A Simple Formula for Predicting the Maintenance Dose of Warfarin with Reference to the Initial Response to Low Dosing at an Outpatient Clinic

Objective The pharmacodynamic effect of warfarin varies among individuals, and its maintenance dose is widely distributed. Although many formulae for predicting the maintenance dose of warfarin have been developed, most of them are complex and not in practical use. Methods and Materials Among 12,738 new patients visiting the Cardiovascular Institute between 2004 and 2009, we identified 127 patients (66.6±8.8 years, 89 men) with atrial fibrillation for whom warfarin was newly started with an initial dose of 2 mg/day and the international normalized ratio (INR) at 1 year after warfarin was started was within the therapeutic range. The prediction models for the maintenance dose were developed by an exponential equation and a first-order equation. Results The initial response of the INR to the dose of 2 mg/day (initial INR) ranged from 1.00-3.24 (mean 1.43), while the maintenance dose of warfarin ranged from 0.5-14 mg (mean 3.8 mg). The maintenance dose showed an exponential correlation to the initial INR: (predicted maintenance dose) =5.522× (initial INR) ^-1.556 (R²=0.795, p<0.001). Excluding the patients with a poor response to the initial dose (initial INR <1.1, n=32) permitted a simple correlation with a first-order approximation: (predicted maintenance dose) =-2.009× (initial INR) +6.172 (R²=0.706, p<0.001). Conclusion We developed a simple formula for predicting the maintenance dose of warfarin using the initial response of the INR to low-dose warfarin.

Evaluation of CYP2C9- and VKORC1-based pharmacogenetic algorithm for warfarin dose in Gaza-Palestine

Aim

To evaluate applicability of CYP2C9*2, *3 and VKORC1-1639G > A based algorithm to predict warfarin stable dose (WSD) in a group of Palestinian patients.

Patients & methods

Warfarin doses were retrospectively calculated for 101 Palestinian patients under warfarin therapy using three models. Performance of the three models was assessed in 47 patients found to take WSD.

Results

Frequency of CYP2C9*2, *3 and VKORC1-1639G > A alleles is 13.6, 0.0 and 46.5% respectively. The international warfarin pharmacogenetics consortium algorithm was more reliable (MAE = 8.9 ± 1.4; R² = 0.350) than both the clinical algorithm (MAE = 10.4 ± 1.4; R² = 0.128;) and the fixed-dose algorithm (MAE = 11.1 ± 1.7).

Conclusion

The international warfarin pharmacogenetics consortium algorithm can be reliably applied for predicting the WSD in Palestinian population.

Influence of common and rare genetic variation on warfarin dose among African-Americans and European-Americans using the exome array

AIM

We conducted a genome-wide association study using the Illumina Exome Array to identify coding SNPs that may explain additional warfarin dose variability.

PATIENTS & METHODS

Analysis was performed after adjustment for clinical variables and genetic factors known to influence warfarin dose among 1680 warfarin users (838 European-Americans and 842 African-Americans). Replication was performed in an independent sample.

RESULTS

We confirmed the influence of known genetic variants on warfarin dose variability. Our study is the first to show the association between rs12772169 and warfarin dose in African-Americans. In addition, genes COX15 and FGF5 showed significant association in European-Americans.

CONCLUSION

We identified some novel genes/SNPs that underpin warfarin dose response. Further replication is needed to confirm our findings.

DNA sensors to assess the effect of VKORC1 and CYP2C9 gene polymorphisms on warfarin dose requirement in Chinese patients with atrial fibrillation

The optimal dose of warfarin depends on polymorphisms in the VKORC1 (the vitamin K epoxide reductase complex subunit (1) and CYP2C9 (cytochrome P450 2C9) genes. To minimize the risk of adverse reactions, warfarin dosages should be adjusted according to results from rapid and simple monitoring methods. However, there are few pharmacogenetic-guided warfarin dosing algorithms that are based on large cohorts from the Chinese population, especially patients with atrial fibrillation. This study aimed to validate a pharmacogenetic-guided warfarin dosing algorithm based on results from a new rapid electrochemical detection method used in a multicenter study. Three SNPs (CYP2C9 *2, *3 and VKORC1 c.-1639G > A) were genotyped by electrochemical detection using a sandwich-type format that included a 3' short thiol capture probe and a 5' ferrocene-labeled signal probe. A total of 1285 samples from four clinical hospitals were evaluated. Concordance rates between the results from the electrochemical DNA biosensor and the sequencing test were 99.8%. The results for gene distribution showed that most Chinese patients had higher warfarin susceptibility because mutant-type and heterozygotes were present in the majority of subjects (99.4%) at locus c.-1639G > A. When the International Warfarin Pharmacogenetics Consortium algorithm was used to estimate therapeutic dosages for 362 patients with AF and the values were compared with their actual dosages, the results revealed that 56.9% were similar to actual dosages (within the 20% range). A novel electrochemical detection method of CYP2C9 *2, *3and VKORC1 c.-1639G > A alleles was evaluated. The warfarin dosing algorithm based on data gathered from a large patient cohort can facilitate the reasonable and effective use of warfarin in Chinese patients with AF.

