Effects of CYP2C9 polymorphisms on the pharmacokinetics of R- and S-phenprocoumon in healthy volunteers

Adverse Drug Reactions in the Emergency Department: Is There a Role for Pharmacogenomic Profiles at Risk?-Results from the ADRED Study

Individual differences in required drug dosages exist based on the pharmacogenomic (PGx) profiles. This study aimed to assess associations between PGx profiles and adverse drug reactions (ADR) that lead to admissions to the emergency department (ED). ADR cases of the prospective multi-center observational trial in EDs (ADRED study) were analyzed (n = 776) together with the relevant PGx phenotypes of the enzymes CYP2D6, CYP2C19, CYP2C9, and VKORC1. Overall, the allele frequency distribution in this cohort did not differ from the population frequencies. We compared the frequencies of phenotypes in the subgroups with the drugs suspected of certain ADR, in the remaining cases. The frequency distribution of CYP2C19 differed for the ADR bleeding cases suspected of clopidogrel (p = 0.020). In a logistic regression analysis, higher CYP2C19 activity (OR (95% CI): 4.97 (1.73−14.27)), together with age (1.05 (1.02−1.08)), showed an impact on the clopidogrel-suspecting ADRs, when adjusting for the clinical parameters. There was a trend for an association of phenprocoumon-risk profiles (low VKORC1 or CYP2C9 activity) with phenprocoumon-suspecting ADRs (p = 0.052). The PGx impact on serious ADRs might be highest in drugs that cannot be easily monitored or those that do not provoke mild ADR symptoms very quickly. Therefore, patients that require the intake of those drugs with PGx variability such as clopidogrel, might benefit from PGx testing.

Phenprocoumon Dose Requirements, Dose Stability and Time in Therapeutic Range in Elderly Patients With CYP2C9 and VKORC1 Polymorphisms

Background

Dose requirements of vitamin K antagonists are associated with CYP2C9 and VKORC1, but, compared to warfarin, less data is available about phenprocoumon. Furthermore, the effects on dose stability and anticoagulation quality are still unclear.

Methods

Aim was to scrutinize phenprocoumon dose requirements, dose stability and anticoagulation quality in association to CYP2C9 and VKORC1 in a natural cohort of elderly primary care patients. As a subgroup within the IDrug study, phenprocoumon treated patients with at least two INR values within three months before enrollment (n = 209) were analyzed concerning average weekly dose, standard deviation of weekly dose (intra-subject variability), constant dose (yes/no), average INR and TTR grouped by CYP2C9 and VKORC1 (and combinations).

Results

Average weekly dose per patient was 14.4 ± 5.3 mg, 11.9 ± 4.0 mg and 11.2 ± 4.3 mg in CYP2C9 wildtypes, *2 and *3 carriers (p < .0001) and 16.0 ± 4.2 mg, 13.3 ± 5.1 mg and 8.0 ± 2.7 mg per week in VKORC1 CC, CT and TT genotypes, respectively (p < .0001). Significant differences concerning intra-subject variability were detected among all groups (p < .0001) with the smallest variability in CYP2C9*3 carriers. TTR medians were 75.4%, 79.4% and 100% in wildtypes, *2 and *3 carriers, respectively (p = 0.0464). The proportion of patients with perfect control was highest among *3 carriers, but this result was not significant (p = 0.0713).

Discussion

Our analyses support the results of previous investigations regarding genotype-associated dose requirements and raise the hypothesis that dose stability and anticoagulation quality may be increased in CYP2C9*3 carriers. However, our data should be treated cautiously due to the small sample size.

Clinical Trial Registration

German Clinical Trials Register, identifier DRKS00006256.

Pharmacogenetics in the Treatment of Cardiovascular Diseases and Its Current Progress Regarding Implementation in the Clinical Routine

There is a special interest in the implementation of pharmacogenetics in clinical practice, although there are some barriers that are preventing this integration. A large part of these pharmacogenetic tests are focused on drugs used in oncology and psychiatry fields and for antiviral drugs. However, the scientific evidence is also high for other drugs used in other medical areas, for example, in cardiology. In this article, we discuss the evidence and guidelines currently available on pharmacogenetics for clopidogrel, warfarin, acenocoumarol, and simvastatin and its implementation in daily clinical practice.

Effects of the genetic variants of organic cation transporters 1 and 3 on the pharmacokinetics of metformin in Jordanians

BACKGROUND

Human response to the antidiabetic metformin is influenced by some factors, such as genetic variants in the SLC22A genes. This study aimed to determine the frequency of main SLC22A1 and SLC22A3 genetic variants and their influence on metformin pharmacokinetics among healthy unrelated Arab Jordanians.

PATIENTS AND METHODS

The SLC22A1 and SLC22A3 genes were genotyped by DNA sequencing of exons 1, 3, 7, and 9 in the SLC22A1 gene and exons 6, 7, and 9 in the SLC22A3 gene. Then, a clinical pharmacokinetic study was conducted on 26 healthy volunteers. The pharmacokinetic parameters were calculated using non-compartmental model analysis. The study was an open-label, randomized study with single 1000 mg metformin administration.

