Phenotypic and genotypic investigations of a healthy volunteer deficient in the conversion of losartan to its active metabolite E-3174

Drug-induced liver injury in COVID-19 treatment: Incidence, mechanisms and clinical management

The COVID-19 outbreak triggered a serious and potentially lethal pandemic, resulting in massive health and economic losses worldwide. The most common clinical manifestations of COVID-19 patients are pneumonia and acute respiratory distress syndrome, with a variety of complications. Multiple organ failure and damage, ultimately leading to patient death, are possible as a result of medication combinations, and this is exemplified by DILI. We hope to summarize DILI caused by the antiviral drugs favipiravir, remdesivir, lopinavir/ritonavir, and hydroxychloroquine in COVID-19 patients in this review. The incidence of liver injury in the treatment of COVID-19 patients was searched on PubMed to investigate DILI cases. The cumulative prevalence of acute liver injury was 23.7% (16.1%-33.1%). We discuss the frequency of these events, potential mechanisms, and new insights into surveillance strategies. Furthermore, we also describe medication recommendations aimed at preserving DILI caused by treatment in COVID-19 patients.

Potential Effects of COVID-19 on Cytochrome P450-Mediated Drug Metabolism and Disposition in Infected Patients

Coronavirus Disease 2019 (COVID-19) has been a global health crisis since it was first identified in December 2019. In addition to fever, cough, headache, and shortness of breath, an intense increase in immune response-based inflammation has been the hallmark of Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV-2) virus infection. This narrative review summarizes and critiques pathophysiology of COVID-19 and its plausible effects on drug metabolism and disposition. The release of inflammatory cytokines (e.g., interleukins, tumor necrosis factor α), also known as ‘cytokine storm’, leads to altered molecular pathophysiology and eventually organ damage in the lung, heart, and liver. The laboratory values for various liver function tests (e.g., alanine aminotransferase, aspartate aminotransferase, total bilirubin, albumin) have indicated potential hepatocellular injury in COVID-19 patients. Since the liver is the powerhouse of protein synthesis and the primary site of cytochrome P450 (CYP)-mediated drug metabolism, even a minor change in the liver function status has the potential to affect the hepatic clearance of xenobiotics. It has now been well established that extreme increases in cytokine levels are common in COVID-19 patients, and previous studies with patients infected with non-SARS-CoV-2 virus have shown that CYP enzymes can be suppressed by an infection-related cytokine increase and inflammation. Alongside the investigational COVID-19 drugs, the patients may also be on therapeutics for comorbidities; especially epidemiological studies have indicated that individuals with hypertension, hyperglycemia, and obesity are more vulnerable to COVID-19 than the average population. This complicates the drug-disease interaction profile of the patients as both the investigational drugs (e.g., remdesivir, dexamethasone) and the agents for comorbidities can be affected by compromised CYP-mediated hepatic metabolism. Overall, it is imperative that healthcare professionals pay attention to the COVID-19 and CYP-driven drug metabolism interactions with the goal to adjust the dose or discontinue the affected drugs as appropriate.

Effect of CYP2C9 genetic polymorphism and breviscapine on losartan pharmacokinetics in healthy subjects

1. Breviscapine was an active ingredient of flavonoid glycosides. Our present study was conducted to evaluate the impact of breviscapine on the pharmacokinetics of losartan and its active metabolite E-3174, and that relationship with the gene polymorphism of CYP2C9 in healthy Chinese volunteers, to provide a basis for clinical rational drug use.2. The genotypes of 217 healthy Chinese subjects were determined using PCR-RFLP. Twelve healthy subjects were selected and were known CYP2C9 genotypes (six CYP2C9*1/*3 and six CYP2C9*1/*1) in a two-phase randomised crossover design study. These subjects were given daily doses of 120 mg (40 mg, three times a day) of breviscapine or a placebo for 14 days, followed by 50 mg losartan on day 15.3. Compared with individuals carrying the CYP2C9*1/*1 genotype, the CYP2C9*1/*3 genotype showed an increase in the AUC_(0-36) (833.6 ± 379.8 ng h ml^-1 vs. 526.1 ± 140.1 ng h ml^-1, p < 0.05) and a decrease in the MR (the metabolic ratio of losartan, AUC_E-3174/AUC_losartan) (2.67 ± 1.40 vs. 4.56 ± 0.83, p < 0.05) of losartan during the placebo treatment phase. Individuals with genotype CYP2C9*1/*3 showed a significant increase in AUC_(0-36) (2335 ± 851.8 ng h ml^-1 vs. 1927 ± 949.5 ng h ml^-1, p < 0.05) and AUC_(0-∞) (2363 ± 875.6 ng h ml^-1 vs. 1966 ± 966.1 ng h ml^-1, p < 0.05) of E-3174 after breviscapine treatment compared to the placebo group.4. In healthy subjects, breviscapine had no significant effect on the pharmacokinetics of losartan. The activity of CYP2C9 enzyme to losartan metabolism was more significant in subjects with CYP2C9*1/*3 than those with CYP2C9*1/*1 genotype.

The cytochrome P450 isoenzyme and some new opportunities for the prediction of negative drug interaction in vivo

Cytochrome (CYP) 450 isoenzymes are the basic enzymes involved in Phase I biotransformation. The most important role in biotransformation belongs to CYP3A4, CYP2D6, CYP2C9, CYP2C19 and CYP1A2. Inhibition and induction of CYP isoenzymes caused by drugs are important and clinically relevant pharmacokinetic mechanisms of drug interaction. Investigation of the activity of CYP isoenzymes by using phenotyping methods (such as the determination of the concentration of specific substrates and metabolites in biological fluids) during drug administration provides the prediction of negative side effects caused by drug interaction. In clinical practice, the process of phenotyping of CYP isoenzymes and some endogenous substrates in the ratio of cortisol to 6β-hydroxycortisol in urine for the evaluation of CYP3A4 activity has been deemed to be a quite promising, safe and minimally invasive method for patients nowadays.

Pharmacogenomics of Drug Metabolizing Enzymes and Transporters: Relevance to Precision Medicine

The interindividual genetic variations in drug metabolizing enzymes and transporters influence the efficacy and toxicity of numerous drugs. As a fundamental element in precision medicine, pharmacogenomics, the study of responses of individuals to medication based on their genomic information, enables the evaluation of some specific genetic variants responsible for an individual’s particular drug response. In this article, we review the contributions of genetic polymorphisms to major individual variations in drug pharmacotherapy, focusing specifically on the pharmacogenomics of phase-I drug metabolizing enzymes and transporters. Substantial frequency differences in key variants of drug metabolizing enzymes and transporters, as well as their possible functional consequences, have also been discussed across geographic regions. The current effort illustrates the common presence of variability in drug responses among individuals and across all geographic regions. This information will aid health-care professionals in prescribing the most appropriate treatment aimed at achieving the best possible beneficial outcomes while avoiding unwanted effects for a particular patient.

