Clinical Pharmacogenetics Implementation Consortium Guidelines for CYP2C9 and VKORC1 genotypes and warfarin dosing

A patient-centered approach to the development and pilot of a warfarin pharmacogenomics patient education tool for health professionals

OBJECTIVE

To describe an exploratory project to develop and pilot a novel patient educational tool that explains the concept of pharmacogenomics and its impact on warfarin dosing that can be utilized by health professionals providing patient counseling.

METHODS

A pharmacogenomics educational tool prototype was developed by an interdisciplinary team. During the pilot of the tool, focus group methodology was used to elicit input from patients based upon their perspectives and experiences with warfarin. Focus group sessions were audio-recorded and transcribed, and the data was analyzed through consensus coding in NVivo.

RESULTS

The focus group participants were generally unfamiliar with the concept of pharmacogenomics but were receptive to the information. They thought the patient education tool was informative and would provide the most benefit to patients newly initiated on warfarin therapy.

CONCLUSIONS

Preliminary results from this exploratory project suggest that implementation and further feasibility testing of this pharmacogenomics patient education tool should be performed in a population of newly initiated patients taking warfarin.

Warfarin dose requirements in a patient with the CYP2C9*14 allele

We describe a 64-year-old male of Indian descent with a history of atrial fibrillation who was started on warfarin after hospital admission for acute stroke. He received genotype-guided warfarin dosing as per the standard-of-care at our hospital, with daily dose recommendations provided by the pharmacogenetics service. Genotyping revealed the rare CYP2C9*1/*14 genotype and warfarin insensitive VKORC1 -1639GG and CYP4F2 433Met/Met genotypes. The patient received an initial warfarin loading dose of 4 mg for 2 days, followed by 2-3 mg/day for the following 11 days. He reached a therapeutic international normalized ratio on day 5, which was maintained over the following week. This report adds to the limited data of the effects of the CYP2C9*14 allele on warfarin dose requirements.

Personalized antiplatelet and anticoagulation therapy: applications and significance of pharmacogenomics

In recent years, substantial effort has been made to better understand the influence of genetic factors on the efficacy and safety of numerous medications. These investigations suggest that the use of pharmacogenetic data to inform physician decision-making has great potential to enhance patient care by reducing on-treatment clinical events, adverse drug reactions, and health care-related costs. In fact, integration of such information into the clinical setting may be particularly applicable for antiplatelet and anticoagulation therapeutics, given the increasing body of evidence implicating genetic variation in variable drug response. In this review, we summarize currently available pharmacogenetic information for the most commonly used antiplatelet (ie, clopidogrel and aspirin) and anticoagulation (ie, warfarin) medications. Furthermore, we highlight the currently known role of genetic variability in response to next-generation antiplatelet (prasugrel and ticagrelor) and anticoagulant (dabigatran) agents. While compelling evidence suggests that genetic variants are important determinants of antiplatelet and anticoagulation therapy response, significant barriers to clinical implementation of pharmacogenetic testing exist and are described herein. In addition, we briefly discuss development of new diagnostic targets and therapeutic strategies as well as implications for enhanced patient care. In conclusion, pharmacogenetic testing can provide important information to assist clinicians with prescribing the most personalized and effective antiplatelet and anticoagulation therapy. However, several factors may limit its usefulness and should be considered.

Genotype-Guided Dosing of Coumarin Anticoagulants

Background:

Coumarin anticoagulants (acenocoumarol, phenprocoumon, and warfarin) are generally used for the prevention of stroke in patients with atrial fibrillation or for the therapy and prevention of venous thromboembolism. However, the safe use of coumarin anticoagulants is restricted by a narrow therapeutic window and large interindividual dosing variations. Some studies found that the effectiveness and safety of coumarin anticoagulants therapy were increased by pharmacogenetic-guided dosing algorithms, while others found no significant effect of genotype-guided therapy.

Methods:

Four electronic databases were searched from January 1, 2000, to March 1, 2014, for randomized controlled trials of patients who received coumarin anticoagulants according to genotype-guided dosing algorithms. The primary outcome was the percentage of time that the international normalized ratio (INR) was within the normal range (2.0-3.0). Secondary outcomes included major bleeding events, thromboembolic events, and INR ≥4 events.

Results:

Eight studies satisfied the inclusion and exclusion criteria. Genotype-guided dosing of coumarin anticoagulants improved the percentage of time within the therapeutic INR range (95% confidence interval [CI], 0.02-0.28; P = .02; I2 = 70%). Subgroup analysis was performed after dividing the nongenotype-guided group into a standard-dose group (95% CI, 0.14-0.49; P = .0004; I2 = 50%) and a clinical variables-guided dosing algorithm group (95% CI, −0.07-0.15; P = .48; I2 = 34%). There is a statistically significant reduction in numbers of secondary outcomes (INR ≥4 events, major bleeding events, and thromboembolic events; 95% CI, 0.79-1.00; P = .04). Subgroup analysis of secondary outcomes showed no significant difference between genotype-guided dosing and clinical variables-guided dosing (95% CI, 0.84-1.10; P = .57; I2 = 11%), but genotype-guided dosing reduced secondary outcomes compared with standard dosing (95% CI, 0.62-0.92; P = .006; I2 = 0%).

Conclusions:

This meta-analysis showed that genotype-guided dosing increased the effectiveness and safety of coumarin therapy compared with standard dosing but did not have advantages compared with clinical variables-guided dosing.

