The CYP2C9 polymorphism: from enzyme kinetics to clinical dose recommendations

A Semi-Mechanistic Population Pharmacokinetic Model of Noscapine in Healthy Subjects Considering Hepatic First-Pass Extraction and CYP2C9 Genotypes

INTRODUCTION

Noscapine is a commonly used cough suppressant, with ongoing research on its anti-inflammatory and anti-tumor properties. The drug has a pronounced pharmacokinetic variability.

OBJECTIVE

This evaluation aims to describe the pharmacokinetics of noscapine using a semi-mechanistic population pharmacokinetic model and to identify covariates that could explain inter-individual pharmacokinetic variability.

METHODS

Forty-eight healthy volunteers (30 men and 18 women, mean age 33 years) were enrolled in a randomized, two-period, two-stage, crossover bioequivalence study of noscapine in two different liquid formulations. Noscapine plasma concentrations following oral administration of noscapine 50 mg were evaluated by a non-compartmental analysis and by a population pharmacokinetic model separately.

RESULTS

Compared to the reference formulation, the test formulation exhibited ratios (with 94.12% confidence intervals) of 0.784 (0.662-0.929) and 0.827 (0.762-0.925) for peak plasma concentrations and area under the plasma concentration-time curve, respectively. Significant differences in p values (< 0.01) were both observed when comparing peak plasma concentrations and area under the plasma concentration-time curve between CYP2C9 genotype-predicted phenotypes. A three-compartmental model with zero-order absorption and first-order elimination process best described the plasma data. The introduction of a liver compartment was able to describe the profound first-pass effect of noscapine. Total body weight and the CYP2C9 genotype-predicted phenotype were both identified as significant covariates on apparent clearance, which was estimated as 958 ± 548 L/h for extensive metabolizers (CYP2C9*1/*1 and *1/*9), 531 ± 304 L/h for intermediate metabolizers with an activity score of 1.5 (CYP2C9*1/*2), and 343 ± 197 L/h for poor metabolizers and intermediate metabolizers with an activity score of 1.0 (CYP2C9*1/*3, *2/*3, and*3/*3).

CONCLUSION

The current work is expected to facilitate the future pharmacokinetic/pharmacodynamic development of noscapine. This study was registered prior to starting at "Deutsches Register Klinischer Studien" under registration no. DRKS00017760.

Comparison of intravenous ibuprofen pharmacokinetics between Caucasian and Chinese populations using physiologically based pharmacokinetics modeling and simulation

BACKGROUND

Intravenous ibuprofen, a nonsteroidal anti-inflammatory drug, is widely used as an antipyretic and analgesic in adults and children. This study was designed to evaluate ethnic differences by comparing the pharmacokinetics of intravenous ibuprofen in Caucasian and Chinese populations using physiologically based pharmacokinetics (PBPK) modeling and simulation.

METHODS

A PBPK model for intravenous ibuprofen was developed in adults and children utilizing the Simcyp Simulator. The model was tested and verified against published literature and unpublished data obtained from the Caucasian adult population, Caucasian pediatric population and Chinese adult population.

RESULTS

The developed PBPK model could adequately pilot the pharmacokinetics of intravenous ibuprofen, and the major observed values were within the 90% prediction interval in both adults and children. Both folding errors of the maximum peak concentration (Cmax) and area under the concentration-time curve (AUC) were 1.5-fold less in the Caucasian and Chinese populations. In addition, no significant differences in weight-normalized Cmax and AUC were observed between the Caucasian and Chinese adult populations. Moreover, there were no obvious pharmacokinetic differences between the Caucasian and Chinese pediatric populations with intravenous infusion (10 minutes) of 10 mg/kg by age group.

CONCLUSION

This study indicates that the pharmacokinetic profile and the parameters of intravenous ibuprofen are analogous in Caucasian and Chinese populations, either adults or children. In addition, this study provides effective evidence that the dosing scheme of intravenous ibuprofen in Chinese children can remain the same as the regimen that the original company (Caldolor®) provided.

Modeling adult COX-2 cerebrospinal fluid pharmacokinetics to inform pediatric investigation

AIM

Hysteresis is reported between plasma concentration and analgesic effect from nonsteroidal anti-inflammatory drugs. It is possible that the temporal delay between plasma and CSF nonsteroidal anti-inflammatory drugs mirrors this hysteresis. The temporal relationship between plasma and CSF concentrations of COX-inhibitors (celecoxib, rofecoxib, valdecoxib) has been described. The purpose of this secondary data analysis was to develop a compartmental model for plasma and CSF disposition of these COX-2 inhibitors.

