Individualizing warfarin therapy

Polypharmacy: a healthcare conundrum with a pharmacogenetic solution

The use of multiple medications is growing at an alarming rate with some reports documenting an average of 12-22 prescriptions being used by individuals ≥50 years of age. The indirect consequences of polypharmacy include exacerbation of drug-drug interactions, adverse drug reactions, increased likelihood of prescribing cascades, chronic dependence, and hospitalizations - all of which have significant health and economic burden. While many practical solutions for reducing polypharmacy have been proposed, they have been met with limited efficacy. This highlights the need for a new systematic approach for fine-tuning dispensing of medications. Pharmacogenetic testing provides an empirical and scientifically rigorous approach for guiding appropriate selection of medicines, with the potential to reduce unnecessary polypharmacy while improving clinical outcomes. The goal of this review article is to provide healthcare providers with an understanding of polypharmacy, its adverse effects on the healthcare system and highlight how pharmacogenetic information can be used to avoid polypharmacy in patients.

Formulation development of a continuously manufactured orodispersible film containing warfarin sodium for individualized dosing

Continuously manufactured orodispersible films (ODFs) offer a promising approach for individualized therapy with an easy to administer solid dosage form. The aim of this study was to develop a long ODF containing warfarin sodium to enable safe and more flexible dosing. Formulation development was conducted systematically for the continuous film coating process. A continuously working pilot-scale coating bench was used for film manufacturing and the viscosities of the polymer solutions were investigated to obtain processible formulations. The investigation of the mechanical properties of the long film was an integral part of the study, because the handling of the long film during flexible dosing differs distinctly from the handling of a single dosed ODF. The secant modulus and the yield stress were evaluated as parameters with high information value about the deformation behavior of the ODF. A long warfarin ODF was successfully produced using the pilot-scale coating bench equipped with an optical probe for in-line film thickness measurement. It was feasible to use the principle of a tape dispenser for flexible and, therefore, individualized dosing as proof of concept. Combining the long ODF with a dosing device allows individualized therapy with warfarin for all age groups manageable by the patient himself.

The Pharmacist's Perspective on Pharmacogenetics Implementation

The future for pharmacogenetics will continue to expand. Pharmacists can apply and incorporate drug knowledge in collaboration with other health providers using pharmacogenetics. Patients benefit with enhanced therapeutic outcomes that could lead to more streamlined drug approaches, fewer follow-up visits, cost savings, and shorter times to achieve therapeutic outcomes. As more drug-gene pathways are discovered and use of this knowledge increases, the potential for algorithm development for medication use will occur, resulting in better patient outcomes, higher standard of care, and reflect evidence-based medicine.

A Novel Admixture-Based Pharmacogenetic Approach to Refine Warfarin Dosing in Caribbean Hispanics

AIM

This study is aimed at developing a novel admixture-adjusted pharmacogenomic approach to individually refine warfarin dosing in Caribbean Hispanic patients.

PATIENTS & METHODS

A multiple linear regression analysis of effective warfarin doses versus relevant genotypes, admixture, clinical and demographic factors was performed in 255 patients and further validated externally in another cohort of 55 individuals.

RESULTS

The admixture-adjusted, genotype-guided warfarin dosing refinement algorithm developed in Caribbean Hispanics showed better predictability (R2 = 0.70, MAE = 0.72mg/day) than a clinical algorithm that excluded genotypes and admixture (R2 = 0.60, MAE = 0.99mg/day), and outperformed two prior pharmacogenetic algorithms in predicting effective dose in this population. For patients at the highest risk of adverse events, 45.5% of the dose predictions using the developed pharmacogenetic model resulted in ideal dose as compared with only 29% when using the clinical non-genetic algorithm (p<0.001). The admixture-driven pharmacogenetic algorithm predicted 58% of warfarin dose variance when externally validated in 55 individuals from an independent validation cohort (MAE = 0.89 mg/day, 24% mean bias).

CONCLUSIONS

Results supported our rationale to incorporate individual's genotypes and unique admixture metrics into pharmacogenetic refinement models in order to increase predictability when expanding them to admixed populations like Caribbean Hispanics.

TRIAL REGISTRATION

ClinicalTrials.gov NCT01318057.

A rolling-horizon pharmacokinetic pharmacodynamic model for warfarin inpatients in transient clinical states

Aim: To design a pharmacokinetic pharmacodynamic model to make individualized and adaptive international normalized ratio (INR) predictions for warfarin inpatients in changing clinical status. Methods: We tested a new model on 60 inpatients at Columbia University. The model personalizes four submodels and minimizes the number of parameters to be estimated. Prediction accuracy was assessed by prediction error, absolute prediction error and percentage absolute prediction error. Results: The INRs were accurately predicted 5 days into the future. Median prediction error: 0.01–0.12; median absolute prediction error: 0.17–0.5 and median percentage absolute prediction error: 9.85–26.06%. Conclusion: Patients exhibit interindividual and intertemporal variability. The model captures the variability and provides accurate and personalized INR predictions.