Harnessing pharmacological knowledge for personalized medicine and pharmacotyping: Challenges and lessons learned

The contribution of the genetic make-up to an individual’s capacity has long been recognized in modern pharmacology as a crucial factor leading to therapy inefficiency and toxicity, negatively impacting the economic burden of healthcare and restricting the monitoring of diseases. In practical terms, and in order for drug prescription to be improved toward meeting the personalized medicine concept in drug delivery, the maximum clinical outcome for most, if not all, patients must be achieved, i.e., pharmacotyping. Such a direction although promising and of high expectation from the society, it is however hardly to be afforded for healthcare worldwide. To overcome any existed hurdles, this means that practical clinical utility of personalized medicine decisions have to be documented and validated in the clinical setting. The latter implies for drug delivery the efficient implementation of previously gained in vivo pharmacology experience with pharmacogenomics knowledge. As an approach to work faster and in a more productive way, the elaboration of advanced physiologically based pharmacokinetics models is discussed. And in better clarifying this topic, the example of tamoxifen is thoroughly presented. Overall, pharmacotyping represents a major challenge in modern therapeutics for which pharmacologists need to work in successfully fulfilling this task.

Drug metabolism for the paediatrician

Drug metabolism importantly determines drug concentrations. The efficacy and safety of many drugs prescribed for children are, therefore, dependent on intraindividual and interindividual variation in drug-metabolising enzyme activity. During growth and development, changes in drug-metabolising enzyme activity result in age-related differences in drug disposition, most pronounced in preterm infants and young infants. The shape of the developmental trajectory is unique to the drug-metabolising enzyme involved in the metabolism of individual drugs. Other factors impacting drug metabolism are underlying disease, drug-drug interactions and genetic variation. The interplay of age with these other factors may result in unexpected variation in drug metabolism in children of different ages. Extrapolation of adult data to guide drug dosing in children should be done with caution. The younger the child, the less reliable is the extrapolation. This review aims to identify the primary sources of variability of drug metabolism in children, the knowledge of which can ultimately guide the practitioner towards effective and safe drug therapy.

Genomic architecture of pharmacological efficacy and adverse events

The pharmacokinetic and pharmacodynamic disciplines address pharmacological traits, including efficacy and adverse events. Pharmacogenomics studies have identified pervasive genetic effects on treatment outcomes, resulting in the development of genetic biomarkers for optimization of drug therapy. Pharmacogenomics-based tests are already being applied in clinical decision making. However, despite substantial progress in identifying the genetic etiology of pharmacological response, current biomarker panels still largely rely on single gene tests with a large portion of the genetic effects remaining to be discovered. Future research must account for the combined effects of multiple genetic variants, incorporate pathway-based approaches, explore gene-gene interactions and nonprotein coding functional genetic variants, extend studies across ancestral populations, and prioritize laboratory characterization of molecular mechanisms. Because genetic factors can play a key role in drug response, accurate biomarker tests capturing the main genetic factors determining treatment outcomes have substantial potential for improving individual clinical care.

Incorporation of pharmacogenomics into routine clinical practice: the Clinical Pharmacogenetics Implementation Consortium (CPIC) guideline development process

The Clinical Pharmacogenetics Implementation Consortium (CPIC) publishes genotype-based drug guidelines to help clinicians understand how available genetic test results could be used to optimize drug therapy. CPIC has focused initially on well-known examples of pharmacogenomic associations that have been implemented in selected clinical settings, publishing nine to date. Each CPIC guideline adheres to a standardized format and includes a standard system for grading levels of evidence linking genotypes to phenotypes and assigning a level of strength to each prescribing recommendation. CPIC guidelines contain the necessary information to help clinicians translate patient-specific diplotypes for each gene into clinical phenotypes or drug dosing groups. This paper reviews the development process of the CPIC guidelines and compares this process to the Institute of Medicine’s Standards for Developing Trustworthy Clinical Practice Guidelines.

Pilot study: incorporation of pharmacogenetic testing in medication therapy management services

Aim: To describe the rationale and design of a pilot study evaluating the integration of pharmacogenetic (PGx) testing into pharmacist-delivered medication therapy management (MTM). Study rationale: Clinical delivery approaches of PGx testing involving pharmacists may overcome barriers of limited physician knowledge about and experience with testing. Study design: We will assess the addition of PGx testing to MTM services for cardiology patients taking three or more medications including simvastatin or clopidogrel. We will measure the impact of MTM plus PGx testing on drug/dose adjustment and clinical outcomes. Factors associated with delivery, such as time to prepare and conduct MTM and consult with physicians will be recorded. Additionally, patient interest and satisfaction will be measured. Anticipated results: We anticipate that PGx testing can be practically integrated into standard a MTM service, providing a viable delivery model for testing. Conclusion: Given the lack of evidence of an effective PGx delivery models, this study will provide preliminary evidence regarding a pharmacist-delivered approach.

Effect of impaired renal function on the maintenance dose of warfarin in Japanese patients

BACKGROUND

Chronic kidney disease (CKD) alters dose-effect relationship not only of drugs eliminated by the kidney but also of some drugs metabolized by the liver and not renally excreted. It is not known whether impaired renal function alters dose-effect relationship of warfarin in Asian patients. It is also unknown whether the maintenance dose of warfarin can be predicted more accurately by incorporating renal function in Asians.

