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

Development of a pharmacogenetic-guided warfarin dosing algorithm for Puerto Rican patients

AIM

This study was aimed at developing a pharmacogenetic-driven warfarin-dosing algorithm in 163 admixed Puerto Rican patients on stable warfarin therapy.

PATIENTS & METHODS

A multiple linear-regression analysis was performed using log-transformed effective warfarin dose as the dependent variable, and combining CYP2C9 and VKORC1 genotyping with other relevant nongenetic clinical and demographic factors as independent predictors.

RESULTS

The model explained more than two-thirds of the observed variance in the warfarin dose among Puerto Ricans, and also produced significantly better 'ideal dose' estimates than two pharmacogenetic models and clinical algorithms published previously, with the greatest benefit seen in patients ultimately requiring <7 mg/day. We also assessed the clinical validity of the model using an independent validation cohort of 55 Puerto Rican patients from Hartford, CT, USA (R(2) = 51%).

CONCLUSION

Our findings provide the basis for planning prospective pharmacogenetic studies to demonstrate the clinical utility of genotyping warfarin-treated Puerto Rican patients.

Genetic and clinical determinants influencing warfarin dosing in children with heart disease

Warfarin is a common anticoagulant with narrow therapeutic window and variable anticoagulation effects. Single gene polymorphisms in cytochrome P450 2C9 (CYP2C9) and vitamin K epoxide reductase (VKORC1) have been shown to impact warfarin dosing in adults. Insufficient data exists on genetic and clinical factors which influence warfarin dosing in children. Pediatric patients with heart disease who received long-term warfarin therapy were tested for VKORC1 and CYP2C9 polymorphisms. Clinical and demographic data were reviewed in those children who achieved stable therapeutic international normalized ratio (INR). Multiple linear regression modeling was used to assess relationships between stable warfarin doses and genetic or clinical variables. Fifty children were tested for VKORC1 and CYP2C9 polymorphisms; 37 patients (M 26: F 11) had complete data, achieved stable therapeutic INR, and were included in dose variability analysis. There were predominance of white race 73% and male sex 70.3%. The mean age was 9.6 years (1.8-18.6 years). The mean weight was 37.8 kg (7.7-95 kg). Fontan physiology and mechanical cardiac valves were two most common indications for chronic warfarin therapy (25/37 or 67.6%). Twelve patients (32.4%) had ≥ 2 indications for warfarin therapy. Three patients had documented venous or arterial clots, and 5 patients had strokes. Congenital heart disease was present in 29 patients (78.4%), including Fontan physiology (20), complex biventricular physiology (4), and congenital mitral valve disease (5). Acquired heart disease was present in 8 patients (21.6%), including Kawasaki disease with coronary aneurysms (3), acquired mitral valve disease (3), and Marfan syndrome (2). Stable warfarin dose (mg/kg/day) was strongly associated with VKORC1 polymorphism (p < 0.0001) and goal therapeutic INR (p = 0.009). Negative correlations were observed between stable warfarin dose and age, weight, height, and BSA (p = 0.04, 0.02, 0.02, and 0.02 respectively). Factors which did not influence warfarin dose included CYP2C9 polymorphism (p = 0.17), concurrent medications (p = 0.85), sex (p = 0.4), race (p = 0.14), congenital heart disease (p = 0.09), and Fontan physiology (p = 0.76). The gene-dose effect was observed in children with homozygous wild type VKORC1 CC, who required higher warfarin dose compared to those carrying heterozygous TC or homozygous TT (p = 0.028 and 0.0004 respectively). The full multiple linear regression model revealed that VKORC1 genotypes accounted for 47% of dosing variability; CYPC29 accounted for 5%. Overall, the combination of VKORC1, CYP2C9, age, and target INR accounted for 82% of dosing variability. In children with heart disease, VKORC1 genotypes, age, and target INR are important determinants influencing warfarin dosing in children with heart disease. Future warfarin dosing algorithm in children should factor both genetic and clinical factors.

Willingness-to-pay and preferences for warfarin pharmacogenetic testing in Chinese warfarin patients and the Chinese general public

Aim: Genetic factors have been found to affect warfarin maintenance dose and a key factor for the successful clinical implementation of warfarin pharmacogenetic testing (WPGT) is economic sustainability. We aimed to estimate the willingness-to-pay (WTP) and preferences for WPGT in Singaporean Chinese subjects. Methods & subjects: A total of 197 warfarin patients and 187 members of the public completed a questionnaire. The discrete choice methodology was used and the choice model was estimated using hierarchical Bayes. Marginal WTP, attribute importance and WTP for three hypothetical WPGTs were calculated from the estimated utilities. Results: Both patients and the public placed most emphasis on side effects, followed by cost, number of International Normalized Ratio tests and ‘nature of test’. WTP for WPGT ranged from S$160 to S$730. Conclusion: WPGTs are likely to be economically sustainable.

Optimal dosing of warfarin and other coumarin anticoagulants: the role of genetic polymorphisms

Coumarin anticoagulants, which include warfarin, acenocoumarol and phenprocoumon, are among the most widely prescribed drugs worldwide. There is now a large body of published data showing that genotype for certain common polymorphisms in the genes encoding the target vitamin K epoxide reductase (G-1639A/C1173T) and the main metabolizing enzyme CYP2C9 (CYP2C9*2 and *3 alleles) are important determinants of the individual coumarin anticoagulant dose requirement. Additional less common polymorphisms in these genes together with polymorphisms in other genes relevant to blood coagulation such as the cytochrome P450 CYP4F2, gamma-glutamyl carboxylase, calumenin and cytochrome P450 oxidoreductase may also be significant predictors of dose, especially in ethnic groups such as Africans where there have been fewer genetic studies compared with European populations. Using relevant genotypes to calculate starting dose may improve safety during the initiation period. Various algorithms for dose calculation, which also take patient age and other characteristics into consideration, have been developed for all three widely used coumarin anticoagulants and are now being tested in ongoing large randomised clinical trials. One recently completed study has provided encouraging results suggesting that calculation of warfarin dose on the basis of individual patient genotype leads to few adverse events and a higher proportion of time within the therapeutic coagulation rate window, but these findings still need confirmation.

