Pharmacogenomics: Increasing the safety and effectiveness of drug therapy

Transcription

1 Pharmacogenomics: Increasing the safety and effectiveness of drug therapy

2 Introduction to personalized medicine The goal of personalized medicine is to individualize health care by using knowledge of patients health history, behaviors, environments, and, most importantly, genetic variation when making clinical decisions. Research on genetics has provided advances that can be used to more accurately predict the risk of developing certain diseases, personalize screening and surveillance protocols and, in some cases, prevent the onset of disease. In certain cases, genetics can also be used to diagnose diseases and tailor therapies and disease management strategies. These advancements in personalized medicine rely on knowledge of how a patient s genotype (genetic makeup) influences his or her phenotype (observable traits or characteristics). Using the principles of personalized medicine, health care providers may be better equipped to move beyond the one-size-fits-all approach that defined much of patient care in the past, to care that is appropriate for unique patient subgroups. 1 Pharmacogenomics One of the most important components of personalized medicine is pharmacogenomics, the study of genetic variations that influence individual response to drugs. Enzymes responsible for drug metabolism and proteins that determine the cellular response to drugs (receptors) are encoded by genes, and can therefore be variable in expression, activity level and function when genetic variations are present. Knowing whether a patient carries any of these variations may help health care professionals individualize drug therapy, decrease the number of adverse drug reactions and increase the effectiveness of drugs. Pharmacogenomics has been characterized as getting the right dose of the right drug to the right patient at the right time. 2 This brochure is intended to introduce the concept of pharmacogenomics to physicians and other health care providers using a case-based approach. Note that the terms pharmacogenomics and pharmacogenetics are often used interchangeably; for this brochure, pharmacogenomics will be used. Genes commonly involved in pharmacogenomic drug metabolism and response There are several genes responsible for differences in drug metabolism and response. Among the most common are the Cytochrome P450 (CYP) genes, encoding enzymes that control the metabolism of more than 70 percent of prescription drugs. People who carry variations in certain CYP genes often do not metabolize drugs at the same rate or extent as in most people, and this can influence response in many ways. Other genes known to affect drug response encode the receptors for regulatory molecules such as neurotransmitters, hormones, cytokines and growth factors, and cellular proteins such as enzymes, transporters, carriers, ion channels, structural proteins and transcription factors. Variations in these genes can lead to poor response and adverse drug reactions by disabling, inactivating, interfering with, or inaccurately inducing the signaling mechanisms or cellular machinery that must function for the body to respond properly to the drug; or by causing side effects that prevent continued use of the drug. 2

3 Selected drugs whose safety and efficacy are affected by gene variations The table below is a partial list of drugs that exhibit reduced therapeutic effectiveness and/or safety concerns in patients carrying certain genetic variations. These variations often make the drug unsafe or unsuitable for patients who carry the variations. This list contains examples of drugs that are affected by inherited genetic variations and by variations that are acquired and present in tumor tissue. Drug Gene(s) Drug Gene(s) Clopidogrel (Plavix ) CYP2C19 Azathiopurine (Imuran ) TPMT Atomoxetine (Strattera ) CYP2D6 Irinotecan (Camptosar ) UGT1A1 Codeine CYP2D6 Cetuximab (Erbitux )* EGFR Tamoxifen (Nolvadex ) CYP2D6 Erlotinib (Tarceva )* EGFR Warfarin (Coumadin ) CYP2C9, VKORC1 Imatinib mesylate (Gleevec )* C-KIT Abacavir (Ziagen ) HLA-B*5701 Panitumumab (Vectibix )* KRAS Carbamazepine (Tegretol ) HLA-B*1502 Trastuzumab (Herceptin )* Her2/neu * Response to these drugs is dependent on genetic variations that are present in tumor tissue. Adapted from U.S. Food and Drug Administration website ( Pharmacogenetics/ucm htm). Accessed February 7, Metabolizer phenotype The metabolizer phenotype describes the patient s ability to metabolize certain drugs and is based on the number and type of functional alleles of certain genes that a patient carries. These genes most commonly encode the CYP enzymes, which are the focus of much of this brochure. The metabolizer phenotype can range from poor, used to describe patients with little or no functional activity of a selected CYP enzyme, to ultra-rapid, used to describe patients with substantially increased activity of a selected CYP enzyme. Depending on the type of CYP variation present, the patient s metabolizer phenotype and the type of drug (active pharmacologic agent or inactive prodrug precursor), therapeutic drug response is often suboptimal. The table on the following page summarizes the effects of CYP variation on therapeutic efficacy. 3 For example, poor metabolizers are unable to metabolize certain drugs efficiently, resulting in a potentially toxic build-up of an active drug or the lack of conversion of a prodrug into an active metabolite. In contrast, in ultra-rapid metabolizers, an active drug is inactivated quickly, leading to a subtherapeutic response, while a prodrug is quickly metabolized, leading to rapid onset of therapeutic effect. 3

4 Effects of CYP variants on therapeutic efficacy: Metabolizer phenotype Poor Ultra-rapid Active drug (inactivated by metabolism) Drug enzyme Inactive Metabolite Increased efficacy; active metabolite may accumulate; usually require lower dose to avoid toxic accumulation Decreased efficacy; active metabolite rapidly inactivated; usually require higher dose to offset inactivation Prodrug (needs metabolism to produce active metabolite) Prodrug enzyme Active Metabolite Decreased efficacy; prodrug may accumulate; may require lower dose to avoid toxic accumulation, or may require alternate drug Increased efficacy; rapid onset of effect; may require lower dose to prevent excessive accumulation of active metabolite Some drugs and foods cause altered metabolizer phenotype Certain drugs mimic the effect of genetic variations, effectively causing changes in metabolizer phenotype. For example, quinidine is an inhibitor of CYP2D6 activity. A patient taking quinidine is therefore a CYP2D6 poor metabolizer, similar to someone who carries a loss-of function variation in CYP2D6. In those patients, drugs that require the activity of CYP2D6, such as atomoxetine, will not be metabolized at the same rate as in most people. 4 Certain foods can also mimic the effects of genetic variations. One of the most common examples is grapefruit juice, which is an inhibitor of CYP3A4. In people regularly drinking grapefruit juice, drugs that require the activity of CYP3A4, such as diazepam, will not be metabolized at the same rate as in most people. 4 Pharmacogenomics in the clinical setting Awareness of the influence of gene variations on patient response to certain drugs can help physicians decide which type of drug therapy may be appropriate, and identify cases in which a patient isn t responding as anticipated to a drug. The examples that follow illustrate three categories for which pharmacogenomic knowledge can help inform therapeutic decisions: predicting and preventing adverse reactions, determining the efficacy of a drug for a particular patient, and predicting the optimal drug dose. Using pharmacogenomics to predict and prevent adverse drug reactions Several drugs can cause severe or life-threatening reactions in patients with variations in genes that encode proteins that metabolize or are targets of the drugs. Knowing about patients genetic variations can help physicians avoid drugs that may cause adverse reactions. On the following two pages are examples of drugs in this category. 4

