Determination of unbound warfarin enantiomers in human plasma and 7-hydroxywarfarin in human urine by chiral stationary-phase liquid chromatography with ultraviolet or fluorescence and on-line circular dichroism detection

A novel, rapid and simple UHPLC-MS/MS method for quantification of warfarin in dried blood spots

Warfarin is a common first line anticoagulant with a narrow therapeutic window. Because of the large blood volume needed, previous warfarin determination methods were not applicable to small animals, such as mice. To reduce the number of small animals used needed, we developed and validated a sensitive rapid assay for the simultaneous detection of warfarin enantiomers in mouse dried blood spot (DBS) samples. Analytes were extracted by tert-butyl methyl ether and then separated by a chiral Cellulose-1 column with a mobile phase of 75% acetonitrile (containing 0.1% formic acid). The total chromatographic run time was 3 min. Negative mode electrospray ionization was used for MS/MS detection, where the monitored ion transitions were m/z 307.1 → 161.0 and 341.1 → 284.0 for warfarin and coumachlor (internal standard) respectively. The calibration curves were linear with a correlation coefficient of ≥0.994 for both enantiomers over a concentration range of 10-1000 ng/mL. The satisfactory accuracy and adequate reproducibility of both warfarin enantiomers were validated in terms of intra- and interday precision with mouse DBS cards. The samples were stable at room temperature for at least 14 days. The validated method was applied to a pharmacokinetic study in mice.

Simultaneous determination of warfarin and 7-hydroxywarfarin in rat plasma by HPLC-FLD

In this study, high-performance liquid chromatography with fluorescence detection (HPLC-FLD) has been used for the first time, for direct determination of warfarin and its major metabolite, 7-hydroxywarfarin, in rat plasma. The simple and sensitive method was developed using Fortis® reversed-phase diphenyl column (150 × 4.6 mm, 3 μm) and a mobile phase composed of phosphate buffer (25 mmol L-1)/methanol/acetonitrile (70:20:10, V/V/V), adjusted to pH 7.4, at a flow rate of 0.8 mL min-1. The diphenyl chemistry of the stationary phase provided a unique selectivity for separating the structurally related aromatic analytes, warfarin and 7-hydroxywarfarin, allowing their successful quantification in the complex plasma matrix. The method was linear over the range 0.01-25 μg mL-1, for warfarin and 7-hydroxywarfarin, and was found to be accurate, precise and selective in accordance with US FDA guidance for bioanalytical method validation. The method was sensitive enough to quantify 0.01 μg mL-1 of warfarin and 7-hydroxywarfarin (LLOQ) using only 100 μL of plasma. The applicability of this method was demonstrated by analyzing samples obtained from rats after oral administration of a single warfarin dose, and studying warfarin and 7-hydroxywarfarin pharmacokinetics.

Simultaneous Determination of Warfarin and Aspirin Contents in Biological Fluids Using Excitation-Emission Matrix Fluorescence Coupled with a Second-order Calibration Method

In the present work, a practical method that combines excitation-emission matrix fluorescence with a second-order calibration method based on an alternating trilinear decomposition (ATLD) algorithm was developed in order to simultaneously and directly determine the contents of warfarin (WAR) and aspirin (ASA) in human plasma and urine samples, even in the presence of unknown interferences. With the pre-estimated component number of 4, the obtained average spiked recoveries were 105.4 ± 7.8 and 104.2 ± 8.3% for WAR, 96.5 ± 2.8 and 91.2 ± 2.3% for ASA in human plasma and urine samples, respectively. Furthermore, the figures of merit were calculated and also inter- and intra-day experiments were performed that proved the proposed method is of great significance to the monitoring of clinical administration and also being a simple sample pretreatment at low-cost.

Correlations between the enantio- and regio-selective metabolisms of warfarin

AIM

To clarify whether the activities of multiple CYPs associated with warfarin metabolism would be correlated with each other.

METHODS

Oral clearances (CLpo) of warfarin enantiomers were estimated in 378 Chinese, Caucasians and African-Americans. The partial metabolic clearances (CLm) for 7-hydroxywarfarin enantiomers were also measured. In addition, CLpo and CLm were determined in a patient on warfarin and rifampicin.

RESULTS

Correlations between CLpo for warfarin enantiomers existed across the three populations. In addition, there was a significant correlation between the CLm for 7-hydroxylation of warfarin enantiomers. Under induced conditions by rifampicin, there were significant correlations between the enantio- and regio-selective metabolisms of warfarin.

CONCLUSION

Metabolic activities of CYP2C9, CYP1A2 and CYP3A4 may be regulated by common transcriptional mechanism(s).