RESULTS

Results showed that volunteers with SLC22A3 rs8187722 variant had higher (χ2, p<0.05) metformin Cmax and AUC values than the wild SLC22A3 volunteers, whereas T½ and Kel were not affected. In addition, volunteers with the heterozygote SLC22A3 rs2292334 variant had significantly higher (χ2, p<0.05) metformin Cmax and AUC and lower Kel values than the wild-type SLC22A3 genotype.

CONCLUSIONS

The SLC22A3 rs8187722 and rs2292334 genetic variants affected metformin pharmacokinetics among a clinical sample of Jordanians. The findings may increase our understanding of the inter-individual and inter-ethnic variations in metformin response.

Personalized antiplatelet and anticoagulation therapy: applications and significance of pharmacogenomics

In recent years, substantial effort has been made to better understand the influence of genetic factors on the efficacy and safety of numerous medications. These investigations suggest that the use of pharmacogenetic data to inform physician decision-making has great potential to enhance patient care by reducing on-treatment clinical events, adverse drug reactions, and health care-related costs. In fact, integration of such information into the clinical setting may be particularly applicable for antiplatelet and anticoagulation therapeutics, given the increasing body of evidence implicating genetic variation in variable drug response. In this review, we summarize currently available pharmacogenetic information for the most commonly used antiplatelet (ie, clopidogrel and aspirin) and anticoagulation (ie, warfarin) medications. Furthermore, we highlight the currently known role of genetic variability in response to next-generation antiplatelet (prasugrel and ticagrelor) and anticoagulant (dabigatran) agents. While compelling evidence suggests that genetic variants are important determinants of antiplatelet and anticoagulation therapy response, significant barriers to clinical implementation of pharmacogenetic testing exist and are described herein. In addition, we briefly discuss development of new diagnostic targets and therapeutic strategies as well as implications for enhanced patient care. In conclusion, pharmacogenetic testing can provide important information to assist clinicians with prescribing the most personalized and effective antiplatelet and anticoagulation therapy. However, several factors may limit its usefulness and should be considered.

Pharmacogenetic-guided dosing of coumarin anticoagulants: algorithms for warfarin, acenocoumarol and phenprocoumon

Coumarin derivatives, such as warfarin, acenocoumarol and phenprocoumon are frequently prescribed oral anticoagulants to treat and prevent thromboembolism. Because there is a large inter-individual and intra-individual variability in dose-response and a small therapeutic window, treatment with coumarin derivatives is challenging. Certain polymorphisms in CYP2C9 and VKORC1 are associated with lower dose requirements and a higher risk of bleeding. In this review we describe the use of different coumarin derivatives, pharmacokinetic characteristics of these drugs and differences amongst the coumarins. We also describe the current clinical challenges and the role of pharmacogenetic factors. These genetic factors are used to develop dosing algorithms and can be used to predict the right coumarin dose. The effectiveness of this new dosing strategy is currently being investigated in clinical trials.

A randomized trial of genotype-guided dosing of acenocoumarol and phenprocoumon

BACKGROUND

Observational evidence suggests that the use of a genotype-guided dosing algorithm may increase the effectiveness and safety of acenocoumarol and phenprocoumon therapy.

METHODS

We conducted two single-blind, randomized trials comparing a genotype-guided dosing algorithm that included clinical variables and genotyping for CYP2C9 and VKORC1 with a dosing algorithm that included only clinical variables, for the initiation of acenocoumarol or phenprocoumon treatment in patients with atrial fibrillation or venous thromboembolism. The primary outcome was the percentage of time in the target range for the international normalized ratio (INR; target range, 2.0 to 3.0) in the 12-week period after the initiation of therapy. Owing to low enrollment, the two trials were combined for analysis. The primary outcome was assessed in patients who remained in the trial for at least 10 weeks.

RESULTS

A total of 548 patients were enrolled (273 patients in the genotype-guided group and 275 in the control group). The follow-up was at least 10 weeks for 239 patients in the genotype-guided group and 245 in the control group. The percentage of time in the therapeutic INR range was 61.6% for patients receiving genotype-guided dosing and 60.2% for those receiving clinically guided dosing (P=0.52). There were no significant differences between the two groups for several secondary outcomes. The percentage of time in the therapeutic range during the first 4 weeks after the initiation of treatment in the two groups was 52.8% and 47.5% (P=0.02), respectively. There were no significant differences with respect to the incidence of bleeding or thromboembolic events.

CONCLUSIONS

Genotype-guided dosing of acenocoumarol or phenprocoumon did not improve the percentage of time in the therapeutic INR range during the 12 weeks after the initiation of therapy. (Funded by the European Commission Seventh Framework Programme and others; EU-PACT ClinicalTrials.gov numbers, NCT01119261 and NCT01119274.).