5 Abacavir Abacavir (Ziagen ) is a nucleoside reverse transcriptase inhibitor used in combination with other antiretrovirals to treat HIV infection. 5 An immunologically-mediated hypersensitivity reaction occurs in 5 8 percent of patients taking abacavir, usually during the first six weeks after initiation of therapy. The hypersensitivity symptoms include a combination of fever, rash, gastrointestinal tract symptoms and respiratory symptoms that become more severe with continued dosing. 6 Discontinuation of abacavir therapy results in reversal of symptoms. The abacavir hypersensitivity described above is associated with a variant allele of the major histocompatibility complex, HLA-B*5701. Patients who carry the HLA-B*5701 allele have an increased risk for developing a hypersensitivity reaction. 6 HLA-B*5701 screening before abacavir treatment results in a significantly reduced number of hypersensitivity cases. 6 The abacavir product labeling recommends genetic testing to detect the presence of HLA-B* In the Warnings and Precautions section, the labeling states: Serious and sometimes fatal hypersensitivity reactions have been associated with ZIAGEN and other abacavir-containing products. Patients who carry the HLA-B*5701 allele are at high risk for experiencing a hypersensitivity reaction to abacavir. Prior to initiating therapy with abacavir, screening for the HLA-B*5701 allele is recommended; this approach has been found to decrease the risk of a hypersensitivity reaction. Screening is also recommended prior to reinitiation of abacavir in patients of unknown HLA-B*5701 status who have previously tolerated abacavir. For HLA-B*5701-positive patients, treatment with an abacavir-containing regimen is not recommended and should be considered only with close medical supervision and under exceptional circumstances when the potential benefit outweighs the risk. (Label updated September 2010) Case study JS is a 35 year-old man who has recently been diagnosed as HIV positive. Before initiating abacavir anti-retroviral therapy, his physician orders genetic testing to determine whether he carries the HLA-B*5701 allele, knowing that JS would develop fever, rash, nausea and fatigue if he carried the variant allele. The test confirmed that JS carries the HLA-B*5701 allele. How did genetic testing help JS and his physician? Knowing that JS carried the HLA-B*5701 variation indicated that he would likely experience a hypersensitivity reaction. JS s physician will probably not include abacavir in his antiretroviral regimen. 5

6 Codeine Codeine is a prodrug with analgesic properties due primarily to its conversion into morphine. 7,8 Conversion to morphine is mediated by the cytochrome P450 enzyme CYP2D6. Variations that decrease the metabolic activity of CYP2D6 result in a poor analgesic response due to the reduced conversion of codeine into morphine, 3 and patients carrying such a variation are considered poor metabolizers and receive little therapeutic benefit from codeine. It is estimated that 5 10 percent of Caucasians are CYP2D6 poor metabolizers; the percentage is approximately 2 3 percent in other racial and ethnic groups. 7,9,10 Variations (such as gene duplications) can also result in increased metabolic activity of CYP2D6; these result in an enhanced analgesic response due to the rapid conversion of codeine into morphine. Patients who carry such variations are at risk for opioid toxicity, which includes moderate to severe central nervous system depression. The prevalence of the CYP2D6 ultra-rapid metabolizer phenotype has been estimated at 1 10 percent in Caucasians, 3 5 percent in African Americans, percent in North Africans, Ethiopians and Arabs, and up to 21 percent in Asians. 3,11 The product labeling (updated in July 2009) of drugs containing codeine include warnings that CYP2D6 ultra-rapid metabolizers may experience overdose symptoms such as extreme sleepiness, confusion or shallow breathing, even at labeled dosage regimens, and encourages physicians to choose the lowest effective dose for the shortest period of time and inform their patients about the risks and the signs of morphine overdose. 11 Of additional concern is the use of codeine in nursing mothers. Product labeling includes the statement that women who are CYP2D6 ultra-rapid metabolizers achieve higher-than-expected levels of morphine in breast milk and potentially dangerously high serum morphine levels in their breastfed infants. A 2007 U.S. Food and Drug Administration (FDA) Public Health Advisory warns about the dangers to neonates of codeine use in breast-feeding mothers and states that the only way to determine prior to drug administration whether a patient is an ultra-rapid metabolizer is by the use of a genetic test. 12 Canadian guidelines also state that genetic testing is available. 13 Case study DS is a 30 year-old woman who gave birth by caesarian section 10 days ago. Her physician prescribed codeine for post-caesarian pain. Despite taking no more than the prescribed dose, DS experienced nausea and dizziness while she was taking codeine. She also noticed that her breastfed infant was lethargic and feeding poorly. When DS mentioned these symptoms to her physician, he recommended that she discontinue codeine use. Within a few days, both DS s and her infant s symptoms were no longer present. How would genetic testing help DS and her physician? Genotyping of DS s CYP2D6 gene may have revealed a duplication of CYP2D6 genes, placing her in the ultra-rapid metabolizer category. Armed with this knowledge, DS s physician would likely have prescribed a different analgesic that would have spared DS and her infant the symptoms of opioid toxicity. 6