Influence of CYP2C9 and VKORC1 on patient response to warfarin: a systematic review and meta-analysis

Background

Warfarin is a highly effective anticoagulant however its effectiveness relies on maintaining INR in therapeutic range. Finding the correct dose is difficult due to large inter-individual variability. Two genes, CYP2C9 and VKORC1, have been associated with this variability, leading to genotype-guided dosing tables in warfarin labeling. Nonetheless, it remains unclear how genotypic information should be used in practice. Navigating the literature to determine how genotype will influence warfarin response in a particular patient is difficult, due to significant variation in patient ethnicity, outcomes investigated, study design, and methodological rigor. Our systematic review was conducted to enable fair and accurate interpretation of which variants affect which outcomes, in which patients, and to what extent.

Methodology/Principal Findings

A comprehensive search strategy was applied and 117 studies included. Primary outcomes were stable dose, time to stable dose and bleeding events. Methodological quality was assessed using criteria of Jorgensen and Williamson and data synthesized in meta-analyses using advanced methods. Pooled effect estimates were significant in most ethnic groups for CYP2C9*3 and stable dose (mutant types requiring between 1.1(0.7–1.5) and 2.3 (1.6–3.0)mg/day). Effect estimates were also significant for VKORC1 and stable dose for most ethnicities, although direction differed between asians and non-asians (mutant types requiring between 0.8(0.4–1.3) and 1.5(1.1–1.8)mg/day more in asians and between 1.5(0.7–2.2) and 3.1(2.7–3.6)mg/day less in non-asians). Several studies were excluded due to inadequate data reporting. Assessing study quality highlighted significant variability in methodological rigor. Notably, there was significant evidence of selective reporting, of outcomes and analysis approaches.

Conclusions/Significance

Genetic associations with warfarin response vary between ethnicities. In order to achieve unbiased estimates in different populations, a high level of methodological rigor must be maintained and studies should report sufficient data to enable inclusion in meta-analyses. We propose minimum reporting requirements, suggest methodological guidelines and provide recommendations for reducing the risk of selective reporting.

Warfarin pharmacogenetics: development of a dosing algorithm for Omani patients

The objective of our present study was to develop a warfarin dosing algorithm for the Omani patients, as performances of warfarin dosing algorithms vary across populations with impact on the daily maintenance dose. We studied the functional polymorphisms of CYP2C9, CYP4F2 and VKORC1 genes to evaluate their impact on the warfarin maintenance dose in an admixed Omani patient cohort with Caucasian, African and Asian ancestries. We observed a 64-fold inter-patient variability for warfarin to achieve stable international normalized ratio in these patients. Univariate analysis revealed that age, gender, weight, atrial fibrillation, deep vein thrombosis/pulmonary embolism and variant genotypes of CYP2C9 and VKORC1 loci were significantly associated with warfarin dose in the studied patient population. However, multiple regression model showed that only the atrial fibrillation, and homozygous CYP2C9 variant genotypes (*2/*3 and *3/*3) and VKORC1 GA and AA genotypes remained significant. A multivariate model, which included demographic, clinical and pharmacogenetic variables together explained 63% of the overall inter-patient variability in warfarin dose requirement in this microgeographically defined, ethnically admixed Omani patient cohort on warfarin. This locally developed model performed much better than the International Warfarin Pharmacogenetics Consortium (IWPC) model as the latter could only explain 34% of the inter-patient variability in Omani patients. VKORC1 3673G>A polymorphism emerged as the single most important predictor of warfarin dose variability, even in this admixed population (partial R(2)=0.45).

Prospective pilot trial of PerMIT versus standard anticoagulation service management of patients initiating oral anticoagulation

We performed a randomised pilot trial of PerMIT, a novel decision support tool for genotype-based warfarin initiation and maintenance dosing, to assess its efficacy for improving warfarin management. We prospectively studied 26 subjects to compare PerMIT-guided management with routine anticoagulation service management. CYP2C9 and VKORC1 genotype results for 13 subjects randomly assigned to the PerMIT arm were recorded within 24 hours of enrolment. To aid in INR interpretation, PerMIT calculates estimated loading and maintenance doses based on a patient's genetic and clinical characteristics and displays calculated S-warfarin plasma concentrations based on planned or administered dosages. In comparison to control subjects, patients in the PerMIT study arm demonstrated a 3.6-day decrease in the time to reach a stabilised INR within the target therapeutic range (4.7 vs. 8.3 days, p = 0.015); a 12.8% increase in time spent within the therapeutic interval over the first 25 days of therapy (64.3% vs. 55.3%, p = 0.180); and a 32.9% decrease in the frequency of warfarin dose adjustments per INR measurement (38.3% vs. 57.1%, p = 0.007). Serial measurements of plasma S-warfarin concentrations were also obtained to prospectively evaluate the accuracy of the pharmacokinetic model during induction therapy. The PerMIT S-warfarin plasma concentration model estimated 62.8% of concentrations within 0.15 mg/l. These pilot data suggest that the PerMIT method and its incorporation of genotype/phenotype information may help practitioners increase the safety, efficacy, and efficiency of warfarin therapeutic management.

Pharmacogenetic aspects of coumarinic oral anticoagulant therapies

Coumarinic oral-anticoagulants (COAs) are commonly used for treatment of thromboembolic events. However, these medications have a narrow therapeutic range and there are large inter-individual variations in drug response. This is especially important in the initial phases of oral-anticoagulant therapy. Recent advancements in pharmacogenetics have established that clinical outcomes in oral-anticoagulant therapy are affected by genetic factors. The allelic variants of genes like cytochrome P450 2C9 (CYP2C9) and vitamin K epoxide reductase complex subunit 1 (VKORC1) are closely associated with maintenance dose of oral anti-coagulants. In addition, GGCX (Gamma-glutamyl carboxylase) polymorphism at position 12970 (rs11676382), CYP4F2 (rs2108622; V433M; 1347 C > T) and Apolipoprotein E (APOE) variants have been shown to explain a small but significant influence on dose requirements. There are large differences in the frequencies of these polymorphisms between different world populations which are also related to the requirements of oral anticoagulants. However, the final drug dosage in an individual is determined by complex sets of genetic and environmental factors and several dosing algorithms which combine clinical and genetic parameters to predict therapeutic COA doses have also been developed. The algorithm based dose prediction shows the importance of pharmacogenetic testing in patients undergoing oral anticoagulant therapies.

