Isolation and characterization of human liver cytochrome P450 2C19: correlation between 2C19 and S-mephenytoin 4'-hydroxylation

A Review of CYP-Mediated Drug Interactions: Mechanisms and In Vitro Drug-Drug Interaction Assessment

Drug metabolism is a major determinant of drug concentrations in the body. Drug-drug interactions (DDIs) caused by the co-administration of multiple drugs can lead to alteration in the exposure of the victim drug, raising safety or effectiveness concerns. Assessment of the DDI potential starts with in vitro experiments to determine kinetic parameters and identify risks associated with the use of comedication that can inform future clinical studies. The diverse range of experimental models and techniques has significantly contributed to the examination of potential DDIs. Cytochrome P450 (CYP) enzymes are responsible for the biotransformation of many drugs on the market, making them frequently implicated in drug metabolism and DDIs. Consequently, there has been a growing focus on the assessment of DDI risk for CYPs. This review article provides mechanistic insights underlying CYP inhibition/induction and an overview of the in vitro assessment of CYP-mediated DDIs.

Anatomical distribution and expression of CYP in humans: Neuropharmacological implications

The cytochrome P450 (CYP450) superfamily is responsible for the metabolism of most xenobiotics and pharmacological treatments generally used in clinical settings. Genetic factors as well as environmental determinants acting through fine epigenetic mechanisms modulate the expression of CYP over the lifespan (fetal vs. infancy vs. adult phases) and in diverse organs. In addition, pathological processes might alter the expression of CYP. In this selective review, we sought to summarize the evidence on the expression of CYP focusing on three specific aspects: (a) the anatomical distribution of the expression in body districts relevant in terms of drug pharmacokinetics (liver, gut, and kidney) and pharmacodynamics, focusing for the latter on the brain, since this is the target organ of psychopharmacological agents; (b) the patterns of expression during developmental phases; and (c) the expression of CYP450 enzymes during pathological processes such as cancer. We showed that CYP isoforms show distinct patterns of expression depending on the body district and the specific developmental phases. Of particular relevance for neuropsychopharmacology is the complex regulatory mechanisms that significantly modulate the complexity of the pharmacokinetic regulation, including the concentration of specific CYP isoforms in distinct areas of the brain, where they could greatly affect local substrate and metabolite concentrations of drugs.

PharmVar GeneFocus: CYP2C19

The Pharmacogene Variation Consortium (PharmVar) catalogues star (*) allele nomenclature for the polymorphic human CYP2C19 gene. CYP2C19 genetic variation impacts the metabolism of many drugs and has been associated with both efficacy and safety issues for several commonly prescribed medications. This GeneFocus provides a comprehensive overview and summary of CYP2C19 and describes how haplotype information catalogued by PharmVar is utilized by the Pharmacogenomics Knowledgebase and the Clinical Pharmacogenetics Implementation Consortium (CPIC).

P450 Pharmacogenetics in Indigenous North American Populations

Indigenous North American populations, including American Indian and Alaska Native peoples in the United States, the First Nations, Métis and Inuit peoples in Canada and Amerindians in Mexico, are historically under-represented in biomedical research, including genomic research on drug disposition and response. Without adequate representation in pharmacogenetic studies establishing genotype-phenotype relationships, Indigenous populations may not benefit fully from new innovations in precision medicine testing to tailor and improve the safety and efficacy of drug treatment, resulting in health care disparities. The purpose of this review is to summarize and evaluate what is currently known about cytochrome P450 genetic variation in Indigenous populations in North America and to highlight the importance of including these groups in future pharmacogenetic studies for implementation of personalized drug therapy.

The influence of CYP2C19 polymorphisms on exacerbating effect of rabeprazole in celecoxib-induced small bowel injury

BACKGROUND

Simultaneous use of proton pump inhibitors (PPIs) has been shown to increase the risk of nonsteroidal anti-inflammatory drug (NSAID)-induced small bowel injury.

AIM

To investigate whether polymorphisms of the cytochrome P450 2C19 gene (CYP2C19), encoding a key metabolising enzyme for PPIs, are associated with small bowel injury induced by celecoxib in combination with the PPI rabeprazole.

METHODS

Study participants included 55 healthy Japanese volunteers, who participated in the PPI-NSAID Kyushu University Study using video capsule endoscopy. For 2 weeks, 26 subjects were treated with celecoxib plus rabeprazole (rabeprazole group), and 29 subjects received celecoxib plus placebo (placebo group). All subjects were genotyped for CYP2C19 using real-time fluorescent polymerase chain reaction. Subjects were sub-classified as poor metabolizers or extensive metabolizers. The incidence and number of small bowel injuries were compared between poor metabolizers and extensive metabolizers in each group.

RESULTS

In the rabeprazole group, the incidence of small bowel injuries was significantly higher in poor metabolizers than in extensive metabolizers (85.7% vs 31.6%, P=.026). The number of mucosal injuries in the rabeprazole group was also significantly higher in poor metabolizers compared with extensive metabolizers (median [range] 3 [0-31] vs 0 [0-7], P=.01). In addition, we found a significant interaction between CYP2C19 genotype and concomitant use of rabeprazole in subjects at risk for celecoxib-induced small bowel injury.

CONCLUSIONS

The CYP2C19 genotype might be associated with the risk of small bowel injury when celecoxib is combined with rabeprazole.

Modulation of benzo[a]pyrene-DNA adduct formation by CYP1 inducer and inhibitor

Benzo[a]pyrene (BaP) is a well-studied pro-carcinogen that is metabolically activated by cytochrome P450 enzymes. Cytochrome P4501A1 (CYP1A1) has been considered to play a central role in the activation step, which is essential for the formation of DNA adducts. This enzyme is strongly induced by many different chemical agents, including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), which binds to the aryl hydrocarbon receptor (AhR). Therefore, AhR activators are suspected to have the potential to aggravate the toxicity of BaP through the induction of CYP1A1. Besides, CYP1A1 inhibitors, including its substrates, are estimated to have preventive effects against BaP toxicity. However, strangely, increased hepatic BaP–DNA adduct levels have been reported in Cyp1a1 knockout mice. Moreover, numerous reports describe that concomitant treatment of AhR activators reduced BaP–DNA adduct formation. In an experiment using several human cell lines, TCDD had diverse modulatory effects on BaP–DNA adducts, both enhancing and inhibiting their formation. In this review, we focus on the factors that could influence the BaP–DNA adduct formation. To interpret these complicated outcomes, we propose a hypothesis that CYP1A1 is a key enzyme for both generation and reduction of (±)-anti-benzo[a]pyrene-7,8-diol-9,10-epoxide (BPDE), the major carcinogenic intermediate of BaP. Conversely, CYP1B1 is thought to contribute only to the metabolic activation of BaP related to carcinogenesis.

