Frequency of cytochrome P450 CYP2C9 variants in a Turkish population and functional relevance for phenytoin

Analysis of diclofenac and its metabolites by high-performance liquid chromatography: relevance of CYP2C9 genotypes in diclofenac urinary metabolic ratios

In humans, diclofenac is metabolised to 4'-hydroxy (OH), 3'-OH and 5-OH metabolites. The polymorphic CYP2C9 is involved in the metabolism of diclofenac to 4'-OH diclofenac and 3'-OH diclofenac. The aim of the present study was to develop a high-performance liquid chromatographic method to simultaneously measure diclofenac and its metabolites in urine, suitable for metabolic studies. After liquid-liquid extraction the compounds were separated in a reversed-phase column and measured by ultraviolet absorption at 282 nm. For all compounds intra-day and inter-day variations were less than 7%, and the limits of quantitation were 0.25 mg/l. No analytical interference with endogenous compounds was found. The relationship between diclofenac metabolic ratios among different CYP2C9 genotypes is reported. The CYP2C9*3/*3 subject had the highest diclofenac/4'-OH ratios. However no difference was found between CYP2C9*2/*2 and *1/*1 genotypes. The chromatographic method developed was sensitive and reliable for the measurement of diclofenac and its metabolites simultaneously in human urine, and is suitable for use in diclofenac metabolism studies.

Influence of age and cytochrome P450 2C9 genotype on the steady-state disposition of diclofenac and celecoxib

OBJECTIVE

To analyse the influence of age and cytochrome P450 (CYP) 2C9 genotype on the steady-state disposition of the standard NSAID diclofenac and the new COX-2 selective inhibitor celecoxib, both of which are metabolised by the polymorphically expressed CYP2C9.

DESIGN

Double-blind randomised crossover study under steady-state conditions.

SUBJECTS

12 young (age 32 +/- 5 years, bodyweight 71 +/- 12kg; mean +/- SD) and 12 elderly (68 +/- 2 years, 82 +/- 15kg) healthy, drug-free, nonsmoking Caucasians of both sexes.

METHODS

All subjects received oral celecoxib (200mg twice daily) and diclofenac (75mg twice daily) for 15 days separated by a drug-free interval of at least 3 weeks. Following the last morning dose, multiple blood samples were taken for 25 hours. Concentrations of celecoxib and diclofenac were measured by specific and sensitive high performance liquid chromatography. Identification of CYP2C9 genotype was performed by genomic DNA sequencing. Pharmacokinetic parameters for total and unbound drugs were individually analysed by noncompartmental techniques.

RESULTS

For diclofenac, area under the concentration-time curve over the dosage interval (AUC(tau)) was larger in young subjects (3.2 +/- 1.0 mg * h/L) than in older individuals (2.4 +/- 0.4 mg * h/L; p < 0.05). As the terminal half-life (t((1/2)Z)) was very similar in both groups (3.9 +/- 4.4 vs 3.5 +/- 3.3 hours), either less complete absorption in the elderly or their higher bodyweight could account for the difference. For celecoxib, AUC(tau) (5.8 +/- 1.7 vs 5.6 +/- 2.3 mg * h/L) and t((1/2)z) (11.8 +/- 8.7 vs 11.2 +/- 2.9 hours) were almost identical in young and older subjects. Plasma protein binding of both NSAIDs was unaffected by age, and apparent oral clearances for unbound drugs were not different between the two groups of healthy subjects. When considering the genotype of all individuals (CYP2C9*1/*1, n = 10; CYP2C9*1/*2, n = 6; CYP2C9*2/*2, n = 2; CYP2C9*1/*3, n = 4; CYP2C9*3/*3, n = 1), no association with any pharmacokinetic parameter of either drug was apparent. Moreover, there was no significant correlation between the AUC values of celecoxib and diclofenac.

CONCLUSIONS

Age and CYP2C9 genotype do not significantly affect the steady-state disposition of celecoxib and diclofenac. This would indicate that both drugs need no dosage reduction in the elderly (at least up to 75 years) and that, besides CYP2C9, additional CYP species contribute to the elimination of both agents.