Urinary antihypertensive drug metabolite screening using molecular networking coupled to high-resolution mass spectrometry fragmentation

INTRODUCTION

Mass spectrometry is the current technique of choice in studying drug metabolism. High-resolution mass spectrometry in combination with MS/MS gas-phase experiments has the potential to contribute to rapid advances in this field. However, the data emerging from such fragmentation spectral files pose challenges to downstream analysis, given their complexity and size.

OBJECTIVES

This study aims to detect and visualize antihypertensive drug metabolites in untargeted metabolomics experiments based on the spectral similarity of their fragmentation spectra. Furthermore, spectral clusters of endogenous metabolites were also examined.

METHODS

Here we apply a molecular networking approach to seek drugs and their metabolites, in fragmentation spectra from urine derived from a cohort of 26 patients on antihypertensive therapy. The mass spectrometry data was collected on a Thermo Q-Exactive coupled to pHILIC chromatography using data dependent analysis (DDA) MS/MS gas-phase experiments.

RESULTS

In total, 165 separate drug metabolites were found and structurally annotated (17 by spectral matching and 122 by classification based on a clustered fragmentation pattern). The clusters could be traced to 13 drugs including the known antihypertensives verapamil, losartan and amlodipine. The molecular networking approach also generated clusters of endogenous metabolites, including carnitine derivatives, and conjugates containing glutamine, glutamate and trigonelline.

CONCLUSIONS

The approach offers unprecedented capability in the untargeted identification of drugs and their metabolites at the population level and has great potential to contribute to understanding stratified responses to drugs where differences in drug metabolism may determine treatment outcome.

Pharmacogenomics Implications of Using Herbal Medicinal Plants on African Populations in Health Transition

The most accessible points of call for most African populations with respect to primary health care are traditional health systems that include spiritual, religious, and herbal medicine. This review focusses only on the use of herbal medicines. Most African people accept herbal medicines as generally safe with no serious adverse effects. However, the overlap between conventional medicine and herbal medicine is a reality among countries in health systems transition. Patients often simultaneously seek treatment from both conventional and traditional health systems for the same condition. Commonly encountered conditions/diseases include malaria, HIV/AIDS, hypertension, tuberculosis, and bleeding disorders. It is therefore imperative to understand the modes of interaction between different drugs from conventional and traditional health care systems when used in treatment combinations. Both conventional and traditional drug entities are metabolized by the same enzyme systems in the human body, resulting in both pharmacokinetics and pharmacodynamics interactions, whose properties remain unknown/unquantified. Thus, it is important that profiles of interaction between different herbal and conventional medicines be evaluated. This review evaluates herbal and conventional drugs in a few African countries and their potential interaction at the pharmacogenomics level.

Sources of interindividual variability

The efficacy, safety, and tolerability of drugs are dependent on numerous factors that influence their disposition. A dose that is efficacious and safe for one individual may result in sub-therapeutic or toxic blood concentrations in other individuals. A major source of this variability in drug response is drug metabolism, where differences in pre-systemic and systemic biotransformation efficiency result in variable degrees of systemic exposure (e.g., AUC, C max, and/or C min) following administration of a fixed dose.Interindividual differences in drug biotransformation have been studied extensively. It is well recognized that both intrinsic (such as genetics, age, sex, and disease states) and extrinsic (such as diet, chemical exposures from the environment, and even sunlight) factors play a significant role. For the family of cytochrome P450 enzymes, the most critical of the drug metabolizing enzymes, genetic variation can result in the complete absence or enhanced expression of a functional enzyme. In addition, up- and down-regulation of gene expression, in response to an altered cellular environment, can achieve the same range of metabolic function (phenotype), but often in a less reliably predictable and time-dependent manner. Understanding the mechanistic basis for drug disposition and response variability is essential if we are to move beyond the era of empirical, trial-and-error dose selection and into an age of personalized medicine that brings with it true improvements in health outcomes in the therapeutic treatment of disease.

Drug-Disease Interactions: Losartan Effect Is Not Downregulated by Rheumatoid Arthritis

Inflammatory conditions, such as rheumatoid arthritis, reduce response to calcium channel and beta-adrenergic antagonists but not the angiotensin II type 1 receptor (AT(1)R) antagonist valsartan. Inflammation also reduces clearance of some drugs or active metabolite, thereby reducing response. Active (n = 14) and controlled rheumatoid arthritis (n = 12) and healthy subjects (n = 12) received losartan (100 mg). Blood pressures were measured, and samples were taken for pharmacokinetic and inflammatory mediator concentration determination. Active disease significantly increased arthritic index, nitric oxide, and Creactive protein. Although no between-group difference in plasma losartan concentration-time curves was observed, concentrations of the active metabolite, EXP 3174, were significantly reduced by arthritis. This, however, was not accompanied by reduced clinical response. One subject produced no detectable concentrations of EXP 3174 likely due to insufficient CYP2C9 activity. Despite reduced concentrations of the active metabolite, AT1R antagonists potency does not appear to be reduced by inflammation.

Drug-drug interactions that reduce the formation of pharmacologically active metabolites: a poorly understood problem in clinical practice

Drug-drug interactions can lead to reduced efficacy of medical treatment. Therapeutic failure may for instance result from combined treatment with an inhibitor of the specific pathway that is responsible for the generation of pharmacologically active drug metabolites. This problem may be overlooked in clinical practice. Several examples of drugs will be discussed -clopidogrel, losartan, tamoxifen and codeine - to illustrate differences in the potential impact on drug treatment in clinical practice. We conclude that the combined use of cytochrome P450-blocking serotonin reuptake inhibitors and tamoxifen or codeine should be avoided, whereas the situation is much more complex regarding the use of proton pump inhibitors together with clopidogrel, and the evidence regarding cytochrome P450 inhibitor-dependent activation of losartan is inconclusive.

Pharmacokinetics and Safety of Olmesartan Medoxomil in Combination with Glibenclamide in Healthy Volunteers

OBJECTIVE

To investigate the pharmacokinetic interactions, safety, and tolerability of the combination of olmesartan medoxomil with glibenclamide.

METHODS

In an open, three-way crossover, phase I trial, 18 healthy adults entered three randomly ordered, seven-day treatment periods. The three treatments comprised once daily administration of (1) olmesartan 20 mg, (2) olmesartan 20 mg plus glibenclamide 3.5 mg, or (3) glibenclamide 3.5 mg.

RESULTS

The combination of olmesartan with glibenclamide did not influence the bioequivalence of the area under the plasma-concentration time curve at steady state during one dosing interval 0 to tau = 24 hours (AUCss,tau) or the maximum steady-state concentration (Css,max) of both substances. Mean AUCss,tau values for olmesartan were 2594.8 ng h/ml for olmesartan alone and 2443.7 ng h/ml in combination with glibenclamide; the corresponding Css,max values were 479.3 ng/ml and 462.7 ng/ml, respectively. For glibenclamide, the mean AUCss,tau values were 525.7 ng x h/ml for monotherapy and 518.7 ng x h/ml for its combination with olmesartan. The median time to reach Css,max (tmax) for glibenclamide was shifted from 2.0 h to 1.0 h when combined with olmesartan, whereas the median tmax values for olmesartan remained unchanged at 1.5 h. During combined treatment with olmesartan plus glibenclamide, no adverse event occurred, and the medications were well tolerated.