Population differences in S-warfarin pharmacokinetics among African Americans, Asians and whites: their influence on pharmacogenetic dosing algorithms

Using population pharmacokinetic analysis (PPK), we attempted to identify predictors of S-warfarin clearance (CL(S)) and to clarify population differences in S-warfarin pharmacokinetics among a cohort of 378 African American, Asian and white patients. Significant predictors of CL(S) included clinical (age, body weight and sex) and genotypic (CYP2C9*2,*3 and *8) factors, as well as African American ethnicity, the median CL(S) being 30% lower in the latter than in Asians and whites (170 versus 243 and 250 ml h^-1, P<0.01). The plasma S-warfarin (Cp(S)) time courses following the genotype-based dosing algorithms simulated using the PPK estimates showed African Americans with CYP2C9*1/*1 and any of the VKORC1 genotypes would have an average Cp(S) at steady state 1.5-1.8 times higher than in Asians and whites. These results indicate warfarin dosing algorithms should be evaluated in each respective ethnic population. Further study of a large African American cohort will be necessary to confirm the present findings.

Validation and Application of a Simple UHPLC-MS-MS Method for the Enantiospecific Determination of Warfarin in Human Urine

A simple and rapid liquid chromatographic-tandem mass spectrometric method has been developed and validated for the enantiospecific determination of R- and S-warfarin in human urine. Warfarin enantiomers were extracted from urine using methyl tert-butyl ether. Chromatographic separation of warfarin enantiomers and the internal standard d5-warfarin was achieved using a Astec Chirobiotic V column with gradient mobile phase at a flow rate of 400 µL/min over 10 min. Detection was performed on a TSQ Quantum Ultra triple quadrupole mass spectrometer equipped with a heated electrospray ionization source. Analytes were detected in negative ionization mode using selected reaction monitoring. Calibration curves were linear with a correlation coefficient of ≥0.996 for both enantiomers over a concentration range of 5-500 ng/mL. The intra- and interday accuracy and precision for both analytes were within ±9.0%. Excellent extraction efficiency and negligible matrix effects were observed. The applicability of the method was demonstrated by successful measurement of warfarin enantiomers in urine of patients with kidney disease. The method is simple, accurate and reproducible and is currently being used to support warfarin pharmacokinetic studies.

Factors influencing pharmacokinetics of warfarin in African-Americans: implications for pharmacogenetic dosing algorithms

AIM

This study attempted to identify predictors of S-warfarin clearance (CL[S]) and to make a pharmacokinetic evaluation of genotype-based dosing algorithms in African-Americans.

METHODS

Using plasma S-warfarin concentration (Cp[S]) at a steady state and eight SNPs previously shown to influence warfarin dose in African-Americans, CL(S) and its predictors were estimated by population pharmacokinetic analysis in 60 African-Americans. The time courses of Cp(S) following either the loading dose or maintenance dose were simulated using the population pharmacokinetic estimates.

RESULTS

CYP2C9*8 and body surface area or body weight were predictors of CL(S) (-30 and -5% per -0.1 m(2)/-10 kg reduction in CL[S], respectively) in African-Americans. Simulations of Cp(S) showed that Cp(S) at steady state was 1.4-times higher in patients with CYP2C9*8 than in those with CYP2C9*1/*1, irrespective of the algorithm for loading dose or maintenance dose.

CONCLUSION

African-Americans possess independent predictors of CL(S), possibly leading to a prediction error of any dosing algorithm that excludes African-specific variant(s). Original submitted 3 September 2014; Revision submitted 3 November 2014.

Simultaneous and stereospecific analysis of warfarin oxidative metabolism using 2D LC/Q-TOF

Background: Warfarin is a widely used racemic anticoagulant with narrow therapeutic range and wide interindividual response to treatment. This is due to the extensive and differential clearance of R- and S-warfarin with the involvement of several polymorphic CYP450 enzymes resulting in the formation of several stereoisomeric oxidative metabolites. Results: A stereospecific 2DLC/Q-TOF method was developed for the simultaneous identification and quantitation of hydroxylated warfarin metabolites from a single sample analysis. Using this method metabolites from rat microsomal and plated hepatocyte incubations with R-, S- and (R/S)-warfarin were estimated. Conclusion: Multiheart cutting with high resolution MS and MS/MS analysis is suggested as a viable approach for achiral-chiral separation of metabolites of warfarin and other chiral or prochiral drugs.

Novel single nucleotide polymorphism in CYP2C9 is associated with changes in warfarin clearance and CYP2C9 expression levels in African Americans

Warfarin is a widely used anticoagulant whose active S-enantiomer is primarily metabolized by the CYP2C9 enzyme. The CYP2C9*2 and CYP2C9*3 alleles are associated with lower warfarin dose requirement and decreased enzyme activity. In contrast, we previously identified a novel single-nucleotide polymorphism (SNP) (rs7089580A > T) in CYP2C9 that is associated with higher warfarin dose requirement in African Americans (AAs). In this study, we examine the effect of rs7089580 on warfarin pharmacokinetics and CYP2C9 expression in 63 AA patients and 32 AA liver tissues, respectively. We found oral clearance of S-warfarin to be higher among carriers of the minor rs7089580 allele (T) compared with wild-type homozygotes (3.73 ± 1.46 vs 2.95 ± 1.39 mL/min; P = 0.04). CYP2C9 messenger RNA expression in liver tissue was also higher among A/T and T/T genotypes compared with A/A (P < 0.02). Our findings indicate that rs7089580 is associated with higher S-warfarin clearance and CYP2C9 expression and may help explain the higher dose requirement of warfarin in AAs. Furthermore, rs7089580 is in complete linkage disequilibrium with the promoter SNP rs12251841 in AAs, which may provide a biologically plausible explanation for the observed effect on CYP2C9 expression levels. Given the many clinically relevant substrates of CYP2C9, identifying polymorphisms that affect expression levels and metabolism across ethnicities is essential for individualization of doses with a narrow therapeutic index.