Implications of pharmacogenetics for individualizing drug treatment and for study design

Adverse drug reactions and ineffective drug treatment are responsible for a large health care burden. Considerable variability in drug response makes the prediction of the individual reaction difficult. Pharmacogenetics can help to individualize drug treatment in accordance with the genetic make-up of the patient. Drug response is best understood as a complex interplay between pharmacokinetics, pharmacodynamics, and other disease-associated factors. There are a large number of genetic variants in the enzymes of phase I and phase II drug metabolism, in drug transporters, and drug targets, all of which account for differences in drug response. The polymorphisms in the cytochrome P450 enzyme system have been investigated most extensively. Genotype-based dose adjustment which should ensure "bioequivalent" drug concentrations in all patients has been derived from pharmacokinetic parameters, but this approach will have to be verified in prospective studies. Drug transport has recently been recognized as a further crucial determinant in pharmacokinetics. The effect of genetics on disease susceptibility and drug treatment has been studied quite extensively; however, hardly any of this progress is at present reflected in routine health care. The integration of pharmacogenetic factors in clinical trials requires novel considerations for study design and data interpretation. It is to be hoped that the new science bioinformatics will (a) help us identify the contribution of genetics to disease and treatment response and will (b) create data-processing devices which help the physician in the face of the enormously expanding scientific knowledge in selecting the best individually adapted treatment for the patient.

Pharmacokinetics of diclofenac and inhibition of cyclooxygenases 1 and 2: no relationship to the CYP2C9 genetic polymorphism in humans

AIMS

The cytochrome P450 enzyme CYP2C9 catalyses the 4'-hydroxylation of the nonsteroidal analgesic drug diclofenac in humans. We studied the influences of the known amino acid variants, CYP2C9*2 (Arg144Cys) and CYP2C9*3 (Ile359Leu), on diclofenac pharmacokinetics after a 50-mg oral dose of diclofenac in healthy volunteers. As a surrogate marker of diclofenac activity, the ex vivo formation of prostaglandin E2 and thromboxane B2, which reflects COX-2 and COX-1 activity, was measured.

METHODS

Genotyping was performed in 516 healthy volunteers to obtain 20 participants with all allelic combinations of the two CYP2C9 variants Arg144Cys (*2) and Ile359Leu (*3). Diclofenac and 4'-hydroxydiclofenac were quantified in plasma by reversed phase h.p.l.c. after oral intake of 50 mg diclofenac. Concentrations of thromboxane B2 (TxB2) and prostaglandin E2 (PGE2) were measured by immunoassays.

RESULTS

There was no evidence of impaired metabolism of oral diclofenac in heterozygous and homozygous carriers of the CYP2C9 alleles *2 and *3 compared with the wild type (mean CL/F (95% CI) 20.5 (11, 30) l h-1 for *1/*1, 29.9 (19, 40) l h-1 for *1/*2, 30.0 (4, 56) l h-1 for *2/*2, 22.6 (12, 33) l h-1 for *1/*3, 23.5 (11, 37) l h-1 for *3/*3 and 37.3 (-15, 89) l h-1 in *2/*3). Furthermore, plasma concentrations of the metabolite 4'-hydroxydiclofenac were not lower in carriers of the CYP2C9 low-activity alleles *2 and *3 compared with carriers of the CYP2C9*1/*1 genotype. Marked diclofenac mediated inhibition of COX-1- and COX-2 activity was detected in all individuals independent of CYP2C9 genotype.

CONCLUSIONS

Polymorphisms of the CYP2C9 gene had no discernible effect on the pharmacokinetics and pharmacodynamics of diclofenac. The question of whether enzymes other than CYP2C9 play a major role in diclofenac 4'-hydroxylation in vivo or whether 4'-hydroxylation is not a rate-limiting step in diclofenac elimination in vivo, or whether the effect of the CYP2C9 polymorphisms is substrate-dependent, needs further investigation.

Phenytoin pharmacokinetics and clinical effects in African children following fosphenytoin and chloramphenicol coadministration

AIMS

Some children with malaria and convulsions also have concurrent bacterial meningitis. Chloramphenicol is used to treat the latter whereas phenytoin is used for convulsions. Since chloramphenicol inhibits the metabolism of phenytoin in vivo, we studied the effects of chloramphenicol on phenytoin pharmacokinetics in children with malaria.