METHODS

Plasma and CSF concentration-time profiles and protein binding data in 10 adult volunteers given oral celecoxib 200 mg, valdecoxib 40 mg and rofecoxib 50 mg were available for study. Nonlinear mixed effects models with a single plasma compartment were used to link a single CSF compartment with a transfer factor and an equilibration rate constant (Keq). To enable predictive modeling in pediatrics, celecoxib pharmacokinetics were standardized using allometry.

RESULTS

Movement of all three unbound plasma COX-2 drugs into CSF was characterized by a common equilibration half-time (T_1/2 keq) of 0.84 h. Influx was faster than efflux and a transfer scaling factor of 2.01 was required to describe conditions at steady-state. Estimated celecoxib clearance was 49 (95% CI 34-80) L/h/70 kg and the volume of distribution was 346 (95% CI 237-468) L/70 kg. The celecoxib absorption half-time was 0.35 h with a lag time of 0.62 h. Simulations predicted a 70-kg adult given oral celecoxib 200 mg with maintenance 100 mg twice daily would have a mean steady-state total (bound and unbound) plasma concentration of 174 μg L^-1 and CSF concentration of 1.1 μg L^-1 . A child (e.g., 25 kg, typically 7 years) given oral celecoxib 6 mg kg^-1 with maintenance of 3 mg kg^-1 twice daily would have 282 and 1.7 μg L^-1 mean plasma and CSF concentrations, respectively.

CONCLUSIONS

Transfer of unbound COX-2 inhibitors from plasma to CSF compartment can be described with a delayed effect model using an equilibration rate constant to collapse observed hysteresis. An additional transfer factor was required to account for passage across the blood-brain barrier. Use of a target concentration strategy for dose and consequent plasma (total and unbound) and CSF concentration prediction could be used to inform pediatric clinical studies.

Torsemide Pharmacometrics in Healthy Adult Populations Including CYP2C9 Genetic Polymorphisms and Various Patient Groups through Physiologically Based Pharmacokinetic-Pharmacodynamic Modeling

Torsemide is a widely used diuretic in clinical practice. In this study, pharmacokinetic (PK) and pharmacodynamic (PD) simulations of torsemide for various population groups and exposure scenarios were performed through human-scale physiologically-based PK-PD (PBPK-PD) modeling of torsemide. For PBPK-PD modeling of torsemide, invitro and clinical data of torsemide reported previously were used. After exposure to clinical doses of torsemide, observed plasma (or serum) concentration and urine torsemide excretion profiles were used as PK-data, and observed urinary sodium excretion rate was used as PD-data. The model was then extended to take into account physiological and biochemical factors according to different CYP2C9 phenotypes or patient populations. The established model captured various torsemide clinical results well. Differences in torsemide PKs and PDs between patient groups or CYP2C9 genetic polymorphisms were modelologically identified. It was confirmed that degrees of differences in torsemide PKs and PDs by disease groups were greater than those according to different CYP2C9 phenotypes. According to torsemide administration frequency or dose change, it was confirmed that although the difference in plasma PKs between groups (healthy adult and patient groups) could increase to 14.80 times, the difference in PDs was reduced to 1.01 times. Results of this study suggested that it is very important to consider disease groups in the setting of torsemide clinical therapy and that it is difficult to predict PD proportionally with only differences in PKs of torsemide between population groups. The PBPK-PD model established in this study is expected to be utilized for various clinical cases involving torsemide application in the future, enabling optimal drug therapy.

