Pharmacogenetics of CYP2C9 and interindividual variability in anticoagulant response to warfarin

Improved stacking ensemble learning based on feature selection to accurately predict warfarin dose

Background

With the rapid development of artificial intelligence, prediction of warfarin dose via machine learning has received more and more attention. Since the dose prediction involve both linear and nonlinear problems, traditional machine learning algorithms are ineffective to solve such problems at one time.

Objective

Based on the characteristics of clinical data of Chinese warfarin patients, an improved stacking ensemble learning can achieve higher prediction accuracy.

Methods

Information of 641 patients from southern China who had reached a steady state on warfarin was collected, including demographic information, medical history, genotype, and co-medication status. The dataset was randomly divided into a training set (90%) and a test set (10%). The predictive capability is evaluated on a new test set generated by stacking ensemble learning. Additional factors associated with warfarin dose were discovered by feature selection methods.

Results

A newly proposed heuristic-stacking ensemble learning performs better than traditional-stacking ensemble learning in key metrics such as accuracy of ideal dose (73.44%, 71.88%), mean absolute errors (0.11 mg/day, 0.13 mg/day), root mean square errors (0.18 mg/day, 0.20 mg/day) and R2 (0.87, 0.82).

Conclusions

The developed heuristic-stacking ensemble learning can satisfactorily predict warfarin dose with high accuracy. A relationship between hypertension, a history of severe preoperative embolism, and warfarin dose is found, which provides a useful reference for the warfarin dose administration in the future.

Impact of genetic and non-genetic factors on hepatic CYP2C9 expression and activity in Hungarian subjects

CYP2C9, one of the most abundant hepatic cytochrome P450 enzymes, is involved in metabolism of 15–20% of clinically important drugs (warfarin, sulfonylureas, phenytoin, non-steroid anti-inflammatory drugs). To avoid adverse events and/or impaired drug-response, CYP2C9 pharmacogenetic testing is recommended. The impact of CYP2C9 polymorphic alleles (CYP2C9*2, CYP2C9*3) and phenoconverting non-genetic factors on CYP2C9 function and expression was investigated in liver tissues from Caucasian subjects (N = 164). The presence of CYP2C9*3 allele was associated with CYP2C9 functional impairment, and CYP2C9*2 influenced tolbutamide 4′-hydroxylase activity only in subjects with two polymorphic alleles, whereas the contribution of CYP2C8*3 was not confirmed. In addition to CYP2C9 genetic polymorphisms, non-genetic factors (co-medication with CYP2C9-specific inhibitors/inducers and non-specific factors including amoxicillin + clavulanic acid therapy or chronic alcohol consumption) contributed to the prediction of hepatic CYP2C9 activity; however, a CYP2C9 genotype–phenotype mismatch still existed in 32.6% of the subjects. Substantial variability in CYP2C9 mRNA levels, irrespective of CYP2C9 genotype, was demonstrated; however, CYP2C9 induction and non-specific non-genetic factors potentially resulting in liver injury appeared to modify CYP2C9 expression. In conclusion, complex implementation of CYP2C9 genotype and non-genetic factors for the most accurate estimation of hepatic CYP2C9 activity may improve efficiency and safety of medication with CYP2C9 substrate drugs in clinical practice.

Comparison of multivariate linear regression and a machine learning algorithm developed for prediction of precision warfarin dosing in a Korean population

BACKGROUND

Personalized warfarin dosing is influenced by various factors including genetic and non-genetic factors. Multiple linear regression (LR) is known as a conventional method to develop predictive models. Recently, machine learning approaches have been extensively implemented for warfarin dosing due to the hypothesis of non-linear association between covariates and stable warfarin dose.

OBJECTIVE

To extend the multiple linear regression algorithm for personalized warfarin dosing in a Korean population and compare with a machine learning--based algorithm.

METHOD

From this cohort study, we collected information on 650 patients taking warfarin who achieved steady state including demographic information, indications, comorbidities, comedications, habits, and genetic factors. The dataset was randomly split into training set (90%) and test set (10%). The LR and machine learning (gradient boosting machine [GBM]) models were developed on the training set and were evaluated on the test set.

RESULT

LR and GBM models were comparable in terms of accuracy of ideal dose (75.38% and 73.85%), correlation (0.77 and 0.73), mean absolute error (0.58 mg/day and 0.64 mg/day), and root mean square error (0.82 mg/day and 0.9 mg/day), respectively. VKORC1 genotype, CYP2C9 genotype, age, and weight were the highest contributors and could obtain 80% of maximum performance in both models.

CONCLUSION

This study shows that our LR and GMB models are satisfactory to predict warfarin dose in our dataset. Both models showed similar performance and feature contribution characteristics. LR may be the appropriate model due to its simplicity and interpretability.

