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

Human pharmacogenomic variations and their implications for antifungal efficacy

Pharmacogenomics is defined as the study of the impacts of heritable traits on pharmacology and toxicology. Candidate genes with potential pharmacogenomic importance include drug transporters involved in absorption and excretion, phase I enzymes (e.g., cytochrome P450-dependent mixed-function oxidases) and phase II enzymes (e.g., glucuronosyltransferases) contributing to metabolism, and those molecules (e.g., albumin, A1-acid glycoprotein, and lipoproteins) involved in the distribution of antifungal compounds. By using the tools of population genetics to define interindividual differences in drug absorption, distribution, metabolism, and excretion, pharmacogenomic models for genetic variations in antifungal pharmacokinetics can be derived. Pharmacogenomic factors may become especially important in the treatment of immunocompromised patients or those with persistent or refractory mycoses that cannot be explained by elevated MICs and where rational dosage optimization of the antifungal agent may be particularly critical. Pharmacogenomics has the potential to shift the paradigm of therapy and to improve the selection of antifungal compounds and adjustment of dosage based upon individual variations in drug absorption, metabolism, and excretion.

CYP2C9 inhibition: impact of probe selection and pharmacogenetics on in vitro inhibition profiles

Drug-drug interactions may cause serious adverse events in the clinical setting, and the cytochromes P450 are the enzyme system most often implicated in these interactions. Cytochrome P450 2C is the second most abundant subfamily of cytochrome P450 enzymes and is responsible for metabolism of almost 20% of currently marketed drugs. The most abundant isoform of this subfamily is CYP2C9, which is the major clearance pathway for the low therapeutic index drugs warfarin and phenytoin. Considering the importance of CYP2C9 to drug-drug interactions, the in vitro-in vivo extrapolation of drug-drug interactions for CYP2C9 may be confounded by the presence of polymorphic variants and the possibility of multiple binding regions within the CYP2C9 active site, leading to the potential for genotype- and substrate-dependent inhibition. To address the issues of genotype-dependent enzyme inhibition as well as probe substrate correlations, the inhibitory potency (Ki) of 28 effector molecules was assessed with five commonly used probes of CYP2C9 in both the CYP2C9.1 and CYP2C9.3 proteins. The inhibition of CYP2C9.1 and CYP2C9.3 by the battery of inhibitors with five substrate probes demonstrated differential inhibition potency not only between the two genotypes but also across substrate probes. Furthermore, the substrate probes fell into three distinct classes depending on genotype, suggesting that multiple probes may be needed to fully assess inhibition of CYP2C9 in vitro. Thus, both genotype and choice of probe substrate must be considered when attempting to predict potential CYP2C9 drug-drug interactions from in vitro data.

Antimicrobial-associated QT interval prolongation: pointes of interest

Until recently, cardiac toxicity manifesting in the form of arrhythmias related to QT interval prolongation was uncommonly appreciated within the antimicrobial class of drugs, but it was well described among antiarrhythmic agents. Antimicrobials that are associated with QT prolongation include the macrolides/ketolides, certain fluoroquinolones and antimalarials, pentamidine, and the azole antifungals. Although, in most cases, mild delays in ventricular repolarization caused by these drugs are clinically unnoticeable, they may serve to amplify the risk for torsades de pointes (TdP) when prescribed in the setting of other risk factors. Conditions or variables that influence proarrhythmic risk include sex, age, electrolyte derangements, structural heart disease, pharmacokinetic/pharmacodynamic interactions, and genetic predisposition. It is important that clinicians be knowledgeable about drugs with QT liability, as well as the risk factors that increase the probability of TdP. Additionally, because TdP remains a difficult-to-measure adverse event, we must rely upon multiple data sources to determine the risk versus the benefit for newly approved drugs.

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.

Allele variants of the cytochrome P450 2C9 genotype in white subjects from the netherlands with serious gastroduodenal ulcers attributable to the use of NSAIDs

BACKGROUND

The most common serious adverse effects (AEs) associated with NSAID therapy are bleeding and perforated gastroduodenal ulcers. These AEs are dose related, and reduced oral clearance of NSAIDs associated with polymorphisms of cytochrome P450 (CYP) would, theoretically, increase the risk for AEs.

OBJECTIVE

The purpose of this study was to determine whether polymorphisms of the CYP2C9 genotype are associated with the development of serious complications of NSAID-related ulcers.

METHODS

We examined the records of patients with complications of serious NSAID-related ulcers who were hospitalized from November 2001 to December 2003. Diagnosis was confirmed by endoscopy or abdominal surgery, and a group of consecutive subjects was identified for genetic analysis. CYP2C9 allele frequencies were determined and compared with those in a matched cohort of subjects receiving stable weekly maintenance doses of oral coumarin anticoagulants. Allele frequencies also were compared with those in matched cohorts from earlier studies.

RESULTS

All 26 subjects with serious NSAID-related ulcers were white, 15 (58%) were female, and the median age was 74.5 years (range, 32-96 years). All 87 subjects in the reference group were white, 24 (28%) were female, and the median age was 69 years (range, 48-81 years). CYP2C9 genotype frequencies did not differ significantly between subjects with serious complications of NSAID-related ulcers and subjects using oral coumarin anticoagulants. The genotype frequencies in both groups were similar to those reported in previous studies in white subjects.

CONCLUSION

The CYP2C9 genotype was not a significant or clinically relevant risk factor in the development of serious NSAID-related ulcers in this group of subjects.

Amiodarone analog-dependent effects on CYP2C9-mediated metabolism and kinetic profiles

CYP2C9 substrates can exhibit both hyperbolic and atypical kinetic profiles, and their metabolism can be activated or inhibited depending on the effector studied. CYP2C9 genetic variants can also affect both substrate turnover and kinetic profile. The present study assessed whether analogs of the effector amiodarone differentially altered the atypical kinetic profile of the substrate naproxen and whether this effect was genotype-dependent. Amiodarone, desethylamiodarone, benzbromarone, and its dimethyl analog (benz(meth)arone) were incubated with naproxen and either CYP2C9.1 or CYP2C9.3. Amiodarone activated naproxen demethylation at lower concentrations, regardless of the CYP2C9 allele, and inhibited metabolism at higher concentrations without altering the kinetic profile. Desethylamiodarone was a potent inhibitor of naproxen demethylation, irrespective of the CYP2C9 allele. Benzbromarone altered naproxen demethylation kinetics from a biphasic profile to that of a hyperbolic form in CYP2C9.1 and CYP2C9.3, resulting in inhibition and activation, respectively. In contrast, benz(meth)arone activated naproxen demethylation in both CYP2C9.1 and CYP2C9.3. In addition, the kinetic profile of naproxen demethylation became more hyperbolic at lower concentrations of benz(meth)arone and then reverted back to biphasic as the benz(meth)arone was increased further. Equilibrium binding and multiple-ligand docking studies were used to propose how such similar compounds exerted very different effects on naproxen metabolism. In summary, effectors of CYP2C9 metabolism can alter not only the degree of substrate turnover (activation or inhibition) but also the kinetic profile of metabolism of CYP2C9 substrates through effects on substrate binding and orientation. In addition, these kinetics effects are concentration- and genotype-dependent.

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.

Allele and genotype frequencies of CYP2C9 in a Korean population

AIMS

To determine the frequencies of the variant alleles and the genotypes of CYP2C9 in a Korean population.

METHODS

Three hundred and fifty-eight healthy Korean subjects were studied. CYP2C9 alleles were detected by polymerase chain reaction-restriction fragment length polymorphism assays and direct sequencing assays.

RESULTS

The allele frequencies were 0.934 for CYP2C9*1, 0.060 for CYP2C9*3 and 0.006 for CYP2C9*13. The CYP2C9*2,*4,*5 and *11 alleles were not detected. The frequencies of the CYP2C9*1/*1, *1/*3 and *1/*13 genotypes were 0.869, 0.120 and 0.011, respectively.