CONCLUSION

With the exception of a slight shift of tmax values for glibenclamide, the concomitant administration of olmesartan medoxomil with glibenclamide had no significant effects on the steady-state pharmacokinetics of either agent. This provides the pharmacokinetic rationale for clinical studies to test the combination therapy of patients with hypertension and type-2 diabetes mellitus with both compounds.

Effect of soy extract administration on losartan pharmacokinetics in healthy female volunteers

BACKGROUND

osartan is metabolized by CYP2C9 and CYP3A4 to an active metabolite, E-3174, which has greater antihypertensive activity than the parent compound. Soy extract has been shown to be an activator of CYP2C9 and CYP3A4 in vitro. Coadministration of soy extract and losartan may therefore alter the pharmacokinetics of losartan and E-3174.

OBJECTIVE

To determine whether, when losartan was used in combination with soy extract, a significant pharmacokinetic interaction would be observed in healthy female volunteers.

METHODS

Eighteen healthy Chinese female volunteers were recruited. In an open-label, 2-phase study, losartan 50 mg was given to each subject, with and without soy extract. Plasma concentrations of losartan and E-3174 were determined by liquid chromatography-tandem mass spectrometry for 12 and 24 hours, respectively. On day 8 through day 21 of the study, following a 7-day washout period, each subject consumed two 1000-mg Genistein Soy Complex tablets orally after meals, twice daily, for 14 days. On day 22, all volunteers received losartan 50 mg and blood samples were collected again.

RESULTS

All subjects completed the study, without adverse drug effects. Over the 14-day pretreatment period, soy extract did not significantly influence the pharmacokinetics of losartan or E-3174. The ratio of the area under the curve of the drug and metabolite after losartan administration, with and without soy extract ingestion, was 0.21 +/- 0.05 and 0.23 +/- 0.05 (mean +/- SD), respectively. The difference was not statistically significant (p = 0.22).

CONCLUSIONS

Our data indicate that a significant interaction between soy extract and losartan is unlikely to occur in females.

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.

The clinical role of genetic polymorphisms in drug-metabolizing enzymes

For most drug-metabolizing enzymes (DMEs), the functional consequences of genetic polymorphisms have been examined. Variants leading to reduced or increased enzymatic activity as compared to the wild-type alleles have been identified. This review tries to define potential fields in the therapy of major medical conditions where genotyping (or phenotyping) of genetically polymorphic DMEs might be beneficial for drug safety or therapeutic outcome. The possible application of genotyping is discussed for depression, cardiovascular diseases and thromboembolic disorders, gastric ulcer, malignant diseases and tuberculosis. Some drugs used for relief of these ailments are metabolized with participation of genetically polymorphic DMEs including CYP2D6, CYP2C9, CYP2C19, thiopurine-S-methyltransferase, dihydropyrimidine dehydrogenase, uridine diphosphate glucuronosyltransferase and N-acetyltransferase type 2. Current evidence suggests that taking genetically determined metabolic capacities of DMEs into account has the potential to improve individual risk/benefit relationship. However, more prospective studies with clinical endpoints are needed before the paradigm of 'personalized medicine' based on DME variants can be established.

Species differences between mouse, rat, dog, monkey and human CYP-mediated drug metabolism, inhibition and induction

Animal models are commonly used in the preclinical development of new drugs to predict the metabolic behaviour of new compounds in humans. It is, however, important to realise that humans differ from animals with regards to isoform composition, expression and catalytic activities of drug-metabolising enzymes. In this review the authors describe similarities and differences in this respect among the different species, including man. This may be helpful for drug researchers to choose the most relevant animal species in which the metabolism of a compound can be studied for extrapolating the results to humans. The authors focus on CYPs, which are the main enzymes involved in numerous oxidative reactions and often play a critical role in the metabolism and pharmacokinetics of xenobiotics. In addition, induction and inhibition of CYPs are compared among species. The authors conclude that CYP2E1 shows no large differences between species, and extrapolation between species appears to hold quite well. In contrast, the species-specific isoforms of CYP1A, -2C, -2D and -3A show appreciable interspecies differences in terms of catalytic activity and some caution should be applied when extrapolating metabolism data from animal models to humans.

Pharmacogenetics, drug-metabolizing enzymes, and clinical practice

The application of pharmacogenetics holds great promise for individualized therapy. However, it has little clinical reality at present, despite many claims. The main problem is that the evidence base supporting genetic testing before therapy is weak. The pharmacology of the drugs subject to inherited variability in metabolism is often complex. Few have simple or single pathways of elimination. Some have active metabolites or enantiomers with different activities and pathways of elimination. Drug dosing is likely to be influenced only if the aggregate molar activity of all active moieties at the site of action is predictably affected by genotype or phenotype. Variation in drug concentration must be significant enough to provide "signal" over and above normal variation, and there must be a genuine concentration-effect relationship. The therapeutic index of the drug will also influence test utility. After considering all of these factors, the benefits of prospective testing need to be weighed against the costs and against other endpoints of effect. It is not surprising that few drugs satisfy these requirements. Drugs (and enzymes) for which there is a reasonable evidence base supporting genotyping or phenotyping include suxamethonium/mivacurium (butyrylcholinesterase), and azathioprine/6-mercaptopurine (thiopurine methyltransferase). Drugs for which there is a potential case for prospective testing include warfarin (CYP2C9), perhexiline (CYP2D6), and perhaps the proton pump inhibitors (CYP2C19). No other drugs have an evidence base that is sufficient to justify prospective testing at present, although some warrant further evaluation. In this review we summarize the current evidence base for pharmacogenetics in relation to drug-metabolizing enzymes.

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.

Linking pharmacovigilance with pharmacogenetics

The ability to identify individuals who are susceptible to adverse drug reactions (ADRs) has the potential to reduce the personal and population costs of drug-related morbidity. Some individuals may show an increased susceptibility to certain ADRs through genetic polymorphisms that alter their responses to various drugs. We wished to establish a methodology that would be acceptable to members of the general population and that would enable estimation of the risks that specific genetic factors confer on susceptibility to specific ADRs. Buccal swabs were selected as a minimally invasive method to obtain cells for DNA extraction. We wished to determine whether DNA of sufficient quantity and quality could be obtained to enable genotyping for two different polymorphic genes that code for enzymes that are widely involved in drug disposition. This article describes a small pilot study of methodology developed in the New Zealand Intensive Medicines Monitoring Programme (IMMP) to link prescription event monitoring (PEM) studies with pharmacogenetics. The methodology involves a nested case-control study design to investigate whether patients with genetic variants in P-glycoprotein (P-gp) and cytochrome P450 (CYP) 2C9 are more susceptible to psychiatric or visual disturbances following cyclo- oxygenase-2 inhibitor use (ADR signals identified in the IMMP database) than matched control patients taking the medication without experiencing any ADRs. It was concluded that the use of buccal swabs is acceptable to patients and provides DNA of sufficient quantity and quality for genotyping. Although no differences in the distribution of genotypes in the case and control populations were found in this small study, case-control studies investigating genetic risks for ADRs using drug cohorts from PEM studies are possible, and there are several areas where population-based studies of genetic risk factors for ADRs are needed. Examples are discussed where research in large populations is required urgently. These are: (i) genetic variations affecting P-gp function; (ii) variations affecting drugs metabolised by CYP2C9 and other polymorphic CYP enzymes; (iii) genetic variation in beta-adrenergic receptors and adverse outcomes from beta-adrenoceptor agonist therapy; and (iv) genetic variation in cardiac cell membrane potassium channels and their association with long QT syndromes and serious cardiac dysrhythmias. Such studies will help to identify factors that increase the risk of unwanted outcomes from drug therapy. They will also help to establish in what circumstances genotyping should be performed prior to commencing drug treatment and in tailoring drug treatment for individual patients.

