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

Influence of CYP2C9 polymorphisms, demographic factors and concomitant drug therapy on warfarin metabolism and maintenance dose

Warfarin is an anticoagulant drug with narrow therapeutic index and high interindividual variability in dose requirement. S-warfarin is metabolized mainly by polymorphic cytochrome P450 (CYP) 2C9. We systematically quantified the influence of CYP2C9 genotype, demographic factors and concomitant drug treatment on warfarin metabolism and maintenance dose. The mean warfarin doses were lower in carriers of one (2.71 mg/day, 59 patients) and two polymorphic alleles (1.64 mg/day, 11 patients) than in carriers of two wild-type alleles (4.88 mg/day, 118 patients). Multiple regression analysis demonstrated that CYP2C9 genotype, age, concomitant treatment with warfarin metabolism inducers and lean body weight contributed significantly to interindividual variability in warfarin dose requirement (adjusted R(2)=0.37). The same factors, except for age, significantly influenced S-warfarin clearance (adjusted R(2)=0.42). These results can serve as a starting point for designing prospective studies in patients in the initiation phase of genotype-based warfarin therapy.

Genetic predictors of the maximum doses patients receive during clinical use of the anti-epileptic drugs carbamazepine and phenytoin

Phenytoin and carbamazepine are effective and inexpensive anti-epileptic drugs (AEDs). As with many AEDs, a broad range of doses is used, with the final "maintenance" dose normally determined by trial and error. Although many genes could influence response to these medicines, there are obvious candidates. Both drugs target the alpha-subunit of the sodium channel, encoded by the SCN family of genes. Phenytoin is principally metabolized by CYP2C9, and both are probable substrates of the drug transporter P-glycoprotein. We therefore assessed whether variation in these genes associates with the clinical use of carbamazepine and phenytoin in cohorts of 425 and 281 patients, respectively. We report that a known functional polymorphism in CYP2C9 is highly associated with the maximum dose of phenytoin (P = 0.0066). We also show that an intronic polymorphism in the SCN1A gene shows significant association with maximum doses in regular usage of both carbamazepine and phenytoin (P = 0.0051 and P = 0.014, respectively). This polymorphism disrupts the consensus sequence of the 5' splice donor site of a highly conserved alternative exon (5N), and it significantly affects the proportions of the alternative transcripts in individuals with a history of epilepsy. These results provide evidence of a drug target polymorphism associated with the clinical use of AEDs and set the stage for a prospective evaluation of how pharmacogenetic diagnostics can be used to improve dosing decisions in the use of phenytoin and carbamazepine. Although the case made here is compelling, our results cannot be considered definitive or ready for clinical application until they are confirmed by independent replication.

Ontogeny of drug metabolizing enzymes in the neonate

Fetal exposure to xenobiotics is modulated to a considerable degree by the metabolic capabilities of the mother and the placenta. However, once liberated from the uterine environment the neonate is instantly exposed to a wide array of new macromolecules in the form of byproducts of cellular metabolism, dietary constituents, environmental toxins and pharmacologic agents. The rapid and efficient biotransformation of these compounds by Phase I and Phase II drug-metabolizing enzymes is an essential process if the infant is to avoid the accumulation of reactive compounds that could produce cellular injury or tissue dysfunction. Genetic polymorphisms and environmental factors are known to contribute dramatically to individual variation in the activity of drug-metabolizing enzymes. More recently, it has become apparent that programmed, developmental, regulatory events occur - independent of genotype - which further add to individual variation in drug metabolism. An appreciation of the impact of ontogeny on the expression and functional activity of the major drug-metabolizing enzymes enables the practicing clinician to predict the ultimate consequence of drug administration in the neonate to help guide optimal drug therapy.

Genetic polymorphism of CYP2C9 and CYP2C19 in a Bolivian population: an investigative and comparative study

OBJECTIVE

Several reports of CYP2C genetic polymorphism demonstrate its potential clinical role in determining both inter-individual and inter-ethnic differences in drug efficacy. We estimated the distribution of CYP2C9 and CYP2C19 common variants in the Bolivian population (a South American population), and compared these data with those from Asian, African, Caucasian and Oceanian populations.

METHODS

Genomic DNA was obtained from 778 unrelated healthy volunteers from Bolivia. The genotypic status of CYP2C9 and CYP2C19 was determined by means of polymerase chain reaction-restriction fragment length polymorphism.

RESULTS

Allelic and genotypic frequencies of CYP2C9 and CYP2C19 were determined for the Bolivian population, and comparison of the data with other ethnic groups revealed a lower CYP2C9*2 frequency (4.8%) than in Caucasians, but a higher frequency than in Asians; frequencies of CYP2C9*3 (3.0%) and CYP2C9 (0.4%) poor metabolizers (PMs) were similar to those seen in Asian populations. Frequencies of CYP2C19*2 (7.8%), CYP2C19*3 (0.1%), and CYP2C19 PMs (1.0%) in the Bolivian population were for the most part lower than in Caucasian, Asian, Oceanian and African populations.

CONCLUSION

This is the first study to investigate a South American population for genetic polymorphism in the CYP2C subfamily. The Bolivian population differs from most other ethnic groups in the incidence of CYP2C9 and CYP2C19 common variants that might be influenced by its admixture characteristics.

Increasing Warfarin Dosage Reductions Associated with Concurrent Warfarin and Repeated Cycles of 5-Fluorouracil Therapy

Concurrent use of warfarin and 5-fluorouracil has resulted in elevated international normalized ratios (INRs). Although this drug interaction is well established in the literature, we found no documented cases that address its effects on anticoagulation parameters in patients requiring repeated cycles of 5-fluorouracil and continuous warfarin therapy. We describe the effect of multiple cycles of 5-fluorouracil administration in a patient receiving warfarin therapy. The patient's INR increased 11-14 days after each cycle of chemotherapy with 5-fluorouracil and leucovorin. In addition, she required additive reductions of 33-42% in her weekly warfarin dose with each chemotherapy cycle to maintain a therapeutic INR (goal range 2.0-3.0). After three cycles of chemotherapy over a 10-week period, the patient's dosage requirements returned to her baseline level (before treatment with 5-fluorouracil and leucovorin had started). Pertinent literature was reviewed to provide supporting evidence for the mechanism and clinical implications of the drug interaction. Based on this report and previous case reports, caution is advised when concurrent warfarin and 5-fluorouracil are prescribed. In addition, patients should be closely monitored for a possible delayed interaction that may occur with each repeated cycle of 5-fluorouracil chemotherapy.

Human variability in xenobiotic metabolism and pathway-related uncertainty factors for chemical risk assessment: a review

This review provides an account of recent developments arising from a database that defined human variability in phase I metabolism (CYP1A2, CYP2A6, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4, hydrolysis, alcohol dehydrogenase), phase II metabolism (N-acetyltransferases, glucuronidation, glycine conjugation, sulphation) and renal excretion. This database was used to derive pathway-related uncertainty factors for chemical risk assessment that allow for human variability in toxicokinetics. Probe substrates for each pathway of elimination were selected on the basis that oral absorption was >95% and that the metabolic route was the primary route of elimination of the compound (60-100% of a dose). Intravenous data were used for compounds for which absorption was variable. Human variability in kinetics was quantified for each compound from published pharmacokinetic studies (after oral and intravenous dosing) in healthy adults and other subgroups of the population using parameters relating to chronic exposure (metabolic and total clearances, area under the plasma concentration-time curve (AUC)) and acute exposure (Cmax) (data not presented here). The pathway-related uncertainty factors were calculated to cover 95%, 97.5% and 99% of the population of healthy adults and of each subgroup. Pathway-related uncertainty factors allow metabolism data to be incorporated into the derivation of health-based guidance values. They constitute an intermediate approach between the general kinetic default factors (3.16) and a chemical-specific adjustment factor. Applications of pathway-related uncertainty factors for chemical risk assessment and future refinements of the approach are discussed. A knowledge-based framework to predict human variability in kinetics for xenobiotics showing a threshold dose below which toxic effects are not observed, is proposed to move away from default assumptions.

