Estimation of Warfarin Maintenance Dose Based on VKORC1 (−1639 G>A) and CYP2C9 Genotypes

ARMS test for diagnosis of CYP2C9 and VKORC1 mutation in patients with pulmonary embolism in Han Chinese

Aims: VKORC1 and CYP2C9 are important genetic factors affecting warfarin dose requirement. Our aim is to establish a simple, rapid and economical method to detect SNPs in the two genes. Materials & methods: Primer Premier 5 was used and a normal primer, a mutational primer and a common primer have been designed using the amplification refractory mutation system for VKORC1 c.-1639G>A (rs9923231), CYP2C9*3 c.1075A>C (rs1057910) and CYP2C9*13 c.269T>C substitutions. The amplification refractory mutation system (ARMS) assay was validated by the restriction enzyme cleavage method of reference or direct sequencing. Results: The ARMS primers designed can distinguish between heterozygotes and homozygotes successfully. In the Han Chinese population, patients with pulmonary embolism allele frequencies of CYP2C9*3 c.1075A>C (rs1057910), *13 c.269T>C and VKORC1 -1639G>A (rs9923231) are 4.3, 0.7 and 8.6%, respectively. Conclusion: The ARMS-PCR method is a simple, economical method that can be used for the rapid detection of SNPs in VKORC1 and CYP2C9.

Ability of VKORC1 and CYP2C9 to predict therapeutic warfarin dose during the initial weeks of therapy

BACKGROUND

CYP2C9 and VKORC1 genotypes predict therapeutic warfarin dose at initiation of therapy; however, the predictive ability of genetic information after a week or longer is unknown. Experts have hypothesized that genotype becomes irrelevant once international normalized ratio (INR) values are available because INR response reflects warfarin sensitivity.

METHODS

We genotyped the participants in the Prevention of Recurrent Venous Thromboembolism (PREVENT) trial, who had idiopathic venous thromboemboli and began low-intensity warfarin (therapeutic INR 1.5-2.0) using a standard dosing protocol. To develop pharmacogenetic models, we quantified the effect of genotypes, clinical factors, previous doses and INR on therapeutic warfarin dose in the 223 PREVENT participants who were randomized to warfarin and achieved stable therapeutic INRs.

RESULTS

A pharmacogenetic model using data from day 0 (before therapy initiation) explained 54% of the variability in therapeutic dose (R(2)). The R(2) increased to 68% at day 7, 75% at day 14, and 77% at day 21, because of increasing contributions from prior doses and INR response. Although CYP2C9 and VKORC1 genotypes were significant independent predictors of therapeutic dose at each weekly interval, the magnitude of their predictive ability diminished over time: partial R(2) of genotype was 43% at day 0, 12% at day 7, 4% at day 14, and 1% at day 21.

CONCLUSION

Over the first weeks of warfarin therapy, INR and prior dose become increasingly predictive of therapeutic dose, and genotype becomes less relevant. However, at day 7, genotype remains clinically relevant, accounting for 12% of therapeutic dose variability.

Personalized healthcare in clotting disorders

In terms of managing thrombotic disorders, genotype-based individualized patient care emerged as early as 1994 when the association of factor V Leiden (G1691A), and later, prothrombin (G20210A), with thrombotic phenotypes were discovered. Since then, genetic tests for specific thrombophilic SNPs have been routinely incorporated into daily practices in both thrombotic risk assessment and clinical decision-making with respect to prophylactic anti-thrombotic therapy. Recently, the area of pharmacogenomics in major anti-thrombotic drugs, such as warfarin and clopidogrel, has been the principal driver for personalized therapy based on one’s own individual characteristics.

Effects of CYP4F2 genetic polymorphisms and haplotypes on clinical outcomes in patients initiated on warfarin therapy

BACKGROUND

A variant in the CYP4F2 gene, rs2108622, has been recently shown to determine stable warfarin dose requirements. CYP4F2 has also been shown recently to metabolize vitamin K.

METHODS

Three hundred and eleven patients were recruited prospectively from two UK hospitals and followed-up for 6 months. Fine mapping of the whole CYP4F2 region was performed to try and define the haplotype structure of CYP4F2. Genotyping was performed on the Sequenom platform. Univariate and multiple regression analyses were undertaken to assess the effect of CYP4F2 on predefined clinical outcomes of warfarin response.

RESULTS

Fifty-nine single nucleotide polymorphisms in the CYP4F2 gene were analyzed. There was a high degree of linkage disequilibrium in the gene with two haplotype blocks. No association was found with warfarin stable dose and rs2108622 in our prospective cohort of patients even after adjustments to reduce patient heterogeneity. Interestingly, a single nucleotide polymorphism (rs2189784), which is in strong linkage disequilibrium with rs2108622, showed an association with time-to-therapeutic international normalized ratio which remained significant after the correction for multiple testing (Pc = 0.03). No association was shown with the haplotypes after false discovery rate correction.

