1'-hydroxylaseand dextromethorphan O-demethylase activities whencoexpressed with rabbit NADPH:P450 oxidoreductase. For bufuralol 1'-hydroxylase, apparent Km values were 4.9, 3.7, and 2.5 microM and the Vmax values were 0.14, 0.034, and 0.60 nmol/min/mg protein for dog liver microsomes, CYP2D15 WT2, and the variant CYP2D15 V1, respectively. For dextromethorphan O-demethylase, apparent Km values were 0.6, 0.6, and 2.0 microM and the Vmax values were 0.18, 0.034, and 0.057 nmol/min/mg protein for dog liver microsomes, CYP2D15 WT2, and the variant CYP2D15 V1, respectively. The human CYP2D6-specific inhibitor quinidine and the rat CYP2D1-specific inhibitor quinine were both shown to be inhibitors of bufuralol 1'-hydroxylase activity for dog liver microsomes, CYP2D15 WT2, and the CYP2D15 V1 variant with nearly equal potency. Thus, the dog expresses a CYP2D ortholog possessing enzymatic activities similar to human CYP2D6, but is affected by the inhibitors quinine and quinidine in a manner closer to that of rat CYP2D1.

Involvement of CYP1A2 in mexiletine metabolism

AIMS

Mexiletine has been reported to be hydroxylated by cytochrome P450 2D6 (CYP2D6) in humans. However, the involvement of CYP1A2 in the metabolism of mexiletine has been proposed based on the interaction with theophylline which is mainly metabolized by CYP1A2. The aim of this study was to clarify the role of human CYP1A2 in mexiletine metabolism.

METHODS

Human CYP isoforms involved in mexiletine metabolism were investigated using microsomes from human liver and B-lymphoblastoid cells expressing human CYPs. The contributions of CYP1A2 and CYP2D6 to mexiletine metabolism were estimated by the relative activity factor (RAF).

RESULTS

Mexiletine p- and 2-hydroxylase activities in human liver microsomes were inhibited by ethoxyresorufin and furafylline as well as quinidine. Mexiletine p- and 2-hydroxylase activities in microsomes from nine human livers correlated significantly with bufuralol 1'-hydroxylase activity (r = 0.907, P < 0.001 and r = 0.886, P < 0.01, respectively). Microsomes of B-lymphoblastoid cells expressing human CYP1A2 exhibited lower mexiletine p- and 2-hydroxylase activities than those expressing human CYP2D6. It was estimated by RAF that the major isoform involved in mexiletine metabolism was CYP2D6, and the contribution of CYPIA2 to both mexiletine p- and 2-hydroxylase activities was 7-30% in human liver microsomes. However, the Km values of the expressed CYP1A2 (approximately 15 microM) were almost identical with those of the expressed CYP2D6 (approximately 22 microM) and human liver microsomes.

CONCLUSIONS

Mexiletine is a substrate of CYP1A2. The data obtained in this study suggest that the interaction of mexiletine with theophylline might be due to competitive inhibition of CYP1A2.

Interactions of amphetamine analogs with human liver CYP2D6

The interaction of fifteen amphetamine analogs with the genetically polymorphic enzyme CYP2D6 was examined. All fourteen phenylisopropylamines tested were competitive inhibitors of CYP2D6 in human liver microsomes. The presence of a methylenedioxy group in the 3,4-positions of both amphetamine (Ki = 26.5 microM) and methamphetamine (Ki = 25 microM) increased the affinity for CYP2D6 to 1.8 and 0.6 microM, respectively. Addition of a methoxy group to amphetamine in the 2-position also increased the affinity for CYP2D6 (Ki = 11.5 microM). The compound with the highest affinity for CYP2D6 was an amphetamine analog (MMDA-2) having both a methoxy group in the 2-position and a methylenedioxy group (Ki = 0.17 microM). Mescaline did not interact with CYP2D6. O-Demethylation of p-methoxyamphetamine (PMA) by CYP2D6 was characterized (Km = 59.2 +/- 22.4 microM, and Vmax = 29.3 +/- 16.6 nmol/mg/hr, N = 6 livers). This reaction was negligible in CYP2D6-deficient liver microsomes, was inhibited stereoselectively by the quinidine/quinine enantiomer pair, and was cosegregated with dextromethorphan O-demethylation (r = 0.975). The inhibitory effect of methylenedioxymethamphetamine (MDMA) was enhanced by preincubation with microsomes, suggesting that MDMA may produce a metabolite complex with CYP2D6. These findings suggest that phenylisopropylamines as a class interact with CYP2D6 as substrates and/or inhibitors. Their use may cause metabolic interactions with other drugs that are CYP2D6 substrates, and the potential for polymorphic oxidation via CYP2D6 may be a source of interindividual variation in their abuse liability and toxicity.