Age- and weight-adjusted warfarin initiation nomogram for ischaemic stroke patients

OBJECTIVES

Specific guidelines for initial dosing of warfarin in ischaemic stroke patients have not been developed. Therefore, we have developed an age- and weight-adjusted warfarin initiation nomogram (AW-WIN) for ischaemic stroke patients and then evaluated the efficacy and safety of AW-WIN compared with physician-determined warfarin dosing (PDWD).

METHODS

The age- and weight-adjusted warfarin initiation nomogram was administered to 104 acute ischaemic stroke patients between January 2008 and February 2009. A historical control group (PDWD) of 96 patients was selected from comparable patients who were discharged with warfarin during the previous year. Time-to-therapeutic international normalized ratios (INRs) and the incidence of excessive anticoagulation were compared in the AW-WIN and PDWD groups.

RESULTS

The general characteristics, risk factors, and stroke mechanism of the AW-WIN and PDWD groups did not differ significantly. The mean time to INR ≥ 2.0 was significantly shorter in the AW-WIN than in the PDWD group (4.9 ± 0.7 vs. 6.2 ± 0.8 days, P = 0.0008). After adjustment for potential confounding variables, the AW-WIN group reached target INR faster than the PDWD group (hazard ratio, 1.76; 95% confidence interval, 1.26-2.45; P = 0.001). The time-to-therapeutic INR ≥1.7 was shorter (P = 0.0002), the proportion of patients with therapeutic INR (2-3) at 5 days was higher (P = 0.002), and the rate of excessive anticoagulation of ≥3.5 INR during hospitalization was lower (P = 0.024) in the AW-WIN than in the PDWD group.

CONCLUSIONS

AW-WIN reduces the time to target INR and the risk of excessive anticoagulation. AW-WIN may be an efficient and safe method of anticoagulation during the acute phase of ischaemic stroke.

Retrospective evidence for clinical validity of expanded genetic model in warfarin dose optimization in a South Indian population

Aim: To optimize warfarin dose in patients at risk for thrombotic events, we have recently developed a pharmacogenomic algorithm, which explained 44.9% of the variability in warfarin dose requirements using age, gender, BMI, vitamin K intake, CYP2C9 (*2 and *3) and VKORC1 (*3, *4 and -1639 G>A) as predictors. The aim of the current study is to develop an expanded genetic model that can explain greater percentage of warfarin variability and that has clinical validity. Patients & methods: CYP2C9*8, CYP4F2 V433M, GGCX G8016A and thyroid status were added to an expanded genetic model (n = 243). Results: The expanded genetic model explained 61% of the variability in warfarin dose requirements, has a prediction accuracy of ±11 mg/week and can differentiate warfarin sensitive and warfarin resistant groups efficiently (areas under receiver operating characteristic curves: 0.93 and 0.998, respectively; p < 0.0001). Higher percentage of International Normalized Ratios in therapeutic range (52.68 ± 4.21 vs 43.80 ± 2.27; p = 0.04) and prolonged time in therapeutic range (61.74 ± 3.18 vs 47.75 ± 5.77; p = 0.03) were observed in subjects with a prediction accuracy of <1 mg/day compared with subjects with prediction accuracy >1 mg/day. In the warfarin-resistant group, primary hypothyroidism was found to induce more resistance while in the warfarin-sensitive group, hyperthyroidism was found to increase sensitivity. Conclusion: The expanded genetic model explains greater variability in warfarin dose requirements and it prolongs time in therapeutic range and minimizes out-of-range International Normalized Ratios. Thyroid status also influences warfarin dose adjustments.

Original submitted 21 March 2012; Revision submitted 16 April 2012

Therapeutic dosing of acenocoumarol: proposal of a population specific pharmacogenetic dosing algorithm and its validation in north Indians

Objectives

To develop a population specific pharmacogenetic acenocoumarol dosing algorithm for north Indian patients and show its efficiency in dosage prediction.

Methods

Multiple and linear stepwise regression analyses were used to include age, sex, height, weight, body surface area, smoking status, VKORC1 -1639 G>A, CYP4F2 1347 G>A, CYP2C9*2,*3 and GGCX 12970 C>G polymorphisms as variables to generate dosing algorithms. The new dosing models were compared with already reported algorithms and also with the clinical data for various performance measures. Odds ratios for association of genotypes with drug sensitive and resistant groups were calculated.

Results

The pharmacogenetic dosing algorithm generated by multiple regression analysis explains 41.4% (p-value <0.001) of dosage variation. Validation of the new algorithm showed its predictive ability to be better than the already established algorithms based on similar variables. Its validity in our population is reflected by increased sensitivity, specificity, accuracy and decreased rates of over- and under- estimation in comparison to clinical data. The VKORC1-1639 G>A polymorphism was found to be strongly associated with acenocoumarol sensitivity according to recessive model.

Conclusions

We have proposed an efficient north India specific pharmacogenetic acenocoumarol dosing algorithm which might become a baseline for personalised medicine approach for treatment of patients in future.

A diagnostic informatics approach for stratifying risk outcome based on combined genotype effects

BACKGROUND

Diagnostic informatics (DI) in the context of personalized medicine involves the integration of molecular information to provide "actionable" diagnostic and therapeutic strategies. In many cases, retrospective predictions of clinical outcomes affected by multiple genes are complicated by not having the relevant genes measured within the same study. Multiplicative effect modeling is a statistical method for estimating the net effect of ≥ 2 independent variables. The authors demonstrate a DI approach that uses multiplicative-effect modeling to combine genetic information from ≥ 2 independent studies to predict a net clinical outcome.

METHODS

As a hypothetical working model, 2 independent studies were selected each reporting on a unique genetic factor proposed to influence the risk of stent thrombosis (ST) among subjects treated with clopidogrel. A multiplicative effect model was used for developing a hypothesis regarding their combined influence on clinical outcome.

RESULTS

Application of multiplicative risk modeling yielded a revised estimated risk of outcomes based on combined genotype. In this scenario, combined genotype revised the categorical risk level (high versus low) estimated from single gene effects for 41.5% of the subjects. Further, the maximum relative risk based on single gene effects was increased from 4.54 to 7.84 based on combined genotype. The revised relative risk values in conjunction with combined genotype frequency estimates provides the data necessary to frame a trial hypothesis and conduct appropriate power analysis to estimate the number of subjects needed to test that hypothesis.

