Warfarin metabolites: stereochemical aspects of protein binding and displacement by phenylbutazone

Differences in teratogenicity of some vitamin K antagonist substances used as human therapeutic or rodenticide are due to major differences in their fate after an oral administration

All vitamin K antagonist active substances used as rodenticides were reclassified in 2016 by the European authorities as active substances "toxic for reproduction", using a "read-across" alternative method based on warfarin, a human vitamin K antagonist drug. Recent study suggested that all vitamin K antagonist active substances are not all teratogenic. Using a neonatal exposure protocol, warfarin evokes skeletal deformities in rats, while bromadiolone, a widely used second-generation anticoagulant rodenticide, failed to cause such effects. Herein, using a rat model we investigated the mechanisms that may explain teratogenicity differences between warfarin and bromadiolone, despite their similar vitamin K antagonist mechanism of action. This study also included coumatetralyl, a first-generation active substance rodenticide. Pharmacokinetic studies were conducted in rats to evaluate a potential difference in the transfer of vitamin K antagonists from mother to fetus. The data clearly demonstrate that warfarin is highly transferred from the mother to the fetus during gestation or lactation. In contrast, bromadiolone transfer from dam to the fetus is modest (5% compared to warfarin). This difference appears to be associated to almost complete uptake of bromadiolone by mother's liver, resulting in very low exposure in plasma and eventually in other peripheric tissues. This study suggests that the pharmacokinetic properties of vitamin K antagonists are not identical and could challenge the classification of such active substances as "toxic for reproduction".

Multiple UDP-glucuronosyltransferases in human liver microsomes glucuronidate both R- and S-7-hydroxywarfarin into two metabolites

The widely used anticoagulant Coumadin (R/S-warfarin) undergoes oxidation by cytochromes P450 into hydroxywarfarins that subsequently become conjugated for excretion in urine. Hydroxywarfarins may modulate warfarin metabolism transcriptionally or through direct inhibition of cytochromes P450 and thus, UGT action toward hydroxywarfarin elimination may impact levels of the parent drugs and patient responses. Nevertheless, relatively little is known about conjugation by UDP-glucuronosyltransferases in warfarin metabolism. Herein, we identified probable conjugation sites, kinetic mechanisms and hepatic UGT isoforms involved in microsomal glucuronidation of R- and S-7-hydroxywarfarin. Both compounds underwent glucuronidation at C4 and C7 hydroxyl groups based on elution properties and spectral characteristics. Their formation demonstrated regio- and enantioselectivity by UGTs and resulted in either Michaelis-Menten or substrate inhibition kinetics. Glucuronidation at the C7 hydroxyl group occurred more readily than at the C4 group, and the reaction was overall more efficient for R-7-hydroxywarfarin due to higher affinity and rates of turnover. The use of these mechanisms and parameters to model in vivo clearance demonstrated that contributions of substrate inhibition would lead to underestimation of metabolic clearance than that predicted by Michaelis-Menten kinetics. Lastly, these processes were driven by multiple UGTs indicating redundancy in glucuronidation pathways and ultimately metabolic clearance of R- and S-7-hydroxywarfarin.

Hydroxywarfarin metabolites potently inhibit CYP2C9 metabolism of S-warfarin

Coumadin (R/S-warfarin) anticoagulant therapy poses a risk to over 50 million Americans, in part due to interpersonal variation in drug metabolism. Consequently, it is important to understand how metabolic capacity is influenced among patients. Cytochrome P450s (P450 or CYP for a specific isoform) catalyze the first major step in warfarin metabolism to generate five hydroxywarfarins for each drug enantiomer. These primary metabolites are thought to reach at least 5-fold higher levels in plasma than warfarin. We hypothesized that hydroxywarfarins inhibit the hydroxylation of warfarin by CYP2C9, thereby limiting enzymatic capacity toward S-warfarin. To test this hypothesis, we investigated the ability of all five racemic hydroxywarfarins to block CYP2C9 activity toward S-warfarin using recombinant enzyme and human liver microsomes. We initially screened for the inhibition of CYP2C9 by hydroxywarfarins using a P450-Glo assay to determine IC(50) values for each hydroxywarfarin. Compared to the substrate, CYP2C9 bound its hydroxywarfarin products with less affinity but retained high affinity for 10- and 4'-hydroxywarfarins, products from CYP3A4 reactions. S-Warfarin steady-state inhibition studies with recombinant CYP2C9 and pooled human liver microsomes confirmed that hydroxywarfarin products from CYP reactions possess the capacity to competitively inhibit CYP2C9 with biologically relevant inhibition constants. Inhibition of CYP2C9 by 7-hydroxywarfarin may be significant given its abundance in human plasma, despite its weak affinity for the enzyme. 10-Hydroxywarfarin, which has been reported as the second most abundant plasma metabolite, was the most potent inhibitor of CYP2C9, displaying approximately 3-fold higher affinity than S-warfarin. These results indicate that hydroxywarfarin metabolites produced by CYP2C9 and other CYPs may limit metabolic capacity toward S-warfarin through competitive inhibition. Subsequent processing of hydroxywarfarins to secondary metabolites, such as hydroxywarfarin glucuronides, could suppress product feedback inhibition, and therefore could play an important role in the modulation of metabolic pathways governing warfarin inactivation and elimination.