CYP2C9 and CYP2C19 genetic polymorphisms: frequencies in the south Indian population

The aim of the study was to establish the frequencies of CYP2C9*1, *2, *3 and CYP2C19*1, *2 and *3 in the south Indian population and to compare them with the inter-racial distribution of the CYP2C9 and CYP2C19 genetic polymorphisms. Genotyping analyses of CYP2C9 and CYP2C19 were conducted in unrelated, healthy volunteers from the three south Indian states of Andhra Pradesh, Karnataka and Kerala, by the polymerase chain reaction-restriction fragment-length polymorphism (PCR-RFLP). The allele frequencies of the populations of these three states were then pooled with our previous genotyping data of Tamilians (also in south India), to arrive at the distribution of CYP2C9 and CYP2C19 alleles in the south Indian population. Frequencies of CYP2C9 and CYP2C19 alleles and genotypes among various populations were compared using the two-tailed Fisher's exact test. The frequencies of CYP2C9*1, *2 and *3 in the south Indian population were 0.88 (95% CI 0.85-0.91), 0.04 (95% CI 0.02-0.06) and 0.08 (95% CI 0.06-0.11), respectively. The frequencies of CYP2C9 genotypes *1/*1, *1/*2, *1/*3, *2/*2, *2/*3 and *3/*3 were 0.78 (95% CI 0.74-0.82), 0.05 (95% CI 0.03-0.07), 0.15 (95% CI 0.12-0.18), 0.01 (95% CI 0.0-0.02), 0.01 (95% CI 0.0-0.02) and 0.0, respectively. CYP2C19*1, *2 and *3 frequencies were 0.64 (95% CI 0.60-0.68), 0.35 (95% CI 0.31-0.39) and 0.01 (95% CI 0.0-0.03), respectively. As a result of a significant heterogeneity, the data on CYP2C19 genotype frequencies were not pooled. The frequency of CYP2C9*2 mutant alleles in south Indians was higher than in Chinese and Caucasians, while CYP2C9*3 was similar to Caucasians. CYP2C19*2 was higher than in other major populations reported so far. The relatively high CYP2C19 poor-metabolizer genotype frequency of 12.6% indicates that over 28 million south Indians are poor metabolizers of CYP2C19 substrates.

A polymorphism in the VKORC1 gene is associated with an interindividual variability in the dose-anticoagulant effect of warfarin

Patients require different warfarin dosages to achieve the target therapeutic anticoagulation. The variability is largely genetically determined, and it can be only partly explained by genetic variability in the cytochrome CYP2C9 locus. In 147 patients followed from the start of anticoagulation with warfarin, we have investigated whether VKORC1 gene mutations have affected doses of drug prescribed to acquire the target anticoagulation intensity. Two synonymous mutations, 129C>T at Cys43 and 3462C>T at Leu120, and 2 missense mutations, Asp38Tyr and Arg151Gln, were identified. None of these mutations was found to affect the interindividual variability of warfarin prescribed. Finally, 2 common polymorphisms were found, 1173C>T in the intron 1 and 3730G>A transition in the 3′ untranslated region (UTR). Regardless of the presence of confounding variables, the mean adjusted dose required of warfarin was higher (6.2 mg) among patients with the VKORC1 1173CC genotype than those of patients carrying the CT (4.8 mg; P = .002) or the TT genotype (3.5 mg; P < .001). In the present setting, VKORC1 and CYP2C9 genetic variants investigated accounted for about a third (r2, 0.353) of the interindividual variability. Genetic variants of the VKORC1 gene locus modulate the mean daily dose of drug prescribed to acquire the target anticoagulation intensity.

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

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

Role of Human Liver Cytochrome P450 2C9 in the Metabolism of a Novel α4β1/α4β7 Dual Antagonist, TR-14035

The metabolism of a novel dual antagonist for alpha4beta1/alpha4beta7 integrin, TR-14035, and the role of polymorphic enzyme responsible for this metabolism were investigated. Human liver microsomes catalyzed the NADPH-dependent metabolism of TR-14035 to a primary metabolite, O-desmethyl TR-14035. This formation was completely blocked by both sulfaphenazole, a selective CYP2C9 inhibitor, and CYP2C9 antibody, whereas potent inhibitors selective for other CYPs exhibited little effects. Of 12 recombinant CYPs examined, O-desmethyl metabolite was principally formed by CYP2C9. CYP1A1, an extrahepatic enzyme, also had this activity (about one-fourth of CYP2C9). Utilizing recombinant CYP2C9*1, K(m) and V(max)/K(m) values of 23.3 microM and 0.284 microL/min/pmol CYP2C9, respectively, were obtained for the O-desmethyl formation, which were quite similar to those in CYP2C9*2 enzyme. In contrast, V(max)/K(m) value in recombinant CYP2C9*3 was approximately one-sixth of CYP2C9*1 and *2. In agreement, kinetics studies using human liver microsomes with CYP2C9*1/*1, *2/*2 and *3/*3 genotypes revealed that the V(max)/K(m) value in *2/*2 microsomes was comparable to that in wild type microsomes, in contrast, that in *3/*3 microsomes was reduced. These results demonstrate CYP2C9 is a primary enzyme mediating the O-desmethylation of TR-14035 in human liver. In homozygotes of CYP2C9*3, the metabolic clearance of TR-14035 should be decreased compared with homozygotes of CYP2C9*1 or 2.

Epoxide hydrolase and CYP2C9 polymorphisms, cigarette smoking, and risk of colorectal carcinoma in the Nurses' Health Study and the Physicians' Health Study

Microsomal epoxide hydrolase (mEH) and cytochrome P450 2C9 (CYP2C9) are involved in the bioactivation and detoxification of polycyclic aromatic hydrocarbons (PAHs) derived from tobacco smoke. Two coding-region mEH variants (Tyr113His, His139Arg) and CYP2C9 variants (Arg144Cys, Ile359Leu) have been described and affect enzyme specific activity. We investigated these polymorphisms and tested interactions with smoking in relationship to risk of colorectal carcinoma in two case-control studies nested in the Nurses' Health Study (NHS) and Physicians' Health Study (PHS) cohorts. mEH Tyr113His and His139Arg polymorphisms were not associated with the risk of cancer among 197 incident cases and 490 controls from the NHS. Among 273 incident cases and 453 controls from the PHS, carrying one or two copies of the 'rapid' 139Arg allele was associated with a significantly reduced risk of colorectal cancer (OR=0.70, 95% CI 0.49--0.99) when compared with His139 wild-type individuals. Risk of colorectal cancer was significantly reduced among men carrying the CYP2C9 *1/*2 genotype (OR=0.62, 95% CI 0.42--0.92) or at least one CYP2C9 variant allele (OR=0.72, 95% CI 0.52--1.00) when compared with *1/*1 wild-type individuals. For women, carrying at least one variant CYP2C9 allele was inversely associated with the risk of colorectal cancer (OR=0.85, 95% CI, 0.57--1.27) when compared with *1/*1 wild-type individuals. No statistically significant genotype-smoking or gene-gene interactions were found in this study. Our results indicate that individuals exposed to tobacco carcinogens were at increased risk of colorectal cancer and that overall risk is related to mEH and CYP2C9 genotype, although the results were not consistent between men and women.