CONCLUSIONS

This DI approach can be used to generate quantitative hypotheses on multiple gene effects derived from independent genotype studies. This approach is useful for estimating parameters needed in designing future studies to evaluate the net effect of ≥ 2 genetic variants on a common clinical endpoint.

[Development of a single-tube PCR-pyrosequencing method for simultaneous and rapid detection of the genetic polymorphism of warfarin metabolizing enzymes]

The purpose of this article is to develop a new high throughput method for detecting genetic polymorphism of warfarin metabolism-related genes rapidly in a single tube. Genomic DNA from human peripheral blood was extracted, and amplified with biotinylated primer to obtain single-stranded templates for pyrosequencing. Then, the single-stranded tem-plates were subjected to Pyrosequencing analysis using PyroMark ID instrument. Simultaneously, Sanger sequencing was also applied to sequence the products as a control to check the reliability of the pyrosequencing result.. The results dis-played that three variants of the warfarin metabolism-related genetic polymorphism (CYP2C9*2, CYP2C9*3, and VKORC1(-1693)) could be simultaneously detected using three different sequencing primers in a single-tube (one test), and 96 tests could be carried out each time. Repeat test and reliability test indicated that the agreement between the pyrosequencing and the Sanger sequencing methods was 100%. . All of these demonstrated that pyrosequencing could accurately and rapidly detect the genetic polymorphism of the warfarin drug metabolism-related genes with high throughput. Compar-ing with simplex pyrosequencing, the method established in the present study was much more economical and timesaving. It has a great value in personalized medical treatment and could be extended to the other genetic diseases.

On pharmacogenomics in pharmacy benefit management

Recently, the separate trajectories of pharmacy benefit management and pharmacogenomics converged. Pharmacogenomic tests have become more widely available for clinical use and at costs within the range of typical health care services. Pharmacy benefit payers continue to seek the precision they can apply to their coverage policies and clinical programs that pharmacogenomics offers. We describe how pharmacogenomics can now make sense as part of a pharmacy benefit and also how pharmacogenomics can be applied in a benefit coverage policy and clinical programs. Detail is provided on clinical program development and implementation processes featuring pharmacogenomics. We also discuss the research needed to support ongoing program development involving pharmacogenomics and describe the current roles of benefit payers and administrators in these research efforts. The legal and ethical dimensions of applying pharmacogenomics in pharmacy benefits are covered and in particular how benefit payers and administrators need to navigate between genetic exceptionalism and applicable laws and regulations. Finally, some thoughts are provided on future opportunities and challenges for pharmacogenomics in pharmacy benefit management and pharmacy in general.

Prediction of warfarin dose reductions in Puerto Rican patients, based on combinatorial CYP2C9 and VKORC1 genotypes

BACKGROUND

The influence of CYP2C9 and VKORC1 polymorphisms on warfarin dose has been investigated in white, Asian, and African American populations but not in Puerto Rican Hispanic patients.

OBJECTIVE

To test the associations between genotypes, international normalized ratio (INR) measurements, and warfarin dosing and gauge the impact of these polymorphisms on warfarin dose, using a published algorithm.

METHODS

A retrospective warfarin pharmacogenetic association study in 106 Puerto Rican patients was performed. DNA samples from patients were assayed for 12 variants in both CYP2C9 and VKORC1 loci by HILOmet PhyzioType assay. Demographic and clinical nongenetic data were retrospectively collected from medical records. Allele and genotype frequencies were determined and Hardy-Weinberg equilibrium (HWE) was tested.

RESULTS

Sixty-nine percent of patients were carriers of at least one polymorphism in either the CYP2C9 or the VKORC1 gene. Double, triple, and quadruple carriers accounted for 22%, 5%, and 1%, respectively. No significant departure from HWE was found. Among patients with a given CYP2C9 genotype, warfarin dose requirements declined from GG to AA haplotypes; whereas, within each VKORC1 haplotype, the dose decreased as the number of CYP2C9 variants increased. The presence of these loss-of-function alleles was associated with more out-of-range INR measurements (OR = 1.38) but not with significant INR >4 during the initiation phase. Analyses based on a published pharmacogenetic algorithm predicted dose reductions of up to 4.9 mg/day in carriers and provided better dose prediction in an extreme subgroup of highly sensitive patients, but also suggested the need to improve predictability by developing a customized model for use in Puerto Rican patients.

CONCLUSIONS

This study laid important groundwork for supporting a prospective pharmacogenetic trial in Puerto Ricans to detect the benefits of incorporating relevant genomic information into a customized DNA-guided warfarin dosing algorithm.

Influence of CYP4F2 genotype on warfarin dose requirement-a systematic review and meta-analysis

INTRODUCTION

Warfarin is a commonly used oral anticoagulant and the dosage is individually adjusted on the basis of the international normalized ratio (INR) monitoring. It is well known that gene polymorphisms of CytochromeP450 (CYP) 2C9 gene and the vitamin K epoxide reductase complex 1 (VKORC1) were significantly associated with warfarin dose. However, the association between Cytochrome P450 4F2 (CYP4F2) polymorphism and warfarin dose requirement is still controversial. This study was to investigate the influence of the CYP4F2 polymorphism, V433M (rs2108622) on warfarin dose for patients by meta-analysis.

METHODS

Strict inclusion and exclusion criteria were set, and the studies prior to December 19, 2010 were searched in PubMed, EMBASE and CNKI. References were examined and experts of primary studies were consulted for additional information. Revman 5.0.2 software was used to analyze the relationship between warfarin maintenance dose and CYP4F2 polymorphism

RESULTS

Thirteen studies were included in the meta-analysis which consisted of Caucasian, Asian and African populations. Compared to individuals with the homozygous CYP4F2 genotype (CC), carriers of CT, TT genotypes required 10.0% (95% confidence interval(CI) 4.0-15.0) and 21.0% (95% CI 9.0-33.0) higher warfarin doses respectively (P value <0.05). In addition, T carriers required 11.0% (95% CI 6.0-17.0) higher warfarin dose than CC genotype.

CONCLUSIONS

Our study showed that polymorphism of CYP4F2 had a moderate but statistically significant association with the variation of interindividual warfarin dose. However, whether CYP4F2 can improve the prediction of warfarin dose warrants need further investigation when combined with environmental factors.