CONCLUSION

The frequency of the CYP2C9*3 allele in the Korean population studied was significantly higher than reported elsewhere, and a novel allele, CYP2C9*13, was found at a frequency of 0.006 (95% confidence interval 0, 0.012). Only three genotypes of CYP2C9, CYP2C9*1/*1,*1/*3 and *1/*13 were observed in this Korean population.

Individualizing analgesic prescription Part I: pharmacogenetics of opioid analgesics

The current use of analgesics is based on the empiric administration of a given drug with clinical monitoring for efficacy and toxicity. However, individual responses to drugs are influenced by a combination of pharmacokinetic and pharmacodynamic processes, and each of these components, in addition to pain perception and processing, seem to be regulated by genetic factors. Whereas polymorphic drug-metabolizing enzymes and drug transporters may affect the pharmacokinetics of drugs, polymorphic drug targets and disease-related pathways may influence the pharmacodynamic action of drugs. After usual dose, drug toxicity, as well as inefficacy, can be observed depending on the polymorphism, the analgesic considered and the presence or absence of active metabolites. Thus, cytochrome P450 (CYP)2D6 polymorphism influences codeine and tramadol analgesic effects, CYP2C9 has an impact on the disposition of some nonsteroidal anti-inflammatory drugs, and opioid receptor polymorphism (118A>G) may reduce morphine potency. Moreover, drug interaction mimics genetic deficiency and contributes to the variability in response to analgesics. This two-part review summarizes the available data on the pharmacokinetic-pharmacodynamic consequences of known polymorphisms of drug-metabolizing enzymes (CYP and uridine diphosphate glucuronosyltransferase), drug transporters (multidrug resistance proteins, multidrug resistance-associated proteins, organic anion-transporting polypeptides, and serotonin transporters), relevant drug targets (such as µ-opioid receptor, serotonin receptor and cyclooxygenases) and other nonopioid biological systems, on currently prescribed central and peripheral analgesics.

Pharmacogenetics of chronic cardiovascular drugs: applications and implications

Cardiovascular disease continues to be a tremendous worldwide problem, and drug therapy is a major modality to attenuate its burden. At present, conditions such as hypertension, dyslipidaemia and heart failure are pharmacologically managed with an empirical trial-and-error approach. However, it has been suggested that this approach fails to adequately address the therapeutic needs of many patients, and pharmacogenetics has been offered as a tool to enhance patient-specific drug therapy. This review outlines pharmacogenetic studies of common cardiovascular drugs, such as diuretics, beta-blockers, angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, statins and warfarin, ultimately highlighting considerations for future research and practice.

A case study of acenocoumarol sensitivity and genotype-phenotype discordancy explained by combinations of polymorphisms in VKORC1 and CYP2C9

AIMS

To determine the cause of a genotype-phenotype discordancy for acenocoumarol sensitivity. Methods A patient, highly sensitive to acenocoumarol, and previously determined to carry only a single CYP2C9*3 allele, was genotyped for additional functionally defective alleles in the CYP2C9 and VKORC1 genes. Family members were also analyzed to trace the pedigree. Results The acenocoumarol-sensitive patient was found to possess, in addition to CYP2C9*3 allele, a CYP2C9*11 allele and the VKORC1 AA diplotype which were all traced back through the parental lines. Conclusions Acenocoumarol sensitivity in this subject is the consequence of inheritance of multiple functionally defective alleles in both the CYP2C9 and VKORC1 genes. The study provides additional data in support of diminished CYP2C9 activity due to the presence of the rare *11 allele.

The effects of human CYP2C8 genotype and fluvoxamine on the pharmacokinetics of rosiglitazone in healthy subjects

AIMS

To determine the effect of CYP2C8 genotype and of fluvoxamine on the pharmacokinetics of rosiglitazone.

METHODS

Twenty-three healthy subjects with the following genotypes were included in a two-phase, open-label, cross-over trial: CYP2C8*3/ *3 (n = 3), CYP2C8*1/ *3 (n = 10) and CYP2C8*1/ *1 (n = 10). In Phase A, the subjects were given 4 mg rosiglitazone as a single oral dose. In Phase B, the subjects were treated with multiple oral doses of 50 mg fluvoxamine maleate for 3 days prior to the single oral administration of 4 mg rosiglitazone. Plasma concentrations of rosiglitazone and relative amounts of N-desmethylrosiglitazone were measured in both phases for 24 h after drug administration.

RESULTS

The pharmacokinetics of rosiglitazone and N-desmethylrosiglitazone were not significantly different between the CYP2C8 genotypic groups. Fluvoxamine caused a statistically significant (P = 0.0066) increase in the AUC(0-infinity) of rosiglitazone, with a geometric mean ratio of 1.21 [95% confidence interval (CI) 1.06-1.39]. The elimination half-life (t(1/2)) was also significantly higher (P = 0.0203) with a geometric mean ratio of 1.38 [95% CI 1.06-1.79]. The coadministration of fluvoxamine had no influence on the pharmacokinetics of N-desmethylrosiglitazone.

CONCLUSION

The importance of the CYP2C8*3 mutation in the in vivo metabolism of rosiglitazone could not be confirmed. Fluvoxamine increased the AUC(0-infinity) and t(1/2) of rosiglitazone moderately and hence may be a weak inhibitor of CYP2C8.

Paradoxical urinary phenytoin metabolite (S)/(R) ratios in CYP2C19*1/*2 patients

Phenytoin (PHT) is primarily metabolized to 5-(4'-hydroxyphenyl)-5-phenylhydantoin (p-HPPH), accounting for 67-88% of an administered dose in humans. p-HPPH is formed by the cytochrome (CYP) 450 enzymes CYP2C9 and CYP2C19, then glucuronidated and excreted into the urine. CYP2C9 catalyses the prochiral formation of (R) and (S)-p-HPPH, and is approximately 40 times more stereoselective towards the formation of the (S) isomer whereas CYP2C19 is not stereoselective. Because of differential stereoselectivity, polymorphisms in the genes can alter the (S)/(R)-p-HPPH ratios. Genotyping for CYP2C9 and CYP2C19 was accomplished by a Taqman based assay. Twelve and twenty-four hour urine samples were collected from 45 epilepsy patients taking PHT under steady-state conditions and (S)/(R) ratios of p-HPPH were determined by chiral HPLC separation. The mean urinary (S)/(R) ratio in the 12-24h urine collection in subjects homozygous for CYP2C9*1/*1, CYP2C19*1/*1 was 24.2+/-3.1(n=21), whereas ratios in CYP2C9*1/*2 and CYP2C9*1/*3 subjects, were 11.1+/-3.3(n=7) and 2.7+/-0.6(n=2), respectively. One CYP2C9*2/*3 patient had a ratio of 2.1. Unexpectedly, CYP2C9*1/*1, CYP2C19*1/*2 subjects had a mean (S)/(R) ratio as low as 12.9+/-1.7(n=12). Our results are generally consistent with single dose PHT studies. However, the (S)/(R)-p-HPPH ratios for the CYP2C9*1/*1, CYP2C19*1/*2 subjects, expected to be in the range of 30-40, were only 12.9, suggesting some undetected linkage disequilibrium between CYP2C9 and CYP2C19 genes that could affect PHT elimination. Furthermore, our study suggests that measurement of urine ratios cannot be used as a marker for genotype determination.