The CYP2C9 polymorphism: from enzyme kinetics to clinical dose recommendations

CYP2C9 is the major human enzyme of the cytochrome P450 2C subfamily and metabolizes approximately 10% of all therapeutically relevant drugs. Two inherited SNPs termed CYP2C9*2 (Arg144Cys) and *3 (Ile359Leu) are known to affect catalytic function. Numerous rare or functionally silent polymorphisms have been identified. About 35% of the Caucasian population carries at least one *2 or *3 allele. CYP2C9 metabolizes several oral hypoglycemics, oral anticoagulants, non-steroidal anti-inflammatory drugs and other drugs, including phenytoin, losartan, fluvastatin, and torsemide. In vitro studies with several drugs indicate that the Cys144 (.2) and Leu359 (.3) variants confer only about 70 and 10% of the intrinsic clearance of the wild-type protein (.1), respectively. The clinical pharmacokinetic implications of these polymorphisms vary depending on the enzymes contribution to total oral clearance. Several studies demonstrated that the CYP2C9 polymorphisms are medically important for non-steroidal anti-inflammatory drugs, for oral hypoglycemics, vitamin K antagonistic oral anticoagulants, and phenytoin. In particular, CYP2C9 polymorphisms should be routinely considered in therapy with oral anticoagulants where severe adverse events at initiation of therapy might be reduced by genotyping. CYP2C9 polymorphisms were also clinically associated with side effects of phenytoin, with gastric bleeding during therapy with non-steroidals and with hypoglycemia under oral hypoglycemic drugs. Data appear mature enough for the routine consideration of CYP2C9 genotypes in therapy with acenocoumarol, phenytoin, warfarin, and some other drugs. Nevertheless, it is advisable before the routine clinical use of these genotype data to rigorously test the benefits of genotype-based therapeutic recommendations by randomized controlled clinical trials.

CYP2C9 genetic polymorphisms and warfarin

The objective of this study was to report 2 cases of CYP2C9 genetic polymorphism and elevated warfarin S:R ratios in patients taking low doses of warfarin, and compare the observed characteristics with those in published reports. Two patients of different age groups and races were evaluated for CYP2C9 genotype and warfarin S:R ratios. The patients had been stabilized on weekly warfarin doses of 10.5 mg and 10 mg, respectively. Each patient was found to have at least 1 variant CYP2C9 allele. Elevated warfarin S:R ratios in both patients provided evidence for impaired metabolism of S-warfarin. This report of a CYP2C9*3 heterozygous individual taking a low dose of warfarin is consistent with previous reports in the literature. This summary of a CYP2C9*6 homozygous individual taking a low dose of warfarin is the first such published report. CYP2C9 genotyping in these patients provided a likely explanation for their continued low warfarin dosage requirements. Awareness of a patient's CYP2C9 genotype may provide an explanation for low warfarin dosage requirements in stable patients and may help in determining the optimal dose in patients being initiated on warfarin.

The novel azole R126638 is a selective inhibitor of ergosterol synthesis in Candida albicans, Trichophyton spp., and Microsporum canis

R126638 is a novel triazole with in vitro activity similar to that of itraconazole against dermatophytes, Candida spp., and Malassezia spp. In animal models of dermatophyte infections, R126638 showed superior antifungal activity. R126638 inhibits ergosterol synthesis in Candida albicans, Trichophyton mentagrophytes, Trichophyton rubrum, and Microsporum canis at nanomolar concentrations, with 50% inhibitory concentrations (IC(50)s) similar to those of itraconazole. The decreased synthesis of ergosterol and the concomitant accumulation of 14 alpha-methylsterols provide indirect evidence that R126638 inhibits the activity of CYP51 that catalyzes the oxidative removal of the 14 alpha-methyl group of lanosterol or eburicol. The IC(50)s for cholesterol synthesis from acetate in human hepatoma cells were 1.4 microM for itraconazole and 3.1 microM for R126638. Compared to itraconazole (IC(50) = 3.5 microM), R126638 is a poor inhibitor of the 1 alpha-hydroxylation of 25-hydroxyvitamin D(3) (IC(50) > 10 microM). Micromolar concentrations of R126638 and itraconazole inhibited the 24-hydroxylation of 25-hydroxyvitamin D(3) and the conversion of 1,25-dihydroxyvitamin D(3) into polar metabolites. At concentrations up to 10 microM, R126638 had almost no effect on cholesterol side chain cleavage (CYP11A1), 11 beta-hydroxylase (CYP11B1), 17-hydroxylase and 17,20-lyase (CYP17), aromatase (CYP19), or 4-hydroxylation of all-trans retinoic acid (CYP26). At 10 microM, R126638 did not show clear inhibition of CYP1A2, CYP2A6, CYP2D6, CYP2C8, CYP2C9, CYP2C10, CYP2C19, or CYP2E1. Compared to itraconazole, R126638 had a lower interaction potential with testosterone 6 beta hydroxylation and cyclosporine hydroxylation, both of which are catalyzed by CYP3A4, whereas both antifungals inhibited the CYP3A4-catalyzed hydroxylation of midazolam similarly. The results suggest that R126638 has promising properties and merits further in vivo investigations for the treatment of dermatophyte and yeast infections.

Association between the CYP2C9 polymorphism and the drug metabolism phenotype

CYP2C9, an isoform of the cytochrome P450 enzyme, is involved in the metabolism of most of the drugs of choice for the treatment of thromboembolic disorders. Functional polymorphism is associated with two variant alleles (alleles *2 and *3) encoding CYP2C9 enzymes with a potentially different catalytic activity. The aim of the study was to determine the frequency of the CYP2C9 polymorphism in a representative sample of the Croatian population (n = 177) and to assess the association between the CYP2C9 polymorphism and the warfarin dose in patients with thromboembolism (n = 181). The CYP2C9 genotype was determined by polymerase chain reaction-restriction fragment length poymorphism (PCR-RFLP). According to the CYP2C9 genotype distribution, 31.2% of the healthy subjects were identified with a heterozygous or homozygous CYP2C9 variant genotype. The frequency of 2C9*2 and 2C9*3 alleles was 12.4% and 3.7%, respectively. There was no gender-related genotype or allele difference. In thromboembolism patients, the frequency of CYP2C9 alleles *2 and *3 was 17.4% and 6.6%, respectively, and did not differ significantly from the control group. Almost half (42.5%) of the patients carried at least one variant CYP2C9 genotype. The allele difference between patient subgroups receiving warfarin doses lower and higher than the optimal warfarin dose (4.1 mg/day) was significant (p = 0.027), especially for allele 2C9*3 (p = 0.019; OR 3.250, 95%, CI 1.263-8.413). Comparison of the warfarin dose between patients with different genotypes yielded a significant dose difference between the patients with wild-type genotype and those with variant genotypes (Kruskall-Wallis, chi2 = 9.745, p = 0.008). The results of the association of each of five genotype combinations with the warfarin maintenance dose revealed it to be significantly related to the genotype (Kruskall-Wallis, chi2 = 12.854, p = 0.025). Expressed as percentage of the mean dose in patients with wild-type alleles, the mean warfarin maintenance dose was 92% in 2C9*2 heterozygotes, 74% in 2C*3 heterozygotes, 61% in 2C9*2 homozygotes, 34% in 2C9*3 homozygotes and 63% in compound heterozygotes for 2C9*2 and 2C9*3. Although the mean maintenance dose in homozygous *2/*2 and compound *2/*3 genotype patients was markedly lower (mean 2.66 mg and 2.75 mg, respectively, vs. 4.37 mg), statistical analysis yielded no significance because of the small number of patients carrying these genotypes. A significantly lower maintenance dose was observed in the subgroup of heterozygous *1/*3 genotype patients (p = 0.022). These preliminary results suggest a significant association of the CYP2C9 polymorphism with the warfarin dose and underline the importance of pre-therapeutic genotyping to identify the subjects likely to develop undesirable drug effects.