Evaluation of the synergistic effect of Allium sativum, Eugenia jambolana, Momordica charantia, Ocimum sanctum, and Psidium guajava on hepatic and intestinal drug metabolizing enzymes in rats

Aims/Background:

This study was to investigated the synergistic effect of polyherbal formulations (PHF) of Allium sativum L., Eugenia jambolana Lam., Momordica charantia L., Ocimum sanctum Linn., and Psidium guajava L. in the inhibition/induction of hepatic and intestinal cytochrome P450 (CYPs) and Phase-II conjugated drug metabolizing enzymes (DMEs). Consumption of these herbal remedy has been extensively documented for diabetes treatment in Ayurveda.

Methodology:

PHF of these five herbs was prepared, and different doses were orally administered to Sprague–Dawley rats of different groups except control group. Expression of mRNA and activity of DMEs were examined by real-time polymerase chain reaction and high performance liquid chromatography in isolated liver and intestine microsomes in PHF pretreated rats.

Results:

The activities of hepatic and intestinal Phase-II enzyme levels increased along with mRNA levels except CYP3A mRNA level. PHF administration increases the activity of hepatic and intestinal UDP-glucuronyltransferase and glutathione S-transferase in response to dose and time; however, the activity of hepatic sulfotransferase increased at higher doses.

Conclusions:

CYPs and Phase-II conjugated enzymes levels can be modulated in dose and time dependent manner. Observations suggest that polyherbal formulation might be a possible cause of herb-drug interaction, due to changes in pharmacokinetic of crucial CYPs and Phase-II substrate drug.

The Use of Long-term Hepatocyte Cultures for Detecting Induction of Drug Metabolising Enzymes: The Current Status

In this report, metabolically competent in vitro systems have been reviewed, in the context of drug metabolising enzyme induction. Based on the experience of the scientists involved, a thorough survey of the literature on metabolically competent long-term culture models was performed. Following this, a prevalidation proposal for the use of the collagen gel sandwich hepatocyte culture system for drug metabolising enzyme induction was designed, focusing on the induction of the cytochrome P450 enzymes as the principal enzymes of interest. The ultimate goal of this prevalidation proposal is to provide industry and academia with a metabolically competent in vitro alternative for long-term studies. In an initial phase, the prevalidation study will be limited to the investigation of induction. However, proposals for other long-term applications of these systems should be forwarded to the European Centre for the Validation of Alternative Methods for consideration. The prevalidation proposal deals with several issues, including: a) species; b) practical prevalidation methodology; c) enzyme inducers; and d) advantages of working with independent expert laboratories. Since it is preferable to include other alternative tests for drug metabolising enzyme induction, when such tests arise, it is recommended that they meet the same level of development as for the collagen gel sandwich long-term hepatocyte system. Those tests which do so should begin the prevalidation and validation process.

Metabolic activation of clopidogrel: in vitro data provide conflicting evidence for the contributions of CYP2C19 and PON1

The recent report that clopidogrel efficacy may be more dependent on paraoxonase-1 (PON1) than on cytochrome P450 2C19 (CYP2C19) activity raises questions about the roles of these and other enzymes in clopidogrel activation. To provide insight into the emerging PON1 versus CYP2C19 debate, this commentary summarizes the clinical evidence on the pharmacokinetic determinants of clopidogrel efficacy. We then review the in vitro studies investigating the enzymes involved in clopidogrel activation, and comment on their strengths and limitations. There is agreement amongst in vitro studies regarding the involvement of CYP1A2 and CYP2B6 in the metabolism of clopidogrel to 2-oxo-clopidogrel. However, the evidence for other CYP enzymes in the first activation step (e.g. CYP2C19 and CYP3A4) is inconsistent and dependent on the in vitro test system and laboratory. All major drug metabolizing CYP enzymes are capable of converting 2-oxo-clopidogrel to sulfenic acid intermediates that subsequently form the active thiol metabolite. However, the extent of CYP involvement in this second step has been challenged, and new evidence suggests that CYP-independent hydrolytic cleavage of the thioester bond may be more important than oxidative metabolism.

Gastrointestinal and hepatotoxicity assessment of an anticancer extract from muricid molluscs

Marine molluscs from the family Muricidae are under development as a potential medicinal food for the prevention of colon cancer and treatment of gynaecological cancers. Here we report the outcome of the first in vivo toxicity assessment on an anticancer extract from a muricid mollusc containing brominated indole derivatives. Mice received the concentrated lipophilic extract by daily oral gavage over a two-week period. Mortality or clinical toxicity symptoms resulting from the extract were not detected during the trial, and there was no difference in the body weight of treated and control mice at the end of the trial. Histological analysis revealed some evidence for mild, idiosyncratic effects on the gastrointestinal tract and liver, including necrosis, fatty change, and inflammation in a small proportion (<40%) of mice. This is likely to result from first-pass hepatic metabolism of tyrindoxyl sulphate combined with second-pass metabolism of indoles. Overall however, oral administration of muricid extract containing brominated indoles does not result in severe clinical toxicity.

Heterologous expression of human cytochrome P450 (CYP) 2C19 in Escherichia coli and establishment of RP-HPLC method to serve as activity marker

In this study, a simple and reliable reverse-phase high-performance liquid chromatography (RP-HPLC) method was established and validated to analyze S-mephenytoin 4-hydroxylase activity of a recombinant CYP2C19 system. This system was obtained by co-expressing CYP2C19 and NADPH-CYP oxidoreductase (OxR) proteins in Escherichia coli (E. coli) cells. In addition to RP-HPLC, the expressed proteins were evaluated by immunoblotting and reduced CO difference spectral scanning. The RP-HPLC assay showed good linearity (r(2) = 1.00) with 4-hydroxymephenytoin concentration from 0.100 to 50.0 μm and the limit of detection was 5.00 × 10(-2) μm. Intraday and interday precisions determined were from 1.90 to 8.19% and from 2.20 to 14.9%, respectively. Recovery and accuracy of the assay were from 83.5 to 85.8% and from 95.0 to 105%. Enzyme kinetic parameters (Km , Vmax and Ki ) were comparable to reported values. The presence of CYP2C19 in bacterial membranes was confirmed by immunoblotting and the characteristic absorbance peak at 450 nm was determined in the reduced CO difference spectral assay. Moreover, the activity level of co-expressed OxR was found to be comparable to that of the literature. As a conclusion, the procedures described here have generated catalytically active CYP2C19 and the RP-HPLC assay developed is able to serve as CYP2C19 activity marker for pharmacokinetic drug interaction study in vitro.