7 Using pharmacogenomics to predict effectiveness Several drugs are subtherapeutic or ineffective in patients with variations in genes that encode drug-metabolizing proteins or targets of the drugs. Knowing about patients genetic variations can help physicians select drug therapies that will be most effective for individual patients. Below is an example of a drug in this category. Clopidogrel Clopidogrel (Plavix ) is a platelet inhibitor used in the treatment of a number of cardiovascular diseases. It is often prescribed for secondary prevention following acute coronary syndromes and for those undergoing percutaneous coronary intervention. However, despite clopidogrel treatment, up to one-quarter of patients experience a subtherapeutic antiplatelet response, resulting in a higher risk for ischemic events. 14,15 Clopidogrel is a prodrug; its antiplatelet properties are exerted once it is converted to an active metabolite. 16 A cytochrome P450 enzyme, CYP2C19, mediates the conversion of clopidogrel into the active metabolite. Patients who carry certain variations in CYP2C19 are considered poor metabolizers and show reduced ability to convert clopidogrel into its active metabolite, resulting in a diminished antiplatelet effect. 16,17 Further, these patients are more likely to have an ischemic event following clopidogrel therapy. 17 Approximately 2 20 percent of patients (depending on ethnicity) are likely to carry CYP2C19 variations. 3,18 A Boxed Warning is included in the product labeling to alert health care providers about its reduced effectiveness in patients who are CYP2C19 poor metabolizers, and to inform health care providers that genetic tests are available to detect genetic variations in CYP2C19 18 : WARNING: DIMINISHED EFFECTIVENESS IN POOR METABOLIZERS The effectiveness of Plavix is dependent on its activation to an active metabolite by the cytochrome P450 (CYP) system, principally CYP2C19. Plavix at recommended doses forms less of that metabolite and has a smaller effect on platelet function in patients who are CYP2C19 poor metabolizers. Poor metabolizers with acute coronary syndrome or undergoing percutaneous coronary intervention treated with Plavix at recommended doses exhibit higher cardiovascular event rates than do patients with normal CYP2C19 function. Tests are available to identify a patient s CYP2C19 genotype; these tests can be used as an aid in determining therapeutic strategy. Consider alternative treatment or treatment strategies in patients identified as CYP2C19 poor metabolizers. (Label updated February 2011). 7

8 Case study JM is a 58 year-old man who recently had an acute myocardial infarction. To prevent subsequent ischemic events, JM s physician recommends antiplatelet therapy and prescribes clopidogrel. Six months later, JM suffered another acute MI, and his physician suspects that the patient has been non-adherent, or alternatively, that clopidogrel therapy may have been ineffective. How would genetic testing help JM and his physician? Determining JM s CYP2C19 genotype may reveal that he carries a variant that diminishes the antiplatelet effect of clopidogrel. If this were the case, alternative anti-platelet therapies may have been considered, reducing the chance that JM would suffer a second cardiac event. In patients taking both clopidogrel and the proton pump inhibitors omeprazole or esomeprazole, diminished antiplatelet activity and adverse cardiac outcomes have been observed. 19 It is thought that omeprazole and esomeprazole inhibit the activity of CYP2C19, making patients who take these drugs de facto CYP2C19 poor metabolizers, similar to someone who carries a loss-of function variation in CYP2C19. In these patients, drugs that require the activity of CYPC19, such as clopidogrel, will not be metabolized at the same rate as in most people. Using pharmacogenomics to predict optimal dose Genetic variations can lead to an altered dosage regimen. Knowing whether a patient carries these genetic variations can assist physicians in determining optimal therapeutic dose. An example of a drug with an altered dosage regimen in patients with certain genetic variations is warfarin. Warfarin Warfarin (Coumadin ) is the most widely-prescribed anticoagulant used to treat and prevent thromboembolic diseases. It is metabolized by the CYP2C9 enzyme, and its anticoagulant effect is mediated by the enzyme VKORC1. Variation in the CYP2C9 gene causes some patients to have slow metabolism of warfarin and a longer half-life of the drug, resulting in higher than usual blood concentrations of warfarin and greater anticoagulant effect. Certain variations in the VKORC1 gene result in reduced activity of the enzyme and subsequently reduced synthesis of coagulation factors. The combination of slow warfarin metabolism caused by CYP2C9 gene variations and reduced coagulation caused by VKORC1 gene variations increases the risk of bleeding during warfarin therapy. Warfarin has a narrow therapeutic index; variations in CYP2C9 and VKORC1, in addition to several other patient characteristics, make it difficult to predict the effective dose. Those carrying certain CYP2C9 and VKORC1 variations are likely to require altered doses and may require prolonged time to reach a stable maintenance dose. The prevalence of these CYP2C9 and VKORC1 variations is variable among racial and ethnic groups: up to 17 percent of Caucasians, 4 percent of African-Americans and less than 2 percent of Asians carry at least one variant of CYP2C9; 20 while 37 percent of Caucasians, 14 percent of African-Americans, and 89 percent of Asians carry at least one variant in VKORC

