Association of pharmacokinetic (CYP2C9) and pharmacodynamic (factors II, VII, IX, and X; proteins S and C; and gamma-glutamyl carboxylase) gene variants with warfarin sensitivity

Effects of coagulation factor VII polymorphisms on warfarin sensitivity and responsiveness in Jordanian cardiovascular patients during the initiation and maintenance phases of warfarin therapy

Purpose

This study aims to investigate the relationships between genetic polymorphisms of the coagulation factor VII (FVII) gene and warfarin responsiveness and sensitivity.

Patients and methods

The study population consisted of 417 subjects (207 Jordanian cardiovascular patients and 210 healthy individuals). Cardiovascular patients were classified into two groups: those sensitive to warfarin dosage (sensitive, moderate, and resistant) and those responsive to warfarin based on International Normalized Ratios (INRs; poor, good, and extensive responders). The HVR4 polymorphism of the FVII gene was genotyped.

Results

Our results showed that there are significant differences between patients and controls according to both genotypic and allelic frequencies (P<0.0001) in the genetic susceptibility study. Moreover, the pharmacogenetics study reported that HVR4 had no association with warfarin sensitivity or responsiveness during the initiation and maintenance phases of therapy, the only significant differences were in the INR outcome measured during the maintenance phase of therapy (P=0.012).

Conclusion

Our data suggests lacking of association between the HVR4 polymorphism in the FVII gene and warfarin sensitivity and responsiveness during the initiation and maintenance phases of therapy. It is possible that these patients carry additional mutations in genes involved in the coagulation pathway.

The impact of R353Q genetic polymorphism in coagulation factor VII on the initial anticoagulant effect exerted by warfarin

BACKGROUND

The initial rise in INR following warfarin is attributed to rapid decline in coagulation factor VII (F7). The R353Q polymorphism in F7 accounts for approximately 1/3 of the variability in F7 activity (FVIIc).

OBJECTIVE

Evaluate the role of R353Q in the initial response to warfarin.

METHODS

Twenty-eight healthy, males, carrying CYP2C9*1/*1 (n = 14), CYP2C9*1/*2 (n = 4) or CYP2C9*1/*3 (n = 10) genotypes, received single 20 mg warfarin. S&R-warfarin concentrations, INR, and FVIIc were monitored periodically for 7 days.

RESULTS

Baseline and maximal INR were 5.6% and 33.5% higher among carriers of the RQ (n = 12) as compared with those carrying the RR (n = 16) genotype (p = 0.032, p = 0.003, respectively). Baseline and nadir FVIIc were 21.6% and 42.0% lower among subjects carrying the RQ as compared with carriers of the RR genotype (p = 0.001, p = 0.007 respectively). In multiple regression analysis, R353Q predicted 36.6% of the variability in peak INR whereas 20.2%, 9.9%, and 5.9% were attributed to VKORC1 genetic polymorphism, cholesterol concentration, and S Warfarin concentration after 24 h, respectively.

CONCLUSIONS

R353Q genetic polymorphism plays a key role in determining the initial response to warfarin. The incorporation of this genetic variant into warfarin loading algorithm should be further investigated.

Impact of GGCX polymorphisms on warfarin dose requirements in atrial fibrillation patients

Background/aim: Warfarin is a common anticoagulant with large interindividual differences and a narrow therapeutic range. The polymorphisms of gamma-glutamyl carboxylase (GGCX) are important genetic factors for warfarin dose requirements. Materials and methods: Polymerase chain reaction and direct sequencing methods were used to detect the GGCX rs699664 genotype in 215 atrial fibrillation (AF) patients with warfarin administration. The effects on warfarin dose by different genotypes were analyzed. A warfarin dosing algorithm was developed based on age, height, CYP2C9, VKORC1, and GGCX genotype. Results: In 215 AF patients, there were 104 cases of wild-type GG genotype (48.4%), 92 cases of GA genotype (42.8%), and 19 cases of AA genotype (8.8%). Patients with the GGCX rs699664 A allele (GA or AA genotypes) needed higher warfarin doses than those with the GG genotype (P < 0.05). A warfarin dosing algorithm showed that age, height, CYP2C9, VKORC1, and GGCX genotype were the best variables for estimating warfarin dose (R2 = 41.2%). Another independent cohort of 60 AF patients showed a significant linear correlation between predicted warfarin maintenance dose and actual dose (R = 0.660, P < 0.01). Conclusion: AF patients with the GA and AA genotypes in GGCX rs699664 required significantly higher warfarin doses. GGCX rs699664 is a potential predictor for the warfarin dose of AF patients.

Dietary reference values for vitamin K

Following a request from the European Commission, the EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA) derives dietary reference values (DRVs) for vitamin K. In this Opinion, the Panel considers vitamin K to comprise both phylloquinone and menaquinones. The Panel considers that none of the biomarkers of vitamin K intake or status is suitable by itself to derive DRVs for vitamin K. Several health outcomes possibly associated with vitamin K intake were also considered but data could not be used to establish DRVs. The Panel considers that average requirements and population reference intakes for vitamin K cannot be derived for adults, infants and children, and therefore sets adequate intakes (AIs). The Panel considers that available evidence on occurrence, absorption, function and content in the body or organs of menaquinones is insufficient, and, therefore, sets AIs for phylloquinone only. Having assessed additional evidence available since 1993 in particular related to biomarkers, intake data and the factorial approach, which all are associated with considerable uncertainties, the Panel maintains the reference value proposed by the Scientific Committee for Food (SCF) in 1993. An AI of 1 μg phylloquinone/kg body weight per day is set for all age and sex population groups. Considering the respective reference body weights, AIs for phylloquinone are set at 70 μg/day for all adults including pregnant and lactating women, at 10 μg/day for infants aged 7-11 months, and between 12 μg/day for children aged 1-3 years and 65 μg/day for children aged 15-17 years.

The association between GGCX, miR-133 genetic polymorphisms and warfarin stable dosage in Han Chinese patients with mechanical heart valve replacement

WHAT IS KNOWN AND OBJECTIVE

Warfarin is a widely used anticoagulant with a narrow therapeutic index. Polymorphisms in the VKORC1, CYP2C9 and CYP4F2 genes have been verified to correlate with warfarin stable dosage (WSD). Whether any other genes or variants affect the dosage is unknown. The aim of our study was to investigate the relationship between GGCX, miR-133 variants and the WSD in Han Chinese patients with mechanical heart valve replacement (MHVR).