Cytochrome P450 enzymes in drug metabolism: regulation of gene expression, enzyme activities, and impact of genetic variation

Cytochromes P450 (CYP) are a major source of variability in drug pharmacokinetics and response. Of 57 putatively functional human CYPs only about a dozen enzymes, belonging to the CYP1, 2, and 3 families, are responsible for the biotransformation of most foreign substances including 70-80% of all drugs in clinical use. The highest expressed forms in liver are CYPs 3A4, 2C9, 2C8, 2E1, and 1A2, while 2A6, 2D6, 2B6, 2C19 and 3A5 are less abundant and CYPs 2J2, 1A1, and 1B1 are mainly expressed extrahepatically. Expression of each CYP is influenced by a unique combination of mechanisms and factors including genetic polymorphisms, induction by xenobiotics, regulation by cytokines, hormones and during disease states, as well as sex, age, and others. Multiallelic genetic polymorphisms, which strongly depend on ethnicity, play a major role for the function of CYPs 2D6, 2C19, 2C9, 2B6, 3A5 and 2A6, and lead to distinct pharmacogenetic phenotypes termed as poor, intermediate, extensive, and ultrarapid metabolizers. For these CYPs, the evidence for clinical significance regarding adverse drug reactions (ADRs), drug efficacy and dose requirement is rapidly growing. Polymorphisms in CYPs 1A1, 1A2, 2C8, 2E1, 2J2, and 3A4 are generally less predictive, but new data on CYP3A4 show that predictive variants exist and that additional variants in regulatory genes or in NADPH:cytochrome P450 oxidoreductase (POR) can have an influence. Here we review the recent progress on drug metabolism activity profiles, interindividual variability and regulation of expression, and the functional and clinical impact of genetic variation in drug metabolizing P450s.

Simultaneous determination of multiple CYP inhibition constants using a cocktail-probe approach

To identify cytochrome P450 (CYP) drug-drug interaction (DDI) potential of a new chemical entity, the use of a specific clinically relevant probe substrate in the presence of a test compound is common place. In early discovery of new chemical entities, a balance of rigor, the ability to predict clinical DDI, and throughput is desired in an in vitro assay. This chapter describes a high-throughput CYP-mediated DDI assay method that balances these characteristics. The method utilizes a cassette approach using a cocktail of five selective probe substrates for the major clinically relevant CYPs involved in drug interactions. CYP1A2, 2C9, 2C19, 2D6, and 3A activities are assessed with liquid chromatography/tandem mass spectrometry (LC-MS/MS) quantification of metabolite formation. The method also outlines specific inhibitors to evaluate dynamic range and as a positive control. The benefits and needs for caution of this method are noted and discussed.

Inhibitory effects of cytochrome P450 enzymes CYP2C8, CYP2C9, CYP2C19 and CYP3A4 by Labisia pumila extracts

ETHNOPHARMACOLOGICAL RELEVANCE

Labisa pumila (LP), popularly known with its local name, Kacip Fatimah, is a well known herb grown in Indochina and Southeast Asia and is traditionally used to regain energy after giving birth in women. The propensity of LP to cause drug-herb interaction via cytochrome P450 (CYP) enzyme system has not been investigated.

AIM OF THE STUDY

To evaluate the in vitro inhibitory effects of various LP extracts (aqueous, ethanol, dichloromethane (DCM) and hexane) on cytochrome P450 2C8 (CYP2C8), CYP2C9, CYP2C19 and CYP3A4 activities.

MATERIALS AND METHODS

Probe substrate-based high performance liquid chromatography (HPLC) methods were established for CYP2C9, CYP2C19 and CYP3A4 whereas a fluorescence-based enzyme assay was established for CYP2C8. The metabolite formations were examined after incubation of probe substrate with respective CYP isoform in the present or absent of LP extracts. The inhibitory effect of LP was characterized with kinetic parameters IC(50) and K(i) values.

RESULTS

LP extracts showed differential effect of CYP activities with the order of inhibitory potency as follows: dichloromethane>hexane>ethanol>aqueous. This differential effect was only observed in CYP2C isoforms but not CYP3A4. Both the hexane and DCM extracts exhibited moderate to potent inhibition towards CYP2C activities in different modes including non-competitive, competive and mixed-type. The DCM effect was notably strong for CYP2C8 and CYP2C9 showing K(i) values of below 1 μg/ml. The selectivity of LP for CYP2C isoforms rather than CYP3A4 may be attributed to the presence of relatively small, lipophilic yet slightly polar compounds within the LP extracts.

CONCLUSIONS

The results of our study revealed that phytoconstituents contained in LP, particularly in hexane and dichloromethane extracts, were able to selectively inhibit CYP2C isoforms. The inactivation was characterized by low K(i) values, in particular, in CYP2C8 and CYP2C9. These in vitro data indicate that LB preparations contain constituents that can potently inhibit CYP2C activities and suggest that this herb should be examined for potential pharmacokinetic drug interactions in vivo.

Metabolism of R- and S-warfarin by CYP2C19 into four hydroxywarfarins

Coumadin (R/S-warfarin) is a highly efficacious and widely used anticoagulant; however, its highly variable metabolism remains an important contributor to uncertainties in therapeutic responses. Pharmacogenetic studies report conflicting findings on the clinical relevance of CYP2C19. A resolution to this controversy is impeded by a lack of de tailon the potential role of CYP2C19 in warfarin metabolism. Consequently, we assessed the efficiency of CYP2C19 metabolism of R- and S-warfarin and explored possible contributions in the liver using in vitro methods. Recombinant CYP2C19 metabolized R- and S-warfarin mainly to 6-, 7-, and 8-hydroxywarfarin, while 4'-hydroxywarfarin was a minormetabolite. Over all R-warfarin metabolism was slightly more efficient than that for S-warfarin. Metabolic pathways thatproduce R-6-, 7-, and 8-hydroxywarfarin in human liver microsomal reactions correlated strongly with CYP2C19 Smephenytoinhydroxylase activity. Similarly, CYP1A2 activity toward phenacetin correlated with formation of R-6 and 7-hydroxywarfarin such that R-8-hydroxywarfarin seems unique to CYP2C19 and possibly a biomarker. In following, CYP2C19 likely impacts R-warfarin metabolism and patient response to therapy. Intriguingly, CYP2C19 may contributeto S-warfarin metabolism in patients, especially when CYP2C9 activity is compromised due to drug interactions orgenetic polymorphisms.