Physiologically based pharmacokinetic modelling to predict the pharmacokinetics of metoprolol in different CYP2D6 genotypes

Metoprolol, a selective β₁-adrenoreceptor blocking agent used in the treatment of hypertension, angina, and heart failure, is primarily metabolized by the CYP2D6 enzyme, which catalyzes α-hydroxylation and O-desmethylation. As CYP2D6 is genetically highly polymorphic and the enzymatic activity differs greatly depending on the presence of the mutant allele(s), the pharmacokinetic profile of metoprolol is highly variable depending on the genotype of CYP2D6. The aim of study was to develop the physiologically based pharmacokinetic (PBPK) model of metoprolol related to CYP2D6 genetic polymorphism for personalized therapy with metoprolol. For PBPK modelling, our previous pharmacogenomic data were used. To obtain kinetic parameters (K_m, V_max, and CL_int) of each genotype, the recombinant CYP enzyme of each genotype was incubated with metoprolol and metabolic rates were assayed. Based on these data, the PBPK model of metoprolol was developed and validated in different CYP2D6 genotypes using PK-Sim^® software. As a result, the input values for various parameters for the PBPK model were presented and the PBPK model successfully described the pharmacokinetics of metoprolol in each genotype group. The simulated values were within the acceptance criterion (99.998% confidence intervals) compared with observed values. The PBPK model developed in this study can be used for personalized pharmacotherapy with metoprolol in individuals of various races, ages, and CYP2D6 genotypes.

Physiologically based pharmacokinetic (PBPK) modeling of piroxicam with regard to CYP2C9 genetic polymorphism

Piroxicam is a non-steroidal anti-inflammatory drug used to alleviate symptoms of osteoarthritis and rheumatoid arthritis. CYP2C9 genetic polymorphism significantly influences the pharmacokinetics of piroxicam. The objective of this study was to develop and validate the piroxicam physiologically based pharmacokinetic (PBPK) model related to CYP2C9 genetic polymorphism. PK-Sim^® version 10.0 was used for the PBPK modeling. The PBPK model was evaluated by predicted and observed plasma concentration-time profiles, fold errors of predicted to observed pharmacokinetic parameters, and a goodness-of-fit plot. The turnover number (k_cat) of CYP2C9 was adjusted to capture the pharmacokinetics of piroxicam in different CYP2C9 genotypes. The population PBPK model overall accurately described and predicted the plasma concentration-time profiles in different CYP2C9 genotypes. In our simulations, predicted AUC_inf in CYP2C9*1/*2, CYP2C9*1/*3, and CYP2C9*3/*3 genotypes were 1.83-, 2.07-, and 6.43-fold higher than CYP2C9*1/*1 genotype, respectively. All fold error values for AUC, C_max, and t_1/2 were included in the acceptance criterion with the ranges of 0.57-1.59, 0.63-1.39, and 0.65-1.51, respectively. The range of fold error values for predicted versus observed plasma concentrations was 0.11-3.13. 93.9% of fold error values were within the two-fold range. Average fold error, absolute average fold error, and root mean square error were 0.93, 1.27, and 0.72, respectively. Our model accurately captured the pharmacokinetic alterations of piroxicam according to CYP2C9 genetic polymorphism.

Impact of genetic and non-genetic factors on hepatic CYP2C9 expression and activity in Hungarian subjects

CYP2C9, one of the most abundant hepatic cytochrome P450 enzymes, is involved in metabolism of 15–20% of clinically important drugs (warfarin, sulfonylureas, phenytoin, non-steroid anti-inflammatory drugs). To avoid adverse events and/or impaired drug-response, CYP2C9 pharmacogenetic testing is recommended. The impact of CYP2C9 polymorphic alleles (CYP2C9*2, CYP2C9*3) and phenoconverting non-genetic factors on CYP2C9 function and expression was investigated in liver tissues from Caucasian subjects (N = 164). The presence of CYP2C9*3 allele was associated with CYP2C9 functional impairment, and CYP2C9*2 influenced tolbutamide 4′-hydroxylase activity only in subjects with two polymorphic alleles, whereas the contribution of CYP2C8*3 was not confirmed. In addition to CYP2C9 genetic polymorphisms, non-genetic factors (co-medication with CYP2C9-specific inhibitors/inducers and non-specific factors including amoxicillin + clavulanic acid therapy or chronic alcohol consumption) contributed to the prediction of hepatic CYP2C9 activity; however, a CYP2C9 genotype–phenotype mismatch still existed in 32.6% of the subjects. Substantial variability in CYP2C9 mRNA levels, irrespective of CYP2C9 genotype, was demonstrated; however, CYP2C9 induction and non-specific non-genetic factors potentially resulting in liver injury appeared to modify CYP2C9 expression. In conclusion, complex implementation of CYP2C9 genotype and non-genetic factors for the most accurate estimation of hepatic CYP2C9 activity may improve efficiency and safety of medication with CYP2C9 substrate drugs in clinical practice.