Sauver JL, Bielinski SJ, Jacobsen DJ, Visscher SL, Wang L, Weinshilboum R, Borah BJ. Comparing outcomes and costs among warfarin-sensitive patients versus warfarin-insensitive patients using The Right Drug, Right Dose, Right Time: Using genomic data to individualize treatment (RIGHT) 10K warfarin cohort

Oral anticoagulant (OAC) therapy has been the main treatment approach for stroke prevention for decades. Warfarin is the most widely prescribed OAC in the United States, but is difficult to manage due to variability in dose requirements across individuals. Pharmacogenomics may mitigate risk concerns related to warfarin use by fostering the opportunity to facilitate individualized medicine approaches to warfarin treatment (e.g., genome-guided dosing). While various economic evaluations exist examining the cost-effectiveness of pharmacogenomics testing for warfarin, few observational studies exist to support these studies, with even fewer using genotype as the main exposure of interest. We examined a cohort of individuals initiating warfarin therapy between 2004 and 2017 and examined bleeding and cost outcomes for the year following initiation using Mayo Clinic’s billing and administrative data, as well the Mayo Clinic Rochester Cost Data Warehouse. Analyses included descriptive summaries, comparison of characteristics across exposure groups, reporting of crude outcomes, and multivariate analyses. We included N = 1,143 patients for analyses. Just over a third of our study population (34.9%) carried a warfarin-sensitive phenotype. Sensitive individuals differed in their baseline characteristics by being of older age and having a higher number of comorbid conditions; myocardial infarction, diabetes, and cancer in particular. The occurrence of bleeding events was not significantly different across exposure groups. No significant differences across exposure groups existed in either the likelihood of incurring all-cause healthcare costs or in the magnitude of those costs. Warfarin-sensitive individuals were no more likely to utilize cardiovascular-related healthcare services; however, they had lower total and inpatient cardiovascular-related costs compared to warfarin-insensitive patients. No significant differences existed in any other categories of costs. We found limited evidence that warfarin-sensitive individuals have different healthcare spending than warfarin-insensitive individuals. Additional real-world studies are needed to support the traditional economic evaluations currently existing in the literature.

Extremely low therapeutic doses of acenocoumarol in a patient with CYP2C9*3/*3 and VKORC1-1639A/A genotype

Vitamin-K antagonists (VKAs) have remained the mainstay of oral anticoagulant therapy for the treatment and prevention of thromboembolism. The management of treatment with VKAs is challenging due to their narrow therapeutic index and the wide interindividual variation in response to therapy. Variants of the CYP2C9 and the VKORC1 gene account for 30–50% of the variability in dosing requirements, and it has been proposed that genotyping of these loci could facilitate management of VKA therapy and minimize risk of overanticoagulation, even in very low doses. We present the first reported case of a patient with the compounded genotype CYP2C9*3*3 and VKORC1-1639A/A under treatment with acenocoumarol, and review of other reported cases with analogous genotypic profiles but under treatment with warfarin.

Lesinurad: Evaluation of Pharmacokinetic and Pharmacodynamic Interactions With Warfarin in Healthy Volunteers

Lesinurad is a selective uric acid reabsorption inhibitor approved for use in combination with xanthine oxidase inhibitors for the treatment of hyperuricemia associated with gout. In vitro, lesinurad was shown to be a weak inhibitor of cytochrome P450 (CYP)2C9 and a weak inducer of CYP3A4. Warfarin is a widely prescribed oral coumarin-based anticoagulant commonly prescribed in gout patients. In an open-label clinical study in healthy adult male subjects, the effects of multiple daily doses of 400 mg lesinurad on the pharmacokinetics and pharmacodynamics of a single dose of 25 mg warfarin (racemic mixture of R- and S- enantiomers) were evaluated. Lesinurad had no effect on the absorption or the exposure (area under the concentration-time curve [AUC] and peak concentration) of the more active S-warfarin enantiomer. A slight reduction (19%) in overall plasma exposure (AUC) was observed for the R-warfarin enantiomer. Lesinurad had no meaningful clinical impact on anticoagulation activity as measured by prothrombin time, activated partial thromboplastin time, and international normalized ratio of prothrombin time and Factor VII clotting activity. Overall, the administration of warfarin in the presence of multiple-dose lesinurad was devoid of clinically significant drug-drug interaction.

Influences of Corydalis decumbens on the Activities of CYP450 Enzymes in Rats with a Cocktail Approach

Corydalis decumbens, a Traditional Chinese Medicine, has been widely used for the alternative and/or complementary therapy of hypertension, arrhythmias rheumatoid arthritis, sciatica, stroke, hemiplegia, paraplegia, and vascular embolism. The aim of this study was to determinate the potential effects of Corydalis decumbens on the five cytochrome P450 (CYP) enzyme activities (CYP1A2, CYP3A4, CYP2C9, CYP2C19, and CYP2D6) by cocktail approach. To evaluate whether concurrent use of Corydalis decumbens interferes with the effect of several prescription drugs, saline (control group) or Corydalis decumbens (XTW group) were administrated via gavage for 7 successive days. A probe cocktail solution (phenacetin, omeprazole, metoprolol, tolbutamide, and midazolam) was given 24 h after the last dose of saline or Corydalis decumbens. A specific and sensitive UHPLC–MS/MS method was validated for the determination of five substrates and their metabolites in control group and XTW group. Our results indicated that Corydalis decumbens could have inductive effects of CYP2C19 and inhibit the activities of CYP1A2 and CYP3A4. However, Corydalis decumbens had no significant influence on CYP2C9 and CYP2D6. The herb-drug interaction should require more attention by careful monitoring and appropriate drug dosing adjustments to the concurrent use of western medications which were metabolized by CYP1A2, CYP2C19, and CYP3A4 in human—Corydalis decumbens, Cytochrome P450, Cocktail, Pharmacokinetics, herb–drug interactions.

