Steady-state clearance rates of warfarin and its enantiomers in therapeutically dosed patients

Impact of organic solvents on cytochrome P450 probe reactions: filling the gap with (S)-Warfarin and midazolam hydroxylation

(S)-Warfarin 7-hydroxylation and midazolam 1'-hydroxylation are among the preferred probe substrate reactions for CYP2C9 and CYP3A4/5, respectively. The impact of solvents on enzyme activity, kinetic parameters, and predicted in vivo hepatic clearance (Cl(H)) associated with each reaction has not been evaluated. The effects of increasing concentrations [0.1-2% (v/v)] of six organic solvents (acetonitrile, methanol, ethanol, dimethyl sulfoxide, acetone, isopropanol) were first tested on each reaction using human liver microsomes (HLMs), human intestinal microsomes (midazolam 1'-hydroxylation only), and recombinant enzymes. Across enzyme sources, relative to water, acetonitrile and methanol had the least inhibitory effect on (S)-warfarin 7-hydroxylation (0-58 and 9-96%, respectively); acetonitrile, methanol, and ethanol had the least inhibitory effect on midazolam 1'-hydroxylation (0-29, 0-22, and 0-20%, respectively). Using HLMs, both acetonitrile and methanol (0.1-2%) decreased the V(max) (32-60 and 24-65%, respectively) whereas methanol (2%) increased the K(m) (100%) of (S)-warfarin-hydroxylation. (S)-Warfarin Cl(H) was underpredicted by 21-65% (acetonitrile) and 13-84% (methanol). Acetonitrile, methanol, and ethanol had minimal to modest impact on both the kinetics of midazolam 1'-hydroxylation (10-24%) and predicted midazolam Cl(H) (2-20%). In conclusion, either acetonitrile or methanol at ≤0.1% is recommended as the primary organic solvent for the (S)-warfarin 7-hydroxylation reaction; acetonitrile is preferred if higher solvent concentrations are required. Acetonitrile, methanol, and ethanol at ≤2% are recommended as primary organic solvents for the midazolam 1'-hydroxylation reaction. This information should facilitate optimization of experimental conditions and improve the interpretation and accuracy of in vitro-in vivo predictions involving these two preferred cytochrome P450 probe substrate reactions.

Stereoselective interactions of warfarin enantiomers with the pregnane X nuclear receptor in gene regulation of major drug-metabolizing cytochrome P450 enzymes

BACKGROUND

Warfarin, an antagonist of vitamin K, is an oral coumarin anticoagulant widely used to control and prevent thromboembolic disorders. Warfarin is clinically available as a racemic mixture of R- and S-warfarin. The S-enantiomer has three to five times greater anticoagulation potency than its optical congener. Recently, vitamin K₂ function has been proposed via the pregnane X receptor (PXR) in osteocytes. PXR acts as a xenobiotic sensor that controls expression of many genes involved in drug/xenobiotic metabolic clearance.

OBJECTIVE

The aim was to examine whether enantiomers of warfarin stereoselectively interact with PXR to up-regulate main drug/xenobiotic-metabolizing enzymes of the cytochrome P450 superfamily.

METHODS

Interactions of warfarin enantiomers with PXR were tested by gene reporter assays and time-resolved fluorescence resonance energy transfer technology (TR-FRET) ligand binding assay. Up-regulation of PXR-target gene mRNAs by warfarin enantiomers was studied using semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) in primary human hepatocytes.

RESULTS

We found that R-warfarin interacts with the PXR nuclear receptor. Consistently, R-warfarin significantly induced CYP3A4 and CYP2C9 mRNAs in cultures of primary human hepatocytes or in LS174T intestinal cells. On the other hand, S-warfarin is a less potent inducer of PXR-target genes in human hepatocytes and activates PXR only at supraphysiological concentrations. In addition, we showed that racemic 10- and 4'-hydroxywarfarins are also highly potent PXR ligands and inducers of CYP3A4 and CYP2C9 mRNA in human hepatocytes.

CONCLUSION

We showed that R-warfarin can significantly up-regulate major drug-metabolizing enzymes CYP3A4 and CYP2C9 in the liver and thus may cause drug-drug interactions (DDI) with co-administered drugs. The results warrant reconsideration of racemic warfarin usage in clinics.

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.

Lack of effect of repeated administration of levetiracetam on the pharmacodynamic and pharmacokinetic profiles of warfarin

The objective of this study was to determine if repeated administration of levetiracetam alters the pharmacokinetics or the pharmacodynamics of warfarin. Forty-two healthy subjects (18-50 years old) were recruited into the study. After a dose-finding phase and a stabilization phase, during which a warfarin treatment was introduced and the dose maintained stable for at least 5 days, 18 male and 8 female subjects were eligible and enrolled. Subjects received warfarin (2.5, 5 or 7.5 mg/day) plus levetiracetam 1000 mg bid, and warfarin plus placebo. The treatment periods were 7 days long and were separated by a 3-day wash-out period. The protein binding and the pharmacokinetic profiles of R- and S-warfarin were assessed at steady state by analysis of blood samples, and the anticoagulant effect was measured using the international normalized ratio (INR). The ratios of the geometric means for AUC(ss) (90% confidence interval) between coadministration of warfarin with levetiracetam or with placebo were 97.17% (92.85%, 101.68%) for R-warfarin and 100.16% (96.43%, 104.02%) for S-warfarin. Results for C(max), C(min) and oral clearance were consistent with those of AUC(ss). In addition, the protein binding of warfarin was not affected by the concomitant treatment. The INR values measured the last 5 days of each period were not statistically altered by the concomitant administration of levetiracetam or placebo: 1.59+/-0.18 for warfarin alone, 1.49+/-0.21 when coadministered with placebo, and 1.55+/-0.23 with levetiracetam (means+/-S.D.). The frequency and profile of adverse events under the concomitant therapy of warfarin and levetiracetam were expected for subjects receiving these drugs, and the coadministration was safe. Moreover, levetiracetam pharmacokinetics after repeated warfarin administration did not differ from those previously reported in healthy volunteers. At the doses administered, there is no evidence of a pharmacokinetic or pharmacodynamic interaction between warfarin and levetiracetam.

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.