So Many Studies, Too Few Subjects: Establishing Functional Relevance of Genetic Polymorphisms on Pharmacokinetics

Based on current literature, greater clarity in defining the magnitude of polymorphism effects on pharmacokinetics can be achieved by addressing key components of study design, including adequate subject numbers per study group. Convincing evidence of functional relevance exists for polymorphisms in genes such as CYP2D6 and UGT1A1, whereas the published evidence for similar effects for CYP3A5, OATP1B1, and ABCB1 is still emerging or equivocal. Polymorphism-associated differences in pharmacokinetic parameters were simulated to incorporate (1) the ratio of group mean parameter values for homozygous wild-type subjects versus homozygous variants, (2) pharmacokinetic variability, and (3) sample size needed to achieve 80% power, assuming 69% coefficient of variation. Subject selection by genotype and choice of probe substrate are also considered. Simulation results and literature examples are incorporated to define key recommendations for future investigations. This will allow for more definitive statements in publications regarding genotype influence on pharmacokinetics.

CYP2C9 and VKORC1 polymorphisms influence warfarin dose variability in patients on long-term anticoagulation

OBJECTIVES

The main aim of this study was to determine whether CYP2C9 and VKORC1 polymorphisms influence warfarin dose variability during initial dose-finding phase and during maintenance treatment after 360 days.

METHODS

Two hundred and six consecutive patients who were beginning warfarin therapy were selected. They were assessed for general and clinical characteristics; prescribed warfarin dose; response to therapy on days 7-10, 30, 60, 180, and 360; adverse events; and CYP2C9 2, 3, 5, 6, 8, 11, and VKORC1 1639G >A assays.

RESULTS

During the first 30 days of anticoagulation, the relative variability of warfarin dose was significantly associated with CYP2C9*2 and CYP2C9*3 polymorphisms (p = 0.02) and with VKORC1 1639G >A genotypes (p = 0.04). Warfarin variability was also statistically different according to predicted metabolic phenotype and to VKORC1 genotypes after 360 days of treatment, and in the phase between 180 and 360 days (long-term dose variability). Both CYP2C9 and VKORC1 polymorphisms were associated with the international normalized ratio (INR) made between 7 and 10 days/initial dose ratio, adjusted for covariates (p < 0.01 and p = 0.02, respectively). Patients carrying VKORC1 and CYP2C9 variants presented lower required dose (at the end of follow-up of 360 days) compared to patients carrying wild-type genotypes (p = 0.04 and p = 0.03, respectively).

CONCLUSIONS

Genetic information on CYP2C9 and VKORC1 is important both for the initial dose-finding phase and during maintenance treatment with warfarin.

CYP2C9 and VKORC1 polymorphisms are differently distributed in the Brazilian population according to self-declared ethnicity or genetic ancestry

BACKGROUND

Warfarin-dosing pharmacogenetic algorithms have presented different performances across ethnicities, and the impact in admixed populations is not fully known.

AIMS

To evaluate the CYP2C9 and VKORC1 polymorphisms and warfarin-predicted metabolic phenotypes according to both self-declared ethnicity and genetic ancestry in a Brazilian general population plus Amerindian groups.

METHODS

Two hundred twenty-two Amerindians (Tupinikin and Guarani) were enrolled and 1038 individuals from the Brazilian general population who were self-declared as White, Intermediate (Brown, Pardo in Portuguese), or Black. Samples of 274 Brazilian subjects from Sao Paulo were analyzed for genetic ancestry using an Affymetrix 6.0(®) genotyping platform. The CYP2C9*2 (rs1799853), CYP2C9*3 (rs1057910), and VKORC1 g.-1639G>A (rs9923231) polymorphisms were genotyped in all studied individuals.

RESULTS

The allelic frequency for the VKORC1 polymorphism was differently distributed according to self-declared ethnicity: White (50.5%), Intermediate (46.0%), Black (39.3%), Tupinikin (40.1%), and Guarani (37.3%) (p<0.001), respectively. The frequency of intermediate plus poor metabolizers (IM+PM) was higher in White (28.3%) than in Intermediate (22.7%), Black (20.5%), Tupinikin (12.9%), and Guarani (5.3%), (p<0.001). For the samples with determined ancestry, subjects carrying the GG genotype for the VKORC1 had higher African ancestry and lower European ancestry (0.14±0.02 and 0.62±0.02) than in subjects carrying AA (0.05±0.01 and 0.73±0.03) (p=0.009 and 0.03, respectively). Subjects classified as IM+PM had lower African ancestry (0.08±0.01) than extensive metabolizers (0.12±0.01) (p=0.02).

CONCLUSIONS

The CYP2C9 and VKORC1 polymorphisms are differently distributed according to self-declared ethnicity or genetic ancestry in the Brazilian general population plus Amerindians. This information is an initial step toward clinical pharmacogenetic implementation, and it could be very useful in strategic planning aiming at an individual therapeutic approach and an adverse drug effect profile prediction in an admixed population.

Pharmacogenetics of anticoagulants

Warfarin, acenocoumarol, and phenprocoumon are among the major anticoagulant drugs worldwide. Because of their low therapeutic index and serious adverse reactions (ADRs), their wide use, and their varying kinetics and pharmacogenetic dependence, it is of great importance to explore further possibilities to forecast the dose beyond conventional INR measurements. Here, we describe particulars of the relative pharmacogenetic influence on the kinetics of these agents, the population distribution of genetics risk groups, and novel data on clinical features with influence on dose requirement and ADR risk. The usefulness of genetic information prior to and soon after start of therapy is also discussed. The current renewed focus on these issues is caused not only because of new genetic knowledge and genotyping facilities but also because of the high rate of serious ADRs. Application of these measures in the care of patients with anticoagulant therapy is important awaiting new therapeutic principles to be introduced, which may take long time still.

