Hydroxywarfarin metabolites potently inhibit CYP2C9 metabolism of S-warfarin

Warfarin Overdose in an Adolescent Not Dependent on Anticoagulation: Reversal Strategy and Kinetics

INTRODUCTION

Warfarin induces coagulopathy. Guidelines protocolize reversal of supratherapeutic international normalized ratio (INR) in patients dependent on anticoagulation, but practices vary for reversing warfarin-induced coagulopathy after overdose in non-warfarin-dependent patients.

CASE REPORT

This is the report of a 15-year-old female who ingested her father's warfarin (100-200 mg) in a self-harm attempt. At hour 24 post-ingestion, her INR was 2.00 and she was admitted for monitoring. Reversal of coagulopathy was initially deferred pending the INR trend. The INR was 5.10 at hour 60 and 2.5 mg oral vitamin K1 (VK1) was given. At hour 85, the INR peaked at 6.67 and she received a second oral dose of 2.5 mg VK1. On day 8, she was medically cleared with an INR of 1.31. On day 11, she developed lower abdominal pain and diarrhea. Imaging revealed a duodenal hematoma, and symptoms improved spontaneously. She was again medically cleared 13 days post-ingestion. Her serum warfarin concentration peaked at 19 mcg/mL at hour 46. Serial warfarin concentrations were obtained, demonstrating first-order elimination kinetics and a 30-hour half-life.

CONCLUSION

A restrictive approach to coagulopathy reversal in non-warfarin-dependent patients with intentional warfarin overdose may result in worsening coagulopathy, bleeding, and lengthy hospital stay. Given the risk for significant, prolonged coagulopathy, these patients should be treated early with VK1, with subsequent serial INR monitoring and probable additional VK1 dosing. Delayed peak warfarin concentrations support consideration of gastrointestinal decontamination in late presenters.

Discovery of Novel Reductive Elimination Pathway for 10-Hydroxywarfarin

Coumadin (R/S-warfarin) anticoagulant therapy is highly efficacious in preventing the formation of blood clots; however, significant inter-individual variations in response risks over or under dosing resulting in adverse bleeding events or ineffective therapy, respectively. Levels of pharmacologically active forms of the drug and metabolites depend on a diversity of metabolic pathways. Cytochromes P450 play a major role in oxidizing R- and S-warfarin to 6-, 7-, 8-, 10-, and 4′-hydroxywarfarin, and warfarin alcohols form through a minor metabolic pathway involving reduction at the C11 position. We hypothesized that due to structural similarities with warfarin, hydroxywarfarins undergo reduction, possibly impacting their pharmacological activity and elimination. We modeled reduction reactions and carried out experimental steady-state reactions with human liver cytosol for conversion of rac-6-, 7-, 8-, 4′-hydroxywarfarin and 10-hydroxywarfarin isomers to the corresponding alcohols. The modeling correctly predicted the more efficient reduction of 10-hydroxywarfarin over warfarin but not the order of the remaining hydroxywarfarins. Experimental studies did not indicate any clear trends in the reduction for rac-hydroxywarfarins or 10-hydroxywarfarin into alcohol 1 and 2. The collective findings indicated the location of the hydroxyl group significantly impacted reduction selectivity among the hydroxywarfarins, as well as the specificity for the resulting metabolites. Based on studies with R- and S-7-hydroxywarfarin, we predicted that all hydroxywarfarin reductions are enantioselective toward R substrates and enantiospecific for S alcohol metabolites. CBR1 and to a lesser extent AKR1C3 reductases are responsible for those reactions. Due to the inefficiency of reactions, only reduction of 10-hydroxywarfarin is likely to be important in clearance of the metabolite. This pathway for 10-hydroxywarfarin may have clinical relevance as well given its anticoagulant activity and capacity to inhibit S-warfarin metabolism.