Dextromethorphan metabolism in Jordanians: dissociation of dextromethorphan O-demethylation from debrisoquine 4-hydroxylation

The concentrations of dextromethorphan (DM) and its metabolites dextrorphan (DRP), 3-methoxymorphinan (MM) and 3-hydroxymorphinan (HM) were measured in 8 h urine samples from 266 unrelated healthy Jordanian subjects following oral administration of 30 mg dextromethorphan hydrobromide and using a rapid, sensitive and precise HPLC method with fluorometric detection. The frequency of the 'poor' metabolizer status of DM-O-demethylation as judged by log DM/DRP was found to be 6.8% with a 95% confidence interval of 3.8-9.8%. There was a strong correlation between log DM/DRP and log total non-O-demethylated compounds (NODM)/total O-demethylated metabolites (ODM) metabolic ratios (r = 0.96, P < 0.01). However, one subject with log DM/DRP of 0.05 that classifies him as a poor metabolizer was found to have a log NODM/ODM of -0.73 which is in the range of extensive metabolizer status suggesting the presence of another cytochrome P450 isoenzyme involved in dextromethorphan O-demethylation. Dextromethorphan N-demethylation to 3-methoxymorphinan was detected in 55.3% of individuals. Furthermore, a dissociation between dextromethorphan O-demethylation and debrisoquine (D) 4-hydroxylation has been observed. Among the 116 subjects phenotyped with both dextromethorphan and debrisoquine, 7 were poor metabolizers of both, three were poor metabolizers of debrisoquine and extensive metabolizers of dextromethorphan whilst 4 were poor metabolizers of dextromethorphan and extensive metabolizers of debrisoquine, one of whom was reclassified as an extensive metabolizer of dextromethorphan using log NODM/ODM to characterize dextromethorphan metabolizer status.

Pharmacokinetic and pharmacodynamic interactions between diltiazem and quinidine

OBJECTIVES

To examine the pharmacokinetic and pharmacodynamic interactions between quinidine and diltiazem because both drugs can inhibit drug metabolism.

METHODS

Twelve fasting, healthy male volunteers (age, 24 +/- 5 years; weight, 75 +/- 10 kg) received a single oral dose of diltiazem (60 mg) or quinidine (200 mg), alone and on a background of the other drug, in a crossover study. Background treatment consisted of 100 mg quinidine twice a day or 90 mg sustained-release diltiazem twice a day for 2 day before the study day.

RESULTS

Pretreatment with diltiazem significantly (p < 0.05) increased the area under the curve of quinidine from 7414 +/- 1965 to 11,213 +/- 2610 ng.hr/ml and increased its terminal elimination half-life (t1/2) from 6.8 +/- 1.1 to 9.3 +/- 1.5 hours. Its oral clearance was decreased from 0.39 +/- 0.1 to 0.25 +/- 0.1 L/hr/kg, whereas the maximal concentration was not significantly affected. Diltiazem disposition was not significantly affected by pretreatment with quinidine. Diltiazem pretreatment increased QTc and PR intervals and decreased heart rate and diastolic blood pressure. No significant pharmacodynamic differences were shown for diltiazem alone versus quinidine pretreatment.

CONCLUSION

Diltiazem significantly decreased the clearance and increased the t1/2 of quinidine, but quinidine did not alter the kinetics of diltiazem with the dose used. No significant pharmacodynamic interaction was shown for the combination that would not be predicted from individual drug administration.

Potent inhibition of yeast-expressed CYP2D6 by dihydroquinidine, quinidine, and its metabolites

The inhibitory effects of dihydroquinidine, quinidine and several quinidine metabolites on cytochrome P450 2D6 (CYP2D6) activity were examined. CYP2D6 heterologously expressed in yeast cells O-demethylated dextromethorphan with a mean Km of 5.4 microM and a Vmax of 0.47 nmol/min/nmol. Quinidine and dihydroquinidine both potently inhibited CYP2D6 metabolic activity (mean Ki = 0.027 and 0.013 microM, respectively) in yeast microsomes and in human liver microsomes. The metabolites, 3-hydroxyquinidine, O-desmethylquinidine and quinidine N-oxide also inhibited CYP2D6, but their Ki values (0.43 to 2.3 microM) were one to two orders of magnitude weaker than the values for quinidine and dihydroquinidine. There was a trend towards an inverse relationship between Ki and lipophilicity (r = -0.90, N = 5, P = 0.07), as determined by the retention-time parameter k' using reverse-phase HPLC. Thus, although the metabolites of quinidine have the capacity to inhibit CYP2D6 activity, quinidine and the impurity dihydroquinidine are the important inhibitors of CYP2D6.