In vitro modulatory effects of Andrographis paniculata, Centella asiatica and Orthosiphon stamineus on cytochrome P450 2C19 (CYP2C19)

ETHNO PHARMACOLOGICAL RELEVANCE

Andrographis paniculata (AP), Centella asiatica (CA) and Orthosiphon stamineus (OS) are three popular herbs traditionally used worldwide. AP is known for the treatment of infections and diabetes and CA is good for wound healing and healthy skin while OS is usually consumed as tea to treat kidney and urinary disorders. Interaction of these herbs with human cytochrome P450 2C19 (CYP2C19), a major hepatic CYP isoform involved in metabolism of many clinical drugs has not been investigated to date.

AIM OF THE STUDY

In this study, the modulatory effects of various extracts and major active constituents of AP, CA and OS on CYP2C19 activities were evaluated.

MATERIALS AND METHODS

S-mephenytoin, the CYP2C19 substrate probe, was incubated in the presence or absence of AP, CA and OS components. The changes in the rate of metabolite (hydroxymephenytoin) formation were subsequently determined by a high-performance liquid chromatography (HPLC)-based enzyme assay to characterize the modulatory effects.

RESULTS

Among the herbal extracts studied, AP ethanol extract and CA dichloromethane extract exhibited mixed type inhibition towards CYP2C19 with K(i) values of 67.1 and 16.4 μg/ml respectively; CA ethanol extract and OS petroleum ether extract competitively inhibited CYP2C19 activity (K(i)=39.6 and 41.5 μg/ml respectively). Eupatorin (a major active constituent of OS) was found to significantly inhibit CYP2C19 by mixed type inhibition (K(i)=7.1 μg/ml or 20.6 μM).

CONCLUSIONS

It was observed that AP, CA and OS inhibited CYP2C19 activity with varying potency. While weak inhibitory effect was observed with AP, moderate to strong inhibition was observed with CA dichloromethane extract and eupatorin, the major OS constituent. Therefore care should be taken when these CA and OS components are co-administered with CYP2C19 substrates (such as omeprazole, proguanil, barbiturates, citalopram, and diazepam).

CYP2C19 genotypes in the pharmacokinetics/pharmacodynamics of proton pump inhibitor-based therapy of Helicobacter pylori infection

IMPORTANCE OF THE FIELD

Proton pump inhibitors (PPIs) are potent gastric acid inhibitors. Therapies with a PPI and antibiotics are used to cure Helicobacter pylori (H. pylori) infection, which is closely related to many gastrointestinal diseases. Most PPIs are mainly metabolized by cytochrome 2C19 (CYP2C19). The genetic polymorphisms of CYP2C19 may lead to the differences in pharmacokinetics (PK), pharmacodynamics (PD) and clinical efficacy of PPIs.

AREAS COVERED IN THIS REVIEW

The roles of PPIs on the eradication of H. pylori are summarized. The impact f CYP2C19 polymorphism on the PK and PD of PPIs is addressed and related to the present status of therapy for H. pylori infection. The opinions on the strategy of PPIs-based therapies of H. pylori infection are provided.

WHAT THE READER WILL GAIN

Update the factors that may influence the PPIs-based therapies of H. pylori infection.

TAKE HOME MESSAGE

The eradication rates of H. pylori infection are significantly different between patients who are CYP2C19 extensive metabolizers and poor metabolizers, partly because of the differences in the PK and PD of PPIs. Nonetheless, the differences can be improved by adjusting the regimens of PPIs and using antibiotics that have less H. pylori-resistance.

Serum concentrations of antidepressant drugs in a naturalistic setting: compilation based on a large therapeutic drug monitoring database

A compilation of therapeutic drug monitoring data for 15 antidepressant drugs in a naturalistic routine clinical setting is presented. A substantial number of serum concentrations, at different daily doses, are outlined, and the intraindividual and overall serum concentration coefficient of variation for a respective substance is presented. Also, concentration comparisons between women and men, and patients older or younger than 65 years are made. The drugs included are amitriptyline (n = 394), citalopram (n = 5457), clomipramine (n = 400), escitalopram (n = 3066), fluoxetine (n = 793), fluvoxamine (n = 165), mianserin (n = 1063), mirtazapine (n = 1427), moclobemide (n = 200), nortriptyline (n = 206), paroxetine (n = 1677), reboxetine (n = 85), sertraline (n = 2998), trimipramine (n = 158), and venlafaxine (n = 1781). Of the 9 drugs exhibiting linear (first order) kinetics, all but reboxetine gave a significant negative dose-to-dose-normalized correlation with concentrations, that is an increased clearance with higher dose. When dose was correlated to the metabolite:parent substance ratio for drugs exhibiting linear kinetics, citalopram and mianserin gave a positive slope, contrary to a negative slope shown for sertraline and venlafaxine. The intraindividual variations of the serum concentrations were lower than the overall variations, and the intraindividual variation of the metabolite:parent substance ratio was lower than the intraindividual variation of respective parent substance (except clomipramine and mianserin). Women had significantly higher serum concentrations than men (significant for citalopram, escitalopram, mianserin, mirtazapine, and venlafaxine), and patients older than 65 years had higher serum concentrations than the younger ones for all drugs except amitriptyline, moclobemide, and trimipramine. By presenting a comprehensive compilation of therapeutic drug monitoring data for each drug, a reference tool is created, in addition to improved pharmacokinetic knowledge of antidepressant drugs.

Rapid identification of the hepatic cytochrome P450 2C19 activity using a novel and noninvasive [13C]pantoprazole breath test