Pharmacogenomics and drug response in cardiovascular disorders

There are a total of 17 families of drugs that are used for treating the heterogeneous group of cardiovascular diseases. We propose a comprehensive pharmacogenomic approach in the field of cardiovascular therapy that considers the five following sources of variability: the genetics of pharmacokinetics, the genetics of pharmacodynamics (drug targets), genetics linked to a defined pathology and its corresponding drug therapies, the genetics of physiologic regulation, and environmental–genetic interactions. Examples of the genetics of pharmacokinetics are presented for phase I (cytochromes P450) and phase II (conjugating enzymes) drug-metabolizing enzymes and for phase III drug transporters. The example used to explain the genetics of pharmacodynamics is glycoprotein IIIa and the response to antiplatelet effects of aspirin. Genetics linked to a defined pathology and its corresponding drug therapies is exemplified by ADRB1, ACE, CETP and APOE and drug response in metabolic syndrome. The examples of cytochrome P450s, APOE and ADRB2 in relation to ethnicity, age and gender are presented to describe genetics of physiologic regulation. Finally, environmental–genetic interactions are exemplified by CYP7A1 and the effects of diet on plasma lipid levels, and by APOE and the effects of smoking in cardiovascular disease. We illustrate this five-tiered approach using examples of cardiovascular drugs in relation to genetic polymorphism.

Lower frequency of CYP2C9*2 in Mexican-Americans compared to Spaniards

Interethnic differences in cytochrome P450 polymorphism might be responsible, at least in part, for the variations in drug disposition between ethnic groups. Of the various CYP2C9 alleles, CYP2C9*2 and CYP2C9*3 have been reported to have altered catalytic activities compared to the wild-type CYP2C9*1. The present study is aimed at analysing the CYP2C9 polymorphism in a Mexican-American compared with a Spanish population. Differences between the two populations of healthy volunteers, Mexican-Americans (n=98 subjects) and Spaniards (n=102 subjects), regarding the CYP2C9 allele frequencies have been found. CYP2C9 genotypes among the studied Mexican-American population are in equilibrium. The 95% CI were, respectively, 0.81-0.90 for CYP2C9*1 (n=169), 0.05-0.13 for CYP2C9*2 (n=16) and 0.031-0.10 for CYP2C9*3 (n=11). CYP2C9*4, *5 and *6 were found in none of the studied subjects. The frequency of CYP2C9*2 was lower among Mexican-Americans compared to Spaniards (P<0.05). The obtained frequency of CYP2C9 alleles is compatible with the genomic assembly of the constitutive potential ethnic origin of this population, and supports the need of pharmacogenetic studies for optimizing the recommended drug dosages to Mexican-Americans.

Potential role of cerebral cytochrome P450 in clinical pharmacokinetics: modulation by endogenous compounds

Cytochrome P450 (CYP) enzymes catalyse phase I metabolic reactions of psychotropic drugs. The main isoenzymes responsible for this biotransformation are CYP1A2, CYP2D6, CYP3A and those of the subfamily CYP2C. Although these enzymes are present in the human brain, their specific role in this tissue remains unclear. However, because CYP enzymatic activities have been reported in the human brain and because brain microsomes have been shown to metabolise the same probe substrates used to assess specific hepatic CYP activities and substrates of known hepatic CYPs, local drug metabolism is believed to be likely. There are also indications that CYP2D6 is involved in the metabolism of endogenous substrates in the brain. This, along with the fact that several neurotransmitters modulate CYP enzyme activities in human liver microsomes, indicates that CYP enzymes present in brain could be under various regulatory mechanisms and that those mechanisms could influence drug pharmacokinetics and, hence, drug response. In this paper we review the presence of CYP1A2, CYP2C9, CYP2D6 and CYP3A in brain, as well as the possible existence of local brain metabolism, and discuss the putative implications of endogenous modulation of these isoenzymes by neurotransmitters.

Flow cytometric assay for genotyping cytochrome p450 2C9 and 2C19: comparison with a microelectronic DNA array

INTRODUCTION

Cytochrome p450 (CYP) 2C9 and 2C19 metabolize a wide range of therapeutically important drugs. Genetic polymorphisms in the CYP2C9 and CYP2C19 genes result in variations in drug response. To correlate the dose required for therapeutic drug efficacy with genotype, accurate and reliable methods for detecting single nucleotide polymorphisms (SNPs) of CYP2C9 and CYP2C19 are required.

STUDY DESIGN

We evaluated two technologies for genotyping CYP2C9 (*2 and *3 alleles) and CYP2C19 (*2 and *3 alleles). We developed a multiplexed flow cytometric assay based on the Luminex xMAP system and oligonucleotide-tagged Universal Array microspheres. The Luminex assay was compared with the eSensor DNA detection system, provided by Motorola Life Sciences. Genotypes determined by the two methods were confirmed by sequence analysis.

RESULTS

Of the 101 whole-genome amplified DNA samples genotyped by the Luminex method, 15 (14.8%) were heterozygous and 1 was homozygous for the CYP2C9*2 polymorphism. For the CYP2C9*3 polymorphism, 13 (12.9%) were heterozygous and 1 was homozygous. Two samples had the CYP2C9*2/*3 genotype. For CYP2C19*2, 17 (16.8%) of the samples were heterozygous and one was homozygous. The CYP2C19*3 polymorphism was not found. Genotypes determined by the Luminex assay were in complete concordance with the eSensor SNP assay results. A dilution study showed that 1.5 ng of nucleic acid was adequate for PCR and subsequent detection of SNPs by the Luminex assay. The within run and between run coefficients of variance (CVs) for allelic ratios determined by the Luminex procedure were found to be <or=4.1% and <or=9.1%, respectively, for the alleles present.

CONCLUSION

Both the in-house Luminex method and the eSensor trade mark DNA detection system reproducibly and unambiguously genotyped SNPs of CYP2C9 and CYP2C19 in the samples tested.

Phenobarbital for the Treatment of Epilepsy in the 21st Century: A Critical Review

SUMMARY

Phenobarbital (PB) is the most widely used antiepileptic drug (AED) in the developing world and remains a popular choice in many industrialized countries. Meta-analyses of randomized controlled trials suggest that few differences in efficacy exist between PB and other established AEDs, but its possible deleterious cognitive and behavioral side effects remain a concern in the developed world. In contrast, high degrees of efficacy and tolerability in everyday clinical use have been demonstrated consistently in observational studies in developing countries. We propose that a pragmatic, comprehensive outcomes program be carried out, perhaps under the aegis of the Global Campaign Against Epilepsy, to optimize the conditions of the use of PB, so that more people around the world can benefit from this cost-effective medication and live more fulfilling lives.