METHODS

A total of 231 patients were enrolled in the study. Blood samples were collected for genotyping. The average WSD among subjects with different GGCX or miR-133 genotypes was compared. Regression analyses were performed to test for any association of genetic polymorphisms with WSD.

RESULTS AND DISCUSSION

The warfarin dosage in patients with the GGCX rs699664 TT and rs12714145 TT genotypes was 3.77±0.93 (95% CI: 3.35-4.19) mg/d and 3.70±1.00 (95% CI: 3.32-4.09) mg/d, respectively. The GGCX rs699664 and rs12714145 genotypes were significantly associated with WSD (P<.05). But they were ruled out in the multivariate regression analysis. There were no significant differences in the average warfarin stable dosage between subjects with MIR133B rs142410335 wild-type and variant genotypes (P>.05).

WHAT IS NEW AND CONCLUSION

The genotypes of GGCX rs699644 and rs12714145 were significantly associated with WSD (P<.05), but their contributions were not significant after accounting for other factors. MIR133B rs142410335 makes no significant contributions to warfarin stable dosage in Han Chinese patients with MHVR neither in univariate regression nor in multivariate regression analyses.

Ayurgenomics for stratified medicine: TRISUTRA consortium initiative across ethnically and geographically diverse Indian populations

BACKGROUND

Genetic differences in the target proteins, metabolizing enzymes and transporters that contribute to inter-individual differences in drug response are not integrated in contemporary drug development programs. Ayurveda, that has propelled many drug discovery programs albeit for the search of new chemical entities incorporates inter-individual variability "Prakriti" in development and administration of drug in an individualized manner. Prakriti of an individual largely determines responsiveness to external environment including drugs as well as susceptibility to diseases. Prakriti has also been shown to have molecular and genomic correlates. We highlight how integration of Prakriti concepts can augment the efficiency of drug discovery and development programs through a unique initiative of Ayurgenomics TRISUTRA consortium.

METHODS

Five aspects that have been carried out are (1) analysis of variability in FDA approved pharmacogenomics genes/SNPs in exomes of 72 healthy individuals including predominant Prakriti types and matched controls from a North Indian Indo-European cohort (2) establishment of a consortium network and development of five genetically homogeneous cohorts from diverse ethnic and geo-climatic background (3) identification of parameters and development of uniform standard protocols for objective assessment of Prakriti types (4) development of protocols for Prakriti evaluation and its application in more than 7500 individuals in the five cohorts (5) Development of data and sample repository and integrative omics pipelines for identification of genomic correlates.

RESULTS

Highlight of the study are (1) Exome sequencing revealed significant differences between Prakriti types in 28 SNPs of 11 FDA approved genes of pharmacogenomics relevance viz. CYP2C19, CYP2B6, ESR1, F2, PGR, HLA-B, HLA-DQA1, HLA-DRB1, LDLR, CFTR, CPS1. These variations are polymorphic in diverse Indian and world populations included in 1000 genomes project. (2) Based on the phenotypic attributes of Prakriti we identified anthropometry for anatomical features, biophysical parameters for skin types, HRV for autonomic function tests, spirometry for vital capacity and gustometry for taste thresholds as objective parameters. (3) Comparison of Prakriti phenotypes across different ethnic, age and gender groups led to identification of invariant features as well as some that require weighted considerations across the cohorts.

CONCLUSION

Considering the molecular and genomics differences underlying Prakriti and relevance in disease pharmacogenomics studies, this novel integrative platform would help in identification of differently susceptible and drug responsive population. Additionally, integrated analysis of phenomic and genomic variations would not only allow identification of clinical and genomic markers of Prakriti for application in personalized medicine but also its integration in drug discovery and development programs.

Pharmacogenetic-guided Warfarin Dosing Algorithm in African-Americans

We aim to develop warfarin dosing algorithm for African-Americans. We explored demographic, clinical, and genetic data from a previously collected cohort of 163 African-American patients with a stable warfarin dose. We explored 2 approaches to develop the algorithm: multiple linear regression and artificial neural network (ANN). The clinical significance of the 2 dosing algorithms was evaluated by calculating the percentage of patients whose predicted dose of warfarin was within 20% of the actual dose. Linear regression model and ANN model predicted the ideal dose in 52% and 48% of the patients, respectively. The mean absolute error using linear regression model was estimated to be 10.8 mg compared with 10.9 mg using ANN. Linear regression and ANN models identified several predictors of warfarin dose including age, weight, CYP2C9 genotype *1/*1, VKORC1 genotype, rs12777823 genotype, rs2108622 genotype, congestive heart failure, and amiodarone use. In conclusion, we developed a warfarin dosing algorithm for African-Americans. The proposed dosing algorithm has the potential to recommend warfarin doses that are close to the appropriate doses. The use of more sophisticated ANN approach did not result in improved predictive performance of the dosing algorithm except for patients of a dose of ≥49 mg/wk.

Impact of GGCX, STX1B and FPGS Polymorphisms on Warfarin Dose Requirements in European-Americans and Egyptians

Genotype-based algorithms that include VKORC1 and CYP2C9 genotypes are less predictive of warfarin dose variability in Africans as opposed to Europeans. Polymorphisms in GGCX, FPGS, or STX1B are associated with warfarin dose requirements in African-Americans. We sought to determine if they influenced warfarin dose in European-Americans, and another African population, specifically Egyptians. We genotyped 529 adults (n = 325 European-Americans, 204 Egyptians) on a stable warfarin dose for GGCX rs12714145 and rs10654848, FPGS rs7856096, and STX1B rs4889606. Rs12714145, rs10654848, and rs7856096 were not associated with warfarin dose, whereas STX1B rs4889606 was a significant determinant in univariate analysis (P < 0.0001) in both cohorts. However, STX1B rs4889606 was in high linkage disequilibrium with VKORC1-1639 G>A, and was no longer significant after including VKORC1-1639 G>A in the regression model. Based on these data, the polymorphisms do not appear to influence, in a clinically important way, warfarin dose requirements in European-Americans and Egyptians.