Analysis of free fractions for chiral drugs using ultrafast extraction and multi-dimensional high-performance affinity chromatography

A multi-dimensional chromatographic approach was developed to measure the free fractions of drug enantiomers in samples that also contained a binding protein or serum. This method, which combined ultrafast affinity extraction with a chiral stationary phase, was demonstrated using the drug warfarin and the protein human serum albumin.

Determinants of the over-anticoagulation response during warfarin initiation therapy in Asian patients based on population pharmacokinetic-pharmacodynamic analyses

To clarify pharmacokinetic-pharmacodynamic (PK-PD) factors associated with the over-anticoagulation response in Asians during warfarin induction therapy, population PK-PD analyses were conducted in an attempt to predict the time-courses of the plasma S-warfarin concentration, Cp(S), and coagulation and anti-coagulation (INR) responses. In 99 Chinese patients we analyzed the relationships between dose and Cp(S) to estimate the clearance of S-warfarin, CL(S), and that between Cp(S) and the normal prothrombin concentration (NPT) as a coagulation marker for estimation of IC₅₀. We also analyzed the non-linear relationship between NPT inhibition and the increase in INR to derive the non-linear index λ. Population analyses accurately predicted the time-courses of Cp(S), NPT and INR. Multivariate analysis showed that CYP2C9*3 mutation and body surface area were predictors of CL(S), that VKORC1 and CYP4F2 polymorphisms were predictors of IC₅₀, and that baseline NPT was a predictor of λ. CL(S) and λ were significantly lower in patients with INR≥4 than in those with INR<4 (190 mL/h vs 265 mL/h, P<0.01 and 3.2 vs 3.7, P<0.01, respectively). Finally, logistic regression analysis revealed that CL(S), ALT and hypertension contributed significantly to INR≥4. All these results indicate that factors associated with the reduced metabolic activity of warfarin represented by CL(S), might be critical determinants of the over-anticoagulation response during warfarin initiation in Asians.

Trial Registration

ClinicalTrials.gov NCT02065388

CYP2C9 promoter region single-nucleotide polymorphisms linked to the R150H polymorphism are functional suggesting their role in CYP2C9*8-mediated effects

Cytochrome P450 2C9 (CYP2C9) c.449G>A (*8) is common in African Americans and is associated with decreased warfarin clearance. We examined the effect of promoter region variants inherited with 449G>A on warfarin clearance, dose requirements, and CYP2C9 expression. In an African American cohort, 449G>A was in linkage disequilibrium with c.-1766T>C (r(2) = 0.89) and c.-1188T>C (D' = 1). The combination of the -1766C and 449A alleles with the -1188CC genotype was associated with lower S-warfarin clearance (0.86 ± 0.22 vs. 1.66 ± 0.75 ml/min/m(2); n = 48; P < 0.01) and dose requirements [33 (25-49) vs. 43 (35-56) mg/week; n = 243; P = 0.03] compared with other genotypes. In liver tissue, alleles with the -1766C/-1188C/449A haplotype showed two-fold decreased mRNA expression compared with reference alleles. In a promoter reporter assay, the -1766C/-1188C haplotype decreased CYP2C9 promoter activity. These data suggest that promoter region polymorphisms inherited with 449G>A decrease CYP2C9 expression and contribute to CYP2C9*8 effects on warfarin clearance and dose requirements.

Development of a chiral micellar electrokinetic chromatography-tandem mass spectrometry assay for simultaneous analysis of warfarin and hydroxywarfarin metabolites: application to the analysis of patients serum samples

The enantioseparation of warfarin (WAR) along with the five positional and optical isomers is challenging because of the difficulty to simultaneously separate and quantitate these chiral compounds. Currently, no effective chiral CE-MS methods exist for the simultaneous enantioseparation of WAR and all its hydroxylated metabolites in a single run. Polymeric surfactants (aka. molecular micelles) are particularly compatible with micellar electrokinetic chromatography-mass spectrometry (MEKC-MS) because they have a wider elution window for enantioseparation and do not interfere with the MS detection of chiral drugs. Using polysodium N-undecenoyl-L,L-leucylvalinate (poly-L,L-SULV) as a chiral pseudophase in MEKC-MS baseline separation of WAR, its five metabolites along with the internal standard was obtained in 45 min. This is in comparison to 100 min required for separation of the same mixture with packed column CEC-MS using a vancomycin chiral stationary phase. Serum samples were extracted with mixed-mode anion-exchange (MAX) cartridge with recoveries of greater than 85.2% for all WAR and hydroxywarfarin (OH-WAR) metabolites. Utilizing the tandem MS and multiple reaction monitoring mode, the MEKC-MS/MS method was used to simultaneously generate calibration curves over a concentration range from 2 to 5000 ng/mL for R- and S-warfarin, 5 to 1000 ng/mL for R- and S-6-, 7-, 8- and 10-OH-WAR and 10 to 1000 ng/mL for R and S-4'-OH-WAR. For the first time, the limits of detection and quantitation for most WAR metabolites by MEKC-MS/MS were found to be at levels of 2 and 5 ng/mL, respectively. The method was successfully applied for the first time to analyze WAR and its metabolites in plasma samples of 55 patients undergoing WAR therapy, demonstrating the potential of chiral MEKC-MS/MS method to accurately quantitate with high sensitivity.