METHODS

Multiple intravenous (i.v.) doses of chloramphenicol succinate (CAP) (25 mg kg-1 6 hourly for 72 h) and a single intramuscular (i.m.) seizure prophylactic dose of fosphenytoin (18 mg kg-1 phenytoin sodium equivalents) were concomitantly administered to 15 African children with malaria. Control children (n = 13) with malaria received a similar dose of fosphenytoin and multiple i.v. doses (25 mg kg-1 8 hourly for 72 h) of cefotaxime (CEF). Blood pressure, heart rate, respiratory rate, oxygen saturation, level of consciousness and convulsion episodes were monitored. Cerebrospinal fluid (CSF) and plasma phenytoin concentrations were determined.

RESULTS

The area under the plasma unbound phenytoin concentration-time curve (AUC(0, infinity ); means (CAP, CEF): 58.5, 47.6 micro g ml-1 h; 95% CI for difference between means: -35.0, 11.4), the peak unbound phenytoin concentrations (Cmax; medians: 1.12, 1.29 micro g ml-1; 95% CI: -0.5, 0.04), the times to Cmax (tmax; medians: 4.0, 4.0 h; 95% CI: -2.0, 3.7), the CSF:plasma phenytoin ratios (means: 0.21, 0.22; 95% CI: -0.8, 0.10), the fraction of phenytoin unbound (means: 0.06, 0.09; 95% CI: -0.01, 0.07) and the cardiovascular parameters were not significantly different between CAP and CEF groups. However, mean terminal elimination half-life (t1/2,z) was significantly longer (23.7, 15.5 h; 95% CI: 1.71, 14.98) in the CAP group compared with the CEF group. Seventy per cent of the children had no convulsions during the study period.

CONCLUSIONS

Concomitant administration of chloramphenicol and a single i.m. dose of fosphenytoin alters the t1/2,z but not the other pharmacokinetic parameters or clinical effects of phenytoin in African children with severe malaria. Moreover, a single i.m. dose of fosphenytoin provides anticonvulsant prophylaxis in the majority of the children over 72 h. However, a larger study would be needed to investigate the effect of concomitant administration of multiple doses of the two drugs in this population of patients.

Clinical significance of the cytochrome P450 2C19 genetic polymorphism

Cytochrome P450 2C19 (CYP2C19) is the main (or partial) cause for large differences in the pharmacokinetics of a number of clinically important drugs. On the basis of their ability to metabolise (S)-mephenytoin or other CYP2C19 substrates, individuals can be classified as extensive metabolisers (EMs) or poor metabolisers (PMs). Eight variant alleles (CYP2C19*2 to CYP2C19*8) that predict PMs have been identified. The distribution of EM and PM genotypes and phenotypes shows wide interethnic differences. Nongenetic factors such as enzyme inhibition and induction, old age and liver cirrhosis can also modulate CYP2C19 activity. In EMs, approximately 80% of doses of the proton pump inhibitors (PPIs) omeprazole, lansoprazole and pantoprazole seem to be cleared by CYP2C19, whereas CYP3A is more important in PMs. Five-fold higher exposure to these drugs is observed in PMs than in EMs of CYP2C19, and further increases occur during inhibition of CYP3A-catalysed alternative metabolic pathways in PMs. As a result, PMs of CYP2C19 experience more effective acid suppression and better healing of duodenal and gastric ulcers during treatment with omeprazole and lansoprazole compared with EMs. The pharmacoeconomic value of CYP2C19 genotyping remains unclear. Our calculations suggest that genotyping for CYP2C19 could save approximately 5000 US dollars for every 100 Asians tested, but none for Caucasian patients. Nevertheless, genotyping for the common alleles of CYP2C19 before initiating PPIs for the treatment of reflux disease and H. pylori infection is a cost effective tool to determine appropriate duration of treatment and dosage regimens. Altered CYP2C19 activity does not seem to increase the risk for adverse drug reactions/interactions of PPIs. Phenytoin plasma concentrations and toxicity have been shown to increase in patients taking inhibitors of CYP2C19 or who have variant alleles and, because of its narrow therapeutic range, genotyping of CYP2C19 in addition to CYP2C9 may be needed to optimise the dosage of phenytoin. Increased risk of toxicity of tricyclic antidepressants is likely in patients whose CYP2C19 and/or CYP2D6 activities are diminished. CYP2C19 is a major enzyme in proguanil activation to cycloguanil, but there are no clinical data that suggest that PMs of CYP2C19 are at a greater risk for failure of malaria prophylaxis or treatment. Diazepam clearance is clearly diminished in PMs or when inhibitors of CYP2C19 are coprescribed, but the clinical consequences are generally minimal. Finally, many studies have attempted to identify relationships between CYP2C19 genotype and phenotype and susceptibility to xenobiotic-induced disease, but none of these are compelling.