A rolling-horizon pharmacokinetic pharmacodynamic model for warfarin inpatients in transient clinical states

Aim: To design a pharmacokinetic pharmacodynamic model to make individualized and adaptive international normalized ratio (INR) predictions for warfarin inpatients in changing clinical status. Methods: We tested a new model on 60 inpatients at Columbia University. The model personalizes four submodels and minimizes the number of parameters to be estimated. Prediction accuracy was assessed by prediction error, absolute prediction error and percentage absolute prediction error. Results: The INRs were accurately predicted 5 days into the future. Median prediction error: 0.01–0.12; median absolute prediction error: 0.17–0.5 and median percentage absolute prediction error: 9.85–26.06%. Conclusion: Patients exhibit interindividual and intertemporal variability. The model captures the variability and provides accurate and personalized INR predictions.

Impact of polymorphisms of the GGCX gene on maintenance warfarin dose in Chinese populations: Systematic review and meta-analysis

The meta-analysis was conducted to investigate the impact of gamma-glutamyl carboxylase (GGCX) on maintenance warfarin dose. 8 studies were included, focusing on the impact of GGCX single nucleotide polymorphisms (SNPs) on mean daily warfarin dose (MDWD). GGCX (rs699664; AA versus GG, GA versus GG, A versus GG) and GGCX (rs12714145; GA versus GG, AA versus GG, A versus GG) showed no significant differences on mean daily warfarin dose (MDWD). This meta-analysis was the first to report the relationship between GGCX SNPs and MDWD in Chinese populations. No evidence could be found in the relationship between SNPs of GGCX (rs699664 and rs12714145) and maintenance warfarin dose.

Warfarin Metabolites in Patients Following Cardiac Valve Implantation: A Contribution of Clinical and Genetic Factors

INTRODUCTION

Warfarin, a racemic mixture of S- and R-enantiomers, is the cornerstone of therapy in patients following cardiac valve replacement. S-warfarin is metabolized to 7-S-hydroxywarfarin by the cytochrome P450 isoform 2C9 encoded by CYP2C9 gene. R-warfarin is metabolized by multiple cytochromes P450. We sought to assess the impact of clinical and genetic factors on circulating warfarin metabolites following valve implantation.

MATERIAL AND METHODS

Venous blood was collected from 120 patients after 3 months since elective mitral and/or aortic valve replacement. Plasma S-warfarin, R-warfarin, S-7-hydroxywarfarin, and R-7-hydroxywarfarin were determined using high-performance liquid chromatography. The S-7-hydroxywarfarin/S-warfarin and S-warfarin/R-warfarin (S/R) ratios, along with warfarin sensitivity index (WSI), defined as INR/S-warfarin ratio, were calculated. Vitamin K epoxide reductase complex subunit 1 (VKORC1) c.-1639A, CYP2C9*3 and CYP2C9*2 alleles were determined using real-time polymerase chain reaction.

RESULTS

The S-warfarin was higher in former smokers (p = 0.047) and the VKORC1 c.-1639A allele carriers (p < 0.0001). The S-7-hydroxywarfarin was lower in carriers of the VKORC1 c.-1639A allele (p = 0.0005) and CYP2C9*3 (p = 0.047). The S-7-hydroxywarfarin/S-warfarin ratio was lower in the carriers of CYP2C9*3 (p = 0.008), but not in those with VKORC1 -c.1639A allele. The S/R ratio was higher in patients with hypertension (p = 0.01). The independent predictors of elevated S/R ratio defined as the upper quartile were diabetes (p = 0.045), CYP2C9*3 (p < 0.0001) and CYP2C9*2 (p = 0.0002). The independent predictors of elevated WSI were current smoking (p = 0.049), implantation of mechanical valve (p = 0.006) and VKORC1c.-1639A allele (p = 0.007).

CONCLUSION

We conclude that not only genetic, but also several clinical factors affect warfarin metabolites in patients following cardiac valve implantation.

Pharmacogenetics of coumarin anticoagulants in Brazilians

INTRODUCTION

Coumarin vitamin K antagonists are the mainstay of anticoagulant therapy in atrial fibrillation, prosthetic heart valves and thromboembolic conditions. Concerns with these drugs include large inter-individual variability in dose requirements, narrow therapeutic index and need to monitor prothrombin time repeatedly.

AREAS COVERED

Pharmacogenetic studies and dosing algorithms for warfarin and phenprocoumon.

EXPERT OPINION

Gene candidate studies in Brazilian patients verified consistently the association of warfarin and pheprocoumon stable dose requirements with CYP2C9 and VKORC1 polymorphisms, and minor or no influence of other pharmacogenes (e.g., CYP4F2 and F7). The predictive power of warfarin and phenprocoumon dosing algorithms developed for Brazilians compares favorably with those reported for other populations. A warfarin dosing algorithm derived for an admixed cohort performed equally well in self-reported White and Black patients, in marked contrast with the considerably poorer performance of other warfarin algorithms in patients of African descent compared to those of European ancestry. This discrepancy is ascribed to the extensive European/African admixture among Brazilians. Prospective studies of clinical utility of coumarin dosing algorithms, in the context of the Brazilian Public Health System, would represent an important counterpart to recently published trials in European and North American cohorts with predominant or exclusive European ancestry.

Is there a role for MDR1, EPHX1 and protein Z gene variants in modulation of warfarin dosage? a study on a cohort of the Egyptian population

BACKGROUND

There is considerable inter-individual variability in warfarin dosages necessary to achieve target therapeutic anticoagulation. Polymorphisms in genes, which master warfarin pharmacokinetics and pharmacodynamics, might influence warfarin dose variation. Genes encoding drug transporters, such as human multidrug resistance (MDR1), as well as epoxide hydrolase 1 (EPHX1), which is a putative subunit of the vitamin K epoxide reductase, and Protein Z (PZ), which is a vitamin K-dependent plasma glycoprotein, are among those candidate genes.

OBJECTIVE

The purpose of this study was to investigate the contribution of MDR1 C3435T, EPHX1 H139R and PZ A-13G gene polymorphisms in warfarin dose variation in a cohort of the Egyptian population.