Effect of CYP2C9 and VKORC1 genetic variations on warfarin dose requirements in Indian patients

BACKGROUND

Warfarin, an oral anticoagulant is used in patients who are at increased risk of developing blood clots. The management of warfarin therapy is challenging because it shows large inter and intra individual variability in patient response due to factors like age, gender, diet, concurrent drug interactions and variations in CYP2C9 and VKORC1 genes. Studies implicate that polymorphisms in VKORC1 and CYP2C9 genes are associated with reduced doses of warfarin. The aim of our current study was to characterize the effects of VKORC1 and CYP2C9 gene variations that contribute to variability in warfarin dosing in Indian patients.

METHODS

Genomic DNA was extracted from 103 patients undergoing warfarin therapy. Their mean daily warfarin dose, INR and demographics were recorded and genotyping of VKORC1 and CYP2C9 gene was performed by PCR-RFLP method.

RESULTS

Individuals with wild type genotypes required highest mean warfarin dosage of 4.72 mg/day while VKORC1 variants required 3.6 mg/day to maintain their therapeutic INR. CYP2C9*2 genotype was not found to affect the warfarin maintenance dosages. The odds ratio for developing supra therapeutic INR in patients carrying VKORC1 variant allele when compared to wild types was 13.96 (95% CI; 4.85 - 44.65. Other factors affecting warfarin dosages were age and weight.

CONCLUSION

Inclusion of pharmacogenetic data along with clinical parameters would help better predict warfarin doses in Indian patients.

Attitudes on warfarin pharmacogenetic testing in Chinese patients and public

INTRODUCTION

Genetic factors affecting warfarin maintenance dose have been well established, but patient acceptance is a necessary consideration for the successful clinical implementation of warfarin pharmacogenetic testing (WPGT). In this study, we aimed to determine the attitudes toward WPGT among Singaporean Chinese.

METHODS

A total of 194 warfarin patients and 187 members of the public completed a structured survey on paper and the Internet, respectively. Attitudes were expressed as willingness to undergo WPGT (single item with 5-point response) and expectations and concerns about WPGT (two multi-item scales). Relationships between attitudes and socio-demographic and clinical variables were explored using Fisher's exact test, Student's t-test, one-way analysis of variance or Pearson's correlation.

RESULTS

Majority of respondents were willing to or neutral about undergoing WPGT. Both patients and public had relatively high expectations (mean [SD]: 3.77 [0.63], and 3.97 [0.55], respectively) and moderately high concerns (mean [SD]: 3.30 [0.69] and 3.33 [0.68], respectively) about WPGT. Willingness to undergo WPGT was associated with gender, educational status, length of warfarin treatment, and number of chronic diseases among warfarin patients, and with history of adverse drug reactions and number of chronic diseases among the public. Higher expectation of WPGT was associated with higher willingness (p < .001 in both populations), while higher concern was associated with lower willingness to undergo WPGT among the public (p = .004) but not among patients (p = .072).

CONCLUSION

Patient acceptance is not a major barrier to clinical implementation of WPGT but patient education is necessary and the ethical, social, and legal issues should be addressed.

Effect of CYP2C9, VKORC1, CYP4F2 and GGCX genetic variants on warfarin maintenance dose and explicating a new pharmacogenetic algorithm in South Indian population

OBJECTIVE

To determine the influence of genetic polymorphisms on warfarin maintenance dose and to explicate an algorithm using the pharmacogenetic and clinical factors to determine the maintenance and/or starting dose of warfarin in South Indian patients receiving warfarin therapy.

METHODS

Patients receiving stabilized warfarin therapy (n=257) were included in the study. Single nucleotide polymorphisms (SNPs) of CYP2C9 (rs1799853 and rs1057910), VKORC1 (rs9923231, rs7196161, rs2884737, rs9934438, rs8050894, rs2359612 and rs7294), CYP4F2 (rs2108622) and GGCX (rs11676382) were genotyped by the quantitative real time-PCR method.