Clinical pharmacokinetics of losartan

Losartan is the first orally available angiotensin-receptor antagonist without agonist properties. Following oral administration, losartan is rapidly absorbed, reaching maximum concentrations 1-2 hours post-administration. After oral administration approximately 14% of a losartan dose is converted to the pharmacologically active E 3174 metabolite. E 3174 is 10- to 40-fold more potent than its parent compound and its estimated terminal half-life ranges from 6 to 9 hours. The pharmacokinetics of losartan and E 3174 are linear, dose-proportional and do not substantially change with repetitive administration. The recommended dosage of losartan 50 mg/day can be administered without regard to food. There are no clinically significant effects of age, sex or race on the pharmacokinetics of losartan, and no dosage adjustment is necessary in patients with mild hepatic impairment or various degrees of renal insufficiency. Losartan, or its E 3174 metabolite, is not removed during haemodialysis. The major metabolic pathway for losartan is by the cytochrome P450 (CYP) 3A4, 2C9 and 2C10 isoenzymes. Overall, losartan has a favorable drug-drug interaction profile, as evidenced by the lack of clinically relevant interactions between this drug and a range of inhibitors and stimulators of the CYP450 system. Losartan does not have a drug-drug interaction with hydrochlorothiazide, warfarin or digoxin. Losartan should be avoided in pregnancy, as is the case with all other angiotensin-receptor antagonists. When given in the second and third trimester of pregnancy, losartan is often associated with serious fetal toxicity. Losartan is a competitive antagonist that causes a parallel rightward shift of the concentration-contractile response curve to angiotensin-II, while E 3174 is a noncompetitive "insurmountable" antagonist of angiotensin-II. The maximum recommended daily dose of losartan is 100mg, which can be given as a once-daily dose or by splitting the same total daily dose into two doses. Losartan reduces blood pressure comparably to other angiotensin-receptor antagonists. Losartan has been extensively studied relative to end-organ protection, with studies having been conducted in diabetic nephropathy, heart failure, post-myocardial infarction and hypertensive patients with left ventricular hypertrophy. The results of these studies have been sufficiently positive to support a more widespread use of angiotensin-receptor antagonists in the setting of various end-organ diseases. Losartan, like other angiotensin-receptor antagonists, is devoid of significant adverse effects.

Differences in drug pharmacokinetics between East Asians and Caucasians and the role of genetic polymorphisms

Interethnic variability in pharmacokinetics can cause unexpected outcomes such as therapeutic failure, adverse effects, and toxicity in subjects of different ethnic origin undergoing medical treatment. It is important to realize that both genetic and environmental factors can lead to these differences among ethnic groups. The International Conference on Harmonization (ICH) published a guidance to facilitate the registration of drugs among ICH regions (European Union, Japan, the United States) by recommending a framework for evaluating the impact of ethnic factors on a drug's effect, as well as its efficacy and safety at a particular dosage and dosage regimen. This review focuses on the pharmacokinetic differences between East Asians and Caucasians. Differences in metabolism between East Asians and Caucasians are common, especially in the activity of several phase I enzymes such as CYP2D6 and the CYP2C subfamily. Before drug therapy, identification of either the genotype and/or the phenotype for these enzymes may be of therapeutic value, particularly for drugs with a narrow therapeutic index. Furthermore, these differences are relevant for international drug approval when regulatory agencies must decide if they accept results from clinical trials performed in other parts of the world.

Discovery of new potentially defective alleles of human CYP2C9

CYP2C9 is a clinically important enzyme, responsible for the metabolism of numerous clinically important therapeutic drugs. In the present study, we discovered 38 single nucleotide polymorphisms in CYP2C9 by resequencing of genomic DNA from 92 individuals from three different racial groups. Haplotype analysis predicted that there are at least 21 alleles of CYP2C9 in this group of individuals. Six new alleles were identified that contained coding changes: L19I (CYP2C9*7), R150H (CYP2C9*8), H251R (CYP2C9*9), E272G (CYP2C9*10), R335W(CYP2C9*11) and P489S (CYP2C9*12). When expressed in a bacterial cDNA expression system, several alleles exhibited altered catalytic activity. CYP2C9*11 appeared to be a putative poor metabolizer allele, exhibiting a three-fold increase in the Km and more than a two-fold decrease in the intrinsic clearance for tolbutamide. Examination of the crystal structure of human CYP2C9 reveals that R335 is located in the turn between the J and J' helices and forms a hydrogen-bonding ion pair with D341 from the J' helix. Abolishing this interaction in CYP2C9*11 individuals could destabilize the secondary structure and alter the substrate affinity. This new putative poor metabolizer (PM) allele was found in Africans. A second potentially PM allele CYP2C9*12 found in a racially unidentified sample also exhibited a modest decrease in the Vmax and the intrinsic clearance for tolbutamide in a recombinant system. Further clinical studies are needed to determine the effect of these new polymorphisms on the metabolism of CYP2C9 substrates.

Clinical relevance of genetic polymorphisms in the human CYP2C9 gene

Cytochrome p450 (CYP) 2C9 hydroxylates about 16% of drugs in current clinical use. Of special interest are those with a narrow therapeutic index, such as S-warfarin, tolbutamide and phenytoin, where impairment in CYP2C9 metabolic activity might cause difficulties in dose adjustment as well as toxicity. Single-nucleotide polymorphisms (SNP) in the CYP2C9 gene have increasingly been recognized as determinants of the metabolic phenotype that underlies interindividual and ethnic differences. Apart from the wild-type protein CYP2C9*1 at least five allelic variants produce allozymes with reduced or deficient metabolic activity. Among white populations only CYP2C9*2 and CYP2C9*3 variants are of significance, with allelic frequencies of 0.08-0.14 and 0.04-0.16, respectively. In contrast, in Africans (African-Americans and Ethiopians) and Asians both variants are much less frequent (0.005-0.04), and CYP2C9*2 has not yet been detected in Asians. CYP2C9*4 has been exclusively identified in Japanese patients, and CYP2C9*5 and *6 were only found among African-Americans with a low allelic frequency of 0.017 and 0.006, respectively. Furthermore in Japanese a CYP2C9 promotor variant of four linked SNPs was correlated with reduced intrinsic clearance of phenytoin in vitro. Subjects who are carriers of one or more variant alleles may be at risk for adverse drug reactions/toxicities when prescribed drugs extensively metabolized by CYP2C9.