Interactions of 7,8-Dihydroxyflavone with Serum Albumin as well as with CYP2C9, CYP2C19, CYP3A4, and Xanthine Oxidase Biotransformation Enzymes

7,8-dihydroxyflavone (DHF) is a flavone aglycone which has beneficial effects in several central nervous system diseases. Most of the pharmacokinetic properties of DHF have been characterized, while only limited information is available regarding its interactions with serum albumin and biotransformation enzymes. In this study, the interactions of DHF with albumin was examined employing fluorescence spectroscopy and ultrafiltration. Furthermore, the inhibitory effects of DHF on cytochrome P450 (CYP2C9, CYP2C19, and CYP3A4) and xanthine oxidase (XO) enzymes were also tested using in vitro models. Our results demonstrate that DHF forms a stable complex with albumin (K = 4.9 × 10⁵ L/mol) and that it is able to displace both Site I and Site II ligands. Moreover, DHF proved to be a potent inhibitor of each enzyme tested, showing similar or slightly weaker effects than the positive controls used. Considering the above-listed observations, the coadministration of DHF with drugs may interfere with the drug therapy due to the development of pharmacokinetic interactions.

Combination index of the concentration and in vivo antagonism activity of racemic warfarin and its metabolites to assess individual drug responses

The present study was undertaken to examine whether in vivo vitamin K epoxide reductase complex 1 (VKOR) "actual" antagonism activity, calculated by the concentrations and the reported anticoagulant activities of the R- and S-warfarin enantiomers and their metabolites, correlates with the weekly dose of warfarin. Five patients under palliative care were enrolled in our study and 20 serum samples were analyzed by an enantioselective high-performance liquid chromatography-ultraviolet detection method. In vivo VKOR inhibition activities of S-warfarin, R-warfarin, 7- and 10-hydroxywarfarin were calculated as the ratio of drug or metabolite concentration to the IC₅₀. The mean drug concentrations (± SD) of S- and R-warfarin, 7-hydroxywarfarin and 10-hydroxywarfarin were 334 ± 154 ng/ml, 370 ± 115 ng/ml, 42 ± 15 ng/ml and 80 ± 44 ng/ml, respectively. Then, in vivo VKOR actual antagonism activities of S- and R-warfarin, 7-hydroxywarfarin and 10-hydroxywarfarin were calculated. Good correlation (R² = 0.69-0.72) was obtained between the weekly warfarin dose and the ratios of INR/actual antagonism activity, while poor correlation was observed between the weekly warfarin dose and INR (R² = 0.32) or the activities (R² = 0.17-0.21). Actual antagonism activities along with the INR correlated well with the warfarin dose. This parameter may be useful for predicting or altering warfarin doses, although further verification in a larger study is required.

Warfarin Dosing and Outcomes in Chronic Kidney Disease: A Closer Look at Warfarin Disposition

BACKGROUND

Chronic Kidney Disease (CKD) is a prevalent worldwide health problem. Patients with CKD are more prone to developing cardiovascular complications such as atrial fibrillation and stroke. This warrants the use of oral anticoagulants, such as warfarin, in this population. While the efficacy and safety of warfarin in this setting remain controversial, a growing body of evidence emphasizes that warfarin use in CKD can be problematic. This review discusses 1) warfarin use, dosing and outcomes in CKD patients; and 2) possible pharmacokinetic mechanisms for altered warfarin dosing and response in CKD.

METHODS

Structured search and review of literature articles evaluating warfarin dosing and outcomes in CKD. Data and information about warfarin metabolism, transport, and pharmacokinetics in CKD were also analyzed and summarized.

RESULTS

The literature data suggest that changes in warfarin pharmacokinetics such as protein binding, nonrenal clearance, the disposition of warfarin metabolites may partially contribute to altered warfarin dosing and response in CKD.

CONCLUSION

Although the evidence to support warfarin use in advanced CKD is still unclear, this synthesis of previous findings may help in improving optimized warfarin therapy in CKD settings.

Evaluation of In Vitro Cytochrome P450 Inhibition and In Vitro Fate of Structurally Diverse N-Oxide Metabolites: Case Studies with Clozapine, Levofloxacin, Roflumilast, Voriconazole and Zopiclone

BACKGROUND AND OBJECTIVES

The role of metabolite(s) to elicit potential clinical drug-drug interaction (DDI) via cytochrome P450 enzymes (CYP) is gaining momentum. In this context, the role of N-oxides for in vitro CYP inhibition has not been evaluated. The objectives of this study were: (a) to examine in vitro CYP inhibition of N-oxides of clozapine, levofloxacin, roflumilast, voriconazole and zopiclone in a tiered approach and (b) evaluate in vitro fate of aforementioned N-oxides examined in recombinant CYPs, human microsomes and hepatocytes.