Regions of homozygosity in three Southeast Asian populations

The genomes of outbred populations were first shown in 2006 to contain regions of homozygosity (ROHs) of several megabases. Further studies have also investigated the characteristics of ROHs in healthy individuals in various populations but there are no studies on Singapore populations to date. This study aims to identify and investigate the characteristics of ROHs in three Singapore populations. A total of 268 samples (96 Chinese, 89 Malays and 83 Indians) are genotyped on Illumina Human 1 M Beadchip and Affymetrix Genome-Wide Human SNP Array 6.0. We use the PennCNV algorithm to detect ROHs. We report an abundance of ROHs (≥500 kb), with an average of more than one hundred regions per individual. On average, the Indian population has the lowest number of ROHs and smallest total length of ROHs per individual compared with the Chinese and Malay populations. We further investigate the relationship between the occurrence of ROHs and haplotype frequency, regional linkage disequilibrium (LD) and positive selection. Based on the results of this data set, we find that the frequency of occurrence of ROHs is positively associated with haplotype frequency and regional LD. The majority of regions detected for recent positive selection and regions with differential LD between populations overlap with the ROH loci. When we consider both the location of the ROHs and the allelic form of the ROHs, we are able to separate the populations by principal component analysis, demonstrating that ROHs contain information on population structure and the demographic history of a population.

Development and comparison of a warfarin-dosing algorithm for Korean patients with atrial fibrillation

BACKGROUND

The pharmacokinetics and pharmacodynamics of warfarin are affected by polymorphisms in the genes coding for cytochrome P450 2C9 (CYP2C9) and vitamin K epoxide reductase complex subunit 1 (VKORC1).

OBJECTIVE

The objective of this study was to develop a pharmacogenetic dosing algorithm for warfarin in Korean patients with atrial fibrillation and to compare it with the published pharmacogenetic dosing algorithms for accuracy to predict warfarin maintenance dose.

METHODS

Clinical and genetic data from 130 Korean patients with atrial fibrillation (mean [SD] age: 66.2 [13.3] years; gender, male/female: 86/44; mean body weight: 66.6 [11.6] kg) were used to create a dosing algorithm, which was validated against an independent group of patients (n = 108; mean age: 67.4 [10.1] years; gender, male/female: 69/39; mean body weight: 66.0 [10.9] kg). Validation cohort data for the 12 previously published dosing algorithms incorporating CYP2C9 and VKORC1 genotype information were also applied.

RESULTS

A multivariate regression model including the variables of age, VKORC1 and CYP2C9 genotype, body surface area, and statin status produced the best model for estimating the warfarin dose (R(2) = 0.62). Among the 12 algorithms that were compared, the predicted doses using algorithms derived from both the Swedish Warfarin Genetics (WARG) study and the Korean population study showed the best correlation with actual warfarin doses. Comparing the percentage of patients whose predicted dosages were within 20% of actual dosages, these algorithms showed similar overall performance.

CONCLUSIONS

This study derived and validated a multivariate regression model for daily warfarin dose requirements in Korean patients with atrial fibrillation. As no algorithm could be considered the best for all dosing ranges, it may be important to consider the characteristics or limitations of each dosing algorithm and the nature of a population in choosing the most appropriate pharmacogenetic dosing.

Influence of genetic, biological and pharmacological factors on warfarin dose in a Southern Brazilian population of European ancestry

AIMS

To investigate the influence of polymorphisms in CYP2C9, VKORC1, CYP4F2 and F2 genes on warfarin dose-response and develop a model including genetic and non-genetic factors for warfarin dose prediction needed for each patient.

METHODS

A total of 279 patients of European ancestry on warfarin medication were investigated. Genotypes for -1639G>A, 1173C>T, and 3730G>A SNPs in the VKORC1 gene, CYP2C9*2 and CYP2C9*3, 1347C>T in the CYP4F2 gene and 494C>T in the F2 gene were determined by allelic discrimination with Taqman 5'-nuclease assays.

RESULTS

The CYP2C9*2 and CYP2C9*3 polymorphisms in the CYP2C9 gene, -1639G>A and 1173C>T in the VKORC1 gene and 494C>T in the F2 gene are responsible for lower anticoagulant doses. In contrast, 1347C>T in the CYP4F2 gene and 3730G>A in the VKORC1 gene are responsible for higher doses of warfarin. An algorithm including genetic, biological and pharmacological factors that explains 63.3% of warfarin dose variation was developed.

CONCLUSION

The model suggested has one of the highest coefficients of determination among those described in the literature.

Genetic factors associated with patient-specific warfarin dose in ethnic Indonesians

Background

CYP2C9 and VKORC1 are two major genetic factors associated with inter-individual variability in warfarin dose. Additionally, genes in the warfarin metabolism pathway have also been associated with dose variance. We analyzed Single Nucleotide Polymorphisms (SNPs) in these genes to identify genetic factors that might confer warfarin sensitivity in Indonesian patients.

Methods

Direct sequencing method was used to identify SNPs in CYP2C9, VKORC1, CYP4F2, EPHX1, PROC and GGCX genes in warfarin-treated patients. Multiple linear regressions were performed to model the relationship warfarin daily dose requirement with genetic and non-genetic variables measured and used to develop a novel algorithm for warfarin dosing.

Results

From the 40 SNPs analyzed, CYP2C9 rs17847036 and VKORC1 rs9923231 showed significant association with warfarin sensitivity. In our study population, no significant correlation could be detected between CYP2C9*3, CYP2C9C-65 (rs9332127), CYP4F2 rs2108622, GGCX rs12714145, EPHX1 rs4653436 and PROC rs1799809 with warfarin sensitivity.

Conclusions

VKORC1 rs9923231 AA and CYP2C9 rs17847036 GG genotypes were associated with low dosage requirements of most patients (2.05 ± 0.77 mg/day and 2.09 ± 0.70 mg/day, respectively). CYP2C9 and VKORC1 genetic variants as well as non-genetic factors such as age, body weight and body height account for 15.4% of variance in warfarin dose among our study population. Additional analysis of this combination could allow for personalized warfarin treatment in ethnic Indonesians.

Evaluation of the effects of VKORC1 polymorphisms and haplotypes, CYP2C9 genotypes, and clinical factors on warfarin response in Sudanese patients

OBJECTIVE

African populations, including the Sudanese, are underrepresented in warfarin pharmacogenetic studies. We designed a study to determine the associations between the polymorphisms and haplotype structures of CYP2C9 and VKORC1 and warfarin dose response in Sudanese patients, one of the most genetically diverse populations in Africa.