CONCLUSION

Although we were unable to demonstrate an association between rs2108622 and stable warfarin dose, our finding of an association between rs2189784 and time-to-therapeutic international normalized ratio is consistent with the recent finding that CYP4F2 plays a role in vitamin K metabolism. However, the effect of CYP4F2 is relatively small in all studies undertaken so far, and thus seems unlikely to be of clinical relevance.

Impact of VKORC1 gene polymorphism on interindividual and interethnic warfarin dosage requirement--a systematic review and meta analysis

INTRODUCTION

Warfarin is the most widely used oral anticoagulant. It has been suggested that anticoagulation effect of warfarin is significantly associated with the polymorphism of certain genes, including Cytochrome P450 complex subunit 2C9 (CYP2C9), Vitamin K Epoxide Reductase Complex Subunit 1 (VKORC1), Gamma-Glutamyl Carboxylase (GGCX) and Apolipoprotein E (APOE) etc. The purpose of the present study was to conduct a systemic review and meta-analysis to investigate the relationship between mean daily warfarin dose (MDWD) and VKORC1 single nucleotide polymorphisms (SNPs).

MATERIALS AND METHODS

Inclusion and exclusion criteria were made, and the studies between 2004 and present were searched. References were examined, and experts were consulted for additional information. Data were extracted. Revman 4.2.10 software was applied to analyze the relationship between MDWD and VKORC1 SNPs.

RESULTS

Total 19 studies were included in the meta-analysis. The frequencies of 1173TT and -1639 AA in Asian patients were higher than those in Caucasian and African populations. Patients with VKORC1 1173 CT and 1173 CC required 44% [95% Confidence Interval (CI); 32%, 56%] and 97% [73%, 122%] higher MDWD than 1173 TT carriers, -1639GA and -1639GG carriers required 52% [41%, 64%] and 102% [85%, 118%] higher MDWD than -1639AA carriers, 3730GA and 3730AA carriers required 27% [3%, 58%] and 52% [3%, 109%] higher MDWD than 3730GG carriers. In addition, 1173C, -1639 G and 3730 A carriers required 63% [44%, 82%], 61% [49%, 73%] and 32% [4%, 59%] higher MDWD than 1173TT, -1639 AA and 3730GG, respectively. Sensitive analyses demonstrated that the impacts of gene polymorphism on warfarin dosage requirement were significantly different between Caucasian and Asian population, and the results of meta-analyses were stable and reliable.

CONCLUSION

This is the first meta-analysis about the impact of VKORC1 gene polymorphism on warfarin dose requirement. Our studies showed that gene polymorphisms of VKORC1 significantly associated with the variation of interindividual warfarin dose requirement variation, and the effects are different in ethnicities.

Genetic and environmental factors determining clinical outcomes and cost of warfarin therapy: a prospective study

BACKGROUND

In this prospective cohort study, we have undertaken a comprehensive evaluation of clinical parameters along with variation in 29 genes (including CYP2C9 and VKORC1) to identify factors determining interindividual variability in warfarin response.

METHODS

Consecutive patients (n=311) were followed up prospectively for 26 weeks. Several outcomes chosen to capture both warfarin efficacy and toxicity were assessed. Univariate and multiple regression analyses were undertaken to assess the combined effect of clinical and genetic factors.

RESULTS

CYP2C9 was the most important gene determining initial anticoagulant control, whereas VKORC1 was more important for stable anticoagulation. Novel associations with some clinical outcomes were found with single nucleotide polymorphisms in the cytochrome 450 genes CYP2C18 and CYP2C19, which were independent of the associations observed with CYP2C9 and in genes encoding CYP3A5, protein S and clotting factor V, although the variability explained by these genes was small. On the basis of the results of microcosting, adverse events were shown to be a significant predictor of total cost.

CONCLUSION

Accurate prediction of warfarin dose requirement needs to take into account multiple genetic and environmental factors, the contributions of which vary in the induction and maintenance phases of treatment.

Interactive modeling for ongoing utility of pharmacogenetic diagnostic testing: application for warfarin therapy

BACKGROUND

The application of pharmacogenetic results requires demonstrable correlations between a test result and an indicated specific course of action. We developed a computational decision-support tool that combines patient-specific genotype and phenotype information to provide strategic dosage guidance. This tool, through estimating quantitative and temporal parameters associated with the metabolism- and concentration-dependent response to warfarin, provides the necessary patient-specific context for interpreting international normalized ratio (INR) measurements.

METHODS

We analyzed clinical information, plasma S-warfarin concentration, and CYP2C9 (cytochrome P450, family 2, subfamily C, polypeptide 9) and VKORC1 (vitamin K epoxide reductase complex, subunit 1) genotypes for 137 patients with stable INRs. Plasma S-warfarin concentrations were evaluated by VKORC1 genotype (-1639G>A). The steady-state plasma S-warfarin concentration was calculated with CYP2C9 genotype-based clearance rates and compared with actual measurements.