Toward individualized pharmaceutical care of East Asians: the value of genetic testing for polymorphisms in drug-metabolizing genes

Research into the relationship between genetics and drug response has focused on polymorphisms in genes that encode drug-metabolizing enzymes, particularly the genes of cytochrome P450 superfamily 2, which affect the clearance of the anticoagulant warfarin, proton pump inhibitors, tricyclic antidepressants, and many other clinically relevant drugs. Much of this work has targeted East Asians, a genetically distinguishable and populous group. Researchers have identified polymorphisms that inactivate gene function, compared polymorphism frequencies in East-Asian and Caucasian populations, and determined the effects on the pharmacokinetic parameters of drugs. Detection in an individual of polymorphisms known to inactivate a drug-metabolizing enzyme is predictive of poor metabolism of drugs processed by that pathway, which itself may be predictive of an atypical drug response. Genetic tests can be used to screen for individuals with poor metabolizer phenotypes, with the ultimate goal of predicting the clinical effects of drugs.

A prospective study of genetic polymorphisms in the cytochrome P-450 2C9 enzyme and the risk for distal colorectal adenoma

BACKGROUND & AIMS

Regular aspirin use is associated with a reduced risk for colorectal adenoma, whereas smoking increases risk. The cytochrome P-450 (CYP) 2C9 (CYP2C9) enzyme is involved in the metabolism of several drugs, including possibly aspirin, and such carcinogens as smoking-related polycyclic aromatic hydrocarbons. Genetic variation in this enzyme may modulate the influence of aspirin and smoking on adenoma risk.

METHODS

We examined the risk for incident distal colorectal adenoma according to CYP2C9 genotype, aspirin use, and smoking in a prospective nested case-control study of women.

RESULTS

Among 394 cases and 396 controls, women with at least 1 variant CYP2C9 allele had a significantly greater risk for adenoma (multivariate odds ratio [OR], 1.56; 95% confidence interval [CI], 1.13-2.15; P = 0.007). Although women who used aspirin regularly (>/=2 standard tablets/wk) experienced a lower risk for adenoma compared with non-regular users, the effect was similar irrespective of genotype. Women who smoked >20 pack-years had an OR of adenoma of 1.50 (95% CI, 1.07-2.12; P = 0.02) compared with those who smoked </=20 pack-years. Among women with at least 1 variant allele who smoked >20 pack-years, the OR of adenoma was 2.50 (95% CI, 1.44-4.38; P = 0.001) compared with women with no variant alleles who smoked </=20 pack-years.

CONCLUSIONS

Polymorphisms in the CYP2C9 enzyme are associated with elevated risk for colorectal adenoma. This observation does not appear to be related to modification of the effect of aspirin on adenoma risk, but may be associated with differential metabolism of environmental carcinogens.

Coadministration of Lopinavir/Ritonavir and Phenytoin Results in Two-Way Drug Interaction Through Cytochrome P-450 Induction

Lopinavir/ritonavir (LPV/RTV) is a CYP3A4 inhibitor and substrate; it also may induce cytochrome P-450 (CYP) isozymes. Phenytoin (PHT) is a CYP3A4 inducer and CYP2C9/CYP2C19 substrate. This study quantified the pharmacokinetic (PK) drug interaction between LPV/RTV and PHT. Open-label, randomized, multiple-dose, PK study in healthy volunteers. Subjects in arm A (n = 12) received LPV/RTV 400/100 mg twice daily (BID) (days 1-10), followed by LPV/RTV 400/100 mg BID + PHT 300 mg once daily (QD) (days 11-22). Arm B (n = 12) received PHT 300 mg QD (days 1-11), followed by PHT 300 mg QD + LPV/RTV 400/100 mg BID (days 12-23). Plasma samples were collected on day 11 and day 22; PK parameters were compared by geometric mean ratio (GMR, day 22:day 11). P values <0.05 were considered significant. Following PHT addition, LPV area under the concentration-time curve (AUC0-12h) decreased from 70.9 +/-37.0 to 49.6 +/- 25.1 microg.h/mL (GMR 0.67, P = 0.011) and C0h decreased from 6.0 +/- 3.2 to 3.6 +/- 2.3 microg/mL (GMR 0.54, P = 0.001). Following LPV/RTV addition, PHT AUC0-24h decreased from 191.0+/-89.2 to 147.8+/-104.5 microg.h/mL (GMR 0.69, P = 0.009) and C0h decreased from 7.0+/-4.0 to 5.3+/-4.1 microg/mL (GMR 0.66, P = 0.033). Concomitant LPV/RTV and PHT use results in a 2-way drug interaction. Phenytoin appears to increase LPV clearance via CYP3A4 induction, which is not offset by the presence of low-dose RTV. LPV/RTV may increase PHT clearance via CYP2C9 induction. Management should be individualized to each patient; dosage or medication adjustments may be necessary.

Discovery of new potentially defective alleles of human CYP2C9

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

The structure of human cytochrome P450 2C9 complexed with flurbiprofen at 2.0-A resolution

The structure of human P450 2C9 complexed with flurbiprofen was determined to 2.0 A by x-ray crystallography. In contrast to other structurally characterized P450 2C enzymes, 2C5, 2C8, and a 2C9 chimera, the native catalytic domain of P450 2C9 differs significantly in the conformation of the helix F to helix G region and exhibits an extra turn at the N terminus of helix A. In addition, a distinct conformation of the helix B to helix C region allows Arg-108 to hydrogen bond with Asp-293 and Asn-289 on helix I and to interact directly with the carboxylate of flurbiprofen. These interactions position the substrate for regioselective oxidation in a relatively large active site cavity and are likely to account for the high catalytic efficiency exhibited by P450 2C9 for the regioselective oxidation of several anionic non-steroidal anti-inflammatory drugs. The structure provides a basis for interpretation of a number of observations regarding the substrate selectivity of P450 2C9 and the observed effects of mutations on catalysis.

A simple method for the detection of CYP2C9 polymorphisms: nested allele-specific multiplex polymerase chain reaction

BACKGROUND

Cytochrome P4502C9 (CYP2C9), a principle drug-metabolizing enzyme is polymorphic in humans and is responsible for important pharmacokinetic and pharmacodynamic variations of CYP2C9 substrates. We developed an allele-specific multiplex polymerase chain reaction (PCR) method for the detection of common CYP2C9 alleles.

METHOD

Genomic DNA was extracted from blood obtained from 40 unrelated healthy Malaysian Indian volunteers. The DNA was subjected to a first PCR that was used to amplify both exons 3 and 7 simultaneously in one reaction tube and a second PCR that was used to detect the polymorphic sites of CYP2C9 alleles using allele-specific primers. Sequencing was performed to validate the test results.

RESULTS

We were successful in amplifying the fragments of interest from the DNA samples. The method was also reproducible and specific. The amplified sequences showed 100% homology to CYP2C9 sequence.

CONCLUSION

This is the first nested allele-specific multiplex PCR method reported to allow for the simultaneously detection of five CYP2C9 alleles.