METHODS

This was a cross-sectional study of patients receiving constant doses of warfarin who had PT-INR within 1.5-3.0 for 3 months or longer.

RESULTS

In a total of 137 participants, the estimated creatinine clearance (eCrCl) was 62.5±25.5 [ml/min] and the warfarin dose was 3.21±1.46 [mg/day] (both mean±standard deviation). There was a significant correlation between warfarin dose and eCrCl (p<0.0001, r(2)=0.23). In a stepwise linear regression with the maintenance dose of warfarin as the dependent variable, eCrCl as well as age, body weight, intra-individual average prothrombin time/international normalized ratio (PT-INR), and genotype of VKORC1 -1639 G>A polymorphism were chosen as independent variables. The coefficient of determination (r(2)) of this formula was 0.47. A regression equation with all the same explanatory variables except for eCrCl had an r(2) of 0.41.

CONCLUSIONS

The maintenance warfarin dose was positively correlated with kidney function as represented by eCrCl in Japanese patients. Incorporating eCrCl improved accuracy of predicting warfarin maintenance dose in this population.

Warfarin-related nephropathy: possible role for the warfarin pharmacogenetic profile

Warfarin-related nephropathy (WRN) is a renal complication of warfarin treatment associated with over-anticoagulation. We describe a case of a 73-year-old man affected by chronic kidney disease, essential hypertension and atrial fibrillation treated with warfarin. The patient presented a rapid course of kidney failure after many episodes of over-anticoagulation, and renal biopsy demonstrated WRN. Interestingly, the patient's warfarin pharmacogenetic profile showed that he was warfarin sensitive. This is the first report describing the presence of gene polymorphisms affecting warfarin metabolism in a subject with a biopsy-proven WRN. The patient was treated with corticosteroids obtaining a partial clinical response.

Priority pharmacogenetics for the African continent: focus on CYP450

Countries in Africa have a high burden of communicable disease, and are experiencing an increase in noncommunicable diseases due to the effects of globalization, industrialization and urbanization. The costs incurred through adverse drug reactions and nonresponsiveness to therapy further aggravates the situation, and the application of pharmacogenetic principles is likely to provide some relief. Having undertaken an extensive evaluation of CYP450 reports in Africa, our objective was to map out areas of need based on regional disease burdens. The data confirms a paucity of CYP450 reports and illustrates large regions for which no population information exists. There is a dire need to address the health problems of Africa, and wide-scale pharmacogenetic profiling of these populations will add significantly to improving patient care on the continent. Priority pharmacogenetics for the African continent gives precedence to the profiling of clinically relevant pharmacogenetic biomarkers, and defines the immediate need in the context of disease burden.

Genetic variant in folate homeostasis is associated with lower warfarin dose in African Americans

The anticoagulant warfarin has >30 million prescriptions per year in the United States. Doses can vary 20-fold between patients, and incorrect dosing can result in serious adverse events. Variation in warfarin pharmacokinetic and pharmacodynamic genes, such as CYP2C9 and VKORC1, do not fully explain the dose variability in African Americans. To identify additional genetic contributors to warfarin dose, we exome sequenced 103 African Americans on stable doses of warfarin at extremes (≤ 35 and ≥ 49 mg/week). We found an association between lower warfarin dose and a population-specific regulatory variant, rs7856096 (P = 1.82 × 10(-8), minor allele frequency = 20.4%), in the folate homeostasis gene folylpolyglutamate synthase (FPGS). We replicated this association in an independent cohort of 372 African American subjects whose stable warfarin doses represented the full dosing spectrum (P = .046). In a combined cohort, adding rs7856096 to the International Warfarin Pharmacogenetic Consortium pharmacogenetic dosing algorithm resulted in a 5.8 mg/week (P = 3.93 × 10(-5)) decrease in warfarin dose for each allele carried. The variant overlaps functional elements and was associated (P = .01) with FPGS gene expression in lymphoblastoid cell lines derived from combined HapMap African populations (N = 326). Our results provide the first evidence linking genetic variation in folate homeostasis to warfarin response.

Preemptive clinical pharmacogenetics implementation: current programs in five US medical centers

Although the field of pharmacogenetics has existed for decades, practioners have been slow to implement pharmacogenetic testing in clinical care. Numerous publications describe the barriers to clinical implementation of pharmacogenetics. Recently, several freely available resources have been developed to help address these barriers. In this review, we discuss current programs that use preemptive genotyping to optimize the pharmacotherapy of patients. Array-based preemptive testing includes a large number of relevant pharmacogenes that impact multiple high-risk drugs. Using a preemptive approach allows genotyping results to be available prior to any prescribing decision so that genomic variation may be considered as an inherent patient characteristic in the planning of therapy. This review describes the common elements among programs that have implemented preemptive genotyping and highlights key processes for implementation, including clinical decision support.