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.

Validation of a new fluorogenic real-time PCR assay for detection of CYP2C9 allelic variants and CYP2C9 allelic distribution in a German population

AIMS

The aim was to develop a fast detection method for the polymorphic alleles related to impaired CYP2C9-mediated metabolism.

METHODS

CYP2C9 genotypes were identified in 118 DNA samples using real-time PCR (LightCycler) followed by melting curve analysis. All samples were re-tested by validated PCR-RFLP methodology.

RESULTS

The concordance between the two methods was 100% for two variant alleles. The frequencies of CYP2C9*1 (wild type), CYP2C9*2 and CYP2C9*3 (with 95% confidence intervals) were 0.81(0.05), 0.14(0.04) and 0.05(0.03), respectively, and are similar to those observed in other Caucasian populations.

CONCLUSIONS

This assay is simple and rapid and may be used for CYP2C9-genotyping in a clinical setting.

Cytochrome p450 phenotyping/genotyping in patients receiving antipsychotics: useful aid to prescribing?

Many antipsychotics, including perphenazine, zuclopenthixol, thioridazine, haloperidol and risperidone, are metabolised to a significant extent by the polymorphic cytochrome P450 (CYP) 2D6, which shows large interindividual variation in activity. Significant relationships between CYP2D6 genotype and steady-state concentrations have been reported for perphenazine, zuclopenthixol, risperidone and haloperidol when used in monotherapy. Other CYPs, especially CYP1A2 and CYP3A4, also contribute to the interindividual variability in the kinetics of antipsychotics and the occurrence of drug interactions. For many antipsychotics, the role of the different CYPs at therapeutic drug concentrations remains to be clarified. Some studies have suggested that poor metabolisers for CYP2D6 would be more prone to oversedation and possibly parkinsonism during treatment with classical antipsychotics, whereas other, mostly retrospective, studies have been negative or inconclusive. For the newer antipsychotics, such data are lacking. Whether phenotyping or genotyping for CYP2D6 or other CYPs can be used to predict an optimal dose range has not been studied so far. Genotyping or phenotyping can today be recommended as a complement to plasma concentration determination when aberrant metabolic capacity (poor or ultrarapid) of CYP2D6 substrates is suspected. The current rapid developments in molecular genetic methodology and pharmacogenetic knowledge can in the near future be expected to provide new tools for prediction of the activity of the various drug-metabolising enzymes. Further prospective clinical studies in well-defined patient populations and with adequate evaluation of therapeutic and adverse effects are required to establish the potential of pharmacogenetic testing in clinical psychiatry.

The interface of preclinical evaluation with clinical testing of antiepileptic drugs: role of pharmacogenomics and pharmacogenetics

Despite the release of eight antiepileptic drugs (AEDs) during the last decade, the incidence of pharmacoresistant epilepsy has changed relatively little. Predicting efficacy and safety of AEDs in people with epilepsy from acute seizure models in rodents is difficult and risky. It is becoming increasingly clear that genetic polymorphisms play an integral role in variability in both antiepileptic drug pharmacokinetics and pharmacodynamics. The publication of the human genome and increasing sophisticated and powerful genetic tools offers new methods for screening drugs and predicting deadly idiosyncratic side effects. In this review the use of pharmacogenomic and pharmacokinetic techniques in the development and monitoring of antiepileptic drug therapy is reviewed. Genetic techniques have the potential of identifying novel drug targets, predicting drug response, and identifying individuals at risk for serious idosyncratic reactions.

