Development and validation of a high-performance liquid chromatographic method for the quantitation of warfarin enantiomers in human plasma

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.

Determination of total and unbound warfarin and warfarin alcohols in human plasma by high performance liquid chromatography with fluorescence detection

Two analytical procedures are presented for the determination of the total content and unbound fraction of both warfarin and warfarin alcohols in human plasma. Chromatographic separation was carried out in isocratic conditions at 25°C on a C-18 reversed-phase column with a mobile phase consisting of a 70% buffer phosphate 25mM at pH=7, 25% methanol and 5% acetonitrile at a flow rate of 1.2mL/min. Fluorescence detection was performed at 390nm (excitation wavelength 310nm). Neither method showed any detectable interference or matrix effect. Inter-day recovery of the total warfarin and warfarin alcohols at a concentration level of 1000ng/mL was 89±3% and 73±3%, respectively, whereas for their unbound fraction (at a concentration level of 10ng/mL) was 66±8% and 90±7%, respectively. The intra- and inter-day precision (assessed as relative standard deviation) was <10% for both methods. The limits of detection were 0.4 and 0.2ng/mL for warfarin and warfarin alcohols, respectively. The methods were successfully applied to a pooled plasma sample obtained from 69 patients undergoing warfarin therapy.

The population pharmacokinetics of R- and S-warfarin: effect of genetic and clinical factors

BACKGROUND

Warfarin is a drug with a narrow therapeutic index and large interindividual variability in daily dosing requirements. Patients commencing warfarin treatment are at risk of bleeding due to excessive anticoagulation caused by overdosing. The interindividual variability in dose requirements is influenced by a number of factors, including polymorphisms in genes mediating warfarin pharmacology, co-medication, age, sex, body size and diet.

AIMS

To develop population pharmacokinetic models of both R- and S-warfarin using clinical and genetic factors and to identify the covariates which influence the interindividual variability in the pharmacokinetic parameters of clearance and volume of distribution in patients on long-term warfarin therapy.

METHODS

Patients commencing warfarin therapy were followed up for 26 weeks. Plasma warfarin enantiomer concentrations were determined in 306 patients for S-warfarin and in 309 patients for R-warfarin at 1, 8 and 26 weeks. Patients were also genotyped for CYP2C9 variants (CYP2C9*1,*2 and *3), two single-nucleotide polymorphisms (SNPs) in CYP1A2, one SNP in CYP3A4 and six SNPs in CYP2C19. A base pharmacokinetic model was developed using NONMEM software to determine the warfarin clearance and volume of distribution. The model was extended to include covariates that influenced the between-subject variability.

RESULTS

Bodyweight, age, sex and CYP2C9 genotype significantly influenced S-warfarin clearance. The S-warfarin clearance was estimated to be 0.144 l h⁻¹ (95% confidence interval 0.131, 0.157) in a 70 kg woman aged 69.8 years with the wild-type CYP2C9 genotype, and the volume of distribution was 16.6 l (95% confidence interval 13.5, 19.7). Bodyweight and age, along with the SNPs rs3814637 (in CYP2C19) and rs2242480 (in CYP3A4), significantly influenced R-warfarin clearance. The R-warfarin clearance was estimated to be 0.125 l h⁻¹ (95% confidence interval 0.115, 0.135) in a 70 kg individual aged 69.8 years with the wild-type CYP2C19 and CYP3A4 genotypes, and the volume of distribution was 10.9 l (95% confidence interval 8.63, 13.2).

CONCLUSIONS

Our analysis, based on exposure rather than dose, provides quantitative estimates of the clinical and genetic factors impacting on the clearance of both the S- and R-enantiomers of warfarin, which can be used in developing improved dosing algorithms.

Determination of plasma warfarin concentrations in Korean patients and its potential for clinical application

BACKGROUND

Warfarin is a widely used oral anticoagulant with broad within- and between-individual dose requirements. Warfarin concentrations can be monitored by assessing its pharmacologic effects on International Normalized Ratio (INR). However, this approach has not been applied in the routine clinical management of patients receiving warfarin therapy. We performed a plasma warfarin assay using high-performance liquid chromatography tandem mass spectrometry (HPLC-MS/MS) to determine if such an assay can be utilized in routine clinical practice.

METHODS

We included a total of 105 patients with atrial fibrillation, and who were receiving warfarin for more than 1 yr. The plasma concentrations of total warfarin and 7-hydroxywarfarin were determined by HPLC-MS/MS (Waters, UK). We assessed the association between warfarin dose, concentration, and INR as well as the effects of these factors on warfarin concentrations.