VKORC1 -1639G>A and CYP2C9*3 are the major genetic predictors of phenprocoumon dose requirement

PURPOSE

Phenprocoumon, similar to other coumarin-derived anticoagulants, is associated with a large variation in the individual dose requirement to achieve stable anticoagulation. Polymorphisms in the vitamin K epoxide reductase complex subunit 1 (VKORC1) and the liver enzyme cytochrome P450 2C9 (CYP2C9) effectively account for the variability in warfarin and acenocoumarol response but are less well-defined pharmacogenetic predictors in phenprocoumon therapy.

METHODS

A retrospective study was performed on 185 outpatients attending anticoagulation clinics in Austria and Germany. These patients were genotyped for the VKORC1 -1639G>A and 3730G>A polymorphisms as well as for the CYP2C9 *2 and *3 polymorphisms using a reverse hybridisation-based teststrip assay.

RESULTS

The VKORC1 -1639A allele, which was present at a frequency of 41.4% in the study cohort, significantly reduced the mean weekly phenprocoumon dose by 3 mg (19%) in the heterozygous and by 6.7 mg (43%) in the homozygous state compared to wild-type carriers (15.5 +/- 6.8 mg, p < 0.0001). A stepwise multiple regression analysis revealed that VKORC1 -1639G>A, age and CYP2C9*3 were the major independent determinants of phenprocoumon dose, accounting for 14.2, 9.1 and 4.7% of its variability, respectively (p </= 0.0007). The CYP2C9*2 polymorphism had a marginal influence (1.4%) and failed to reach statistical significance (p = 0.062). The VKORC1 3730G>A genotype had no additional predictive power for individual dose variability.

CONCLUSION

Similar to warfarin and acenocoumarol, the VKORC1 -1639G>A polymorphism had the highest impact on the maintenance dose of phenprocoumon. The factor age was the second most important predictor and explained a greater percentage of the variability than CYP2C9 genotype.

Polymorphisms of human cytochrome P450 2C9 and the functional relevance

Human cytochrome P450 2C9 (CYP2C9) accounts for ∼20% of hepatic total CYP content and metabolizes ~15% clinical drugs such as phenytoin, S-warfarin, tolbutamide, losartan, and many nonsteroidal anti-inflammatory agents (NSAIDs). CYP2C9 is highly polymorphic, with at least 33 variants of CYP2C9 (*1B through *34) being identified so far. CYP2C9*2 is frequent among Caucasians with ~1% of the population being homozygous carriers and 22% are heterozygous. The corresponding figures for the CYP2C9*3 allele are 0.4% and 15%, respectively. There are a number of clinical studies addressing the impact of CYP2C9 polymorphisms on the clearance and/or therapeutic response of therapeutic drugs. These studies have highlighted the importance of the CYP2C9*2 and *3 alleles as a determining factor for drug clearance and drug response. The CYP2C9 polymorphisms are relevant for the efficacy and adverse effects of numerous NSAIDs, sulfonylurea antidiabetic drugs and, most critically, oral anticoagulants belonging to the class of vitamin K epoxide reductase inhibitors. Warfarin has served as a practical example of how pharmacogenetics can be utilized to achieve maximum efficacy and minimum toxicity. For many of these drugs, a clear gene-dose and gene-effect relationship has been observed in patients. In this regard, CYP2C9 alleles can be considered as a useful biomarker in monitoring drug response and adverse effects. Genetic testing of CYP2C9 is expected to play a role in predicting drug clearance and conducting individualized pharmacotherapy. However, prospective clinical studies with large samples are warranted to establish gene-dose and gene-effect relationships for CYP2C9 and its substrate drugs.

Genetic polymorphisms of cytochrome P450 2C9 causing reduced phenprocoumon (S)-7-hydroxylationin vitroandin vivo

The effect of cytochrome P450 (CYP) 2C9 polymorphisms on the stereoselective biotransformation of the oral anticoagulant phenprocoumon (PPC) to inactive, monohydroxylated metabolites was studied in vitro and in vivo. In human liver microsomes, the (S)-7-hydroxylation--being the major metabolic pathway--was significantly compromised in a gene-dose-dependent manner in samples expressing the CYP2C9*2 or CYP2C9*3 allele. The CYP2C9*3/*3 genotype corresponded to an almost fourfold lower (S)-7-hydroxylation rate than CYP2C9*1/*1 (wild-type). The intrinsic clearance of human recombinant CYP2C9*2 and CYP2C9*3 for the (S)-7-hydroxylation was 28.9 and 50.9% lower than of CYP2C9*1, respectively. The area under the plasma concentration-time curve (AUC) of PPC metabolites after oral intake of 12 mg racemic PPC was significantly lower in volunteers expressing the CYP2C9*2 or CYP2C9*3 allele. Increasing plasma AUC metabolic ratios (parent compound/metabolite) in CYP2C9*2 and CYP2C9*3 variant allele carriers were found for each hydroxylation reaction and the CYP2C9*3/*3 genotype corresponded to an about 10-fold higher metabolic ratio of PPC (S)-7-hydroxylation relative to CYP2C9*1/*1. CYP2C9 polymorphisms cause a markedly compromised PPC (S)-7-hydroxylation. However, PPC metabolism appears overall less influenced by CYP2C9 genotype compared with other oral anticoagulants and it may thus be a valuable alternative for therapeutic anticoagulation of patients expressing CYP2C9 variant alleles.