Respiratory review of 2014: pulmonary thromboembolism

Venous thromboembolism (VTE), which includes pulmonary embolism and deep vein thrombosis, is an important cause of morbidity and mortality. The aim of this review is to summarize the findings from clinically important publications over the last year in the area of VTE. In this review, we discuss 11 randomized controlled trials published from March 2013 to April 2014. The COAG and the EU-PACT trials indicate that pharmacogenetic testing has either no usefulness in the initial dosing of vitamin K antagonists or marginal usefulness in the Caucasian population. Recent clinical trials with novel oral anticoagulants (NOACs) have demonstrated that the efficacy and safety of rivaroxaban, apixaban, edoxaban, and dabigatran are not inferior to those of conventional anticoagulants for the treatment of VTE. The PEITHO and ULTIMA trials suggested that rescue thrombolysis or catheter-directed thrombolysis may maximize the clinical benefits and minimize the bleeding risk. Lastly, riociguat has a proven efficacy in treating chronic thromboembolic pulmonary hypertension. In the future, NOACs, riociguat, and catheter-directed thrombolysis have the potential to revolutionize the management of patients with VTE.

Progress towards the integration of pharmacogenomics in practice

Understanding the role genes and genetic variants play in clinical treatment response continues to be an active area of research with the goal of common clinical use. This goal has developed into today's industry of pharmacogenomics, where new drug-gene relationships are discovered and further characterized, published and then curated into national and international resources for use by researchers and clinicians. These efforts have given us insight into what a pharmacogenomic variant is, and how it differs from human disease variants and common polymorphisms. While publications continue to reveal pharmacogenomic relationships between genes and specific classes of drugs, many challenges remain toward the goal of widespread use clinically. First, the clinical guidelines for pharmacogenomic testing are still in their infancy. Second, sequencing technologies are changing rapidly making it somewhat unclear what genetic data will be available to the clinician at the time of care. Finally, what and when to return data to a patient is an area under constant debate. New innovations such as PheWAS approaches and whole genome sequencing studies are enabling a tsunami of new findings. In this review, pharmacogenomic variants, pharmacogenomic resources, interpretation clinical guidelines and challenges, such as WGS approaches, and the impact of pharmacogenomics on drug development and regulatory approval are reviewed.

Warfarin pharmacogenetics

The cytochrome P450 (CYP) 2C9 and vitamin K epoxide reductase complex 1 (VKORC1) genotypes have been strongly and consistently associated with warfarin dose requirements, and dosing algorithms incorporating genetic and clinical information have been shown to be predictive of stable warfarin dose. However, clinical trials evaluating genotype-guided warfarin dosing produced mixed results, calling into question the utility of this approach. Recent trials used surrogate markers as endpoints rather than clinical endpoints, further complicating translation of the data to clinical practice. The present data do not support genetic testing to guide warfarin dosing, but in the setting where genotype data are available, use of such data in those of European ancestry is reasonable. Outcomes data are expected from an on-going trial, observational studies continue, and more work is needed to define dosing algorithms that incorporate appropriate variants in minority populations; all these will further shape guidelines and recommendations on the clinical utility of genotype-guided warfarin dosing.

Drug-gene interactions: inherent variability in drug maintenance dose requirements

The influence of genetics on pharmacokinetics can introduce variability among individual patients that may cause treatment failure, toxicity, or both. Such variability, specifically in clearance rates, can influence drug maintenance dosing regimens.

Biomarkers: refining diagnosis and expediting drug development - reality, aspiration and the role of open innovation

In the last decade, there have been intensive efforts to invent, qualify and use novel biomarkers as a means to improve success rates in drug discovery and development. The biomarkers field is maturing and this article considers whether these research efforts have brought about the expected benefits. The characteristics of a clinically useful biomarker are described and the impact this area of research has had is evaluated by reviewing a few, key examples of emerging biomarkers. There is evidence that the impact has been genuine and is increasing in both the drug and the diagnostic discovery and development processes. Beneficial impact on patient health outcomes seems relatively limited thus far, with the greatest impact in oncology (again, both in terms of novel drugs and in terms of more refined diagnoses and therefore more individualized treatment). However, the momentum of research would indicate that patient benefits are likely to increase substantially and to broaden across multiple therapeutic areas. Even though this research was originally driven by a desire to improve the drug discovery and development process, and was therefore funded with this aim in mind, it seems likely that the largest impact may actually come from more refined diagnosis. Refined diagnosis will facilitate both better allocation of healthcare resources and the use of treatment regimens which are optimized for the individual patient. This article also briefly reviews emerging technological approaches and how they relate to the challenges inherent in biomarker discovery and validation, and discusses the role of public/private partnerships in innovative biomarker research.

Pharmacogenetic association study of warfarin safety endpoints in Puerto Ricans

OBJECTIVE

This study was intended to determine the incidence rate of warfarin-related adverse events (e.g., bleeding) in Puerto Ricans and whether a genetic association between warfarin pharmacogenes and any of these adverse events was observed over the initiation period (i.e., the first 90 days of therapy).

METHODS

We conducted an observational, retrospective cohort study of pharmacogenetic association in 122 warfarin-treated, male, Puerto Rican patients (69.9 +/- 9.6 years) from the Veterans Affair Caribbean Healthcare System (VACHS) who consented to participate. Genotyping was performed using the CYP2C9 and VKORC1 assays by Luminex. Event-free survival curves were estimated using the Kaplan-Meier method and analyzed by log-rank test. Cox regression models were constructed and hazard ratios (HR) calculated.

RESULTS

Carriers of functional CYP2C9 and VKORC1 polymorphisms demonstrated a higher incidence rate of multiple adverse events (i.e., 5.2 vs. 1.0 cases per 100 patient-months; RR = 4.8, p = 0.12) than did wild types. A significant association was observed between multiple adverse events and carrier status (HR = 2.5; 95% CI: 1.0-6.3, p = 0.04). However, no significant associations between genotypes and individual outcomes over the first 90 days of therapy were found.