Tailored on demand anti-coagulant dosing: An in vitro and in vivo evaluation of 3D printed purpose-designed oral dosage forms

Coumarin therapy has been associated with high levels of inter- and intra-individual variation in the required dose to reach a therapeutic anticoagulation outcome. Therefore, a dynamic system that is able to achieve accurate delivery of a warfarin dose is of significant importance. Here we assess the ability of 3D printing to fabricate and deliver tailored individualised precision dosing using in-vitro and in-vivo models. Sodium warfarin loaded filaments were compounded using hot melt extrusion (HME) and further fabricated via fused deposition modelling (FDM) 3D printing to produce capsular-ovoid-shaped dosage forms loaded at 200 or 400 µg dose. The solid dosage forms and comparator warfarin aqueous solutions were administered by oral gavage to Sprague-Dawley rats. A novel UV imaging approach indicated that the erosion of the methacrylate matrix was at a rate of 16.4 and 15.2 µm/min for horizontal and vertical planes respectively. In vivo, 3D printed forms were as proportionately effective as their comparative solution form in doubling plasma exposure following a doubling of warfarin dose (184% versus 192% respectively). The 3D printed ovoids showed a lower C_max of warfarin (1.51 and 3.33 mg/mL versus 2.5 and 6.44 mg/mL) and a longer T_max (6 and 3.7 versus 4 and 1.5 h) in comparison to liquid formulation. This work demonstrates for the first time in vivo, the potential of FDM 3D printing to produce a tailored specific dosage form and to accurately titrate coumarin dose response to an individual patient.

Pharmacogenetic dosing of warfarin in the Han-Chinese population: a randomized trial

Aim: This study aimed to determine clinical utility of genotype-guided dosing for warfarin in Han-Chinese. Methods: A total of 320 patients were randomly assigned International Warfarin Pharmacogenetic Consortium algorithm, Taiwan algorithm and optimal clinical care arms. The primary outcome of the study was the percentage of time in the therapeutic range during the first 90 days of treatment. Results: The percentage of time in the therapeutic range of the clinical care group in the first 2 weeks was significantly higher than the algorithm groups. This difference was no longer observed after 4 weeks. No difference in excessive anticoagulation (international normalized ratio ≥4.0) and adverse events was observed. Conclusion: Genotype-guided dosing did not provide significant benefit. Loading dose with frequent international normalized ratio monitoring could provide sufficient control of anticoagulation.

Genetic Polymorphisms and in Vitro Functional Characterization of CYP2C8, CYP2C9, and CYP2C19 Allelic Variants

Genetic variations in CYP 2C (CYP2C) subfamily, CYP2C8, CYP2C9, and CYP2C19 contribute to interindividual variability in the metabolism of clinically used drugs. Changes in the drug metabolizing activity of CYP2C members may cause unexpected and serious adverse drug reactions and inadequate therapeutic effects. Therefore, CYP2C gene polymorphism is used as a genome biomarker for predicting responsiveness to administered drugs. The most direct method for understanding the extent of the effects of CYP2C gene polymorphism on drug pharmacokinetics is by evaluating the blood and urine concentrations of the drug in subjects. However, in vivo tests are highly invasive, and considering the risk of adverse drug reactions, the burden on the patient may be significant. In addition, examining the functions of rare variant enzymes with an allele frequency of ≤1% requires at least several hundred subjects. Furthermore, it is extremely difficult to evaluate the functions of all variant enzymes in an in vivo test. On the other hand, in vitro enzyme activity can be evaluated using a heterologous expression system to avoid the aforementioned problems. In vitro tests are extremely important as they complement in vivo information. This review focuses on recent findings of in vitro studies on 3 highly polymorphic CYP2C members: CYP2C8, CYP2C9, and CYP2C19.

Pharmacogenomics Genome Wise Association Clinical Studies

Pharmacogenomics deals with drug responses in individual based on genetic variation in genome. Based on genetic variations, drugs may produce more or less therapeutic effect, and same way in side effects also. Physicians can use information about your genetic makeup to choose those drugs and drug doses to get better therapy. Optimizing drug therapy and rational dose adjustment with respect to genetic makeup will maximize drug efficacy and minimal adverse effects. This broken traditional ‘trial and error' method of ‘one drug fits all', and ‘one dose fits all' which contributing to 25–50% of drug toxicity or treatment failures. This will contribute to improve the ways in which existing drugs are used, genomic research will lead to drug development to produce new drugs that are highly effective without serious side effects. This approach to bring personalized medicine more practice and drug combinations are optimized for each individual' genetic makeup.