RESULTS

The mean maintenance dose of warfarin in 105 patients was 4.1 +/-1.3 mg/day (range, 1.7-8.0 mg/day) and their mean plasma warfarin concentration was 1.3+/-0.5 mg/L. We defined a concentration range of 0.6-2.6 mg/L (corresponding to the 2.5th to 97.5th percentile range of the Plasma warfarin levels in the 74 patients showing INR within target range) as the therapeutic range for warfarin. The correlation of warfarin dose with warfarin concentration (r(2)=0.259, P<0.001) was higher than that with INR (r(2)=0.029, P=0.072).

CONCLUSIONS

There was a significant correlation between warfarin dose and plasma warfarin concentrations in Korean patients with atrial fibrillation. Hence, plasma warfarin monitoring can help determine dose adjustments and improve our understanding of individual patient response to warfarin treatment.

The pharmacokinetics and pharmacodynamics of warfarin in combination with ambrisentan in healthy volunteers

AIMS

Ambrisentan is an oral, propanoic acid-based endothelin receptor antagonist often co-administered with warfarin to patients with pulmonary arterial hypertension. The aim of this study was to evaluate the potential for ambrisentan to affect warfarin pharmacokinetics and pharmacodynamics.

METHODS

In this open-label cross-over study, 22 healthy subjects received a single dose of racemic warfarin 25 mg alone and after 8 days of ambrisentan 10 mg once daily. Assessments included exposure (AUC(0-last)) and maximum plasma concentration (C(max)) for R- and S-warfarin, and International Normalized Ratio maximum observed value (INR(max)) and area under the curve (INR(AUC(0-last))). The effects of warfarin on ambrisentan steady-state pharmacokinetics and the safety of ambrisentan/warfarin co-administration were assessed. Data are presented as geometric mean ratios.

RESULTS

Ambrisentan had no significant effects on the AUC(0-last) of R-warfarin [104.7; 90% confidence interval (CI) 101.7, 107.7) or S-warfarin (101.6; 90% CI 98.4, 105.0). Similarly, ambrisentan had no significant effects on the C(max) of R-warfarin (91.6; 90% CI 86.2, 97.4) or S-warfarin (89.9; 90% CI 84.8, 95.3). Consistent with these observations, little pharmacodynamic change was observed for INR(max) (85.3; 90% CI 82.4, 88.2) or INR(AUC(0-last)) (93.0; 90% CI 90.8, 95.3). In addition, co-administration of warfarin did not alter ambrisentan steady-state pharmacokinetics. Adverse events were infrequent, and there were no bleeding adverse events.

CONCLUSIONS

Multiple doses of ambrisentan had no clinically relevant effects on the pharmacokinetics and pharmacodynamics of a single dose of warfarin. Therefore, significant dose adjustments of either drug are unlikely to be required with co-administration.

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.

Supercritical fluid chromatography–tandem mass spectrometry for fast bioanalysis of R/S-warfarin in human plasma

Chiral separation for the analysis of enantiomers in biological fluids by HPLC often takes relatively long chromatography time compared to achiral analysis. The advantage of fast mass transfer in packed-column supercritical fluid chromatography (pSFC) and the high-flow compatibility of APCI-MS/MS were applied to develop a fast bioanalytical method for R/S-warfarin in human plasma. Presented here are the main challenges encountered during method development of a semi-automated liquid extraction SFC-MS/MS method. The selection of internal standard, robustness of the SFC equipment, and carryover issues are discussed. The method has high-throughput: the chromatography time is at least two-fold faster than the our fastest previous method; and the liquid/liquid extraction time of 96 samples is less than 20 min using a Tecan Genesis RSP 100 pipetting station and a Tomtec Quadra-96 workstation. The standard curve range was 13.6-2500 ng/ml. Precision of QC concentrations from four validation runs was 7.0% for R-warfarin and 6.0% C.V. for S-warfarin; and the bias was 3.7 and 3.2% R.E., respectively. The method is sensitive, accurate, selective and robust, and was applied to a drug-interaction clinical study with rapid turnaround of sample analysis.

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.

Effect of ginkgo and ginger on the pharmacokinetics and pharmacodynamics of warfarin in healthy subjects

AIM

The aim of this study was to investigate the effect of two common herbal medicines, ginkgo and ginger, on the pharmacokinetics and pharmacodynamics of warfarin and the independent effect of these herbs on clotting status.