RESULTS

The plasma S-warfarin concentration required to yield the target INR response is significantly (P < 0.05) associated with VKORC1 -1639G>A genotype (GG, 0.68 mg/L; AG, 0.48 mg/L; AA, 0.27 mg/L). Modeling of the plasma S-warfarin concentration according to CYP2C9 genotype predicted 58% of the variation in measured S-warfarin concentration: Measured [S-warfarin] = 0.67(Estimated [S-warfarin]) + 0.16 mg/L.

CONCLUSIONS

The target interval of plasma S-warfarin concentration required to yield a therapeutic INR can be predicted from the VKORC1 genotype (pharmacodynamics), and the progressive changes in S-warfarin concentration after repeated daily dosing can be predicted from the CYP2C9 genotype (pharmacokinetics). Combining the application of multivariate equations for estimating the maintenance dose with genotype-guided pharmacokinetics/pharmacodynamics modeling provides a powerful tool for maximizing the value of CYP2C9 and VKORC1 test results for ongoing application to patient care.

Warfarin-dosing algorithm based on a population pharmacokinetic/pharmacodynamic model combined with Bayesian forecasting

Aims: To develop a novel warfarin-dosing algorithm based on a previous population pharmacokinetic/pharmacodynamic (PK/PD) model with Bayesian forecasting to facilitate warfarin therapy. Materials & methods: Using information on CYP2C9 and VKORC1 genotypes, S-warfarin level, dose and international normalized ratio (INR) of prothrombin time, individual PK (apparent clearance of S-warfarin [CLs]) and PD (concentration resulting in 50% of Emax [EC50]) parameters were determined by Bayesian forecasting for 45 Japanese patients. Maintenance doses were described by multiple linear regression using individually estimated PK/PD parameters and INR values. The validity of the model and a comparison with other dosing methods were evaluated by bootstrap resampling and a cross-validation method. Results: The plasma concentration of S-warfarin and INR were accurately predicted from individual PK/PD parameters. The following final regression model for maintenance dose was obtained; maintenance dose = 11.2 × CLs + 0.91 × EC50 + 2.36 × INR – 9.67, giving a strong correlation between actual and predicted maintenance doses (r2 = 0.944). Bootstrap resampling and cross-validation showed robustness and a superior predictive performance compared with other dosing methods. On the other hand, the predictability without actual measurements (S-warfarin and INR values) and Bayesian inference was comparable to other dosing methods. Conclusion: A novel algorithm, based on the population PK/PD model combined with Bayesian forecasting, gave precise predictions of maintenance dose, leading to individualized warfarin therapy.

Predicting warfarin maintenance dose in patients with venous thromboembolism based on the response to a standardized warfarin initiation nomogram

BACKGROUND

Polymorphisms in the VKORC1 and CYP2C9 genes influence warfarin requirements. It has been suggested that dosing algorithms incorporating them might outperform usual care. Standardized warfarin initiation nomograms are safe and effective and patients' responses to them could be used to predict warfarin requirements without the need for genetic testing.

OBJECTIVES

To develop a model to predict warfarin dose requirements based on the response to a standard nomogram without using genetic testing.

PATIENTS/METHODS

We included 363 outpatients with acute venous thromboembolism who were started on treatment using a standardized warfarin nomogram and achieved a stable maintenance warfarin dose defined as a dose prescribed twice consecutively after two consecutive INR measurements between 2.0 and 3.0. Linear regression was used to derive equations predicting the maintenance dose and models were validated using non-parametric bootstrapping and tested in an independent cohort.

RESULTS

Three models were constructed for patients completing the nomogram until day 3 (warfarin dose (mg week(-1)) = Exp [2.737 + 1.896(INR(3)(-1))-0.008(Age)]; R2adj = 0.462), day 5 (warfarin dose (mg week(-1)) = Exp[2.261 + 2.412(INR(3)(-1)) -0.285(DeltaINR(5-3))]; R2adj = 0.603) and day 8 (warfarin dose (mg week(-1)) = Exp[1.574 + 1.788(INR(8)(-1)) + 0.024(cumulated warfarin dose until nomogram day 7)]; R2adj = 0.643), where Exp is the exponential function; INR3 and INR8 are the INR on days 3 or 8 of the nomogram, and DeltaINR(5-3) is the difference in the INR on days 5 and 3. All models were internally and externally validated and were accurate to within 25% of the actual dose in >60% of patients.

CONCLUSION

Maintenance warfarin dose can be accurately predicted using individual response to a standard warfarin initiation nomogram without the need for costly genetic testing.

Genetic testing before anticoagulation? A systematic review of pharmacogenetic dosing of warfarin

BACKGROUND

Genotype-guided initial warfarin dosing may reduce over-anticoagulation and serious bleeding compared to a one-dose-fits-all dosing method.

OBJECTIVE

The objective of this review was to investigate the safety and efficacy of genotype-guided dosing of warfarin in reducing the occurrence of serious bleeding events and over-anticoagulation.

DATA SOURCES

The authors searched PubMed, EMBASE and International Pharmaceutical Abstracts through January 23, 2009, without language restrictions. Selected articles were randomized trials comparing pharmacogenetic dosing of warfarin versus a "standard" dose control algorithm in adult patients taking warfarin for the first time.