Impact of genetic polymorphisms in CYP2C8 and rosiglitazone intake on the urinary excretion of dihydroxyeicosatrienoic acids

The cytochrome P450 enzymes CYP2C8, CYP2C9 and CYP2J2 generate 8,9-, 11,12-, and 14,15-epoxyeicosatrienic acid (EET) from arachidonic acid, and these EETs are then hydrolyzed to dihydroxyeicosatrienoic acids (DHET) before excretion into the urine. It is unknown how genetic polymorphisms affect formation of these diuretic, vasodilatory and anti-inflammatory eicosanoids, and whether the CYP2C8 substrate rosiglitazone inhibits their formation. Methods: A panel of 14, 13 and four carriers of the CYP2C8 genotypes *1/*1, *1/*3 and *3/*3, respectively was preselected for this study. Daily morning oral doses of 8 mg rosiglitazone were administered for 15 days. Urine was collected prior to rosiglitazone, and for 24 h after the first and last administration of rosiglitazone. Urinary EETs and DHETs were analyzed by tandem mass spectrometry. Results:Carriers of the high-activity CYP2C8*3 allele had higher excretion of all three DHETs (p < 0.01 for 11,12-DHET, p < 0.05 for 14,15-DHET), whereas carriers of the low-activity CYP2C8 haplotype C (genotypes GCGA at positions rs2275622, rs7909236, rs1113129 and rs11572080) had lower DHET excretion in urine before and during rosiglitazone. Rosiglitazone intake leads to a decrease in DHET excretion by approximately 10% (p < 0.02). Urinary excretion of unhydrolyzed EETs was below the limit of quantification of 50 pg/ml in all samples. Conclusion: The data consistently indicate that genetic variation in CYP2C8 moderately modulates-EET formation as reflected in urinary DHET excretion. This might impact cardiovascular functions, and may be one mechanism explaining the influence of CYP polymorphisms on myocardial infarction and hypertension.

Depressive und Angststörungen bei somatischen Krankheiten

Depressiv-ängstliche Störungen sind bei den unterschiedlichen somatischen Erkrankungen häufig. Sie sind nicht nur als Reaktion auf die Situation der Erkrankung zu verstehen, sondern in ein komplexes Bedingungsgefüge eingebettet. Sie sind besonders häufig bei Erkrankungen, die das Zentralnervensystem oder endokrine Regulationssysteme direkt betreffen. Es besteht ein enger Zusammenhang zur Chronizität, Schwere und Prognose der Erkrankung. Eigenständige Effekte von diversen pharmakologischen Substanzgruppen sind wahrscheinlich.

Pharmacogenetics of oral anticoagulants: the opportunity for individualized drug treatment of greater safety

Oral antivitamin K (AVK) anticoagulants are widely prescribed for the prophylaxis and treatment of a number of thromboembolic disorders. They constitute a major cause of iatrogenic accidents because of their narrow therapeutic index and consequently increase both thrombotic and bleeding risk; thus, regular monitoring is required. Patient and environmental factors affect the anticoagulation response and it has become evident that the wide interindividual variation in AVK response is also partly genetically determined. The main enzyme responsible for the metabolism of AVKs is hepatic cytochrome P450 CYP2C9. Vitamin K epoxide reductase complex subunit I (VKORC1) is a key enzyme in the vitamin K cycle; it is required to regenerate the cofactor essential for the activation of vitamin K-dependent clotting factors and is the target enzyme of AVK inhibition. Genetic variations affecting both CYP2C9 and VKORC1 are associated with variability in drug response and may explain differences in dose requirements. Genotyping for CYP2C9 and VKORC1 variants before initiation of treatment could allow clinicians to develop dosing protocols and identify patients at higher risk for AVK complications such as bleeding.

Influence of cytochrome P450 polymorphisms on drug therapies: pharmacogenetic, pharmacoepigenetic and clinical aspects

The polymorphic nature of the cytochrome P450 (CYP) genes affects individual drug response and adverse reactions to a great extent. This variation includes copy number variants (CNV), missense mutations, insertions and deletions, and mutations affecting gene expression and activity of mainly CYP2A6, CYP2B6, CYP2C9, CYP2C19 and CYP2D6, which have been extensively studied and well characterized. CYP1A2 and CYP3A4 expression varies significantly, and the cause has been suggested to be mainly of genetic origin but the exact molecular basis remains unknown. We present a review of the major polymorphic CYP alleles and conclude that this variability is of greatest importance for treatment with several antidepressants, antipsychotics, antiulcer drugs, anti-HIV drugs, anticoagulants, antidiabetics and the anticancer drug tamoxifen. We also present tables illustrating the relative importance of specific common CYP alleles for the extent of enzyme functionality. The field of pharmacoepigenetics has just opened, and we present recent examples wherein gene methylation influences the expression of CYP. In addition microRNA (miRNA) regulation of P450 has been described. Furthermore, this review updates the field with respect to regulatory initiatives and experience of predictive pharmacogenetic investigations in the clinics. It is concluded that the pharmacogenetic knowledge regarding CYP polymorphism now developed to a stage where it can be implemented in drug development and in clinical routine for specific drug treatments, thereby improving the drug response and reducing costs for drug treatment.