METHODS

Eighty-four patients whose international normalized ratio (INR) was in the range of 2-3, 41 males and 43 females, with a mean (±SD) age of 40.9 (13.3) years were recruited into this study. MDR1 C3435T, EPHX1 H139R and PZ A-13G gene polymorphisms were detected by polymerase chain reaction-restriction fragment length polymorphism. Primarily, linear regression analysis, including the variables age, gender, MDR1 C3435T, EPHX1 H139R and combined MDR1 C3435T, EPHX1 H139R and PZ A-13G genotypes, was used to assess the effective factors for warfarin maintenance dose. Secondly, the previously examined cytochrome P450 (CYP) 2C9 A1075C and vitamin K epoxide reductase complex subunit 1 (VKORC1) C1173T were added to the regression analysis.

RESULTS

Warfarin dose/week was not influenced by each of the MDR1 C3435T, EPHX1 H139R, and PZ A-13G gene polymorphisms when examined separately. However, when these single nucleotide polymorphisms (SNPs) were combined, MDR1 TT/EPHX1 RH,RR/PZ AA subjects showed statistically significant increase in warfarin dose/week when compared with MDR1 CC/EPHX1 RH,RR/PZ AA subjects [median (25th-75th percentiles): 49.0 (42.0-59.5) vs. 35.0 (24.5-42.0) mg/week, respectively] (p = 0.014). In contrast, in the presence of wild-type EPHX1 HH, there was a decrease in warfarin dose/week in MDR1 TT subjects when compared with CT and CC subjects [median (25th-75th percentiles): 22.0 (17.5-30.6), 42.0 (35.0-49.0) and 42.0 (28.0-54.3) mg/week, respectively] (p = 0.005 and 0.030, respectively). Age had a significant contribution (p = 0.048) to the overall variability in warfarin dose. Calculated weekly dose = 52.928 - (0.289 × age) + (9.709 × combined genotype). The multivariate linear regression equation of warfarin maintenance dose accounted for about 8 % of variation in dose (R (2) = 0.079), age accounted for 5 % of variation, while combined genotypes added the extra 3 %. However, the new regression equation accounted for 20.9 % of variation in dose. Age accounted for 5 %, while VKORC1 C1173T accounted for an extra 13 % of variation and MDR1 C3435T accounted for the remaining 3 % of variation. Calculated dose = 64.909 - (0.282 × age) - (13.390 × VKORC1) - (7.164 × MDR1). Correlation analysis showed a close and significant relationship between the calculated and actual warfarin dose (r = 0.457; p < 0.0005).

CONCLUSION

Warfarin dose/week was significantly influenced by the combined MDR1 C3435T and EPHX1 H139R gene polymorphism since no polymorphism of PZ A-13G SNP was detected in our studied Egyptian population. Future studies with larger sample size will be needed to confirm our findings before definitive conclusions can be made.

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.

Variability of warfarin dose response associated with CYP2C9 and VKORC1 gene polymorphisms in Chinese patients

OBJECTIVE

To investigate the prevalence and implication of cytochrome P450, family 2, subfamily C, polypeptide 9 (CYP2C9) variants and vitamin K epoxide reductase complex, subunit 1 (VKORC1)-1639 G > A polymorphisms in Chinese patients receiving warfarin therapy.

METHODS

Chinese Han patients requiring oral warfarin therapy were consecutively enrolled. Correlations between CYP2C9*1, *2, *3, *4, *5 variants and VKORC1-1639 G > A polymorphisms, fourth-day international normalized ratios (INRs) and warfarin maintenance dose were investigated.

RESULTS

Out of 101 patients, there were no significant differences in fourth-day INR or warfarin daily maintenance doses between patients with CYP2C9*1*1 and CYP2C9*1*3 genotypes. Patients with the VKORC1-1639 AA genotype had a higher fourth-day INR (1.87 ± 0.14) than those with the VKORC1-1639 AG genotype (1.32 ± 0.15). Warfarin maintenance dose for patients with the VKORC1-1639 AA genotype (2.40 ± 0.70 mg/day) was significantly lower than for patients with the VKORC1-1639 AG genotype (4.83 ± 0.70 mg/day).

CONCLUSIONS

Unlike Caucasian populations, VKORC1-1639 G > A polymorphisms in the Chinese population may be the dominant genetic factors associated with warfarin response variability.

A review of pharmacogenetics of adverse drug reactions in elderly people

Older adults are more susceptible to the prevalence of therapeutic failure and adverse drug reactions (ADRs). Recent advances in genomic research have shed light on the crucial role of genetic variants, mainly involving genes encoding drug-metabolizing enzymes, drug transporters and genes responsible for a compound's mechanism of action, in driving different treatment responses among individuals, in terms of therapeutic efficacy and safety. The interindividual variations of these genes may account for the differences observed in drug efficacy and the appearance of ADRs in elderly people. The advent of whole genome mapping techniques has allowed researchers to begin to characterize the genetic components underlying serious ADRs. The identification and validation of these genetic markers will enable the screening of patients at risk of serious ADRs and to establish personalized treatment regimens.The aim of this review was to provide an update on the recent developments in geriatric pharmacogenetics in clinical practice by reviewing the available evidence in the PubMed database to September 2012. A Pubmed search was performed (years 1999-2012) using the following two search strategies: ('pharmacogenomic' OR 'pharmacogenetic ') AND ('geriatric' or 'elderly ') AND 'adverse drug reactions'; [gene name] AND ('geriatric' or 'elderly ') AND 'adverse drug reactions', in which the gene names were those contained in the Table of Pharmacogenomic Biomarkers in Drug Labels published online by the US Food and Drug Administration ( http://www.fda.gov/drugs/scienceresearch/researchareas/pharmacogenetics/ucm083378.htm ). Reference lists of included original articles and relevant review articles were also screened. The search was limited to studies published in the English language.

Effects of torsemide on pharmacodynamics and pharmacokinetics of warfarin in humans and rats

OBJECTIVES

This study aimed to evaluate the effects of torsemide on warfarin therapy in humans and rats.

METHODS

For the animal study, rats were orally dosed with warfarin (0.13 mg/kg, control group) or warfarin (0.13 mg/kg) with torsemide (2 mg/kg, low dose group and 10 mg/kg, high dose group). The pharmacodynamic response of warfarin was assessed by measuring the international normalized ratio (INR) for 5 consecutive days following drug administration. For the human study, 191 patients on warfarin with mechanical heart valves were followed up retrospectively. The stable dose was calculated as the mean dose in INR levels of 2-3 for 3 consecutive times.