Losartan and E3174 pharmacokinetics in cytochrome P450 2C9*1/*1, *1/*2, and *1/*3 individuals

STUDY OBJECTIVE

To determine if differences in the pharmacokinetics of losartan and its pharmacologically active E3174 metabolite exist among individuals expressing the cytochrome P450 (CYP) 2C9*1/*1, *1/*2, and *1/*3 genotypes.

DESIGN

Single-dose pharmacokinetic study.

SETTING

University general clinical research center.

SUBJECTS

Fifteen healthy volunteers, five from each genotype: CYP2C9*1/*1, *1/*2, and *1/*3.

INTERVENTION

A single oral dose of losartan 50 mg.

MEASUREMENTS AND MAIN RESULTS

Plasma and urine samples were collected for 24 hours, and losartan and E3174 pharmacokinetic data were compared across the three genotypes. Orthostatic blood pressure was measured over 12 hours after dosing. No significant differences were observed among the three groups in losartan or E3174 area under the plasma concentration-time curve, losartan or E3174 elimination half-life, or losartan oral clearance. A significant association between CYP2C9 genotype and losartan to E3174 formation clearance was observed, such that 50% of the variability was accounted for by the genotype. No significant relationship between that genotype and blood pressure was observed at any time.

CONCLUSION

Differences in the pharmacokinetics of losartan and its active E3174 metabolite were not observed in healthy subjects with the genotype of CYP2C9*1/*2 and *1/*3 compared with those expressing *1/*1. Alterations in losartan dosing in CYP2C9*1/*2 and *1/*3 individuals does not appear necessary.

Interactions between antiretroviral drugs and drugs used for the therapy of the metabolic complications encountered during HIV infection

Treatment of HIV infection with potent combination antiretroviral therapy has resulted in major improvement in overall survival, immune function and the incidence of opportunistic infections. However, HIV infection and treatment has been associated with the development of metabolic complications, including hyperlipidaemia, diabetes mellitus, hypertension, lipodystrophy and osteopenia. Safe pharmacological treatment of these complications requires an understanding of the drug-drug interactions between antiretroviral drugs and the drugs used in the treatment of metabolic complications. Since formal studies of most of these interactions have not been performed, predictions must be based on our understanding of the metabolism of these agents. All HIV protease inhibitors are metabolised by and inhibit cytochrome P450 (CYP) 3A4. Ritonavir is the most potent inhibitor of CYP3A4. Ritonavir and nelfinavir also induce a host of CYP isoforms as well as some conjugating enzymes. The non-nucleoside reverse transcriptase inhibitor delavirdine potently inhibits CYP3A4, whereas nevirapine and efavirenz are inducers of CYP3A4. Drug interaction studies have been performed with HIV protease inhibitors and HMG-CoA reductase inhibitors. Coadministration of ritonavir plus saquinavir to HIV-seronegative volunteers resulted in increased exposure to simvastatin acid by 3059%. Atorvastatin exposure increased by 347%, but exposure to active atorvastatin increased by only 79%. Conversely, pravastatin exposure decreased by 50%. Similar results have been obtained with combinations of simvastatin and atorvastatin with other HIV protease inhibitors. Thus, the lactone prodrugs simvastatin and lovastatin should not be used with HIV protease inhibitors. Atorvastatin may be used with caution. Although there are no formal studies available, calcium channel antagonists and repaglinide may have significant interactions and toxicity when used with HIV protease inhibitors because of their metabolism by CYP3A4. Sulfonylurea drugs utilise mainly CYP2C9 for metabolism, and this isoenzyme may be induced by ritonavir and nelfinavir with a resulting decrease in efficacy of the sulfonylurea. Losartan may have increased effect when coadministered with ritonavir and nelfinavir because of the induction of CYP2C9 and the expected increase in formation of the active metabolite, E-3174. Overall, well-designed drug-drug interaction studies at steady state are needed to determine whether antiretroviral drugs may be safely coadministered with many of the drugs used in the treatment of the metabolic complications of HIV infection.

Cytochrome P450 in vitro reaction phenotyping: a re-evaluation of approaches used for P450 isoform identification

Marker substrates, chemical inhibitors, and inhibitory antibodies are important tools for the identification of cytochrome P450 (P450) isoform responsible for the metabolism of therapeutic agents in vitro. In view of the versatile and nonspecific nature of P450 enzymes, many of the marker substrates and chemical inhibitors used for P450 in vitro reaction phenotyping are isoform selective but not specific. Recently, the use of marker substrate and chemical inhibitors in CYP2D6 in vitro reaction phenotyping was questioned by Granvil et al. (2002). In comparison of a panel of 15 recombinant P450 enzymes, they found that in addition to CYP2D6, CYP1A1 is also capable of catalyzing the formation of 4-hydroxylated metabolite of debrisoquine and that the intrinsic clearance of debrisoquine by CYP2D6-mediated 4-hydroxylation is only about twice that by CYP1A1. In their study, they have also demonstrated that quinidine inhibits both CYP2D6- and CYP1A1-mediated debrisoquine 4-hydroxylation. In view of these important findings, we have reevaluated various approaches used to identify P450 isoform(s) responsible for the metabolism of therapeutic agents. While acknowledging the value of inhibitory antibodies in P450-phenotyping studies, it is our opinion that in well conducted in vitro experiments, isoform-selective chemical inhibitors can also provide valuable and reliable information. Hopefully, future efforts may produce even better P450 isoform-selective marker substrates and inhibitors.

Extrapolation of diclofenac clearance from in vitro microsomal metabolism data: role of acyl glucuronidation and sequential oxidative metabolism of the acyl glucuronide

Diclofenac is eliminated predominantly (approximately 50%) as its 4'-hydroxylated metabolite in humans, whereas the acyl glucuronide (AG) pathway appears more important in rats (approximately 50%) and dogs (>80-90%). However, previous studies of diclofenac oxidative metabolism in human liver microsomes (HLMs) have yielded pronounced underprediction of human in vivo clearance. We determined the relative quantitative importance of 4'-hydroxy and AG pathways of diclofenac metabolism in rat, dog, and human liver microsomes. Microsomal intrinsic clearance values (CL(int) = V(max)/K(m)) were determined and used to extrapolate the in vivo blood clearance of diclofenac in these species. Clearance of diclofenac was accurately predicted from microsomal data only when both the AG and the 4'-hydroxy pathways were considered. However, the fact that the AG pathway in HLMs accounted for ~75% of the estimated hepatic CL(int) of diclofenac is apparently inconsistent with the 4'-hydroxy diclofenac excretion data in humans. Interestingly, upon incubation with HLMs, significant oxidative metabolism of diclofenac AG, directly to 4'-hydroxy diclofenac AG, was observed. The estimated hepatic CL(int) of this pathway suggested that a significant fraction of the intrahepatically formed diclofenac AG may be converted to its 4'-hydroxy derivative in vivo. Further experiments indicated that this novel oxidative reaction was catalyzed by CYP2C8, as opposed to CYP2C9-catalyzed 4'-hydroxylation of diclofenac. These findings may have general implications in the use of total (free + conjugated) oxidative metabolite excretion for determining primary routes of drug clearance and may question the utility of diclofenac as a probe for phenotyping human CYP2C9 activity in vivo via measurement of its pharmacokinetics and total 4'-hydroxy diclofenac urinary excretion.