REVIEW METHODS

Two reviewers independently extracted data and assessed study quality using a validated instrument. The primary outcomes were major bleeding and time spent within the therapeutic range International Normalized Ratio (INR). Secondary outcomes included minor bleeding, thrombotic events and other measures of anticoagulation quality.

RESULTS

Three of 2,014 studies (423 patients) met the inclusion and exclusion criteria. Differences in study quality, dosing algorithms, length of follow-up and outcome measures limited meta-analysis. Summary estimates revealed no statistically significant difference in bleeding rates or time within the therapeutic range INR. The highest quality study found no significant difference in primary or secondary outcomes, although there was a trend towards more rapid achievement of a stable dose (14.1 vs. 19.6 days, p = 0.07) in the pharmocogenetic arm.

CONCLUSIONS

We did not find sufficient evidence to support the use of pharmacogenetics to guide warfarin therapy. Additional clinical trials are needed to define the optimal approach to use warfarin pharmacogenetics in clinical practice.

A Multiplex Assay for Detecting Genetic Variations in CYP2C9, VKORC1, and GGCX Involved in Warfarin Metabolism

Background: Patients differ in responses to warfarin, which is commonly prescribed to treat thromboembolic events. Genetic variations in the cytochrome P450, family 2, subfamily C, polypeptide 9 (CYP2C9), vitamin K epoxide reductase complex, subunit 1 (VKORC1), and gamma-glutamyl carboxylase (GGCX) genes have been shown to contribute to impaired metabolism of warfarin.

Methods: We designed a custom multiplex single-nucleotide polymorphism (SNP) panel to interrogate the CYP2C9 *2, *3, VKORC1 (–1639G→A), and GGCX (1181T→G) alleles simultaneously in a single sample by use of single-base extension and capillary electrophoresis after genomic DNA extraction and PCR amplification.

Results: Our assay successfully detected various genotypes from known controls and 24 unknown samples. It was found to be 100% concordant with sequencing results.

Conclusions: Our multiplexed SNP panel can be successfully used in genotyping of patient blood samples. Results can be combined with other clinical parameters in an algorithm for warfarin dosing. These data provide a proof-in-principle of multiplexed SNP analysis using rapid single-base extension and capillary electrophoresis, and warrant additional validation using a larger cohort of patient samples.

Validation of clinical testing for warfarin sensitivity: comparison of CYP2C9-VKORC1 genotyping assays and warfarin-dosing algorithms

Responses to warfarin (Coumadin) anticoagulation therapy are affected by genetic variability in both the CYP2C9 and VKORC1 genes. Validation of pharmacogenetic testing for warfarin responses includes demonstration of analytical validity of testing platforms and of the clinical validity of testing. We compared four platforms for determining the relevant single nucleotide polymorphisms (SNPs) in both CYP2C9 and VKORC1 that are associated with warfarin sensitivity (Third Wave Invader Plus, ParagonDx/Cepheid Smart Cycler, Idaho Technology LightCycler, and AutoGenomics Infiniti). Each method was examined for accuracy, cost, and turnaround time. All genotyping methods demonstrated greater than 95% accuracy for identifying the relevant SNPs (CYP2C9 *2 and *3; VKORC1 -1639 or 1173). The ParagonDx and Idaho Technology assays had the shortest turnaround and hands-on times. The Third Wave assay was readily scalable to higher test volumes but had the longest hands-on time. The AutoGenomics assay interrogated the largest number of SNPs but had the longest turnaround time. Four published warfarin-dosing algorithms (Washington University, UCSF, Louisville, and Newcastle) were compared for accuracy for predicting warfarin dose in a retrospective analysis of a local patient population on long-term, stable warfarin therapy. The predicted doses from both the Washington University and UCSF algorithms demonstrated the best correlation with actual warfarin doses.

A regulatory science perspective on warfarin therapy: a pharmacogenetic opportunity

Warfarin is a challenging drug to accurately dose, both initially and for maintenance, because of its narrow therapeutic range, wide interpatient variability, and long list of factors that can influence dosing. Two million people in the United States are initiated on warfarin therapy annually, and this number is steadily increasing because of the increase in number of eligible patients. Recently, warfarin was reported to be the fourth leading cause of adverse events. The U.S. Food and Drug Administration recognizes that the adverse event rate of warfarin can be improved through better initial dosing, because many of the serious adverse events of warfarin occur soon after starting treatment. A substantial number of studies demonstrate that common variants of two genes, VKORC1 and CYP2C9, along with other nongenetic factors, correlate significantly with warfarin dosing. The genotypes of VKORC1 and CYP2C9 alone account for nearly 3 times more of the variability ( approximately 30%) in warfarin dosing than do age, weight, gender, and other clinical factors combined ( approximately 12%). Therefore, the purpose of this report is to review the current recommendations for warfarin therapy that involve genetic testing.