Pharmacokinetic and pharmacodynamic interaction of Danshen-Gegen extract with warfarin and aspirin

ETHNOPHARMACOLOGICAL RELEVANCE

Danshen-Gegen (DG) product has clinically been proven to be an effective agent for heart-tonic efficacy by our previous research. In the mean time, herb-drug interactions between DG product and its commonly co-administered drugs, such as aspirin or warfarin need to be explored to ensure its safe clinical usage.

AIM OF THE STUDY

Current study aims to investigate whether DG extract interacts with warfarin or aspirin when administered concomitantly to ensure the safety and efficacy of their usage.

MATERIAL AND METHODS

Five groups of SD rats (n=6/group) received DG alone (0.15 g/kg, human relevant dose), warfarin alone (0.2 mg/kg), warfarin (0.2 mg/kg) in combination with DG (0.15 g/kg), aspirin alone (10.3 mg/kg), or aspirin (10.3 mg/kg) in combination with DG (0.15 g/kg), respectively. DG product was given twice daily for 5 day. Warfain or aspirin were given once daily for 5 day. DG morning doses were given 2 h post that of warfarin/aspirin. Following first dosing on day 5, plasma samples were collected at different time points. For the pharmacodynamic measurement, whole blood was collected at 30 min after DG dosing or at 2.5 h after warfarin/aspirin dosing, and the prothrombin time assay was conducted.

RESULTS

Co-administration of DG with warfarin could significantly decrease the C(max), AUC and the prothrombin time of warfarin (p<0.05). In the mean time, the C(max) and AUC of danshensu, the active bioavailable component of DG were significantly increased (p<0.01) in presence of warfarin. Co-administration of DG with aspirin could significantly increase the C(max) and AUC of both aspirin (p<0.01) and its metabolite salicylic acid (p<0.01) and significantly decrease the prothrombin time of aspirin (p<0.05). However, the pharmacokinetics parameters of danshensu were not significantly affected by aspirin.

CONCLUSION

Our animal study indicated that co-administration of DG with warfarin/aspirin can cause significant pharmacokinetic and pharmacodynamic herb-drug interactions in rat.

Blood-direct InvaderPlus® as a new method for genetic testing

Aim: This study compared the efficiency of Blood-direct InvaderPlus® with existing methods for detecting genetic polymorphisms by using clinical samples, and to determine whether this new method can be used for future clinical studies. Materials & methods: We developed a new method, called Blood-direct InvaderPlus, for detecting genetic polymorphisms. Whole-blood samples were subjected to genetic analysis with the new method and also the conventional method. Results: The results obtained with the new method were consistent with those of direct sequencing. Blood-direct InvaderPlus was able to detect gene polymorphisms of UGT1A1*6, *27, *28, *60, VKORC1 -1639G>A, VKORC1 1173T>C and CYP2C9 *2 and *3 much faster than the conventional methods. Conclusion: Blood-direct InvaderPlus allowed accurate, simple and rapid detection of genetic polymorphisms.

Prospective pilot trial of PerMIT versus standard anticoagulation service management of patients initiating oral anticoagulation

We performed a randomised pilot trial of PerMIT, a novel decision support tool for genotype-based warfarin initiation and maintenance dosing, to assess its efficacy for improving warfarin management. We prospectively studied 26 subjects to compare PerMIT-guided management with routine anticoagulation service management. CYP2C9 and VKORC1 genotype results for 13 subjects randomly assigned to the PerMIT arm were recorded within 24 hours of enrolment. To aid in INR interpretation, PerMIT calculates estimated loading and maintenance doses based on a patient's genetic and clinical characteristics and displays calculated S-warfarin plasma concentrations based on planned or administered dosages. In comparison to control subjects, patients in the PerMIT study arm demonstrated a 3.6-day decrease in the time to reach a stabilised INR within the target therapeutic range (4.7 vs. 8.3 days, p = 0.015); a 12.8% increase in time spent within the therapeutic interval over the first 25 days of therapy (64.3% vs. 55.3%, p = 0.180); and a 32.9% decrease in the frequency of warfarin dose adjustments per INR measurement (38.3% vs. 57.1%, p = 0.007). Serial measurements of plasma S-warfarin concentrations were also obtained to prospectively evaluate the accuracy of the pharmacokinetic model during induction therapy. The PerMIT S-warfarin plasma concentration model estimated 62.8% of concentrations within 0.15 mg/l. These pilot data suggest that the PerMIT method and its incorporation of genotype/phenotype information may help practitioners increase the safety, efficacy, and efficiency of warfarin therapeutic management.

Prediction of warfarin dose reductions in Puerto Rican patients, based on combinatorial CYP2C9 and VKORC1 genotypes

BACKGROUND

The influence of CYP2C9 and VKORC1 polymorphisms on warfarin dose has been investigated in white, Asian, and African American populations but not in Puerto Rican Hispanic patients.

OBJECTIVE

To test the associations between genotypes, international normalized ratio (INR) measurements, and warfarin dosing and gauge the impact of these polymorphisms on warfarin dose, using a published algorithm.