Pharmacogenomics and personalized medicine: a review focused on their application in the Chinese population

The field of pharmacogenomics was initiated in the 1950s and began to thrive after the completion of the human genome project 10 years ago. Thus far, more than 100 drug labels and clinical guidelines referring to pharmacogenomic biomarkers have been published, and several key pharmacogenomic markers for either drug safety or efficacy have been identified and subsequently adopted in clinical practice as pre-treatment genetic tests. However, a tremendous variation of genetic backgrounds exists between different ethnic groups. The application of pharmacogenomics in the Chinese population is still a long way off, since the published guidelines issued by the organizations such as US Food and Drug Administration require further confirmation in the Chinese population. This review highlights important pharmacogenomic discoveries in the Chinese population and compares the Chinese population with other nations regarding the pharmacogenomics of five most commonly used drugs, ie, tacrolimus, cyclosporine A, warfarin, cyclophosphamide and azathioprine.

Combination analysis in genetic polymorphisms of drug-metabolizing enzymes CYP1A2, CYP2C9, CYP2C19, CYP2D6 and CYP3A5 in the Japanese population

The Cytochrome P450 is the major enzyme involved in drug metabolism. CYP enzymes are responsible for the metabolism of most clinically used drugs. Individual variability in CYP activity is one important factor that contributes to drug therapy failure. We have developed a new straightforward TaqMan PCR genotyping assay to investigate the prevalence of the most common allelic variants of polymorphic CYP enzymes CYP1A2, CYP2C9, CYP2C19, CYP2D6 and CYP3A5 in the Japanese population. Moreover, we focused on the combination of each genotype for clinical treatment. The genotype analysis identified a total of 139 out of 483 genotype combinations of five genes in the 1,003 Japanese subjects. According to our results, most of subjects seemed to require dose modification during clinical treatment. In the near future, modifications should be considered based on the individual patient genotype of each treatment.

[Genomics from bench to bedside: a change in perspective]

While genomics is sometimes presented as an area of ​​research where results can be rapidly translated into clinical practice, the facts are more ambiguous. To illustrate some of the pitfalls of translation, this article focuses on the applications of genome-wide association studies (GWAS) results. Following a brief scientific contextualization of GWAS, two emblematic examples are presented as illustrations. The case of Crohn's disease emphasizes the limits of GWAS results for individual risk prediction. The case of warfarin highlights the difficulties of demonstrating the clinical utility of genetic data in treatment decisions. The article outlines the simplification of disease causation that underlies the GWAS methodology. Whereas this reductionist approach is fruitful for exploratory research purposes, it shows its limits when applied to clinical conditions.

Effect of CYP2C9 and VKORC1 genetic variations on warfarin dose requirements in Indian patients

BACKGROUND

Warfarin, an oral anticoagulant is used in patients who are at increased risk of developing blood clots. The management of warfarin therapy is challenging because it shows large inter and intra individual variability in patient response due to factors like age, gender, diet, concurrent drug interactions and variations in CYP2C9 and VKORC1 genes. Studies implicate that polymorphisms in VKORC1 and CYP2C9 genes are associated with reduced doses of warfarin. The aim of our current study was to characterize the effects of VKORC1 and CYP2C9 gene variations that contribute to variability in warfarin dosing in Indian patients.

METHODS

Genomic DNA was extracted from 103 patients undergoing warfarin therapy. Their mean daily warfarin dose, INR and demographics were recorded and genotyping of VKORC1 and CYP2C9 gene was performed by PCR-RFLP method.

RESULTS

Individuals with wild type genotypes required highest mean warfarin dosage of 4.72 mg/day while VKORC1 variants required 3.6 mg/day to maintain their therapeutic INR. CYP2C9*2 genotype was not found to affect the warfarin maintenance dosages. The odds ratio for developing supra therapeutic INR in patients carrying VKORC1 variant allele when compared to wild types was 13.96 (95% CI; 4.85 - 44.65. Other factors affecting warfarin dosages were age and weight.

CONCLUSION

Inclusion of pharmacogenetic data along with clinical parameters would help better predict warfarin doses in Indian patients.

Association of genetic polymorphisms with warfarin dose requirements in Chinese patients

OBJECTIVE

Warfarin is a commonly used anticoagulant with a narrow therapeutic range and large interindividual differences in dosing requirements. Previously, studies have identified that the interindividual variability was influenced by varieties of factors, including age, body size, vitamin K intake, interacting medications, as well as genetic variants. We aimed to investigate the effect of single-nucleotide polymorphisms (SNPs) on the interindividual variability of warfarin dose requirements in Chinese patients.

METHODS

The study population consisted of 300 patients with a stable maintenance dose of warfarin. We examined SNPs in eight genes involving in the biotransformation and mode of action of warfarin (i.e., CYP4F2, CYP2C19, APOE, CALU, EPHX1, PROC, CYP2C9, and GGCX) using the SNaPshot assay.

RESULTS

The mean daily warfarin dose in patients carrying CYP2C19 rs3814637CC, CYP2C9 rs1057910AA, and GGCX rs699664AA genotype was 3.39, 3.34, and 3.51 mg/day, respectively, which was higher than those carrying CYP2C19 rs3814637TT, CYP2C9 rs1057910CC, and rs699664GG genotype (2.00, 0.81, and 3.09 mg/day, respectively).