Allele and genotype frequencies of polymorphic cytochromes P450 (CYP2C9, CYP2C19, CYP2E1) and dihydropyrimidine dehydrogenase (DPYD) in the Egyptian population

AIMS

The goal of this study was to determine the frequencies of important allelic variants of CYP2C9, CYP2C19, CYP2E1 and DPYD in the Egyptian population and compare them with the frequencies in other ethnic populations.

METHODS

Genotyping of CYP2C9 (*2 and *3), CYP2C19 (*2 and *3), c2 variant of CYP2E1 and DPYD alleles (*2 A-*6 ) was carried out in a total of 247 unrelated Egyptian subjects. An allele-specific fluorogenic 5' nuclease chain reaction assay was applied for detection of CYP2C9 and CYP2C19 variants. Other variants of the CYP2E1 and DPYD genes were determined using polymerase chain reaction (PCR)-restriction fragment length polymorphism and allele-specific PCR based assays.

RESULTS

CYP2C9 allele frequencies in 247 Egyptian subjects were 0.820 for CYP2C9*1, 0.120 for CYP2C9*2 and 0.060 for CYP2C9*3. For CYP2C19, the frequencies of the wild type (CYP2C19*1) and the nonfunctional (*2 and *3) alleles were 0.888, 0.110 and 0.002, respectively. CYP2C19*3, which is considered an Asian mutation, was detected in one subject (0.40%) who was heterozygous (*1/*3). Two subjects (0.80%) were homozygous for *2/*2, while no compound heterozygotes (*2/*3) or homozygotes for *3 were detected. For CYP2E1, only four subjects (1.70%) had the rare c2 variant, expressed heterozygously, giving an allele frequency of 0.009. Five variants of DPYD were analysed, with no splice sites (*2 A) or DeltaC1897 (*3) found in this population. The frequencies of other variants were 0.028, 0.115 and 0.090 for *4, *5 and *6, respectively.

CONCLUSIONS

Comparing our data with that obtained in several Caucasian, African-American and Asian populations, we found that Egyptians resemble Caucasians with regard to allelic frequencies of the tested variants of CYP2C9, CYP2C19, CYP2E1 and DPYD. Our results may help in better understanding the molecular basis underlying ethnic differences in drug response, and contribute to improved individualization of drug therapy in the Egyptian population.

The predictive value of MDR1, CYP2C9, and CYP2C19 polymorphisms for phenytoin plasma levels

Phenytoin, an anticonvulsant, exhibits nonlinear pharmacokinetics with large interindividual differences. Because of its small therapeutic range with the risk of therapeutic failure or adverse drug effects in susceptible persons, therapeutic drug monitoring is frequently applied. The interindividual differences in dose response can partially be explained by known genetic polymorphisms in the metabolic enzyme CYP2C9 but a large deal of individual variability remains still unexplained. Part of this variability might be accounted for by variable uptake of phenytoin, which is a substrate of p-glycoprotein, encoded by the human MDR1 gene. We evaluated, whether phenytoin plasma levels correlate with a polymorphism in the MDR1 gene, C3435T, which is associated with intestinal PGP activity. Genotyping and analyses of plasma levels of phenytoin and metabolites in 96 healthy Turkish volunteers showed that the MDR1C > T3435 polymorphism affects phenytoin plasma levels (P = 0.064) and the metabolic ratio of p-HPPH vs phenytoin (MDR1*TT genotype, P = 0.026). The MDR1*CC genotype is more common in volunteers with low phenytoin levels (P < or = 0.001, chi2 test). A combined analysis of variable alleles of CYP2C9, 2C19 and MDR1 revealed that the number of mutant CYP2C9 alleles is a major determinant, the number of MDR1*T alleles further contributes to the prediction of phenytoin plasma levels and CYP2C19*2 does not explain individual variability. The regression equation that fitted the data best included the number of mutant CYP2C9 and MDR*T alleles as predictory variables and explained 15.4% of the variability of phenytoin data (r2 = 0.154, P = 0.0002). Furthermore, analysis of CYP2C9 and MDR1 genotypes in 35 phenytoin-treated patients recruited from therapeutic drug monitoring showed that combined CYP2C9 and MDR1 analysis has some predictive value not only in the controlled settings of a clinical trial, but also in the daily clinical practice.