MATERIAL AND METHODS

The effect of the CYP2C9 polymorphisms (*2, *3, *5, *6, *8, *9, and *11), 20 VKORC1 tag SNPs and haplotypes, and clinical covariates were comprehensively assessed in 203 Sudanese warfarin-treated patients.

RESULTS

Patients with the CYP2C9*2,*5,*6, or *11 variant required a daily warfarin dose that was 21% lower than those with CYP2C9*1/*1 (4.7 vs 5.8 mg/day, P < 0.001). SNPs around the VKORC1 and POL3S genes were divided into two haplotype blocks in Sudanese populations. According to multiple linear regression results, rs8050984, rs7294, and rs7199949 in the VKORC1 and POL3S genes (P <0.001, <0.001, <0.001, respectively), CYP2C9 genotype (*2, *5, *6, *11; P < 0.001), body weight (P = 0.04), target INR (P = 0.007), and concurrent medications (P = 0.029) could explain about 36.7% of the total warfarin dose variation.

CONCLUSION

Our data revealed that VKORC1 and CYP2C9 polymorphisms are important factors that influence warfarin dose response in Sudanese patients. Our data suggest that combinations of the SNPs may improve predictions of warfarin dose requirements.

Comparison of warfarin pharmacogenetic dosing algorithms in a racially diverse large cohort

AIMS

Multiple warfarin pharmacogenetic dosing algorithms have been reported to date. However, there is only limited information available on the performance of the algorithms that can be used with the results of a US FDA-cleared warfarin pharmacogenetic test. We compared the performance of warfarin pharmacogenetic dosing algorithms in a large racially diverse cohort.

MATERIALS & METHODS

Warfarin pharmacogenetic dosing algorithms were identified using the PubMed database. Patient information from the International Warfarin Pharmacogenetics Consortium database was used to predict therapeutic warfarin doses according to each algorithm. By using bootstrapping analysis, the performance of algorithms was tested by comparing the mean absolute error and mean percentage of patients whose predicted dose fell within 20% of actual dose (percentage within 20%) in the entire cohort, and by race and therapeutic dose range.

RESULTS

A total of 13 algorithms and 1940 patients were included in the study. Overall, all the algorithms had similar performances (mean absolute error: 10.3 mg/week and mean percentage within 20%-41.4%). However, algorithms derived from racially mixed populations tended to perform better than those derived from single race populations. Mixed population algorithms had the lowest mean absolute error and the highest percentage within 20% across the racial groups. Most algorithms performed better in the intermediate-dose range (between 21 and 49 mg/week) than in the low (≤21 mg/week) or high-(≥49 mg/week) range.

CONCLUSION

Published warfarin pharmacogenetic algorithms performed similarly, although mixed population algorithms tended to perform better than race-specific algorithms.

Risk modeling strategies for pharmacogenetic studies

Pharmacogenetic risk models offer great promise as treatment decision tools; however, their uptake in routine clinical practice is so far disappointing, not least due to the lack of evidence of their benefit in randomized controlled trials and other types of studies. Prior to conducting such a study, it is imperative that the model’s predictive capability is first of all proven, and that it is shown to be superior to the most appropriate alternative model. When demonstrating predictive capability, clinical implications of applying the model should be a key consideration, and the Decision Curve Analysis method takes this into account for binary outcomes. Furthermore, when comparing a novel model to the best alternative, methods such as Net Reclassification Improvement or Integrated Discrimination Difference are recommended as they provide a more reliable comparison than other methods currently in common use. Where outcome is continuous, such as therapeutic dose, assessing a model’s performance is generally more intuitive and straightforward since the aim is to achieve a predicted dose as close as possible to the true therapeutic dose.

In pediatric patients, age has more impact on dosing of vitamin K antagonists than VKORC1 or CYP2C9 genotypes

Anticoagulation with vitamin K antagonists (VKAs) is problematic because of difficulties in safely managing dosing. Polymorphisms in cytochrome P450 2C9 (CYP2C9) and vitamin K epoxide reductase genes (VKORC1) have been shown to affect VKA dosing in adults. The association of these polymorphisms on VKA dosing in children has not been investigated. The objective of the study was to assess associations of CYP2C9 and VKORC1 polymorphisms and clinical variables on VKA dosing in children. A nonselected cohort of pediatric patients receiving VKA were tested for CYP2C9 and VKORC1 polymorphisms, and clinical data were collected. Multiple linear regression modeling was used to assess relationships of VKA dose with genetic and clinical variables. Fifty-nine patients were recruited; 55.9% were receiving warfarin, and 44.1% were on phenprocoumon. There was a negative association of age with VKA dose (P < .001). Comparing VKORC1 genotypes, the AA group required significantly lower daily doses than GG group (P = .011). In the full model including age, VKORC1 and CYP2C9 genotypes accounted for 38% of dose variation. Age explained 28.3% of VKA dose variations; VKORC1 and CYP2C9 explained only 3.7% and 0.4%, respectively. In children, the most critical factor in determining VKA dose is age. VKORC1/CYP2C9 genotypes only marginally explain dose variations.

Extending and evaluating a warfarin dosing algorithm that includes CYP4F2 and pooled rare variants of CYP2C9

OBJECTIVE

Warfarin dosing remains challenging because of its narrow therapeutic window and large variability in dose response. We sought to analyze new factors involved in its dosing and to evaluate eight dosing algorithms, including two developed by the International Warfarin Pharmacogenetics Consortium (IWPC).

METHODS

we enrolled 108 patients on chronic warfarin therapy and obtained complete clinical and pharmacy records; we genotyped single nucleotide polymorphisms relevant to the VKORC1, CYP2C9, and CYP4F2 genes using integrated fluidic circuits made by Fluidigm.

RESULTS

When applying the IWPC pharmacogenetic algorithm to our cohort of patients, the percentage of patients within 1 mg/d of the therapeutic warfarin dose increases from 54% to 63% using clinical factors only, or from 38% using a fixed-dose approach. CYP4F2 adds 4% to the fraction of the variability in dose (R) explained by the IWPC pharmacogenetic algorithm (P<0.05). Importantly, we show that pooling rare variants substantially increases the R for CYP2C9 (rare variants: P=0.0065, R=6%; common variants: P=0.0034, R=7%; rare and common variants: P=0.00018; R=12%), indicating that relatively rare variants not genotyped in genome-wide association studies may be important. In addition, the IWPC pharmacogenetic algorithm and the Gage (2008) algorithm perform best (IWPC: R=50%; Gage: R=49%), and all pharmacogenetic algorithms outperform the IWPC clinical equation (R=22%). VKORC1 and CYP2C9 genotypes did not affect long-term variability in dose. Finally, the Fluidigm platform, a novel warfarin genotyping method, showed 99.65% concordance between different operators and instruments.

