Pharmacogenetics in clinical practice: considerations for testing

Pharmacogenetics is a rapidly evolving field that will undoubtedly lead to the development of pharmacogenetic tests. Such tests will need to be assimilated into healthcare systems, but represent a further call on scarce healthcare resources. Therefore, in order for a pharmacogenetic test to fulfill its potential beyond the laboratory and into the clinical environment, it must prove itself on a wide range of multifaceted criteria. The test must have proven and reproducible analytical and clinical validity, and stand up to critical appraisal of clinical utility and cost-effectiveness. Pharmacogenetic testing can be considered to be a form of screening, and the experience that has been gained to date in evaluating other forms of screening tests may prove beneficial in evaluating pharmacogenetic technology. It is essential that the goals of pharmacogenetic testing are defined as early as possible to ensure that appropriate studies can be designed to provide the evidence base, and thereby enable appropriate evaluation of the technology by clinicians and healthcare administrators for incorporation into clinical practice. This review focuses on issues that will need to be considered in the scientific assessment of pharmacogenetic testing.     

Genetic influences of angiotensin-converting enzyme inhibitor response: an opportunity for personalizing therapy?

The angiotensin-converting enzyme (ACE) inhibitors are a cornerstone drug therapy in the current treatment of patients with hypertension, stable coronary artery disease and heart failure. Individualizing therapy of ACE inhibitors with clinical risk factors in low-risk patients with stable coronary artery disease is not feasible. The concept of pharmacogenetics, by studying patient factors more individually, offers a first glimpse in the quest for the ‘holy grail’ of personalized medicine. As such, genetic targets in the direct pharmacodynamic pathway of ACE inhibitors, the renin–angiotensin–aldosterone system, is a plausible candidate for such an approach. In the past few decades, results of pharmacogenetic studies were scarce and inconsistent. However, recently the first reports of larger pharmacogenetic studies are now confirming that the ‘pharmacogenetic approach’ might be feasible in the future. The current review focuses on the recent developments in pharmacogenetic research in response to ACE inhibitors in patients with stable coronary artery disease.     

Comparison of the TaqMan and LightCycler systems in pharmacogenetic testing: evaluation of CYP2C9*2/*3 polymorphisms.

BACKGROUND: Pharmacogenetic testing for drug-metabolizing enzymes is not yet widely used in clinical practice. METHODS: In an attempt to facilitate the application of this procedure, we have compared two real-time PCR-based methods, the TaqMan and the LightCycler for the pharmacogenetic evaluation of CYP2C9*2/*3 polymorphisms. RESULTS AND CONCLUSION: Both procedures are suitable for pharmacogenetic studies. The TaqMan procedure was less expensive in terms of cost per sample, but the TaqMan apparatus is more expensive than the LightCycler apparatus.     

Why, when, and how should pharmacogenetics be applied in clinical studies?: current and future approaches to study designs

The growing interest in incorporating pharmacogenetics (PGx) into drug development and clinical practice raises several questions: which study designs best reveal relevant pharmacogenetic biomarkers, best clarify specific hypotheses in PGx, and result in the largest gain of clinical evidence in this field? In this review, we present and compare a variety of PGx-related study designs. The type and quality of evidence gained by each category of study design is evaluated, and an appropriate timeline for the integration of pharmacogenetic studies into drug development is proposed. A summary of the pros and cons of the different study designs might help investigators decide how best to incorporate PGx into drug research. Using different scenarios to explain how genetic polymorphisms influence drug action, we illustrate how this knowledge can be translated into individualized drug choices, individualized dosage determination based on pharmacogenetic diagnostics, and other types of monitoring in order to make drug therapies safer and more effective.     

Pharmacogenetics: using DNA to optimize drug therapy

Pharmacogenetics is a growing field of research that focuses on the interaction between genetics and drug therapy. Relationships between genetic variation and drug effect have been observed for a growing number of commonly used drugs. Validation studies may soon define the use of these relationships in clinical practice, moving the field toward routine application. Currently, there are only a few pharmacogenetic diagnostic tests available, and clinical guidelines for pharmacogenetically tailored therapy are lacking. It is likely that guidelines for pharmacogenetic dosing of certain commonly used drugs such as warfarin, codeine, and inhaled beta agonists will become available within the next few years.     