Decreased warfarin clearance associated with the CYP2C9 R150H (*8) polymorphism

The cytochrome P450 (CYP) 2C9 R150H (*8) allele occurs commonly in African Americans and is associated with lower warfarin dose requirements. We conducted a pharmacokinetic study to examine whether the CYP2C9*8 allele impacts warfarin clearance in African-American patients. We also conducted an in vitro kinetic study of S-warfarin 7-hydroxylation using complementary DNA (cDNA)-expressed CYP2C9 enzymes. We observed a 30% reduction in the unbound oral clearance of S-warfarin and a 25% lower R- to S-warfarin plasma concentration ratio in patients with the CYP2C9*8 allele (n = 12) as compared to CYP2C9*1 homozygotes (n = 26). Consistent with these findings, the in vitro intrinsic clearance of S-warfarin was 30% lower with the cDNA-expressed R150H protein as compared to the wild-type protein. These data show that the R150H variant protein expressed by the CYP2C9*8 allele is associated with lower S-warfarin clearance. This finding provides clinical and experimental evidence to explain the lower warfarin dose requirements in patients with the CYP2C9*8 allele.

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.

Assays and applications in warfarin metabolism: what we know, how we know it and what we need to know

INTRODUCTION

Coumadin (R/S-warfarin) is the most widely prescribed oral anticoagulant in the world; nevertheless, its clinical use is complicated by unpredictability in dose requirements to achieve and maintain optimal anticoagulation. Variations in warfarin metabolism among patients contribute to unpredictability in therapeutic responses. Studying the clinical relevance of warfarin metabolism poses a significant analytical challenge. Warfarin is given to patients as an equal mixture of R and S enantiomers. Both drugs undergo extensive metabolism through different pathways to generate > 20 structurally similar isomeric metabolites.

AREAS COVERED

The article discusses how analytical methods have evolved to effectively resolve and quantify individual metabolites. The authors also discuss how the application of these methods has identified clinically relevant metabolic pathways for warfarin and fostered the investigation of clinical biomarkers for patient responses to therapy. The article additionally presents the power of these methods and how aspects of warfarin metabolism have led to the use of warfarin as a phenotyping probe for multiple drug metabolizing enzymes.

EXPERT OPINION

Progress in these areas has been hampered by shortcomings in analytical methods and a narrow focus on one metabolic pathway. Recent advances in liquid chromatographic-mass spectral methods can rapidly analyze most warfarin metabolites. It is now possible to effectively assess alternate metabolic pathways and expand biomarker analyses for clinical and phenotyping applications.

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.

Effects of folic acid supplementation on the pharmacokinetics and anticoagulant effect of warfarin: an open-label, prospective study of long-term administration in adults

BACKGROUND

Folic acid supplementation in patients with folic acid deficiency has been associated with increased clearance of phenytoin to its cytochrome P450 (CYP) 2C9-mediated metabolite, 5-(4'-hydroxyphenyl)-5-phenylhydantoin.

OBJECTIVE

The aim of this study was to determine whether folic acid supplementation increases the dosage requirement of the CYP2C9 substrate warfarin, and the formation clearance of the CYP2C9-mediated product, (S)-7-hydroxywarfarin.

METHODS

Patients aged >or=18 years with folic acid deficiency who were receiving long-term treatment with a stable dosage of warfarin were studied prospectively, before and 30 to 60 days after the initiation of supplementation with folic acid. Warfarin dosage and international normalized ratio (INR) were documented, and the formation clearance of (S)- and (R)-7-hydroxywarfarin and the oral clearance of (S)- and (R)-warfarin were determined.

RESULTS

Twenty-four white patients (14 males; mean (SD) age, 55.0 [19.7] years; body mass index, 30.64 [6.8] kg/m(2)) were enrolled. Treatment with folic acid was associated with a significantly increased mean (SD) formation clearance of (S)-7-hydroxywarfarin (1.096 [0.816] vs 1.608 [1.302] mL/min; P = 0.048). Before folic acid supplementation, the mean (SD) warfarin dosage was 5.98 (2.12) mg/d, and the INR was 2.51 (0.55). During supplementation, the warfarin dosage was 6.17 (2.31) mg/d and the INR was 2.63 (0.65) (both, P = NS vs before supplementation).

CONCLUSIONS

Folic acid supplementation was associated with significantly increased formation clearance of (S)-7-hydroxywarfarin. Changes in warfarin dosage requirements and INR were nonsignificant.