CONCLUSION

These findings indicate that individuals carrying the CYP2C19 rs3814637CC or CYP2C9 rs1057910AA or GGCX rs699664AA genotype needed higher warfarin doses in the Chinese population.

Pharmacogenetics of warfarin: challenges and opportunities

Since the introduction in the 1950s, warfarin has become the commonly used oral anticoagulant for the prevention of thromboembolism in patients with deep vein thrombosis, atrial fibrillation or prosthetic heart valve replacement. Warfarin is highly efficacious; however, achieving the desired anticoagulation is difficult because of its narrow therapeutic window and highly variable dose response among individuals. Bleeding is often associated with overdose of warfarin. There is overwhelming evidence that an individual's warfarin maintenance is associated with clinical factors and genetic variations, most notably polymorphisms in cytochrome P450 2C9 and vitamin K epoxide reductase subunit 1. Numerous dose-prediction algorithms incorporating both genetic and clinical factors have been developed and tested clinically. However, results from major clinical trials are not available yet. This review aims to provide an overview of the field of warfarin which includes information about the drug, genetics of warfarin dose requirements, dosing algorithms developed and the challenges for the clinical implementation of warfarin pharmacogenetics.

CYP2C9 Mutation Affecting the Individual Variability of Warfarin Dose Requirement

Warfarin is a frequently prescribed anticoagulant in rehabilitation patients. Adverse drug reactions of warfarin were reported as bleeding and cutaneous microvascular thrombosis. Major bleeding, such as intracranial hemorrhage and psoas hematoma, in patients receiving anticoagulation therapy is a rare condition, but sometimes very serious complication that can even be fatal. Patient-specific factors (eg, age, body size, race, concurrent diseases, and medications) explain some of the individual variability in warfarin dose, but genetic factors, which influence warfarin response, explain a significantly higher proportion of the variability in the dose. There are two identified genes that are responsible for the main proportion of the genetic effect: CYP2C9, which codes for the enzyme cytochrome P450 2C9 that metabolizes S-warfarin, and VKORC1, which codes for warfarin's target, vitamin K epoxide reductase. We report a case of intolerance to warfarin dosing, due to impaired drug metabolism in a patient with CYP2C9^*1/^*3 and VKORC 1173TT. Fortunately, there are no severe complications.

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

INTRODUCTION

Coumadin (R/S-warfarin) is the most widely prescribed oral anticoagulant in the world; nevertheless, its clinical use is complicated by unpredictability in dose requirements to achieve and maintain optimal anticoagulation. Variations in warfarin metabolism among patients contribute to unpredictability in therapeutic responses. Studying the clinical relevance of warfarin metabolism poses a significant analytical challenge. Warfarin is given to patients as an equal mixture of R and S enantiomers. Both drugs undergo extensive metabolism through different pathways to generate > 20 structurally similar isomeric metabolites.

AREAS COVERED

The article discusses how analytical methods have evolved to effectively resolve and quantify individual metabolites. The authors also discuss how the application of these methods has identified clinically relevant metabolic pathways for warfarin and fostered the investigation of clinical biomarkers for patient responses to therapy. The article additionally presents the power of these methods and how aspects of warfarin metabolism have led to the use of warfarin as a phenotyping probe for multiple drug metabolizing enzymes.

EXPERT OPINION

Progress in these areas has been hampered by shortcomings in analytical methods and a narrow focus on one metabolic pathway. Recent advances in liquid chromatographic-mass spectral methods can rapidly analyze most warfarin metabolites. It is now possible to effectively assess alternate metabolic pathways and expand biomarker analyses for clinical and phenotyping applications.

Risk modeling strategies for pharmacogenetic studies

Pharmacogenetic risk models offer great promise as treatment decision tools; however, their uptake in routine clinical practice is so far disappointing, not least due to the lack of evidence of their benefit in randomized controlled trials and other types of studies. Prior to conducting such a study, it is imperative that the model’s predictive capability is first of all proven, and that it is shown to be superior to the most appropriate alternative model. When demonstrating predictive capability, clinical implications of applying the model should be a key consideration, and the Decision Curve Analysis method takes this into account for binary outcomes. Furthermore, when comparing a novel model to the best alternative, methods such as Net Reclassification Improvement or Integrated Discrimination Difference are recommended as they provide a more reliable comparison than other methods currently in common use. Where outcome is continuous, such as therapeutic dose, assessing a model’s performance is generally more intuitive and straightforward since the aim is to achieve a predicted dose as close as possible to the true therapeutic dose.