Development of the PGx-Passport: A Panel of Actionable Germline Genetic Variants for Pre-Emptive Pharmacogenetic Testing

Pre-emptive pharmacogenetics (PGx) testing of a panel of germline genetic variants represents a new model for personalized medicine. Clinical impact of PGx testing is maximized when all variant alleles for which actionable clinical guidelines are available are included in the test panel. However, no such standardized panel has been presented to date, impeding adoption, exchange, and continuity of PGx testing. We, therefore, developed such a panel, hereafter called the PGx-Passport, based on the actionable Dutch Pharmacogenetics Working Group (DPWG) guidelines. Germline-variant alleles were systematically selected using predefined criteria regarding allele population frequencies, effect on protein functionality, and association with drug response. A PGx-Passport of 58 germline variant alleles, located within 14 genes (CYP2B6, CYP2C9, CYP2C19, CYP2D6, CYP3A5, DPYD, F5, HLA-A, HLA-B, NUDT15, SLCO1B1, TPMT, UGT1A1, and VKORC1) was composed. This PGx-Passport can be used in combination with the DPWG guidelines to optimize drug prescribing for 49 commonly prescribed drugs.

Distribution of polymorphisms in the CYP2C9 gene and CYP2C19/CYP2C9 haplotypes among Venezuelan populations

BACKGROUND

Polymorphisms with decreased enzyme activity of their gene products have been reported in region CYP2C with population variations in haplotype structure.

AIM

To estimate the allelic and genotypic frequencies of variants CYP2C9*2 and CYP2C9*3 and of CYP2C9/CYP2C19 haplotypes in Venezuelan populations.

SUBJECTS AND METHODS

Six hundred and thirty-four individuals from nine admixed populations (AP) and the Warao indigenous group were studied. Allelic frequencies, linkage disequilibrium and genetic distances for haplotypes were calculated and compared within Venezuela and with data available in the literature.

RESULTS

Heterogeneity in the distribution of CYP2C9 alleles and CYP2C9/CYP2C19 haplotypes among the AP and the Warao was observed. The joint frequency of haplotypes, with at least one non-functional variant, shows values in AP between 21-41%, while in Warao it reaches 5%. The haplotype that includes the Asian and rare Latin America CYP2C19*3 allele was detected in most AP and in Warao. Pairwise Fst values showed that the Warao was an outlier compared with the AP, while these are closer to European-derived populations. No significant correlation was found between haplotype frequencies and admixture.

CONCLUSIONS

These results support the need to understand the distribution of genomic biomarkers related to the metabolism of drugs, for planning national public health strategies.

CYP2C9 genotype vs. metabolic phenotype for individual drug dosing--a correlation analysis using flurbiprofen as probe drug

Currently, genotyping of patients for polymorphic enzymes responsible for metabolic elimination is considered a possibility to adjust drug dose levels. For a patient to profit from this procedure, the interindividual differences in drug metabolism within one genotype should be smaller than those between different genotypes. We studied a large cohort of healthy young adults (283 subjects), correlating their CYP2C9 genotype to a simple phenotyping metric, using flurbiprofen as probe drug. Genotyping was conducted for CYP2C9*1, *2, *3. The urinary metabolic ratio MR (concentration of CYP2C9-dependent metabolite divided by concentration of flurbiprofen) determined two hours after flurbiprofen (8.75 mg) administration served as phenotyping metric. Linear statistical models correlating genotype and phenotype provided highly significant allele-specific MR estimates of 0.596 for the wild type allele CYP2C9*1, 0.405 for CYP2C9*2 (68 % of wild type), and 0.113 for CYP2C9*3 (19 % of wild type). If these estimates were used for flurbiprofen dose adjustment, taking 100 % for genotype *1/*1, an average reduction to 84 %, 60 %, 68 %, 43 %, and 19 % would result for genotype *1/*2, *1/*3, *2/*2, *2/*3, and *3/*3, respectively. Due to the large individual variation within genotypes with coefficients of variation ≥ 20 % and supposing the normal distribution, one in three individuals would be out of the average optimum dose by more than 20 %, one in 20 would be 40 % off. Whether this problem also applies to other CYPs and other drugs has to be investigated case by case. Our data for the given example, however, puts the benefit of individual drug dosing to question, if it is exclusively based on genotype.