METHODS

A retrospective warfarin pharmacogenetic association study in 106 Puerto Rican patients was performed. DNA samples from patients were assayed for 12 variants in both CYP2C9 and VKORC1 loci by HILOmet PhyzioType assay. Demographic and clinical nongenetic data were retrospectively collected from medical records. Allele and genotype frequencies were determined and Hardy-Weinberg equilibrium (HWE) was tested.

RESULTS

Sixty-nine percent of patients were carriers of at least one polymorphism in either the CYP2C9 or the VKORC1 gene. Double, triple, and quadruple carriers accounted for 22%, 5%, and 1%, respectively. No significant departure from HWE was found. Among patients with a given CYP2C9 genotype, warfarin dose requirements declined from GG to AA haplotypes; whereas, within each VKORC1 haplotype, the dose decreased as the number of CYP2C9 variants increased. The presence of these loss-of-function alleles was associated with more out-of-range INR measurements (OR = 1.38) but not with significant INR >4 during the initiation phase. Analyses based on a published pharmacogenetic algorithm predicted dose reductions of up to 4.9 mg/day in carriers and provided better dose prediction in an extreme subgroup of highly sensitive patients, but also suggested the need to improve predictability by developing a customized model for use in Puerto Rican patients.

CONCLUSIONS

This study laid important groundwork for supporting a prospective pharmacogenetic trial in Puerto Ricans to detect the benefits of incorporating relevant genomic information into a customized DNA-guided warfarin dosing algorithm.

The Creating an Optimal Warfarin Nomogram (CROWN) Study

A significant proportion of warfarin dose variability is explained by variation in the genotypes of the cytochrome P450 CYP2C9 and the vitamin K epoxide reductase complex, VKORC1, enzymes that influence warfarin metabolism and sensitivity, respectively. We sought to develop an optimal pharmacogenetic warfarin dosing algorithm that incorporated clinical and genetic information. We enroled patients initiating warfarin therapy. Genotyping was performed of the VKORC1, -1639G>A, the CYP2C9*2, 430C>T, and the CYP2C9*3, 1075C>A genotypes. The initial warfarin dosing algorithm (Algorithm A) was based upon established clinical practice and published warfarin pharmacogenetic information. Subsequent dosing algorithms (Algorithms B and Algorithm C) were derived from pharmacokinetic / pharmacodynamic (PK/PD) modelling of warfarin dose, international normalised ratio (INR), clinical and genetic factors from patients treated by the preceding algorithm(s). The primary outcome was the time in the therapeutic range, considered an INR of 1.8 to 3.2. A total of 344 subjects are included in the study analyses. The mean percentage time within the therapeutic range for each subject increased progressively from Algorithm A to Algorithm C from 58.9 (22.0), to 59.7 (23.0), to 65.8 (16.9) percent (p = 0.04). Improvement also occurred in most secondary endpoints, which included the per-patient percentage of INRs outside of the therapeutic range (p = 0.004), the time to the first therapeutic INR (p = 0.07), and the time to achieve stable therapeutic anticoagulation (p < 0.001). In conclusion, warfarin pharmacogenetic dosing can be optimised in real time utilising observed PK/PD information in an adaptive fashion.

Genotyping three SNPs affecting warfarin drug response by isothermal real-time HDA assays

BACKGROUND

The response to the anticoagulant drug warfarin is greatly affected by genetic polymorphisms in the VKORC1 and CYP2C9 genes. Genotyping these polymorphisms has been shown to be important in reducing the time of the trial and error process for finding the maintenance dose of warfarin thus reducing the risk of adverse effects of the drug.

METHOD

We developed a real-time isothermal DNA amplification system for genotyping three single nucleotide polymorphisms (SNPs) that influence warfarin response. For each SNP, real-time isothermal Helicase Dependent Amplification (HDA) reactions were performed to amplify a DNA fragment containing the SNP. Amplicons were detected by fluorescently labeled allele specific probes during real-time HDA amplification.

RESULTS

Fifty clinical samples were analyzed by the HDA-based method, generating a total of 150 results. Of these, 148 were consistent between the HDA-based assays and a reference method. The two samples with unresolved HDA-based test results were repeated and found to be consistent with the reference method.

CONCLUSION

The HDA-based assays demonstrated a clinically acceptable performance for genotyping the VKORC1 -1639G>A SNP and two SNPs (430C>T and 1075A>C) for the CYP2C9 enzyme (CYP2C9*2 and CYP2C9*3), all of which are relevant in warfarin pharmacogenentics.

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.

CYP2C9 and VKORC1 genotypes in Puerto Ricans: A case for admixture-matching in clinical pharmacogenetic studies

BACKGROUNDS

Admixture is of great relevance to the clinical application of pharmacogenetics and personalized medicine, but unfortunately these studies have been scarce in Puerto Ricans. Besides, allele frequencies for clinically relevant genetic markers in warfarin response (i.e., CYP2C9 and VKORC1) have not yet been fully characterized in this population. Accordingly, this study is aimed at investigating whether a correlation between overall genetic similarity and CYP2C9 and/or VKORC1 genotypes could be established.

METHODS

98 DNA samples from Puerto Ricans were genotyped for major CYP2C9 and VKORC1 polymorphisms and tested on a physiogenomic (PG)-array to infer population structure and admixture pattern.