We tested the hypothesis that the stable isotope [(13)C]pantoprazole is O-demethylated by cytochrome P450 CYP2C19 and that the (13)CO(2) produced and exhaled in breath as a result can serve as a safe, rapid, and noninvasive phenotyping marker of CYP2C19 activity in vivo. Healthy volunteers who had been genotyped for the CYP2C19(*)2, CYP2C19(*)3, and CYP2C19(*)17 alleles were administered a single oral dose of [(13)C]pantoprazole sodium-sesquihydrate (100 mg) with 2.1 g of sodium bicarbonate. Exhaled (13)CO(2) and (12)CO(2) were measured by IR spectroscopy before (baseline) and 2.5 to 120 min after dosing. Ratios of (13)CO(2)/(12)CO(2) after [(13)C]pantoprazole relative to (13)CO(2)/(12)CO(2) at baseline were expressed as change over baseline (DOB). Maximal DOB, DOB(15) to DOB(120), and area under the DOB versus time curve (AUC(0-120) and AUC(0-infinity)) were significantly different among three genotype groups (CYP2C19(*)1/(*)1, n = 10; CYP2C19(*)1/(*)2 or CYP2C19(*)1/(*)3, n = 10; and CYP2C19(*)2/(*)2, n = 5) with predicted extensive metabolizers (EMs), intermediate metabolizers (IMs), and poor metabolizers (PMs) of CYP2C19, respectively (Kruskal-Wallis test, p < 0.01); linear regression analysis indicated a gene-dose effect relationship (r(2) ranged between 0.236 and 0.522; all p < 0.05). These breath test indices were significantly lower in PMs than IMs (p < 0.05) or EMs (p < 0.01) of CYP2C19. [(13)C]Pantoprazole plasma exposure showed significant inverse correlation with breath test indices in the respective subjects (Pearson r = -0.74; p = 0.038). These feasibility data suggest that the [(13)C]pantoprazole breath test is a reliable, rapid, and noninvasive probe of CYP2C19 and seems to be a useful tool to optimize drug therapy metabolized by CYP2C19.

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.

Role of cytochrome P450 in drug interactions

Drug-drug interactions have become an important issue in health care. It is now realized that many drug-drug interactions can be explained by alterations in the metabolic enzymes that are present in the liver and other extra-hepatic tissues. Many of the major pharmacokinetic interactions between drugs are due to hepatic cytochrome P450 (P450 or CYP) enzymes being affected by previous administration of other drugs. After coadministration, some drugs act as potent enzyme inducers, whereas others are inhibitors. However, reports of enzyme inhibition are very much more common. Understanding these mechanisms of enzyme inhibition or induction is extremely important in order to give appropriate multiple-drug therapies. In future, it may help to identify individuals at greatest risk of drug interactions and adverse events.

Comparative 1-Substituted Imidazole Inhibition of Cytochrome P450 Isozyme-Selective Activities in Human and Mouse Hepatic Microsomes

Inhibition of cytochrome P450(CYP)-selective reactions in a single human and a single mouse hepatic microsome preparation by fourteen 1-substituted imidazoles provides a simultaneous ranking of reaction susceptibility to a specific imidazole and the relative inhibitory potency of the imidazoles for a given reaction. CYP3A4/5 activity was inhibited (IC(50) <5 microM) by the greatest number of imidazoles, followed closely by CYP2C9. Seven imidazoles exhibited IC(50) values for CYP3A4/5 <0.3 microM (none for CYP2C9) and were exclusively above 300 MW. Nafimidone (MW, 236) exhibited an IC(50) value <0.3 microM towards CYP2D6 and CYP1A2 reactions. CYP2E1 and CYP2A6 were exclusively inhibited (IC(50) <5 microM) by imidazoles with MWs below approximately 200. In general, mouse activities exhibited lower IC(50) values than in human microsomes.

Role of human cytochrome P450 3A4 in the metabolism of DA–8159, a new erectogenic

The purpose of this paper was to characterize cytochrome P450 (CYP) enzymes involved in N-dealkylation of a new oral erectogenic, DA-8159 to DA-8164, a major circulating active metabolite, in human liver microsomes and to investigate the inhibitory potential of DA-8159 on CYP enzymes. CYP3A4 was identified as the major enzyme responsible for DA-8159 N-dealkylation to DA-8164 based on correlation analysis and specific CYP inhibitor and antibody-mediated inhibition study in human liver microsomes, and DA-8159 metabolism in cDNA expressed CYP enzymes. There is the possibility of drug-drug interactions when prescribing DA-8159 concomitantly with known inhibitors or inducers of CYP3A4. DA-8159 was found to be only a very weak inhibitor of eight major CYPs (1A2, 2A6, 2C8, 2C9, 2C19, 2D6, 2E1 and 3A4), the largest inhibition occurring against CYP2D6 (IC5o 67.7 microM) in human liver microsomes. Drug-drug interactions would not be predicted on the basis of DA-8159 inhibiting the metabolism of coadministered drugs.

Identification of cytochrome P450 forms involved in the 4-hydroxylation of valsartan, a potent and specific angiotensin II receptor antagonist, in human liver microsomes

Valsartan is known to be excreted largely as unchanged compound and is minimally metabolized in man. Although the only notable metabolite is 4-hydroxyvaleryl metabolite (4-OH valsartan), the responsible enzyme has not been clarified at present. The current in vitro studies were conducted to identify the cytochrome P450 (CYP) enzymes involved in the formation of 4-OH valsartan. Valsartan was metabolized to 4-OH valsartan by human liver microsomes and the Eadie-Hofstee plots were linear. The apparent Km and Vmax values for the formation of 4-OH valsartan were 41.9-55.8 microM and 27.2-216.9 pmol min(-1) mg(-1) protein, respectively. There was good correlation between the formation rates of 4-OH valsartan and diclofenac 4'-hydroxylase activities (representative CYP2C9 activity) of 11 individual microsomes (r = 0.889). No good correlation was observed between any of the other CYP enzyme marker activities (CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4 and CYP4A). Among the recombinant CYP enzymes examined (CYPs 1A2, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6, 2E1, 3A4, 3A5 and 4A11), CYP2C9 notably catalysed 4-hydroxylation of valsartan. For the specific CYP inhibitors or substrates examined (furafylline, diclofenac, S(+)-mephenytoin, quinidine and troleandomycin), only diclofenac inhibited the formation of 4-OH valsartan. These results showed that CYP2C9 is the only form responsible for 4-hydroxylation of valsartan in human liver microsomes. Although CYP2C9 is involved in valsartan metabolism, CYP-mediated drug-drug interaction between valsartan and other co-administered drugs would be negligible.

The development of drug metabolism research as expressed in the publications of ASPET: Part 3, 1984-2008

The dramatic changes in drug metabolism research in the last 25 years are well documented in the publications of the American Society for Pharmacology and Experimental Therapeutics (ASPET). New analytical tools combined with modern molecular biological techniques have provided unprecedented access to the workings of the cell. A field that concentrated on only a handful of primary enzymes now has a list of hundreds in its purview. Genetic variation, environmental impact, and molecular diversity have all come under study in attempts to follow the fate of drugs and chemicals. Examples from ASPET journals will be used to illustrate the dramatic advancements in the field.

Development of an in vitro drug-drug interaction assay to simultaneously monitor five cytochrome P450 isoforms and performance assessment using drug library compounds

INTRODUCTION

Inhibition of cytochrome P450 (CYP) is a principal mechanism for metabolism-based drug-drug interactions (DDIs). This article describes a robust, high-throughput CYP-mediated DDI assay using a cocktail of 5 clinically relevant probe substrates with quantification by liquid chromatography/tandem mass spectrometry (LC/MS-MS).