CONCLUSION

CYP4F2 and pooled rare variants of CYP2C9 significantly improve the ability to estimate warfarin dose.

Genotyping three SNPs affecting warfarin drug response by isothermal real-time HDA assays

BACKGROUND

The response to the anticoagulant drug warfarin is greatly affected by genetic polymorphisms in the VKORC1 and CYP2C9 genes. Genotyping these polymorphisms has been shown to be important in reducing the time of the trial and error process for finding the maintenance dose of warfarin thus reducing the risk of adverse effects of the drug.

METHOD

We developed a real-time isothermal DNA amplification system for genotyping three single nucleotide polymorphisms (SNPs) that influence warfarin response. For each SNP, real-time isothermal Helicase Dependent Amplification (HDA) reactions were performed to amplify a DNA fragment containing the SNP. Amplicons were detected by fluorescently labeled allele specific probes during real-time HDA amplification.

RESULTS

Fifty clinical samples were analyzed by the HDA-based method, generating a total of 150 results. Of these, 148 were consistent between the HDA-based assays and a reference method. The two samples with unresolved HDA-based test results were repeated and found to be consistent with the reference method.

CONCLUSION

The HDA-based assays demonstrated a clinically acceptable performance for genotyping the VKORC1 -1639G>A SNP and two SNPs (430C>T and 1075A>C) for the CYP2C9 enzyme (CYP2C9*2 and CYP2C9*3), all of which are relevant in warfarin pharmacogenentics.

Patient Factors That Influence Warfarin Dose Response

Warfarin has long been the mainstay of oral anticoagulation therapy for the treatment and prevention of venous and arterial thrombosis. The narrow therapeutic index of warfarin, and the complex number of factors that influence international normalized ratio (INR) response, makes optimization of warfarin therapy challenging. Determination of the appropriate warfarin dose during initiation and maintenance therapy requires an understanding of patient factors that influence dose response: age, body weight, nutritional status, acute and chronic disease states, and changes in concomitant drug therapy and diet. This review will examine specific clinical factors that can affect the pharmacokinetics and pharmacodynamics of warfarin, as well as the role of pharmacogenetics in optimizing warfarin therapy.

Implementing genotype-guided antithrombotic therapy

Genotyping has the potential to improve the efficacy and safety of major antithrombotic drugs. For warfarin, the stable maintenance dose varies from 1-10 mg/day. The VKORC1 -1639G>A allele and the CYP2C9*2 and *3 alleles (cumulative frequency: 90% in Asians, 65% in Europeans and 20% in Africans), explain 45% of response variability in European and 30% in African populations. The large clinical trials COAG and EU-PACT will define the extent to which pharmacogenetic dosing affects the safety and efficacy of warfarin and coumarin derivatives. The platelet inhibitor clopidogrel requires activation by the CYP2C19 enzyme. CYP2C19*2 and *3 alleles (cumulative frequency: 20-50%) produce null enzyme activity, and their presence attenuates platelet inhibition and increases cardiovascular events. The US FDA-mandated drug labeling recognizes the relevance of genotyping in the selection and dosing of both warfarin and clopidogrel.

Comparative performance of gene-based warfarin dosing algorithms in a multiethnic population

SUMMARY BACKGROUND

Gene-based warfarin dosing algorithms have largely been developed in homogeneous populations, and their generalizability has not been established.

OBJECTIVES

We sought to assess the performance of published algorithms in a racially diverse and multiethnic sample, and determine if additional clinical variables or genetic variants associated with dose could enhance algorithm performance.

PATIENTS AND METHODS

In 145 compliant patients on warfarin with a goal international normalized ratio (INR) of 2-3, stable, therapeutic doses were compared with predicted doses using 12 reported algorithms that incorporated CYP2C9 and VKORC1 variants. Additional covariates tested with each model included race, concurrent medications, medications known to interact with warfarin and previously described CYP4F2, CALU and GGCX variants.

RESULTS

The mean patient age was 67 +/- 14 years; 90 (62%) were male. Eighty-two (57%) were Caucasian, 28 (19%) African-American, 20 (14%) Hispanic and 15 (10%) Asian. The median warfarin dose was 35 mg per week (interquartile range 23-53 mg per week). Gene-based dosing algorithms explained 37-55% of the variation in warfarin dose requirements. Neither the addition of race, number of concurrent medications nor the number of concurrent medications interacting with warfarin enhanced algorithm performance. Similarly, consideration of CYP4F2, CALU or GGCX variant genotypes did not improve algorithms.

CONCLUSIONS

Existing gene-based dosing algorithms explained between approximately one-third and one-half of the variability in warfarin dose requirements in this racially and ethnically diverse cohort. Additional clinical and recently described genetic variants associated with warfarin dose did not enhance prediction in our patient population.

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.

Warfarin pharmacogenetics: a single VKORC1 polymorphism is predictive of dose across 3 racial groups

Warfarin-dosing algorithms incorporating CYP2C9 and VKORC1 -1639G>A improve dose prediction compared with algorithms based solely on clinical and demographic factors. However, these algorithms better capture dose variability among whites than Asians or blacks. Herein, we evaluate whether other VKORC1 polymorphisms and haplotypes explain additional variation in warfarin dose beyond that explained by VKORC1 -1639G>A among Asians (n = 1103), blacks (n = 670), and whites (n = 3113). Participants were recruited from 11 countries as part of the International Warfarin Pharmacogenetics Consortium effort. Evaluation of the effects of individual VKORC1 single nucleotide polymorphisms (SNPs) and haplotypes on warfarin dose used both univariate and multi variable linear regression. VKORC1 -1639G>A and 1173C>T individually explained the greatest variance in dose in all 3 racial groups. Incorporation of additional VKORC1 SNPs or haplotypes did not further improve dose prediction. VKORC1 explained greater variability in dose among whites than blacks and Asians. Differences in the percentage of variance in dose explained by VKORC1 across race were largely accounted for by the frequency of the -1639A (or 1173T) allele. Thus, clinicians should recognize that, although at a population level, the contribution of VKORC1 toward dose requirements is higher in whites than in nonwhites; genotype predicts similar dose requirements across racial groups.