QTc interval, CYP2D6 and CYP2C9 genotypes and risperidone plasma concentrations

The role of certain drug metabolizing enzymes in cardiotoxicity, such as CYP2D6 for thioridazine, has been suggested. Risperidone has been shown to inhibit the delayed rectifier leading to lengthening of cardiac repolarization. The heart-rate corrected QT (QTc) interval lengthening has been reported in psychiatric patients receiving risperidone under steady-state conditions. CYP2D6 is involved in the metabolism of risperidone to 9-hydroxy (OH)-risperidone. CYP2C9 enzyme is also involved in the metabolism of several psychotropic drugs, although there are no data about its implication in risperidone metabolism. The present study aimed to evaluate the influence of CYP2D6 and CYP2C9 genotypes, and plasma concentrations of risperidone and 9-OH-risperidone on the QTc interval in patients under steady-state conditions. The relevance of CYP2D6 and CYP2C9 genotypes on risperidone metabolism was also analysed. Thirty-five White European psychiatric patients receiving risperidone monotherapy were studied. QTc interval was longer (p < 0.05) in subjects with one active CYP2D6 gene compared to those with two. The number of CYP2D6 active genes was related to the dose-corrected plasma concentration of risperidone (p < 0.05), the active moiety (risperidone plus 9-OH-risperidone) (p < 0.05) and the risperidone/9-OH-risperidone ratio (p < 0.05). CYP2C9 genotypes were not related to plasma concentrations of risperidone or 9-OH-risperidone, nor QTc interval. The results suggest that CYP2D6, but not CYP2C9, may be related to QTc lengthening during treatment with risperidone. The effect of the CYP2D6 genotype in risperidone metabolism is also shown.

Pharmacogenetics of cytochrome P450 and its applications in drug therapy: the past, present and future

The field of cytochrome P450 pharmacogenetics has progressed rapidly during the past 25 years. All the major human drug-metabolizing P450 enzymes have been identified and cloned, and the major gene variants that cause inter-individual variability in drug response and are related to adverse drug reactions have been identified. This information now provides the basis for the use of predictive pharmacogenetics to yield drug therapies that are more efficient and safer. Today, we understand which drugs warrant dosing based on pharmacogenetics to improve drug treatment. It is anticipated that, in the future, genotyping could be used to personalize drug treatment for vast numbers of subjects, decreasing the cost of drug treatment and increasing the efficacy of drugs and health in general. I estimate that such personalized P450 gene-based treatment would be relevant for 10-20% of all drug therapy.

Reproducibility over time of the urinary diclofenac/4'-OH diclofenac ratio among different CYP2C9 genotypes

Diclofenac has been used for the evaluation of CYP2C9 activity in vitro as well as in vivo with varying results. The present study was aimed at evaluating the reproducibility of the urinary diclofenac/4'-OH diclofenac ratio among different CYP2C9 genotypes in healthy volunteers. The study of CYP2C9 genotypes in the family of a CYP2C9*3/*3 subject is also reported. The urinary diclofenac/4'-OH diclofenac ratio was determined on two occasions within a period of 9-12 months, and was found to be correlated (r = 0.83, p < 0.05). The mean (+/- SD) of diclofenac/4'-OH diclofenac ratio was 1.5 times higher among subjects carrying CYP2C9*3 allele (CYP2C9*1/*3 and CYP2C9*2/*3 genotypes) (0.91 +/- 0.28), compared to CYP2C9*1/*1 subjects (0.60 +/- 0.11). The results show that the urinary diclofenac/4'-OH diclofenac ratio might be used to study CYP2C9 in humans. The data agree with previous studies showing that the CYP2C9*3 allelic variant seems to cause a decreased CYP2C9 hydroxylation capacity.

Cytochrome P450 2C9 phenotyping using low-dose tolbutamide

OBJECTIVES

The hypoglycaemic drug tolbutamide is used for assessment of CYP2C9 activity in vivo. However, therapeutically active doses of 500 mg bear the risk of hypoglycaemia, and a tolbutamide-derived parameter based on a single plasma or urine concentration reflecting CYP2C9 activity accurately is lacking.

METHODS

We examined tolbutamide and its metabolites 4'-hydroxy-tolbutamide and carboxytolbutamide in plasma and urine of 26 healthy, male volunteers up to 24 h after intake of 125 mg tolbutamide using liquid chromatography-tandem mass spectrometry. CYP2C9 genotypes were determined by sequencing of exons 3 and 7. Raw plasma and urine data were compared with pharmacokinetic parameters, CYP2C9 genotypes, and data from a study in 23 volunteers with all six CYP2C9*1-*3 combinations who received 500 mg tolbutamide.

RESULTS

Plasma clearance and tolbutamide plasma concentrations 24 h after drug intake reflected the genotypes: 0.85 l/h and 1.70 microg/ml (95% confidence interval, CI, 0.80-0.89 l/h and 1.50-1.90 microg/ml) for CYP2C9*1 homozygotes (n=15), 0.77 l/h and 2.14 microg/ml (95%CI, 0.67-0.88 l/h and 1.64-2.63 microg/ml) for *1/*2 genotypes (n=7), 0.60 l/h and 3.13 microg/ml (95%CI, 0.58-0.62 l/h and 2.68-3.58 microg/ml) for *1/*3 genotypes (n=3), and 0.57 l/h and 3.27 microg/ml in the single *2/*2 carrier. Natural logarithms of tolbutamide plasma concentrations 24 h after intake correlated to plasma clearance (r(2)=0.84, P<0.0000001). This correlation was confirmed in the comparison data set (r(2)=0.97, P<0.0000001).

CONCLUSIONS

A low dose of 125 mg tolbutamide can safely and accurately be used for CYP2C9 phenotyping. As a simple metric for CYP2C9 activity, we propose to determine tolbutamide in plasma 24 h after drug intake.

Genetic polymorphisms of CYP2C9 and CYP2C19 in the Beninese and Belgian populations

AIMS

To investigate the distribution of cytochrome P450 2C9 (CYP2C9) and 2C19 (CYP2C19) genotype frequencies in the Beninese and Belgian Caucasian populations.

METHODS

Beninese (n = 111) and Belgian (n = 121) were genotyped for CYP2C9*2, *3, *4, *5, and *11 as well as for CYP2C19*2 and*3.

RESULTS

The distribution of alleles was: CYP2C9*1: 95.5 vs. 82.2% (P < 0.001); CYP2C9*2: 0 vs. 10% (P < 0.001); CYP2C9*3: 0 vs. 7.4% (P < 0.01); CYP2C9*4: both 0%; CYP2C9*5: 1.8 vs. 0% (P = 0.05); and CYP2C9*11: 2.7 vs. 0.4% (P < 0.05). The frequencies of the CYP2C19*2 allele were 13 vs. 9.1%, respectively. CYP2C19*3 was not detected in either population. The 95% confidence intervals for the differences of frequencies of CYP2C9*1, CYP2C9*2, CYP2C9*3, CYP2C9*4, CYP2C9*5, CYP2C9*11, CYP2C19*1, CYP2C19*2 and CYP2C19*3 between Belgian and Beninese were 7%, 19%; - 14%, - 6%; - 11%, - 4%; - 1%, 1%; 0%, 4%; 0%, 5%; - 10%, 2%; - 2%, 10%; - 1%; respectively. The distributions of CYP2C9 genotypes in the Beninese and Belgian individuals were: CYP2C9*1/*1: 91 vs. 67% (P < 0.00001); CYP2C9*1/*2: 0 vs. 18.2% (P < 0.0001); CYP2C9*1/*3: 0 vs. 11.6% (P < 0.001); CYP2C9*1/*5: 3.6 vs. 0% (P = 0.05); CYP2C9*1/*11: 5.4 vs. 0.8% (P = 0.05); CYP2C9*2/*3: 0 vs. 1.6% (NS); CYP2C9*3/*3: 0 vs. 0.8% (NS). The distributions of CYP2C19 genotypes between these ethnic groups were: CYP2C19*1/*1: 73.9 vs. 83.5% (NS); CYP2C19*1/*2: 26.1 vs. 14.9% (P < 0.05); CYP2C9*2/*2: 0 vs. 1.6% (NS).