KEY FINDINGS

In the animal study, the INR, maximum plasma concentration (Cmax ) and area under the plasma drug concentration-time curve (AUC0-∞ ) of (S)-warfarin in the high dose group were significantly higher than in other groups (P < 0.05). Compared with the control group, Cmax and AUC0-∞ of (R)-warfarin in the high and low dose groups were higher, whereas the volume of distribution/bioavailability and clearance/bioavailability were significantly lower (P < 0.05). In the univariate analysis of the clinical study, diuretics significantly lowered stable warfarin doses (P = 0.016) (5.07 ± 1.78 mg/day vs 5.77 ± 1.81 mg/day). After controlling confounding variables, the effects of diuretics were found to lower the warfarin dose by 0.464 mg.

CONCLUSIONS

It was concluded that warfarin dose needs to be lowered when it is used concomitantly with diuretics.

Pharmacogenetics of warfarin: challenges and opportunities

Since the introduction in the 1950s, warfarin has become the commonly used oral anticoagulant for the prevention of thromboembolism in patients with deep vein thrombosis, atrial fibrillation or prosthetic heart valve replacement. Warfarin is highly efficacious; however, achieving the desired anticoagulation is difficult because of its narrow therapeutic window and highly variable dose response among individuals. Bleeding is often associated with overdose of warfarin. There is overwhelming evidence that an individual's warfarin maintenance is associated with clinical factors and genetic variations, most notably polymorphisms in cytochrome P450 2C9 and vitamin K epoxide reductase subunit 1. Numerous dose-prediction algorithms incorporating both genetic and clinical factors have been developed and tested clinically. However, results from major clinical trials are not available yet. This review aims to provide an overview of the field of warfarin which includes information about the drug, genetics of warfarin dose requirements, dosing algorithms developed and the challenges for the clinical implementation of warfarin pharmacogenetics.

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.

Personalized approach of medication by indirect anticoagulants tailored to the patient-Russian context: what are the prospects?

Indirect anticoagulants such as warfarin are the ‘gold standard’ for prevention and treatment of thromboembolic complications in patients at risk (in atrial fibrillation of valvular and nonvalvular etiology, the presence of artificial heart valves, orthopedic and trauma interventions, and other pathological conditions). A wide range of doses required to achieve a therapeutic effect indicates the need for a personalized approach to the appointment of warfarin. In addition to the dependence on the patient's clinical characteristics (sex, age, smoking status, diagnosis), there is a clear association between the warfarin dose and the carriage of certain allelic variants of key genes that makes it possible to apply molecular genetic testing for individual dose adjustment. This provides a more rapid target anticoagulant effect and also reduces the risk of bleeding associated with a possible overdose of warfarin. Implementation of this approach will allow more wide and safe application of indirect anticoagulants in Russia for needy patients.

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.

Association of the GGCX (CAA)16/17 repeat polymorphism with higher warfarin dose requirements in African Americans

OBJECTIVE

Little is known about genetic contributors to higher than usual warfarin dose requirements, particularly for African Americans. This study tested the hypothesis that the γ-glutamyl carboxylase (GGCX) genotype contributes to warfarin dose requirements greater than 7.5 mg/day in an African American population.

METHODS

A total of 338 African Americans on a stable dose of warfarin were enrolled. The GGCX rs10654848 (CAA)n, rs12714145 (G>A), and rs699664 (p.R325Q); VKORC1 c.-1639G>A and rs61162043; and CYP2C9*2, *3, *5, *8, *11, and rs7089580 genotypes were tested for their association with dose requirements greater than 7.5 mg/day alone and in the context of other variables known to influence dose variability.

RESULTS

The GGCX rs10654848 (CAA)16 or 17 repeat occurred at a frequency of 2.6% in African Americans and was overrepresented among patients requiring greater than 7.5 mg/day versus those who required lower doses (12 vs. 3%, P=0.003; odds ratio 4.0, 95% confidence interval, 1.5-10.5). The GGCX rs10654848 genotype remained associated with high dose requirements on regression analysis including age, body size, and VKORC1 genotype. On linear regression, the GGCX rs10654848 genotype explained 2% of the overall variability in warfarin dose in African Americans. An examination of the GGCX rs10654848 genotype in warfarin-treated Caucasians revealed a (CAA)16 repeat frequency of only 0.27% (P=0.008 compared with African Americans).

CONCLUSION

These data support the GGCX rs10654848 genotype as a predictor of higher than usual warfarin doses in African Americans, who have a 10-fold higher frequency of the (CAA)16/17 repeat compared with Caucasians.

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

Vitamin K nutrition, metabolism, and requirements: current concepts and future research

In 2001, the US Food and Nutrition Board concluded that there were insufficient data with which to establish a RDA for vitamin K, in large part because of a lack of robust endpoints that reflected adequacy of intake. Knowledge of the relative bioavailability of multiple vitamin K forms was also poor. Since then, stable isotope methodologies have been applied to the assessment of the bioavailability of the major dietary form of vitamin K in its free state and when incorporated into a plant matrix. There is a need for stable isotope studies with enhanced sensitivity to expand knowledge of the bioavailability, absorption, disposition, and metabolism of different molecular forms of vitamin K. Another area for future research stems from evidence that common polymorphisms or haplotypes in certain key genes implicated in vitamin K metabolism might affect nutritional requirements. Thus far, much of this evidence is indirect via effects on warfarin dose requirements. In terms of clinical endpoints, vitamin K deficiency in early infancy continues to be a leading cause of intracranial bleeding even in developed countries and the reasons for its higher prevalence in certain Asian countries has not been solved. There is universal consensus for the need for vitamin K prophylaxis in newborns, but the effectiveness of any vitamin K prophylactic regimen needs to be based on sound nutritional principles. In contrast, there is still a lack of suitable biomarkers or clinical endpoints that can be used to determine vitamin K requirements among adults.

Genetic polymorphisms are associated with variations in warfarin maintenance dose in Han Chinese patients with venous thromboembolism

AIM

Warfarin is a clinical anticoagulant that requires periodic monitoring because it is associated with adverse outcomes. Personalized medicine, which is based on pharmacogenetics, holds great promise in solving these types of problems. It aims to provide the tools and knowledge to tailor drug therapy to an individual patient, with the potential of increasing safety and efficacy of medications.