9 The warfarin product labeling (updated January 2010) states The patient s CYP2C9 and VKORC1 genotype information, when available, can assist in selection of the starting dose, and includes the following table of expected therapeutic doses for patients with different combinations of CYP2C9 and VKORC1 variations: 22 Range of Expected Therapeutic Warfarin Doses (in mg) for CYP2C9 and VKORC1 Genotypes VKORC1 CYP2C9 genotype genotype *1/*1 *1/*2 *1/*3 *2/*2 *2/*3 *3/*3 GG AG AA Ranges are derived from multiple published clinical studies. Other clinical factors (e.g., age, race, body weight, sex, concomitant medications, and comorbidities) are generally accounted for, along with genotype, in the ranges expressed in the table. Adapted from Warfarin drug labeling, The product labeling suggests that this table of expected therapeutic doses can be used to assist prescribers in choosing initial warfarin dose for patients whose CYP2C9 and VKORC1 genotypes are known. For example, if genetic testing reveals that a patient carries the *1/*3 variation of CYP2C9 and the AG variation of VKORC1, his or her expected therapeutic warfarin dose is likely to be in the 3 4 mg range. Dosing algorithms that rely on clinical features such as age, sex and weight, along with genotype, can also assist in the determination of optimal dose (visit where one such algorithm can be found). Genotype is one factor of many that contribute to variation in a patient s response to warfarin. Careful monitoring of INR is still required during titration to steady state and monitoring of long-term therapy. Case study ML is a 65 year-old woman who has recently been diagnosed with atrial fibrillation. To reduce the risk of stroke and other thrombotic events, ML s physician recommends warfarin therapy. To estimate the initial dose, ML s clinical characteristics (age, sex, weight, diet) were considered. However, ML will need to return to the clinic every day for INR monitoring until a stable dose is determined, and then every few weeks thereafter for maintenance monitoring. How would genetic testing help ML and her physician? Determination of ML s CYP2C9 and VKORC1 genotype would reveal whether she carries any variations that alter her ability to metabolize and respond to warfarin. Knowing about any gene variations before initiating therapy allows for more accurate initial dosing and faster INR stabilization, and can reduce the risk for bleeding or clotting events. 9

10 Pharmacogenomics and genetic testing Genetic tests that detect variations in the genes that control metabolism and response to certain drugs are commercially available. Costs and insurance coverage are variable, depending on the type of test ordered. Your hospital and reference laboratories as well as clinical geneticists are valuable sources of information on the best test to order and interpretation of results. It is important to note that the field of pharmacogenomics is still developing, with new findings being rapidly reported. Clinical trials and other studies focusing on the benefits, risks and cost-effectiveness of using genetic information to inform drug therapy are underway. In the meantime, physicians and health care providers should be familiar with the concept that genetic variations can cause their patients to respond unexpectedly to drug therapy, and that in some cases, it may be appropriate to use genetic testing to guide therapeutic decisions. For more information on pharmacogenomics, visit the following websites: UC San Diego Skaggs School of Pharmacy and Pharmaceutical Sciences Pharmacogenomics Education Program ( American College of Clinical Pharmacology The Future of Medicine: Pharmacogenomics ( Indiana University School of Medicine, Division of Clinical Pharmacology Defining Genetic Influences on Pharmacologic Responses ( U.S. Food and Drug AdministrationGenomics ( ResearchAreas/Pharmacogenetics/default.htm) Pharmacogenomics Research Network ( Acknowledgements The American Medical Association, the Arizona Center for Education and Research on Therapeutics (AzCERT), and the Critical Path Institute thank the American Academy of Family Physicians, the American College of Clinical Pharmacy, the American Society for Clinical Pharmacology and Therapeutics, Medco Research Institute, the Personalized Medicine Coalition, and several individuals with expertise in pharmacogenomics for their helpful review comments. This brochure was partially funded by a grant from the Agency for Healthcare Research and Quality to AzCERT. The authors of this document declare no conflicts of interest. The information contained in this document is not intended to be a treatment recommendation or an endorsement of any particular product. 10

11 References 1. Collins FS The Future of Personalized Medicine. NIH Medline Plus 5(1): Lesko L. Regulatory Perspective on Warfarin Relabeling with Genetic Information. May Presentation at the 9th National Conference on Anti-Coagulation Therapy. downloads/drugs/scienceresearch/researchareas/ Pharmacogenetics/ucm pdf. Accessed June 23, Belle DJ, Singh H. (2008). Genetic Factors in Drug Metabolism. Am Fam Physician 77(11): Indiana University Division of Clinical Pharmacology. P450 Drug interaction Table Accessed June 23, GlaxoSmithKline. Ziagen (abacavir sulfate) product labeling assets/us_ziagen.pdf. Accessed February 8, Mallal S, Phillips E, Carosi G, Molina JM, Workman C, Tomazic J et al. (2008). HLA-B*5701 screening for hypersensitivity to abacavir. N Engl J Med 358(6): Madadi P, Ross CJD, Hayden MR, Carleton BC, Gaedigk A, Leeder JS, Koren G. (2009). Pharmacogenomics of Neonatal Opioid Toxicity Following Maternal Use of Codeine During Breastfeeding: A Case-Control Study. Clin Pharmacol Ther 85(1): Willmann S, Edginton AN, Coboeken K, Ahr A, Lippert J. (2009). Risk to the Breast-Fed Neonate From Codeine Treatment to the Mother: A Quantitative Mechanistic Modeling Study. Clin Pharmacol Ther 86(6): Rollason V, Samer C, Piguet V, Dayer P, Desmeules J. (2008). Pharmacogenomics of analgesics: Toward the individualization or prescription. Pharmacogenomics 9(7): Ingelman-Sundberg M, Sim SC, Gomez A, Rodriguez-Antona C. (2007). Influence of cytochrome P450 polymorphisms on drug therapies: Pharmacogeneic, pharmacoepigenetic, and clinical aspects. Clin Pharmacol Ther 116: Roxane Laboratories Inc. Codeine sulfate product labeling docs/label/2009/022402s000lbl.pdf. Accessed March 3, US Food and Drug Administration. (2007). Public Health Advisory: Use of Codeine By Some Breastfeeding Mothers May Lead To Life-Threatening Side Effects In Nursing Babies. PostmarketDrugSafetyInformationfor Patientsand Providers/default.htm. Accessed February 8, Madadi P, Moretti M, Djokanovic N, Bozzo P, Nulman I, Ito S, Koren G. (2009). Guidelines for maternal codeine use during breastfeeding. Can Fam Physician 55(11): Ellis KJ, Stouffer GA, McLeod HL, Lee CR. Clopidogrel pharmacogenomics and risk of inadequate platelet inhibition: U.S. FDA recommendations. (2009). Pharmacogenomics 10(11): Simon T, Verstuyft C, Mary-Krause M, Quteineh L, Drouet E, Meneveau N et al. (2009). Genetic determinants of response to clopidogrel and cardiovascular events. N Engl J Med 360(4): Shuldiner AR, O Connell JR, Bliden KP, Gandhi A, Ryan K, Horenstein RB et al. (2009) Association of cytochrome P450 2C19 genotype with the antiplatelet effect and clinical efficacy of clopidogrel therapy. JAMA 302(8): Mega JL, Close SL, Wiviott SD, Shen L, Hockett RD, Brandt JT et al. Cytochrome p-450 polymorphisms and response to clopidogrel. N Engl J Med 2009; 360(4): Bristol Meyers Squibb/Sanofi Pharmaceuticals Partnership. Plavix (clopidogrel bisulfate) product labeling docs/label/2011/020839s052lbl.pdf. Accessed February 8, Ho PM, Maddox TM, Wang L, Fihn SD, Jesse RL, Peterson ED, Rumsfeld JS. (2009). Risk of Adverse Outcomes Associated With Concomitant Use of Clopidogrel and Proton Pump Inhibitors Following Acute Coronary Syndrome. JAMA 301(9): Gage, BF. (2006). Pharmacogenetics-Based Coumarin Therapy. Hematology ASH Educ Prog Book Reider MJ, Reiner AP, Gage BF, Nickerson DA, Eby CS, McLeod HL et al. (2005). Effect of VKORC1 Haplotypes on Transcriptional Regulation and Warfarin Dose. N Engl J Med 352(22): Bristol Meyers Squibb Company. Coumadin (warfarin sodium) product labeling label/2010/009218s108lbl.pdf. Accessed February 8,