RESULTS

Analysis affirmed that Puerto Ricans are broadly admixed. A genetic distance dendrogram was constructed by clustering those subjects with similar genetic profiles. Individual VKORC1 and CYP2C9 genotypes were visually overlaid atop the three dendrogram sectors. Sector-1, representing Amerindian ancestry, showed higher VKORC1 -1639G>A variant frequency than the rest of the population (p=0.051). Although CYP2C9*3 allele frequencies matched the expected HapMap values, admixture may explain deviations from published findings regarding VKORC1 -1639G>A and CYP2C9*2 allele frequencies in sector-3.

CONCLUSIONS

Results suggest that the observed inter-individual variations in ancestral contributions have significant implications for the way each Puerto Rican responds to warfarin therapy. Our findings provide valuable evidence on the importance of controlling for admixture in pharmacogenetic studies of Puerto Rican Hispanics.

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.

Implementing genotype-guided antithrombotic therapy

Genotyping has the potential to improve the efficacy and safety of major antithrombotic drugs. For warfarin, the stable maintenance dose varies from 1-10 mg/day. The VKORC1 -1639G>A allele and the CYP2C9*2 and *3 alleles (cumulative frequency: 90% in Asians, 65% in Europeans and 20% in Africans), explain 45% of response variability in European and 30% in African populations. The large clinical trials COAG and EU-PACT will define the extent to which pharmacogenetic dosing affects the safety and efficacy of warfarin and coumarin derivatives. The platelet inhibitor clopidogrel requires activation by the CYP2C19 enzyme. CYP2C19*2 and *3 alleles (cumulative frequency: 20-50%) produce null enzyme activity, and their presence attenuates platelet inhibition and increases cardiovascular events. The US FDA-mandated drug labeling recognizes the relevance of genotyping in the selection and dosing of both warfarin and clopidogrel.

Warfarin genotyping reduces hospitalization rates results from the MM-WES (Medco-Mayo Warfarin Effectiveness study)

OBJECTIVES

This study was designed to determine whether genotype testing for patients initiating warfarin treatment will reduce the incidence of hospitalizations, including those due to bleeding or thromboembolism.

BACKGROUND

Genotypic variations in CYP2C9 and VKORC1 have been shown to predict warfarin dosing, but no large-scale studies have prospectively evaluated the clinical effectiveness of genotyping in naturalistic settings across the U.S.

METHODS

This national, prospective, comparative effectiveness study compared the 6-month incidence of hospitalization in patients receiving warfarin genotyping (n = 896) versus a matched historical control group (n = 2,688). To evaluate for temporal changes in the outcomes of warfarin treatment, a secondary analysis compared outcomes for 2 external control groups drawn from the same 2 time periods.

RESULTS

Compared with the historical control group, the genotyped cohort had 31% fewer hospitalizations overall (adjusted hazard ratio [HR]: 0.69, 95% confidence interval [CI]: 0.58 to 0.82, p < 0.001) and 28% fewer hospitalizations for bleeding or thromboembolism (HR: 0.72, 95% CI: 0.53 to 0.97, p = 0.029) during the 6-month follow-up period. Findings from a per-protocol analysis were even stronger: 33% lower risk of all-cause hospitalization (HR: 0.67, 95% CI: 0.55 to 0.81, p < 0.001) and 43% lower risk of hospitalization for bleeding or thromboembolism (HR: 0.57, 95% CI: 0.39 to 0.83, p = 0.003) in patients who were genotyped. During the same period, there was no difference in outcomes between the 2 external control groups.

CONCLUSIONS

Warfarin genotyping reduced the risk of hospitalization in outpatients initiating warfarin. (The Clinical and Economic Impact of Pharmacogenomic Testing of Warfarin Therapy in Typical Community Practice Settings [MHSMayoWarf1]; NCT00830570).

Warfarin sensitivity genotyping: a review of the literature and summary of patient experience