Pharmacogenomics of acetylsalicylic acid and other nonsteroidal anti-inflammatory agents: clinical implications

PURPOSE

Pharmacogenomics investigates interindividual genetic variability in the DNA sequence of drug targets, drug-metabolizing enzymes or disease genes, RNA expression, or protein translation of genes affecting drug response and drug safety. Aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs) are among the most commonly prescribed medications with well-documented variation in patient response in terms of efficacy and safety. This variation may in part be explained by pharmacogenomics.

METHODS

In this paper I review data on the pharmacogenomics of aspirin and other NSAIDs focusing on clinical implications.

RESULTS

Existing scientific evidence supports the pharmacogenomic basis of interindividual variation in treatment response to aspirin and NSAIDs, with clinical implications for antiplatelet action, cancer chemoprevention, and drug safety. However, further research efforts are needed before knowledge on the pharmacogenomics of aspirin and NSAIDs can be implemented in clinical practice.

CONCLUSION

The outcome of these research efforts would be anticipated to have added value for both science and society, contributing to the enhanced efficacy and safety of these agents through patient selection.

Impact of genetic polymorphisms in CYP2C9 and CYP2C19 on the pharmacokinetics of clinically used drugs

Human cytochrome P450 (CYP) is a superfamily of hemoproteins which oxidize a number of endogenous compounds and xenobiotics. The human CYP2C subfamily consists of four members: CYP2C8, CYP2C9, CYP2C18 and CYP2C19. CYP2C9 and CYP2C19 are important drug-metabolizing enzymes and together metabolize approximately 20% of therapeutically used drugs. Forty-two allelic variants for CYP2C9 and 34 for CYP2C19 have been reported. The frequencies of these variants show marked inter-ethnic variation. The functional consequences of genetic polymorphisms have been examined, and many studies have shown the clinical importance of these polymorphisms. Current evidence suggests that taking the genetically determined metabolic capacity of CYP2C9 and CYP2C19 into account has the potential to improve individual risk/benefit relationships. However, more prospective studies with clinical endpoints are needed before the paradigm of "personalized medicine" based on the variants can be established. This review summarizes the currently available important information on this topic.

Novel drug metabolism indices for pharmacogenetic functional status based on combinatory genotyping of CYP2C9, CYP2C19 and CYP2D6 genes

AIMS

We aim to demonstrate clinical relevance and utility of four novel drug-metabolism indices derived from a combinatory (multigene) approach to CYP2C9, CYP2C19 and CYP2D6 allele scoring. Each index considers all three genes as complementary components of a liver enzyme drug metabolism system and uniquely benchmarks innate hepatic drug metabolism reserve or alteration through CYP450 combinatory genotype scores.

METHODS

A total of 1199 psychiatric referrals were genotyped for polymorphisms in the CYP2C9, CYP2C19 and CYP2D6 gene loci and were scored on each of the four indices. The data were used to create distributions and rankings of innate drug metabolism capacity to which individuals can be compared. Drug-specific indices are a combination of the drug metabolism indices with substrate-specific coefficients.

RESULTS

The combinatory drug metabolism indices proved useful in positioning individuals relative to a population with regard to innate drug metabolism capacity prior to pharmacotherapy. Drug-specific indices generate pharmacogenetic guidance of immediate clinical relevance, and can be further modified to incorporate covariates in particular clinical cases.

CONCLUSIONS

We believe that this combinatory approach represents an improvement over the current gene-by-gene reporting by providing greater scope while still allowing for the resolution of a single-gene index when needed. This method will result in novel clinical and research applications, facilitating the translation from pharmacogenomics to personalized medicine, particularly in psychiatry where many drugs are metabolized or activated by multiple CYP450 isoenzymes.