The clinical role of genetic polymorphisms in drug-metabolizing enzymes

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

VKORC1 and CYP2C9 genotypes and phenprocoumon anticoagulation status: interaction between both genotypes affects dose requirement

In a prospective follow-up study of the effects of VKORC1 and CYP2C9 genotypes on the anticoagulation status of patients, we assessed the CYP2C9 and the VKORC1 C1173T genotypes of patients during the initial 6 months of phenprocoumon treatment. We used linear regression models and Cox proportional hazard models to determine the effects of the VKORC1 and CYP2C9 genotypes on phenprocoumon dose requirements, overanticoagulation, and time to achieve stability. Allele frequencies of interest within the cohort (N=281) were 40.8% VKORC1 T-1173, 12.8% CYP2C9*2, and 6.9% CYP2C9*3. In patients with the VKORC1 CC genotype, carriers of a CYP2C9 polymorphism needed dosages that were nearly 30% lower than those for CYP2C9*1/*1 patients (P<0.001). In patients with a VKORC1 polymorphism, differences between carriers of a CYP2C9 polymorphism and CYP2C9*1/*1 were far smaller and largely not statistically significant. A larger part of the variability in dose requirement was explained by the VKORC1 genotype than by the CYP2C9 genotype (28.7% and 7.2%, respectively). Carriers of a combination of a CYP2C9 polymorphism and a VKORC1 polymorphism had a strongly increased risk of severe overanticoagulation (hazard ratio (HR) 7.20, P=0.002). Only carriers of a CYP2C9*2 allele had a decreased chance to achieve stability compared to CYP2C9*1/*1 patients (HR 0.61, P=0.004). In conclusion, the VKORC1 genotype modifies the effect of the CYP2C9 genotype on phenprocoumon dose requirements. A combination of polymorphisms of both genotypes is associated with a strongly increased risk of overanticoagulation, whereas delayed stabilization is mainly associated with the CYP2C9 genotype.

Pharmacogenetics, drug-metabolizing enzymes, and clinical practice

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

Obesity is associated with a slower response to initial phenprocoumon therapy whereas CYP2C9 genotypes are not

BACKGROUND

Initiation of phenprocoumon therapy is associated with a variable individual response. The CYP2C9 genotype has been shown to influence the response to warfarin therapy, but such an effect on phenprocoumon therapy remains uncertain.

METHOD

Two hundred sixty hospital patients started on phenprocoumon were recruited for this study. Body mass index (BMI), waist and hip circumference, dietary habits, comorbidity, and comedication were initially assessed. A 5' exonuclease assay (TaqManR) was used to analyze the presence of five polymorphisms of the CYP2C9 gene in each of the study patients. Study endpoints included the time necessary to achieve the international normalized ratio (INR) target (INR >2) and the total drug amount required to attain target INR. For 250 of 260 patients, the subsequent required daily maintenance dose of phenprocoumon was also recorded.

RESULTS

Both the necessary time and total dose required to attain target INR correlated significantly with BMI. The leaner the patient, the shorter the required time interval [BMI <22 (n=31), 5.48+/-2.49 days; BMI 22-25 (n=70), 6.09+/-2.40; BMI 25-30 (n=113), 6.76+/-3.61; BMI >30 (n=46), 8.50+/-5.75; p=0.001] and the lower the required dosage until the therapeutic range was achieved [BMI <22 (n=31), 23.8+/-12.1 mg; BMI 22-25 (n=70), 25.9+/-11.4 mg; BMI 25-30 (n=113), 29.6+/-25.2; BMI >30 (n=46), 35.8+/-19.7; p=0.027]. Overweight and waist circumference as a surrogate marker for abdominal fat were also associated significantly with these two parameters. Moreover, obesity was associated with a lower body-weight-adjusted maintenance dosage. All CYP2C9 genotypes that were tested failed to reveal an association with individual response variability.

CONCLUSION

Patient obesity appears to directly correspond to the amount of phenprocoumon required during initiation of therapy. The CYP2C9 genotype was not shown to influence the necessary therapeutic dosage.

The pharmacogenetics of coumarin therapy

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

Pharmacogenetics of oral anticoagulants

The use of oral anticoagulants (OA) is problematic due to its association with hemorrhagic complications. OA metabolism relies on the CYP2C9 complex. Genetic variations compromising metabolic competence of this complex may explain the risk of excessive and hazardous anticoagulation. A pharmacogenetics-based approach to this issue could be beneficial for choosing adequate dose and duration of treatment, in addition to having a better understanding of pharmacological interactions to which OA are susceptible. However, evidence from several basic and clinical studies indicates that both a complicated system of regulation of expression of multiple genes and the influence of a wide variety of epigenetic factors could be responsible for adverse drug reactions associated with the use of OA. Emphasis on understanding the gene-environment interactions could attain new paths to facilitate the use of these important drugs in the quotidian clinical practice.

