Abstract
As genotyping and genetic testing become more sophisticated, accessible, and costeffective, these tools hold great promise for predicting and improving responses to medications.
This is the first in a series of two articles about pharmacogenomics and drug response. Part 2 will discuss the challenges that impede the clinical use of pharmacogenomics.
Introduction
A current focus of pharmacogenomic research is to explore the effect of inter-individual genetic differences on the pharmacokinetics, pharmacodynamics, efficacy, and safety of drug treatments.1 The ultimate goal of these efforts is to develop personalized, genetic-based strategies that will optimize therapeutic outcomes.1 Gene–drug associations have been identified for anticonvulsant, anti-infective, cancer, cardiovascular, opioid, proton-pump inhibitor, and psychotropic drugs, as well as other types of treatments.1,3–8 Many genetic variants have been identified that are known to alter cytochrome P450 (CYP) enzymes and drug receptors, transporters, and targets.2,3 These modifications can greatly influence pharmacokinetics, dose requirements, and other factors that affect therapeutic outcomes.1–4 The clinical application of such pharmacogenetic findings holds great promise in improving drug efficacy and safety.1
How Do Pharmacogenomics and Pharmacogenetics Differ?
Both pharmacogenomics and pharmacogenetics concern the effect of genetic variations on drug metabolism and response, but these fields do differ.9 Pharmacogenomics is the “genome-wide analysis of genetic determinants of drug-metabolizing enzymes, receptors, transporters, and targets that influence therapeutic efficacy and safety and drug-related phenotypes.”3,4,6,9,10 Pharmacogenomic research involves scanning the whole genome to find single-nucleotide polymorphisms (SNPs) that might be associated with drug response without necessarily knowing the specific function of the identified SNPs.2 Whereas pharmacogenomics involves the study of genes in all chromosomes,3,10 pharmacogenetics is the study of specific SNPs in distinct genes with known functions that are plausibly connected to drug response.
Although the two terms are often used interchangeably, pharmacogenomics is becoming the preferred term to describe genetic variations—either inherited (germline), acquired (somatic), or both—that influence drug response.1
Goals of Pharmacogenomics and Pharmacogenetics
Prior to the introduction of pharmacogenomics, treatment choices were traditionally made based on a patient’s medical history and pathology.7 Possible influences on drug response that are usually considered when making treatment decisions include age, sex, disease, environmental factors, diet, and drug interactions.3,7 Dose adjustments are typically made based on a patient’s age, sex, organ function, body weight, and body surface area.7 However, even when these factors are taken into account, drug response still often varies among patients, ranging from positive outcomes to fatal adverse reactions.7
These long-established clinical parameters will continue to be used to guide drug treatment.6 However, they don’t directly consider genetic factors, which can account for 20% to 40% of inter-individual differences in drug metabolism and response.6 In fact, for certain drugs or drug classes, genetic factors have been shown to be the most important influence on drug treatment outcomes.6
Therefore, the goal of pharmacogenomic and pharmaco-genetic research is to apply current knowledge about genetic variants that influence drug response to develop personalized treatment strategies that maximize therapeutic efficacy and safety.1,5–7 Personalized drug therapy is especially desirable when prescribing a drug with a narrow therapeutic index or when drug toxicity can be life-threatening.5,7 For example, antineoplastic, anticoagulant, and anti-HIV therapies are often administered at maximally tolerated doses that are typically chosen from population averages.5 However, this approach can result in toxicity in up to one-third of patients, and a significant portion of the people treated can exhibit poor or no response.5
Genetic factors account for 20% to 40% of inter-individual differences in metabolism and response. For certain drugs or drug classes, they are the most important influence on treatment outcomes.
Severe adverse drug reactions are one of the most common reasons for hospital admissions in the U.S.6,7 Adverse drug reactions rank as the fourth leading cause of death in the U.S. and are responsible for 100,000 deaths annually.6,7 Genetic testing for drug response is expected to reduce the risk of hospitalization by as much as 30%.5 To this end, multidisciplinary teams of laboratory, clinical, and computational researchers are working together to personalize drug treatment by incorporating an individual’s genetic information (both germline and somatic) into existing prescribing models.7 A patient’s genome needs to be identified only once in a lifetime, which makes pharmacogenetic screening a potentially very potent, cost-effective diagnostic tool.1
Genetic testing for drug response is expected to reduce the risk of hospitalization by as much as 30%.