Pharmacokinetic variability of long-acting stimulants in the treatment of children and adults with attention-deficit hyperactivity disorder

Methylphenidate- and amfetamine-based stimulants are first-line pharmacotherapies for attention-deficit hyperactivity disorder, a common neurobehavioural disorder in children and adults. A number of long-acting stimulant formulations have been developed with the aim of providing once-daily dosing, employing various means to extend duration of action, including a transdermal delivery system, an osmotic-release oral system, capsules with a mixture of immediate- and delayed-release beads, and prodrug technology. Coefficients of variance of pharmacokinetic measures can estimate the levels of pharmacokinetic variability based on the measurable variance between different individuals receiving the same dose of stimulant (interindividual variability) and within the same individual over multiple administrations (intraindividual variability). Differences in formulation clearly impact pharmacokinetic profiles. Many medications exhibit wide interindividual variability in clinical response. Stimulants with low levels of inter- and intraindividual variability may be better suited to provide consistent levels of medication to patients. The pharmacokinetic profile of stimulants using pH-dependent bead technology can vary depending on food consumption or concomitant administration of medications that alter gastric pH. While delivery of methylphenidate with the transdermal delivery system would be unaffected by gastrointestinal factors, intersubject variability is nonetheless substantial. Unlike the beaded formulations and, to some extent (when considering total exposure) the osmotic-release formulation, systemic exposure to amfetamine with the prodrug stimulant lisdexamfetamine dimesylate appears largely unaffected by such factors, likely owing to its dependence on systemic enzymatic cleavage of the precursor molecule, which occurs primarily in the blood involving red blood cells. The high capacity but as yet unidentified enzymatic system for conversion of lisdexamfetamine dimesylate may contribute to its consistent pharmacokinetic profile. The reasons underlying observed differential responses to stimulants are likely to be multifactorial, including pharmacodynamic factors. While the use of stimulants with low inter- and intrapatient pharmacokinetic variability does not obviate the need to titrate stimulant doses, stimulants with low intraindividual variation in pharmacokinetic parameters may reduce the likelihood of patients falling into subtherapeutic drug concentrations or reaching drug concentrations at which the risk of adverse events increases. As such, clinicians are urged both to adjust stimulant doses based on therapeutic response and the risk for adverse events and to monitor patients for potential causes of pharmacokinetic variability.

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.

10 years of oral anticoagulant pharmacogenomics: what difference will it make? A critical appraisal

Since the first report on warfarin pharmacogenetics in 1999, genetic variants have emerged as an important predictor of warfarin maintenance doses before therapy is initiated, raising expectations of greatly improved clinical outcomes. However, much of the information on warfarin sensitivity conveyed by genetic variants is captured by early international normalized ratio values traditionally used to guide dose titration. Thus, inclusion of early international normalized ratios in prediction models reduces the contribution of genetics. Moreover, in large population cohorts, genetics explained only 20–30% of variance in warfarin doses. Finally, even pharmacogenetic prediction models did not predict doses reliably in the majority of at-risk patients with warfarin requirements at the low or high end of the dose range. Currently, the clinical utility and cost–effectiveness of pharmacogenetic-based dosing are being assessed in large prospective trials in various settings. In the interim, enthusiasm for warfarin pharmacogenetics should not supersede strict adherence to traditional measures used to optimize coumarin anticoagulation.

A genome-wide association study confirms VKORC1, CYP2C9, and CYP4F2 as principal genetic determinants of warfarin dose

We report the first genome-wide association study (GWAS) whose sample size (1,053 Swedish subjects) is sufficiently powered to detect genome-wide significance (p<1.5×10⁻⁷) for polymorphisms that modestly alter therapeutic warfarin dose. The anticoagulant drug warfarin is widely prescribed for reducing the risk of stroke, thrombosis, pulmonary embolism, and coronary malfunction. However, Caucasians vary widely (20-fold) in the dose needed for therapeutic anticoagulation, and hence prescribed doses may be too low (risking serious illness) or too high (risking severe bleeding). Prior work established that ∼30% of the dose variance is explained by single nucleotide polymorphisms (SNPs) in the warfarin drug target VKORC1 and another ∼12% by two non-synonymous SNPs (*2, *3) in the cytochrome P450 warfarin-metabolizing gene CYP2C9. We initially tested each of 325,997 GWAS SNPs for association with warfarin dose by univariate regression and found the strongest statistical signals (p<10⁻⁷⁸) at SNPs clustering near VKORC1 and the second lowest p-values (p<10⁻³¹) emanating from CYP2C9. No other SNPs approached genome-wide significance. To enhance detection of weaker effects, we conducted multiple regression adjusting for known influences on warfarin dose (VKORC1, CYP2C9, age, gender) and identified a single SNP (rs2108622) with genome-wide significance (p = 8.3×10⁻¹⁰) that alters protein coding of the CYP4F2 gene. We confirmed this result in 588 additional Swedish patients (p<0.0029) and, during our investigation, a second group provided independent confirmation from a scan of warfarin-metabolizing genes. We also thoroughly investigated copy number variations, haplotypes, and imputed SNPs, but found no additional highly significant warfarin associations. We present power analysis of our GWAS that is generalizable to other studies, and conclude we had 80% power to detect genome-wide significance for common causative variants or markers explaining at least 1.5% of dose variance. These GWAS results provide further impetus for conducting large-scale trials assessing patient benefit from genotype-based forecasting of warfarin dose.