Interethnic and Intraethnic Variability of CYP2C8 and CYP2C9 Polymorphisms in Healthy Individuals

Cytochrome P450 (CYP) superfamily members CYP2C8 and CYP2C9 are polymorphically expressed enzymes that are involved in the metabolic inactivation of several drugs, including, among others, antiepileptics, NSAIDs, oral hypoglycemics, and anticoagulants. Many of these drugs have a narrow therapeutic index, and growing evidence indicates a prominent role of CYP2C8 and CYP2C9 polymorphisms in the therapeutic efficacy and in the development of adverse effects among patients treated with drugs that are CYP2C8 or CYP2C9 substrates. In this review, we summarize present knowledge on human variability in the frequency of variant CYP2C8 and CYP2C9 alleles. Besides an expected interethnic variability in allele frequencies, a large intraethnic variability exists. Among Asian subjects, for example, statistically significant differences (p < 0.0001) in CYP2C9*3 allele frequencies between Chinese and Japanese individuals have been reported. In addition, individuals from East Asia present different allele frequencies for CYP2C9*2 and CYP2C9*3 compared with South Asian subjects (p < 0.0001). Among Caucasian Europeans, statistically significant differences for the frequency of CYP2C8*3, CYP2C9*2, and CYP2C9*3 exist (p < 0.0001). This indicates that Asian individuals or Caucasian European individuals cannot be considered as homogeneous groups regarding CYP2C8 or CYP2C9 allele frequencies. Caucasian American subjects also show a large variability in allele frequencies, which is likely to be related to ethnic ancestry. A higher frequency of variant CYP2C8 and CYP2C9 alleles is expected among Caucasian Americans with South European ancestry than in individuals with North European ancestry. The findings summarized in this review suggest that among individuals with Asian or European ancestry, intraethnic differences in the risk of developing adverse effects with drugs that are CYP2C8 or CYP2C9 substrates are to be expected. In addition, the observed intraethnic variability reinforces the need for proper selection of control subjects and points against the use of surrogate control groups for studies involving association of CYP2C8 or CYP2C9 alleles with adverse drug reactions or spontaneous diseases.

Metabolic capabilities of cytochrome P450 enzymes in Chinese liver microsomes compared with those in Caucasian liver microsomes

AIM

The most common causes of variability in drug response include differences in drug metabolism, especially when the hepatic cytochrome P450 (CYP) enzymes are involved. The current study was conducted to assess the differences in CYP activities in human liver microsomes (HLM) of Chinese or Caucasian origin.

METHODS

The metabolic capabilities of CYP enzymes in 30 Chinese liver microsomal samples were compared with those of 30 Caucasian samples utilizing enzyme kinetics. Phenacetin O-deethylation, coumarin 7-hydroxylation, bupropion hydroxylation, amodiaquine N-desethylation, diclofenac 4'-hydroxylation (S)-mephenytoin 4'-hydroxylation, dextromethorphan O-demethylation, chlorzoxazone 6-hydroxylation and midazolam 1'-hydroxylation/testosterone 6β-hydroxylation were used as probes for activities of CYP1A2, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1 and CYP3A, respectively. Mann-Whitney U test was used to assess the differences.

RESULTS

The samples of the two ethnic groups were not significantly different in cytochrome-b(5) concentrations but were significantly different in total CYP concentrations and NADPH-P450 reductase activity (P < 0.05). Significant ethnic differences in intrinsic clearance were observed for CYP1A2, CYP2C9, CYP2C19 and CYP2E1; the median values of the Chinese group were 54, 58, 26, and 35% of the corresponding values of the Caucasian group, respectively. These differences were associated with differences in Michaelis constant or maximum velocity. Despite negligible difference in intrinsic clearance, the Michaelis constant of CYP2B6 appeared to have a significant ethnic difference. No ethnic difference was observed for CYP2A6, CYP2C8, CYP2D6 and CYP3A.

CONCLUSIONS

These data extend our knowledge on the ethnic differences in CYP enzymes and will have implications for drug discovery and drug therapy for patients from different ethnic origins.

Genetically based impairment in CYP2C8- and CYP2C9-dependent NSAID metabolism as a risk factor for gastrointestinal bleeding: is a combination of pharmacogenomics and metabolomics required to improve personalized medicine?

Polymorphisms in CYP2C8 and CYP2C9 are common in all the human populations and many CYP2C8 and CYP2C9 gene variations cause decreased enzyme activity towards the NSAIDs aceclofenac, celecoxib, diclofenac, ibuprofen, indomethazine, lornoxicam, meloxicam, naproxen, piroxicam, tenoxicam and valdecoxib. This impairment in drug biodisposition alters drug pharmacokinetics, with carriers of detrimental mutations displaying increased values of AUC and decreased drug clearance. Individuals carrying the gene variants CYP2C8*3 (rs11572080; rs10509681), CYP2C9*2 (rs1799853) or CYP2C9*3 (rs1057910) show increased risk of developing acute gastrointestinal bleeding during the use of NSAID that are CYP2C8 or CYP2C9 substrates. However, it is not known whether parent drugs or products of alternative metabolic pathways are responsible for bleeding. We present an overview of the current knowledge of relevant polymorphisms of CYP2C8 and CYP2C9 genes, their association with NSAID metabolism and pharmacokinetics and a meta-analysis that confirms the clinical significance of these gene variations with regard to gastrointestinal bleeding.