CYP2C9 allelic variants: ethnic distribution and functional significance

Cytochrome P-450 (CYP) 2C9 CYP2C9 is a polymorphically expressed enzyme responsible for the metabolism of several clinically important drugs, some with a low therapeutic index. This review summarizes the structure-function relationship of the CYP2C9 promoter and coding regions, known polymorphisms, the functional significance of various CYP2C9 alleles in vitro and in vivo, and their population frequencies. In addition, possible molecular mechanisms underlying ethnic variability in the metabolism of CYP2C9 substrate drugs are discussed.

Cytochrome P450 2C9 polymorphisms: a comprehensive review of the in-vitro and human data

The discovery of six distinct polymorphisms in the genetic sequence encoding for the cytochrome P450 2C9 (CYP2C9) protein has stimulated numerous investigations in an attempt to characterize their population distribution and metabolic activity. Since the CYP2C9*1, *2 and *3 alleles were discovered first, they have undergone more thorough investigation than the recently identified *4, *5 and *6 alleles. Population distribution data suggest that the variant *2 and *3 alleles are present in approximately 35% of Caucasian individuals; however, these alleles are significantly less prevalent in African-American and Asian populations. In-vitro data have consistently demonstrated that the CYP2C9*2 and *3 alleles are associated with significant reductions in intrinsic clearance of a variety of 2C9 substrates compared with CYP2C9*1; however, the degree of these reductions appear to be highly substrate-dependent. In addition, multiple in-vivo investigations and clinical case reports have associated genotypes expressing the CYP2C9*2 and *3 alleles with significant reductions in both the metabolism and daily dose requirements of selected CYP2C9 substrates. Individuals expressing these variant genotypes also appear to be significantly more susceptible to adverse events with the narrow therapeutic index agents warfarin and phenytoin, particularly during the initiation of therapy. These findings have subsequently raised numerous questions regarding the potential clinical utility of genotyping for CYP2C9 prior to initiation of therapy with these agents. However, further clinical investigations evaluating the metabolic consequences in individuals expressing the CYP2C9*2, *3, *4, *5, or *6 alleles are required before large-scale clinical genotyping can be recommended.

Impact of CYP2C9 and CYP2C19 polymorphisms on tolbutamide kinetics and the insulin and glucose response in healthy volunteers

Tolbutamide is known to be metabolized by cytochrome P450 2C9 (CYP2C9), and the effects of the CYP2C9 amino acid polymorphisms *2 (Arg144Cys) and *3 (Ile359Leu) could be important for drug treatment with tolbutamide and for use of tolbutamide as a CYP2C9 test drug. Tolbutamide pharmacokinetics and plasma insulin and glucose concentrations were studied in 23 healthy volunteers with all six combinations of the CYP2C9 alleles *1, *2 and *3, including two subjects with the combined CYP2C9*1/*1 and CYP2C19*2/*2 genotype. Volunteers received a single oral dose of 500 mg tolbutamide, followed by 75 g oral glucose at 1, 4.5 and 8 h after tolbutamide administration. Pharmacokinetic analysis was performed using a computer program for regression analysis of nonlinear mixed effects models. The mean oral clearances of tolbutamide were 0.97 (95% confidence interval [CI] 0.89-1.05), 0.86 (95% CI 0.79-0.93), 0.75 (95% CI 0.69-0.81), 0.56 (95% CI 0.51-0.61), 0.45 (95% CI 0.41-0.49) and 0.15 (95% CI 0.14-0.16) l/h in carriers of CYP2C9 genotypes 1/*1, *1/*2, *2/*2, *1/*3, *2/*3 and *3/*3, respectively. Tolbutamide pharmacokinetics in carriers of the functionally deficient CYP2C19*2/*2 genotype were not different from those in the CYP2C19 highly active genotype. Elimination in the six CYP2C9 genotype groups could be expressed as the linear combination of three constants (0.05, 0.04, 0.01 h(-1), which were specific to the respective CYP2C9 alleles *1, *2 and *3, thus indicating a co-dominant mode of inheritance. Insulin and glucose concentration-time curves did not change with differing CYP2C9 genotypes. Tolbutamide was confirmed as a substrate of the genetically polymorphic enzyme CYP2C9. The pronounced differences in pharmacokinetics due to the amino acid variants did not significantly affect plasma insulin and glucose concentrations in healthy volunteers.

Pharmacogenetics of warfarin elimination and its clinical implications

Warfarin is one of the most widely prescribed oral anticoagulants. However, optimal use of the drug has been hampered by its >10-fold interpatient variability in the doses required to attain therapeutic responses. Pharmacogenetic polymorphism of cytochrome P450 (CYP) may be associated with impaired elimination of warfarin and exaggerated anticoagulatory responses to the drug in certain patients. Clinically available warfarin is a racemic mixture of (R)- and (S)-warfarin, and the (S)-enantiomer has 3 to 5 times greater anticoagulation potency than its optical congener. Both enantiomers are eliminated extensively via hepatic metabolism with low clearance relative to hepatic blood flow. CYP2C9 is almost exclusively responsible for the metabolism of the pharmacologically more active (S)-enantiomer. Several human allelic variants of CYP2C9 have been cloned, designated as CYP2C9*1 (reference sequence or wild-type allele), CYP2C9*2, CYP2C9*3 and CYP2C9*4, respectively. The allelic frequencies for these variants differ considerably among different ethnic populations. Caucasians appear to carry the CYP 2C9*2 (8 to 20%) and CYP2C9*3 (6 to 10%) variants more frequently than do Asians (0% and 2 to 5%, respectively). The metabolic activities of the CYP2C9 variants have been investigated in vitro. The catalytic activity of CYP2C9*3 expressed from cDNA was significantly less than that of CYP2C9*1. Human liver microsomes obtained from individuals heterozygous for CYP2C9*3 showed significantly reduced (S)-warfarin 7-hydroxylation as compared with those obtained from individuals genotyped as CYP2C9*1. The influence of the CYP2C9*3 allele on the in vivo pharmacokinetics of (S)-warfarin has been studied in Japanese patients. Patients with the homozygous CYP2C9*3 genotype, as well as those with the heterozygous CYP2C9*1/*3 genotype, had significantly reduced clearance of (S)-warfarin (by 90 and 60%, respectively) compared with those with homozygous CYP2C9*1. The maintenance dosages of warfarin required in Japanese patients with heterozygous and homozygous CYP2C9*3 mutations were significantly lower than those in patients with CYP2C9*1/*1. In addition, 86% of British patients exhibiting adequate therapeutic responses with lower maintenance dosages of warfarin (<1.5 mg/day) had either the CYP2C9*2 or CYP2C9*3 mutation singly or in combination, whereas only 38% of randomly selected patients receiving warfarin carried the corresponding mutations. Furthermore, the former group showed more frequent episodes of major bleeding associated with warfarin therapy. These data indicate that the CYP2C9*3 allele may be associated with retarded elimination of (S)-warfarin and the resulting clinical effects. However, relationships between CYP2C9 genotype, enzyme activity, metabolism of probe substrates, dosage requirements and bleeding complications should be interpreted with caution, and further studies are required.