The antithrombotic benefits of warfarin are countered by a narrow therapeutic index that contributes to excessive bleeding or cerebrovascular clotting and stroke in some patients. This article reviews the current literature describing warfarin sensitivity genotyping and compares the results of that review to the findings of our study in 189 patients at Mayo Clinic conducted between June 2001 and April 2003. For the review of the literature, we identified relevant peer-reviewed articles by searching the Web of Knowledge using key word warfarin-related adverse event. For the 189 Mayo Clinic patients initiating warfarin therapy to achieve a target international normalized ratio (INR) in the range of 2.0 to 3.5, we analyzed the CYP2C9 (cytochrome P450 2C9) and VKORC1 (vitamin K epoxide reductase complex, subunit 1) genetic loci to study the relationship among the initial warfarin dose, steady-state dose, time to achieve steady-state dose, variations in INR, and allelic variance. Results were compared with those previously reported in the literature for 637 patients. The relationships between allelic variants and warfarin sensitivity found in our study of Mayo Clinic patients are fundamentally the same as in those reported by others. The Mayo Clinic population is predominantly white and shows considerable allelic variability in CYP2C9 and VKORC1. Certain of these alleles are associated with increased sensitivity to warfarin. Polymorphisms in CYP2C9 and VKORC1 have a considerable effect on warfarin dose in white people. A correlation between steady-state warfarin dose and allelic variants of CYP2C9 and VKORC1 has been demonstrated by many previous reports and is reconfirmed in this report. The allelic variants found to most affect warfarin sensitivity are CYP2C9*1*1-VKORC1BB (less warfarin sensitivity than typical); CYP2C9*1*1-VKORC1AA (considerable variance in INR throughout initiation); CYP2C9*1*2-VKORC1AB (more sensitivity to warfarin than typical); CYP2C9*1*3-VKORC1AB (much more sensitivity to warfarin than typical); CYP2C9*1*2-VKORC1AB (much more sensitivity to warfarin than typical); CYP2C9*1*3-VKORC1AA (much more sensitivity to warfarin than typical); and CYP2C9*2*2-VKORC1AB (much more sensitivity to warfarin than typical). Although we were unable to show an association between allelic variants and initial warfarin dose or dose escalation, an association was seen between allelic variant and steady-state warfarin dose. White people show considerable variance in CYP2C9 allele types, whereas people of Asian or African descent infrequently carry CYP2C9 allelic variants. The VKORC1AA allele associated with high warfarin sensitivity predominates in those of Asian descent, whereas white people and those of African descent show diversity, carrying either the VKORC1BB, an allele associated with low warfarin sensitivity, or VKORC1AB or VKORC1AA, alleles associated with moderate and high warfarin sensitivity, respectively.

Interactive modeling for ongoing utility of pharmacogenetic diagnostic testing: application for warfarin therapy

BACKGROUND

The application of pharmacogenetic results requires demonstrable correlations between a test result and an indicated specific course of action. We developed a computational decision-support tool that combines patient-specific genotype and phenotype information to provide strategic dosage guidance. This tool, through estimating quantitative and temporal parameters associated with the metabolism- and concentration-dependent response to warfarin, provides the necessary patient-specific context for interpreting international normalized ratio (INR) measurements.

METHODS

We analyzed clinical information, plasma S-warfarin concentration, and CYP2C9 (cytochrome P450, family 2, subfamily C, polypeptide 9) and VKORC1 (vitamin K epoxide reductase complex, subunit 1) genotypes for 137 patients with stable INRs. Plasma S-warfarin concentrations were evaluated by VKORC1 genotype (-1639G>A). The steady-state plasma S-warfarin concentration was calculated with CYP2C9 genotype-based clearance rates and compared with actual measurements.

RESULTS

The plasma S-warfarin concentration required to yield the target INR response is significantly (P < 0.05) associated with VKORC1 -1639G>A genotype (GG, 0.68 mg/L; AG, 0.48 mg/L; AA, 0.27 mg/L). Modeling of the plasma S-warfarin concentration according to CYP2C9 genotype predicted 58% of the variation in measured S-warfarin concentration: Measured [S-warfarin] = 0.67(Estimated [S-warfarin]) + 0.16 mg/L.

CONCLUSIONS

The target interval of plasma S-warfarin concentration required to yield a therapeutic INR can be predicted from the VKORC1 genotype (pharmacodynamics), and the progressive changes in S-warfarin concentration after repeated daily dosing can be predicted from the CYP2C9 genotype (pharmacokinetics). Combining the application of multivariate equations for estimating the maintenance dose with genotype-guided pharmacokinetics/pharmacodynamics modeling provides a powerful tool for maximizing the value of CYP2C9 and VKORC1 test results for ongoing application to patient care.

A regulatory science perspective on warfarin therapy: a pharmacogenetic opportunity

Warfarin is a challenging drug to accurately dose, both initially and for maintenance, because of its narrow therapeutic range, wide interpatient variability, and long list of factors that can influence dosing. Two million people in the United States are initiated on warfarin therapy annually, and this number is steadily increasing because of the increase in number of eligible patients. Recently, warfarin was reported to be the fourth leading cause of adverse events. The U.S. Food and Drug Administration recognizes that the adverse event rate of warfarin can be improved through better initial dosing, because many of the serious adverse events of warfarin occur soon after starting treatment. A substantial number of studies demonstrate that common variants of two genes, VKORC1 and CYP2C9, along with other nongenetic factors, correlate significantly with warfarin dosing. The genotypes of VKORC1 and CYP2C9 alone account for nearly 3 times more of the variability ( approximately 30%) in warfarin dosing than do age, weight, gender, and other clinical factors combined ( approximately 12%). Therefore, the purpose of this report is to review the current recommendations for warfarin therapy that involve genetic testing.