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.

Warfarin metabolism and anticoagulant effect: a prospective, observational study of the impact of CYP2C9 genetic polymorphism in the presence of drug-disease and drug-drug interactions

BACKGROUND

Cytocbrome P450 (CYP) 2C9 polymorphism affects the warfarin dosage requirement in stable outpatients. However, it is not known whether the CYP2C9 genotype contributes to the variability in warfarin dosage in the presence of drug-disease and drug-drug interactions.

OBJECTIVE

The aim of this study was to examine the effects of CYP2C9 genetic polymorphism on warfarin dosage requirements in patients with severe comorbid conditions and those treated with medications that potentially interact with warfarin.

METHODS

This prospective, observational study was conducted at Hadassah University Hospital, Jerusalem, Israel. Data from consecutive patients treated with warfarin for at least 3 months and admitted to the internal medicine ward were eligible for inclusion. Clinical data, international normalized ratio (INR), and warfarin dosage were recorded from medical records. The CYP2C9 genotype was determined using polymerase chain reaction restriction fragment length polymorphism, and plasma concentrations of (S)- and (R)-warfarin were determined by high-performance liquid chromatography using chiral methods.

RESULTS

One hundred nineteen subjects (52% women) were studied. Mean age was 65.8 years (95% CI, 63.1-68.4), and weight was 74.9 kg (95% CI, 72.1-77.7). The mean warfarin dosage was 33% lower in patients with the CYP2C9-*1/*3 genotype (mean [SEM], 0.045 [0.006] mg/kg x d(-1)) compared with the CYP2C9-*1/*1 genotype (0.067 [0.004] mg/kg x d(-1)) (P=0.008); an intermediate value was found for the CYP2C9-*1/*2 genotype (0.062 [0.008] mg/kg x d(-1)). However, despite the lower dosage, INR was significantly higher in patients with the *1/*3 genotype (mean [95% CI], 3.29 [2.44-4.14]) (n=18) compared with the *1/*1 genotype (2.52 [2.34-2.71]) (n=64) (P=0.029). In addition to genotype, older age, congestive heart failure (CHF), and treatment with antibiotics were associated with lower warfarin dosages, whereas treatment with drug-metabolism inducers was associated with higher warfarin dosages. In addition, the ratios of (S)- to (R)-warfarin concentrations were significantly higher in patients with *1/*3 compared with those in patients with the *1/*1 genotype.

CONCLUSIONS

In this study population of patients with severe comorbid conditions and those treated with medications that potentially interact with warfarin, CYP2C9 *1/*3 genotype, older age, CHF, and the use of antibiotics were associated with lower warfarin dosage requirements. The CYP2C9*1/*3 genotype, compared with CYP2C9 *1/*1, was associated with 33% lower mean warfarin dosage requirements and higher INR values, which were higher than the upper therapeutic range of INR (ie, 3). Genetic CYP2C9 polymorphism contributed to the variability in warfarin dosage requirements in the presence of drug-disease and drug-drug interactions.

Simultaneous Determination of Warfarin Enantiomers and Its Metabolite in Human Plasma by Column-Switching High-Performance Liquid Chromatography With Chiral Separation

A simple and sensitive column-switching high-performance liquid chromatographic method for the simultaneous determination of warfarin enantiomers and their metabolites, 7-hydroxywarfarin enantiomers, in human plasma is described. Warfarin enantiomers, 7-hydroxywarfarin enantiomers, and an internal standard, diclofenac sodium, were extracted from 1 mL of a plasma sample using diethyl ether-chloroform (80:20, v/v). The extract was injected onto column I (TSK precolumn BSA-C8, 5 microm, 10 mm x 4.6 mm inside diameter) for cleanup and column II (Chiralcel OD-RH analytical column, 150 mm x 4.6 mm inside diameter) coupled with a guard column (Chiralcel OD-RH guard column, 10 mm x4.6 mm inside diameter) for separation. The mobile phase consisted of phosphate buffer-acetonitrile (84:16 v/v, pH 2.0) for clean-up and phosphate buffer-acetonitrile (45:55 v/v, pH 2.0) for separation. The peaks were monitored with an ultraviolet detector set at a wavelength of 312 nm, and total time for chromatographic separation was approximately 25 minutes. The validated concentration ranges of this method were 3 to 1000 ng/mL for (R)- and (S)-warfarin and 3 to 200 ng/mL for (R)- and (S)-7-hydroxywarfarin. Intra- and interday coefficients of variation were less than 4.4% and 4.9% for (R)-warfarin and 4.8% and 4.0% for (S)-warfarin, and 5.1% and 4.2% for (R)-7-hydroxywarfarin and 5.8% and 5.0% for (S)-7-hydroxywarfarin at the different concentrations. The limit of quantification was 3 ng/mL for both warfarin and 7-hydroxywarfarin enantiomers. This method was suitable for therapeutic drug monitoring of warfarin enantiomers and was applied in a pharmacokinetic study requiring the simultaneous determination of warfarin enantiomers and its metabolite, 7-hydroxywarfarin enantiomers, in human volunteers.