METHODS

The assay consisted of human liver microsomes and a cocktail of probe substrates metabolized by the five major CYP isoforms (tacrine for CYP1A2, diclofenac for CYP2C9, (S)-mephenytoin for CYP2C19, dextromethorphan for CYP2D6 and midazolam for CYP3A4). The assay was fully automated in both 96- and 384-well formats.

RESULTS

A series of experiments were conducted to define the optimal kinetic parameters and solvent concentrations, as well as, to assess potential reactant and product interference. The assay was validated against known CYP inhibitors (miconazole, sulfaphenazole, ticlopidine, quinidine, ketoconazole, itraconazole, fluoxetine) and evaluated in a screening environment by testing 9494 compounds.

DISCUSSION

Our findings show that this assay has application in early stage drug discovery to economically, reliably and accurately assess compounds for DDIs.

PhRMA white paper on ADME pharmacogenomics

Pharmacogenomic (PGx) research on the absorption, distribution, metabolism, and excretion (ADME) properties of drugs has begun to have impact for both drug development and utilization. To provide a cross-industry perspective on the utility of ADME PGx, the Pharmaceutical Research and Manufacturers of America (PhRMA) conducted a survey of major pharmaceutical companies on their PGx practices and applications during 2003-2005. This white paper summarizes and interprets the results of the survey, highlights the contributions and applications of PGx by industrial scientists as reflected by original research publications, and discusses changes in drug labels that improve drug utilization by inclusion of PGx information. In addition, the paper includes a brief review on the clinically relevant genetic variants of drug-metabolizing enzymes and transporters most relevant to the pharmaceutical industry.

Limited frequency of the CYP2C19*17 allele and its minor role in a Japanese population

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

A novel CYP2C19 gene variant, CYP2C19*17, is associated with increased metabolic activity. Ethnic differences in the frequency of the variant allele have been reported. However, the frequency of the CYP2C19*17 allele has not been studied in the Japanese population.

WHAT THIS STUDY ADDS

In a population of 265 healthy Japanese subjects, a low frequency (1.3%) of the CYP2C19*17 allele was observed. The limited frequency of the *17 allele and the absence of a subject homozygous for *17 indicated that CYP2C19*17 would play a minor role in a Japanese population.

AIMS

We investigated the CYP2C19*17 allelic frequency in Japanese subjects, and evaluated whether CYP2C19*17 is an important determinant of interindividual variability of CYP2C19 activity.

METHODS

We enrolled 265 subjects to determine their CYP2C19 genotype and plasma metabolic ratio following a single dose of 40 mg omeprazole.

RESULTS

Seven subjects heterozygous for CYP2C19*17 and no *17/*17 subjects resulted in the CYP2C19*17 frequency being 1.3%. These heterozygotes had moderate metabolic activities when compared with the metabolic ratio of the other subjects.

CONCLUSIONS

The low frequency of CYP2C19*17 and the absence of *17/*17 indicates that CYP2C19*17 plays a minor role in the Japanese population.

Review article: The prevalence and clinical relevance of cytochrome P450 polymorphisms

BACKGROUND

Most drugs currently used in clinical practice are effective in only 25% to 60% of patients, while adverse drug reactions (ADRs) as a consequence of treatment are estimated to cost billions of US dollars and tens of thousands of deaths.

AIM

To review the prevalence and clinical significance of cytochrome P450 polymorphisms.

RESULTS

The cytochrome P450 enzyme families 1-3 are responsible for 70 to 80% of all phase I dependent drug metabolisms. In 90% metabolic activity dependents on six enzymes: CYP1A2, CYP3A, CYP2C9, CYP2C19, CYP2D6 and CYP2E1. Polymorphisms in the CYP450 gene can influence metabolic activity of the subsequent enzymes. A poor metabolizer (PM) has no or very poor enzyme activity. A consequence of PM is drug toxicity if no other metabolic route is available, or when multiple drugs are metabolized by the same cytochrome. In that case dose reduction is an option to prevent toxic effects.

CONCLUSIONS

In the future genotyping should be considered to identify patients who might be at risk of severe toxic responses, in order to guide appropriate individual dosage. Medical therapy should be a close cooperation between clinicians, pharmacologists and laboratory specialists, leading to reduced therapeutic errors, ADRs and health care costs.

Dynamic expression of epoxyeicosatrienoic acid synthesizing and metabolizing enzymes in the primate corpus luteum

Epoxyeicosatrienoic acids (EpETrEs), produced from arachidonic acid via cytochrome P450 (CYP) epoxygenases, regulate inflammation, angiogenesis, cellular proliferation, ion transport and steroidogenesis. EpETrE actions are regulated through their metabolism to diols (dihydroxyeicosatrienoic acids; DiHETrE) via the enzyme soluble epoxide hydrolase (EPHX2). We set out to determine, therefore, whether EpETrE generating (epoxygenases CYP2C8, 2C9, 2C19, 2J2, 1A2 and 3A4) and metabolizing (EPHX2) enzymes are expressed in the primate corpus luteum (CL). CL were isolated from rhesus macaques during the early (day 3-5 post-LH surge), mid (day 6-8), mid-late (day 10-12), late (day 14-16) and very-late (day 17-19: menses) luteal phase of natural menstrual cycles. EPHX2 mRNA levels peaked in mid-late CL (5-fold when compared with early CL, P<0.05) and remained elevated in the late CL. Ablation of pituitary LH secretion and luteal steroid synthesis significantly reduced (P<0.05) EPHX2 mRNA levels in the mid-late CL, with progestin replacement being insufficient to restore its level of expression to control values. EPHX2 protein was localized to large and small luteal cells, as well as vascular endothelial cells. The EpETrE-generating CYP epoxygenase 2J2, 2C9 and 3A4 genes were also expressed in the macaque CL. While CYP2J2 mRNA levels did not significantly change through the luteal phase, CYP2C9 and CYP3A4 levels were significantly (P<0.05) higher in the mid-late phase when compared with the early phase. CYP2C9, 2J2 and 3A4 proteins were each localized to the large luteal cells, with 2C9 and 2J2 also being present in the small luteal, stromal and endothelial cells. These studies demonstrate for the first time that an EpETrE generating and metabolizing system exists in the primate CL, with the latter being regulated by LH and steroid hormone(s).