12 For more information contact: American Medical Association The American Medical Association, with its mission to promote the art and science of medicine and the betterment of public health, is a non-profit organization that unites physicians nationwide to work on the most important professional and public health issues. Arizona Center for Education and Research on Therapeutics The Arizona Center for Education and Research on Therapeutics is an independent, collaborative program of the Critical Path Institute and the University of Arizona College of Pharmacy. Its mission is to improve therapeutic outcomes and reduce adverse events caused by drug interactions. Critical Path Institute The Critical Path Institute is an independent, non-profit organization dedicated to bringing scientists from the FDA, industry and academia together to improve the path for innovative new drugs, diagnostic tests and devices to reach patients in need American Medical Association. All rights reserved :5/11:jt

13 REVIEW CME CREDIT EDUCATIONAL OBJECTIVE: Readers will discern the implications of pharmacogenomic testing as it emerges JOSEPH P. KITZMILLER, MD, PhD Department of Pharmacology, Division of Clinical Trials, College of Medicine, The Ohio State University, Columbus, OH DAVID K. GROEN, MD Department of Pharmacology, Division of Clinical Trials, College of Medicine, The Ohio State University, Columbus, OH MITCH A. PHELPS, PhD Division of Pharmaceutics Resources, College of Pharmacy, The Ohio State University, Columbus, OH WOLFGANG SADEE, Dr rer nat Chairman, Department of Pharmacology; Director, Program in Pharmacogenomics, College of Medicine, The Ohio State University, Columbus, OH Pharmacogenomic testing: Relevance in medical practice Why drugs work in some patients but not in others ABSTRACT Genetics may account for much of the variability in our patients responses to drug therapies. This article offers the clinician an up-to-date overview of pharmacogenomic testing, discussing implications and limitations of emerging validated tests relevant to the use of warfarin (Coumadin), clopidogrel (Plavix), statins, tamoxifen (Nolvadex), codeine, and psychotropic drugs. It also discusses the future role of pharmacogenomic testing in medicine. KEY POINTS Polymorphisms that affect the pharmacokinetics and pharmacodynamics of specific drugs are common. Testing for certain polymorphisms before prescribing certain drugs could help avoid adverse drug effects and improve efficacy. Pharmacogenomic testing has only recently begun to enter clinical practice, and routine testing is currently limited to a few clinical scenarios. However, additional applications may be just around the corner. Many pharmacogenomic tests are available, but testing has not yet been recommended for most drugs. Needed are large-scale trials to show that routine testing improves patient outcomes. doi: /ccjm.78a n many patients, certain drugs do not I work as well as expected, whereas in other patients they cause toxic effects, even at lower doses. For some patients, the reason may be genetic. Sizeable minorities of the population carry genetic variants polymorphisms that affect their response to various drugs. Thanks to genetic research, our understanding of the variability of drug response has advanced markedly in the last decade. Many relevant polymorphisms have been identified, and tests for some of them are available. See related editorial, page 241 Armed with the knowledge of their patients genetic status, physicians could predict their response to certain drugs, leading to better efficacy, fewer adverse drug reactions, and a better cost-benefit ratio. The possible impact is substantial, since more than half of the drugs most commonly involved in adverse drug reactions are metabolized by polymorphic enzymes. 1 Adverse drug reactions remain a significant detriment to public health, having a substantial impact on rates of morbidity and death and on healthcare costs. 2 8 In the United States, adverse drug reactions are a leading cause of death in hospitalized patients 4 and are annually responsible for hundreds of thousands of deaths and hundreds of billions of dollars in added costs. 2,4,6 8 But the era of truly individualized medicine is not here yet. For most drugs, pharmacogenomic testing has not been endorsed by expert committees (and insurance companies will not CLEVELAND CLINIC JOURNAL OF MEDICINE VOLUME 78 NUMBER 4 APRIL