Types of Genetic Variants That Can Influence Drug Response
The number of pharmacogenetic associations known to affect drug response has steadily increased over the years.11 Genetic polymorphisms have been identified for many proteins that are significant in clinical pharmacology, including enzymes, drug receptors, transporters, and targets.2,3 These polymorphisms can cause alterations in the amount, structure, binding, and/or function of these proteins, affecting how drugs interact with them.2,3 Genetic variants can alter the pharmacokinetics and pharmacodynamics of a drug, potentially affecting both drug efficacy and toxicity.4 Evidence indicates that genetic factors can account for an estimated 20% to 95% of drug metabolism and response.1
The majority of observed DNA sequence variations are due to SNPs, which are single base-pair substitution mutations.11 These occur every 100 to 300 base pairs and account for 90% of all human genetic variations.11 The location of the SNPs in relation to a particular gene determines whether or not the normal function of a gene is affected.11 The net effect of an SNP on gene function can also depend on whether one or both copies of a gene are affected by a variant; therefore, one of the aims of pharmacogenetic testing is to identify heterozygous and homozygous SNPs.11
Genetic variation can also occur in “non-SNP” polymorphisms, better known as structural variations (SVs).9 SVs consist of small (fewer than 10 base pairs) insertions or deletions (indels), copy number variations (CNVs), and inversions.10 These genetic modifications occur less frequently than SNPs but have greater repercussions because they encompass larger regions of genomic variation than SNPs do.1,10 For example, if an indel occurs in the coding region of a gene, this can lead to completely aberrant, nonfunctional proteins.1,10 Both SNPs and SVs are thought to play a role to varying degrees with respect to individual phenotypic drug response outcomes, such as drug sensitivity, resistance, and toxicity.1
Effect of Genetic Variants on Pharmacokinetics And Pharmacodynamics
CYP450 enzymes, each of which is coded for by a different gene, are responsible for metabolizing the majority of drugs.2,3,6,10 Genetic polymorphisms identified in CYP450 genes affect the metabolism of up to 25% of all drug therapies.2,3,6 Polymorphisms in the genetic coding sequences for these enzymes can influence drug response, causing it to be normal, increased, reduced, or even completely neutralized.10 For example, genetic variations in CYP2D6 and other genes that encode for drug-metabolizing enzymes (DMEs) have been correlated with four different metabolism phenotypes: ultra-rapid metabolizers (UMs), normal/extensive metabolizers (EMs), intermediate metabolizers (IMs), and poor metabolizers (PMs).3,9 Now that DNA-based CYP450 enzyme tests are clinically available, genetic screening can identify these metabolism phenotypes in individuals.9
Genetic polymorphisms identified in CYP450 genes affect the metabolism of up to 25% of all drug therapies.