Overview of pharmacogenetics in anticoagulation therapy

Although used for many years, a detailed understanding of the mechanism of action and metabolism of anticoagulants has become available only recently. After the addition of pharmacogenetic data to the drug label by the U.S. Food and Drug Administration, interest in the pharmacogenetics of warfarin and its clinical application has grown exponentially. Dosing algorithms have been developed and continue to be refined that incorporate the polymorphisms of P450 2C9 and vitamin K epoxide reductase. Widespread adoption of these algorithms has been slow because of factors such as physician education, timely testing, complexity of dosing calculations, dietary variations, and other confounding variables. Although most useful before the first dose, these tests are also being used to explain labile responses to warfarin. Current protocols are capable of predicting a large portion of interindividual dosing variation and, as more data become available, truly personalized dosing of warfarin should be achievable, improving patient safety and clinical efficacy.

Pharmacogenetic testing for warfarin sensitivity

With the US Food and Drug Administration's recent label change of warfarin to include genetic testing for warfarin sensitivity, manufacturers are developing assays, and laboratories are offering testing. This article describes the genetic variants for which testing is available. Current technologies and assays are compared, including considerations for laboratories in choosing a method. Finally, laboratory issues that apply to all methods, such as quality control and proficiency testing as well as service issues including turn-around-time requirements are discussed.

VKORC1 polymorphisms, haplotypes and haplotype groups on warfarin dose among African-Americans and European-Americans

BACKGROUND

Although the influence of VKORC1 and CYP2C9 polymorphisms on warfarin response has been studied, variability in dose explained by CYP2C9 and VKORC1 is lower among African-Americans compared with European-Americans. This has lead investigators to hypothesize that assessment of VKORC1 haplotypes may help capture a greater proportion of the variability in dose for this under-represented group. However, the inadequate representation of African-Americans and the assessment of a few VKORC1 polymorphisms have hindered this effort.

METHODS

To determine if VKORC1 haplotypes or haplotype groups explain a higher variability in warfarin dose, we comprehensively assessed VKORC1 polymorphisms in 273 African-Americans and 302 European-Americans. The influence of VKORC1 polymorphisms, race-specific haplotypes and haplotype groups on warfarin dose was evaluated in race-stratified multivariable analyses after accounting for CYP2C9 (*2, *3, *5, *6 and *11) and clinical covariates.

RESULTS

VKORC1 explained 18% (30% with CYP2C9) variability in warfarin dose among European-Americans and 5% (8% with CYP2C9) among African-Americans. Four common haplotypes in European-Americans and twelve in African-Americans were identified. In each race VKORC1 haplotypes emerged into two groups: low-dose (Group A) and high-dose (Group B). African-Americans had a lower frequency of Group A haplotype (10.6%) compared with European-Americans (35%, p < 0.0001).The variability in dose explained by VKORC1 haplotype or haplotype groups was similar to that of a single informative polymorphism.

CONCLUSIONS

Our findings support the use of CYP2C9, VKORC1 polymorphisms (rs9934438 or rs9923231) and clinical covariates to predict warfarin dose in both African- and European-Americans. A uniform set of common polymorphisms in CYP2C9 and VKORC1, and limited clinical covariates can be used to improve warfarin dose prediction for a racially diverse population.

Ethnic differences in warfarin maintenance dose requirement and its relationship with genetics

Warfarin is a highly efficacious drug, but management of warfarin is difficult, in part because of the large interindividual maintenance dose differences. Warfarin dose requirements differ by race and it has been suggested that some of these differences are owing to genetic diversity. For example, persons of African descent have lower allele frequencies of the CYP2C9*2 and *3 and VKORC1 1173T allele, which have been associated with lower warfarin dose requirements in Caucasians. Since there is currently debate whether genetic information should be used in clinical practice to determine the starting dose for a warfarin initiator, it is of great importance to determine whether everyone will benefit from this knowledge.

Influence of CYP2C9 and VKORC1 on warfarin dose, anticoagulation attainment and maintenance among European-Americans and African-Americans

AIMS

The influence of CYP2C9 and VKORC1 on warfarin dose, time to target International Normalized Ratio (INR), time to stabilization, and risk of over-anticoagulation (INR: > 4) was assessed after adjustment for clinical factors, intraindividual variation in environmental factors and unobserved heterogeneity.

MATERIALS & METHODS

Common CYP2C9 and VKORC1 polymorphisms were assessed in 302 European-Americans and 273 African-Americans receiving warfarin. Race-stratified multivariable analyses evaluated the influence of CYP2C9 and VKORC1 on warfarin response.

RESULTS & CONCLUSION

CYP2C9 and VKORC1 accounted for up to 30% of the variability in warfarin dose among European-Americans and 10% among African-Americans. Neither CYP2C9 nor VKORC1 influenced the time to target INR or stabilization among patients of either race, and neither influenced the risk of over-anticoagulation among African-Americans. The risk of over-anticoagulation was higher among European-Americans with variant VKORC1 1173C/T (p < 0.01) and marginally significant among those with variant CYP2C9 (p = 0.08) genotype. Although CYP2C9 and VKORC1 genotyping can facilitate individualized initiation of warfarin dose in African and European-Americans, the ability to predict the risk of over-anticoagulation is inconsistent across race. Identification of other factors that can predict such risk consistently in a racially diverse group will facilitate individualized maintenance of warfarin therapy.

