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

A physiologically-based pharmacokinetic/pharmacodynamic modeling approach for drug-drug-gene interaction evaluation of S-warfarin with fluconazole

Warfarin is a widely used anticoagulant, and its S-enantiomer has higher potency compared to the R-enantiomer. S-warfarin is mainly metabolized by cytochrome P450 (CYP) 2C9, and its pharmacological target is vitamin K epoxide reductase complex subunit 1 (VKORC1). Both CYP2C9 and VKORC1 have genetic polymorphisms, leading to large variations in the pharmacokinetics (PKs) and pharmacodynamics (PDs) of warfarin in the population. This makes dosage management of warfarin difficult, especially in the case of drug-drug interactions (DDIs). This study provides a whole-body physiologically-based pharmacokinetic/PD (PBPK/PD) model of S-warfarin for predicting the effects of drug-drug-gene interactions on S-warfarin PKs and PDs. The PBPK/PD model of S-warfarin was developed in PK-Sim and MoBi. Drug-dependent parameters were obtained from the literature or optimized. Of the 34 S-warfarin plasma concentration-time profiles used, 96% predicted plasma concentrations within twofold range compared to observed data. For S-warfarin plasma concentration-time profiles with CYP2C9 genotype, 364 of 386 predicted plasma concentration values (~94%) fell within the twofold of the observed values. This model was tested in DDI predictions with fluconazole as CYP2C9 perpetrators, with all predicted DDI area under the plasma concentration-time curve to the last measurable timepoint (AUClast) ratio within twofold of the observed values. The anticoagulant effect of S-warfarin was described using an indirect response model, with all predicted international normalized ratio (INR) within twofold of the observed values. This model also incorporates a dose-adjustment method that can be used for dose adjustment and predict INR when warfarin is used in combination with CYP2C9 perpetrators.

Safflower yellow for injection enhances anti-coagulation of warfarin in rats: implications in pharmacodynamics and pharmacokinetics

This study investigated whether Safflower Yellow for injection (SYI) would affect the anticoagulation of warfarin in rats.Wistar male rats were divided into six groups randomly and administered with SYI (9 mg/kg, intraperitoneal injection) in single-dose and steady-dose warfarin (0.2 mg/kg, oral gavage), respectively. The pharmacodynamic parameters of PT and APTT were measured by a coagulation analyser. R/S-warfarin concentration was measured by UHPLC-MS/MS, and pharmacokinetic parameters calculated using DAS 2.0 software.The single-dose study demonstrated that SYI, alone or co-administered with warfarin, could significantly increase PT, INR, and APTT values (p < 0.01). R-warfarin Cmax, AUC, and t1/2 values increased by 9.25% (p > 0.05), 25.96% (p < 0.01), and 26.17% (p < 0.01), respectively, whereas the CL/F value reduced by 22.22% (p < 0.01) in the presence of SYI. Meanwhile, S-warfarin Cmax, AUC, and t1/2 values increased by 37.41%, 32.11%, and 31.73% (all p < 0.01), respectively, whereas the CL/F value reduced by 33.33% (p < 0.01). The steady-dose study showed that PT, INR, APTT, and the concentrations of R/S-warfarin increased significantly when SYI was co-administered with warfarin (p < 0.01).SYI can enhance warfarin's anticoagulation intensity and decelerate its metabolism in rats.

Simultaneous Characterization and Determination of Warfarin and Its Hydroxylation Metabolites in Rat Plasma by Chiral Liquid Chromatography-Tandem Mass Spectrometry