CYP2C9*8 is prevalent among African-Americans: implications for pharmacogenetic dosing

AIMS

Although the frequencies of pharmacogenetic variants differ among racial groups, most pharmacogenetic algorithms for genotype-guided warfarin dosing only include two CYP2C9 alleles (*2 and *3) and a single VKORC1 allele (g.-1639G>A or g.1173C>T) commonly found among Caucasians. Therefore, this study sought to identify other CYP2C9 and VKORC1 alleles important in warfarin dose variability and to determine their frequencies in different racial and ethnic groups.

MATERIALS & METHODS

The CYP2C9 and VKORC1 genes were sequenced in selected sensitive (< 21 mg/week) and resistant (> 49 mg/week) individuals with discrepant therapeutic and algorithm-predicted warfarin doses based on prior CYP2C9 and VKORC1 genotyping. The CYP2C9 and VKORC1 allele frequencies were determined in healthy, racially self-identified blood donors.

RESULTS

Sequencing identified an African-American male with a lower than predicted therapeutic warfarin dose (14.4 mg/week), previously genotyped as CYP2C9*1/*1, who was homozygous for CYP2C9*8 (c.449G>A; p.R150H). Genotyping 600 African-American alleles identified CYP2C9*8 as their most frequent variant CYP2C9 allele (0.047), and the combined allele frequency of CYP2C9*2, *3, *5, *6, *8 and *11 was 0.133. Given most warfarin pharmacogenetic dosing algorithms only include CYP2C9*2 and *3, the inclusion of CYP2C9*8 alone could reclassify the predicted metabolic phenotypes of almost 10% of African-Americans, or when combined with CYP2C9*5, *6 and *11, more than 15%. In addition, the African-American VKORC1 g.-1639A allele frequency was 0.108 and three g.1331G>A (p.V66M) carriers were identified.

CONCLUSIONS

CYP2C9*8 is prevalent among African-Americans ( approximately 1 in 11 individuals). Thus, in this racial group, the incorporation of CYP2C9*8 into genotyping panels may improve dose prediction of CYP2C9-metabolized drugs, including warfarin.

Point-of-care testing and molecular diagnostics: miniaturization required

Turnaround time for molecular diagnostic tests is critical in detecting infectious agents, in determining a patient's ability to metabolize a drug or drug class, and in detecting minimal residual disease. These applications would benefit from the development of a point-of-care device for nucleic acid extraction, amplification, and detection. The ideal device would have a low cost per test, use a disposable unit use device for all steps in the assay, be portable, and provide a result that requires no interpretation. The creation of such a device requires miniaturization of current technologies and the use of microfluidics, microarrays, and small-diameter capillary tubes to reduce reagent volumes and simplify heat conduction by convection during nucleic acid amplification. This ideal device may be available in 3 to 5 years and will revolutionize and expand the global availability of molecular diagnostic assays.

VKORC1 diplotype-derived dosing model to explain variability in warfarin dose requirements in Asian patients

SUMMARY

Warfarin-induced bleeding complications and high inter-patient variability are major hindrances to oral anticoagulant therapy. The present study identifies the influence of VKORC1 diplotypes, CYP2C9 and CYP2C19 variants on warfarin disposition and dose requirements in Chinese patients (n=107). The study subjects were genotyped for VKORC1, CYP2C9 and CYP2C19 polymorphic variants. Weekly warfarin dose requirements and S-warfarin clearance were stratified by VKORC1, CYP2C9 and CYP2C19 pharmacogenetics. The major VKORC1 diplotypes were H1-H1 (62%), H1-H7 (18%) and H1-H(*)(b) (10%). Warfarin dose requirements were significantly lower in patients with VKORC1 H1-H1 and H1-H(*)(a) diplotypes compared to patients harboring the H1-H7 and H1-H(*)(b) diplotypes (P<0.05). Hepatic tissues with H1-H1 diplotype had significantly lower expression of VKORC1 mRNA compared with liver tissues carrying the H1-H7 and H1-H(*)(b) diplotypes (P=0.006). The percent variability explained by VKORC1 diplotype status was 59.1% while the CYP2C9 genotype status accounted for 6.9% variability in warfarin dose requirements. Patient age and weight were significant covariates accounting for 29% and 8.6% of warfarin dose variability, respectively. The present study shows that VKORC1 diplotype status, CYP2C9 genotype, age and weight are significant covariates, accounting for 73.4% of interindividual variability in warfarin dose requirements among Chinese patients. Translation of these findings into clinical guidelines for warfarin dosing may be required to assess its impact on the safety and efficacy of warfarin.

Rapid genotyping of single nucleotide polymorphisms influencing warfarin drug response by surface-enhanced laser desorption and ionization time-of-flight (SELDI-TOF) mass spectrometry

Warfarin exhibits significant interindividual variability in dosing requirements. Different drug responses are partly attributed to the single nucleotide polymorphisms (SNPs) that influence either drug action or drug metabolism. Rapid genotyping of these SNPs helps clinicians to choose appropriate initial doses to quickly achieve anticoagulation effects and to prevent complications. We report a novel application of surface-enhanced laser desorption and ionization time-of-flight mass spectrometry (SELDI-TOF MS) in the rapid genotyping of SNPs that impact warfarin efficacy. The SNPs were first amplified by PCR and then underwent single base extension to generate the specific SNP product. Next, genetic variants displaying different masses were bound to Q10 anionic proteinChips and then genotyped by using SELDI-TOF MS in a multiplex fashion. SELDI-TOF MS offered unique properties of on-chip sample enrichment and clean-ups, which streamlined the testing procedures and eliminated many tedious experimental steps required by the conventional MS-based method. The turn-around time for genotyping three known warfarin-related SNPs, CYP2C9*2, CYP2C9*3, and VKORC1 3673G>A by SELDI-TOF MS was less than 5 hours. The analytical accuracy of this method was confirmed both by bidirectional DNA sequencing and by comparing the genotype results (n = 189) obtained by SELDI-TOF MS to reports from a clinical reference laboratory. This new multiplex genotyping method provides an excellent clinical laboratory platform to promote personalized medicine in warfarin therapy.