Warfarin and UDP-glucuronosyltransferases: writing a new chapter of metabolism

The widely prescribed anticoagulant, Coumadin (racemic R/S-warfarin), Bristol-Myers Squibb Company, Clinton, NY has a narrow therapeutic range and wide interindividual response due, in part, to drug metabolism. Early identification of hydroxywarfarins (OHWARs), especially S-7-OHWAR, as major metabolites fostered studies characterizing cytochrome P450s responsible for those reactions. Nevertheless, phase II metabolism by sulfotransferases and, especially uridine diphosphate (UDP)-glucuronosyltransferases (UGTs), marks the next chapter in warfarin inactivation and clearance. Rodents converted OHWARs to glucuronides (O-GLUC), including high levels of 4'-, 7-, and 8-O-GLUC. Similarly, humans generated significant levels of glucuronides following treatment with warfarin. 7-O-GLUC was a major metabolite, while 6- and 8-O-GLUC were minor ones. Surprisingly, warfarin glucuronidation accounted for up to 13% of metabolites. This capacity in humans derives from several UGTs, as shown by studies with recombinant enzymes and racemic warfarin and OHWARs. 7-OHWAR was a high-affinity substrate for UGT1A1, compared to other UGTs. UGT1A1 and UGT1A10 also glucuronidated 6-OHWAR. Of five UGT1A enzymes, UGT1A10 was approximately 7-fold more efficient than the rest. Broad substrate specificity for UGT1A10 derives, in part, from an active site-binding motif, specifically F90-M91-V92-F93. Unlike glucuronidation, less is known about sulfonation of warfarin and its metabolites, except that low capacities are shown by rats and, possibly, humans. Collectively, phase I and II metabolic steps create pathways for inactivating and eliminating warfarin that require elucidation. These findings will ultimately enrich our understanding of warfarin metabolism and facilitate the interpreting of metabolic profiles of patients. This knowledge will possibly avoid complications during warfarin therapy related to metabolism by personalizing therapy for the patient.

Effect of rutin on warfarin anticoagulation and pharmacokinetics of warfarin enantiomers in rats

Objectives

The effects of the flavonoid rutin on the anticoagulant activity of oral warfarin and the protein binding and pharmacokinetics of its enantiomers were investigated in rats.

Methods

A single dose of racemic warfarin, 1.5 mg/kg, was administered orally to rats either alone or on day 5 of an 8-day oral regimen of rutin, 1 g/kg daily.

Results

Rutin reduced the anticoagulant effect of racemic warfarin, evident as a 31% reduction in the area under the prothrombin complex activty–time curve (P < 0.05).

Key findings

Rutin had no apparent effect on pre-treatment baseline blood coagulation. It enhanced the in-vitro serum protein binding of S- and R-warfarin (reflected by 40% and 26% reductions in unbound fraction, respectively), and thus restricted distribution by 33 and 21%, respectively. Treatment with rutin significantly decreased the elimination half-life of S-warfarin by 37% as a result of the 69% increase in unbound clearance of the S-enantiomer. This effect was attributed to a significant 77% increase in the unbound formation clearance of the overall oxidative and reductive metabolites, and an increase in the unbound renal clearance of the more potent S-enantiomer of warfarin.

Conclusions

Concurrent rutin administration is likely to reduce the anticoagulant effect of racemic warfarin, reflecting a significant decrease in the elimination half-life of the more potent S-enantiomer.