The role of CYP2D6 in primary and secondary oxidative metabolism of dextromethorphan: in vitro studies using human liver microsomes

1. The enzyme kinetics of dextromethorphan O-demethylation in liver microsomes from three extensive metabolisers (EM) with respect to CYP2D6 indicated high (Km1 2.2-9.4 microM) and low (Km2 55.5-307.3 microM) affinity sites whereas microsomes from two poor metabolisers (PM) indicated a single site (Km 560 and 157 microM). Similar differences were shown for 3-methoxymorphinan O-demethylation to 3-hydroxymorphinan (Km 6.9-9.6 microM in EM subjects; Km 307 and 213 microM in PM subjects). 2. Dextromethorphan O-demethylation was inhibited competitively by quinidine (Ki 0.1 microM), rac-perhexiline (Ki 0.4 microM), dextropropoxyphene (Ki 6 microM), rac-methadone (Ki 8 microM), and 3-methoxymorphinan (Ki 15 microM). These compounds were also potent inhibitors of 3-methoxymorphinan O-demethylation with IC50 values ranging from 0.02-12 microM. Anti-LKM1 serum inhibited both dextromethorphan and 3-methoxymorphinan O-demethylations in a titre-dependent manner. 3. The Michaelis-Menten constant for dextromethorphan N-demethylation to 3-methoxymorphinan (Km 632-977 microM) and dextrorphan N-demethylation to 3-hydroxymorphinan (Km 1571-4286 microM) did not differ between EM and PM microsomes. These N-demethylation reactions were not inhibited by quinidine and rac-methadone or LKM1 antibodies. 4. Dextromethorphan and 3-methoxymorphinan are metabolised by the same P450 isoform, CYP2D6, whereas the N-demethylation reactions are not carried out by CYP2D6.

Characterization of dextromethorphan N-demethylation by human liver microsomes. Contribution of the cytochrome P450 3A (CYP3A) subfamily

In an effort to identify the human cytochromes P450 involved in the N-demethylation of dextromethorphan, the kinetics of 3-methoxymorphinan formation were studied in microsomal enzyme systems. Under initial rate conditions, 3-methoxymorphinan formation demonstrated single enzyme Michaelis-Menten kinetics using microsomes obtained from three human livers (Km: 0.52-0.71 mM; Vmax: 375-812 pmol/mg protein/min). B-lymphoblastoid cells expressing CYP3A4 incubated with 0.4 mM dextromethorphan catalyzed the formation of 3-methoxymorphinan at a rate of 22 pmol product/mg protein/min. Midazolam, a prototypic substrate for CYP3A4 and CYP3A5, competitively inhibited dextromethorphan N-demethylation by two human liver microsomal samples with Ki values of 46 +/- 10 and 63 +/- 8 microM. At a dextromethorphan concentration of 0.4 mM, gestodene (100 microM) inhibited 3-methoxymorphinan formation by approximately 50%. Immunoinhibition of dextromethorphan N-demethylation using rabbit anti-CYP3A4 antibodies resulted in a 60% decrease in 3-methoxymorphinan formation at a dextromethorphan concentration of 0.4 mM. Additional inhibition studies using furafylline, coumarin, sulfaphenazole, mephenytoin, quinidine, and diethyldithiocarbamic acid, which are selective inhibitors of CYP1A2, CYP2A6, CYP2C8/9, CYP2Cmp, CYP2D6, and CYP2E1, respectively, demonstrated no substantial inhibition of dextromethorphan N-demethylation. Correlation analysis was performed using the rate of 3-methoxymorphinan formation at a concentration of 1 mM dextromethorphan and immunoquantified levels of CYP1A2, CYP2A6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4, and CYP3A5 and their associated characteristic catalytic activities. A significant correlation was observed between dextromethorphan N-demethylase activity and midazolam 1'- and 4-hydroxylase activity (r2 = 0.77 and 0.69 respectively, N = 19, P < 0.01); the exclusion of those samples containing both CYP3A4 and CYP3A5 increased the correlation significantly (r2 = 0.87 and 0.91 respectively, N = 12, P < 0.01). In the absence of CYP3A5, a significant correlation was observed between 3-methoxymorphinan formation and the sample's erythromycin N-demethylase activity (r2 = 0.94, N = 12, P < 0.01), testosterone 6 beta-hydroxylase activity (r2 = 0.96, N = 7, P < 0.01) and relative immunoquantified levels of CYP3A4 (r2 = 0.96, N = 12, P < 0.01). Inclusion of those samples expressing CYP3A5 in addition to CYP3A4 reduced the magnitude of the observed correlation. No significant correlation between 3-methoxymorphinan formation and the sample's relative immunoquantified levels of or form-selective activity associated with CYP1A2, CYP2A6, CYP2C8, CYP2C9, CYP2C19 (or CYP2Cmp), CYP2D6, and CYP2E1 was observed. In conclusion, dextromethorphan N-demethylation appears to be catalyzed primarily by CYP3A4 and to a lesser extent by CYP3A5 in vitro in humans.(ABSTRACT TRUNCATED AT 400 WORDS)