METHODS

This was an open label, three-way crossover randomized study in 12 healthy male subjects, who received a single 25 mg dose of warfarin alone or after 7 days pretreatment with recommended doses of ginkgo or ginger from herbal medicine products of known quality. Dosing with ginkgo or ginger was continued for 7 days after administration of the warfarin dose. Platelet aggregation, international normalized ratio (INR) of prothrombin time, warfarin enantiomer protein binding, warfarin enantiomer concentrations in plasma and S-7-hydroxywarfarin concentration in urine were measured. Statistical comparisons were made using anova and the 90% confidence intervals (CIs) of the ratio of log transformed parameters are reported.

RESULTS

INR and platelet aggregation were not affected by administration of ginkgo or ginger alone. The mean (95% CI) apparent clearances of S-warfarin after warfarin alone, with ginkgo or ginger were 189 (167-210) ml h(-1), 200 (173-227) ml h(-1) and 201 (171-231) ml h(-1), respectively. The respective apparent clearances of R-warfarin were 127 (106-149) ml h(-1), 126 (111-141) ml h(-1) and 131 (106-156) ml h(-1). The mean ratio (90% CI) of apparent clearance for S-warfarin was 1.05 (0.98-1.21) and for R-warfarin was 1.00 (0.93-1.08) when coadministered with ginkgo. The mean ratio (90% CI) of AUC(0-168) of INR was 0.93 (0.81-1.05) when coadministered with ginkgo. The mean ratio (90% CI) of apparent clearance for S-warfarin was 1.05 (0.97-1.13) and for R-warfarin was 1.02 (0.95-1.10) when coadministered with ginger. The mean ratio (90% CI) of AUC(0-168) of INR was 1.01 (0.93-1.15) when coadministered with ginger. The mean ratio (90% CI) for S-7-hydroxywarfarin urinary excretion rate was 1.07 (0.85-1.32) for ginkgo treatment, and 1.00 (0.81-1.23) for ginger coadministration suggesting these herbs did not affect CYP2C9 activity. Ginkgo and ginger did not affect the apparent volumes of distribution or protein binding of either S-warfarin or R-warfarin.

CONCLUSIONS

Ginkgo and ginger at recommended doses do not significantly affect clotting status, the pharmacokinetics or pharmacodynamics of warfarin in healthy subjects.

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.

Effect of St John's wort and ginseng on the pharmacokinetics and pharmacodynamics of warfarin in healthy subjects

M: The aim of this study was to investigate the effect of St John's wort and ginseng on the pharmacokinetics and pharmacodynamics of warfarin.

METHODS

This was an open-label, three-way crossover randomized study in 12 healthy male subjects, who received a single 25-mg dose of warfarin alone or after 14 days' pretreatment with St John's wort, or 7 days' pretreatment with ginseng. Dosing with St John's wort or ginseng was continued for 7 days after administration of the warfarin dose. Platelet aggregation, international normalized ratio (INR) of prothrombin time, warfarin enantiomer protein binding, warfarin enantiomer concentrations in plasma and S-7-hydroxywarfarin concentration in urine were measured. Statistical comparisons were made using anova and 90% confidence intervals are reported.

RESULTS

INR and platelet aggregation were not affected by treatment with St John's wort or ginseng. The apparent clearances of S-warfarin after warfarin alone or with St John's wort or ginseng were, respectively, 198 +/- 38 ml h(-1), 270 +/- 44 ml h(-1) and 220 +/- 29 ml h(-1). The respective apparent clearances of R-warfarin were 110 +/- 25 ml h(-1), 142 +/- 29 ml h(-1) and 119 +/- 20 ml h(-1) [corrected]. The mean ratio and 90% confidence interval (CI) of apparent clearance for S-warfarin was 1.29 (1.16, 1.46) and for R-warfarin it was 1.23 (1.11, 1.37) when St John's wort was coadministered. The mean ratio and 90% CI of AUC(0-168) of INR was 0.79 (0.70, 0.95) when St John's wort was coadministered. St John's wort and ginseng did not affect the apparent volumes of distribution or protein binding of warfarin enantiomers.

CONCLUSIONS

St John's wort significantly induced the apparent clearance of both S-warfarin and R-warfarin, which in turn resulted in a significant reduction in the pharmacological effect of rac-warfarin. Coadministration of warfarin with ginseng did not affect the pharmacokinetics or pharmacodynamics of either S-warfarin or R-warfarin.