MATERIALS & METHODS

In the present study we analyzed genotypes of 14 SNPs for seven genes using DNA from 297 Han Chinese venous thromboembolism patients treated with warfarin.

RESULTS

Multiple regression analyses revealed that CYP2C9 genotype (p = 0.001), VKORC1 genotype (p < 0.001), age (p < 0.01) and weight (p < 0.001) were all associated with warfarin dose requirements, which can explain 37.4% of the variability of warfarin dose among Han Chinese patients. Meanwhile, in the validation cohort, the predicted warfarin daily dose was calculated using the best model with a 64.5% predicted dose being acceptable (-1 mg/day ≤Δwarfarin dose ≤1 mg/day).

CONCLUSION

We developed a pharmacogenetic dose algorithm for warfarin treatment that uses genotypes from two genes (VKORC1 and CYP2C9) and clinical variables to predict therapeutic maintenance doses in Chinese patients with venous thromboembolism. The validity of the dosing algorithm was confirmed in a cohort of venous thromboembolism patients on warfarin therapy.

New anticoagulants: how to deal with treatment failure and bleeding complications

Conventional anticoagulants have proven efficacy in the management of thromboembolism. Their adverse effects and a narrow therapeutic window, necessitating regular need for monitoring, however, have long been an incentive for the development of safer anticoagulants without compromising efficacy. Over the last decade or so several new parenteral and oral anticoagulants have been launched with efficacy comparable with conventional agents. From fondaparinux to its long acting derivative idraparinux, and the factor Xa inhibitor rivaroxaban to the direct thrombin inhibitor dabigatran, the advent of new anticoagulants is radically changing anticoagulation. For conventional anticoagulants, despite their shortcomings, effective methods of reversing their anticoagulant effects exist. Moreover, strategies to deal with the occurrence of fresh thrombotic events in the face of therapeutic anticoagulation with the conventional agents have also been addressed. Nevertheless, for the new anticoagulants, the optimal management of these complications remains unknown. This review explores these issues in the light of current evidence.

CYP4F2 gene polymorphism as a contributor to warfarin maintenance dose in Japanese subjects

WHAT IS KNOWN AND OBJECTIVE

Polymorphisms in the gene encoding CYP4F2 may partly explain the variability in warfarin maintenance dose by altering the metabolism of vitamin K. To determine the genetic factors that cause large inter-patient variability in warfarin efficacy, we investigated the relationship between serum warfarin concentration and CYP4F2 V433M (1347C>T, rs2108622) polymorphism in Japanese subjects.

METHODS

Gene variations in VKORC1, CYP2C9 and CYP4F2 were analysed in 126 Japanese patients treated with warfarin. The daily dosage of warfarin, concentration of S- and R-warfarin in plasma, and prothrombin time international normalized ratio (PT-INR) was used as the pharmacokinetic and pharmacodynamic indices.

RESULTS AND DISCUSSION

The maintenance dose of warfarin was larger in the CYP4F2 1347 CT genotype group (3·59±1·80 mg/day, P=0·027) than in the CYP4F2 CC genotype group (2·88±1·00 mg/day). CYP4F2 1347C>T polymorphism significantly affected serum R-warfarin concentration when the VKORC1-1639 genotypes are AG and GG.

WHAT IS NEW AND CONCLUSION

Although a significant inter-patient difference in warfarin maintenance dose was observed between the CYP4F2 CC and CT genotypes, serum S-warfarin concentration was not significantly different between them. An effect of CYP4F2 V433M polymorphism on warfarin maintenance dose was observed but was relatively small when compared to the effects of CYP2C9 and VKOR polymorphism.

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.

Translational aspects of genetic factors in the prediction of drug response variability: a case study of warfarin pharmacogenomics in a multi-ethnic cohort from Asia

Genetic markers displaying highly significant statistical associations with complex phenotypes may not necessarily possess sufficient clinical validity to be useful. Understanding the contribution of these markers beyond readily available clinical biomarkers is particularly important in pharmacogenetics. We demonstrate the utility of genetic testing using the example of warfarin in a multi-ethnic setting comprising of three Asian populations that are broadly representative of the genetic diversity for half of the population in the world, especially as distinct interethnic differences in warfarin dose requirements have been previously established. We confirmed the roles of three well-established loci (CYP2C9, VKORC1 and CYP4F2) in explaining warfarin dosage variation in the three Asian populations. In addition, we assessed the relationship between ethnicity and the genotypes of these loci, observing strong correlations at VKORC1 and CYP4F2. Subsequently, we established the additional utility of these genetic factors in predicting warfarin dose beyond ethnicity and clinical biomarkers through performing a series of systematic cross-validation analyses of the relative predictive accuracies of various fixed-dose regimen, clinical and genetic models. Through a pharmacogenetics model for warfarin, we show the importance of genetic testing beyond readily available clinical biomarkers in predicting dose requirements, confirming the role of genetic profiling in personalized medicine.

Genetic polymorphism in metabolism and host defense enzymes: implications for human health risk assessment

Genetic polymorphisms in xenobiotic metabolizing enzymes can have profound influence on enzyme function, with implications for chemical clearance and internal dose. The effects of polymorphisms have been evaluated for certain therapeutic drugs but there has been relatively little investigation with environmental toxicants. Polymorphisms can also affect the function of host defense mechanisms and thus modify the pharmacodynamic response. This review and analysis explores the feasibility of using polymorphism data in human health risk assessment for four enzymes, two involved in conjugation (uridine diphosphoglucuronosyltransferases [UGTs], sulfotransferases [SULTs]), and two involved in detoxification (microsomal epoxide hydrolase [EPHX1], NADPH quinone oxidoreductase I [NQO1]). This set of evaluations complements our previous analyses with oxidative and conjugating enzymes. Of the numerous UGT and SULT enzymes, the greatest likelihood for polymorphism effect on conjugation function are for SULT1A1 (*2 polymorphism), UGT1A1 (*6, *7, *28 polymorphisms), UGT1A7 (*3 polymorphism), UGT2B15 (*2 polymorphism), and UGT2B17 (null polymorphism). The null polymorphism in NQO1 has the potential to impair host defense. These highlighted polymorphisms are of sufficient frequency to be prioritized for consideration in chemical risk assessments. In contrast, SNPs in EPHX1 are not sufficiently influential or defined for inclusion in risk models. The current analysis is an important first step in bringing the highlighted polymorphisms into a physiologically based pharmacokinetic (PBPK) modeling framework.