Four novel defective alleles and comprehensive haplotype analysis of CYP2C9 in Japanese

Genetic variations in cytochrome P450 2C9 (CYP2C9) are known to contribute to interindividual and interethnic variability in response to clinical drugs such as warfarin. In the present study, CYP2C9 from 263 Japanese subjects was resequenced, resulting in the discovery of 62 variations including 32 novel ones. In addition to the two known non-synonymous single nucleotide polymorphisms (SNPs), Ile359Leu (*3; allele frequency=0.030) and Leu90Pro (*13; 0.002), seven novel non-synonymous SNPs, Leu17Ile (0.002), Lys118ArgfsX9 (*25; 0.002), Thr130Arg (*26; 0.002), Arg150Leu (*27; 0.004), Gln214Leu (*28; 0.002), Pro279Thr (*29; 0.002) and Ala477Thr (*30; 0.002), were found. Functional characterization of novel alleles using a mammalian cell expression system in vitro revealed that *25 was a null allele and that *26, *28 and *30 were defective alleles. The *26 product showed a 90% decrease in the Vmax value but little change in the Km value towards diclofenac. Both *28 and *30 products showed two-fold higher Km values and three-fold lower Vmax values than the *1 allele, suggesting the importance of Gln214 and Ala477 for substrate recognition. Linkage disequilibrium and haplotype analyses were performed using the detected variations. Only five haplotypes (frequency >0.02) accounted for most (>87%) of the inferred haplotypes, and they were closely associated with the haplotypes of CYP2C19 in Japanese. Although the haplotype structure of CYP2C9 was rather simple in Japanese, the haplotype distribution was quite different from those previously reported in Caucasians and Africans. Taken together, novel defective alleles and detailed haplotype structures would be useful for determining metabolic phenotypes of CYP2C9 substrate drugs in Japanese and probably Asians.

The pharmacogenetics of coumarin therapy

Vitamin K antagonists (coumarins) are widely-used oral anticoagulants for the prevention of venous thromboembolism and strokes. Wide inter-individual variation in dose response and frequent bleeds characterize the initiation of coumarin therapy. Over the past 10 years both genetic and nongenetic determinants of coumarin dose response have been identified. A comprehensive pharmacogenetics approach to warfarin therapy has the potential to improve the safety and efficiency of warfarin initiation.

Lopinavir/ritonavir induces the hepatic activity of cytochrome P450 enzymes CYP2C9, CYP2C19, and CYP1A2 but inhibits the hepatic and intestinal activity of CYP3A as measured by a phenotyping drug cocktail in healthy volunteers

OBJECTIVE

The effect of lopinavir/ritonavir (LPV/r) administration on cytochrome P450 (CYP) enzyme activity was quantified using a phenotyping biomarker cocktail. Changes in CYP2C9, CYP2C19, CYP3A, CYP1A2, N-acetyltransferase-2 (NAT-2), and xanthine oxidase (XO) activities were evaluated using warfarin (WARF) + vitamin K, omeprazole (OMP), intravenous (IV) and oral (PO) midazolam (MDZ), and caffeine (CAF).

DESIGN

: Open-label, multiple-dose, pharmacokinetic study in healthy volunteers.

METHODS

Subjects (n = 14) simultaneously received PO WARF 10 mg, vitamin K 10 mg, OMP 40 mg, CAF 2 mg/kg, and IV MDZ 0.025 mg/kg on days (D) 1 and 14, and PO MDZ 5 mg on D2 and D15. LPV/r (400/100 mg twice daily) was administered on D4-17. CYP2C9 and CYP2C19 activities were quantified by S-WARF AUC0-inf and OMP/5-hydroxy OMP ratio, respectively. CYP1A2, NAT-2, and XO activities were quantified by urinary CAF metabolite ratios. Hepatic and intestinal + hepatic CYP3A activities were quantified by IV (CL) and PO (CL/F) MDZ clearance, respectively.

RESULTS

After LPV/r therapy, CYP2C9, CYP2C19, and CYP1A2 activity increased by 29%, 100%, and 43% (P = 0.001, 0.046, and 0.001), respectively. No changes were seen in NAT-2 or XO activity. Hepatic and intestinal + hepatic CYP3A activity decreased by 77% (P < 0.001) and 92% (P = 0.001), respectively.

CONCLUSION

LPV/r therapy results in modest induction of CYP1A2 and CYP2C9 and potent induction of CYP2C19 activity. Increasing doses of concomitant medications metabolized by these enzymes may be necessary. LPV/r inhibited intestinal CYP3A to a greater extent than hepatic CYP3A activity. Doses of concomitant CYP3A substrates should be reduced when combined with LPV/r, although intravenously administered compounds may require less of a relative dose reduction than orally administered compounds.

Effect of CYP2C9 genetic polymorphisms on the efficacy and pharmacokinetics of glimepiride in subjects with type 2 diabetes

Glimepiride, a sulfonylurea hypoglycemic agent, is metabolized by cytochrome P450 2C9 (CYP2C9) which is known to have genetic polymorphisms. To examine the effects of CYP2C9 genetic polymorphisms on the safety and efficacy of glimepiride in patients with type 2 diabetes, the responses to the glimepiride were measured in Japanese type 2 diabetic patients with the different CYP2C9 genotype. The reduction in the HbA(1c) was significantly larger (P<0.05) among the CYP2C9*1/*3 subjects than among the CYP2C9*1/*1 subjects. The long-term observations of 2 patients with a CYP2C9*1/*3 suggested that subjects with a CYP2C9*1/*3 respond well to glimepiride during the initial phase of treatment, but 1 patient have shown the weight gain over the long-term treatment. The pharmacokinetic study showed that the area under the concentration-time curve for glimepiride in the CYP2C9*1/*3 subjects was approximately 2.5-fold higher than that of the CYP2C9*1/*1 subjects. The intrinsic clearance of glimepiride by the CYP2C9*3 enzyme was lower than that by the CYP2C9*1 enzyme. These results suggested that the lower hydroxylation activity of glimepiride in the subject with type 2 diabetes and CYP2C9*1/*3 led to a marked elevation in the plasma concentrations of glimepiride and a stronger pharmacological effect of glimepiride.

Frequency of CYP2C9 polymorphisms affecting warfarin metabolism in a large anticoagulant clinic cohort

OBJECTIVES

To assess allele frequency and genotype distribution of CYP2C9 polymorphisms in patients (n = 189) attending an anticoagulant clinic in comparison to control patients (n = 177) and also to assess if the patients with variant genotypes require lower doses of warfarin.

METHODS

Genotyping of the common CYP2C9 variants *2 and *3 was carried out by multiplexed PCR-RFLP while the *5 and *6 allele variants were genotyped by singleton PCR-RFLP. DNA sequencing was used to confirm genotype in all specimens with *3, *4 and *6 alleles.

RESULTS

CYP2C9 allele frequencies in patients were 0.81 for *1, 0.11 for *2 and 0.08 for *3, compared to 0.88, 0.08 and 0.04, respectively, in controls. Patients with *1/*3 and *X/*X (where *X is *2 or *3) genotypes required 32 to 67% less warfarin in comparison to patients with the normal *1/*1 genotype. Other alleles were observed in less than 1% of subjects.

CONCLUSIONS

Allele frequencies and genotypes for CYP2C9*2 and *3 variants in patients on warfarin are not statistically different from controls whether or not they are stratified for ethnicity. Less common genotypes (*4, *5, *6) do not contribute significantly to warfarin sensitivity among patients attending a routine anticoagulation clinic. CYP2C9 genotype predicts warfarin dosage even in an uncontrolled, retrospective survey of unselected patients on warfarin therapy.

Association between human atrial natriuretic peptide Val7Met polymorphism and baseline blood pressure, plasma trough irbesartan concentrations, and the antihypertensive efficacy of irbesartan in rural Chinese patients with essential hypertension

BACKGROUND

Individual variations in the pharmacokinetics and pharmacodynamics of antihypertensive drugs are influenced by genetic and environmental factors. The ANP gene, which encodes the precursor of atrial natriuretic peptide (ANP), is among the candidate genes for genetic susceptibility to hypertension.

OBJECTIVE

This study examined the relationship between ANP Val7Met polymorphism (Single Nucleotide Polymorphism database ID: rs5063) and baseline blood pressure (BP), plasma trough irbesartan concentrations, and the antihypertensive efficacy of irbesartan in rural Chinese patients with essential hypertension.