Breast cancer treatment and ovarian failure: risk factors and emerging genetic determinants

Most premenopausal women diagnosed with primary breast cancer receive adjuvant chemotherapy, and many experience chemotherapy-induced ovarian failure (CIOF). CIOF is associated with menopausal symptoms, fertility concerns and long-term implications including bone loss. Ironically, CIOF might confer a disease-specific benefit to women whose breast cancers express hormone receptors. Risk factors of CIOF include the woman's age at the time of therapy, and the type, dose and schedule of chemotherapy. Because inherited genetic factors have an important role in determining who will experience CIOF, genetic testing has the potential to provide optimal counselling about risks and possible interventions.

An evaluation of the cytochrome p450 inhibition potential of lisdexamfetamine in human liver microsomes

The human cytochrome P450 (P450) system is implicated in many drug interactions. Lisdexamfetamine dimesylate (NRP104), the proposed generic name for a new agent under investigation for treatment of attention deficit hyperactivity disorder, was recently analyzed for inhibitory drug-drug interactions with seven major P450 isoforms using pooled human liver microsomes. Probe substrates were used near the K(m) concentration values reported in the literature for CYP1A2 (phenacetin), CYP2A6 (coumarin), CYP2B6 (bupropion), CYP2C9 (tolbutamide), CYP2C19 ([S]-mephenytoin), CYP2D6 (dextromethorphan), and CYP3A4 (midazolam and testosterone), and lisdexamfetamine was evaluated at concentrations ranging from 0.01 to 100 muM for its ability to inhibit the activity of these seven P450 isoforms. NADPH was added to one set of samples to initiate metabolic reactions, which were then terminated by adding organic solvent, vortexing the samples, and placing them on ice. The relevant substrates were then introduced to both sets of samples so that the percentage of remaining activity could be measured and compared. In addition, these samples were compared with other samples with the same concentrations of lisdexamfetamine but without preincubation. None of the seven P450 isoforms showed any concentration-dependent inhibition. Comparison of results from microsomes preincubated with and without NADPH showed no mechanism-based inhibition. Neither concentration-dependent nor mechanism-based inhibition caused by time-dependent inactivation of human P450 isoforms was shown for lisdexamfetamine during in vitro testing. The evidence suggests that lisdexamfetamine has a low potential for drug-drug interactions or initiation of drug-drug interactions.

Cynomolgus Monkey Cytochrome P450 2C43: cDNA Cloning, Heterologous Expression, Purification and Characterization

The cDNA of cytochrome P450 (CYP) 2C43 was cloned from cynomolgus monkey liver by RT-PCR. The deduced amino acid sequence showed 93% and 91% identity to human CYP2C9 and CYP2C19, respectively. The cDNA was expressed in Escherichia coli and purified by a series of chromatography steps, yielding a specific content of 11.5 nmol P450/mg protein. The substrate specificity of the purified CYP2C43 was examined in a reconstitution system comprising NADPH-P450 reductase, lipid, cytochrome b(5) and CYP2C marker substrates. The purified CYP2C43 showed high activity for testosterone 17-oxidation and progesterone 21-hydroxylation, which were also observed for CYP2C19 but not CYP2C9. In addition, CYP2C43 showed activity for (S)-mephenytoin 4'-hydroxylation, a marker reaction for CYP2C19. With CYP2C9 marker substrates, CYP2C43 exhibited low activity for diclofenac 4'-hydroxylation and no activity for tolbutamide p-methylhydroxylation. Therefore, in terms of substrate specificity, our results indicate that CYP2C43 is similar to CYP2C19, rather than CYP2C9.

Combinatorial pharmacogenetics

Combinatorial pharmacogenetics seeks to characterize genetic variations that affect reactions to potentially toxic agents within the complex metabolic networks of the human body. Polymorphic drug-metabolizing enzymes are likely to represent some of the most common inheritable risk factors associated with common 'disease' phenotypes, such as adverse drug reactions. The relatively high concordance between polymorphisms in drug-metabolizing enzymes and clinical phenotypes indicates that research into this class of polymorphisms could benefit patients in the near future. Characterization of other genes affecting drug disposition (absorption, distribution, metabolism and elimination) will further enhance this process. As with most questions concerning biological systems, the complexity arises out of the combinatorial magnitude of all the possible interactions and pathways. The high-dimensionality of the resulting analysis problem will often overwhelm traditional analysis methods. Novel analysis techniques, such as multifactor dimensionality reduction, offer viable options for evaluating such data.

Novel mutations in the cytochrome P450 2C19 gene: a pitfall of the PCR-RFLP method for identifying a common mutation

CYP2C19 is a clinically important enzyme involved in the metabolism of therapeutic drugs such as (S)-mephenytoin, omeprazole, proguanil, and diazepam. Individuals can be characterized as either extensive metabolizers (EM) or poor metabolizers (PM) on the basis of CYP2C19 enzyme activity. The PM phenotype occurs in 2-5% of Caucasian populations, but at higher frequencies (18-23%) in Asians. CYP2C19*2 and CYP2C19*3, which are single-nucleotide polymorphisms of CYP2C19, are the main cause of PM phenotyping in homozygotes or compound heterozygotes. We report two novel mutations in the CYP2C19 gene identified by direct sequencing and subcloning procedures. One of these mutations was considered to be CYP2C19*3 by polymerase chain reaction restriction fragment length polymorphism (PCR-RFLP). This result suggests that mutations classed as CYP2C19*3 might include other mutations. Further studies are needed to clarify the relationship between these novel mutations and enzyme activity.

DEVELOPMENT OF NOVEL FURANOCOUMARIN DIMERS AS POTENT AND SELECTIVE INHIBITORS OF CYP3A4

Grapefruit juice has been found to cause an increase in the oral bioavailability of many therapeutic agents. Such interactions are believed to result from the mechanism-based inhibition of CYP3A4 activity in the intestine. Furanocoumarin dimers present in the juice have been found to be extremely potent inhibitors of CYP3A4 activity. The aim of this work was to synthesize and test a series of dimers with a view to defining the relationship between structure and inhibitory activity and establish whether they might make suitable probes of CYP3A4 activity. Eleven furanocoumarin dimers were synthesized and evaluated as inhibitors of CYP3A4 using human liver microsomes, with testosterone as the marker substrate. Four of the most potent dimers were also investigated for their effects on CYP3A4 activity in the human intestine and on five additional hepatic cytochrome P450 isoforms. The dimers showed potent dose-dependent inhibition of CYP3A4 activity in both liver and intestine; IC50 values ranged from 0.021 +/- 0.002 to 0.146 +/- 0.041 microM (mean +/- S.D. n = 3). Of the four dimers evaluated further, all showed time-dependent inhibition of CYP3A4 activity. 88Prop showed moderate inhibition of both CYP2C19 and CYP1A2 with IC50 values of 4.42 +/- 0.01 and 1.98 +/- 0.34 microM, 88Octa was found to inhibit CYP2C19 (IC50 = 3.16 +/- 0.01 microM) and 58Prop to inhibit CYP1A2 (IC50 = 2.39 +/- 0.77 microM). Minimal inhibition of CYP2D6 and CYP2C9 was observed (IC50 > 10 microM). In conclusion, all the dimers tested were extremely potent inhibitors of CYP3A4 activity. In particular, dimer 55EE was highly selective toward the enzyme, suggesting that this compound is a suitable probe for determining the contribution of CYP3A4 to drug metabolism.