CONCLUSION

The association of CYP2C9 and VKORC1 genotypes and risks for adverse events due to exposure to warfarin was examined for the first time in Puerto Ricans. Despite a lack of association with individual events in this study population, our findings revealed a potential utility of genotyping for the prevention of multiple adverse events during warfarin therapy.

Determinants of the over-anticoagulation response during warfarin initiation therapy in Asian patients based on population pharmacokinetic-pharmacodynamic analyses

To clarify pharmacokinetic-pharmacodynamic (PK-PD) factors associated with the over-anticoagulation response in Asians during warfarin induction therapy, population PK-PD analyses were conducted in an attempt to predict the time-courses of the plasma S-warfarin concentration, Cp(S), and coagulation and anti-coagulation (INR) responses. In 99 Chinese patients we analyzed the relationships between dose and Cp(S) to estimate the clearance of S-warfarin, CL(S), and that between Cp(S) and the normal prothrombin concentration (NPT) as a coagulation marker for estimation of IC₅₀. We also analyzed the non-linear relationship between NPT inhibition and the increase in INR to derive the non-linear index λ. Population analyses accurately predicted the time-courses of Cp(S), NPT and INR. Multivariate analysis showed that CYP2C9*3 mutation and body surface area were predictors of CL(S), that VKORC1 and CYP4F2 polymorphisms were predictors of IC₅₀, and that baseline NPT was a predictor of λ. CL(S) and λ were significantly lower in patients with INR≥4 than in those with INR<4 (190 mL/h vs 265 mL/h, P<0.01 and 3.2 vs 3.7, P<0.01, respectively). Finally, logistic regression analysis revealed that CL(S), ALT and hypertension contributed significantly to INR≥4. All these results indicate that factors associated with the reduced metabolic activity of warfarin represented by CL(S), might be critical determinants of the over-anticoagulation response during warfarin initiation in Asians.

Trial Registration

ClinicalTrials.gov NCT02065388

Implementation and utilization of genetic testing in personalized medicine

Clinical genetic testing began over 30 years ago with the availability of mutation detection for sickle cell disease diagnosis. Since then, the field has dramatically transformed to include gene sequencing, high-throughput targeted genotyping, prenatal mutation detection, preimplantation genetic diagnosis, population-based carrier screening, and now genome-wide analyses using microarrays and next-generation sequencing. Despite these significant advances in molecular technologies and testing capabilities, clinical genetics laboratories historically have been centered on mutation detection for Mendelian disorders. However, the ongoing identification of deoxyribonucleic acid (DNA) sequence variants associated with common diseases prompted the availability of testing for personal disease risk estimation, and created commercial opportunities for direct-to-consumer genetic testing companies that assay these variants. This germline genetic risk, in conjunction with other clinical, family, and demographic variables, are the key components of the personalized medicine paradigm, which aims to apply personal genomic and other relevant data into a patient’s clinical assessment to more precisely guide medical management. However, genetic testing for disease risk estimation is an ongoing topic of debate, largely due to inconsistencies in the results, concerns over clinical validity and utility, and the variable mode of delivery when returning genetic results to patients in the absence of traditional counseling. A related class of genetic testing with analogous issues of clinical utility and acceptance is pharmacogenetic testing, which interrogates sequence variants implicated in interindividual drug response variability. Although clinical pharmacogenetic testing has not previously been widely adopted, advances in rapid turnaround time genetic testing technology and the recent implementation of preemptive genotyping programs at selected medical centers suggest that personalized medicine through pharmacogenetics is now a reality. This review aims to summarize the current state of implementing genetic testing for personalized medicine, with an emphasis on clinical pharmacogenetic testing.

Clinical pharmacogenetics implementation consortium guidelines for CYP2C9 and HLA-B genotypes and phenytoin dosing

Phenytoin is a widely used antiepileptic drug with a narrow therapeutic index and large interpatient variability, partly due to genetic variations in the gene encoding cytochrome P450 (CYP)2C9 (CYP2C9). Furthermore, the variant allele HLA-B*15:02, encoding human leukocyte antigen, is associated with an increased risk of Stevens-Johnson syndrome and toxic epidermal necrolysis in response to phenytoin treatment. We summarize evidence from the published literature supporting these associations and provide recommendations for the use of phenytoin based on CYP2C9 and/or HLA-B genotype (also available on PharmGKB: http://www.pharmgkb.org). The purpose of this guideline is to provide information for the interpretation of HLA-B and/or CYP2C9 genotype tests so that the results can guide dosing and/or use of phenytoin. Detailed guidelines for the use of phenytoin as well as analyses of cost-effectiveness are out of scope. Clinical Pharmacogenetics Implementation Consortium (CPIC) guidelines are periodically updated at http://www.pharmgkb.org.

Warfarin pharmacogenetics: to genotype or not to genotype, that is the question

Genotype is well recognized to influence the dose of warfarin necessary for therapeutic anticoagulation. Recent randomized controlled trials evaluating the clinical utility of genotype-guided warfarin dosing have produced varying results. We review the design and results of the recent clinical trials, assess the impact of their findings on warfarin dosing, and examine unanswered questions related to clinical implementation of warfarin pharmacogenetics.