METHODS

Patients with essential hypertension who had taken no antihypertensive medications within 4 weeks of study initiation received oral irbesartan 150 mg/d for 4 weeks. Genotyping was performed for all patients. BP was measured before dosing on the 1st and 28th days of treatment. Plasma irbesartan concentrations were measured using high-performance liquid chromatography with fluorescent detection. Antihypertensive efficacy was defined as attainment of a diastolic BP (DBP) <90 mm Hg (DBP analysis), a systolic BP (SBP) <140 mm Hg (SBP analysis), and a DBP <90 mm Hg and SBP <140 mm Hg (DBP and SBP analysis).

RESULTS

The study included 756 patients, 621 with the Val/Val genotype and 135 with the Val/Met+Met/Met genotypes. There were no significant differences in age, body mass index, sex, education level, occupation, alcohol consumption, or smoking status between the 2 groups. Patients with the Val/Met+Met/Met genotypes had a significantly lower mean baseline DBP compared with those with the Val/Val genotype (adjusted regression coefficient [SE]: -2.5 [1.0] mm Hg; P = 0.012) and significantly lower mean steady-state plasma trough irbesartan concentrations (adjusted regression coefficient: -12.6 [4.1]; P = 0.002). No significant association was found between antihypertensive efficacy and Val7Met polymorphism in the overall population, but in an analysis by baseline DBP status, patients with the Val/Met+Met/Met genotype a baseline DBP > or =100 mm Hg had significantly smaller reductions in DBP (adjusted regression coefficient: -5.7 [1.4] mm Hg; P < 0.001) and SBP compared with those with the Val/Val genotype and a baseline DBP > or =100 mm Hg (adjusted regression coefficient: -9.8 [2.9] mm Hg; P < 0.001).

CONCLUSION

The findings of this study suggest that in these rural Chinese patients with essential hypertension, ANP Val7Met polymorphism may be a genetic marker for baseline DBP, plasma irbesartan concentrations, and the antihypertensive efficacy of short-term irbesartan therapy.

Metabolism and transport of oxazaphosphorines and the clinical implications

The oxazaphosphorines including cyclophosphamide (CPA), ifosfamide (IFO), and trofosfamide represent an important group of therapeutic agents due to their substantial antitumor and immuno-modulating activity. CPA is widely used as an anticancer drug, an immunosuppressant, and for the mobilization of hematopoetic progenitor cells from the bone marrow into peripheral blood prior to bone marrow transplantation for aplastic anemia, leukemia, and other malignancies. New oxazaphosphorines derivatives have been developed in an attempt to improve selectivity and response with reduced toxicity. These derivatives include mafosfamide (NSC 345842), glufosfamide (D19575, beta-D-glucosylisophosphoramide mustard), NSC 612567 (aldophosphamide perhydrothiazine), and NSC 613060 (aldophosphamide thiazolidine). This review highlights the metabolism and transport of these oxazaphosphorines (mainly CPA and IFO, as these two oxazaphosphorine drugs are the most widely used alkylating agents) and the clinical implications. Both CPA and IFO are prodrugs that require activation by hepatic cytochrome P450 (CYP)-catalyzed 4-hydroxylation, yielding cytotoxic nitrogen mustards capable of reacting with DNA molecules to form crosslinks and lead to cell apoptosis and/or necrosis. Such prodrug activation can be enhanced within tumor cells by the CYP-based gene directed-enzyme prodrug therapy (GDEPT) approach. However, those newly synthesized oxazaphosphorine derivatives such as glufosfamide, NSC 612567 and NSC 613060, do not need hepatic activation. They are activated through other enzymatic and/or non-enzymatic pathways. For example, both NSC 612567 and NSC 613060 can be activated by plain phosphodiesterase (PDEs) in plasma and other tissues or by the high-affinity nuclear 3'-5' exonucleases associated with DNA polymerases, such as DNA polymerases and epsilon. The alternative CYP-catalyzed inactivation pathway by N-dechloroethylation generates the neurotoxic and nephrotoxic byproduct chloroacetaldehyde (CAA). Various aldehyde dehydrogenases (ALDHs) and glutathione S-transferases (GSTs) are involved in the detoxification of oxazaphosphorine metabolites. The metabolism of oxazaphosphorines is auto-inducible, with the activation of the orphan nuclear receptor pregnane X receptor (PXR) being the major mechanism. Oxazaphosphorine metabolism is affected by a number of factors associated with the drugs (e.g., dosage, route of administration, chirality, and drug combination) and patients (e.g., age, gender, renal and hepatic function). Several drug transporters, such as breast cancer resistance protein (BCRP), multidrug resistance associated proteins (MRP1, MRP2, and MRP4) are involved in the active uptake and efflux of parental oxazaphosphorines, their cytotoxic mustards and conjugates in hepatocytes and tumor cells. Oxazaphosphorine metabolism and transport have a major impact on pharmacokinetic variability, pharmacokinetic-pharmacodynamic relationship, toxicity, resistance, and drug interactions since the drug-metabolizing enzymes and drug transporters involved are key determinants of the pharmacokinetics and pharmacodynamics of oxazaphosphorines. A better understanding of the factors that affect the metabolism and transport of oxazaphosphorines is important for their optional use in cancer chemotherapy.

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.

Non-steroidal anti-inflammatory drugs for cancer prevention: promise, perils and pharmacogenetics

Aspirin and other non-steroidal anti-inflammatory drugs (NSAIDs) show indisputable promise as chemopreventive agents. Possible targets include cancers of the colon, stomach, breast and lung. However, recent studies raise concern about potential cardiovascular toxicity associated with the use of NSAIDs that specifically target the enzyme cyclooxygenase 2. These findings, and others that show that inherited genetic characteristics might determine preventive success, argue for new strategies that are tailored to individual medical history and genetic make-up.

Contribution of CYP2C9 to variability in vitamin K antagonist metabolism

CYP2C9 is the third most important cytochrome P450 (CYP) in terms of number of drugs metabolised. A considerable amount of information on this isoform is now available with respect to its structural biology, the mechanisms by which it can be induced and the existence of a range of variant alleles, which are often functionally significant. CYP2C9 makes a very important contribution to metabolism of vitamin K antagonist anticoagulants, and is the main oxidising enzyme for S-warfarin and S-acenocoumarol as well as contributing to phenprocoumon metabolism. A large number of studies have now shown that CYP2C9 genotype predicts dose requirement for both warfarin and acenocoumarol, with a possible contribution for phenprocoumon. Patients with variant alleles are likely to require a lower dose and may be at risk of overcoagulation and resultant bleeding, especially during the induction phase of therapy. Although CYP2C9 genotype is clearly a predictor of vitamin K antagonist dose requirement, especially in Caucasian populations in whom variant alleles are common, a number of recent studies have shown that age, genotype for the gene encoding the target gene vitamin K epoxide reductase and concomitant drugs are equally important factors in determining dose. There is a need for prospective studies to assess the value of predicting dose requirement on the basis of all these factors, including the CYP2C9 genotype.

Inhibitory Effect of 5-Fluorouracil on Cytochrome P450 2C9 Activity in Cancer Patients

Drug interactions have been reported between 5-fluorouracil and cytochrome P450 2C9 (CYP2C9) substrates, S-warfarin and phenytoin. This study was performed to determine the influence of 5-fluorouracil on cytochrome P450 2C9 (CYP2C9) activity in colorectal cancer patients (n=17) receiving 5-fluorouracil. Losartan was used as a marker to assess CYP2C9 activity. Losartan and its CYP2C9 dependent metabolite, E-3174, were determined in urine. The ratios of urinary losartan/E-3174 before and after the 5-fluorouracil treatment were compared for each patient. Genotyping was performed to detect the CYP2C9*2 and CYP2C9*3. At the end of the first cycle of 5-fluorouracil, losartan/E-3174 ratio was increased by 28.0% compared to the pre-treatment values (P=0.15). In five patients recruited for phenotyping after three 5-fluorouracil cycles, the metabolic ratio was increased significantly by 5.3 times (P=0.03). The results suggest that in most patients 5-fluorouracil inhibited CYP2C9 activity. This inhibition was more pronounced when the total administered dose increased. This finding may help explain the mechanism of interaction between 5-fluorouracil and CYP2C9 substrates.