Dietary vitamin K influences intra-individual variability in anticoagulant response to warfarin

The relationship between dietary intake of vitamin K, fat, plasma vitamin K concentrations and anticoagulation response to warfarin within individuals, as well as the contribution of dietary vitamin K to differences in warfarin dose requirements between individuals were investigated in 53 patients on warfarin therapy who had stably controlled anticoagulation. Each patient completed a dietary record of all foods consumed on a daily basis for 4 weeks. Each week a blood sample was taken for measurement of the international normalized ratio (INR), plasma vitamin K, triglycerides and warfarin enantiomer concentrations. The patients' genotype for CYP2C9 was also determined. Regression analysis of the data showed that, for each increase of 100 microg in the daily dietary intake of vitamin K averaged over 4 d, the INR was reduced by 0.2. There was no correlation between warfarin daily dose and average daily dietary vitamin K intake when calculated over 28 d. The regression model for warfarin dose showed that, while dietary vitamin K had no effect, CYP2C9 genotype (P = 2%) and age (P < 1%) significantly contributed to inter-patient variability in warfarin dose requirements. A consistent intake of vitamin K could reduce intrapatient variability in anticoagulation response and thus improve the safety of warfarin therapy.

High-performance liquid-chromatographic determination of warfarin enantiomers in plasma with automated on-line sample enrichment

A sensitive and selective chiral high performance liquid chromatographic method was developed for the direct determination of R- and S-warfarin enantiomers in human plasma. The method involved direct injection of human plasma onto a semipermeable surface (SPS) guard column, washing the proteins from the column with aqueous acetonitrile and back flushing the analytes onto a reversed phase ovomucoid silica HPLC column using switching valves. After separation, the analytes were simultaneously detected and quantitated with a fluorometer. The recoveries of R-warfarin from human plasma at 25 and 2500 ng/ml were 98.9% and 88.1%, respectively. The recoveries of S-warfarin at 25 and 2500 ng/ml were 105.4% and 93.9%, respectively. Using 100 microl of human plasma, the lower limit of quantification for both R- and S-warfarins was 25 ng/ml. Linear responses in analyte/internal standard peak height ratios were observed for analyte concentrations ranging from 25 to 2500 ng/ml for both enantiomers. Fluorescence chromatograms of drug-free human plasma showed no interfering peaks with retention times similar to those for R- and S-warfarins and the internal standard. Results from a 3-day validation study for both enantiomers demonstrated excellent precision (1.7-9.0%) and accuracy (97-109%) across the calibration range.

Development and validation of a sensitive and robust LC-tandem MS method for the analysis of warfarin enantiomers in human plasma

A liquid chromatography-tandem mass spectrometry method (LC-MS-MS) was developed and validated for measuring warfarin (WAR) enantiomers (R-WAR and S-WAR) in human EDTA plasma. Liquid-liquid extraction using ethyl ether was used to extract the analytes from the plasma. Baseline resolution of S- and R-WAR as well as the internal standard enantiomers (S- and R-p-ClWAR, S-IS and R-IS) was achieved on a beta-cyclodextrin column with a mobile phase of acetonitrile-acetic acid-triethylamine (1000:3:2.5, v/v/v). The retention times are 6.9, 8.0, 7.0, and 7.9 min for S-WAR, R-WAR, S-IS and R-IS, respectively. The detection was by monitoring S- and R-WAR at m/z 307-->161 and S- and R-IS at m/z 341-->161 using (-) ESI. The standard curve range was 1-100 ng ml(-1) for both S- and R-WAR. The inter-day precision and accuracy of the quality control (QC) samples were <7.3% relative standard deviation (RSD) and <7.3% bias for S-WAR, and <6.5% RSD and <5.8% bias for R-WAR, respectively. Analyte stability during sample processing and storage were established. Method ruggedness was demonstrated by the reproducible performance from analysis of clinical samples.