14 PHARMACOGENOMIC TESTING Pharmacogenomic testing is beginning to affect the way medicine is practiced pay for it), since we still lack evidence that clinical outcomes improve. This, we hope, will change as ongoing clinical trials are completed. FIGURE 1 describes the various stages involved in translational pharmacogenomic research. 11 In the meantime, physicians can educate their patients and promote efforts to incorporate genomic information into standard clinical decision-making. This article offers an overview of pharmacogenomic testing, discussing implications and limitations of a few validated tests. Specifically, we will discuss testing that is relevant when using warfarin (Coumadin), clopidogrel (Plavix), statins, tamoxifen (Nolvadex), codeine, and psychotropic medications, as well as the future role of pharmacogenomic testing in medicine. WHAT IS PHARMACOGENOMICS? Pharmacogenomics is the study of how genetic factors relate to interindividual variability of drug response. Many clinicians may not be familiar with the background and terminology used in the pharmacogenomic literature. Below, a brief review of the terminology is followed by a schematic describing the various stages of research involved in pharmacogenomics and the advancement of a test into standard practice. The review and schematic may be helpful for evaluating the clinical significance of pharmacogenomics-related articles. From genotype to phenotype Genotype refers to the coding sequence of DNA base pairs for a particular gene, and phenotype (eg, disease or drug response) refers to a trait resulting from the protein product encoded by the gene. The name of a gene often refers to its protein product and is italicized (eg, the CYP3A4 gene encodes for the CYP3A4 enzyme). Two alleles per autosomal gene (one paternal and one maternal) form the genotype. Heterozygotes possess two different alleles, and homozygotes possess two of the same alleles. The most common allele in a population is referred to as the wild type, and allele frequencies can vary greatly in different populations. 9 Most sequence variations are single nucleotide polymorphisms (SNPs, pronounced snips ), a single DNA base pair substitution that may result in a different gene product. SNPs can be classified as structural RNA polymorphisms (srsnps), regulatory polymorphisms (rsnps), or polymorphisms in coding regions (csnps) 10 : srsnps alter mrna processing and translation, rsnps alter transcription, and csnps alter protein sequence and function. Recently, genetic associations with a phenotype have been done on a large scale, with millions of SNPs measured in each of many subjects. This approach, called a genomewide association study or GWAS, has revealed countless candidate genes for clinical traits, but only a few have resulted in a practical clinical application. SNPs may by themselves exert a pharmacokinetic effect (ie, how the body processes the drug), a pharmacodynamic effect (ie, how the drug affects the body), or both, or they may act in concert with other genetic factors. Pharmacodynamic effects can result from a pharmacokinetic effect or can result from variations in a pharmacologic target. Establishing a genotype-phenotype association can involve clinical studies, animal transgenic studies, or molecular and cellular functional assays. Clinical applications are emerging Although pharmacogenomic testing is beginning to affect the way medicine is practiced, it is recommended, or at least strongly suggested, by labeling mandated by the US Food and Drug Administration (FDA) for only a few clinical scenarios, mostly in the treatment of cancer and human immunodeficiency virus (TABLE 1). However, applications are also being developed for a few widely prescribed drugs and drug classes in primary care. We will therefore focus our discussion on the advantages and limitations of a few of these examples for which clinical applications may be emerging. WARFARIN: IMPORTANCE OF CYP2C9, VKORC1 Warfarin is used for the long-term treatment and prevention of thromboembolic events. 244 CLEVELAND CLINIC JOURNAL OF MEDICINE VOLUME 78 NUMBER 4 APRIL 2011

15 KITZMILLER AND COLLEAGUES How a genetic test becomes a standard of care: Typical advancement pathway Identify candidate genetic variant The US Food and Drug Administration (FDA) lists genetic tests validated for accuracy and reproducibility at ResearchAreas/Pharmacogenetics. Determine penetrance of genetic variant Allele frequency is an important determinant of clinical significance; even genetic variants associated with large pharmacokinetic or pharmacodynamic effects may not have sufficient clinical utility if they occur only rarely. Determine genotype-phenotype association Measured biologic or physiologic effects (associations) are often viewed as surrogates for clinical response; however, clinical responses do not always correspond as predicted. Determine clinical significance Clinical trials that compare implementation of genetic testing to the current standard of care should be used for determining clinical significance. FIGURE 1 Determine cost-effectiveness The cost-effectiveness of implementing a pharmacogenomic testing application should compare favorably with current standard practice. Cost-effectiveness assessments measure a variety of outcomes, including clinical events and quality-adjusted life-years. Standard of care The FDA can require genetic information to be added to a product label (eg, codeine), recommend the use of genetic testing data if available (eg, warfarin [Coumadin]), or even recommend genetic testing prior to prescribing a medication to specific patient populations eg, HLA-B genotyping prior to prescribing carbamazepine (Tegretol, Equetro) to genetically at-risk patients. 11 About 20% of patients starting warfarin are hospitalized in the first 6 months because of bleeding This drug has a narrow therapeutic window and shows substantial interpatient dose variability. The start of warfarin therapy is associated with one of the highest rates of adverse events and emergency room visits of any single drug. 12 More than 2 million patients start warfarin each year in the United States alone, 13 and about 20% of them are hospitalized within the first 6 months because of bleeding due to overanticoagulation. 14 The findings from a recent study suggest that pharmacogenomic testing may eventually allow more patients to safely benefit from warfarin therapy. In this large, nationwide, prospective study, hospitalization rates were 30% lower when pharmacogenomic testing was used. 14 However, no reduction was seen in the time needed to reach the target international normalized ratio (INR) or in the need for INR checks at 6 months. Furthermore, CLEVELAND CLINIC JOURNAL OF MEDICINE VOLUME 78 NUMBER 4 APRIL