METHODS

CYP enzymes evaluated in the work included: CYP1A2, 2B6, 2C9, 2C19, 2D6 and 3A4 using standard procedures for incubation with appropriate probe substrates. The initial cutoff for CYP inhibition was ≥50% using 2 and 10 µM concentrations of various N-oxide metabolites (Tier 1). IC₅₀ values were constructed for the CYP pathway(s) that showed ≥50% inhibition (Tier 2). In addition, co-incubation of N-oxides with parent was performed to evaluate potentiation of CYP inhibition (Tier 3).

RESULTS

N-oxides of clozapine (CYP2B6/2C19) and voriconazole (CYP2C9/3A4) showed CYP inhibition ≥50%. Clozapine-N-oxide inhibited CYP2B6 and CYP2C19 pathways with IC₅₀ of 8.3 and 8.7 µM, respectively. Voriconazole-N-oxide inhibited CYP2B6 and CYP2C19 pathways with IC₅₀ of 10.5 and 11.2 µM, respectively. Co-incubation of clozapine-N-oxide with clozapine potentiated CYP2B6/2C19 pathways; however, incubation of voriconazole-N-oxide with voriconazole did not appear to potentiate the CYP pathways because parent caused an inhibition of almost 80%. None of the N-oxides appeared to further undergo biotransformation as judged by the in vitro metabolic fate experiments (stage 2).

CONCLUSIONS

Clinical DDI potential of specific CYP enzymes needs to be considered arising due to circulatory concentrations of certain N-oxides depending on the dose size and/or frequency of dosing of the respective parent drugs.

7-Hydroxylation of warfarin is strongly inhibited by sesamin, but not by episesamin, caffeic and ferulic acids in human hepatic microsomes

Warfarin is a commonly used anticoagulant drug and is a derivate of coumarin. Cytochrome P450 2C9 (CYP2C9) plays the key role in transformation of coumarin and thus, influences determination of warfarin dosage. A number of factors including dietary compounds such as sesamin, caffeic acid and ferulic acids can regulate the activity of CYP2C9. The present study tested the hypothesis that sesamin, episesamin, caffeic acid and ferulic acid decreases the rate of warfarin 7-hydroxylation via inhibition of hepatic CYP2C9. The experiments were conducted on hepatic microsomes from human donors. It was demonstrated that the rate of 7-hydroxylation of warfarin was significantly decreased in the presence of sesamin in the range of concentrations from 5 to 500 nM, and was not affected by episesamin, caffeic acid and ferulic acid in the same range of concentrations. The kinetic analysis indicated non-competitive type of inhibition by sesamin with Ki = 202 ± 18 nM. In conclusion, the results of our in vitro study revealed that sesamin was able to inhibit formation of a major metabolite of warfarin, 7-hydroxywarfarin. The potentially negative consequences of the consumption of high amounts of sesamin-containing food or dietary supplements in warfarin-treated patients need to be further studied.

Pharmacokinetics of Tecarfarin and Warfarin in Patients with Severe Chronic Kidney Disease

Chronic kidney disease (CKD) complicates warfarin anticoagulation partially through its effect on CYP2C9 activity. Tecarfarin, a novel vitamin K antagonist, is not metabolized by CYP2C9. To evaluate the effect of CKD on their metabolism, we measured PK parameters of warfarin and tecarfarin in subjects with and without CKD. CKD subjects with estimated glomerular filtration rate < 30 mL/min not on dialysis (n = 13) were matched to healthy volunteers (HVs) (n = 10). Each subject was randomized to either warfarin 10 mg or tecarfarin 30 mg and was later crossed over to the other drug. PK parameters were measured following each drug. Mean plasma concentrations of (S)-warfarin and (R,S)-warfarin were higher (44 and 27%, respectively) in the subjects with CKD than in the healthy subjects. Both of these values fell outside of the 90% confidence interval of equivalence. For tecarfarin, the difference was less than 15% higher. Elimination half-life (t_1/2) increased by 20% for (S)-warfarin and by 8% for (R,S)-warfarin and decreased by 8% for tecarfarin. The mean plasma concentration for tecarfarin's inactive metabolite ATI-5900 increased by approximately eightfold. CKD increased the effect of CYP2C9 genetic variation on (S)-warfarin and (R,S)-warfarin metabolism. Tecarfarin exposure was similar between the HVs and the CKD subjects regardless of CYP2C9 genotype. There were neither serious adverse events (SAEs) nor treatment-emergent adverse events (TEAEs) for any subject in the study. CKD inhibits metabolism of (S)-warfarin and (R,S)-warfarin, but not tecarfarin. The safety of repeated dosing of tecarfarin in CKD patients remains unknown. However, if the PK findings of this single-dose study are present with repeated dosing, tecarfarin may lead to dosing that is more predictable than warfarin in CKD patients who require anticoagulation therapy.