Drug interaction studies with dexlansoprazole modified release (TAK-390MR), a proton pump inhibitor with a dual delayed-release formulation: results of four randomized, double-blind, crossover, placebo-controlled, single-centre studies

BACKGROUND AND OBJECTIVE

Most proton pump inhibitors are extensively metabolized by cytochrome P450 (CYP) isoenzymes, as are many other drugs, giving rise to potential drug-drug interactions. Dexlansoprazole modified release (MR) [TAK-390MR] is a modified-release formulation of dexlansoprazole (TAK-390), an enantiomer of lansoprazole, which employs an innovative Dual Delayed Release technology designed to prolong the plasma dexlansoprazole concentration-time profile following once-daily oral administration. As with lansoprazole, dexlansoprazole is metabolized mainly by CYP3A and CYP2C19. Based on in vitro studies, dexlansoprazole has the potential to inhibit activity of these isoenzymes and also may induce human hepatic CYP1A and CYP2C9 activity. To determine whether dexlansoprazole has an effect on these isoenzymes in vivo, drug interaction studies with dexlansoprazole MR were conducted.

METHODS

Four separate randomized, double-blind, two-way crossover, placebo-controlled, single-centre studies were conducted in healthy volunteers to evaluate the effect of dexlansoprazole on the pharmacokinetics of four test substrates (diazepam, phenytoin, theophylline [administered as intravenous aminophylline] and warfarin), which were selected based on in vitro and/or in vivo data that suggest a potential drug interaction with CYP isoenzymes or potentially coadministered narrow therapeutic index drugs. In each study, dexlansoprazole MR 90 mg or placebo was administered once daily for 9 or 11 days in each period. Subjects received a single dose of test substrate in each study period. Pharmacokinetic parameters of the test substrates were estimated using noncompartmental methods. A conclusion of no effect of dexlansoprazole MR on the test substrate was made if the 90% confidence intervals (CIs) for the ratios of the central values for the observed maximum plasma drug concentration (C(max)) and the area under the plasma concentration-time curve (AUC) of test substrate administered with dexlansoprazole MR versus placebo were within 0.80-1.25 based on an analysis of variance model. The potential for a pharmacodynamic interaction was also assessed for warfarin using prothrombin time, measured as the international normalized ratio. Routine safety assessments were conducted in these studies.

RESULTS

Mean C(max) and AUC values were generally similar for each test substrate when administered with multiple once-daily doses of dexlansoprazole MR or placebo. The 90% CIs for the bioavailability of these test substrates administered with dexlansoprazole MR relative to that obtained when the substrates were administered with placebo were within the bioequivalency range of 0.80-1.25, indicating that multiple doses of dexlansoprazole MR had no effect on the pharmacokinetics of these drugs. Additionally, dexlansoprazole MR had no effect on the pharmacodynamics of warfarin. Administration of these drugs with dexlansoprazole MR 90 mg or placebo was well tolerated; the only serious adverse event, which led to a subject's discontinuation from the study, was considered unrelated to study drugs.

CONCLUSIONS

Coadministration of dexlansoprazole MR with diazepam, phenytoin or theophylline did not affect the pharmacokinetics of these drugs, and therefore is unlikely to alter the pharmacokinetic profile of other drugs metabolized by CYP2C19, CYP2C9, CYP1A2 and perhaps CYP3A. Additionally, dexlansoprazole MR coadministered with warfarin did not affect the pharmacokinetics of the warfarin enantiomers and had no effect on the anticoagulant activity of warfarin. Dexlansoprazole MR was well tolerated in these trials of healthy subjects.

The value of CYP2D6 and OPRM1 pharmacogenetic testing for opioid therapy

In managing pain, clinicians working with the more than 80 million people in the United States who suffer annually from serious pain face decisions about choosing the most appropriate pharmacologic agent, to contemplating nonpharmacologic modalities. This article focuses on opioid use for pain management, their risks of toxicity and addiction, adverse reactions, undertreatment for fear of addiction, and integration of novel diagnostics, such as the pharmacogenetic biomarkers CYP2D6 and OPRM1 as holding promise for assessing a patient's risk of adverse events or likelihood of efficacy. Incorporation of such biomarkers is emerging on the forefront of personalized medicine, and has the potential to dramatically improve the utility and efficacy of both current and future pain management strategies.

High carrier prevalence of combinatorial CYP2C9 and VKORC1 genotypes affecting warfarin dosing

BACKGROUND

Polymorphisms in the cytochrome P450 2C9 (CYP2C9) and vitamin K epoxide reductase complex subunit 1 (VKORC1) genes significantly alter the effective warfarin dose. The CYP2C9*2 (430C>T), CYP2C9*3 (1075A>C) and VKORC1 -1639 G>A polymorphisms affect warfarin dose through altered metabolism (CYP2C9) and sensitivity (VKORC1).

OBJECTIVE

We determined the frequencies of SNPs in the CYP2C9 and VKORC1 genes in a clinical outpatient population and the carrier prevalences for a variety of genotype combinations to gauge the impact of these polymorphisms on warfarin dosage using published algorithms.

METHOD

A total of 127 patients from an outpatient clinic at Hartford Hospital (Hartford, CT, USA) were genotyped for five SNPs in the CYP2C9 gene and seven SNPs in the VKORC1 gene using Luminex^® technology.