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.

Pharmacogenetics: the therapeutic drug monitoring of the future?

Genetic variability in drug response occurs as a result of molecular alterations at the level of drug-metabolising enzymes, drug targets/receptors, and drug transport proteins. In this paper, we discuss the possibility that therapeutic drug monitoring (TDM) in the future will involve not the mere measurement and interpretation of drug concentrations but will include both traditional TDM and pharmacogenetics-oriented TDM. In contrast to traditional TDM, which cannot be performed until after a drug is administered to the patient. pharmacogenetics-oriented TDM can be conducted even before treatment begins. Other advantages of genotyping over traditional TDM include, but are not limited to, the following: (i) it does not require the assumption of steady-state conditions (or patient compliance) for the interpretation of results; (ii) it can often be performed less invasively (with saliva, hair root or buccal swab samples); (iii) it can provide predictive value for multiple drugs [e.g. a number of cytochrome P450 (CYP) 2D6, CYP2C 19 or CYP2C9 substrates] rather than a single drug; (iv) it provides mechanistic, instead of merely descriptive, information; and (v) it is constant over an individual's lifetime (and not influenced by concurrent drug administration, alteration in hormonal levels or disease states). Pharmacogenetic information can be applied a priori for initial dose stratification and identification of cases where certain drugs are simply not effective. However, traditional TDM will still be required for all of the reasons that we use it now. In current clinical practice, pharmacogenetic testing is performed for only a few drugs (e.g. mercaptopurine, thioguanine, azathioprine, trastuzumab and tacrine) and in a limited number of teaching hospitals and specialist academic centres. We propose that other drugs (e.g. warfarin, phenytoin, codeine, oral hypoglycaemics, tricyclic antidepressants, aminoglycosides, digoxin, cyclosporin, cyclophosphamide, ifosfamide, theophylline and clozapine) are potential candidates for pharmacogenetics-oriented TDM. However, prospective studies of phaymacogenetics-oriented TDM must be performed to determine its efficacy and cost effectiveness in optimising therapeutic effects while minimising toxicity. In the future, in addition to targeting a patient's drug concentrations within a therapeutic range, pharmacists are likely to be making dosage recommendations for individual drugs on the basis of the individual patient's genotype. As we enter the era of personalised drug therapy, we will be able to identify not only the best drug to be administered to a particular patient, but also the most effective and safest dosage from the outset of therapy.

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.

Major bleeding caused by warfarin in a genetically susceptible patient

A 90-year-old woman was hospitalized for gastrointestinal bleeding. Although she had been receiving only warfarin 5 mg/day, her international normalized ratio (INR) was 66. Warfarin was discontinued, and her INR fell to 3.7 after transfusion of fresh-frozen plasma. However, it rose again spontaneously to 7.5. Eleven days after the last dose of warfarin had been administered, it was still detectable in the patient's plasma, indicating that impaired warfarin clearance may have caused an enhanced anticoagulation effect. Genetic analysis of the cytochrome P450 (CYP) isoenzyme 2C9, which mediates the major deactivating pathway of S-warfarin, revealed that the patient was a compound heterozygote carrying two variant alleles: CYP2C9*2 and CYP2C9*3. The patient's enhanced sensitivity to warfarin 5 mg/day can be ascribed to decreased clearance of S-warfarin secondary to genetic alteration of the gene encoding CYP2C9, resulting in a life-threatening complication.

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.

Genetic polymorphism of cytochrome P450 2C9 in a Caucasian and a black African population

AIMS

CYP2C9 is a major enzyme in human drug metabolism and the polymorphism observed in the corresponding gene may affect the therapeutic outcome during treatment with several drugs. The distribution of variant CYP2C9 alleles was therefore investigated in an Italian and an Ethiopian population.

METHODS

Allele-specific PCR analysis was carried out in order to determine the frequencies of the two most common variant alleles, CYP2C9*2 and CYP2C9*3 in genomic DNA isolated from 157 Italians and 150 Ethiopians.