Plasma pharmacokinetics of warfarin enantiomers in cats

The purpose of this study was to determine the dispositions of S-warfarin and R-warfarin in normal cats following intravenous and oral administrations of racemic warfarin. Citrated blood samples were collected from 10 cats prior to and at times 5, 15, and 30 min, 1, 2, 3, 4, 5, 6, 12, 24, 36, 48, 72, 96, and 120 h following a single intravenous bolus of 0.5 mg/kg of racemic warfarin. After a 21-day washout period, samples were then similarly collected in three groups of four cats for 120 h following oral administration of 0.1, 0.25, and 0.5 mg/kg racemic warfarin. S-warfarin and R-warfarin were detected using a high-performance liquid chromatography assay validated for cat plasma. Drug concentration-time curves were subjected to non-compartmental analysis. Median pharmacokinetic parameters associated with the intravenous administration of 0.5 mg/kg racemic warfarin were as follows: t1/2 (S:28.2, R:18.3 h), area under the plasma concentration-time curve (AUC; S:33.0, R:24.6 h*microg/mL), area under the moment curve (AUMC; S:1889, R:527.8 h*h*microg/mL), and mean residence time (MRT; S:38.7, R:20.9 h). For each parameter, S-warfarin was significantly different from R-warfarin (P<0.05). Warfarin was absorbed rapidly after oral administration, and the dosage did not affect the time to maximum concentration (S:0.87, R:0.75 h). Oral dosage significantly influenced maximum plasma concentration (ng/mL, S:1267, R:1355 at 0.5 mg/kg; S:614.9, R:679.4 at 0.25 mg/kg; S:250.5, R:367.6 at 0.1 mg/kg), AUC (h*microg/mL, S:45.12, R:30.91 at 0.5 mg/kg; S:22.98:, R:18.99 at 0.25 mg/kg; S:3.922, R:3.570 at 0.1 mg/kg) and AUMC (h*h*microg/mL, S:2135, R:1062 at 0.5 mg/kg; S:943.1, R:599.9 at 0.25 mg/kg; S:132.2, R:59.03 at 0.1 mg/kg), but not t1/2 (S:23.5, R:11.6 h) nor MRT (S:26.3, R:13.5 h). Both warfarin enantiomers were highly (>96.5%) protein-bound. Quantitation of the warfarin content in commercially available tablets indicated an unequal distribution of the drug throughout the tablet.

Pharmacodynamics of warfarin in cats

The overall purpose of this study was to evaluate the pharmacodynamic response to warfarin in cats. The specific aim was to determine if a log-linear indirect response model (Nagashima et al., 1969) used to describe the in vivo effect of warfarin in humans could be applied to cats. The pharmacokinetics of racemic warfarin were described using a non-compartmental approach. The relationship between prothrombin complex activity (PCA) and normalized prothrombin time (PTR) was defined for feline plasma under our experimental conditions, and determined to be: %PCA=12.38+648 e-PTR/0.492. These data were then integrated and used to predict the warfarin dose associated with therapeutic anti-coagulation defined as an International Normalized Ratio (INR) of 2.0-3.0. The maximum prothrombinopenic response to warfarin in cats after a single intravenous dose of 0.5 mg/kg occurred at 24-48 h. Pharmacodynamic modeling suggested that each cat had a narrow therapeutic range of the steady-state concentration of total warfarin required to appropriately block prothrombin complex synthesis (median: 265.2-358.7 ng/mL). The median daily dose range predicted to yield therapeutic concentrations of warfarin was 0.061-0.088 mg/kg per day. Wide inter-individual variations in both pharmacokinetics and pharmacodynamic response suggest that a more optimal dosing of warfarin may be possible with the development of individual pharmacokinetic/pharmacodynamic algorithms, analogous to those currently employed in human patients.

Validation of a method for the determination of (R)-warfarin and (S)-warfarin in human plasma using LC with UV detection

A sensitive and selective chiral high-performance liquid chromatography (HPLC) method was developed for the determination of (R)-warfarin and (S)-warfarin in human plasma. (R)- and (S)-warfarin and the internal standard (oxybenzone) were extracted from human plasma that had been made acidic with 1 N sulfuric acid into ethyl ether. The ethyl ether layer was removed and evaporated, and the residue was reconstituted in 200 microl of acetonitrile. A 50-microl aliquot was injected onto the HPLC system. Separation was achieved on a beta-cyclodextrin column (250 x 4.6 mm, 5 microm) with a mobile phase composed of acetonitrile:glacial acetic acid:triethylamine (1000:3:2.5, v/v/v). Detection was by ultraviolet absorbance at 320 nm. Late-eluting peaks were diverted from the analytical column by using a beta-cyclodextrin precolumn (50 x 4.6 mm, 5 microm) and a column switching device. The retention times of (R)- and (S)-warfarin and the internal standard were approximately 7.7, 6.9 and 4.0 min, respectively. The run time was 15 min. The assay was linear in concentration ranges of 12.5-2500 ng/ml for (R)- and (S)-warfarin in human plasma. The analysis of quality control samples for (R)- and (S)-warfarin (25.0, 400 and 2000 ng/ml) demonstrated excellent precision with relative standard deviations (R.S.D.) for (R)-warfarin of 10.9, 2.8, and 2.8%, respectively (n = 18), and for (S)-warfarin of 7.0, 2.4 and 2.6%, respectively (n = 18). The method was accurate with all overall (n = 18) mean concentrations being less than 6.0% from nominal at all quality control sample concentrations.