RESULTS

The polymorphism frequencies were 10.2, 7.9 and 37.4% for the functionally deficient CYP2C9*2, CYP2C9*3 and VKORC1 -1639 G>A polymorphisms, respectively. Combining prevalence of combinatorial genotypes, 18% were carriers of both CYP2C9 and VKORC1 polymorphisms, 13% were CYP2C9 polymorphism carriers only, 42.5% were VKORC1 carriers only, and the remaining 27% were noncarriers for either gene. Based on published warfarin dosing algorithms, carriers of 1, 2, 3 and 4 functionally deficient polymorphisms predict reductions of 1.0 to 1.6, 2.0 to 2.9, 2.9 to 3.7, and 3.6 to 4.4 mg/day, respectively, in warfarin dose.

CONCLUSION

Overall, 73% of the population carried at least one polymorphism predicting deficient warfarin metabolism or responsiveness and 18% were carriers for polymorphisms in both genes studied. Combinatorial genotyping of CYP2C9 and VKORC1 can allow for individualized dosing of warfarin amongst patients with gene polymorphisms potentially reducing the risk of accentuated responses and bleeding.

Pharmacogenetic testing of CYP2C9 and VKORC1 alleles for warfarin

American College of Medical Genetics statements and guidelines are designed primarily as an educational resource for medical geneticists and other health care professionals to help them provide quality medical genetic services. Adherence to these standards and guidelines does not necessarily ensure a successful medical outcome. These statements and guidelines should not be considered inclusive of all proper procedures and tests or exclusive of other procedures and tests that are reasonably directed to obtaining the same results. In determining the propriety of any specific procedure or test, the health care professional should apply his or her own professional judgment to the specific clinical circumstances presented by the individual patient or specimen. It may be prudent, however, to document in the patient's record the rationale for any significant deviation from these standards and guidelines. Warfarin (Coumadin) is a potent drug that when used judiciously and monitored closely, leads to substantial reductions in morbidity and mortality from thromboembolic events. However, even with careful monitoring, initiation of warfarin dosing is associated with highly variable responses between individuals and challenges achieving and maintaining levels within the narrow therapeutic range that can lead to adverse drug events. Variants of two genes, CYP2C9 and VKORC1, account for 30-50% of the variability in dosing of warfarin; thus, many believe that testing of these genes will aid in warfarin dosing recommendations. Evidence about this test is evolving rapidly, as is its translation into clinical practice. In an effort to address this situation, a multidisciplinary expert group was organized in November 2006 to evaluate the role of CYP2C9 and VKORC1 testing in altering warfarin-related therapeutic goals and reduction of adverse drug events. A recently completed Rapid-ACCE (Analytical, Clinical Validity, Clinical Utility, and Ethical, Legal, and Social Implications) Review, commissioned to inform this work group, was the foundation for this analysis. From this effort, specific recommendations for the appropriate use of CYP2C9 and VKORC1 testing were developed and are presented here. The group determined that the analytical validity of these tests has been met, and there is strong evidence to support association between these genetic variants and therapeutic dose of warfarin. However, there is insufficient evidence, at this time, to recommend for or against routine CYP2C9 and VKORC1 testing in warfarin-naive patients. Prospective clinical trials are needed that provide direct evidence of the benefits, disadvantages, and costs associated with this testing in the setting of initial warfarin dosing. Although the routine use of warfarin genotyping is not endorsed by this work group at this time, in certain situations, CYP2C9 and VKORC1 testing may be useful, and warranted, in determining the cause of unusual therapeutic responses to warfarin therapy.

Pharmacogenetics: data, concepts and tools to improve drug discovery and drug treatment

Variation in the human genome is a most important cause of variable response to drugs and other xenobiotics. Susceptibility to almost all diseases is determined to some extent by genetic variation. Driven by the advances in molecular biology, pharmacogenetics has evolved within the past 40 years from a niche discipline to a major driving force of clinical pharmacology, and it is currently one of the most actively pursued disciplines in applied biomedical research in general. Nowadays we can assess more than 1,000,000 polymorphisms or the expression of more than 25,000 genes in each participant of a clinical study -- at affordable costs. This has not yet significantly changed common therapeutic practices, but a number of physicians are starting to consider polymorphisms, such as those in CYP2C9, CYP2C19, CYP2D6, TPMT and VKORC1, in daily medical practice. More obviously, pharmacogenetics has changed the practices and requirements in preclinical and clinical drug research; large clinical trials without a pharmacogenomic add-on appear to have become the minority. This review is about how the discipline of pharmacogenetics has evolved from the analysis of single proteins to current approaches involving the broad analyses of the entire genome and of all mRNA species or all metabolites and other approaches aimed at trying to understand the entire biological system. Pharmacogenetics and genomics are becoming substantially integrated fields of the profession of clinical pharmacology, and education in the relevant methods, knowledge and concepts form an indispensable part of the clinical pharmacology curriculum and the professional life of pharmacologists from early drug discovery to pharmacovigilance.