Recently, geneticists have begun assaying hundreds of thousands of genetic markers covering the entire human genome to systematically search for and identify genes that cause disease. We have extended this “genome-wide association study” (GWAS) method by assaying ∼326,000 markers in 1,053 Swedish patients in order to identify genes that alter response to the anticoagulant drug warfarin. Warfarin is widely prescribed to reduce blood clotting in order to protect high-risk patients from stroke, thrombosis, and heart attack. But patients vary widely (20-fold) in the warfarin dose needed for proper blood thinning, which means that initial doses in some patients are too high (risking severe bleeding) or too low (risking serious illness). Our GWAS detected two genes (VKORC1, CYP2C9) already known to cause ∼40% of the variability in warfarin dose and discovered a new gene (CYP4F2) contributing 1%–2% of the variability. Since our GWAS searched the entire genome, additional genes having a major influence on warfarin dose might not exist or be found in the near-term. Hence, clinical trials assessing patient benefit from individualized dose forecasting based on a patient's genetic makeup at VKORC1, CYP2C9 and possibly CYP4F2 could provide state-of-the-art clinical benchmarks for warfarin use during the foreseeable future.

Potential roles for pharmacists in pharmacogenetics

OBJECTIVES

To highlight areas of pharmacogenetics in which pharmacists may play a role and to describe those roles in the context of specific examples from a major academic medical center.

DATA SOURCES

Literature search (PubMed) and personal interviews for the University of California at San Francisco case examples.

DATA SYNTHESIS

The field of pharmacogenetics presents a wide range of opportunities for pharmacists. Specific roles for pharmacists are likely to fall within three major domains: developing research methodologies and setting research directions, establishing the value of pharmacogenetic testing in clinical practice, and participating in education and infrastructure development that moves pharmacogenetic technologies toward implementation.

CONCLUSION

As drug therapy experts, pharmacists are in a unique position to push the frontiers of pharmacogenetics in both the research and clinical practice environments.

CYP2C9 Genotype-guided Warfarin Prescribing Enhances the Efficacy and Safety of Anticoagulation: A Prospective Randomized Controlled Study

Warfarin anticoagulation effect is characterized by marked variability, some of which has been attributed to CYP2C9 polymorphisms. This study prospectively examines whether a priori knowledge of CYP2C9 genotype may improve warfarin therapy. Patients were randomly assigned to receive warfarin by a validated algorithm ("control", 96 patients) or CYP2C9 genotype-adjusted algorithms ("study", 95 patients). The first therapeutic international normalized ratio and stable anticoagulation were reached 2.73 and 18.1 days earlier in the study group, respectively (P<0.001). The faster rate of initial anticoagulation was driven by a 28% higher daily dose in the study group (P<0.001). Study group patients spent more time within the therapeutic range (80.4 vs 63.4%, respectively, P<0.001) and experienced less minor bleeding (3.2 vs 12.5%, P<0.02, respectively). In conclusion, CYP2C9 genotype-guided warfarin therapy is more efficient and safer than the "average-dose" protocol. Future research should focus on construction of algorithms that incorporate other polymorphisms (VKORC1), host factors, and environmental influences.

A case report of a patient carrying CYP2C9*3/4 genotype with extremely low warfarin dose requirement

We report a case of intolerance to warfarin dosing due to impaired drug metabolism in a patient with CYP2C9*3/*4. A 73-yr-old woman with atrial fibrilation was taking warfarin. She attained a high prothrombin time international normalized ratio (INR) at the standard doses during the induction of anticoagulation and extremely low dose of warfarin (6.5 mg/week) was finally chosen to reach the target INR. Genotyping for CYP2C9 revealed that this patient had a genotype CYP2C9*3/*4. This is the first Korean compound heterozygote for CYP2C9*3 and *4. This case suggests the clinical usefulness of pharmacogenetic testing for individualized dosage adjustments of warfarin.

Should pharmacogenomic studies be required for new drug approval?

The field of pharmacogenetics has existed since the 1950s, when it was demonstrated that some drug effects could differ substantially among race and ethnic groups, and that some drug metabolizing enzyme activities were inherited. During the 1990s, application of molecular biology to the study of inherited drug-related phenotypes proved the genetic basis of several genetic polymorphisms. Genomic technology has now demonstrated that germline genetic variability among humans is extremely common. The combined weight of proven examples whereby pharmacogenetics affects drugs, and the possibility of even more examples being elucidated in the coming decades, dictates that pharmacogenetics be incorporated into the drug approval process. It is our contention that minimal pharmacogenetic testing should be required for all new drug applications to the Food and Drug Administration (FDA). This would include a requirement for germline DNA to be prospectively collected from all subjects participating in preapproval clinical trials. For drugs that are metabolized by enzymes whose genes have clearly inactivating polymorphisms, clinical trial participants should be genotyped for those polymorphisms.

[Clinical pharmacogenomics for CYP2C8 and CYP2C9: general concepts and application to the use of NSAIDs]

OBJECTIVE

To study the major mutations in genes CYP2C8 and CYP2C9, their frequency in populations of diverse ethnical descent, their analysis methods, and the major drugs with affected metabolism, with a special emphasis on NSAIDs.

METHOD

Repeated searches of Pubmed (January 1966-January 2006) and Scholar Google were performed. All searches were restricted to studies in humans, and papers not written in Spanish or English were excluded.