Warfarin is extensively used for venous thromboembolism and other coagulopathies. In clinical settings, warfarin is administered as a mixture of S- and R-warfarin, and both enantiomers are metabolized by multiple cytochrome P450 enzymes into many hydroxylation metabolites. Due to the high degree of structural similarity of hydroxylation metabolites, their profile possesses significant challenges. The previous methods generally suffer from lacking baseline resolution and/or involving complex analysis processes. To overcome this limitation, a sensitive and specific chiral liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was developed to simultaneously identify warfarin and hydroxywarfarins enantiomers. Chromatographic separation was achieved on a HYPERSIL CHIRAL-OT column. The mass spectrometric detection was carried out in negative ion MRM mode with electrospray ionization source. The optimized method exhibited satisfactory within-run and between-run accuracy and precision with lower limit of quantification (LLOQ) of 10.0 ng/mL and 1.0 ng/mL for warfarin and 7-, 10(R)-OH-warfarin enantiomers, respectively. Linear responses of warfarin enantiomers and 7-, and 10(R)-OH-warfarin enantiomers in rat plasma were observed over the range of 10.0–8000 ng/mL, and 1.00–800 ng/mL, respectively. The analytes were shown to be stable in various experimental conditions in rat plasma. Protein precipitation was used in sample preparation without a matrix effect. This method was successfully applied to pharmacokinetic study for quantitating the concentrations of S/R-warfarin, S/R-7-OH-warfarin, and S/R-10(R)-OH-warfarin and relatively quantitating 3′-, 4-, 6-, and 8-OH warfarin enantiomers in rat plasma.

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.

The Impairment in Kidney Function in the Oral Anticoagulation Era. A Pathophysiological Insight

The need for anticoagulation in patients with atrial fibrillation (AF) is fundamental to prevent thromboembolic events. Direct oral anticoagulants (DOACs) recently demonstrated to be superior, or at least equal, to Warfarin in reducing the risk for stroke/systemic embolism and preventing major bleeding and intracranial hemorrhages. The AF population often suffers from chronic kidney disease (CKD). Indeed, the relationship between AF and renal function is bidirectional: AF can trigger kidney failure, while kidney impairment can promote alterations able to enhance AF. Therefore, there are concerns regarding prescriptions of anticoagulants to patients with AF and CKD. The worsening in kidney function can be effectively due to anticoagulants administration. Warfarin has been recognized to promote acute kidney injury in case of excessive anticoagulation levels. Nevertheless, further mechanisms can induce the chronic worsening of renal function, thus leading to terminal kidney failure as observed in post-hoc analysis from registration trials and dedicated observational studies. By contrast, DOACs seem to protect kidneys from injuries more efficiently than Warfarin, although they still continue to play a role in promoting some kidney lesions. However, the exact mechanisms remain unknown. This narrative review aimed to discuss the influence of oral anticoagulants on renal impairment as well as to overview potential pathophysiological mechanisms related to this clinical complication.

Association between four microRNA binding site-related polymorphisms and the risk of warfarin-induced bleeding complications

Bleeding is the most serious complication of warfarin anticoagulation therapy and is known to occur even at patients with therapeutic international normalized ratio (INR) range. Recently, it has been shown that microRNAs play a significant role in pharmacogenetics by regulating genes that are critical for drug function. Interaction between microRNAs and these target genes could be affected by single-nucleotide polymorphisms (SNPs) located in microRNA-binding sites. This study focused on 3′-untranslated region (3′-UTR) SNPs of the genes involved in the warfarin action and the occurrence of bleeding complications in an Iranian population receiving warfarin. A total of 526 patients under warfarin anticoagulation therapy with responding to the therapeutic dose and maintenance of the INR in the range of 2.0-3.5 in three consecutive blood tests were included in the study. Four selected 3'-UTR SNPs (rs12458, rs7294, rs1868774 and rs34669593 located in GATA4, VKORC1, CALU and GGCX genes, respectively) with the potential to disrupt/eliminate or enhance/create microRNA-binding site were genotyped using a simple PCR-based restriction fragment length polymorphism (PCR-RFLP) method. Patients with the rs12458 AT or TT genotypes of the GATA4 gene had a lower risk of bleeding compared to patients with the AA genotype (adjusted OR: 0.478, 95% CI: 0.285-0.802, P= 0.005, OR: 0.416, 95% CI: 0.192-0.902, P= 0.026, respectively). 3'-UTR polymorphisms in other genes were not significantly associated with the risk of bleeding complications. In conclusion, the SNP rs12458A>T in the 3′UTR region of GATA4 is associated with the incidence of warfarin-related bleeding at target range of INR, likely by altering microRNA binding and warfarin metabolism. Further genetics association studies are needed to validate these findings before they can be implemented in clinical settings.