Contribution of CYP2C9 to variability in vitamin K antagonist metabolism

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

Anticoagulation in patients with non-valvular atrial fibrillation: an evaluation of stability and early factors that predict longer-term stability on warfarin in a large UK population

OBJECTIVE

To determine the proportion of patients with non-valvular atrial fibrillation (NVAF) treated with warfarin that achieved a 6-month period within the target INR range (stability). To then evaluate any associations between stability and outcome and to determine whether stability can be predicted by clinical factors at an early stage in warfarin treatment.

METHODS

This study was a record linkage study in 1513 patients with NVAF treated with warfarin for a minimum of 6-months, carried out in a large UK population. The main outcome measures were stability (defined as six months within the target INR range [2.0-3.0]), thromboembolic and bleeding event rates and mortality. Secondary outcome measures were the predictive value of baseline characteristics and other treatment variables.

RESULTS

Stability was achieved in 52% of the study group. Standardised mean survival was significantly higher in the group who achieved stability (Delta = 16.91 months, p < 0.001) with a hazard ratio of 4.36 (p < 0.001). The stable group had a lower rate of both thromboembolic events (0.8% vs. 2.3% per patient year) and bleeds recorded on inpatient diagnoses (0.4% vs. 1.2% per patient year). Failure to achieve stable control was associated with age (Odds Ratio [OR] 1.011 (95% Confidence Interval [CI] 1.001-1.021)) and morbidity at baseline (OR 1.015; 95% CI 1.007-1.022). An increase in mean time between visits (OR 0.939; 95% CI 0.926-0.952) and the percentage time in range (OR 0.889; 95% CI 0.879-0.900) was associated with a decrease in the chance of instability. Greater variability in INR was also associated with a failure to achieve stability (OR 1.518; 95% CI 1.427-1.615). Receiver Operator Characteristic (ROC) analysis using data from the first three months of treatment demonstrated good discrimination of stability using age and morbidity at baseline and percentage time in range and frequency of visits during the first three months of treatment (area under curve [AUC] 0.780; standard error [SE] 0.012; 95% CI 0.757-0.803).

CONCLUSIONS

Many patients never achieved a period of 6-months stability and were at increased risk of thromboembolic events and bleeds. Age, morbidity at baseline and variability of INR control in the first three months could be used to predict instability using warfarin. This study infers that patients should be treated more intensively in the early stages of warfarinisation in order to improve outcome.

Stereospecific pharmacokinetic characterisation of phenprocoumon metabolites, and mass-spectrometric identification of two novel metabolites in human plasma and liver microsomes

Phenprocoumon belongs to the group of vitamin K antagonists (VKAs), for example warfarin and acenocoumarol. It is widely used for therapeutic anticoagulation and clinically administered as a racemate. Both enantiomers are partially metabolized by the polymorphic CYP2C9 enzyme. The pharmacokinetics are, however, substantially less dependent on CYP2C9 activity or genotype than for other CYP2C9-metabolised VKAs, and pharmacokinetic differences for the enantiomers are only minor. We have investigated the stereospecific pharmacokinetics of the monohydroxylated phenprocoumon metabolites in human plasma by achiral-chiral LC-LC-MS-MS coupling. In addition to the known metabolites, 4'-, 6-, and 7-hydroxyphenprocoumon, two other monohydroxylated metabolites (M1 and M2) were detected in plasma and human liver microsomal incubations. One of these was identified as 2'-hydroxyphenprocoumon by comparison with synthetic standards; the other seemed to be a side-chain-hydroxylated derivative. Analysis of enantiomeric metabolite ratios after a single oral dose of phenprocoumon revealed changes over time with an overall preponderance of the respective (R) enantiomers. The minor role of CYP2C9 in 4'-hydroxy-PPC formation and the effect of CYP2C9 genotype for (S)-6- and (S)-7-hydroxy-PPC were confirmed. M1 and M2 are formed highly stereoselectively, without dependence on CYP2C9 genotype. These may be interpreted as alternative metabolic pathways that render phenprocoumon less dependent on CYP2C9 activity or genotype.

Achiral–chiral LC/LC–MS/MS coupling for determination of chiral discrimination effects in phenprocoumon metabolism