A rapid method for the quantification of the enantiomers of Warfarin, Phenprocoumon and Acenocoumarol by two-dimensional-enantioselective liquid chromatography/electrospray tandem mass spectrometry

We describe a new fully validated enantioselective LC-MS/MS method for stereospecific quantification of both the racemic forms of Warfarin (WF), Phenprocoumon and Acenocoumarol in human plasma. Measurement specificity was assessed by using different blank donor plasma samples, where no interfering reagent peak appeared at the retention time (RT) of the targeted analytes. Response was linear for all analytes. Typical linear regression coefficients have >0.99. The recoveries ranged from 98% to 118%. Determinations in 10 normal healthy individuals revealed a high reproducibility of RTs. These findings confer to the method suitability for large population studies.

Separation and determination of warfarin enantiomers in human plasma using a novel polymeric surfactant for micellar electrokinetic chromatography-mass spectrometry

Warfarin is a widely used oral anticoagulant which is mostly administrated as a racemic mixture containing equal amount of R- and S-enantiomers. The two enantiomers are shown to exhibit significant differences in pharmacokinetics and pharmacodynamics. In this study, a new chiral micellar electrokinetic chromatography-mass spectrometry (MEKC-MS) method has been developed using a polymeric chiral surfactant, polysodium N-undecenoyl-L,L-leucyl-valinate (poly-L,L-SULV), as a pseudostationary phase for the chiral separation of (+/-)-warfarin (WAR) and (+/-)-coumachlor (COU, internal standard). Under optimum MEKC-MS conditions, the enantio-separation of both (+/-)-WAR and (+/-)-COU was achieved within 23 min. Calibration curves were linear (R=0.995 for (R)-WAR and R=0.989 for (S)-WAR) over the concentration range 0.25-5.0 microg/mL. The MS detection was found to be superior over the commonly used UV detection in terms of selectivity and sensitivity with LOD as low as 0.1 microg/mL in human plasma. The method was successfully applied to determine WAR enantiomeric ratio in patients' plasma undergoing warfarin therapy.

Significant pharmacokinetic and pharmacodynamic interaction of warfarin with the NO-independent sGC activator HMR1766

HMR1766 is a new nitric oxide (NO)-independent activator of soluble guanylyl cyclase (sGC) in development for the treatment of cardiovascular diseases and chronic heart failure. A significant fraction of patients to be treated with HMR1766 is expected to be maintained on warfarin. Because HMR1766 is an inhibitor and warfarin a substrate of CYP2C9, the authors studied whether warfarin pharmacokinetics and pharmacodynamics are influenced by HMR1766. Eighteen healthy males were to receive a single oral dose of 20 mg warfarin each under steady-state conditions of HMR1766 or placebo. Plasma concentrations of HMR1766, (R)- and (S)-warfarin, and its 7-hydroxy-metabolites were determined using high-performance liquid chromatography and prothrombin time, and the international standardized ratio was determined by the nephelometric method. (S)-Warfarin AUC(inf) and t(1/2) were 106,471 h x microg/L and 82.92 hours versus 33,148 h x microg/L under HMR1766 and 31.72 hours under placebo, and the maximum decrease in prothrombin time values after warfarin dosing was 58.75% versus 39.94%. These data demonstrate a CYP2C9-mediated pharmacokinetic interaction with pharmacodynamic, clinically relevant consequences, which might require warfarin dose adjustment.

Elevated warfarin metabolism in warfarin-resistant roof rats (Rattus rattus) in Tokyo

Wild roof rats (Rattus rattus) live in proximity to human habitats, and they may carry numerous pathogens of infectious diseases. Pest control is important for public health, and warfarin is a commonly used rodenticide worldwide. However, continual use of warfarin may cause drug resistance in rodents and lead to failure of their control, especially in urbanized areas. In warfarin-resistant rats, the warfarin level in plasma was significantly lower after oral administration than that in the control warfarin-sensitive rats. Warfarin is metabolized by cytochrome P450 (P450), and hydroxylation of warfarin by P450 isoforms was significantly higher in warfarin-resistant rats (2-fold). Western blot analysis indicated that the level of CYP3A2 expression in warfarin-resistant rats was significantly larger than in warfarin-sensitive rats. The NADPH-P450 reductase activities in resistant rats were 8-fold higher than those in sensitive rats. In vivo, the administration of the P450 potent inhibitor proadifen (SKF-525A) increased the mortality of warfarin in the warfarin-resistant roof rats. We concluded that the mechanism of warfarin resistance in Tokyo roof rats is caused by increased clearance of warfarin.

Different contributions of polymorphisms in VKORC1 and CYP2C9 to intra- and inter-population differences in maintenance dose of warfarin in Japanese, Caucasians and African-Americans

OBJECTIVE

To investigate pharmacokinetic and pharmacodynamic factors associated with population differences in warfarin doses needed to achieve anticoagulation, in particular the possible involvement of genetic variability in vitamin K epoxide reductase (VKOR) and CYP2C9.