New oral anticoagulants vs. warfarin treatment: no need for pharmacogenomics?

For patients requiring long-term anticoagulation, oral vitamin K antagonists (VKAs) such as warfarin have overwhelming efficacy data and present significant challenges. In addition to the potential exposure to numerous drug-drug and drug-food interactions, patients receiving warfarin require frequent monitoring. It had been hoped that the integration of pharmacogenomic with clinical information would improve anticoagulation control with warfarin, but trials have not supported this aim. Novel oral anticoagulants (NOACs) offer both advantages and disadvantages and deserve consideration in appropriate patients.

Frequency of CYP2C9 and VKORC1 gene polymorphisms and their influence on warfarin dose in Egyptian pediatric patients

INTRODUCTION

Warfarin is a widely used anticoagulant that shows a high inter-individual variability in the dose needed to achieve target anticoagulation. In adults, common genetic variants in the cytochrome P450-2C9 (CYP2C9) and vitamin K epoxide reductase complex (VKORC1) enzymes, in addition to non-genetic factors, explain this dose variability. In children, data about warfarin pharmacogenetics are limited and inconsistent.

METHODS

CYP2C9 (*2 and *3) alleles and the VKORC1 (C1173T and G-1639A) polymorphisms were studied by multiplex real time polymerase chain reaction in 41 pediatric patients who received stable warfarin maintenance dose.

RESULTS

The allele frequency of the studied genes was CYP2C9*2 (0.085), CYP2C9*3 (0.12), VKORC1 1173T (0.52), and VKORC1 -1639A (0.54). In univariate analysis, patients' age, weight, and height were significantly (p < 0.0001) associated with warfarin maintenance dose. However, CYP2C9 and VKORC1 gene polymorphisms did not affect warfarin dose. In multivariate analysis, age was found to be the only significant determinant of daily warfarin maintenance dose (p = 0.045).

CONCLUSION

Age was the most significant determinant of warfarin dosage in this preliminary study including Egyptian pediatric patients. Further studies involving larger numbers of children are warranted to determine the true impact of genetic factors on warfarin doses in pediatric patients.

Clinical utility of genetic testing in pediatric drug-resistant epilepsy: a pilot study

RATIONALE

The utility of genetic testing in pediatric drug-resistant epilepsy (PDRE), its yield in "real life" clinical practice, and the practical implications of such testing are yet to be determined.

GOAL

To start to address the above gaps in our knowledge as they apply to a patient population seen in a tertiary care center.

METHODS

We retrospectively reviewed our experience with the use of clinically available genetic tests in the diagnosis and management of PDRE in one clinic over one year. Genetic testing included, depending on clinical judgment, one or more of the following: karyotype, chromosomal microarray, single gene sequencing, gene sequencing panels, and/or whole exome sequencing (WES).

RESULTS

We were more likely to perform genetic testing in patients with developmental delay, epileptic encephalopathy, and generalized epilepsy. In our unique population, the yield of specific genetic diagnosis was relatively high: karyotype 14.3%, microarray 16.7%, targeted single gene sequencing 15.4%, gene panels 46.2%, and WES 16.7%. Overall yield of diagnosis from at least one of the above tests was 34.5%. Disease-causing mutations that were not clinically suspected based on the patients' phenotypes and representing novel phenotypes were found in 6.9% (2/29), with an additional 17.2% (5/29) demonstrating pharmacologic variants. Three patients were incidentally found to be carriers of recessive neurologic diseases (10.3%). Variants of unknown significance (VUSs) were identified in 34.5% (10/29).

CONCLUSIONS

We conclude that genetic testing had at least some utility in our patient population of PDRE, that future similar larger studies in various populations are warranted, and that clinics offering such tests must be prepared to address the complicated questions raised by the results of such testing.

Understanding and preventing drug-drug and drug-gene interactions

Concomitant administration of multiple drugs can lead to unanticipated drug interactions and resultant adverse drug events with their associated costs. A more thorough understanding of the different cytochrome P450 isoenzymes and drug transporters has led to new methods to try to predict and prevent clinically relevant drug interactions. There is also an increased recognition of the need to identify the impact of pharmacogenetic polymorphisms on drug interactions. More stringent regulatory requirements have evolved for industry to classify cytochrome inhibitors and inducers, test the effect of drug interactions in the presence of polymorphic enzymes, and evaluate multiple potentially interacting drugs simultaneously. In clinical practice, drug alert software programs have been developed. This review discusses drug interaction mechanisms and strategies for screening and minimizing exposure to drug interactions. We also provide future perspectives for reducing the risk of clinically significant drug interactions.

The emerging era of pharmacogenomics: current successes, future potential, and challenges

The vast range of genetic diversity contributes to a wonderful array of human traits and characteristics. Unfortunately, a consequence of this genetic diversity is large variability in drug response between people, meaning that no single medication is safe and effective in everyone. The debilitating and sometimes deadly consequences of adverse drug reactions (ADRs) are a major and unmet problem of modern medicine. Pharmacogenomics can uncover associations between genetic variation and drug safety and has the potential to predict ADRs in individual patients. Here we review pharmacogenomic successes leading to changes in clinical practice, as well as clinical areas probably to be impacted by pharmacogenomics in the near future. We also discuss some of the challenges, and potential solutions, that remain for the implementation of pharmacogenomic testing into clinical practice for the significant improvement of drug safety.