Pharmacogenetics of warfarin: development of a dosing algorithm for brazilian patients

A dosing algorithm including genetic (VKORC1 and CYP2C9 genotypes) and nongenetic factors (age, weight, therapeutic indication, and cotreatment with amiodarone or simvastatin) explained 51% of the variance in stable weekly warfarin doses in 390 patients attending an anticoagulant clinic in a Brazilian public hospital. The VKORC1 3673G>A genotype was the most important predictor of warfarin dose, with a partial R(2) value of 23.9%. Replacing the VKORC1 3673G>A genotype with VKORC1 diplotype did not increase the algorithm's predictive power. We suggest that three other single-nucleotide polymorphisms (SNPs) (5808T>G, 6853G>C, and 9041G>A) that are in strong linkage disequilibrium (LD) with 3673G>A would be equally good predictors of the warfarin dose requirement. The algorithm's predictive power was similar across the self-identified "race/color" subsets. "Race/color" was not associated with stable warfarin dose in the multiple regression model, although the required warfarin dose was significantly lower (P = 0.006) in white (29 +/- 13 mg/week, n = 196) than in black patients (35 +/- 15 mg/week, n = 76).

Influence of CYP2C9 and VKORC1 on warfarin dose, anticoagulation attainment and maintenance among European-Americans and African-Americans

AIMS

The influence of CYP2C9 and VKORC1 on warfarin dose, time to target International Normalized Ratio (INR), time to stabilization, and risk of over-anticoagulation (INR: > 4) was assessed after adjustment for clinical factors, intraindividual variation in environmental factors and unobserved heterogeneity.

MATERIALS & METHODS

Common CYP2C9 and VKORC1 polymorphisms were assessed in 302 European-Americans and 273 African-Americans receiving warfarin. Race-stratified multivariable analyses evaluated the influence of CYP2C9 and VKORC1 on warfarin response.

RESULTS & CONCLUSION

CYP2C9 and VKORC1 accounted for up to 30% of the variability in warfarin dose among European-Americans and 10% among African-Americans. Neither CYP2C9 nor VKORC1 influenced the time to target INR or stabilization among patients of either race, and neither influenced the risk of over-anticoagulation among African-Americans. The risk of over-anticoagulation was higher among European-Americans with variant VKORC1 1173C/T (p < 0.01) and marginally significant among those with variant CYP2C9 (p = 0.08) genotype. Although CYP2C9 and VKORC1 genotyping can facilitate individualized initiation of warfarin dose in African and European-Americans, the ability to predict the risk of over-anticoagulation is inconsistent across race. Identification of other factors that can predict such risk consistently in a racially diverse group will facilitate individualized maintenance of warfarin therapy.

High carrier prevalence of combinatorial CYP2C9 and VKORC1 genotypes affecting warfarin dosing

BACKGROUND

Polymorphisms in the cytochrome P450 2C9 (CYP2C9) and vitamin K epoxide reductase complex subunit 1 (VKORC1) genes significantly alter the effective warfarin dose. The CYP2C9*2 (430C>T), CYP2C9*3 (1075A>C) and VKORC1 -1639 G>A polymorphisms affect warfarin dose through altered metabolism (CYP2C9) and sensitivity (VKORC1).

OBJECTIVE

We determined the frequencies of SNPs in the CYP2C9 and VKORC1 genes in a clinical outpatient population and the carrier prevalences for a variety of genotype combinations to gauge the impact of these polymorphisms on warfarin dosage using published algorithms.

METHOD

A total of 127 patients from an outpatient clinic at Hartford Hospital (Hartford, CT, USA) were genotyped for five SNPs in the CYP2C9 gene and seven SNPs in the VKORC1 gene using Luminex^® technology.

RESULTS

The polymorphism frequencies were 10.2, 7.9 and 37.4% for the functionally deficient CYP2C9*2, CYP2C9*3 and VKORC1 -1639 G>A polymorphisms, respectively. Combining prevalence of combinatorial genotypes, 18% were carriers of both CYP2C9 and VKORC1 polymorphisms, 13% were CYP2C9 polymorphism carriers only, 42.5% were VKORC1 carriers only, and the remaining 27% were noncarriers for either gene. Based on published warfarin dosing algorithms, carriers of 1, 2, 3 and 4 functionally deficient polymorphisms predict reductions of 1.0 to 1.6, 2.0 to 2.9, 2.9 to 3.7, and 3.6 to 4.4 mg/day, respectively, in warfarin dose.

CONCLUSION

Overall, 73% of the population carried at least one polymorphism predicting deficient warfarin metabolism or responsiveness and 18% were carriers for polymorphisms in both genes studied. Combinatorial genotyping of CYP2C9 and VKORC1 can allow for individualized dosing of warfarin amongst patients with gene polymorphisms potentially reducing the risk of accentuated responses and bleeding.