RESULTS

The mean daily maintenance dose of warfarin was found to be 4.7 ± 2.1 mg/day. Patients with the CYP2C9*1/*2, *1/*3 and *2/*3 variant genotypes required a 51.0 (2.8 mg), 60.9 (2.3 mg) and 62.2 % (2.2 mg) lower daily maintenance dose of warfarin, respectively, than those patients with the CYP2C9*1/*1 wild-type genotype (5.2 mg) (p<0.0001). The genetic variants of CYP2C9, VKORC1 and GGCX were associated with decreased warfarin dose, except for rs7196161, rs7294 and rs2108622 which were associated with an increased warfarin dose. Genetic variations of CYP2C9 (*2 and *3), VKORC1 (rs9923231, rs7294, rs9934438 and rs2359612), CYP4F2, GGCX and non-genetic factors such as age, body weight, clinical status (post mechanical valve replacement) could explain up to 62.1 % of the overall variation (adjusted r (2) 60.2 %, p<0.0001) in warfarin maintenance dose.

CONCLUSION

Genetic polymorphisms of CYP2C9, VKORC1, CYP4F2 and GGCX are important predictive factors of warfarin maintenance dose, and the developed algorithm will be useful to predict the required maintenance and/or starting warfarin dose in South Indian populations.

The population attributable fraction as a measure of the impact of warfarin pharmacogenetic testing

AIM

We aimed to estimate the population impact of warfarin pharmacogenetic testing (WPGT) across multiple populations.

MATERIALS & METHODS

We used the expanded International Warfarin Pharmacogenetics Consortium data set and genotype frequencies from HapMap to simulate dose distributions for each CYP2C9/VKORC1 genotype combination in the different races, and calculated the population attributable fraction as a measure of population impact of WPGT. WPGT was compared to both clinical and fixed-dose algorithms to estimate the benefits of WPGT.

RESULTS

Our dose simulation revealed different dose requirements in difference races and considerable overlap in dose distributions of different genotype combinations. Population attributable fraction calculations suggest that complete implementation of WPGT can reduce inaccurate dosing by 18-24% in white individuals. However black, Japanese and Chinese patients do not benefit from WPGT, especially when compared against a race-specific fixed dose.

CONCLUSION

Our findings support WPGT in white individuals but not in black, Japanese and Chinese individuals.

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.

A systems approach to designing effective clinical trials using simulations

BACKGROUND

Pharmacogenetics in warfarin clinical trials have failed to show a significant benefit in comparison with standard clinical therapy. This study demonstrates a computational framework to systematically evaluate preclinical trial design of target population, pharmacogenetic algorithms, and dosing protocols to optimize primary outcomes.

METHODS AND RESULTS

We programmatically created an end-to-end framework that systematically evaluates warfarin clinical trial designs. The framework includes options to create a patient population, multiple dosing strategies including genetic-based and nongenetic clinical-based, multiple-dose adjustment protocols, pharmacokinetic/pharmacodynamics modeling and international normalization ratio prediction, and various types of outcome measures. We validated the framework by conducting 1000 simulations of the applying pharmacogenetic algorithms to individualize dosing of warfarin (CoumaGen) clinical trial primary end points. The simulation predicted a mean time in therapeutic range of 70.6% and 72.2% (P=0.47) in the standard and pharmacogenetic arms, respectively. Then, we evaluated another dosing protocol under the same original conditions and found a significant difference in the time in therapeutic range between the pharmacogenetic and standard arm (78.8% versus 73.8%; P=0.0065), respectively.

CONCLUSIONS

We demonstrate that this simulation framework is useful in the preclinical assessment phase to study and evaluate design options and provide evidence to optimize the clinical trial for patient efficacy and reduced risk.

Validation of pharmacogenetic algorithms and warfarin dosing table in Egyptian patients

BACKGROUND

Warfarin remains a difficult drug to use due to the large variability in dose response. Clear understanding of the accuracy of warfarin pharmacogenetic dosing methods might lead to appropriate control of anticoagulation.

OBJECTIVE

This study aims to evaluate the accuracy of warfarin dosing table and two pharmacogenetic algorithms, namely the algorithms of Gage et al. (Clin Pharmacol Ther 84:326-331, 2008), and the International Warfarin Pharmacogenetics Consortium algorithm (IWPC) in a real Egyptian clinical setting. Additionally, three non-pharmacogenetic dosing methods (the Gage, IWPC clinical algorithms and the empiric 5 mg/day dosing) were evaluated.

SETTING

Sixty-three Egyptian patients on a stable therapeutic warfarin dose were included. Patients were recruited from the outpatient clinic of the critical care medicine department.