Recent chromatographic and electrophoretic enantioseparations of cardiovascular drugs

The chromatographic and electrophoretic enantiomeric separation and analysis of several clinically used cardiovascular drugs have been reviewed. Several examples of recently reported applications of enantioselective analysis and various cardiovascular agents are presented.

Cyclodextrins and enantiomeric separations of drugs by liquid chromatography and capillary electrophoresis: basic principles and new developments

Investigation of individual drug enantiomers is required in pharmacokinetic and pharmacodynamic studies of drugs with a chiral centre. Cyclodextrins (CDs) are extensively used in high-performance liquid chromatography as stationary phases bonded to a solid support or as mobile phase additives in HPLC and capillary electrophoresis (CE) for the separation of chiral compounds. We describe here the basis for the liquid chromatographic and capillary electrophoretic resolution of drug enantiomers and the factors affecting their enantiomeric separation. This review covers the use of CDs and some of their derivatives in studies of compounds of pharmacological interest.

Development and validation of chiral high-performance liquid chromatographic methods for the quantitation of valsartan and of the tosylate of valinebenzyl ester

A stereospecific HPLC method for the quantitation of CGP 49309 in samples of its corresponding enantiomer valsartan has been developed and validated. The enantiomeric separation was achieved on a 5 micron silica-bonded, alpha 1-acid glycoprotein column (Chiral AGP) with a phosphate buffer, pH 7, containing 2% (v/v) 2-propanol as a mobile phase. The linearity was established in the range 0.1-4% (r > 0.999). The limit of quantitation was 0.1% and the limit of detection was 0.04%. The accuracy of the method was found to be 96.7% (average). For the precision (repeatability), a relative standard deviation value of 2.4% was found. Similarly, a stereoselective HPLC method was also developed and validated for the quantitation of the enantiomer of the starting material used for the synthesis of valsartan, namely (R)-valinebenzyl ester tosylate. Baseline resolution of the enantiomers of valinebenzyl ester tosylate could be achieved on the chiral crown ether column Crownpak CR (Daicel) at 50 degrees C using water-methanol-trifluoroacetic acid (850:150:1, v/v) as a mobile phase. The linearity was established in the range 0.5-5% (r > 0.999). The accuracy of the method was found to be 100.5% (average). For the precision (repeatability), a relative standard deviation value of 3.4% was found. Both methods were found to be suitable for the analysis of the respective analytes.

Stereospecific determinations of (+/- )-DU-124884 and its metabolites (+/- )-KC-9048 in human plasma by liquid chromatography

(+)-DU-124884 is a 5-HT1-like receptor agonist under investigation for drug development. A sensitive, stereospecific LC method was developed for the analysis of (+)-DU-124884, its optical isomer (-)-DU-124884 and their N-dealkylated metabolites, (+/- )-KC-9048, in human plasma. A plasma sample was treated with triethylamine in methanol and the proteins were precipitated by acetonitrile. The supernatant was evaporated to dryness under nitrogen. The analytes and internal standard (acebutolol) formed diastereomers with (S)-(+)-1-(1-naphthyl)ethyl isocyanate immediately. The diastereomers formed were extracted into diethyl ether. They were completely resolved from each other and from matrix peaks on a Microsorb silica column with a mobile phase of methanol-chloroform-hexane (8:12:80, v/v/v) in a run time of 26 min. Detection was by fluorescence with excitation wavelength at 320 nm and emission wavelength at 440 nm. The linearity range is 0.1-200 ng ml-1 (r > 0.99). The limit of quantitation is 0.1 ng ml-1 and the detection limit is 0.02 ng ml-1 (signal-to-noise ratio = 3). The interday precision and accuracy of quality control samples were 5.5-7.6% RSD (relative standard deviation) and 0 to+4% bias for (+)-DU-124884, 5.5-7.9% RSD and 0 to +4% bias for (-)-DU-124884, 4.5-6.5% RSD and -7 to 0% bias for (+)-KC-9048 and 4.5-7.5% RSD and -7 to 0% bias for (-)-KC-9048. Consistent recovery from different lots of human plasma, parallelism of the method, stabilities of on-system, reinjection, bench-top, freeze-thaw cycles and sample storage were established.