Use of genetic and nongenetic factors in warfarin dosing algorithms

Despite the fact that warfarin has been used as an anticoagulant for many years, the safety profile for this drug has been poor. Inappropriate dosing continues to contribute to significant morbidity and mortality due to thrombotic disease and bleeding. Therefore, there is a need for the development, characterization and implementation of dosing algorithms using a patient's demographic information and genotype. Recently, polymorphisms in two genes, cytochrome P450 2C9 and vitamin K epoxide reductase complex 1, have been shown to affect warfarin's pharmacogenomics and pharmacodynamics, respectively. Adding genotypes to a dosing algorithm may enable better prediction of initial warfarin dosing than use of demographic data alone. An advisory committee of the US FDA voted on November 14, 2005, to require manufacturers of warfarin to relabel their product, indicating that genotyping is recommended prior to drug administration. The exact date when this recommendation will be enacted remains to be determined. Successful implementation of pharmacogenomics into clinical practice requires genotyping results that can be translated directly into clinical decisions. The development of a warfarin dosing algorithm that includes genotyping may be the means to achieve this goal.

Genelex Corporation

Comprehensive, personalized medication management and pharmacogenetic testing are important existing opportunities to reduce adverse medication events and improve overall healthcare outcomes. A primary barrier to the adoption of personalized pharmacology is the inadequacy of existing patient records, drug interaction tools and the ‘interpretation gap’ – the lack of physician decision support tools needed to interpret DNA test reports. GeneMedRx, an algorithm-driven, gene–drug interaction software, closes this gap. It helps physicians optimize medication regimens by correlating the genetic makeup of the patient with all the medicines they are taking. Portable personal health records created by GeneMedRx are the core product required for a much-needed comprehensive program of personalized medication management.

Pharmacogenomics and drug development: the impact of US FDA postapproval tracking on clinical pharmacology

Severe adverse drug reactions to commonly prescribed drugs such as Vioxx® have led to a call for increased scrutiny in deciding which patients are given which drugs, and how much drug they should receive. A personalized approach to medicine offers a larger variety of drugs and doses that would be prescribed only to a subgroup of patients. Pharmacogenomics could help divide patients into these subgroups based on variation in the genes either causing the disease or encoding the principle drug-metabolizing enzymes. Given the cost and infrastructure associated with assembling genetic data, drug sponsors, regulatory agencies and clinicians each play a role in the collection, storage and oversight of pharmacogenetic information. The 110th Congress is in the process of making changes to the drug-approval process and the role of genetics in that process.

CYP4F2 genetic variant alters required warfarin dose

Warfarin is an effective, commonly prescribed anticoagulant used to treat and prevent thrombotic events. Because of historically high rates of drug-associated adverse events, warfarin remains underprescribed. Further, interindividual variability in therapeutic dose mandates frequent monitoring until target anticoagulation is achieved. Genetic polymorphisms involved in warfarin metabolism and sensitivity have been implicated in variability of dose. Here, we describe a novel variant that influences warfarin requirements. To identify additional genetic variants that contribute to warfarin requirements, screening of DNA variants in additional genes that code for drug-metabolizing enzymes and drug transport proteins was undertaken using the Affymetrix drug-metabolizing enzymes and transporters panel. A DNA variant (rs2108622; V433M) in cytochrome P450 4F2 (CYP4F2) was associated with warfarin dose in 3 independent white cohorts of patients stabilized on warfarin representing diverse geographic regions in the United States and accounted for a difference in warfarin dose of approximately 1 mg/day between CC and TT subjects. Genetic variation of CYP4F2 was associated with a clinically relevant effect on warfarin requirement.

Warfarin pharmacogenetics: CYP2C9 and VKORC1 genotypes predict different sensitivity and resistance frequencies in the Ashkenazi and Sephardi Jewish populations

Warfarin is a widely used anticoagulant that has a narrow therapeutic range because of both genetic and environmental factors. CYP2C9( *)2 (p.R144C), CYP2C9( *)3 (p.I359L), and the VKORC1 promoter (g.-1639G-->A) polymorphisms occur frequently in patients who are warfarin "sensitive" and require lower doses, whereas patients with VKORC1 missense mutations are warfarin "resistant" and require higher doses. To compare the CYP2C9 and VKORC1 allele and genotype frequencies among 260 Ashkenazi (AJ) and 80 Sephardi Jewish (SJ) individuals, we genotyped six CYP2C9 and eight VKORC1 alleles by using the Tag-It Mutation Detection Kit and PCR-RFLP assays. The "sensitive"CYP2C9( *)2 and ( *)3 alleles had significantly higher frequencies in SJ than in AJ individuals, 0.194 and 0.144 versus 0.127 and 0.081, respectively (p <or= 0.001). In contrast, the VKORC1 p.D36Y mutation, which predicts warfarin "resistance," had a significantly higher frequency in AJ than in SJ individuals, 0.043 versus 0.006, respectively (p <or= 0.025). Of note, 11.3% of AJ individuals predicted to be CYP2C9 extensive metabolizers and 8.7% of those predicted to be intermediate and poor metabolizers were VKORC1 p.D36Y carriers who require markedly higher warfarin doses. Thus, approximately 10% of all AJ individuals would be misclassified when only genotyping CYP2C9( *)2, ( *)3, and VKORC1 g.-1639G-->A, underscoring the importance of screening for p.D36Y prior to initiating warfarin anticoagulation in AJ individuals. Taken together, our findings show that approximately 85% of AJ and approximately 90% of SJ individuals have at least one "sensitive" (CYP2C9( *)2, ( *)3, VKORC1 g.-1639G-->A) or "resistant" (VKORC1 p.D36Y) allele, indicating that each group has different warfarin pharmacogenetics and would benefit from genotype-based dose predictions.