VKORC1 and CYP2C9 genotypes are predictors of warfarin-related outcomes in children

BACKGROUND

Despite substantial evidence supporting a pharmacogenetic approach to warfarin therapy in adults, evidence on the importance of genetics in warfarin therapy in children is limited, particularly for clinical outcomes. We assessed the contribution of CYP2C9/VKORC1/CYP4F2 genotypes and variation in other genes involved in vitamin K and coagulation pathways to warfarin dose and related clinical outcomes in children.

PROCEDURE

Clinical and genetic data for 93 children (age ≤ 18 years) who received warfarin therapy were obtained. DNA was genotyped for 93 selected single nucleotide polymorphisms using a custom assay.

RESULTS

With a median age of 4.8 years, our cohort included more young children than most previous studies. Overall, 76.3% of dose variability was explained by weight, indication, VKORC1-1639G/A and CYP2C9 *2/*3, with genotypes accounting for 21.1% of variability. There was a strong correlation (R(2) = 0.68; P < 0.001) between actual and predicted warfarin dose using a pediatric genotype-based dosing model. VKORC1 genotype had a significant impact on time to therapeutic international normalized ratio (INR) (P = 0.047) and time to over-anticoagulation (INR > 4; P = 0.024) during the initiation of therapy. CYP2C9*3 carriers were also at increased risk of major bleeding while receiving warfarin (adjusted OR = 11.28). An additional variant in CYP2C9 (rs7089580) was significantly associated with warfarin dose (P = 0.020) in a multivariate clinical and genetic model.

CONCLUSIONS

This study confirms the importance of VKORC1/CYP2C9 genotypes for warfarin dosing in a young pediatric cohort and demonstrates an impact of genetic factors on clinical outcomes in children. Furthermore, we identified an additional variant in CYP2C9 of potential relevance for warfarin dosing in children.

Towards the clinical implementation of pharmacogenetics in bipolar disorder

BACKGROUND

Bipolar disorder (BD) is a psychiatric illness defined by pathological alterations between the mood states of mania and depression, causing disability, imposing healthcare costs and elevating the risk of suicide. Although effective treatments for BD exist, variability in outcomes leads to a large number of treatment failures, typically followed by a trial and error process of medication switches that can take years. Pharmacogenetic testing (PGT), by tailoring drug choice to an individual, may personalize and expedite treatment so as to identify more rapidly medications well suited to individual BD patients.

DISCUSSION

A number of associations have been made in BD between medication response phenotypes and specific genetic markers. However, to date clinical adoption of PGT has been limited, often citing questions that must be answered before it can be widely utilized. These include: What are the requirements of supporting evidence? How large is a clinically relevant effect? What degree of specificity and sensitivity are required? Does a given marker influence decision making and have clinical utility? In many cases, the answers to these questions remain unknown, and ultimately, the question of whether PGT is valid and useful must be determined empirically. Towards this aim, we have reviewed the literature and selected drug-genotype associations with the strongest evidence for utility in BD.

SUMMARY

Based upon these findings, we propose a preliminary panel for use in PGT, and a method by which the results of a PGT panel can be integrated for clinical interpretation. Finally, we argue that based on the sufficiency of accumulated evidence, PGT implementation studies are now warranted. We propose and discuss the design for a randomized clinical trial to test the use of PGT in the treatment of BD.

Personalized medicine, availability, and group disparity: an inquiry into how physicians perceive and rate the elements and barriers of personalized medicine

BACKGROUND

The success of personalized medicine depends on factors influencing the availability and implementation of its new tools to individualize clinical care. However, little is known about physicians' views of the availability of personalized medicine across racial/ethnic groups and the relationship between perceived availability and clinical implementation. This study examines physicians' perceptions of key elements/tools and potential barriers to personalized medicine in connection with their perceptions of the availability of the latter across subpopulations.

METHODS

Study subjects consisted of physicians recruited from Cincinnati Children's Hospital Medical Center and UC Health. An electronic survey conducted from September 2012 to November 2012 recruited 104 physicians. Wilcoxon rank sum analysis compared groups.

RESULTS

Physicians were divided about whether personalized medicine contributes to health equality, as 37.4% of them believe that personalized medicine is currently available only for some subpopulations. They also rated the importance of racial/ethnic background almost as high as the importance of genetic information in the delivery of personalized medicine. Actual elements of personalized medicine rated highest include family history, drug-drug interaction alerts in medical records, and biomarker measurements to guide therapy. Costs of gene-based therapies and genetic testing were rated the most significant barriers. The ratings of several elements and barriers were associated with perceived availability of personalized medicine across subpopulations.

CONCLUSION

While physicians hold differing views about the availability and implementation of personalized medicine, they likewise establish complex relationships between race/ethnicity and personalized medicine that may carry serious implications for its clinical success.