Polymorphic variant alleles of many important CYP450 genes have been discovered in different populations.9 Notable discoveries include two enzymes from the CYP2C sub-family: CYP2C9 and CYP2C19.10 The enzyme coded for by the CYP2C9 allele metabolizes many clinically relevant drugs, including warfarin, phenytoin (Dilantin, Pfizer), tolbutamide, losartan (Cozaar, Merck), and others.9 Like other CYP450 genes, the CYP2C9 gene is highly polymorphic. Genetic variations of the CYP2C9 allele have been strongly associated with inter-individual warfarin dosing variability.5
At least 27 variant alleles for CYP2C19 have also been identified, with the most extensively described being CYP2C19*2 and CYP2C19*3.9 The CYP2C19*2 allele was originally associated with impaired mephenytoin metabolism, and it has also since been linked to a reduction in active clopidogrel metabolites.9 This causes higher platelet aggregation and adverse clinical outcomes in certain clopidogrel-treated, cardiovascular patient populations compared with non-carriers.9 With respect to CYP2C19*2, a splicing defect in exon 5 occurs, resulting in early termination of protein synthesis.7 Similarly, CYP2C19*3 is a premature stop codon SNP, which results in a truncated protein.3 The functional effect of these polymorphisms is a complete loss of enzyme activity.3,9 Other CYP2C19 alleles also result in a loss (CYP2C19*4–*8), reduced (CYP2C19*9, *10, and *12), or increased (CYP2C19*17) metabolic enzyme activity.3 Variation in genes coding for drug metabolism enzyme aren’t the sole determinant of drug response.9 Polymorphisms in genes that code for drug receptors, transporters, and targets also play a role.9 For example, a variant allele in the solute carrier organic anion transporter family member 1B1 (SLCO1B1) gene has been associated with statin-induced myalgia.9 A common variant in the VKORC1 gene (vitamin K epoxide reductase complex subunit 1), which encodes a target of warfarin, has also been strongly associated with inter-individual dosing variability for that drug.5,9
Specific Pharmacogenetic Associations Known to Alter Drug Response
Genetic Variants and Response to Anticonvulsant Drugs
Carbamazepine has been linked to dose-dependent side effects as well as life-threatening idiosyncratic adverse reactions.3 It is metabolized primarily by enzymes encoded by the CYP3A4 gene to the active metabolite, carbamazepine-10,11-epoxide.3
Variation in genes coding for drug metabolism enzyme aren’t the sole determinant of drug response. Polymorphisms in genes that code for drug receptors, transporters, and targets also play a role.
The human leukocyte antigen (HLA) gene has been a primary focus in the study of hypersensitivity and Stevens–Johnson syndrome/toxic epidermal necrolysis (SJS/TEN) reactions that occur in response to carbamazepine treatment, which tend to be familial.3 Investigators discovered an association between the HLA-B*1502 allele and the risk of SJS/TEN, particularly in Asians who are prescribed carbamazepine.3 In a landmark study of 44 Han Chinese patients, a 100% association between the HLA-B*1502 allele and carbamazepine-induced SJS/TEN was observed.3 However, this allele has not been shown to increase the risk for carbamazepine-induced hypersensitivity in Caucasians.3
Other HLA alleles have been identified as potential markers for carbamazepine-induced hypersensitivity reactions in other populations.3 For example, the HLA-A*3101 allele has been associated with carbamazepine-induced hypersensitivity reactions in Japanese and European populations.3 In addition, the HLA-B*1511 allele was discovered to be a risk factor for carbamazepine-induced SJS/TEN in Japanese patients.3 Another study reported an association between the HLA-B*1518, HLA-B*5901, and HLA-C*0704 alleles and severe cutaneous adverse drug reactions.3
Physicians could avoid many adverse drug reactions if they were capable of predicting an individual’s response to treatment.
Before carbamazepine treatment is initiated in high-risk patients, genetic testing is recommended.3 In 2007, the FDA recommended that all patients of Asian ancestry be screened for the HLA-B*1502 allele before they begin carbamazepine therapy.3 This recommendation is stated in the boxed warning section of the carbamazepine product information.3
Table 1 includes a comprehensive list of drug labels that have been revised by the FDA to include information about pharmacogenomic biomarkers.12
Table 1.
Pharmacogenomic Biomarkers in Drug Labels
This table lists FDA-approved drugs with labels that have been revised to include pharmacogenomic biomarker information. The revisions describe the effect of these biomarkers on drug exposure and clinical response variability; risk of adverse events; genotype-specific dosing; mechanisms of drug action; and polymorphic gene drug targets and disposition. Some, but not all, of the revised labels include specific treatment approaches to be taken in response to pharmacogenetic data. For more complete information, please consult the relevant drug labels.