Association of VKORC1 and CYP2C9 polymorphisms with warfarin dose requirements in Japanese patients

Warfarin is the most commonly used oral anticoagulant for treatment of thromboembolism, but adjustment of the dose appropriate to each patient is not so easy because of the large inter-individual variation in dose requirement. We analyzed single nucleotide polymorphism (SNP) genotypes of the VKORC1 and CYP2C9 genes using DNA from 828 Japanese patients treated with warfarin, and investigated association between SNP genotype and warfarin-maintenance dose. Five SNPs in VKORC1, 5' flanking-1413A > G, intron 1-136T > C, intron 2+124C > G, intron 2+837T > C and exon 3 343G > A, were in absolute linkage disequilibrium, and showed a significant association with daily warfarin dose of these patients. The median warfarin dose of patients with homozygosity for the minor allele was 4.0 mg/day, which is significantly higher than those heterozygous for the minor allele (3.5 mg/day) or those homozygous for the major allele (2.5 mg/day; P = 5.1 x 10(-11) in the case of intron 1-136T > C SNP). We then genotyped the CYP2C9 gene for the Japanese common genetic variant, CYP2C9*3 and, based on the genotype of these two genes, classified patients into three categories, which we call "warfarin-responsive index." The median warfarin daily dose varied significantly in this classification according to the warfarin-responsive index (2.0 mg/day for index 0 group, 2.5 mg/day for index 1 group, and 3.5 mg/day for index 2 group; P = 4.4 x 10(-13)). Thus, analysis of the combination of VKORC1 and CYP2C9 genotypes should identify warfarin-sensitive patients who require a lower dose of drug, allowing personalized warfarin treatment.

Interaction of warfarin with drugs, natural substances, and foods

Variability in the anticoagulant response to warfarin is an ongoing clinical dilemma. Fluctuations in dietary vitamin K are an important source of variance, and the need for constancy in vitamin K intake is routinely emphasized for warfarin-treated patients. Anticoagulant response is also influenced by a number of drugs that induce or inhibit warfarin metabolism, as well as by genetic polymorphisms that may modulate expression or activity of CYP2C9, the isoform mediating clearance of S-warfarin. The possible role of dietary factors other than vitamin K, as well as of herbal medicines or supplements as contributors to the instability of anticoagulation in warfarin-treated patients, has received recent attention. St. John's wort and possibly some ginseng formulations may have the potential to diminish warfarin anticoagulation, apparently by inducing CYP2C9 activity. Otherwise, there is no reliable evidence to indicate that any dietary component (other than vitamin K) or any herbal product has an effect on the anticoagulant response to warfarin. Scientific conclusions on this important therapeutic issue should be based on valid scientific data rather than unvalidated case reports.

Marked interindividual variability in the response to selective inhibitors of cyclooxygenase-2

BACKGROUND & AIMS

Variability in response to drugs may influence both efficacy and safety. Cyclooxygenase (COX)-2 inhibitors pose a cardiovascular risk by potentially increasing the likelihood of thrombosis, hypertension, and atherogenesis. Differences between individuals in the response to COX-2 inhibitors would be expected to influence their susceptibility to cardiovascular complications. We examined the variability in degree and selectivity of COX-2 inhibition in humans in response to celecoxib and rofecoxib.

METHODS

Fifty healthy volunteers received placebo, rofecoxib (25 mg), and celecoxib (200 mg), randomized by order. COX-1 and COX-2 inhibition was determined using ex vivo and in vivo indices of enzymatic activity. A subset of 5 individuals underwent 5 replicate studies to estimate variability in drug response both within and between subjects.

RESULTS

Despite the higher COX-2 selectivity of rofecoxib in vitro, the average selectivity attained by 25 mg rofecoxib and 200 mg celecoxib in vivo were not different. However, there was considerable variability at an individual level in the degree of COX-2 inhibition and selectivity attained by both drugs. Approximately one third of the variability was attributable to differences between individuals, suggesting the contribution of genetic sources of variance, such as candidate polymorphisms detected in COX-1 and CYP2C9.

CONCLUSIONS

The actual degree of selectivity for inhibition of COX-2 achieved by the coxibs relates both to chemical properties of the drug and to factors within an individual that modulate drug response. These sources of variability might be exploited to identify patients uniquely susceptible to benefit or at developing risk of cardiovascular complications.

A case of intolerance to warfarin dosing in an intermediate metabolizer of CYP2C9

We report a case of intolerance to warfarin dosing due to impaired drug metabolism in a patient heterozygous for the CYP2C9*3 allele. A 30-year-old woman with an artificial cardiac pacemaker was taking warfarin to prevent thromboembolism. This patient had an extremely elevated international normalized ratio (INR) of prothrombin time (PT) following standard doses of warfarin and experienced difficulties during the induction of anticoagulation. Genotyping for CYP2C9 revealed that this patient was an intermediate metabolizer with genotype CYP2C9*1/*3. This case suggests the clinical usefulness of pharmacogenetic testing for individualized dosage adjustments of warfarin.

Functional characterization of novel allelic variants of CYP2C9 recently discovered in southeast Asians

CYP2C9 was recently resequenced in 150 Asian subjects from Singapore. Several new coding variants were reported, and these variants are now named CYP2C9*14 (R125H), CYP2C9*15 (S162X), CYP2C9*16 (T299A), CYP2C9*17 (P382S), CYP2C9*18 (D397A), and CYP2C9*19 (Q454H). The CYP2C9*18 variant also contained an I359L change previously associated with the CYP2C9*3 allele. In this study, we assessed the functional consequences of the new coding changes. cDNAs containing each of the new coding changes were constructed by site-directed mutagenesis and expressed in a bacterial cDNA expression system, the allelic proteins were partially purified, and their ability to hydroxylate a prototype CYP2C9 substrate was assayed. Expression of cDNAs in Escherichia coli containing either the D397A change or the S162X (premature stop codon) could not be detected either spectrally or at the apoprotein level. CYP2C9.14 and CYP2C9.16 exhibited 80 to 90% lower catalytic activity toward tolbutamide at two substrate concentrations compared with wild-type CYP2C9.1. Kinetic analysis confirmed that CYP2C9.14 and CYP2C9.16 have a higher Km and a >90% lower intrinsic clearance of tolbutamide compared with wild-type CYP2C9.1. Both CYP2C9.17 and CYP2C9.19 proteins exhibited modest 30 to 40% decreases in catalytic activity toward tolbutamide. Thus, CYP2C9*15 and CYP2C9*18 may represent null alleles, whereas CYP2C9*14 and CYP2C9*16 allelic variants produce proteins that are clearly catalytically defective in vitro, indicating the existence of new defective putative alleles of CYP2C9 in Asians.

Genetics of Drug Resistance

Drug resistance in epilepsy affects about a third of patients and is an important clinical problem, associated with increased morbidity and mortality. It is important to consider carefully the definition of drug resistance. Recent interest in the field has focused on the potential molecular mechanisms underlying drug resistance. Environmental and seizure-related acquired causes are likely to contribute to the multifaceted basis of resistance in most cases. Genetic causes have attracted particular attention, partly because they may allow prediction of drug resistance and, potentially, rational treatment strategies. Gene mutations, however, are unlikely to cause many cases of drug resistance. However, common variation in genes probably will turn out to generate an important contribution to drug resistance phenomena. Associations between common variations in a number of genes and clinical drug resistance have now been published. However, to date, none of these associations has been unequivocally replicated by others to the extent that the original association has been accepted. Some of these associations are considered. Despite this apparently uninspiring record, the genetics of drug resistance are likely to prove productive in the near future, but their pursuit will require painstaking studies and multicenter collaboration.