Novel isomeric metabolite profiles correlate with warfarin metabolism phenotype during maintenance dosing in a pilot study of 29 patients

: For this pilot study, we leveraged metabolite patterns for warfarin patients to more accurately assess clinically relevant differences in drug metabolism. We tested our hypothesis that plasma metabolite levels correlate with the influence of clinical factors on R-warfarin and S-warfarin metabolism (warfarin metabolic phenotype). We recruited 29 patients receiving a maintenance dose and testing within targeted therapeutic range. We determined their CYP2C9 and vitamin K epoxide reductase genotype and profiled 14 isomeric forms of warfarin and its metabolites. We employed three novel types of clearance ratios using analyte levels to perform multiple-linear regression analyses with clinical factors impacting drug metabolism and dose-responses. Competitive clearance ratios correlated with seven clinical factors including lifestyle choices (smoking), genetics (CYP2C9 and vitamin K epoxide reductase 1), and drug interactions (omeprazole) along with age, weight, and malignancy. Significant competitive clearance ratio correlations (P = 0.04 to < 0.001) explained 21-95% variability. Their performances surpassed that of oxidative and metabolic clearance ratios based on the number and significance of correlations. Competitive clearance ratios may accurately assess significance of factors on maintaining levels of pharmacologically active forms of the drug and metabolites related to dose-responses and thus provide a strategy to minimize adverse events and improve safety during anticoagulant therapy. This unique capacity could provide a strategy in a future, higher power study with a larger cohort of patients to more accurately assess the significance of clinical factors on active drug levels contributing to warfarin dose-responses.

Regioselectivity significantly impacts microsomal glucuronidation efficiency of R/S-6, 7-, and 8-hydroxywarfarin

Coumadin (R/S-warfarin) metabolism plays a critical role in patient response to anticoagulant therapy. Several cytochrome P450s oxidize warfarin into R/S-6-, 7-, 8-, 10, and 4'-hydroxywarfarin that can undergo subsequent glucuronidation by UDP-glucuronosyltransferases (UGTs); however, current studies on recombinant UGTs cannot be adequately extrapolated to microsomal glucuronidation capacities for the liver. Herein, we estimated the capacity of the average human liver to glucuronidate hydroxywarfarin and identified UGTs responsible for those metabolic reactions through inhibitor phenotyping. There was no observable activity toward R/S-warfarin, R/S-10-hydroxywarfarin or R/S-4'-hydroxywarfarin. The observed metabolic efficiencies (V_max/K_m) toward R/S-6-, 7-, and especially 8-hydroxywarfarin indicated a high glucuronidation capacity to metabolize these compounds. UGTs demonstrated strong regioselectivity toward the hydroxywarfarins. UGT1A6 and UGT1A1 played a major role in R/S-6- and 7-hydroxywarfarin glucuronidation, respectively, whereas UGT1A9 accounted for almost all of the generation of the R/S-8-hydroxywarfarin glucuronide. In summary, these studies expanded insights to glucuronidation of hydroxywarfarins by pooled human liver microsomes, novel roles for UGT1A6 and 1A9, and the overall degree of regioselectivity for the UGT reactions.