| Drug | Therapeutic Area | Biomarker | Label Sections |
|---|---|---|---|
| Abacavir | Antivirals | HLA-B*5701 | Boxed Warning, Contraindications, Warnings and Precautions, Patient Counseling Information |
| Ado-trastuzumab emtansine | Oncology | ERBB2 (HER2) | Indications and Usage, Warnings and Precautions, Adverse Reactions, Clinical Pharmacology, Clinical Studies |
| Aripiprazole | Psychiatry | CYP2D6 | Clinical Pharmacology, Dosage and Administration |
| Arsenic trioxide | Oncology | PML/RARα | Boxed Warning, Clinical Pharmacology, Indications and Usage, Warnings |
| Atomoxetine | Psychiatry | CYP2D6 | Dosage and Administration, Warnings and Precautions, Drug Interactions, Clinical Pharmacology |
| Atorvastatin | Metabolic and Endocrinology | LDL receptor | Indications and Usage, Dosage and Administration, Warnings and Precautions, Clinical Pharmacology, Clinical Studies |
| Azathioprine | Rheumatology | TPMT | Dosage and Administration, Warnings and Precautions, Drug Interactions, Adverse Reactions, Clinical Pharmacology |
| Boceprevir | Antivirals | IL28B | Clinical Pharmacology |
| Brentuximab vedotin | Oncology | CD30 | Indications and Usage, Description, Clinical Pharmacology |
| Busulfan | Oncology | Ph Chromosome | Clinical Studies |
| Capecitabine | Oncology | DPD | Contraindications, Precautions, Patient Information |
| Carbamazepine | Neurology | HLA-B*1502 | Boxed Warning, Warnings and Precautions |
| Carisoprodol | Musculoskeletal | CYP2C19 | Clinical Pharmacology, Special Populations |
| Carvedilol | Cardiovascular | CYP2D6 | Drug Interactions, Clinical Pharmacology |
| Celecoxib | Analgesics | CYP2C9 | Dosage and Administration, Drug Interactions, Use in Specific Populations, Clinical Pharmacology |
| Cetuximab (1) | Oncology | EGFR | Indications and Usage, Warnings and Precautions, Description, Clinical Pharmacology, Clinical Studies |
| Cetuximab (2) | Oncology | KRAS | Indications and Usage, Dosage and Administration, Warnings and Precautions, Adverse Reactions, Clinical Pharmacology, Clinical Studies |
| Cevimeline | Dermatology and Dental | CYP2D6 | Drug Interactions |
| Chlordiazepoxide and amitriptyline | Psychiatry | CYP2D6 | Precautions |
| Chloroquine | Anti-infectives | G6PD | Precautions |
| Cisplatin | Oncology | TPMT | Clinical Pharmacology, Warnings, Precautions |
| Citalopram (1) | Psychiatry | CYP2C19 | Drug Interactions, Warnings |
| Citalopram (2) | Psychiatry | CYP2D6 | Drug Interactions |
| Clobazam | Neurology | CYP2C19 | Clinical Pharmacology, Dosage and Administration, Use in Specific Populations |
| Clomipramine | Psychiatry | CYP2D6 | Drug Interactions |
| Clopidogrel | Cardiovascular | CYP2C19 | Boxed Warning, Dosage and Administration, Warnings and Precautions, Drug Interactions, Clinical Pharmacology |
| Clozapine | Psychiatry | CYP2D6 | Drug Interactions, Clinical Pharmacology |
| Codeine | Analgesics | CYP2D6 | Warnings and Precautions, Use in Specific Populations, Clinical Pharmacology |
| Crizotinib | Oncology | ALK | Indications and Usage, Warnings and Precautions, Adverse Reactions, Clinical Pharmacology, Clinical Studies |
| Dapsone (1) | Dermatology | G6PD | Warnings and Precautions, Adverse Reactions, Use in Specific Populations, Patient Counseling Information |
| Dapsone (2) | Anti-infectives | G6PD | Precautions, Adverse Reactions, Overdosage |
| Dasatinib | Oncology | Ph Chromosome | Indications and Usage, Clinical Studies, Patient Counseling Information |