METHODS

CYP2C9 and VKORC1 polymorphisms were genotyped by real time PCR system. Predicted doses by all dosing methods were calculated and compared with the actual therapeutic warfarin doses.

RESULTS

The Gage algorithm (adjusted R(2) = 0.421, and mean absolute error (MAE) = 3.3), and IWPC algorithm (adjusted R(2) = 0.419, MAE = 3.2) produced better accuracy than did the warfarin dosing table (adjusted R(2) = 0.246, MAE = 3.5), the two clinical algorithms (R(2) = 0.24, MAE = 3.7) and the fixed dose approach (MAE = 3.9). However, all dosing models produced comparable clinical accuracy with respect to proportion of patients within 1 mg/day of actual dose (ideal dose). Non-pharmacogenetic methods severely over-predicted dose (defined as ≥2 mg/day more than actual dose) compared to the three pharmacogenetic models. In comparison to non-pharmacogenetic methods, the three pharmacogenetic models performed better regarding the low dose group in terms of percentage of patients within ideal dose. In the high dose group, none of the dosing models predicted warfarin doses within ideal dose.

CONCLUSION

Our study showed that genotype-based dosing improved prediction of warfarin therapeutic dose beyond that available with the fixed-dose approach or the clinical algorithms, especially in the low-dose group. However, the two pharmacogenetic algorithms were the most accurate.

Analysis of R- and S-hydroxywarfarin glucuronidation catalyzed by human liver microsomes and recombinant UDP-glucuronosyltransferases

Coumadin (R-, S-warfarin) is a challenging drug to accurately dose, both initially and for maintenance, because of its narrow therapeutic range and wide interpatient variability and is typically administered as a racemic (Rac) mixture, which complicates the biotransformation pathways. The goal of the current work was to identify the human UDP-glucuronosyltransferases (UGTs) involved in the glucuronidation of the separated R- and S-enantiomers of 6-, 7-, and 8-hydroxywarfarin and the possible interactions between these enantiomers. The kinetic and inhibition constants for human recombinant 1A family UGTs toward these separated enantiomers have been assessed using high-performance liquid chromatography (HPLC)-UV-visible analysis, and product confirmations have been made using HPLC-mass spectrometry/mass spectrometry. We found that separated R- and S-enantiomers of 6-, 7-, and 8-hydroxywarfarin demonstrate significantly different glucuronidation kinetics and can be mutually inhibitory. In some cases significant substrate inhibition was observed, as shown by K(m), V(max), and K(i), comparisons. In particular, UGT1A1 and extrahepatic UGT1A10 have significantly higher capacities than other isoforms for S-7-hydroxywarfarin and R-7-hydroxywarfarin glucuronidation, respectively. Activity data generated using a set of well characterized human liver microsomes supported the recombinant enzyme data, suggesting an important (although not exclusive) role for UGT1A1 in glucuronidation of the main warfarin metabolites, including Rac-6- and 7-hydroxywarfarin and their R- and S-enantiomers in the liver. This is the first demonstration that the R- and S-enantiomers of hydroxywarfarins are glucuronidated, with significantly different enzymatic affinity and capacity, and supports the importance of UGT1A1 as the major hepatic isoform involved.

Vitamin K antagonists--current concepts and challenges

Vitamin K antagtonists (VKAs) have, for decades, been the corner-stone of anticoagulation in the outpatient setting. While the long half life makes once daily administration practical, close monitoring of VKA effect is necessary because these medicines have a narrow therapeutic index. Despite inter-individual variations in response to VKA doses, the increasing availability of specialized anticoagulation monitoring systems, along with a better understanding of potential drug and dietary interactions, has made the use of VKAs safer and less burdensome. In the future, newer classes of oral anticoagulants and genomic-based dosing strategies may further expand or improve the management options for many patients at risk for thromboembolism.

Confirmation of ALDH2 as a Major locus of drinking behavior and of its variants regulating multiple metabolic phenotypes in a Japanese population

BACKGROUND

Normative alcohol use (or drinking behavior) influences the risk of cardiovascular disease in a multi-faceted manner. To identify susceptibility gene variants for drinking behavior, a 2-staged genome-wide association study was performed in a Japanese population.