CYP2C9, CYP2C19, ABCB1 (MDR1) genetic polymorphisms and phenytoin metabolism in a Black Beninese population

The genetically polymorphic cytochrome P450 2C9 (CYP2C9) metabolizes many important drugs. Among them, phenytoin has been used as a probe to determine CYP2C9 phenotype by measuring the urinary excretion of its major metabolite, S-enantiomer of 5-(4-hydroxyphenyl)-5-phenylhydantoin (p-HPPH). Phenytoin pharmacokinetic is also dependent on the activity of CYP2C19 and p-glycoprotein (ABCB1). To determine the influence of CYP2C9, CYP2C19 and ABCB1 genetic polymorphisms on phenytoin metabolism in a Black population, 109 healthy Beninese subjects received a single 300 mg oral dose of phenytoin. Blood was drawn 4 h after drug intake and urine was collected during the first 8 h. Plasma phenytoin and urine S- and R-enantiomers of p-HPPH were determined by high-performance liquid chromatography. Urinary excretion of (S)-p-HPPH [defined as urinary volumex(S)-p-HPPH urinary concentration] and PMR (defined as the ratio of p-HPPH in urine to 4 h phenytoin plasma concentration), both markers of CYP2C9 activity, were used to determine the functional relevance of new variants of CYP2C9 (*5, *6, *8, *9 and *11) in this population. Plasma phenytoin concentration was significantly associated with ABCB1 haplotype/genotype (P=0.05, Kruskal-Wallis test) and levels increased significantly in the genotype order: wild-type, T3421A and Block-2 genotypes (P=0.015, Jonckheere-Terpstra test). Urinary excretion of (S)-p-HPPH and PMR were significantly associated with the CYP2C9 genotype (P=0.001, analysis of variance (ANOVA) and P<0.0001, Kruskal-Wallis test, respectively) and decreased in the order: CYP2C9*1/*1, CYP2C9*1/*9, CYP2C9*9/*9, CYP2C9*1/*8, CYP2C9*8/*9, CYP2C9*9/*11, CYP2C9*1/*5, CYP2C9*6/*9, CYP2C9*1/*6, CYP2C9*8/*11, CYP2C9*5/*8 and CYP2C9*5/*6 (P<0.001, Jonckheere-Terpstra test). A combined analysis of CYP2C9, 2C19 and ABCB1 revealed that only ABCB1 predicted phenytoin concentration at 4 h and explained 8% of the variability (r=0.08, P=0.04). On the other hand, only CYP2C9 was predictive for the urinary excretion of (S)-p-HPPH and PMR (r=0.21, P=0.001 and r=0.25, P<0.001, respectively). Furthermore, significant relation was found between urinary excretion of (R)-p-HPPH and CYP2C9 genotype (P=0.035) and levels significantly increased in the genotype order: CYP2C9*1/*9, CYP2C9*1/*1, CYP2C9*9/*11, CYP2C9*1/*8 and CYP2C9*1/*5 (P<0.001, Jonckheere-Terpstra test). In summary, the present study demonstrates that, in a Black population, CYP2C9*5, *6, *8 and *11 variants, but not CYP2C9*9, are associated with a decreased phenytoin metabolism. The data also confirm the limited contribution of MDR1 gene to inter-individual phenytoin pharmacokinetic variation.

Association of Vitamin K epoxide reductase complex 1 (VKORC1) variants with warfarin dose in a Hong Kong Chinese patient population

OBJECTIVE

To evaluate the association of VKORC1 genetic variants with warfarin dose requirements in a Hong Kong Chinese patient population.

METHODS

A retrospective study of Hong Kong Chinese patients chronically maintained on warfarin was conducted. Single nucleotide polymorphisms (SNPs) in VKORC1 and CYP2C9 were genotyped. Stable warfarin dose data were retrieved from patient medical records.

RESULTS

Sixty-nine patients were included in the analysis. VKORC1 haplotypes H1 (group A) and H7 (group B) were most common, accounting for 86% and 13% of all haplotypic variation in this cohort. Patients carrying at least one copy of a VKORC1 group B haplotype (n = 16) required a significantly higher stable warfarin dose (5.17+/-1.53 mg/day) than patients that were homozygous for group A haplotypes (n = 53; 2.93+/-1.22 mg; P < 0.001). In the VKORC1 A/A group, four patients (5.8%) were heterozygous for CYP2C9*3 and had a lower dose requirement (1.94+/-0.43 mg) than patients that exhibited the CYP2C9 *1/*1 genotype (3.01+/-1.23 mg), P = 0.004. In multivariate analysis, VKORC1 and CYP2C9 explained 31% and 7.9% of the variability in warfarin dose, respectively.

CONCLUSIONS

VKORC1 genotype is the dominant genetic influence on inter-individual variability in warfarin dose in Hong Kong Chinese. The lower mean dose of warfarin in Chinese, relative to Europeans, appears to be a reflection of their preponderance of the 'low-dose' VKORC1 H1/H1 (homozygous group A) genotype.

Predicting drug response and toxicity based on gene polymorphisms

The sequencing of the human genome has allowed the identification of thousands of gene polymorphisms, most often single nucleotide polymorphims (SNP), which may play an important role in the expression level and activity of the corresponding proteins. When these polymorphisms occur at the level of drug metabolising enzymes or transporters, the disposition of the drug may be altered and, consequently, its efficacy may be compromised or its toxicity enhanced. Polymorphisms can also occur at the level of proteins directly involved in drug action, either when the protein is the target of the drug or when the protein is involved in the repair of drug-induced lesions. There again, these polymorphisms may lead to alterations in drug efficacy and/or toxicity. The identification of functional polymorphisms in patients undergoing chemotherapy may help the clinician prescribe the optimal drug combination or schedule and predict with more accuracy the response to these prescriptions. We have recorded in this review the polymorphisms that have been identified up till now in genes involved in anticancer drug activity. Some of them appear especially important in predicting drug toxicity and should be determined in routine before drug administration; this is the case of the most common variations of thiopurine methyltransferase for 6-mercaptopurine and of dihydropyrimidine dehydrogenase for fluorouracil. Other appear determinant for drug response, such as the common SNPs found in glutathione S-transferase P1 or xereoderma pigmentosum group D enzyme for the activity of oxaliplatin. However, confusion factors may exist between the role of gene polymorphisms in cancer risk or overall prognosis and their role in drug response.

CYP2C9 genotype-dependent effects on in vitro drug-drug interactions: switching of benzbromarone effect from inhibition to activation in the CYP2C9.3 variant

The CYP2C9.3 variant exhibits marked decreases in substrate turnover compared with the wild-type enzyme, but little is known regarding the effect this variant form may have on the occurrence of drug-drug interactions. To examine this possibility, the effect of the potent CYP2C9 inhibitor, benzbromarone, was studied with regard to CYP2C9.1- and CYP2C9.3-mediated flurbiprofen metabolism to evaluate whether the variant enzyme exhibits differential inhibition kinetics. Although benzbromarone inhibited CYP2C9.1 activity as expected, CYP2C9.3-mediated flurbiprofen 4'-hydroxylation was activated in the presence of benzbromarone. T1 relaxation studies revealed little change in distances of flurbiprofen protons from the heme iron of either CYP2C9.1 or CYP2C9.3 in the presence of benzbromarone compared with flurbiprofen alone. Spectral binding studies were also performed to investigate whether benzbromarone affected substrate binding, with the addition of benzbromarone having little effect on flurbiprofen-binding affinity in both CYP2C9.1 and CYP2C9.3. Docking studies with the 2C9.1 structure crystallized with a closed active site identified multiple but overlapping subsites with sufficient space for benzbromarone binding in the enzyme when flurbiprofen was positioned closest to the heme. If the closed conformation of 2C9.3 is structurally similar to 2C9.1, as expected for the conservative I359L mutation, then the dynamics of benzbromarone binding may account for the switching of drug interaction effects. In conclusion, the I359L amino acid substitution found in CYP2C9.3 not only reduces metabolism compared with CYP2C9.1 but can also dramatically alter inhibitor effects, suggesting that differential degrees of drug inhibition interactions may occur in individuals with this variant form of CYP2C9.