Identification of a synonymous polymorphism within the cytochrome P4502C9 gene that interferes with identification of the CYP2C9*2 allele

Cytochrome P450 2C9 (CYP4502C9) genotyping is useful in dosage adjustments for several critical drug therapies, including warfarin. Potential interference compromising these genotyping results could lead to inappropriate dose adjustments that may result in adverse drug reactions. During routine clinical CYP4502C9 genotyping using multiplex allele-specific primer extension, an ambiguous result was obtained for determination of the CYP2C9 430C>T substitution, which defines the CYP2C9*2 allele. In this one patient sample submitted for CYP2C9 genotyping, the ratio for the variant 430T allele signal to the total signal (C+T alleles) was 0.29. This is above the expected ratio to be classified as wild-type (<0.15) and below the minimum expected ratio (>0.3) when the genotype is heterozygous at the 430 position. The mean fluorescence intensity for the 430C allele was consistent with that observed in subjects who are heterozygous at this nucleotide position. However, the corresponding signal for the 430T allele indicated the absence of the CYP2C9*2 allele. This suggests the assay was not able to determine the correct nucleotide at position 430 for one of the two alleles in this patient. Subsequent sequencing to investigate the allele-specific primer extension failure revealed the presence of a rare C>T nucleotide substitution at position 429. We tested this subject's CYP2C9 genotype using AvaII restriction endonuclease digestion and found that this rare substitution causes false-positive identification of the CYP2C9*2 allele when using this method. We developed a DpnII endonuclease digestion assay to specifically detect the CYP2C9 429C>T substitution and tested 100 randomly selected samples obtained from unrelated individuals. The 429C>T polymorphism was not identified in this sample set, which indicates an allele frequency of less than 2.0% (95% confidence interval, 0.0-1.8%) in the general population. Despite the rarity of this polymorphism, it has important implications for the accuracy of assays using allele-specific primers and the Ava II restriction endonuclease, when it occurs, which are two common methods currently applied for detecting the presence of the CYP2C9*2 allele.

Pharmacogenetics of oral anticoagulants: the opportunity for individualized drug treatment of greater safety

Oral antivitamin K (AVK) anticoagulants are widely prescribed for the prophylaxis and treatment of a number of thromboembolic disorders. They constitute a major cause of iatrogenic accidents because of their narrow therapeutic index and consequently increase both thrombotic and bleeding risk; thus, regular monitoring is required. Patient and environmental factors affect the anticoagulation response and it has become evident that the wide interindividual variation in AVK response is also partly genetically determined. The main enzyme responsible for the metabolism of AVKs is hepatic cytochrome P450 CYP2C9. Vitamin K epoxide reductase complex subunit I (VKORC1) is a key enzyme in the vitamin K cycle; it is required to regenerate the cofactor essential for the activation of vitamin K-dependent clotting factors and is the target enzyme of AVK inhibition. Genetic variations affecting both CYP2C9 and VKORC1 are associated with variability in drug response and may explain differences in dose requirements. Genotyping for CYP2C9 and VKORC1 variants before initiation of treatment could allow clinicians to develop dosing protocols and identify patients at higher risk for AVK complications such as bleeding.

Translation of pharmacogenetics into clinically relevant testing modalities

Pharmacogenetics (PGx) relies on the genetic makeup of an individual to predict drug response and efficacy, as well as potential adverse drug events. Significant advances in PGx research have been made since inherited differences in response to such drugs as isoniazid and succinylcholine were explored in the 1950s, and the clinical utility and application of PGx are especially apparent in some subspecialty areas of chemotherapeutic, psychotropic drug, and anticoagulant therapies.

Estimation of Warfarin Maintenance Dose Based on VKORC1 (−1639 G>A) and CYP2C9 Genotypes

Background: CYP2C9 polymorphisms are associated with decreased S-warfarin clearance and lower maintenance dosage. Decreased expression of VKORC1 resulting from the −1639G&gt;A substitution has also been implicated in lower warfarin dose requirements. We investigated the additional contribution of this polymorphism to the variance in warfarin dose.

Methods: Sixty-five patients with stable anticoagulation were genotyped for CYP2C9 and VKORC1 with Tag-It™ allele-specific primer extension technology. Plasma S-warfarin concentrations and warfarin maintenance dose were compared among patients on the basis of the VKORC1 −1639G&gt;A genotype.

Results: Eighty percent of CYP2C9*1/*1 patients stabilized on &lt;4.0 mg/day warfarin had at least 1 VKORC1 −1639A allele. Mean warfarin doses (SD) were 6.7 (3.3), 4.3 (2.2), and 2.7 (1.2) mg/day for patients with the VKORC1 −1639GG, GA, and AA genotypes, respectively. Steady-state plasma concentrations of S-warfarin were lowest in patients with the VKORC1 −1639AA genotype and demonstrated a positive association with the VKORC1 −1639G allele copy number (trend P = 0.012). A model including VKORC1 and CYP2C9 genotypes, age, sex, and body weight accounted for 61% of the variance in warfarin daily maintenance dose.

Conclusions: The VKORC1 −1639A allele accounts for low dosage requirements of most patients without a CYP2C9 variant. Higher plasma S-warfarin concentrations corresponding to increased warfarin maintenance dosages support a hypothesis for increased expression of the VKORC1 −1639G allele. VKORC1 and CYP2C9 genotypes, age, sex, and body weight account for the majority of variance in warfarin dose among our study population.