METHODS

Warfarin maintenance dose, unbound plasma S-warfarin concentration [Cu(S)] and INR were determined in 157 Caucasians, 172 Japanese, and 36 African-Americans stably anticoagulated patients. In a subset (n = 166), fully carboxylated plasma normal prothrombin levels (NPT) were also measured. Genotyping for seven CYP2C9 (CYP2C9*1 through 6 and *11) and seven VKORC1 variants were performed in 115 Caucasians and 64 Japanese patients and 66 healthy African-Americans. Multivariate analysis was performed to identify covariates associated with warfarin requirement.

RESULTS

The relationship between NPT and Cu(S) indicated Japanese are more susceptible to inhibition of NPT production by S-warfarin than the other two populations. VKORC1 1173 C > T had a greater frequency in Japanese (89.1%) than Caucasians (42.2%) and African-Americans (8.6%). CYP2C9 variants with reduced metabolizing ability were less frequent in Japanese compared to the other two populations. The median warfarin dose was significantly higher in Caucasians than Japanese patients (5.5 versus 3.5 mg/day), however, when matched for CYP2C9*1 homozygosity, no difference in dose was observed between VKORC1 genotype-matched groups. Furthermore, VKORC1 1173C > T and CYP2C9 (*2/*3/*11) genotypes, age and weight were identified as independent covariates contributing to interpatient variability in warfarin dosage.

CONCLUSIONS

Both VKORC1 and CYP2C9 polymorphisms contribute to inter-population difference in warfarin doses among the three populations, but their contribution to intra-population variability may differ within each population.

Sensitivity improvement of circular dichroism detection in HPLC by using a low-pass electronic noise filter: Application to the enantiomeric determination purity of a basic drug

The quality control of chiral drugs requires the determination of their enantiomeric purity. Nowadays, circular dichroism (CD) spectroscopy is gaining increasing importance in pharmaceutical analysis because of the commercially available CD detector in liquid chromatography. The separation of the two enantiomers of a basic drug (efaroxan) was achieved by high performance liquid chromatography using an amylose-derivated column with both UV and CD detections. A baseline-resolved separation (resolution: 5) was obtained after optimization of the mobile phase composition with hexane-ethanol-diethylamine (90:10:0.05; v/v/v). The use of a commercial low-pass electronic noise filter of the CD signal has improved the signal-to-noise ratio by a factor twelve and allowed the quantitation of each enantiomer in the 1.25-300 microg ml(-1) concentration range. The CD linear calibration curve, expressed in terms of stereoisomer height ratio versus concentration ratio, was plotted over the 0.4-6% range. A correlation coefficient greater than 0.999 was obtained by least-squares regression and the limit of detection for the distomer/eutomer ratio was estimated at 0.14%. Although the method validation showed good repeatability on the retention times (RSD < 0.9%), on the peak height ratios (RSD < 8.7%) of each enantiomer only up to 99.2% enantiomeric purity was achieved.

Determination of warfarin enantiomers and hydroxylated metabolites in human blood plasma by liquid chromatography with achiral and chiral separation

An assay comprising two simple, selective and isocratic HPLC methods with UV detection was developed and validated for measuring warfarin enantiomers and all five warfarin monohydroxylated metabolites in patient blood plasma. Following liquid/liquid extraction from 1 ml of blood plasma a baseline separation of analytes was achieved on chiral (alpha(1) acid glycoprotein - AGP) and achiral (C(18)) column. Both methods were consistent (R.S.D.<6.9% for warfarin enantiomers and<8.9% for monohydroxylated metabolites) and linear (r>0.998). The limits of detection were 25 ng/ml for warfarin enantiomers, 25 ng/ml for 4'-, 10-, 6- and 7-hydroxywarfarin, 35 ng/ml for 8-hydroxywarfarin and 50 ng/ml for racemic warfarin. In a clinical study in 204 patients, it was confirmed that the assay is appropriate for evaluation of influences of genetic polymorphisms, demographic factors and concomitant drug treatment on warfarin metabolism.

Comparative Pharmacokinetics of Vitamin K Antagonists

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

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

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

5′-Flanking region polymorphisms of CYP2C9 and their relationship to S-warfarin metabolism in white and Japanese patients

White and Japanese patients require different warfarin dosages to achieve therapeutic anticoagulation, but this can be only partly explained by genetic variability in the coding region of CYP2C9—a critical enzyme in the drug's metabolism. Accordingly, analysis of the -2.1-kb 5′-flanking region of CYP2C9 was undertaken in 22 white and 38 Japanese patients whose unbound oral clearance of S-warfarin had been previously determined. Thirteen single nucleotide polymorphisms (SNPs) were identified, some of which were in linkage disequilibrium with functionally defective coding region variants. Those 5′-flanking patterns linked with at least one CYP2C9*3 allele or CYP2C9*2/*3 were associated with reduced CYP2C9 activity and warfarin dose. Japanese patients possessing the wild-type promoter and coding sequences had significantly (P &lt; .01) greater CYP2C9 activity than white patients with the corresponding genotype. In conclusion, either unidentified polymorphisms further upstream in the promoter region or environmental factor(s) account for the differences in the warfarin doses between whites and Japanese. (Blood. 2004;103: 3055-3057)