Pharmacogenetics in drug regulation: promise, potential and pitfalls

Pharmacogenetic factors operate at pharmacokinetic as well as pharmacodynamic levels-the two components of the dose-response curve of a drug. Polymorphisms in drug metabolizing enzymes, transporters and/or pharmacological targets of drugs may profoundly influence the dose-response relationship between individuals. For some drugs, although retrospective data from case studies suggests that these polymorphisms are frequently associated with adverse drug reactions or failure of efficacy, the clinical utility of such data remains unproven. There is, therefore, an urgent need for prospective data to determine whether pre-treatment genotyping can improve therapy. Various regulatory guidelines already recommend exploration of the role of genetic factors when investigating a drug for its pharmacokinetics, pharmacodynamics, dose-response relationship and drug interaction potential. Arising from the global heterogeneity in the frequency of variant alleles, regulatory guidelines also require the sponsors to provide additional information, usually pharmacogenetic bridging data, to determine whether data from one ethnic population can be extrapolated to another. At present, sponsors explore pharmacogenetic influences in early clinical pharmacokinetic studies but rarely do they carry the findings forward when designing dose-response studies or pivotal studies. When appropriate, regulatory authorities include genotype-specific recommendations in the prescribing information. Sometimes, this may include the need to adjust a dose in some genotypes under specific circumstances. Detailed references to pharmacogenetics in prescribing information and pharmacogenetically based prescribing in routine therapeutics will require robust prospective data from well-designed studies. With greater integration of pharmacogenetics in drug development, regulatory authorities expect to receive more detailed genetic data. This is likely to complicate the drug evaluation process as well as result in complex prescribing information. Genotype-specific dosing regimens will have to be more precise and marketing strategies more prudent. However, not all variations in drug responses are related to pharmacogenetic polymorphisms. Drug response can be modulated by a number of non-genetic factors, especially co-medications and presence of concurrent diseases. Inappropriate prescribing frequently compounds the complexity introduced by these two important non-genetic factors. Unless prescribers adhere to the prescribing information, much of the benefits of pharmacogenetics will be squandered. Discovering highly predictive genotype-phenotype associations during drug development and demonstrating their clinical validity and utility in well-designed prospective clinical trials will no doubt better define the role of pharmacogenetics in future clinical practice. In the meantime, prescribing should comply with the information provided while pharmacogenetic research is deservedly supported by all concerned but without unrealistic expectations.

Simultaneous genotyping of CYP2C9*2, *3, and 5′ flanking region (C-1189T) polymorphisms in a Spanish population through a new minisequencing multiplex single-base extension analysis

OBJECTIVE

To conduct a descriptive study on the prevalence of relevant cytochrome P450 2C9 (CYP2C9) polymorphisms--the *2, *3, and 5' flanking region (C-1189T)--in a Spanish population using a new minisequencing fluorescent method through a multiplex single base extension (SBE) analysis.

METHOD

The method simultaneously and accurately genotypes the CYP2C9 polymorphisms studied and is available as a commercial protocol (SNaPshot). Various strategies, including restriction fragment length polymorphism (RFLP) and Taqman, were used to validate the methodology.

RESULTS

The frequencies of alleles CYP2C9*2 (12%) and *3 (6.2%) were similar to those described for other Caucasian populations. The frequency of allele t at the 5' flanking region was 62%, which is close to the percentage reported in Japanese and French populations. The four haplotypes inferred in our samples and their frequencies were consistent with those reported in other studies.

CONCLUSION

Our results confirm previously reported Caucasian frequencies for the CYP2C9*2 and *3 alleles and, for the first time, provide data on the frequency of the CYP2C9 5' flanking region (C-1189T), a recently described polymorphism, in a Spanish population. The SBE technique detects unequivocally the three polymorphisms in a single reaction, which makes it suitable for the analysis of CYP2C9 in the many therapeutic situations in which it is involved.

Factors Affecting the Maintenance Stable Warfarin Dosage in Hong Kong Chinese Patients

BACKGROUND

Multiple factors can affect the anticoagulation effect of warfarin. The objective of this study was to determine the relationship between different clinical factors and outcomes of warfarin therapy in Hong Kong Chinese patients.

METHODS

The study was conducted at the anticoagulation clinic of the Prince of Wales Hospital from 1 April to 31 December 2003. Clinical data collected included demographics, indications of warfarin, dietary vitamin K consumption, and drug-drug interactions. Blood samples were obtained for the genetic polymorphism analysis of CYP 2 C 9. Linear and multiple regression analysis were used for statistical analysis to determine the correlation between variables and the importance of various factors as the determinants of warfarin dosage requirement.

RESULTS

A total of 63 patients were recruited. The mean warfarin dosage was 3.30+/-2.23 mg/day. The warfarin dosage ranged from 0.75 to 12 mg/day. The mean age was 59+/-14 years old. Age, dietary vitamin K consumption, chronic heart failure, atrial fibrillation, hypertension, smoking and drinking status were found to be factors statistically significant affecting warfarin dosage. We detected no single nucleotide polymorphism in CYP 2 C 9 exon 4.

CONCLUSION

Age, dietary vitamin K consumption, warfarin indication for atrial fibrillation, co-morbid with CHF, smoking and drinking status were found to be the factors that affected the warfarin requirement in Hong Kong Chinese patients. However, the genetic polymorphism in exon 4 of CYP 2 C 9 may not be associated with the warfarin sensitivity in this patient population.

Genetic polymorphism of CYP2C9 in a Vietnamese Kinh population

Cytochrome P450 2C9 (CYP2C9) shows genetic polymorphism with high interethnic variation, but no report has addressed the genetic polymorphism in the Vietnamese population. In the present study, the distribution of 2 common allelic variations of CYP2C9 was investigated in Vietnamese Kinh population, a major ethnic group in Vietnam. Genomic DNA from 157 Vietnamese subjects was amplified by polymerase chain reaction, and the presence of CYP2C9*2 and CYP2C9*3 allelic variants was determined by pyrosequencing. Among 157 Vietnamese subjects, no subject with the CYP2C9*2 allele was detected, but 7 subjects were heterozygous for the CYP2C9*3 allele. The allele frequency of CYP2C9*3 was 2.2% in the Vietnamese Kinh population. This genotype distribution was well correlated with previous reports suggesting no occurrence of CYP2C9*2 in Asians. These results suggest that CYP2C9*2 may be absent in Vietnamese Kinh population and that CYP2C9*3 is major allelic variant that causes interindividual variation of drug responses to CYP2C9 substrate drugs in the Vietnamese Kinh population.

CYP2C9 gene variants, drug dose, and bleeding risk in warfarin-treated patients: a HuGEnet systematic review and meta-analysis

PURPOSE

Two common variant alleles of the cytochrome CYP2C9 (CYP2C9*2 and CYP2C9*3) lead to reduced warfarin metabolism in vitro and in vivo. The study objective was to examine the strength and quality of existing evidence about CYP2C9 gene variants and clinical outcomes in warfarin-treated patients.

METHODS

The study was a systematic review and meta-analysis. Multiple electronic databases were searched, references identified from bibliographies were sought, and experts and authors of primary studies were also contacted. Strict review inclusion criteria were determined. Three reviewers independently extracted data using prepiloted proformas.