Acoustic coupling of transverse waves as a mechanism for the label-free detection of protein-small molecule interactions

An on-line acoustic transverse wave device has been used to study the binding interactions of human serum albumin with the small molecule drug, warfarin. Four linking systems for the covalent attachment of the protein to the surface of the gold electrode of the sensor were employed, namely thioctic acid, cysteamine, an N-hydroxysuccinimide ester and 11-mercaptoundecanoic acid. All the attachment protocols involve the ability of thiols to form gold-sulfur bonds at the metal surface. The functional group present at the distal end of each thiol was chemically activated in order to facilitate covalent attachment of the protein. On-line sensor measurements of acoustic parameters show that the binding of warfarin to the protein can be detected, and depending on the linking monolayer used three of four possible combinations of changes in series resonance frequency and motional resistance are observed. Calculations of possible mass and thickness viscoelastic effects demonstrate that these conventional notions are invalid in terms of an explanation of the acoustic signals observed for the warfarin-protein interaction. The responses are ascribed to acoustic coupling phenomena.

Effect of ubidecarenone on warfarin anticoagulation and pharmacokinetics of warfarin enantiomers in rats

Interaction between the antioxidant ubidecarenone and the oral anticoagulant warfarin enantiomers was investigated in rats. The decreased hypoprothrombinemic response, assessed by means of percent changes of prothrombin complex activity and clotting factor VII activity, to warfarin, was observed following oral administration of 1.5 mg/kg racemic warfarin to rats during an 8-day oral regimen (10 mg/kg daily) of ubidecarenone. The antioxidant had no apparent effect on the in vitro rat serum protein binding of warfarin enantiomers. Treatment with ubidecarenone did not affect the absorption and distribution of the S- and R-enantiomers of warfarin, but produced a significant increase in the total serum clearance values of both R- and S-warfarin in rats. This effect was more pronounced with R-warfarin than with S-warfarin. The increased clearance values are attributable to acceleration of certain metabolic pathways and renal excretion of the warfarin enantiomers.

Site I on human albumin: differences in the binding of (R)- and (S)-warfarin

The binding of drugs known to interact with area I on human serum albumin (HSA) was investigated using a chiral stationary phase obtained by anchoring HSA to a silica matrix. In particular, this high-pressure affinity chromatography selector was employed to study the binding properties of the individual enantiomers of warfarin. The pH and composition of the mobile phase modulate the enantioselective binding of warfarin. Displacement chromatography experiments evidenced significant differences in the binding of the warfarin enantiomers to site I. The (S)-enantiomer was shown to be a direct competitor for (R)-warfarin, while (R)-warfarin was an indirect competitor for the (S)-enantiomer. Salicylate directly competed with (R)-warfarin and indirectly with (S)-warfarin. This behavior was confirmed by difference CD experiments, carried out with the same [HSA]/[drug] system in solution.

Protein-binding high-performance frontal analysis of (R)- and (S)-warfarin on HSA with and without phenylbutazone

Applicability of high-performance frontal analysis (HPFA) to the stereoselective study of drug-drug interaction upon plasma protein binding has been investigated. Racemic warfarin and phenylbutazone were used as model drugs. An on-line HPFA/HPLC system consisting of a HPFA column (diol-silica column), an extraction column, and a chiral separation column was developed, and human serum albumin solution containing racemic warfarin and/or phenylbutazone was injected directly to the HPFA column. When the injection volume was large enough, the binding equilibrium in the sample solution was reproduced in the column, and consequently a plateau region appeared on the chromatogram. This plateau region contains unbound drug(s). A given volume of eluent in the plateau part was transferred into the extraction column by column-switching. The concentrated drug(s) was then transferred to the chiral separation column to determine the unbound concentrations of the enantiomers and/or the competitor. The results agreed with those obtained by a conventional ultrafiltration-HPLC method. The influence of phenylbutazone upon the protein binding of warfarin is enantioselective. In warfarin and human serum albumin mixed solution, the unbound concentration of (R)-warfarin was 1.22 times higher than that of the S-isomer. By addition of phenylbutazone, the unbound concentration of (S)-warfarin increased more than that of (R)-warfarin, resulting in the reversed enantioselectivity, i.e., the unbound concentration of (S)-warfarin became 1.19 times larger than that of (R)-warfarin. The present method was also applicable to human plasma samples.