Polymorphism in the DNA repair enzyme XRCC1: utility of current database and implications for human health risk assessment

Genetic polymorphisms are increasingly recognized as sources of variability not only in toxicokinetic but also in toxicodynamic response to environmental agents. XRCC1 is involved in base excision repair (BER) of DNA; it has variant genotypes that are associated with modified repair function. This analysis focuses on four polymorphisms: three in the coding region that affect protein structure and one in an upstream regulatory sequence that affects gene expression. The Arg399Gln variant is the most widely studied with evidence supporting a quantitative effect of genotype on phenotype. The homozygous variant (Gln/Gln) can have 3-4-fold diminished capacity to remove DNA adducts and oxidized DNA damage. This variant is relatively common in Caucasians and Asians where approximately 10% are homozygous variant. In contrast, the Arg194Trp variant appears to protect against genotoxic effects although the degree to which DNA repair is enhanced by this polymorphism is uncertain. The homozygous variant is rare in Caucasians and African Americans but it is present at 7% in Asians. A third coding region polymorphism at codon 280 appears to decrease repair function but additional quantitative information is needed and the homozygous variant is rare across populations studied. A polymorphism in an upstream promoter binding sequence (-77T>C) appears to lower XRCC1 levels by decreasing gene expression. Based upon genotype effect on phenotype and allele frequency, the current analysis finds that the codon 399 and upstream (-77) polymorphisms have the greatest potential to affect the toxicodynamic response to DNA damaging agents. However, the implications for risk assessment are limited by the likelihood that polymorphisms in multiple BER genes interact to modulate DNA repair.

Influence of GGCX genotype on warfarin dose requirements in Chinese patients

INTRODUCTION

It has been widely accepted that genetic factors were the major sources of the variation in warfarin dose. This study is intended to investigate whether the 3261G>A variation in GGCX gene influences stable warfarin dose in Chinese patient population.

MATERIALS AND METHODS

A total of 217 patients with stable warfarin dose were enrolled. Genomic DNA was extracted from each subject and the genotype of GGCX 3261G>A was determined by using of denaturing high-performance liquid chromatography (DHPLC). Least significant difference tests (LSDs) were used to compare dose with genotypes. Analysis of variance (ANVOA) was used to calculate the proportion of warfarin dose that could be explained by variation in genotype.

RESULTS

In the total of 217 subjects, 84 patients (38.7%) were GG homozygote, whereas 117 (53.9%) were GA heterozygote and 16 (7.4%) were AA homozygote. Patients with the GGCX 3261AA genotype had a significantly higher average daily maintenance dose (3.39 ± 1.40 mg) than those with the GG genotype (2.69 ± 1.07 mg; P=0.027), and GGCX 3261G>A explains 2.3% of the univariate warfarin dose variance.

CONCLUSION

GGCX 3261G>A may affect warfarin dose requirements, and showed a small but significant effect on warfarin dose in a Chinese patient population.

Prediction of phenprocoumon maintenance dose and phenprocoumon plasma concentration by genetic and non-genetic parameters

Purpose

The anticoagulation response to vitamin K antagonists is characterised by high inter-individual variability. The impact of single nucleotide polymorphisms (SNPs) in several genes of enzymes involved in the vitamin K cycle on phenprocoumon dose variability and phenprocoumon plasma concentrations is still under investigation.

Methods

We assessed the influence of VKORC1 c.-1639G>A, CYP2C9*2, CYP2C9*3, CYP4F2 c.1297G>A, CALU c.*4A>G, EPHX1 c.337T>C, GGCX c.214+597G>A, F7 c.-402G>A, F7 c.-401G>T, PROC c.-228C>T and PROC c.-215G>A along with clinical and demographic parameters on steady-state phenprocoumon therapy in 75 patients. A prediction model was developed for total phenprocoumon plasma concentrations and daily phenprocoumon doses required for therapeutic anticoagulation.

Results

The VKORC1 c.-1639 genotype was the main predictor of the phenprocoumon daily dose (adjusted R² = 37.6%) and the total phenprocoumon concentration (adjusted R² = 38.3%). CYP2C9 affected the phenprocoumon concentration, but not the dose requirements. SNPs in the other genes of the vitamin K cycle, concomitant medication, nicotine use and alcohol consumption did not predict phenprocoumon concentrations and phenprocoumon dose requirements in a multiple linear regression model. Phenprocoumon concentrations were predicted by VKORC1 c.-1639, CYP2C9 genotype, age and BMI. The final prediction model for the daily phenprocoumon dose requirements comprised VKORC1 c.-1639 genotype, age and height accounting for 48.6% of the inter-individual variability.

Conclusions

A rough prediction of phenprocoumon maintenance doses can be achieved by a limited set of parameters (VKORC1, age, height). The investigated SNPs in CYP4F2, CALU, EPHX1, GGCX, F7, and PROC did not improve the predictive value of a pharmacogenetic-based dosing equation for phenprocoumon.

VKORC1-1639G>A, CYP2C9, EPHX1691A>G genotype, body weight, and age are important predictors for warfarin maintenance doses in patients with mechanical heart valve prostheses in southwest China

OBJECTIVE

To investigate the contribution of genetic polymorphisms of vitamin K epoxide reductase complex subunit 1 gene VKORC1-1639G>A, cytochrome P450 2C9 gene (CYP2C9), EPHXI, and clinical factors to warfarin sensitivity in southwest Chinese Han patients with mechanical heart valve prostheses.

METHODS

A total of 127 patients with mechanical heart valve prostheses who have been followed up at our department during the past 23 years were enrolled in this study and compared to a control group that consisted of 133 randomly selected healthy blood donors. These Chinese patients met stable warfarin dosage requirements and had reached the target international normalized ratio (INR) of 1.5-2.0. PCR and direct sequencing were carried out to identify the polymorphisms of VKORC1-1639G>A (rs9923231), CYP2C9*3 (rs1057910), CYP2C9 IVS3-65G>C (rs9332127), and EPHX1691A>G (rs4653436). In addition, total and free (non-protein-bound) warfarin concentrations were analyzed.