Dihydropyrimidine dehydrogenase deficiency: impact of pharmacogenetics on 5-fluorouracil therapy

Through the use of pharmacogenetic studies, interindividual variability in response (efficacy and toxicity) to 5-fluorouracil (5-FU) chemotherapy has been linked to the rate-limiting enzyme in the drug's catabolic pathway, known as dihydropyrimidine dehydrogenase (DPD). This pharmacogenetic syndrome, known as "DPD deficiency," results in excessive amounts of 5-FU available to be anabolized to its active metabolites and is relatively undetectable by clinical observation prior to 5-FU administration. Extensive studies have associated both profound and partial deficiency in DPD activity with severe, unanticipated toxicity after 5-FU administration, while research on the molecular basis behind DPD deficiency has been linked to various sequence variants of the DPYD gene. Due to the widespread use of 5-FU, the severity of toxicity associated with DPD deficiency, and the prevalence of DPD deficiency in the population, extensive research is continually being performed to develop quick and accurate phenotypic and genotypic assays suitable for clinical settings that would allow clinicians to identify patients susceptible to adverse 5-FU reactions.     

Translation of pharmacogenetic knowledge into cancer therapeutics

The US Food and Drug Administration has recommended changes in the labeling for 6-mercaptopurine, irinotecan, and, most recently, tamoxifen to include pharmacogenetic information on treatment outcome. With the increased availability of pharmacogenetic testing, oncologists are now able to identify individuals at risk for severe treatment toxicity or poor treatment response. However, there are knowledge gaps to fill before rationalized therapy based on pharmacogenetics can be fully integrated into clinical practice. This review gives an overview of pharmacogenetic methods and focuses on the application of pharmacogenetic knowledge in oncology, using the examples of 6-mercaptopurine, irinotecan, tamoxifen, and gefitinib.     

Physician barriers to incorporating pharmacogenetic treatment strategies for nicotine dependence into clinical practice

Advances in genomics research may improve health outcomes by tailoring treatment according to patients' genetic profiles. The treatment of nicotine dependence, in particular, may soon encompass pharmacogenetic treatment models. Realizing the benefits of such treatment strategies may depend on physicians' preparedness to incorporate genetic testing into clinical practice. This article describes barriers to clinical integration of pharmacogenetic treatments that will need to be addressed to realize the benefits of individualized smoking-cessation treatment.     

Global requirements for DNA sample collections: results of a survey of 204 ethics committees in 40 countries

The Industry Pharmacogenomics Working Group has an interest in attaining a better understanding of global requirements for sample collections intended for pharmacogenetics research. To have adequately powered pharmacogenetics studies representative of the clinical trial population, it is important to collect DNA samples from a majority of consenting study participants under many institutional review board/ethics committee (IRB/EC) jurisdictions. A survey was distributed to gather information from local and central IRBs/ECs. The survey included questions related to the approval of pharmacogenetics studies, collection and banking of samples, and return of data to subjects. A total of 204 responses were received from global IRBs/ECs with pharmacogenetic experience. The data show that requirements for approval of pharmacogenetic research differ between IRBs/ECs within and between countries but not between regions of the United States. A better understanding of differing requirements should facilitate global sample collection of DNA for pharmacogenetics research and may provide the basis for harmonized regulations for collection of genetic samples in the future.     

Aminoglycoside-induced translational read-through in disease: overcoming nonsense mutations by pharmacogenetic therapy

A third of inherited diseases result from premature termination codon mutations. Aminoglycosides have emerged as vanguard pharmacogenetic agents in treating human genetic disorders due to their unique ability to suppress gene translation termination induced by nonsense mutations. In preclinical and pilot clinical studies, this therapeutic approach shows promise in phenotype correction by promoting otherwise defective protein synthesis. The challenge ahead is to maximize efficacy while preventing interaction with normal protein production and function.     