Global variation in CYP2C8-CYP2C9 functional haplotypes

We have studied the global frequency distributions of 10 single nucleotide polymorphisms (SNPs) across 132 kb of CYP2C8 and CYP2C9 in ∼2500 individuals representing 45 populations. Five of the SNPs were in noncoding sequences; the other five involved the more common missense variants (four in CYP2C8, one in CYP2C9) that change amino acids in the gene products. One haplotype containing two CYP2C8 coding variants and one CYP2C9 coding variant reaches an average frequency of 10% in Europe; a set of haplotypes with a different CYP2C8 coding variant reaches 17% in Africa. In both cases these haplotypes are found in other regions of the world at <1%. This considerable geographic variation in haplotype frequencies impacts the interpretation of CYP2C8/CYP2C9 association studies, and has pharmacogenomic implications for drug interactions.

Relative impact of genotype and enzyme induction on the metabolic capacity of CYP2C9 in healthy volunteers

Pharmacokinetics in individual subjects is determined by genes and environment. The relative contributions of enzyme induction and inherited genomic variation to cytochrome P450 enzyme 2C9 (CYP2C9) activity are unknown. In 130 volunteers, CYP2C9 activity was measured in vivo using tolbutamide as a probe drug. Tolbutamide was administered orally, and the pharmacokinetics of the drug was analyzed twice--before and after four doses of 450 mg rifampin. Mean total apparent clearances (Cl/F) in the genotype groups CYP2C9*1/*1, *1/*2, *1/*3, *2/*3, and *3/*3 before rifampin were 0.78, 0.74, 0.52, 0.40, and 0.13 l/h, respectively. After rifampin administration, these clearances increased in all genotype groups by a median factor of 1.9 (range 1.1-4.8). The combined effects of genes and environment could be predicted by a simple additive model. Thus, enzyme induction resulted in an approximately twofold difference in CYP2C9 activity, irrespective of the CYP2C9 genotypes. But the difference in activity levels between the CYP2C9*1/*1 and *3/*3 genotypes before the administration of rifampin was sixfold.

[Clinical pharmacogenomics for CYP2C8 and CYP2C9: general concepts and application to the use of NSAIDs]

OBJECTIVE

To study the major mutations in genes CYP2C8 and CYP2C9, their frequency in populations of diverse ethnical descent, their analysis methods, and the major drugs with affected metabolism, with a special emphasis on NSAIDs.

METHOD

Repeated searches of Pubmed (January 1966-January 2006) and Scholar Google were performed. All searches were restricted to studies in humans, and papers not written in Spanish or English were excluded.

RESULTS

Ten allelic variants of CYP2C8 and 24 of CYP2C have been reported. Not all of them exert a relevant effect on drug metabolism. In Caucasians 22% of CYP2C8 genes and 31% of CYP2C9 genes have mutations. In Asians fewer than 1% and nearly 3% are mutated, respectively. Major identification methods include endonuclease digestion, PCR, pyrosequencing, and microarrays. Not all NSAIDs are exclusive substrates for CYP2C8/9. The usefulness of allelic variant analysis varies with each individual drug. The risk for digestive hemorrhage associated with the CYP2C9 genotype is particularly relevant when using aceclofenac, celecoxib, diclofenac, ibuprofen, indomethacin, lornoxicam, piroxicam, or naproxen.

CONCLUSIONS

Although CYP2C8/9 activity plays an essential role in the metabolism of and clinical response to many NSAIDs, the use of pharmacogenomic techniques is not equally useful for all these drugs.

Can pharmacogenetics help rescue drugs withdrawn from the market?