16 PHARMACOGENOMIC TESTING TABLE 1 FDA-mandated product labeling: Genetic biomarkers and their clinical context C-KIT (cytokine receptor) Imatinib mesylate (Gleevec) is indicated for the treatment of patients with Kit (CD117)-positive unresectable tumors or metastatic malignant gastrointestinal stromal tumors (GIST). CCR5 (C-C chemokine receptor type 5) Maraviroc (Selzentry) is indicated for treatment-experienced adult patients infected with CCR5-tropic HIV-1 infection. Chromosome 5q Lenalidomide (Revlimid) is indicated for the treatment of patients with transfusion-dependent anemia due to low- or intermediate-risk myelodysplastic syndromes associated with a deletion 5q cytogenic abnormality with or without additional cytogenic abnormalities. Familial hypercholesterolemia Atorvastatin (Lipitor) dosage adjustment is necessary for children who have homozygous or heterozygous familial hypercholesterolemia. G6PD (glucose-6-phosphate dehydrogenase) Testing for G6PD deficiency is recommended in high-risk populations before starting treatment with rasburicase (Elitek). HER2/neu (human epidermal growth factor receptor 2) HER2 testing is recommended before starting treatment with trastuzumab (Herceptin). HLA-B*1502 (major histocompatibility complex, class I, B) HLA-B testing is recommended in high-risk populations before starting treatment with carbamazepine (Tegretol, Equetro). HLA-B*5701 (major histocompatibility complex, class I, B) HLA-B testing is recommended before starting treatment with abacavir (Ziagen). Philadelphia (Ph1) chromosome Dasatinib (Sprycel) is indicated for treatment of adults with Philadelphia-chromosome-positive acute lymphoblastic leukemia with resistance or intolerance to prior therapy. TPMT (thiopurine methyltransferase) TPMT testing is recommended before starting treatment with azathioprine (Imuran). Adapted from US Food and Drug Administration (FDA). Table of pharmacogenomic biomarkers in drug labels. Pharmacogenetics/ucm htm. Accessed 2/3/2011. this study used historical control data, and some or all of the reduction in hospitalization rates may be attributed to more frequent INR checks in the patients who underwent testing than in the historical control group. A relationship between warfarin dose requirements and the genetic status of CYP2C9, which encodes a major drug-metabolizing enzyme, has been demonstrated in retrospective and prospective studies S-warfarin is metabolized by CYP2C9, which is polymorphic Warfarin contains equal amounts of two isomers, designated S and R. S-warfarin, which is more potent, is metabolized principally by CYP2C9, while R-warfarin is metabolized by CYP1A2, CYP2C19, and CYP3A4. People who possess two copies of the wild type CYP2C9 gene CYP2C9*1 metabolize warfarin very well and so are called extensive warfarin metabolizers. Carriers of the allelic variants CYP2C9*2 and CYP2C9*3 (which have point mutations in exons 3 and 7 of CY- P2C9, respectively), have less capacity. Compared with those who are homozygous for the wild-type gene, homozygous carriers of CY- P2C9*3 clear S-warfarin at a rate that is 90% lower, and those with the CYP2C9*1/*3, CY- P2C9*1/*2, CYP2C9*2/*2, or CYP2C9*2/*3 genotypes clear it at a rate 50% to 75% lower. A meta-analysis of 12 studies found that the CYP2C9 genotype accounted for 12% of the interindividual variability of warfarin dose requirements. 18 About 8% of whites carry at least one copy of CYP2C9*2, as do 1% of African Americans; the allele is rare in Asian populations. The frequency of CYP2C9*3 is 6% in whites, 1% in African Americans, and 3% in Asians. 19,20 People with CYP2C9*4 or CYP2C9*5 have a diminished capacity to clear warfarin; however, these variants occur so infrequently that their clinical relevance may be minimal. Warfarin s target, VKOR, is also polymorphic Genetic variation in warfarin s pharmacologic target, vitamin K 2,3-epoxide reductase (VKOR), also influences dose requirements. Warfarin decreases the synthesis of vitamin- K-dependent clotting factors by inhibiting 246 CLEVELAND CLINIC JOURNAL OF MEDICINE VOLUME 78 NUMBER 4 APRIL 2011

17 KITZMILLER AND COLLEAGUES TABLE 2 Range of expected warfarin doses based on CYP2C9 and VKORC1 genotypes VKORC1 genotype CYP2C9 genotype *1/*1 *1/*2 *1/*3 *2/*2 *2/*3 *3/*3 GG 5 7 mg 5 7 mg 3 4 mg 3 4 mg 3 4 mg mg AG 5 7 mg 3 4 mg 3 4 mg 3 4 mg mg mg AA 3 4 mg 3 4 mg mg mg mg mg This table and the following comments were taken directly from the warfarin prescribing information. Ranges are derived from multiple published clinical studies. Other factors (eg, age, race, body weight, sex, concomitant medications, and comorbidities) are generally accounted for along with genotype in the ranges expressed in the table. The VKORC1 1639G>A (rs ) variant is used in this table. Other coinherited VKORC1 variants may also be important determinants of warfarin dose. Patients with CYP2C9 *1/*3, *2/*2, *2/*3, and *3/*3 may require a prolonged time (< 2 to 4 weeks) to achieve the maximum international normalized ratio effect for a given dosage regimen. 24 VKOR. This inhibition depends on the patient s C1 subunit gene, VKORC1. Patients with a guanine-to-adenine SNP 1,639 bases upstream of VKORC1 ( 1639G>A) need lower warfarin doses an average of 25% lower in those with the GA genotype (ie, one allele has guanine in the 1639 position and the other allele has adenine in that position) and 50% lower in those with the AA genotype compared with the wild-type genotype GG. 21 This promoter SNP, positioned upstream (ie, before the gene-coding region), greatly influences VKORC1 expression. A meta-analysis of 10 studies found that the VKORC1 polymorphism accounts for 25% of the interindividual variation in warfarin dose. 18 In one study, the frequency of the AA genotype in a white population was 14%, AG 47%, and GG 39%; in a Chinese population the frequency of AA was 82%, AG 18%, and GG 0.35%. 22 CYP4F2 and GGCX also affect warfarin s dose requirements Genetic variations in the enzymes CYP4F2 and gamma-glutamyl carboxylase (GGCX) also influence warfarin dose requirements. Although the data are limited and the effects are smaller than those of CYP2C9 and VKORC1, people with a SNP in CYP4F2 need 8% higher doses of warfarin, while those with a SNP in GGCX need 6% lower doses. 23 CYP2C9 and VKORC1 testing is available Currently, the clinical pharmacogenetic tests relevant for warfarin use are for CYP2C9 and VKORC1. 10 The FDA has approved four warfarin pharmacogenetic test kits, but most third-party payers are reluctant to reimburse for testing because it is not currently considered a standard of care. Testing typically costs a few hundred dollars, but it should become less expensive as it becomes more commonplace. The current FDA-approved product label for warfarin does not recommend routine pharmacogenomic testing for determining initial or maintenance doses, but it does acknowledge that dose requirements are influenced by CYP2C9 and VKORC1 and states that genotype information, when available, can assist in selecting the starting dose. 24 The product label includes a table (TABLE 2) of expected therapeutic warfarin doses based on CYP2C9 and VKORC1 genotypes, which can be used when choosing the initial dose for patients whose genetic status is known. A well-developed warfarin-dosing model incorporating traditional clinical factors and patient genetic status is available on the nonprofit Web site 25 Clinical trials of warfarin pharmacogenomic testing are under way Although genetic status can greatly influence an individual patient s warfarin dosing re- Patients with CYP2C9 variants or VKORC1 variants may need lower doses of warfarin CLEVELAND CLINIC JOURNAL OF MEDICINE VOLUME 78 NUMBER 4 APRIL