Dosing algorithm for warfarin using CYP2C9 and VKORC1 genotyping from a multi-ethnic population: comparison with other equations

Objectives: Polymorphism in the genes for cytochrome (CYP)2C9 and the vitamin K epoxide reductase complex subunit 1 (VKORC1) affect the pharmacokinetics and pharmacodynamics of warfarin. We developed and validated a warfarin-dosing algorithm for a multi-ethnic population that predicts the best dose for stable anticoagulation, and compared its performance against other regression equations. Methods: We determined the allele and haplotype frequencies of genes for CYP2C9 and VKORC1 on 167 Caucasian, African–American, Asian and Hispanic patients on warfarin. On a subset where complete data were available (n = 92), we developed a dosing equation that predicts the actual dose needed to maintain target anticoagulation using demographic variables and genotypes. This regression was validated against an independent group of subjects. We also applied our data to five other published warfarin-dosing equations. Results: The allele frequency for CYP2C9*2 and *3 and the A allele for VKORC1 3673 was similar to previously published reports. For Caucasians and Asians, VKORC1 SNPs were in Hardy–Weinberg linkage equilibrium. Some VKORC1 SNPs among the African–American population and one SNP among Hispanics were not in equilibrium. The linear regression of predicted versus actual warfarin dose produced r-values of 0.71 for the training set and 0.67 for the validation set. The regression coefficient improved (to r = 0.78 and 0.75, respectively) when rare genotypes were eliminated or when the 7566 VKORC1 genotype was added to the model. All of the regression models tested produced a similar degree of correlation. The exclusion of rare genotypes that are more associated with certain ethnicities improved the model. Conclusion: Minor improvements in algorithms can be observed with the inclusion of ethnicity and more CYP2C9 and VKORC1 SNPs as variables. Major improvements will likely require the identification of new gene associations with warfarin dosing.

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.

Automation of complex assays: pharmacogenetics of warfarin dosing

AutoGenomics, Inc. (Carlsbad, CA, USA) have developed a multiplex microarray assay for genotyping both VKORC1 and CYP2C9 using the INFINITI(™) Analyzer. Multiple alleles in each DNA sample are analyzed by polymerase chain reaction amplification, followed by detection primer extension using the INFINITI Analyzer. The INFINITI Analyzer performs single-nucleotide polymorphism (SNP) analysis using universal oligonucleotides immobilized on the biochip. To genotype broader ethnic groups, genomic DNA from whole blood was tested for nine SNPs for VKORC1 and six for CYP2C9 genotypes. Information related to all 15 SNPs is needed to determine dosing of population of diverse ethnic origin. The INFINITI system provides genotyping information for same day dosing of warfarin.

Fungal colonization of gastric mucosa and its clinical relevance

BACKGROUND

The aim of our study was to evaluate the incidence of fungi in the stomach in patients with gastric ulcer and chronic gastritis in comparison to healthy humans, and to identify the fungus species isolated from these patients and their susceptibility to antifungal agents. We also assessed the coincidence of the presence of antifungal antibodies and fungal mannan antigen in serum with the concentration of fungi in the stomach.

MATERIAL AND METHODS

We investigated 293 patients, aged 20-80, who visited the Gastroenterology Outpatient Clinic at the Jagellonian University's Collegium Medicum in Cracow, complaining of dyspeptic symptoms or clinical manifestations of ulcer disease. The examinations included endoscopy of the upper part of the alimentary tract with sampling of gastric contents, as well as surface brushing and biopsy from the bottom of the ulceration for mycological analysis. Also, biopsy specimens from the margin of the ulceration or inflammatory mucosa were collected for histological examination and urease testing.

RESULTS

Gastric mucosa and stomach contents are often an area of fungal colonization, which was detected in 54.2% of the gastric ulcer cases and 10.3% of the chronic gastritis cases. The most frequently isolated fungus species was Candida albicans, although other fungi, previously considered rare or uncommon, were also found. A difference in growth in vitro between the C. albicans, C. tropicalis and C. lusitaniae strains was discovered: C. albicans and C. tropicalis grew from pH 2.0, while C. lusitaniae grew from pH 3.0. This finding suggests differentiation in the properties of these fungi.

CONCLUSIONS

The lack of correlation between the concentration of fungi, the titre of antifungal antibodies and the presence of fungal antigen in serum suggests that fungal colonization is secondary in nature.