Pharmacogenetics of antiplatelets and anticoagulants: a report on clopidogrel, warfarin and dabigatran

Genetic polymorphisms are thought to contribute to the wide intraindividual variability in antiplatelet and anticoagulant drug response. Pharmacogenetics is the study of how genetic variants influence drug response and how the adoption of a more personalized approach in antiplatelet and anticoagulant therapy may help to minimize harmful drug effects and optimize care for individual patients. However, due to sometimes conflicting evidence, the uptake of pharmacogenetics in the clinical setting has been slow. In this article, we review the genetic mechanisms contributing to the variability in response to three commonly used and emerging antiplatelet and anticoagulant drug therapies, namely clopidogrel, warfarin and dabigatran. We will focus on common genetic variants that influence the absorption, metabolism and/or action of these agents, including CYP2C19 (*2, *3 and *17), CYP3A4, CYP3A5, CYP2C9, ABCB1, P2RY12, CYP2C9 (*2/*3), VKORC1 and CESI.

Prioritizing genomic applications for action by level of evidence: a horizon-scanning method

As evidence accumulates on the use of genomic tests and other health-related applications of genomic technologies, decision makers may increasingly seek support in identifying which applications have sufficiently robust evidence to suggest they might be considered for action. As an interim working process to provide such support, we developed a horizon-scanning method that assigns genomic applications to tiers defined by availability of synthesized evidence. We illustrate an application of the method to pharmacogenomics tests.

A model predicting fluindione dose requirement in elderly inpatients including genotypes, body weight, and amiodarone

Indandione VKAs have been widely used for decades, especially in Eastern Europe and France. Contrary to coumarin VKAs, the relative contribution of individual factors to the indandione-VKA response is poorly known. In the present multicentre study, we sought to develop and validate a model including genetic and non-genetic factors to predict the daily fluindione dose requirement in elderly patients in whom VKA dosing is challenging. We prospectively recorded clinical and therapeutic data in 230 Caucasian inpatients mean aged 85 ± 6 years, who had reached international normalized ratio stabilisation (range 2.0-3.0) on fluindione. In the derivation cohort (n=156), we analysed 13 polymorphisms in seven genes potentially involved in the pharmacological effect or vitamin-K cycle (VKORC1, CYP4F2, EPHX1) and fluindione metabolism/transport (CYP2C9, CYP2C19, CYP3A5, ABCB1). We built a regression model incorporating non-genetic and genetic data and evaluated the model performances in a separate cohort (n=74).Body-weight, amiodarone intake, VKORC1, CYP4F2, ABCB1 genotypes were retained in the final model, accounting for 31.5% of dose variability. None influence of CYP2C9 was observed. Our final model showed good performances: in 83.3% of the validation cohort patients, the dose was accurately predicted within 5 mg, i.e.the usual step used for adjusting fluindione dosage. In conclusion, in addition to body-weight and amiodarone-intake, pharmacogenetic factors (VKORC1, CYP4F2, ABCB1) related to the pharmacodynamic effect and transport of fluindione significantly influenced the dose requirement in elderly patients while CYP2C9 did not. Studies are required to know whether fluindione could be an alternative VKA in carriers of polymorphic CYP2C9 alleles, hypersensitive to coumarins.

Pharmacogenetics in clinical practice: how far have we come and where are we going?

Recent years have seen great advances in our understanding of genetic contributors to drug response. Drug discovery and development around targeted genetic (somatic) mutations has led to a number of new drugs with genetic indications, particularly for the treatment of cancers. Our knowledge of genetic contributors to variable drug response for existing drugs has also expanded dramatically, such that the evidence now supports clinical use of genetic data to guide treatment in some situations, and across a variety of therapeutic areas. Clinical implementation of pharmacogenetics has seen substantial growth in recent years and groups are working to identify the barriers and best practices for pharmacogenetic-guided treatment. The advances and challenges in these areas are described and predictions about future use of genetics in drug therapy are discussed.

Assessment of a pharmacogenomic marker panel in a polypharmacy population identified from electronic medical records

BACKGROUND

The ADME Core Panel assays 184 variants across 34 pharmacogenes, many of which are difficult to accurately genotype with standard multiplexing methods.

METHODS

We genotyped 326 frequently medicated individuals of European descent in Vanderbilt's biorepository linked to de-identified electronic medical records, BioVU, on the ADME Core Panel to assess quality and performance of the assay. We compared quality control metrics and determined the extent of direct and indirect marker overlap between the ADME Core Panel and the Illumina Omni1-Quad.

RESULTS

We found the quality of the ADME Core Panel data to be high, with exceptions in select copy number variants and markers in certain genes (notably CYP2D6). Most of the common variants on the ADME panel are genotyped by the Omni1, but absent rare variants and copy number variants could not be accurately tagged by single markers.

CONCLUSION

Our frequently medicated study population did not convincingly differ in allele frequency from reference populations, suggesting that heterogeneous clinical samples (with respect to medications) have similar allele frequency distributions in pharmacogenetics genes compared with reference populations.

Institutional profile: University of Florida and Shands Hospital Personalized Medicine Program: clinical implementation of pharmacogenetics

The University of Florida and Shands Hospital recently launched a genomic medicine program focused on the clinical implementation of pharmacogenetics called the Personalized Medicine Program. We focus on a pre-emptive, chip-based genotyping approach that is cost effective, while providing experience that will be useful as genomic medicine moves towards genome sequence data for patients becoming available. The Personalized Medicine Program includes a regulatory body that is responsible for ensuring that evidence-based examples are moved to clinical implementation, and relies on clinical decision support tools to provide healthcare providers with guidance on use of the genetic information. The pilot implementation was with CYP2C19–clopidogrel and future plans include expansion to additional pharmacogenetic examples, along with aiding in implementation in other health systems across Florida.