Cytochrome P4502C9 (CYP2C9) allele frequencies in Canadian Native Indian and Inuit populations

CYP2C9 is the major P450 2C enzyme in human liver and contributes to the metabolism of a number of clinically important substrate drugs. This polymorphically expressed enzyme has been studied in Caucasian, Asian, and to some extent in African American populations, but little is known about the genetic variation in Native American populations. We therefore determined the 2C9*2 (Arg144Cys) and 2C9*3 (Ile359Leu) allele frequencies in 153 Native Canadian Indian (CNI) and 151 Inuit subjects by PCR-RFLP techniques. We also present genotyping data for two reference populations, 325 Caucasian (white North American) and 102 Chinese subjects. Genotyping analysis did not reveal any 2C9*4 alleles in the CNI, Inuit, Caucasian, or Chinese individuals. The 2C9*2 allele appears to be absent in Chinese and Inuit populations, but was present in CNI and Caucasian subjects at frequencies of 0.03 and 0.08–0.15, respectively. The 2C9*3 allele was not detected in the Inuit group, but occured in the CNI group (f = 0.06) at a frequency comparable to that of other ethnic groups. This group of Inuit individuals are the first population in which no 2C9*2 or *3 alleles have been detected so far. Therefore, these alleles may be extremely rare or absent, and unless other novel polymorphisms exist in this Inuit group one would not anticipate any CYP2C9 poor metabolizer subjects among this population.Key words: CYP2C9, polymorphism, genotyping, ethnic diversity.

Clinical relevance of genetic polymorphisms in the human CYP2C subfamily

The human CYP2Cs are an important subfamily of P450 enzymes that metabolize approximately 20% of clinically used drugs. There are four members of the subfamily, CYP2C8, CYP2C9, CYP2C19, and CYP2C18. Of these CYP2C8, CYP2C9, and CYP2C19 are of clinical importance. The CYP2Cs also metabolize some endogenous compounds such as arachidonic acid. Each member of this subfamily has been found to be genetically polymorphic. The most well-known of these polymorphisms is in CYP2C19. Poor metabolizers (PMs) of CYP2C19 represent approximately 3-5% of Caucasians, a similar percentage of African-Americans and 12-100% of Asian groups. The polymorphism affects metabolism of the anticonvulsant agent mephenytoin, proton pump inhibitors such as omeprazole, the anxiolytic agent diazepam, certain antidepressants, and the antimalarial drug proguanil. Toxic effects can occur in PMs exposed to diazepam, and the efficacy of some proton pump inhibitors may be greater in PMs than EMs at low doses of these drugs. A number of mutant alleles exist that can be detected by genetic testing. CYP2C9 metabolizes a wide variety of drugs including the anticoagulant warfarin, antidiabetic agents such as tolbutamide, anticonvulsants such as phenytoin, and nonsteroidal anti-inflammatory drugs. The incidence of functional polymorphisms is much lower, estimated to be 1/250 in Caucasians and lower in Asians. However, the clinical consequences of these rarer polymorphisms can be severe. Severe and life-threatening bleeding episodes have been reported in CYP2C9 PMs exposed to warfarin. Phenytoin has been reported to cause severe toxicity in PMs. New polymorphisms have been discovered in CYP2C8, which metabolizes taxol (paclitaxel). Genetic testing is available for all of the known CYP2C variant alleles.

Phenytoin metabolic ratio: a putative marker of CYP2C9 activity in vivo

CYP2C9 mediates the oxidative metabolism of approximately 10% of drugs, some of which are characterized by a narrow therapeutic index. We aimed to validate genotype method and phenotype methodology, for evaluation of CYP2C9 activity in vivo. Thirty-one healthy subjects (22 male) received a single 300 mg dose of phenytoin. Blood was drawn periodically and urine was collected at intervals for 96 h. Plasma phenytoin and 5-(4-hydroxyphenyl)-5-phenylhydantoin (p-HPPH) and urine S and R enantiomers of p-HPPH were determined by high-performance liquid chromatography. CYP2C9 genotyping was obtained by polymerase chain reaction followed by digestion with Sau96I and StyI for the identification of CYP2C9*2 and CYP2C9*3, respectively. Eighteen subjects were CYP2C9*1 homozygous, seven were CYP2C9*2 heterozygous, four were CYP2C9*3 heterozygous, one was CYP2C9*2 homozygous and one was compound CYP2C9*2/CYP2C9*3 heterozygous. The allele frequencies of CYP2C9*1, CYP2C9*2 and CYP2C9*3 were 0.76 [95% confidence interval (CI) 0.73-0.79], 0.16 (95% CI 0.13-0.19) and 0.08 (95% CI 0.05-0.11), respectively. The CYP2C9-mediated production of (S)-p-HPPH represented the major metabolic pathway of phenytoin biotransformation as its excretion accounted for 95.6 + 0.9% of 'total' p-HPPH excretion over the 96 h collection interval. Phenytoin metabolic clearance to produce (S)-p-HPPH (PMC), correlated significantly with (S)-p-HPPH (or 'total' p-HPPH) content in 0-8, 0-12 and 0-24 urine collections (r = 0.88, 0.85 and 0.89, respectively) and with phenytoin metabolic ratio (PMR) defined as the ratio of urine (S)-p-HPPH (or 'total' p-HPPH) to mid-interval plasma phenytoin (r = 0.90, 0.88 and 0.94, respectively). PMC and PMR exhibited a gene-dose effect so that the highest and lowest values were noted in homozygous subjects CYP2C9*1 and subjects carrying two defective alleles, respectively, whereas heterozygous subjects had intermediate values. CYP2C9 genotyping and several phenytoin metabolic indices are correlated with CYP2C9 activity in vivo. The utility of phenytoin to predict the metabolism of other CYP2C9 substrates justifies further evaluation.