METHODS AND RESULTS

In the stage-1 scan, 733 cases and 729 controls were genotyped with 456,827 SNP markers. The associated loci without redundancy of linkage disequilibrium were further examined in the stage-2 general population panel comprising 2,794 drinkers (≥ once per week), 1,521 chance drinkers (< once per week), and 1,351 non-drinkers. Along with genome-wide exploration, we aimed to replicate the trait association of a candidate gene SNP previously reported (rs1229984 in ADH1B). A cluster of 12 SNPs on 12q24 were found to significantly (P<5×10(-8)) associate with drinking behavior in stage 1, among which rs671 (a Glu-to-Lys substitution at position 504) in the ALDH2 gene showed the strongest association (odds ratio (OR)=0.16, P=3.6×10(-211) in the joint analysis). The association was also replicated for rs1229984 (OR=1.20, P<3.6×10(-4)). Furthermore, ALDH2 504Lys was associated with several metabolic traits, eg, lower levels of high-density lipoprotein cholesterol and liver enzymes-AST, ALT, and γGTP-by interacting with alcohol intake.

CONCLUSIONS

Our results confirm ALDH2 as a major locus regulating drinking behavior in the Japanese, indicating that the ALDH2 504Lys variant exerts pleiotropic effects on risk factors of cardiovascular disease among drinkers.

Proposal of pharmacogenetics-based warfarin dosing algorithm in Korean patients

Warfarin is a commonly prescribed anticoagulant drug for the prevention of thromboembolic disorders. We investigated the contribution of genetic variations of four genes and clinical factors to warfarin dose requirement and provided a warfarin-dosing algorithm based on genetic and clinical variables in Korean patients. We recruited 564 Korean patients on stable anticoagulation. Single nucleotide polymorphisms (SNPs) for the VKORC1, CYP2C9, CYP4F2 and GGCX were analyzed. Using multiple regression analysis, we developed a model to predict the warfarin requirement. The SNPs of VKORC1, CYP2C9, CYP4F2 and GGCX showed significant correlation with warfarin dose. Patients with the 3730AA genotype received significantly higher doses of warfarin than those with the 3730GG (P=0.0001). For CYP2C9, the highest maintenance dose was observed in the patients with wild-type genotype compared with the variant allele carriers (P<0.0001). The multiple regression model including age, gender, body surface area (BSA), international normalized ratio (INR) and four genetic polymorphisms accounted for 35% of total variations in warfarin dose (R(2)=0.3499; P<0.0001). This study shows that age, gender, BSA, INR and VKORC1, CYP2C9 and CYP4F2 polymorphism affect warfarin dose requirements in Koreans. Translation of this knowledge into clinical guidelines for warfarin prescription may contribute to improve the efficacy and safety of warfarin treatment for Korean patients.

A genome-wide association study of hypertension-related phenotypes in a Japanese population

BACKGROUND

Large-scale genome-wide association studies (GWAS) have been successful in identifying genes that contribute to common diseases and phenotypes. A GWAS of hypertension-related phenotypes in a Japanese population was conducted in the current study.

METHODS AND RESULTS

A total of 936 participants were recruited from the Suita Study and a GWAS with 538,732 single nucleotide polymorphisms (SNP) was performed. The phenotypes included were systolic and diastolic blood pressure (SBP and DBP), body mass index (BMI), waist-to-hip ratio (WHR), plasma renin activity (PRA), plasma aldosterone concentration (PAC), plasma brain natriuretic peptide (BNP) concentration and alcohol consumption (AC). The SNP exceeding the genome-wide significance level were subjected to subsequent association studies using samples available from the Suita Study and Nomura Study. There is no master gene in the Japanese population that profoundly affects SBP, DBP, BMI, WHR, PRA and PAC. AC was influenced by the functional polymorphism in ALDH2, which affected BP levels in men. The BNP concentration was influenced by a polymorphism in the 3' region of the gene encoding for BNP. However, this polymorphism did not influence blood pressure (BP). Six SNP were identified to be associated with hypertension in both the Suita and Nomura studies.

CONCLUSIONS

Although several candidate SNP relevant to hypertension and those influencing AC and BNP were identified, our middle-sized GWAS indicated that there is no master gene in Japanese people that profoundly affects BP-related phenotypes.

Testing of VKORC1 and CYP2C9 alleles to guide warfarin dosing. Test category: pharmacogenomic (treatment)

Warfarin is an oral anticoagulant that is widely prescribed to prevent thromboembolic events in persons at increased risk. The optimal dose is difficult to establish because it can vary 10-fold among individuals due to clinical and demographic factors. Testing for variants of the vitamin K epoxide reductase complex 1 (VKORC1) and cytochrome P450 2C9 (CYP2C9) genes has been proposed for use in guiding the initial dose of warfarin, thus achieving optimal dosing more quickly and with lower risk of bleeding.