CONCLUSIONS

Differences of allele frequencies between Beninese and Belgian populations were statistically significant for CYP2C9*2, *3, *5 and *11, but not for CYP2C9*4 or for CYP2C19*2 and *3.

Induction of human CYP2C9 by rifampicin, hyperforin, and phenobarbital is mediated by the pregnane X receptor

Human CYP2C9 is important in the metabolism of numerous clinically used drugs such as the anticoagulant warfarin, the anticonvulsant phenytoin, antidiabetic drugs such as tolbutamide and glipizide, the hypertensive agent losartan, and numerous nonsteroidal anti-inflammatory drugs. Several studies have reported that certain drugs such as rifampicin and phenobarbital induce CYP2C9, but the molecular basis for this induction remains unknown. In the present study, we demonstrate that the human pregnane X receptor (hPXR) mediates induction of CYP2C9 by the prototype drugs rifampicin, hyperforin (found in St. John's Wart), and phenobarbital. Deletion and mutagenesis studies with luciferase reporter constructs showed that a functional PXR-responsive element located -1839/-1824 base pairs upstream from the translation start site was the primary binding site mediating the rifampicin induction of CYP2C9. This site was previously described as a constitutive androstane receptor-responsive element (CAR-RE). Mutational analysis of 3- and 12-kilobase CYP2C9 promoter fragments indicated that this proximal binding site was essential for rifampicin inducibility, although a cooperative effect could be attributed to a second CAR-RE located at -2899/-2883. In summary, we have demonstrated rifampicin induction of CYP2C9 promoter constructs that is consistent with the magnitude of induction of CYP2C9 protein and mRNA reported in vivo and in primary human hepatocytes, and we have identified the cis-element essential for this response. This is the first report to demonstrate that the nuclear receptor PXR mediates induction of CYP2C9 with rifampicin, phenobarbital, and hyperforin.

Genetic predisposition to acute gastrointestinal bleeding after NSAIDs use

Impaired drug metabolism is a major cause of adverse drug reactions, and it is often caused by mutations at genes coding for drug-metabolising enzymes. Two amino-acid polymorphisms of cytochrome P4502C9 (CYP2C9), an enzyme involved in the metabolism of several nonsteroidal anti-inflammatory drugs (NSAIDs), were studied in 94 individuals with acute bleeding after NSAIDs use and 124 individuals receiving NSAIDs with no adverse effects. The frequency of CYP2C9 variant alleles was increased in overall bleeding patients, with a significant trend to higher risk with increasing number of variant alleles (P=0.02). The odds ratio for bleeding patients receiving CYP2C9 substrates (n=33) was 2.5 for heterozygous and 3.7 for homozygous carriers of mutations (P<0.015), suggesting that the inherited impairment of CYP2C9 activity increases the risk for severe adverse drug reactions after NSAIDs use.

Review article: chemoprevention of colorectal cancer

Colorectal cancer is a disease with a high mortality at present, due to the late stage at which many cases present. Attention is therefore focusing on preventative strategies for colorectal cancer given that polyps appear to be identifiable and treatable precursor lesions of this disease. Endoscopic polypectomy has been shown to reduce the incidence of colorectal cancer and there is a good case for endoscopic screening of the general population. However, this will require a large amount of manpower and resources and its success will also depend on the overall compliance of the population. Epidemiological studies have shown that individuals reporting a regular intake of aspirin and other non-steroidal anti-inflammatory drugs have a reduced risk of developing colorectal polyps and cancer. Similarly, a number of natural substances, such as calcium and folate, when supplemented regularly in the diet, have also been linked to a possible decreased incidence of colorectal cancer. This has led to the concept of using such agents to reduce the number of cases of colorectal cancer. In this article, we review the current evidence for the use of these and other agents for the chemoprevention of colorectal cancer, together with theories as to their possible mechanisms of action.

Clinical Pharmacology

The dose of a drug is a major determinant of its safety, and establishing a safe dose of a novel drug is a prime objective during clinical development. The design of pre-marketing clinical trials precludes the representation of important subpopulations such as children, the elderly and people with co-morbidities. Therefore, postmarketing surveillance (PMS) activities are required to monitor the safety profile of drugs in real clinical practice. Furthermore, individual variations in pharmacogenetic profiles, the immune system, drug metabolic pathways and drug-drug interactions are also important factors in the occurrence of adverse drug reactions. Thus, the safety of a drug is a major clinical consideration before and after it is marketed. A multidisciplinary approach is required to enhance the safety profile of drugs at all stages of development, including PMS activities. Clinical pharmacology encompasses a range of disciplines and forms the backbone of drug safety consideration during clinical drug development. In this review we give an overview of the clinical drug development process and consider its limitations. We present a discussion of several aspects of clinical pharmacology and their application to enhancing drug safety. Pharmacokinetic-pharmacodynamic modelling provides a method of predicting a clinically safe dose; consideration of drug pharmacokinetics in special populations may enhance safe therapeutics in a wider spectrum of patients, while pharmacogenetics provides the possibility of genotype-specific therapeutics. Pharmacovigilance activities are also discussed. Given the complex nature and unpredictability of type B reactions, PMS activities are crucial in managing the risks drugs pose to the general population. The various aspects of clinical pharmacology discussed make a strong case for this field as the backbone of optimising and promoting safe development and use of drugs.

Association of pharmacokinetic (CYP2C9) and pharmacodynamic (factors II, VII, IX, and X; proteins S and C; and gamma-glutamyl carboxylase) gene variants with warfarin sensitivity

We analyzed mutations of 7 vitamin K-dependent protein and cytochrome P450 2C9 genes in 45 patients and investigated whether any contribute to the large interpatient variability in the warfarin dose-effect relationship. Total clearance and daily dose, INR and INR/Cp, were used as pharmacokinetic and pharmacodynamic indexes, respectively. Patients were grouped by genotype based on a single polymorphism and combinations of polymorphisms. Among the 30 sequence variants identified, CYP2C9*3, 165Thr-->Met of the factor II gene, -402G-->A, (37-bp repeat)n, and -746T-->C of the factor VII gene, and (CAA repeat)n of the gamma-glutamyl carboxylase gene were selected as candidate polymorphisms. As the analysis of single polymorphisms implied, the highest INR/Cp mean values and the lowest warfarin maintenance doses were observed in patients homozygous for the 165Met, -402G, (37-bp repeat)6 and -746T alleles. Multiple regression analysis revealed that warfarin sensitivity was independently associated with -402G-->A, (CAA repeat)n, CYP2C9*3, and 165Thr-->Met, which accounted for 50% of variance. These results suggest that part of the considerable interpatient variation is attributable to genetic variation, and the combined genotyping of CYP2C9 and certain vitamin K-dependent protein genes is useful for predicting anticoagulant responses.

Influence of CYP2C9 genotypes on the formation of a hepatotoxic metabolite of valproic acid in human liver microsomes

The present study investigated the effect of cytochrome P450 2C9 (CYP2C9) genetic polymorphism on the biotransformation of valproic acid (VPA) to its hepatotoxic metabolite, 4-ene-VPA, and compared that to the formation of the inactive 4-OH-VPA and 5-OH-VPA. cDNA-expressed CYP2C9(*)2 and CYP2C9(*)3 variants were less efficient than the CYP2C9(*)1 wild type in catalyzing the formation of these metabolites, as assessed by the ratio of Vmax and apparent Km (in vitro intrinsic clearance). The reduced efficiency by CYP2C9(*)2 was due to a reduced Vmax, whereas, in the case of CYP2C9(*)3, it was the result of increased apparent Km. The formation rates of 4-ene-VPA, 4-OH-VPA, and 5-OH-VPA in human liver microsomes were reduced by 29, 28, and 31%, respectively, in samples with one mutated CYP2C9 allele, and by 61, 73, and 58%, respectively, in samples with two mutated CYP2C9 alleles. Overall, the homozygote and heterozygote CYP2C9(*)2 and CYP2C9(*)3 genotypes may compromise hepatic VPA biotransformation.