Pharmacogenetics of small-molecule tyrosine kinase inhibitors: Optimizing the magic bullet

Cancer treatment has undergone revolutionary changes during the past decade, as a result of the introduction of tyrosine kinase inhibitors (TKIs) that selectively inhibit growth factor pathways critical for tumor growth. Unexpected toxicity profiles and disappointing response rates to these 'magic bullets' have prompted research to identify markers that can predict toxicity or response to such agents. This review discusses the results of pharmacogenetic studies that have used germline DNA to assess the effects of various polymorphisms on currently available small-molecule TKIs. In these studies, polymorphisms in the EGFR gene (ie, EGFR CA-repeat and -216G>T) have consistently been associated with response to the EGFR-blocking TKIs gefitinib and, to a lesser extent, erlotinib. In addition, results from studies investigating polymorphisms in drug transporting enzymes (ie, ABCB1 1236T>C, 2677G>T/A and 3435C>T, and ABCG2 421C>A) suggest such polymorphisms are relevant for the pharmacokinetics of the TKIs; however, some conflicting findings on these polymorphisms have been published. The clinical impact of polymorphisms in EGFR and in drug transporting enzymes needs to be evaluated and validated in order for these pharmacogenetic markers to be applied successfully to individualize treatment in the clinic.     

Harnessing economic drivers for successful clinical implementation of pharmacogenetic testing

Before pharmacogenetic (PGx) testing has a major impact on clinical practice, two levels of evidence must be generated. First, studies demonstrating the links between genetic variation and response to medications in defined populations are needed, along with development of valid tests to measure these specific variants. Second, studies should be conducted to evaluate whether PGx testing improves health outcomes for patients and whether the decision to test is cost-effective relative to usual care. This latter set of questions is typically of greatest relevance to clinicians and payers, the ultimate gatekeepers for the clinical integration of pharmacogenetics. To date, nearly all of the research efforts and funding for PGx have been focused on the first set of issues-getting the science right. However, now is the time to increase our research efforts on the second set of issues-to improve the PGx evidence base for both clinical and economic decision making.     

How to manage individualized drug therapy: application of pharmacogenetic knowledge of drug metabolism and transport.

Significant fractions of health budgets must be spent for treatment of drug side effects and for inefficient drug therapy. Hereditary variants in drug metabolizing enzymes, drug transporters, and drug targets are important determinants of drug response and toxicity and may therefore aid in selection and dosage of drugs. Today there is extensive knowledge of genetic polymorphisms of cytochrome P450 (CYP) enzymes 2A6, 2C9, 2C19, and 2D6; of phase-2 enzymes such as thiopurine S-methyltransferase; and more recently of drug transporters such as the MDR-1 gene-product P-glycoprotein, affecting a significant share of currently used drugs. However, application of pharmacogenetic knowledge to clinical routine is limited in current practice. To promote the application of pharmacogenetic knowledge in clinical routine, research on genotype-based dose adjustments is still necessary - as is the promotion of faster and cheaper genotype analyses. Furthermore, the benefits of CYP genotype-directed drug therapy should be evaluated in properly designed prospective studies. Once such steps have been successfully taken, drug therapy could well become more prevention-directed and patient-tailored than it is possible today, replacing the current "one drug in one dose for one disease" strategy by a more individualized approach.     

Laboratory and clinical outcomes of pharmacogenetic vs. clinical protocols for warfarin initiation in orthopedic patients