The impact of non-genetic and genetic factors on a stable warfarin dose in Thai patients

Purpose

The aim of this study was to investigate the contributions of non-genetic and genetic factors on the variability of stable warfarin doses in Thai patients.

Methods

A total of 250 Thai patients with stable warfarin doses were enrolled in the study. Demographics and clinical data, e.g., age, body mass index, indications for warfarin and concomitant medications, were documented. Four single nucleotide polymorphisms in the VKORC1 − 1639G > A, CYP2C9*3, CYP4F2 rs2108622, and UGT1A1 rs887829 genes were detected from gDNA using TaqMan allelic discrimination assays.

Results

The patients with variant genotypes of VKORC1 − 1639G > A required significantly lower warfarin stable weekly doses (SWDs) than those with wild-type genotype (p < 0.001). Similarly, the patients with CYP2C9*3 variant allele required significantly lower warfarin SWDs than those with homozygous wild-type (p = 0.006). In contrast, there were no significant differences in the SWDs between the patients who carried variant alleles of CYP4F2 rs2108622 and UGT1A1 rs887829 as compared to wild-type allele carriers. Multivariate analysis, however, showed that CYP4F2 rs2108622 TT genotype accounted for a modest part of warfarin dose variability (1.2%). In contrast, VKORC1 − 1639G > A, CYP2C9*3, CYP4F2 rs2108622 genotypes and non-genetic factors accounted for 51.3% of dose variability.

Conclusions

VKORC1 − 1639G > A, CYP2C9*3, and CYP4F2 rs2108622 polymorphisms together with age, body mass index, antiplatelet drug use, amiodarone use, and current smoker status explained 51.3% of individual variability in stable warfarin doses. In contrast, the UGT1A1 rs887829 polymorphism did not contribute to dose variability.

Electronic supplementary material

The online version of this article (doi:10.1007/s00228-017-2265-8) contains supplementary material, which is available to authorized users.

Influence of UDP-Glucuronosyltransferase Polymorphisms on Stable Warfarin Doses in Patients with Mechanical Cardiac Valves

AIM

This study aimed to evaluate the effect of uridine diphosphate (UDP)-glucuronosyltransferase (UGT) polymorphisms on warfarin dosing requirements in patients with mechanical cardiac valves.

METHODS

A total of 191 patients with stable warfarin doses from the EAST Group of Warfarin were included in this study. The influence of genetic polymorphisms on stable warfarin doses was investigated by genotyping 6 single nucleotide polymorphisms (SNPs): vitamin K epoxide reductase complex 1 (VKORC1) rs9934438, cytochrome P450 (CYP) 2C9 rs1057910, CYP4F2 rs2108622, and UGT1A1 (rs887829, rs4148323, and rs4124874). An additional subgroup analysis was carried out using patients with wild-type homozygote carriers of CYP2C9.

RESULTS

One UGT1A1 SNP of rs887829 (C>T) exhibited significant association with stable warfarin doses in the study population and subgroup. Patients with the T allele in UGT1A1 rs887829 (CT or TT) required higher doses than those with the CC genotype in the study population (6.3 ± 2.4 mg vs. 5.2 ± 1.6 mg, P = 0.003). Similarly, in the subpopulation of AA carriers in the CYP2C9 gene, patients with the T allele required significantly higher doses of warfarin than those with other genotypes of rs887829 (6.5 ± 2.4 vs. 5.3 ± 1.5 mg, P = 0.002). Approximately 45.1% of overall interindividual variability in warfarin dose requirement was explained by the multivariate regression model. VKORC1, CYP2C9, UGT1A1 rs887829, age, and CYP4F2 accounted for 28.2%, 6.6%, 5.5%, 3.0%, and 1.8% of the variability, respectively.

CONCLUSION

Our results suggest that UGT1A1 could be a determinant of stable warfarin doses.