Stereospecific Metabolism of R- and S-Warfarin by Human Hepatic Cytosolic Reductases

Coumadin (rac-warfarin) is the most commonly used anticoagulant in the world; however, its clinical use is often challenging because of its narrow therapeutic range and interindividual variations in response. A critical contributor to the uncertainty is variability in warfarin metabolism, which includes mostly oxidative but also reductive pathways. Reduction of each warfarin enantiomer yields two warfarin alcohol isomers, and the corresponding four alcohols retain varying levels of anticoagulant activity. Studies on the kinetics of warfarin reduction have often lacked resolution of parent-drug enantiomers and have suffered from coelution of pairs of alcohol metabolites; thus, those studies have not established the importance of individual stereospecific reductive pathways. We report the first steady-state analysis of R- and S-warfarin reduction in vitro by pooled human liver cytosol. As determined by authentic standards, the major metabolites were 9R,11S-warfarin alcohol for R-warfarin and 9S,11S-warfarin alcohol for S-warfarin. R-warfarin (Vmax 150 pmol/mg per minute, Km 0.67 mM) was reduced more efficiently than S-warfarin (Vmax 27 pmol/mg per minute, Km 1.7 mM). Based on inhibitor phenotyping, carbonyl reductase-1 dominated R-and S-warfarin reduction, followed by aldo-keto reductase-1C3 and then other members of that family. Overall, the carbonyl at position 11 undergoes stereospecific reduction by multiple enzymes to form the S alcohol for both drug enantiomers, yet R-warfarin undergoes reduction preferentially. This knowledge will aid in assessing the relative importance of reductive pathways for R- and S-warfarin and factors influencing levels of pharmacologically active parent drugs and metabolites, thus impacting patient dose responses.

Development of an Ecofriendly Anticoagulant Rodenticide Based on the Stereochemistry of Difenacoum

Difenacoum, an antivitamin K anticoagulant, has been widely used as rodenticide to manage populations of rodents. Difenacoum belongs to the second generation of anticoagulant, and, as all the molecules belonging to the second generation of anticoagulant, difenacoum is often involved in primary poisonings of domestic animals and secondary poisonings of wildlife by feeding contaminated rodents. To develop a new and ecofriendly difenacoum, we explored in this study the differences in properties between diastereomers of difenacoum. Indeed, the currently commercial difenacoum is a mixture of 57% of cis-isomers and 43% of trans-isomers. Cis- and trans-isomers were thus purified on a C18 column, and their respective pharmacokinetic properties and their efficiency to inhibit the coagulation of rodents were explored. Tissue persistence of trans-isomers was shown to be shorter than that of cis-isomers with a half-life fivefold shorter. Efficiency to inhibit the vitamin K epoxide reductase activity involved in the coagulation process was shown to be similar between cis- and trans-isomers. The use of trans-isomers of difenacoum allowed to drastically reduce difenacoum residues in liver and other tissues of rodents when the rodent is moribund. Therefore, secondary poisonings of wildlife should be decreased by the use of difenacoum largely enriched in trans-isomers.

The emerging threat of superwarfarins: history, detection, mechanisms, and countermeasures

Superwarfarins were developed following the emergence of warfarin resistance in rodents. Compared to warfarin, superwarfarins have much longer half-lives and stronger affinity to vitamin K epoxide reductase and therefore can cause death in warfarin-resistant rodents. By the mid-1970s, the superwarfarins brodifacoum and difenacoum were the most widely used rodenticides throughout the world. Unfortunately, increased use was accompanied by a rise in accidental poisonings, reaching >16,000 per year in the United States. Risk of exposure has become a concern since large quantities, up to hundreds of kilograms of rodent bait, are applied by aerial dispersion over regions with rodent infestations. Reports of intentional use of superwarfarins in civilian and military scenarios raise the specter of larger incidents or mass casualties. Unlike warfarin overdose, for which 1-2 days of treatment with vitamin K is effective, treatment of superwarfarin poisoning with vitamin K is limited by extremely high cost and can require daily treatment for a year or longer. Furthermore, superwarfarins have actions that are independent of their anticoagulant effects, including both vitamin K-dependent and -independent effects, which are not mitigated by vitamin K therapy. In this review, we summarize superwarfarin development, biology and pathophysiology, their threat as weapons, and possible therapeutic approaches.

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.

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.

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.