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

We analyzed mutations of 7 vitamin K-dependent protein and cytochrome P450 2C9 genes in 45 patients and investigated whether any contribute to the large interpatient variability in the warfarin dose-effect relationship. Total clearance and daily dose, INR and INR/Cp, were used as pharmacokinetic and pharmacodynamic indexes, respectively. Patients were grouped by genotype based on a single polymorphism and combinations of polymorphisms. Among the 30 sequence variants identified, CYP2C9*3, 165Thr-->Met of the factor II gene, -402G-->A, (37-bp repeat)n, and -746T-->C of the factor VII gene, and (CAA repeat)n of the gamma-glutamyl carboxylase gene were selected as candidate polymorphisms. As the analysis of single polymorphisms implied, the highest INR/Cp mean values and the lowest warfarin maintenance doses were observed in patients homozygous for the 165Met, -402G, (37-bp repeat)6 and -746T alleles. Multiple regression analysis revealed that warfarin sensitivity was independently associated with -402G-->A, (CAA repeat)n, CYP2C9*3, and 165Thr-->Met, which accounted for 50% of variance. These results suggest that part of the considerable interpatient variation is attributable to genetic variation, and the combined genotyping of CYP2C9 and certain vitamin K-dependent protein genes is useful for predicting anticoagulant responses.

Stereoselective chromatography of cardiovascular drugs: an update

This review reports the latest achievements in chromatographic enantioseparations of various classes of cardiovascular drugs and selected applications of these methods in pharmaceutical and clinical analysis. The use of these drugs as test compounds for new chiral stationary phases and different parameters of chromatographic processes is also presented.

Pharmaceutical and biomedical applications of enantioseparations using liquid chromatographic techniques

The chiral separation methods using liquid chromatographic techniques can be divided into two categories: one is a direct method, which is based on a diastereomer formation on stationary phase or in mobile phase. The other is an indirect method, which is based on a diasteromer formation by reaction with a homochiral reagent. The enantiomer separation on a chiral stationary phases followed by derivatization with an achiral reagent is also dealt with this review article as the indirect method. The pharmaceutical and biomedical applications of enantioseparations using the direct and indirect methods have been considered in this review.

Pharmacogenetics of warfarin elimination and its clinical implications

Warfarin is one of the most widely prescribed oral anticoagulants. However, optimal use of the drug has been hampered by its >10-fold interpatient variability in the doses required to attain therapeutic responses. Pharmacogenetic polymorphism of cytochrome P450 (CYP) may be associated with impaired elimination of warfarin and exaggerated anticoagulatory responses to the drug in certain patients. Clinically available warfarin is a racemic mixture of (R)- and (S)-warfarin, and the (S)-enantiomer has 3 to 5 times greater anticoagulation potency than its optical congener. Both enantiomers are eliminated extensively via hepatic metabolism with low clearance relative to hepatic blood flow. CYP2C9 is almost exclusively responsible for the metabolism of the pharmacologically more active (S)-enantiomer. Several human allelic variants of CYP2C9 have been cloned, designated as CYP2C9*1 (reference sequence or wild-type allele), CYP2C9*2, CYP2C9*3 and CYP2C9*4, respectively. The allelic frequencies for these variants differ considerably among different ethnic populations. Caucasians appear to carry the CYP 2C9*2 (8 to 20%) and CYP2C9*3 (6 to 10%) variants more frequently than do Asians (0% and 2 to 5%, respectively). The metabolic activities of the CYP2C9 variants have been investigated in vitro. The catalytic activity of CYP2C9*3 expressed from cDNA was significantly less than that of CYP2C9*1. Human liver microsomes obtained from individuals heterozygous for CYP2C9*3 showed significantly reduced (S)-warfarin 7-hydroxylation as compared with those obtained from individuals genotyped as CYP2C9*1. The influence of the CYP2C9*3 allele on the in vivo pharmacokinetics of (S)-warfarin has been studied in Japanese patients. Patients with the homozygous CYP2C9*3 genotype, as well as those with the heterozygous CYP2C9*1/*3 genotype, had significantly reduced clearance of (S)-warfarin (by 90 and 60%, respectively) compared with those with homozygous CYP2C9*1. The maintenance dosages of warfarin required in Japanese patients with heterozygous and homozygous CYP2C9*3 mutations were significantly lower than those in patients with CYP2C9*1/*1. In addition, 86% of British patients exhibiting adequate therapeutic responses with lower maintenance dosages of warfarin (<1.5 mg/day) had either the CYP2C9*2 or CYP2C9*3 mutation singly or in combination, whereas only 38% of randomly selected patients receiving warfarin carried the corresponding mutations. Furthermore, the former group showed more frequent episodes of major bleeding associated with warfarin therapy. These data indicate that the CYP2C9*3 allele may be associated with retarded elimination of (S)-warfarin and the resulting clinical effects. However, relationships between CYP2C9 genotype, enzyme activity, metabolism of probe substrates, dosage requirements and bleeding complications should be interpreted with caution, and further studies are required.