Mechanism of Drug-Drug Interactions Between Warfarin and Statins

The anticoagulant drug warfarin and the lipid-lowering statin drugs are commonly co-administered to patients with cardiovascular diseases. Clinically significant drug-drug interactions (DDIs) between these drugs have been recognized through case studies for many years, but the biochemical mechanisms causing these interactions have not been explained fully. Previous theories include kinetic alterations in cytochrome P-450-mediated drug metabolism or disturbances of drug-protein binding, leading to anticoagulant activity of warfarin; however, neither the enantioselective effects on warfarin metabolism nor the potential disruption of drug transporter function have been well investigated. This study investigated the etiology of the DDIs between warfarin and statins. Liquid chromatography-mass spectrometry methods were developed and validated to quantify racemic warfarin, 6 of its hydroxylated metabolites, and pure enantiomers of warfarin; these methods were applied to study the role of different absorption, distribution, metabolism, and excretion properties, leading to DDIs. Plasma protein binding displacement of warfarin was performed in the presence of statins using equilibrium dialysis method. Substrate kinetics of warfarin and pure enantiomers were performed with human liver microsomes to determine the kinetic parameters (Km and Vmax) for the formation of all 6 hydroxywarfarin metabolites, inhibition of warfarin metabolism in the presence of statins, was determined. Uptake transport studies of warfarin were performed using overexpressing HEK cell lines and efflux transport using human adenocarcinoma colonic cell line cells. Fluvastatin significantly displaced plasma protein binding of warfarin and pure enantiomers; no other statin resulted in significant displacement of warfarin. All the statins that inhibited the formation of 10-hydroxywarfarin, atorvastatin, pitavastatin, and simvastatin were highly potent compared to other statins; in contrast, only fluvastatin was found to be a potent inhibitor of formation of 7-hydroxy warfarin. Uptake and efflux drug transporters do not play any role in these DDIs. The results showed that DDIs between warfarin and statins are primarily caused by cytochrome P-450 inhibition.

Cytochrome P450-mediated warfarin metabolic ability is not a critical determinant of warfarin sensitivity in avian species: In vitro assays in several birds and in vivo assays in chicken

Coumarin-derivative anticoagulant rodenticides used for rodent control are posing a serious risk to wild bird populations. For warfarin, a classic coumarin derivative, chickens have a high median lethal dose (LD50), whereas mammalian species generally have much lower LD50. Large interspecies differences in sensitivity to warfarin are to be expected. The authors previously reported substantial differences in warfarin metabolism among avian species; however, the actual in vivo pharmacokinetics have yet to be elucidated, even in the chicken. In the present study, the authors sought to provide an in-depth characterization of warfarin metabolism in birds using in vivo and in vitro approaches. A kinetic analysis of warfarin metabolism was performed using liver microsomes of 4 avian species, and the metabolic abilities of the chicken and crow were much higher in comparison with those of the mallard and ostrich. Analysis of in vivo metabolites from chickens showed that excretions predominantly consisted of 4'-hydroxywarfarin, which was consistent with the in vitro results. Pharmacokinetic analysis suggested that chickens have an unexpectedly long half-life despite showing high metabolic ability in vitro. The results suggest that the half-life of warfarin in other bird species could be longer than that in the chicken and that warfarin metabolism may not be a critical determinant of species differences with respect to warfarin sensitivity.

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.

Induction of liver cytochrome P450s by Danshen-Gegen formula is the leading cause for its pharmacokinetic interactions with warfarin

ETHNOPHARMACOLOGICAL RELEVANCE

Although the increased usage of herbal medicine leading to herb-drug interactions is well reported, the mechanism of such interactions between herbal medicines with conventionally prescribed drugs such as warfarin is not yet fully understood. Our previous rat in vivo study demonstrated that co-administration of Danshen-Gegen Formula (DGF), a Radix Salvia miltiorrhiza (Danshen) and Radix Puerariae lobatae (Gegen) containing Chinese medicine formula recently developed for the treatment of cardiovascular disease, with warfarin could cause significant herb-drug interactions. The current study aims to explore the pharmacokinetics-based mechanism of the DGF-warfarin interactions during absorption, distribution and metabolism processes.