Observations over the later half of the last century have suggested that genetic factors may be the prime determinant of drug response, at least for some drugs. Retrospectively gathered data have provided further support to the notion that genotype-based prescribing will improve the overall efficacy rates and minimize adverse drug reactions (ADRs), making personalized medicine a reality. During the last 16 years, 38 drugs have been withdrawn from major markets due to safety concerns. Inevitably, a question arises as to whether it might be possible to 'rescue' some of these drugs by promoting genotype-based prescribing. However, ironically pharmacogenetics has not perceptibly improved the risk/benefit of a large number of genetically susceptible drugs that are already in wide clinical use and are associated with serious ADRs. Drug-induced hepatotoxicity and QT interval prolongation (with or without torsade de pointes) account for 24 (63%) of these 38 drug withdrawals. In terms of the number of drugs implicated, both these toxicities are on the increase. Many others have had to be withdrawn due to their inappropriate use. This paper discusses the criteria that a drug would need to fulfill, and summarizes the likely regulatory requirements, before its pharmacogenetic rescue can be considered to be realistic. One drug that fulfils these criteria is perhexiline (withdrawn worldwide in 1988) and is discussed in some detail. For the majority of these 38 drugs there are, at present, no candidates for genetic traits to which the toxicity that led to their withdrawal may be linked. For a few other drugs where a potential candidate for a genetic trait might explain the toxicity of concern, the majority of patients who experienced the index toxicity had easily managed nongenetic risk factors. It may be possible to rescue these drugs simply by careful attention to their dose, interaction potential and prescribing patterns, but without the need for any pharmacogenetic test. In addition, the pharmacogenetic rescue of drugs might not be as effective as anticipated as hardly any pharmacogenetic test is known to have the required test efficiency to promote individualized therapy. Multiple pathways of drug elimination, contribution to toxicity by metabolites as well as the parent drug, gene-gene interactions, multiple mechanisms of toxicity and inadequate characterization of phenotype account for this lack of highly predictive tests. The clinical use of tests that lack the required efficiency carries the risks of over- or under-dosing some patients, denying the drug to others and decreasing physician vigilance of patients. Above all, at present, prescribing physicians lack an adequate understanding of pharmacogenetics and its limitations. It is also questionable whether their prescribing will comply with the requirements for pretreatment pharmacogenetic tests to make pharmacogenetic rescue a realistic goal.

Genetic variation at the CYP2C locus and its association with torsemide biotransformation

In 97 unselected volunteers and two additional homozygous carriers of CYP2C9(*)3, we investigated the oral clearance of torsemide in relation to 37 polymorphisms at the CYP2C gene locus. Torsemide total oral clearance was linearly associated with the number of CYP2C9(*)3 alleles (geometric mean: 59, 40 and 20 ml/min in carriers of no, one and two alleles) and so were the methyl- and ring-hydroxylation but not the carboxylation clearance. Haplotypes including the CYP2C9(*)3 allele were similarly associated with the clearances but no other variant and no haplotype not including the CYP2C9(*)3 variant. The extended haplotype length (EHL) of the CYP2C9 haplotypes was positively associated with higher activity of the gene product. Torsemide total oral clearance was predictable with r(2)=82.1% using plasma concentrations at 0.5, 1, 2 and 24 h. In conclusion, torsemide's biotransformation strongly depended on the CYP2C9(*)3 variant but no other. Higher clearance CYP2C9 haplotypes appear to be evolutionarily selected.

High carrier prevalence of deficient and null alleles of CYP2 genes in a major USA hospital: implications for personalized drug safety

Many drugs are metabolized by highly polymorphic cytochrome P450 (CYP) enzymes. Among these enzymes, members of the CYP2 family coded by the CYP2D6, CYP2C9 and CYP2C19 genes are best amenable to the precise prediction of an individual's innate capacity to metabolize drugs by DNA typing of inherited null and deficient alleles. We determined the frequency of these alleles and the prevalence of their carriers in a New England, USA, tertiary care center to assess underlying population genetic features for the practice of personalized medicine. We determined that 54, 25 and 27% are carriers of at least one deficient or null allele for the CYP2D6, CYP2C9 and CYP2C19 genes, respectively. Furthermore, 6% of individuals are carriers of two null alleles for CYP2D6 and are predicted to have no biochemical activity for this isoenzyme. These results support the implementation of DNA typing of CYP2 genes to diagnose adverse drug reactions and to prevent a substantial number of patients being prescribed drugs they cannot adequately metabolize.