RESULTS AND CONCLUSIONS

There were great interindividual differences in warfarin maintenance dosage (ranging from 0.6 to 8.4 mg/day) among the 127 patients with mechanical heart valve prostheses. VKORC1-1639G>A, CYP2C9, EPHX1691A>G polymorphism, body weight, and age were found to affect the dose demands. Multiple linear regression models incorporating genetic polymorphisms of VKORC1, CYP2C9, EPHX1691A>G, and the nongenetic factors of age and body weight were developed, and explained up to 76.8% of the total variation (adjusted R (2) of 0.743) in warfarin maintenance doses in southwest Chinese patients with mechanical heart valve prostheses.

Pharmacogenetics of anticoagulants

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

Gamma-glutamyl carboxylase and its influence on warfarin dose

Via generation of vitamin K-dependent proteins, gamma-glutamyl carboxylase (GGCX) plays a critical role in the vitamin K cycle. Single nucleotide polymorphisms (SNPs) in GGCX, therefore, may affect dosing of the vitamin K antagonist, warfarin. In a multi-centered, cross-sectional study of 985 patients prescribed warfarin therapy, we genotyped for two GGCX SNPs (rs11676382 and rs12714145) and quantified their relationship to therapeutic dose. GGCX rs11676382 was a significant (p=0.03) predictor of residual dosing error and was associated with a 6.1% reduction in warfarin dose (95% CI: 0.6%-11.4%) per G allele. The prevalence was 14.1% in our predominantly (78%) Caucasian cohort, but the overall contribution to dosing accuracy was modest (partial R2 = 0.2%). GGCX rs12714145 was not a significant predictor of therapeutic dose (p = 0.26). GGCX rs11676382 is a statistically significant predictor of warfarin dose, but the clinical relevance is modest. Given the potentially low marginal cost of adding this SNP to existing genotyping platforms, we have modified our non-profit website (www.WarfarinDosing.org) to accommodate knowledge of this variant.

Pharmacogenetic testing and therapeutic drug monitoring are complementary tools for optimal individualization of drug therapy

Genetic factors contribute to the phenotype of drug response, but the translation of pharmacogenetic outcomes into drug discovery, drug development or clinical practice has proved to be surprisingly disappointing. Despite significant progress in pharmacogenetic research, only a few drugs, such as cetuximab, dasatinib, maraviroc and trastuzumab, require a pharmacogenetic test before being prescribed. There are several gaps that limit the application of pharmacogenetics based upon the complex nature of the drug response itself. First, pharmacogenetic tests could be more clinically applicable if they included a comprehensive survey of variation in the human genome and took into account the multigenic nature of many phenotypes of drug disposition and response. Unfortunately, much of the existing research in this area has been hampered by limitations in study designs and the nonoptimal selection of gene variants. Secondly, although responses to drugs can be influenced by the environment, only fragmentary information is currently available on how the interplay between genetics and environment affects drug response. Third, the use of a pharmacogenetic test as a standard of care for drug therapy has to overcome significant scientific, economic, commercial, political and educational barriers, among others, in order for clinically useful information to be effectively communicated to practitioners and patients. Meanwhile, the lack of efficacy is in this process is quite as costly as drug toxicity, especially for very expensive drugs, and there is a widespread need for clinically and commercially robust pharmacogenetic testing to be applied. In this complex scenario, therapeutic drug monitoring of parent drugs and/or metabolites, alone or combined with available pharmacogenetic tests, may be an alternative or complementary approach when attempts are made to individualize dosing regimen, maximize drug efficacy and enhance drug safety with certain drugs and populations (e.g. antidepressants in older people).

Genetic determinants of warfarin dosing in the Han-Chinese population

Warfarin, a widely prescribed oral anticoagulant, is used for the prevention of thromboembolism. Polymorphisms in CYP2C9 and VKORC1 have been shown to be associated with warfarin dose requirements. However, it is likely that other genes could also affect warfarin dose. Aims: In this study, we aimed to identify additional genes influencing warfarin dosing in the Han-Chinese population. Materials & methods: In this study, we screened for SNPs in 13 genes (VKORC1, CYP2C9, CYP2C18, PROC, APOE, EPHX1, CALU, GGCX, ORM1, ORM2, factor II, factor VII and CYP4F2) and tested their associations with warfarin dosing with univariate and multiple regression analysis. Results: Polymorphisms in the VKORC1 gene have the strongest effects on warfarin dose, followed by CYP2C9*3. In addition, our results showed that CYP2C18, PROC and EPHX1 have small but significant associations with warfarin dose. In multiple regression analysis, PROC and EPHX1 explained 3% of the dose variation. The incorporation of these two genes into warfarin dosing algorithms could improve the accuracy of prediction in the Han-Chinese population.

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.

Association of sequence variations in vitamin K epoxide reductase and gamma-glutamyl carboxylase genes with biochemical measures of vitamin K status

Genetic factors, specifically the VKORC1 and GGCX genes, have been shown to contribute to the interindividual variability in response to the vitamin K-antagonist, warfarin, which influences the dose required to achieve the desired anticoagulation response. These differences in warfarin sensitivity may be explained by differences in vitamin K status. Men and women (n=416, 60-80 y), primarily of European descent, were genotyped for common polymorphisms in VKORC1 and GGCX. Cross-sectional associations exist between polymorphisms and biochemical markers of vitamin K [plasma phylloquinone, percent undercarboxylated osteocalcin (%ucOC)]. VKORC1 rs8050894 GG homozygotes had significantly higher cross-sectional measures of plasma phylloquinone than carriers of the CG or CC genotypes (plasma phylloquinone geometric means: GG 0.874+/-0.092 versus CG/CC 0.598+/- 0.044; p=0.020), whereas carriers of VKORC1 rs7294 AA or AG had significantly lower plasma phylloquinone concentrations compared to GG homozygotes (plasma phylloquinone geometric means: 0.579+/-0.045 versus 0.762+/-0.057; p=0.035). Cross-sectional analyses also revealed that heterozygous carriers of GGCX rs10187424 and rs7568458 had significantly lower %ucOC relative to either homozygous group. Polymorphisms in genes encoding enzymes involved in vitamin K metabolism may modulate plasma concentrations of phylloquionone and percent carboxylation of osteocalcin.