| Denileukin diftitox | Oncology | CD25 | Indications and Usage, Warnings and Precautions, Clinical Studies |
| Desipramine | Psychiatry | CYP2D6 | Drug Interactions |
| Dexlansoprazole (1) | Gastroenterology | CYP2C19 | Clinical Pharmacology, Drug Interactions |
| Dexlansoprazole (2) | Gastroenterology | CYP1A2 | Clinical Pharmacology |
| Dextromethorphan and quinidine | Neurology | CYP2D6 | Clinical Pharmacology, Warnings and Precautions |
| Diazepam | Psychiatry | CYP2C19 | Drug Interactions, Clinical Pharmacology |
| Doxepin | Psychiatry | CYP2D6 | Precautions |
| Drospirenone and ethinyl estradiol | Reproductive | CYP2C19 | Precautions, Drug Interactions |
| Eltrombopag (1) | Hematology | Factor V Leiden (FV) | Warnings and Precautions |
| Eltrombopag (2) | Hematology | Antithrombin III deficiency (SERPINC1) | Warnings and Precautions |
| Erlotinib | Oncology | EGFR | Clinical Pharmacology |
| Esomeprazole | Gastroenterology | CYP2C19 | Drug Interactions, Clinical Pharmacology |
| Everolimus | Oncology | ERBB2 (HER2) | Indications and Usage, Boxed Warning, Adverse Reactions, Use in Specific Populations, Clinical Pharmacology, Clinical Studies |
| Exemestane | Oncology | ER &/ PGR | Indications and Usage, Dosage and Administration, Clinical Studies, Clinical Pharmacology |
| Fluorouracil (1) | Dermatology | DPD | Contraindications, Warnings |
| Fluorouracil (2) | Oncology | DPD | Warnings |
| Fluoxetine | Psychiatry | CYP2D6 | Warnings, Precautions, Clinical Pharmacology |
| Fluoxetine and olanzapine | Psychiatry | CYP2D6 | Drug Interactions, Clinical Pharmacology |
| Flurbiprofen | Rheumatology | CYP2C9 | Clinical Pharmacology, Special Populations |
| Fluvoxamine | Psychiatry | CYP2D6 | Drug Interactions |
| Fulvestrant | Oncology | ER | Indications and Usage, Patient Counseling Information |
| Galantamine | Neurology | CYP2D6 | Special Populations |
| Gefitinib | Oncology | EGFR | Clinical Pharmacology |
| Iloperidone | Psychiatry | CYP2D6 | Clinical Pharmacology, Dosage and Administration, Drug Interactions, Specific Populations, Warnings and Precautions |
| Imatinib (1) | Oncology | C-Kit | Indications and Usage, Dosage and Administration Clinical Pharmacology, Clinical Studies |
| Imatinib (2) | Oncology | Ph Chromosome | Indications and Usage, Dosage and Administration, Clinical Pharmacology, Clinical Studies |
| Imatinib (3) | Oncology | PDGFR | Indications and Usage, Dosage and Administration, Clinical Studies |
| Imatinib (4) | Oncology | FIP1L1-PDGFRα | Indications and Usage, Dosage and Administration, Clinical Studies |
| Imipramine | Psychiatry | CYP2D6 | Drug Interactions |
| Indacaterol | Pulmonary | UGT1A1 | Clinical Pharmacology |
| Irinotecan | Oncology | UGT1A1 | Dosage and Administration, Warnings, Clinical Pharmacology |
| Isosorbide and hydralazine | Cardiovascular | NAT1; NAT2 | Clinical Pharmacology |
| Ivacaftor | Pulmonary | CFTR (G551D) | Indications and Usage, Adverse Reactions, Use in Specific Populations, Clinical Pharmacology, Clinical Studies |
| Lansoprazole | Gastroenterology | CYP2C19 | Drug Interactions, Clinical Pharmacology |
| Lapatinib | Oncology | ERBB2 (HER2) | Indications and Usage, Clinical Pharmacology, Patient Counseling Information |
| Lenalidomide | Hematology | Chromosome 5q | Boxed Warning, Indications and Usage, Clinical Studies, Patient Counseling |
| Letrozole | Oncology | ER &/ PGR | Indications and Usage, Adverse Reactions, Clinical Studies, Clinical Pharmacology |