Dextromethorphan O-demethylation polymorphism in Jordanians

The O-demethylation of dextromethorphan (DMT) to dextrorphan (DRP) was studied in 241 unrelated, healthy Jordanian volunteers (171 males, 70 females). Urine was collected for 8 h following a single oral dose of DMT bromhydrate 30 mg. A thin-layer chromatographic (TLC) technique was used to identify the metaboliser phenotype. The frequency of the poor metaboliser phenotype was found to be 2.9% (approximate 95% confidence interval 0.8-5.0%). Applying the Hardy-Weinberg Law, the frequency of the recessive autosomal gene controlling poor metabolism was 0.17 (95% confidence interval 0.108-0.232).

Amiodarone N-deethylation in human liver microsomes: involvement of cytochrome P450 3A enzymes (first report)

Experiments were conducted on three different human liver samples to identify the cytochrome P450 isozyme which is involved in the biotransformation of the class III antiarrhythmic agent, amiodarone, into its major metabolite, desethylamiodarone (DEA). The classic P450 inhibitors, SKF 525A, metyrapone, and carbon monoxide provided a significant reduction in the in vitro formation of DEA by human hepatic microsomes. Amiodarone N-deethylase activities expressed by intrinsic clearance values were similar in all the livers used, although two livers were genotyped as extensive and one as a poor metabolizer for the cytochrome P450 CYP2D6 gene. DEA production was strongly inhibited (more than 80%) by the anti-P450 3A4 antibody, but not by anti-LKM1-positive serum. It seems therefore that the P450 3A subfamily is certainly implicated in human hepatic amiodarone N-deethylation.

The metabolism of mexiletine in relation to the debrisoquine/sparteine-type polymorphism of drug oxidation

1. The relationship between the metabolism of the antiarrhythmic drug mexiletine and the debrisoquine/sparteine-type polymorphism was studied in vitro, using microsomes from six human livers, and in vivo, in nine healthy drug-free volunteers with wide variation in their ability to hydroxylate debrisoquine. 2. There was a strong and similar correlation between the formation rate of both major mexiletine metabolites, p-hydroxymexiletine (PHM) and hydroxymethylmexiletine (HMM), and the high affinity component of dextromethorphan O-demethylase activity in human liver microsomes (rs = 0.94; P less than 0.01). 3. There were marked interindividual differences in the amounts of PHM and HMM excreted in the urine over 48 h after a single 200 mg oral dose of mexiletine hydrochloride. Recoveries of both metabolites were correlated inversely with the debrisoquine/4-hydroxydebrisoquine (D/HD) urinary metabolic ratio (rs = -0.83; P = 0.006 and rs = -0.85; P = 0.004, respectively) and were lower in poor metabolisers of debrisoquine (PM) than in extensive metabolisers (EM). Moreover, PM had the highest values of mexiletine/PHM and mexiletine/HMM urinary ratios. In addition, there was a strong correlation between these two indices of mexiletine hydroxylation and the D/HD metabolic ratios (rs = 0.92; P = 0.001 and rs = 0.90; P = 0.001, respectively). 4. After mexiletine pretreatment, the values for D/HD ratio were significantly increased in EM while corresponding values in PM were similar. 5. These findings are in accordance with previous in vitro data suggesting that PHM and HMM formation is predominantly catalyzed by the genetically variable human liver cytochrome P450IID6 isoenzyme responsible for the debrisoquine/sparteine-type polymorphism of drug oxidation.(ABSTRACT TRUNCATED AT 250 WORDS)