MATERIALS AND METHODS

Caco-2 cell monolayer model and rat in situ intestinal perfusion model were used to study the DGF-warfarin interactions during the intestinal absorption processes. Male Sprague-Dawley rats were orally administered warfarin in presence and absence of DGF for consecutive 5 days. The microsomal activity and expression of the liver CYP isozymes were determined and compared among different treatment groups. Blood from the rats administered DGF was employed to evaluate effects of DGF on the plasma protein binding of warfarin.

RESULTS

Absorption studies demonstrated that DGF could potentially increase the intestinal absorption of warfarin (32% and 75% increase of warfarin Papp in Caco-2 and intestinal perfusion models, respectively) via altering the regional pH environment in GI tract. DGF administration could lead to significant increase in liver microsomal activity and mRNA expression of CYP1A1 and CYP2B1, indicating the potential induction on the liver metabolism of warfarin by DGF. Moreover, it has been proven by ex vivo study that the single-dose administration of DGF could decrease the protein binding of warfarin in plasma by at least 11.6%.

CONCLUSION

Collectively, current study demonstrated that DGF could significantly induce the liver phase I metabolism of warfarin, and to a less extent, potentially increase the intestinal absorption and decrease the plasma protein binding of warfarin. The inductive effects of DGF on the liver phase I metabolism of warfarin may be dominantly responsible for the DGF-warfarin pharmacokinetics interactions.

A cellular system for quantitation of vitamin K cycle activity: structure-activity effects on vitamin K antagonism by warfarin metabolites

Warfarin and other 4-hydroxycoumarins inhibit vitamin K epoxide reductase (VKOR) by depleting reduced vitamin K that is required for posttranslational modification of vitamin K-dependent clotting factors. In vitro prediction of the in vivo potency of vitamin K antagonists is complicated by the complex multicomponent nature of the vitamin K cycle. Here we describe a sensitive assay that enables quantitative analysis of γ-glutamyl carboxylation and its antagonism in live cells. We engineered a human embryonic kidney (HEK) 293-derived cell line (HEK 293-C3) to express a chimeric protein (F9CH) comprising the Gla domain of factor IX fused to the transmembrane and cytoplasmic regions of proline-rich Gla protein 2. Maximal γ-glutamyl carboxylation of F9CH required vitamin K supplementation, and was dose-dependently inhibited by racemic warfarin at a physiologically relevant concentration. Cellular γ-glutamyl carboxylation also exhibited differential VKOR inhibition by warfarin enantiomers (S > R) consistent with their in vivo potencies. We further analyzed the structure-activity relationship for inhibition of γ-glutamyl carboxylation by warfarin metabolites, observing tolerance to phenolic substitution at the C-5 and especially C-6, but not C-7 or C-8, positions on the 4-hydroxycoumarin nucleus. After correction for in vivo concentration and protein binding, 10-hydroxywarfarin and warfarin alcohols were predicted to be the most potent inhibitory metabolites in vivo.

Effects of etravirine on the pharmacokinetics and pharmacodynamics of warfarin in rats

BACKGROUND AND PURPOSE

Warfarin is often used with etravirine (ETV) to prevent HIV-related thromboembolic events. As both warfarin and ETV bind to plasma proteins and are metabolized by hepatic cytochrome P450s, they are likely to interact. Hence, we evaluated the effect of ETV on the pharmacokinetics and blood clotting time of racemic warfarin in rats.

EXPERIMENTAL APPROACH

Two groups of male Sprague-Dawley rats, in which the jugular vein had been cannulated, were studied. The control group (n = 10) received 1 mg·kg(-1) racemic warfarin i.v., and the test group (n = 13) 1 mg·kg(-1) of racemic warfarin followed by 25 mg·kg(-1) ETV i.v. Serial blood samples were collected for up to 144 h and the blood clotting time (calculated as international normalized ratio [INR]) measured in blood plasma at each sample point. Plasma concentrations of R-warfarin, S-warfarin, R-7-hydroxywarfarin and S-7-hydroxywarfarin were measured by a LC/MS/MS method using a chiral lux cellulose-1 column. Pharmacokinetic parameters were analysed using non-compartmental methods.