Effect of grapefruit juice on the pharmacokinetics of losartan and its active metabolite E3174 in healthy volunteers

Grapefruit juice (GJ), a cytochrome P450 (CYP) 3A4 inhibitor, may affect the pharmacokinetics of drugs metabolized through CYP 3A4. Losartan, an angiotensin II antagonist, is converted into its main active metabolite E3174 by CYP 3A4 and CYP 2C9. The effect of GJ on losartan pharmacokinetics was assessed in a randomized crossover trial. Losartan was given to 9 volunteers with and without GJ. Concentrations of losartan and its E3174 metabolite were determined in serum by a high-performance liquid chromatography method (HPLC). Significant differences were observed in some of the pharmacokinetic parameters of losartan and its metabolite E3174 after losartan administration with and without co-administered GJ. The lag time (time to drug appearance in serum) of losartan increased significantly with co-administered GJ. The mean residence time (MRT) and half-life (t(1/2)) of the E3174 metabolite were significantly longer and the area under the concentration--time curve (AUC) of the E3174 metabolite was significantly smaller after concomitant GJ administration. The ratio AUC(losartan)/AUC(E3174) was significantly increased after concurrent grapefruit juice intake. The increased lag time of losartan and the increased MRT and t1/2 and decreased AUC of E3174 were considered indicative of simultaneous CYP 3A4 inhibition and P-glycoprotein activation. The significantly increased AUC(losartan)/AUC(E3174) ratio, however, indicates reduced losartan conversion to E3174 by CYP 3A4 metabolism as a result of co-administered GJ.

Identification and functional characterization of a new CYP2C9 variant (CYP2C9*5) expressed among African Americans

CYP2C9 is a polymorphic gene for which there are four known allelic variants; CYP2C9*1, CYP2C9*2, CYP2C9*3, and CYP2C9*4. In the present study, DNA from 140 European Americans and 120 African Americans was examined by single-strand conformational polymorphism and restriction fragment length polymorphism analyses, resulting in the identification of a new CYP2C9 variant, CYP2C9*5. This variant is derived from a C1080G transversion in exon 7 of CYP2C9 that leads to an Asp360Glu substitution in the encoded protein. The CYP2C9*5 variant was found to be expressed only in African Americans, such that approximately 3% of this population carries the CYP2C9*5 allele. The variant was expressed in, and purified from, insect cells infected with a recombinant baculovirus. Comparative kinetic studies using the purified wild-type protein CYP2C9*1; the Ile359Leu variant, CYP2C9*3; and the Asp360Glu variant, CYP2C9*5 were carried out using (S)-warfarin, diclofenac, and lauric acid as substrates. The major effect of the Asp360Glu mutation was to increase the K(m) value relative to that of CYP2C9*1 for all three substrates: 12-fold higher for (S)-warfarin 7-hydroxylation, 5-fold higher for the 4'-hydroxylation of diclofenac, and 3-fold higher for the omega-1 hydroxylation of lauric acid. V(max) values differed less than K(m) values between the CYP2C9*1 and CYP2C9*5 proteins. In vitro intrinsic clearances for CYP2C9*5, calculated as the ratio of V(max)/K(m), ranged from 8 to 18% of CYP2C9*1 values. The corresponding ratio for CYP2C9*3 was 4 to 13%. Accordingly, the in vitro data suggest that carriers of the CYP2C9*5 allele would eliminate CYP2C9 substrates at slower rates relative to persons expressing the wild-type protein.

Molecular basis of ethnic differences in drug disposition and response

Ethnicity is an important demographic variable contributing to interindividual variability in drug metabolism and response. In this rapidly expanding research area many genetic factors that account for the effects of ethnicity on pharmacokinetics, pharmacodynamics, and drug safety have been identified. This review focuses on recent developments that have improved understanding of the molecular mechanisms responsible for such interethnic differences. Genetic variations that may provide a molecular basis for ethnic differences in drug metabolizing enzymes (CYP 2C9, 2C19, 2D6, and 3A4), drug transporter (P-glycoprotein), drug receptors (adrenoceptors), and other functionally important proteins (eNOS and G proteins) are discussed. A better understanding of the molecular basis underlying ethnic differences in drug metabolism, transport, and response will contribute to improved individualization of drug therapy.

In-vitro metabolism of celecoxib, a cyclooxygenase-2 inhibitor, by allelic variant forms of human liver microsomal cytochrome P450 2C9: correlation with CYP2C9 genotype and in-vivo pharmacokinetics

In-vitro studies were conducted to assess the impact of CYP2C9 genotype on the metabolism (methyl hydroxylation) and pharmacokinetics of celecoxib, a novel cyclooxygenase-2 inhibitor and CYP2C9 substrate. When compared to cDNA-expressed wild-type CYP2C9 (CYP2C9*1), the Vmax/Km ratio for celecoxib methyl hydroxylation was reduced by 34% and 90% in the presence of recombinant CYP2C9*2 and CYP2C9*3, respectively. These data indicated that the amino acid substitution at position 359 (Ile to Leu) elicited a more pronounced effect on the metabolism of celecoxib than did a substitution at position 144 (Arg to Cys). The Vmax/Km ratio was also decreased in microsomes of livers genotyped CYP2C9*1/*2 (47% decrease, mean of two livers), or CYP2C9*1/*3 (59% decrease, one liver). In all cases, these changes were largely reflective of a decrease in Vmax, with a minimal change in Km. Based on simulations of the in-vitro data obtained with the recombinant CYP2C9 proteins, it was anticipated that the pharmacokinetics of celecoxib (as a much as a five-fold increase in plasma AUC) would be altered (versus CYP2C9*1/*1 subjects) in subjects genotyped heterozygous or homozygous for the CYP2C9*2 (Cys144) or CYP2C9*3 (Leu359) allele. In a subsequent clinical study, the AUC of celecoxib was increased (versus CYP2C9*1/*1 subjects) approximately 2.2-fold (range, 1.6-3-fold) in two CYP2C9*1/*3 subjects and one CYP2C9*3/*3 subject receiving a single oral dose (200 mg) of the drug. In contrast, there was no significant change in celecoxib AUC in two subjects genotyped CYP2C9*1/*2.

Phenotyping of drug-metabolizing enzymes in adults: a review of in-vivo cytochrome P450 phenotyping probes

Cytochrome P450 phenotyping provides valuable information about real-time activity of these important drug-metabolizing enzymes through the use of specific probe drugs. Despite more than 20 years of research, few conclusions regarding optimal phenotyping methods have been reached. Caffeine offers many advantages for CYP1A2 phenotyping, but the widely used caffeine urinary metabolic ratios may not be the optimal method of measuring CYP1A2 activity. Several probes of CYP2C9 activity have been suggested, but little information exists regarding their use, largely due to the narrow therapeutic index of most CYP2C9 probes. Mephenytoin has long been considered the standard CYP2C19 phenotyping probe, but problems such as sample stability and adverse effects have prompted the investigation of potential alternatives, such as omeprazole. Several well-validated CYP2D6 probes are available, including dextromethorphan, debrisoquin and sparteine, but, in most cases, dextromethorphan may be preferred due to its wide safety margin and availability. Chlorzoxazone remains the only CYP2E1 probe that has received much study. However, questions concerning phenotyping method and involvement of other enzymes have impaired its acceptance as a suitable CYP2E1 phenotyping probe. CYP3A phenotyping has been the subject of numerous investigations, reviews and commentaries. Nevertheless, much controversy regarding the selection of an ideal CYP3A probe remains. Of all the proposed methods, midazolam plasma clearance and the erythromycin breath test have been the most rigorously studied and appear to be the most reliable of the available methods. Despite the limitations of many currently available probes, with continued research, phenotyping will become an even more valuable research and clinical resource.