RESULTS

Ten allelic variants of CYP2C8 and 24 of CYP2C have been reported. Not all of them exert a relevant effect on drug metabolism. In Caucasians 22% of CYP2C8 genes and 31% of CYP2C9 genes have mutations. In Asians fewer than 1% and nearly 3% are mutated, respectively. Major identification methods include endonuclease digestion, PCR, pyrosequencing, and microarrays. Not all NSAIDs are exclusive substrates for CYP2C8/9. The usefulness of allelic variant analysis varies with each individual drug. The risk for digestive hemorrhage associated with the CYP2C9 genotype is particularly relevant when using aceclofenac, celecoxib, diclofenac, ibuprofen, indomethacin, lornoxicam, piroxicam, or naproxen.

CONCLUSIONS

Although CYP2C8/9 activity plays an essential role in the metabolism of and clinical response to many NSAIDs, the use of pharmacogenomic techniques is not equally useful for all these drugs.

Frequency of CYP2C9 genotypes among Omani patients receiving warfarin and its correlation with warfarin dose

OBJECTIVES

This study was conducted to determine the frequency of CYP2C9 alleles in Omani patients receiving warfarin and to correlate genotyping data with warfarin dosage. The Omani population has Asian and African ethnicities.

METHODS

CYP2C9 genotypes were determined by the polymerase chain reaction restriction fragment length polymorphism method. Non-parametric Kruskal-Wallis test was used to compare groups of continuous data for significance differences.

RESULTS

Genotyping data showed that 12.7 and 5.8% of the samples were heterozygous for the CYP2C9*2 and CYP2C9*3 alleles, respectively. The CYP2C9*2 allele frequency was 0.074 in our population. It was 0.029 for CYP2C9*3.

CONCLUSION

This is the first report on the presence of CYP2C9*2 allele homozygocity in any Asian or African population.

Association of warfarin dose with genes involved in its action and metabolism

We report an extensive study of variability in genes encoding proteins that are believed to be involved in the action and biotransformation of warfarin. Warfarin is a commonly prescribed anticoagulant that is difficult to use because of the wide interindividual variation in dose requirements, the narrow therapeutic range and the risk of serious bleeding. We genotyped 201 patients for polymorphisms in 29 genes in the warfarin interactive pathways and tested them for association with dose requirement. In our study, polymorphisms in or flanking the genes VKORC1, CYP2C9, CYP2C18, CYP2C19, PROC, APOE, EPHX1, CALU, GGCX and ORM1-ORM2 and haplotypes of VKORC1, CYP2C9, CYP2C8, CYP2C19, PROC, F7, GGCX, PROZ, F9, NR1I2 and ORM1-ORM2 were associated with dose (P < 0.05). VKORC1, CYP2C9, CYP2C18 and CYP2C19 were significant after experiment-wise correction for multiple testing (P < 0.000175), however, the association of CYP2C18 and CYP2C19 was fully explained by linkage disequilibrium with CYP2C9*2 and/or *3. PROC and APOE were both significantly associated with dose after correction within each gene. A multiple regression model with VKORC1, CYP2C9, PROC and the non-genetic predictors age, bodyweight, drug interactions and indication for treatment jointly accounted for 62% of variance in warfarin dose. Weaker associations observed for other genes could explain up to approximately 10% additional dose variance, but require testing and validation in an independent and larger data set. Translation of this knowledge into clinical guidelines for warfarin prescription will be likely to have a major impact on the safety and efficacy of warfarin.

A PK-PD model for predicting the impact of age, CYP2C9, and VKORC1 genotype on individualization of warfarin therapy

The aim of this study was to characterize the relationship between warfarin concentrations and international normalized ratio (INR) response and to identify predictors important for dose individualization. S- and R-warfarin concentrations, INR, and CYP2C9 and VKORC1 genotypes from 150 patients were used to develop a population pharmacokinetic/pharmacodynamic model in NONMEM. The anticoagulant response was best described by an inhibitory E(MAX) model, with S-warfarin concentration as the only exposure predictor for response. Delay between exposure and response was accounted for by a transit compartment model with two parallel transit compartment chains. CYP2C9 genotype and age were identified as predictors for S-warfarin clearance, and VKORC1 genotype as a predictor for warfarin sensitivity. Predicted INR curves indicate important steady-state differences between patients with different sets of covariates; differences that cannot be foreseen from early INR assessments alone. It is important to account for CYP2C9 and VKORC1 genotypes and age to improve a priori and a posteriori individualization of warfarin therapy.

Individualizing warfarin therapy

Warfarin is the most commonly prescribed oral anticoagulant for the treatment and prevention of thromboembolic events. The correct maintenance dose of warfarin for a given patient is difficult to predict, the drug carries a high risk of toxicity, and variability among patients means that the safe dose range differs widely between individuals. Recent pharmacogenetic studies indicate that the routine incorporation of genetic testing into warfarin therapy protocols could substantially ease both the financial and health risks currently associated with this treatment. In particular, the variability in warfarin dose requirement is now recognized to be due, in large part, to polymorphisms in two genes: cytochrome P450 2C9 and the vitamin K epoxide reductase complex subunit 1. The development of algorithms that integrate all of the relevant genetic and physical factors into comprehensive, individualized predictive models for warfarin dose could be used to translate the results of pharmacogenetic testing into actionable clinical application.