Summary. Background: Warfarin is commonly prescribed for prophylaxis and treatment of thromboembolism after orthopedic surgery. During warfarin initiation, out‐of‐range International Normalized Ratio (INR) values and adverse events are common. Methods: In orthopedic patients beginning warfarin therapy, we developed and prospectively validated pharmacogenetic and clinical dose refinement algorithms to revise the estimated therapeutic dose after 4 days of therapy. Results: The pharmacogenetic algorithm used the cytochrome P450 (CYP) 2C9 genotype, smoking status, peri‐operative blood loss, liver disease, INR values and dose history to predict the therapeutic dose. The R² was 82% in a derivation cohort (n = 86) and 70% when used prospectively (n = 146). The R² of the clinical algorithm that used INR values and dose history to predict the therapeutic dose was 57% in a derivation cohort (n = 178) and 48% in a prospective validation cohort (n = 146). In 1 month of prospective follow‐up, the percent time spent in the therapeutic range was 7% higher (95% CI: 2.7–11.7) in the pharmacogenetic cohort. The risk of a laboratory or clinical adverse event was also significantly reduced in the pharmacogenetic cohort (Hazard Ratio 0.54; 95% CI: 0.30–0.97). Conclusions: Warfarin dose adjustments that incorporate genotype and clinical variables available after four warfarin doses are accurate. In this non‐randomized, prospective study, pharmacogenetic dose refinements were associated with more time spent in the therapeutic range and fewer laboratory or clinical adverse events. To facilitate gene‐guided warfarin dosing we created a non‐profit website, http://www.WarfarinDosing.org .     

Pharmacogenetics of antipsychotic treatment response and side effects

Antipsychotic drugs are particularly interesting in pharmacogenetic studies as they are associated with a large interindividual variability in terms of response and side effects and, therefore, frequently need to be discontinued, requiring switches to other antipsychotics. Any information that allows the prediction of outcome to a given antipsychotic in a particular patient will, therefore, be of great help for the clinician to minimize time and find the right drug for the right patient, thus optimizing response and minimizing side effects. This will also have a substantial impact on compliance and doctor-patient relationships. Moreover, antipsychotic drug treatments are often required for life-long treatment and are also frequently prescribed to the more 'vulnerable' populations: children, adolescents and the elderly. This article focuses on some important studies performed with candidate gene variants associated with antipsychotic response. In addition, important findings in pharmacogenetic studies of antipsychotic-induced side effects will be briefly summarized, such as antipsychotic treatment induced tardive dyskinesia and weight gain.     

Cardiovascular pharmacogenomics of adrenergic receptor signaling: clinical implications and future directions

G-protein-coupled receptors (GPCRs) are the targets for many drugs, and genetic variation in coding and noncoding regions is apparent in many such receptors. In this superfamily, adrenergic receptors (ARs) were among the first in which single-nucleotide polymorphisms (SNPs) were discovered, and studies including in vitro mutagenesis, genetically modified mouse models, human ex vivo and in vitro studies and pharmacogenetic association studies were conducted. The signal transduction in these receptors includes amplification steps, desensitization, crosstalk, and redundancies, enabling potential mitigation of the size of the clinical effect for a single variant in a single gene. Nevertheless, convincing evidence has emerged that several variants have an impact on therapy, with certain caveats as to how the results are to be interpreted. Here we review these results for selected ARs and associated regulatory kinases relative to the pharmacogenomics of β-blocker treatment for hypertension and heart failure. We emphasize the linking of clinical results to molecular mechanisms, discuss study design limitations, and offer some recommendations for future directions.     

Use of pharmacogenetic and clinical factors to predict the therapeutic dose of warfarin

Initiation of warfarin therapy using trial-and-error dosing is problematic. Our goal was to develop and validate a pharmacogenetic algorithm. In the derivation cohort of 1,015 participants, the independent predictors of therapeutic dose were: VKORC1 polymorphism -1639/3673 G>A (-28% per allele), body surface area (BSA) (+11% per 0.25 m(2)), CYP2C9(*)3 (-33% per allele), CYP2C9(*)2 (-19% per allele), age (-7% per decade), target international normalized ratio (INR) (+11% per 0.5 unit increase), amiodarone use (-22%), smoker status (+10%), race (-9%), and current thrombosis (+7%). This pharmacogenetic equation explained 53-54% of the variability in the warfarin dose in the derivation and validation (N= 292) cohorts. For comparison, a clinical equation explained only 17-22% of the dose variability (P < 0.001). In the validation cohort, we prospectively used the pharmacogenetic-dosing algorithm in patients initiating warfarin therapy, two of whom had a major hemorrhage. To facilitate use of these pharmacogenetic and clinical algorithms, we developed a nonprofit website, http://www.WarfarinDosing.org.