RESULTS

In all, 11 studies meeting review inclusion criteria were identified (3029 patients). Nine were included in the meta-analyses (2775 patients). Random effects meta-analyses were performed; statistical heterogeneity and inconsistency was assessed. Twenty percent of patients studied carry a variant allele: CYP2C9*2 12.2% (9.7%-15.0%) and CYP2C9*3, 7.9% (6.5%-9.7%). Mean difference in daily warfarin dose: for CYP2C9*2, the reduction was 0.85 mg (0.60-1.11 mg), a 17% reduction. For CYP2C9*3, the reduction was 1.92 mg (1.37-2.47 mg), a 37% reduction. For CYP2C9*2 or *3, the reduction was 1.47 mg (1.24-1.71 mg), a 27% reduction. The relative bleeding risk for CYP2C9*2 was 1.91 (1.16-3.17) and for CYP2C9*3 1.77 (1.07-2.91). For either variant, the relative risk was 2.26 (1.36-3.75).

CONCLUSIONS

Patients with CYP2C9*2 and CYP2C9*3 alleles have lower mean daily warfarin doses and a greater risk of bleeding. Testing for gene variants could potentially alter clinical management in patients commencing warfarin. Evidence for the clinical utility and cost-effectiveness of genotyping is needed before routine testing can be recommended.

Drug bioactivation, covalent binding to target proteins and toxicity relevance

A number of therapeutic drugs with different structures and mechanisms of action have been reported to undergo metabolic activation by Phase I or Phase II drug-metabolizing enzymes. The bioactivation gives rise to reactive metabolites/intermediates, which readily confer covalent binding to various target proteins by nucleophilic substitution and/or Schiff's base mechanism. These drugs include analgesics (e.g., acetaminophen), antibacterial agents (e.g., sulfonamides and macrolide antibiotics), anticancer drugs (e.g., irinotecan), antiepileptic drugs (e.g., carbamazepine), anti-HIV agents (e.g., ritonavir), antipsychotics (e.g., clozapine), cardiovascular drugs (e.g., procainamide and hydralazine), immunosupressants (e.g., cyclosporine A), inhalational anesthetics (e.g., halothane), nonsteroidal anti-inflammatory drugs (NSAIDSs) (e.g., diclofenac), and steroids and their receptor modulators (e.g., estrogens and tamoxifen). Some herbal and dietary constituents are also bioactivated to reactive metabolites capable of binding covalently and inactivating cytochrome P450s (CYPs). A number of important target proteins of drugs have been identified by mass spectrometric techniques and proteomic approaches. The covalent binding and formation of drug-protein adducts are generally considered to be related to drug toxicity, and selective protein covalent binding by drug metabolites may lead to selective organ toxicity. However, the mechanisms involved in the protein adduct-induced toxicity are largely undefined, although it has been suggested that drug-protein adducts may cause toxicity either through impairing physiological functions of the modified proteins or through immune-mediated mechanisms. In addition, mechanism-based inhibition of CYPs may result in toxic drug-drug interactions. The clinical consequences of drug bioactivation and covalent binding to proteins are unpredictable, depending on many factors that are associated with the administered drugs and patients. Further studies using proteomic and genomic approaches with high throughput capacity are needed to identify the protein targets of reactive drug metabolites, and to elucidate the structure-activity relationships of drug's covalent binding to proteins and their clinical outcomes.

Role of CYP2C9 and its variants (CYP2C9*3 and CYP2C9*13) in the metabolism of lornoxicam in humans

CYP2C9 is an important member of the cytochrome P450 enzyme superfamily with some 12 CYP2C9 alleles (*1-*12) being previously reported. Recently, we identified a new CYP2C9 allele with a Leu90Pro mutation in a Chinese poor metabolizer of lornoxicam [Si D, Guo Y, Zhang Y, Yang L, Zhou H, and Zhong D (2004) Pharmacogenetics 14:465-469]. The new allele, designated CYP2C9*13, was found to occur in approximately 2% of the Chinese population. To examine enzymatic activity of the CYP2C9*13 allele, kinetic parameters for lornoxicam 5'-hydroxylation were determined in COS-7 cells transiently transfected with pcDNA3.1 plasmids carrying wild-type CYP2C9*1, variant CYP2C9*3, and CYP2C9*13 cDNA. The protein levels of cDNA-expressed CYP2C9*3 and *13 in postmitochondrial supernatant (S9) from transfected cells were lower than those from wild-type CYP2C9*1. Mean values of Km and Vmax for CYP2C9*1, *3, and *13 were 1.24, 1.61, and 2.79 microM and 0.83, 0.28, and 0.22 pmol/min/pmol, respectively. Intrinsic clearance values (Vmax/Km) for variant CYP2C9*3 and CYP2C9*13 on the basis of CYP2C9 protein levels were separately decreased to 28% and 12% compared with wild type. In a subsequent clinical study, the AUC of lornoxicam was increased by 1.9-fold and its oral clearance (CL/F) decreased by 44% in three CYP2C9*1/*13 subjects, compared with CYP2C9*1/*1 individuals. This suggests that the CYP2C9*13 allele is associated with decreased enzymatic activity both in vitro and in vivo.

The effect of the cytochrome P450 CYP2C8 polymorphism on the disposition of (R)-ibuprofen enantiomer in healthy subjects

AIMS

To study the effect of CYP2C8*3, the most common CYP2C8 variant allele on the dis-position of (R)-ibuprofen and the association of CYP2C8*3 with variant CYP2C9 alleles.

METHODS

Three hundred and fifty-five randomly selected Spanish Caucasians were screened for the common CYP2C8 and CYP2C9 mutations. The pharmacokinetics of (R)-ibuprofen were studied in 25 individuals grouped into different CYP2C8 genotypes.

RESULTS

The allele frequency of CYP2C8*3 (0.17) was found to be higher than that reported for other Caucasian populations (P = 0.0001). The frequencies of CYP2C9*2 and CYP2C9*3 were 0.19 (0.16-0.21) and 0.10 (0.08-0.12), respectively. An association between CYP2C8*3 and CYP2C9*2 alleles was observed, occurring together at a frequency 2.4-fold higher than expected for a random association of alleles (P = 0.0001). The presence of the CYP2C8*3 allele was found to influence the pharmacokinetics of (R)-ibuprofen in a gene-dose effect manner. Thus, after administration of 400 mg ibuprofen, the plasma half-life (95% confidence intervals) for individuals with genotypes CYP2C8*1/*1, CYP2C8*1/*3 and CYP2C8*3/*3, was 2.0 h (1.8-2.2), 4.2 h (1.9-6.5; P < 0.05) and 9.0 h (7.8-10.2; P < 0.002), respectively. A statistically significant trend with respect to the number of variant CYP2C8*3 alleles was also observed for the area under the concentration-time curve (P < 0.025), and drug clearance (P < 0.03).

CONCLUSION

Polymorphism of the CYP2C8 gene was found to be common, with nearly 30% of the population studied carrying the variant CYP2C8*3 allele. The presence of the latter caused a significant effect on the disposition of (R)-ibuprofen. This suggests that a substantial proportion of Caucasian subjects may show alterations in the disposition of drugs that are CYP2C8 substrates.

Clinical and toxicological relevance of CYP2C9: drug-drug interactions and pharmacogenetics

CYP2C9 is a major cytochrome P450 enzyme that is involved in the metabolic clearance of a wide variety of therapeutic agents, including nonsteroidal antiinflammatories, oral anticoagulants, and oral hypoglycemics. Disruption of CYP2C9 activity by metabolic inhibition or pharmacogenetic variability underlies many of the adverse drug reactions that are associated with the enzyme. CYP2C9 is also the first human P450 to be crystallized, and the structural basis for its substrate and inhibitor selectivity is becoming increasingly clear. New, ultrapotent inhibitors of CYP2C9 have been synthesised that aid in the development of quantitative structure-activity relationship (QSAR) models to facilitate drug redesign, and extensive resequencing of the gene and studies of its regulation will undoubtedly help us understand interindividual variability in drug response and toxicity controlled by this enzyme.