RESULTS

The frequencies of CYP2C9*1 (80%), CYP2C9*2 (11%) and CYP2C9*3 (9%) found in the Italian population were similar to other Caucasian groups. However in the Ethiopian population CYP2C9*1, CYP2C9*2 and CYP2C9*3 were present at a frequency of 94, 4 and 2% respectively. The 95% confidence intervals in CYP2C9*1, CYP2C9*2 and CYP2C9*3 between Italians and Ethiopians were 0.098, 0.176, 0.040, 0.098 and 0.040, 0.098, respectively.

CONCLUSIONS

Our results indicate that the Ethiopian population has a unique relative distribution of the CYP2C9 alleles, which is not similar to any other ethnic group hitherto described.

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.

Relationships between non-occupational cadmium exposure and expression of nine cytochrome P450 forms in human liver and kidney cortex samples

This study was undertaken to assess associations between age, gender, cigarette smoke and non-workplace cadmium exposure, and liver pathology and inter-individual variation in cytochrome P450 (CYP) expression in human tissues. Autopsy specimens of twenty-eight Queensland residents whose ages ranged from 3 to 89 years were analyzed for the presence of nine CYP protein isoforms by immunoblotting. All subjects were Caucasians and their liver cadmium contents ranged from 0.11 to 3.95 microg/g wet weight, while their kidney cadmium contents were in the range of 2 to 63 microg/g wet weight. CYP1A2, CYP2A6, CYP2D6, CYP3A4, and CYP3A5 were detected in liver but not in kidney, and CYP1A1 and CYP1B1 were not found in liver or kidney. Lowered liver CYP2C8/19 protein contents were found to be associated with liver pathology. Importantly, we show elevated levels of CYP2C9 protein to be associated with cadmium accumulation in liver. No mechanism that explains this association is apparent, but there are two possibilities that require further study. One is that variation in CYP2C9 protein levels may be, in part, attributed to an individual's non-workplace exposure to cadmium, or an individual's CYP2C9 genotype may be a risk factor for cadmium accumulation. A positive correlation was found between liver CYP3A4 protein and subject age. Levels of liver CYP1A2 protein, but not other CYP forms, were increased in people more exposed to cigarette smoke, but there was no association between CYP1A2 protein and cadmium. CYP2A6 protein was found in all liver samples and CYP2A6 gene typing indicated the absence of CYP2A6 null allele (CYP2A6(D)) in this sample group, confirming very low prevalence of homozygous CYP2A6(D) in Caucasians. CYP2A6 gene types W/W, W/C, and C/C were not associated with variations in liver microsomal CYP2A6 protein. CYP2D6 protein was absent in all twenty-five kidney samples tested but was detectable in liver samples of all but two subjects, indicating the prevalence of the CYP2D6 null allele (CYP2D6(D)) in this sample group to be about 7%, typical of Caucasian populations.

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.

The effect of genetic polymorphism of cytochrome P450 CYP2C9 on phenytoin dose requirement

The cytochrome P450 enzyme CYP2C9 catalyses the metabolism of numerous therapeutic agents, including the anti-epileptic drug phenytoin. CYP2C9 is genetically polymorphic: two allelic variants are known, CYP2C9*2 and CYP2C9*3, differing from the wild-type CYP2C9*1 by a single point mutation. Both mutant alleles are associated with markedly impaired metabolic capacity for many CYP2C9 substrates compared to the wild-type, resulting in raised serum drug levels upon a given dose. Because this may be relevant in treatment with phenytoin, we studied the effect of CYP2C9 genotype on phenytoin dose requirement in a group of 60 epileptic patients on long-term phenytoin therapy. CYP2C9 genotyping was performed by polymerase chain reaction analysis, phenytoin serum concentrations were measured by high-performance liquid chromatography analysis and related to the maintenance doses. For patients carrying at least one mutant CYP2C9 allele (n = 17), the mean phenytoin dose required to achieve a therapeutic serum concentration was about 37% lower than the mean dose required by wild-type individuals (199 mg/day versus 314 mg/day; P < 0.01). A low maintenance dose (< 200 mg/day) sufficed for 47% of carriers, while 58% of normals required a high dose (> 300 mg/day) for an effective serum level. The results show that there is a strong association between CYP2C9 allelic variants and phenytoin dose requirement. Since phenytoin has a narrow therapeutic index and genotyping may be carried out rapidly and at low cost, dosage adjustment based on CYP2C9 genotype, especially at the induction of therapy, would be of value in order to lower the risk of concentration dependent drug intoxications in carriers.