18 PHARMACOGENOMIC TESTING FDA: Poor CYP2C19 metabolizers may not benefit from clopidogrel quirement, routine prospective pharmacogenomic testing is not endorsed by the FDA or by other expert panels 26 because there is currently insufficient evidence to recommend for or against it. Several large prospective trials are under way. For example, the National Heart, Lung, and Blood Institute began a prospective trial in about 1,200 patients to evaluate the use of clinical plus genetic information to guide the initiation of warfarin therapy and to improve anticoagulation control for patients. 27 The results, expected in September 2011, and those of other large prospective trials should provide adequate evidence for making recommendations about the clinical utility of routine pharmacogenetic testing for guiding warfarin therapy. Several recent cost-utility and cost-effectiveness studies have attempted to quantify the value of pharmacogenomic testing for warfarin therapy, but their analyses are largely limited because the benefit (clinical utility) is yet to be sufficiently characterized. The relevance of such analyses may soon be drastically diminished, as several non-vitamin-k-dependent blood thinners such as rivaroxaban (Xarelto), dabigatran (Pradaxa), and apixaban are poised to enter clinical practice. 31 CLOPIDOGREL IS ACTIVATED BY CYP2C19 Clopidogrel, taken by about 40 million patients worldwide, is used to prevent atherothrombotic events and cardiac stent thrombosis when given along with aspirin. Clopidogrel is a prodrug, and to do its job it must be transformed to a more active metabolite (FIGURE 2). CYP2C19 is responsible for its metabolic activation, and CYP2C19 loss-of-function alleles appear to be associated with higher rates of recurrent cardiovascular events in patients receiving clopidogrel. At least one loss-of-function allele is carried by 24% of the white non-hispanic population, 18% of Mexicans, 33% of African Americans, and 50% of Asians. Homozygous carriers, who are poor CYP2C19 metabolizers, make up 3% to 4% of the population. 32 Studies of clopidogrel pharmacogenomics A recent genome-wide association study conducted in a cohort of 429 healthy Amish persons revealed a SNP in CYP2C19 to be associated with a diminished response to clopidogrel and to account for 12% of the variation in drug response. 33 Traditional factors (the patient s age, body-mass index, and cholesterol level) combined accounted for less than 10% of the variation. Findings were similar in a subsequent investigation in 227 cardiac patients receiving clopidogrel: 21% of those with the variant had a cardiovascular ischemic event or died during a 1-year follow-up period compared with 10% of those without the variant (hazard ratio 2.42, P =.02). 33 A 12-year prospective study investigating clopidogrel efficacy in 300 cardiac patients under the age of 45 used cardiovascular death, nonfatal myocardial infarction, and urgent coronary revascularization as end points. It concluded that the only independent predictor of these events was the patient s CYP2C19 status. 34 A study in 2,200 patients with recent myocardial infarction examined whether any of the known allelic variations that modulate clopidogrel s absorption (ABCB1), metabolic activation (CYP3A4/5 and CYP2C19), or biologic activity (P2RY12 and ITGB3) was associated with a higher rate of the combined end point of all-cause mortality, nonfatal myocardial infarction, or stroke. None of the SNPs in CYP3A4/5, P2RY12, or ITGB3 that were evaluated was associated with a higher risk at 1 year. However, the allelic variations modulating clopidogrel s absorption (ABCB1) and metabolism (CYP2C19) were associated with higher event rates. Patients with two variant ABCB1 alleles had a higher adjusted hazard ratio (95% confidence interval [CI] ) than those with the wild-type allele. Patients who had one or two CYP2C19 loss-of-function alleles had a higher event rate than those with two wild-type alleles (95% CI and , respectively). 35 Conversely, researchers from the Population Health Research Institute found no association between poor-metabolizer status and treatment outcomes when CYP2C19 analysis was retrospectively added to the findings of two large clinical trials (combined N > 5,000). However, patients with acute coronary syndrome benefited more from clopido- 248 CLEVELAND CLINIC JOURNAL OF MEDICINE VOLUME 78 NUMBER 4 APRIL 2011

19 KITZMILLER AND COLLEAGUES MM Why some drugs don t work in some patients Pharmacogenomic research investigates how patients genetics influence their response to medication. Genetic variability can influence how quickly a drug is metabolized, how well a drug works, or even the likelihood of side effects. The example below depicts how genetic variation can influence the conversion of a prodrug. The normal wild-type allele (CYP2C19*1) results in a normalfunctioning CYP2C19 enzyme capable of converting the inactive prodrug clopidogrel (Plavix) to its active metabolite. Clopidogrel is a prodrug Active metabolite Functional enzyme CYP2C19 Variant alleles (mainly CYP2C19*2, CYP2C19*3, and CYP2C19*17) result in poor-metabolizer status (having little or no CYP2C19 function). For CYP2C19*2, the single-nucleotide polymorphism (SNP) is substitution of adenine for guanine at position 681 of CYP2C19 (681G > A). The resulting CYP2C19 protein is unable to convert clopidogrel to its active form, and drug efficacy is reduced. Clopidogrel remains inactive FIGURE 2 SNP CYP2C19*2 protein is enzymatically inactive Medical Illustrator: Beth Halasz CCF 2011 CLEVELAND CLINIC JOURNAL OF MEDICINE VOLUME 78 NUMBER 4 APRIL