Many physiological processes show a high degree of stereoselectivity, including the metabolism of xenobiotics as catalyzed by cytochrome P450 enzymes. An analysis of these chiral discrimination effects in drug metabolism is essential for an in-depth understanding of metabolic pathways that differ between enantiomers of a given chiral drug or metabolite thereof. Achiral chromatographic separation and structural identification followed by chiral analysis of metabolites from blood specimens usually requires a time-consuming multistage analytical technique. In an effort to optimize such a complicated analytical scheme, a novel two-dimensional online achiral-chiral liquid chromatography-tandem mass spectrometry (LC/LC-MS/MS) coupling method was developed by using a peak parking technique in combination with a makeup flow system. Metabolites were separated in the first dimension using a C18 reversed-phase system. A makeup eluent of water/methanol (95/5) was split into the flow before storing the metabolites separately on chiral cartridges. Subsequently, the metabolite enantiomers were eluted backward onto the analytical chiral column and separated, and the ratio of enantiomers was determined. The method was successfully validated with respect to limit of detection, linearity, intra- and interday accuracy, and precision. In the course of a human volunteer study investigating the influence of CYP (cytochrome) 2C9 genetic polymorphism on phenprocoumon (PPC) metabolism, we used this new two-dimensional online analytical technique for the analysis of PPC metabolites in plasma. The enantiomeric forms of 4'-, 6-, and 7-hydroxy-PPC metabolites as well as two novel metabolites were identified, and the ratio of the enantiomers was calculated. We found that the enantiomeric ratio for the different metabolites in the plasma sample of each measured individual differs markedly from a nearly 100% chiral discrimination for the two new putative metabolites. This new analytical coupling method possesses general utility in the analysis of chiral discrimination effects, particularly as it relates to pharmacokinetics and dynamics, a scientific field that is rapidly becoming an area of concern and interest.

Pharmacogenetics of oral anticoagulants

Understanding the basis for the observed wide interindividual variation in the dose requirement for the oral anticoagulants should facilitate the safer use of these widely prescribed drugs. Factors that include patient age, body weight and concomitant medications are well known to affect dose requirement and in a large number of recent pharmacogenetic studies, it has been demonstrated that the common variant CYP2C9 alleles are also good predictors of dose requirement, although they fail to account for all the observed interindividual variability. Since pharmacokinetic factors seem to be major contributors to the variability, the possibility that additional CYP2C9 polymorphisms or other polymorphisms affecting anticoagulant disposition could also predict dose is discussed. In addition, the relevance with regard to dose requirement of polymorphism in the recently discovered vitamin K epoxide reductase gene, which encodes the target gene for oral anticoagulants, is considered.

Comparative Pharmacokinetics of Vitamin K Antagonists

Vitamin K antagonists belong to the group of most frequently used drugs worldwide. They are used for long-term anticoagulation therapy, and exhibit their anticoagulant effect by inhibition of vitamin K epoxide reductase. Each drug exists in two different enantiomeric forms and is administered orally as a racemate. The use of vitamin K antagonists is complicated by a narrow therapeutic index and an unpredictable dose-response relationship, giving rise to frequent bleeding complications or insufficient anticoagulation. These large dose response variations are markedly influenced by pharmacokinetic aspects that are determined by genetic, environmental and possibly other yet unknown factors. Previous knowledge in this regard principally referred to warfarin. Cytochrome P450 (CYP) 2C9 has clearly been established as the predominant catalyst responsible for the metabolism of its more potent S-enantiomer. More recently, CYP2C9 has also been reported to catalyse the hydroxylation of phenprocoumon and acenocoumarol. However, the relative importance of CYP2C9 for the clearance of each anticoagulant substantially differs. Overall, the CYP2C9 isoenzyme appears to be most important for the clearance of warfarin, followed by acenocoumarol and, lastly, phenprocoumon. The less important role of CYP2C9 for the clearance of phenprocoumon is due to the involvement of CYP3A4 as an additional catalyst of phenprocoumon hydroxylation and significant excretion of unchanged drug in bile and urine, while the elimination of warfarin and acenocoumarol is almost completely by metabolism. Consequently, the effects of CYP2C9 polymorphisms on the pharmacokinetics and anticoagulant response are also least pronounced in the case of phenprocoumon; this drug seems preferable for therapeutic anticoagulation in poor metabolisers of CYP2C9. In addition to these vitamin K antagonists, oral thrombin inhibitors are currently under clinical development for the prevention and treatment of thromboembolism. Of these, ximelagatran has recently gained marketing authorisation in Europe. These novel drugs all feature some major advantages over traditional anticoagulants, including a wide therapeutic interval, the lack of anticoagulant effect monitoring and a low drug-drug interaction potential. However, they are also characterised by some pitfalls. Amendments of traditional anticoagulant therapy, including self-monitoring of international normalised ratio values or prospective genotyping for individual dose-tailoring may contribute to the continuous use of warfarin, phenprocoumon and acenocoumarol in the future.

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.

Determination of phenprocoumon, warfarin and their monohydroxylated metabolites in human plasma and urine by liquid chromatography–mass spectrometry after solid-phase extraction

A high-performance liquid chromatography-mass spectrometry (HPLC-MS) method for the quantification of phenprocoumon, warfarin, and their known monohydroxylated metabolites in human plasma and urine was developed using a simple, selective solid-phase extraction scheme. Chromatographic separation was achieved on a reversed-phase Luna C18 column and step gradient elution resulted in a total run time of about 13 min. Limits of quantification (LOQ) were < or = 40 nM for the parent compounds and < or = 25 nM for the metabolites and the limit of detection (LOD) was < or = 2.5 nM for all analytes. Average recovery was 84% (+/- 3.7) and 74% (+/- 13.2) in plasma and urine, respectively. Intra- and inter-day coefficients of variation were < or = 8.6 and < or = 10.6% in plasma and urine, respectively. The method was successfully applied to the analysis of phenprocoumon samples from four healthy volunteers and should prove useful for future comparative studies of warfarin and phenprocoumon pharmacokinetics.