Frequency of cytochrome P450 2C9 mutant alleles in a Korean population

AIMS

To determine the frequencies of CYP2C9 variants in the Korean population and compare them with the frequencies in other ethnic populations.

METHODS

Genotyping of CYP2C9*2 and CYP2C9*3 allelic variants was carried out in 574 Korean subjects by PCR and restriction fragment length pattern analysis.

RESULTS

Thirteen of 574 subjects (2.3%) were heterozygous for CYP2C9*3 (Ile359Leu), but no subjects with a CYP2C9*2 allele or homozygous for CYP2C9*3 were identified. The allele frequency of CYP2C9*3 in Korean subjects (0.0113, 95% CI 0.0066-0.0193) was similar to that of other East Asian populations, but was considerably lower than that of Caucasian populations.

CONCLUSIONS

CYP2C9*3 seems to be an allelic variant related to the functional polymorphism of CYP2C9, but this variant is rarely seen among Koreans compared with Caucasians. Routine genotyping of the CYP2C9*2 allele is considered to be unnecessary in Korean and East Asians, because this allele appears to be extremely rare or absent in these populations.

How to manage individualized drug therapy: application of pharmacogenetic knowledge of drug metabolism and transport

Significant fractions of health budgets must be spent for treatment of drug side effects and for inefficient drug therapy. Hereditary variants in drug metabolizing enzymes, drug transporters, and drug targets are important determinants of drug response and toxicity and may therefore aid in selection and dosage of drugs. Today there is extensive knowledge of genetic polymorphisms of cytochrome P450 (CYP) enzymes 2A6, 2C9, 2C19, and 2D6; of phase-2 enzymes such as thiopurine S-methyltransferase; and more recently of drug transporters such as the MDR-1 gene-product P-glycoprotein, affecting a significant share of currently used drugs. However, application of pharmacogenetic knowledge to clinical routine is limited in current practice. To promote the application of pharmacogenetic knowledge in clinical routine, research on genotype-based dose adjustments is still necessary - as is the promotion of faster and cheaper genotype analyses. Furthermore, the benefits of CYP genotype-directed drug therapy should be evaluated in properly designed prospective studies. Once such steps have been successfully taken, drug therapy could well become more prevention-directed and patient-tailored than it is possible today, replacing the current "one drug in one dose for one disease" strategy by a more individualized approach.

Pharmacogenetic diagnostics of cytochrome P450 polymorphisms in clinical drug development and in drug treatment

The current use and future perspectives of molecular genetic characterisation of cytochrome P450 enzymes (CYP) for drug development and drug treatment are summarised. CYP genes are highly polymorphic and the enzymes play a key role in the elimination of the majority of drugs from the human body. Frequent variants of some enzymes, CYP2A6, 2C9, 2C19 and 2D6, should be analysed in participants of clinical trials whenever these enzymes may play a role. It is suggested that a CYP genotype certificate is handed out to the volunteers or patients to avoid replicate analyses, and to allow that this information is available for future research and also for treatment with eventually needed drugs. Guidelines on what CYP alleles have to be analysed in drug development, as well as on analytical validation and CYP genotype data handling will be required. Treatment with several drugs may be improved by prior genotyping. The concepts and problems of CYP genotype-based clinical dose recommendations are presented and illustrated for selected drugs. The requirement for prospective trials on the medical and economic benefits of routine CYP genotyping is emphasised. Specific operationally defined recommendations dependent on genotype are a prerequisite for such studies and this review presents tentative CYP genotype-based dose recommendations systematically calculated from published data. Because of the multiplicity of factors involved, these doses will not be the optimal doses for each given individual, but should be more adequate than doses generally recommended for an average total population. Those CYP alleles and polymorphically metabolised drugs which are currently most interesting in drug development and drug treatment are reviewed, and more complete information is available from websites cited in this article.