Hepatic drug metabolism and transport in patients with kidney disease

The disposition of many drugs is altered in patients with acute (AKD) and chronic kidney disease (CKD). A decline in renal clearance of several drugs has been correlated significantly with residual renal function (ie, creatinine clearance) of subjects. Reductions in nonrenal clearance of some compounds also have been reported and associated with clearance of markers of oxidative and/or conjugative metabolism or P-glycoprotein-mediated transport. Although initial accounts of reduced hepatic microsomal cytochrome P-450 (CYP) content and activity in animal models of AKD and CKD were published almost 25 years ago, it is only in the last decade that technical advances in molecular biology and clinical pharmacology have enabled researchers to begin to characterize the phenotypic expression of individual enzymes and, importantly, distinguish the molecular and/or genetic basis for these changes. The selective modulation of hepatic CYP enzyme activity observed in kidney disease is caused, at least in part, by differentially altered expression of several CYP isoforms. This review summarizes data available through June 2003 regarding the effect of AKD and CKD on drug metabolism. Knowledge of the impact and nature of these alterations associated with kidney disease may facilitate the individualization of medication management in this patient population.

Clinical relevance of genetic polymorphisms in the human CYP2C9 gene

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

Differential expression and function of CYP2C isoforms in human intestine and liver

This study aimed to characterize the intestinal and hepatic expression and function of CYP2C enzymes in the same set of subjects. CYP2C isoform-specific quantitative reverse transcription-polymerase chain reaction assays, Western immunoblotting and marker reactions of CYP2C8, CYP2C9 and CYP2C19 activities were employed to investigate expression and activity of the CYP2C isoforms in samples of small intestine and liver obtained from 15 patients undergoing gastrectomy or pancreatoduodenectomy. The rank order for CYP2C mRNA expression in the intestine was CYP2C9 = CYP2C18 > CYP2C19 > CYP2C8, whereas that in the liver was CYP2C9 > CYP2C8 > CYP2C18 > CYP2C19. The rank order for expression of CYP2C protein in the intestine was CYP2C9 > CYP2C19 > CYP2C8 (content below limit of quantification) > CYP2C18 (not detected) and that in the liver was CYP2C9 > CYP2C8 > CYP2C19 > CYP2C18 (not detected). The CYP2C9 protein content was approximately 10-fold higher in the liver than in the intestine (P < 0.001). The CLint for the formation of D-703 from verapamil (marker of CYP2C8 activity) was 7.6-fold higher (P < 0.001) and that for the diclofenac 4'-hydroxylation (marker of CYP2C9 activity) was 6.1-fold higher (P < 0.001) in the liver than in the intestine. Apart from a borderline positive correlation (r = 0.58, P = 0.0504) between the intestinal and hepatic CLint for the diclofenac 4'-hydroxylation, no intra-individual relationships between these tissues with respect to expression or activity of different CYP2C isoforms were found. Collectively, these results show that CYP2C8, CYP2C9 and CYP2C19 are expressed as functional enzymes in the human small intestine, and further suggest that CYP2C genes are independently regulated in human intestine and liver. Although, overall, the expression and activity of CYP2C enzymes is lower in the gut than in the liver, the surface area of the proximal small intestine is large and intestinal CYP2C9 and CYP2C19 may well contribute to the first-pass metabolism of their substrate drugs.

Polymorphisms of drug-metabolizing enzymes CYP2C9, CYP2C19, CYP2D6, CYP1A1, NAT2 and of P-glycoprotein in a Russian population

OBJECTIVE

The frequency of functionally important mutations and alleles of genes coding for xenobiotic metabolizing enzymes shows a wide ethnic variation. However, little is known of the frequency distribution of the major allelic variants in the Russian population.

METHODS

Using polymerase chain reaction/restriction fragment length polymorphism (PCR/RFLP) genotyping assays and the real-time PCR with fluorescent probes, the frequencies of functionally important variants of the cytochromes P450 (CYP) 2C9, 2C19, 2D6, 1A1 as well as arylamine N-acetyltransferase 2 (NAT2) and P-glycoprotein (MDR1) were determined in a sample of 290 Russian volunteers derived from Voronezh area.

RESULTS

CYP2C9*2 and * 3 alleles were found with allelic frequencies of 10.5% and 6.7%, respectively. The novel intron-2 T>C mutation at exon 2 +73 bp occurred in 24.8% of alleles. CYP2C19*2 and *3 alleles occurred in 11.4% and 0.3%, respectively. Six persons (2.1%) carried two of these CYP2C19 alleles responsible for poor metabolizing activity. Of all subjects, 5.9% were CYP2D6 poor metabolizers, whereas 3.4% were addressed to ultra-rapid metabolizers (CYP2D6*1x2/*1). The CYP1A1*2A allele was found in 4.7%, *2B in 5.0%, *4 in 2.6%, and the 5'-mutations -3219C>T, -3229G>A, and the novel -4335G>A in 6.0%, 2.9% and 26.0% of alleles, respectively. Genotyping of eight different single nucleotide polymorphisms in the NAT2 gene provided in 58.0% a genotype associated with slow acetylation. The MDR1 triple variants G2677T and G2677A in exon 21 had an allelic frequency of 41.9% and 3.3%, respectively, and the variant C3435T in exon 26 one of 54.3%. Frequencies of functionally important haplotypes were calculated.

CONCLUSION

The overview of allele distribution of important xenobiotic-metabolizing enzymes among a Russian population shows similarity to other Caucasians. The data will be useful for clinical pharmacokinetic investigations and for drug dosage recommendations in the Russian population.

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

STUDY OBJECTIVE

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

DESIGN

Single-dose pharmacokinetic study.

SETTING

University general clinical research center.

SUBJECTS

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

INTERVENTION

A single oral dose of losartan 50 mg.

MEASUREMENTS AND MAIN RESULTS

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

CONCLUSION

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

Human MDR1 polymorphism: G2677T/A and C3435T have no effect on MDR1 transport activities

The two most frequently observed single nucleotide polymorphisms (SNPs) of the human multidrug resistance 1 (MDR1) gene are 2677G/T/A (893Ala/Ser/Thr) and 3435C/T (no amino acid substitution). In this study, six forms of MDR1 cDNAs with the SNPs were expressed in LLC-PK1 cells and their transport activities were determined. Nearly identical amounts of the recombinant MDR1 proteins were expressed in the established cell lines using the Flp recombinase, which integrates a gene of interest at a specific genomic location. Four structurally diverse compounds: verapamil, digoxin, vinblastine and cyclosporin A, were examined for transcellular transport activities and intracellular accumulation. No significant differences were observed between cells expressing five polymorphic types of the MDR1 cDNAs (2677G/3435T, 2677A/3435C, 2677A/3435T, 2677T/3435C, 2677T/3435T) and cells expressing the wild-type (2677G/3435C). These results suggested that the two frequently observed MDR1 SNPs had no effect on the transport activities of MDR1 proteins expressed in LLC-PK1 cells in vitro, and other genetic or environmental factors might control the expression of MDR1 and the in vivo activity of MDR1.