KEY RESULTS

ETV significantly increased, by threefold, the systemic clearance and volume of distribution of S-warfarin, but not those of R-warfarin. ETV decreased the total AUC of warfarin, but had no effect on its elimination half-life. ETV also increased the systemic clearance of both R-7-hydroxywarfarin and S-7-hydroxywarfarin but only increased the volume of distribution of R-7-hydroxywarfarin. Interestingly, the effect of warfarin on blood clotting time (INR) was significantly increased in the presence of etravirine.

CONCLUSION AND IMPLICATIONS

Our data suggest that etravirine may potentiate the anticoagulant effect of warfarin and this could have clinical significance.

Metabolism of R- and S-warfarin by CYP2C19 into four hydroxywarfarins

Coumadin (R/S-warfarin) is a highly efficacious and widely used anticoagulant; however, its highly variable metabolism remains an important contributor to uncertainties in therapeutic responses. Pharmacogenetic studies report conflicting findings on the clinical relevance of CYP2C19. A resolution to this controversy is impeded by a lack of de tailon the potential role of CYP2C19 in warfarin metabolism. Consequently, we assessed the efficiency of CYP2C19 metabolism of R- and S-warfarin and explored possible contributions in the liver using in vitro methods. Recombinant CYP2C19 metabolized R- and S-warfarin mainly to 6-, 7-, and 8-hydroxywarfarin, while 4'-hydroxywarfarin was a minormetabolite. Over all R-warfarin metabolism was slightly more efficient than that for S-warfarin. Metabolic pathways thatproduce R-6-, 7-, and 8-hydroxywarfarin in human liver microsomal reactions correlated strongly with CYP2C19 Smephenytoinhydroxylase activity. Similarly, CYP1A2 activity toward phenacetin correlated with formation of R-6 and 7-hydroxywarfarin such that R-8-hydroxywarfarin seems unique to CYP2C19 and possibly a biomarker. In following, CYP2C19 likely impacts R-warfarin metabolism and patient response to therapy. Intriguingly, CYP2C19 may contributeto S-warfarin metabolism in patients, especially when CYP2C9 activity is compromised due to drug interactions orgenetic polymorphisms.

In vitro-to-in vivo predictions of drug-drug interactions involving multiple reversible inhibitors

INTRODUCTION

Predictions of drug-drug interactions (DDIs) are commonly performed for single inhibitors, but interactions involving multiple inhibitors also frequently occur. Predictions of such interactions involving stereoisomer pairs, parent/metabolite combinations and simultaneously administered multiple inhibitors are increasing in importance. This review provides the framework for predicting inhibitory DDIs of multiple inhibitors with any combination of reversible inhibition mechanism.

AREAS COVERED

The review provides an overview of the reliability of the in vitro determined reversible inhibition mechanism. Furthermore, the article provides a method to predict DDIs for multiple reversible inhibitors that allows substituting the inhibition constant (K(i)) with an inhibitor affinity (IC(50)) value determined at S << K(M).

EXPERT OPINION

A better understanding and the prediction methods of DDIs, resulting from multiple inhibitors, are important. The inhibition mechanism of a reversible inhibitor is often equivocal across studies and unreliable. Determination of the K(i) requires the assignment of reversible inhibition mechanism but in vitro-to-in vivo prediction of DDI risk can be achieved for multiple inhibitors from estimates of the inhibitor affinity (IC(50)) only, regardless of the inhibition mechanism.

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.

Warfarin toxicity and individual variability-clinical case

Warfarin is a widely used anticoagulant in the treatment and prevention of thrombosis, in the treatment for chronic atrial fibrillation, mechanical valves, pulmonary embolism, and dilated cardiomyopathy. It is tasteless and colorless, was used as a poison, and is still marketed as a pesticide against rats and mice. Several long-acting warfarin derivatives-superwarfarin anticoagulants-such as brodifacoum, diphenadione, chlorophacinone, bromadiolone, are used as pesticides and can produce profound and prolonged anticoagulation. Several factors increase the risk of warfarin toxicity. However, polymorphisms in cytochrome P450 genes and drug interactions account for most of the risk for toxicity complications. Each person is unique in their degree of susceptibility to toxic agents. The toxicity interpretation and the health risk of most toxic substances are a subject of uncertainty. Genetically determined low metabolic capacity in an individual can dramatically alter the toxin and metabolite levels from those normally expected, which is crucial for drugs with a narrow therapeutic index, like warfarin. Personalized approaches in interpretation have the potential to remove some of the scientific uncertainties in toxicity cases.