Practical application of mammalian cytochrome P450

Heterologous expression systems play an important role in the analysis of structure-function relationships of mammalian P450s. In addition, these expression systems allow practical application of mammalian P450s. Genetically engineered fused enzymes between mammalian P450 and yeast NADPH-P450 reductase have possible applications in bioconversion processes. Combined use of techniques reported thus far could produce steroid hormones in the recombinant yeast cells harboring four P450 species, CYP11A1, CYP17A1, CYP21B1 and CYP11B1. In an Escherichia coli expression system, the technology of the construction of the mitochondrial P450 electron transport chain has been established. The recombinant E. coli cells expressing CYP27B1, adrenodoxin and NADPH-adrenodoxin reductase would be applicable to a bioconversion process to produce 1alpha,25-dihydroxyvitamin D3. We also demonstrated the usefulness of heterologous expression systems for human liver microsomal P450s for the prediction of drug metabolism in the human body. Microsomal fractions prepared from recombinant yeast, insect and mammalian cells are commercially available and play an important role in preclinical drug development. Application of mammalian P450 to bioremediation with genetic engineering has also been developed. Thus, mammalian P450s appear to have great potential for a wide range of practical applications.

The Metabolic Profile of Azimilide in Man: In Vivo and in Vitro Evaluations

The metabolic fate of azimilide in man is unusual as it undergoes a cleavage in vivo resulting in the formation of two classes of structurally distinct metabolites. During a metabolite profiling study conducted in human volunteers to assess the contribution of all pathways to the clearance of (14)C-azimilide, greater than 82% of radioactivity was recovered in urine (49%-58%) and feces (33%). Urine, feces, and plasma were profiled for metabolites. A cleaved metabolite, 4-chloro-2-phenyl furoic acid was present at high concentration in plasma (metabolite/parent AUC ratio approx. 4), while other plasma metabolites, azimilide N-oxide (metabolite/parent AUC ratio 0.001), and a cleaved hydantoin metabolite (metabolite/parent AUC ratio = 0.3) were present at lower concentrations than azimilide. In urine, the cleaved metabolites were the major metabolites, (> 35% of the dose) along with phenols (as conjugates, 7%-8%), azimilide N-oxide (4%-10%), a butanoic acid metabolite (2%-3%), and desmethyl azimilide (2%). A limited investigation of fecal metabolites indicated that azimilide (3%-5%), desmethyl azimilide (1%-3%), and the butanoic acid metabolite (< 1%) were present. Contributing pathways for metabolism of azimilide, identified through in vitro and in-vivo studies, were CYPs 1A1 (est. 28%), 3A4/5 (est. 20%), 2D6 (< 1%), FMO (est. 14%), and cleavage (35%). Enzyme(s) involved in the cleavage of azimilide were not identified.

The relationship between the pharmacology of antiepileptic drugs and human gene variation: an overview

Individual differences in clinical responsiveness to antiepileptic drugs are due to a complex interaction between environmental factors and genetic variation. Considerable interest has arisen in exploiting advances in molecular genetics to improve drug therapy for epilepsy and many other diseases; however, practical application of pharmacogenetics has been difficult to realize. Attempts to define gene variants that are associated with therapeutic (or adverse) effects of antiepileptic drugs rely currently on the prior identification of candidate genes and the subsequent evaluation of the distribution of allelic variants between individuals who have a "good" versus a "poor" clinical response. Many factors can adversely affect interpretation of such data, and careful consideration must be given to the design of genetic association studies involving candidate genes. Candidate genes may be identified in a number of ways; however, for studies of drugs, application of knowledge derived from basic pharmacology can suggest focused and testable hypotheses that are based on the fundamental principles of drug action. Thus, studies of genetic variation as they relate to proteins involved in antiepileptic drug kinetics and dynamics will identify key polymorphisms in endogenous molecules that determine degrees of drug efficacy and toxicity. Delineation of these effects in the coming years will promote enhanced success in the treatment of epilepsy.

CYP isoforms involved in the metabolism of clarithromycin in vitro: comparison between the identification from disappearance rate and that from formation rate of metabolites

To clarify whether CYP2C19 is involved in the overall metabolism of clarithromycin (CAM) or not, in vitro studies using human liver microsomes and recombinant CYPs were performed by an approach based on the disappearance rate of parent compound from the incubation mixture. In addition, the results of disappearance rate were compared with those obtained from the formation rates of the major metabolites of CAM, 14-(R)-hydroxy-CAM and N-demethyl-CAM. The intrinsic clearance (CL(int)) values determined from the disappearance of CAM in nine different human liver microsomes were highly correlated with the testosterone 6beta-hydroxylation activity (r=0.957, p<0.001). The CL(int) of CAM was markedly reduced by selective inhibitors of CYP3A4 (ketoconazole and troleandomycin) and by polyclonal antibodies raised against CYP3A4/5 in human liver microsomes. Among the 11 isoforms of recombinant human CYP, only CYP3A4 revealed the metabolic activity for the disappearance of CAM. These results were fairly consistent with those obtained from the conventional approach based on the formation of major metabolites of CAM. Comparison of the kinetic parameters estimated from the disappearance rate of CAM and the formation rates of 14-(R)-hydroxy-CAM and N-demethyl-CAM indicates that N-demethylation and 14-(R)-hydroxylation account for 65% of CL(int) derived from the disappearance of CAM in human liver microsomes. The findings suggest that CYP3A4 plays a predominant role in the overall metabolic clearance of CAM as well as in the formation of 14-(R)-hydroxy-CAM and N-demethyl-CAM. CYP2C19 does not appear to be involved in the overall metabolism of CAM at least in human liver microsomes. A combination of the disappearance rate of a parent compound and the formation rate of metabolites appears to be a useful approach for estimating the percentage contribution of the formation of metabolites to the overall metabolic clearance of a parent compound in vitro.