| Maraviroc | Antivirals | CCR5 | Indications and Usage, Warnings and Precautions, Clinical Pharmacology, Clinical Studies, Patient Counseling Information |
| Mercaptopurine | Oncology | TPMT | Dosage and Administration, Contraindications, Precautions, Adverse Reactions, Clinical Pharmacology |
| Metoprolol | Cardiovascular | CYP2D6 | Precautions, Clinical Pharmacology |
| Modafinil | Psychiatry | CYP2D6 | Drug Interactions |
| Mycophenolic acid | Transplantation | HGPRT | Precautions |
| Nefazodone | Psychiatry | CYP2D6 | Drug Interactions |
| Nilotinib (1) | Oncology | Ph Chromosome | Indications and Usage, Patient Counseling Information |
| Nilotinib (2) | Oncology | UGT1A1 | Warnings and Precautions, Clinical Pharmacology |
| Nortriptyline | Psychiatry | CYP2D6 | Drug Interactions |
| Omeprazole | Gastroenterology | CYP2C19 | Dosage and Administration, Warnings and Precautions, Drug Interactions |
| Panitumumab (1) | Oncology | EGFR | Indications and Usage, Warnings and Precautions, Clinical Pharmacology, Clinical Studies |
| Panitumumab (2) | Oncology | KRAS | Indications and Usage, Clinical Pharmacology, Clinical Studies |
| Pantoprazole | Gastroenterology | CYP2C19 | Clinical Pharmacology, Drug Interactions, Special Populations |
| Paroxetine | Psychiatry | CYP2D6 | Clinical Pharmacology, Drug Interactions |
| PEG-interferon alfa-2b | Antivirals | IL28B | Clinical Pharmacology |
| Perphenazine | Psychiatry | CYP2D6 | Clinical Pharmacology, Drug Interactions |
| Pertuzumab | Oncology | ERBB2 (HER2) | Indications and Usage, Warnings and Precautions, Adverse Reactions, Clinical Studies, Clinical Pharmacology |
| Phenytoin | Neurology | HLA-B*1502 | Warnings |
| Pimozide | Psychiatry | CYP2D6 | Warnings, Precautions, Contraindications, Dosage and Administration |
| Prasugrel | Cardiovascular | CYP2C19 | Use in Specific Populations, Clinical Pharmacology, Clinical Studies |
| Pravastatin | Metabolic and Endocrinology | APOE2 | Clinical Studies, Use in Specific Populations |
| Propafenone | Cardiovascular | CYP2D6 | Clinical Pharmacology |
| Propranolol | Cardiovascular | CYP2D6 | Precautions, Drug Interactions, Clinical Pharmacology |
| Protriptyline | Psychiatry | CYP2D6 | Precautions |
| Quinidine | Antiarrhythmics | CYP2D6 | Precautions |
| Rabeprazole | Gastroenterology | CYP2C19 | Drug Interactions, Clinical Pharmacology |
| Rasburicase | Oncology | G6PD | Boxed Warning, Contraindications |
| Rifampin, isoniazid, and pyrazinamide | Anti-infectives | NAT1; NAT2 | Adverse Reactions, Clinical Pharmacology |
| Risperidone | Psychiatry | CYP2D6 | Drug Interactions, Clinical Pharmacology |
| Sodium phenylacetate and sodium benzoate | Gastroenterology | UCD (NAGS; CPS; ASS; OTC; ASL; ARG) | Indications and Usage, Description, Clinical Pharmacology |
| Sodium phenylbutyrate | Gastroenterology | UCD (NAGS; CPS; ASS; OTC; ASL; ARG) | Indications and Usage, Dosage and Administration, Nutritional Management |
| Tamoxifen (1) | Oncology | ER | Indications and Usage, Precautions, Medication Guide |
| Tamoxifen (2) | Oncology | Factor V Leiden (FV) | Warnings |
| Tamoxifen (3) | Oncology | Prothrombin mutations (F2) | Warnings |
| Telaprevir | Antivirals | IL28B | Clinical Pharmacology |
| Terbinafine | Antifungals | CYP2D6 | Drug Interactions |
| Tetrabenazine | Neurology | CYP2D6 | Dosage and Administration, Warnings, Clinical Pharmacology |
| Thioguanine | Oncology | TPMT | Dosage and Administration, Precautions, Warnings |
| Thioridazine | Psychiatry | CYP2D6 | Precautions, Warnings, Contraindications |
| Ticagrelor | Cardiovascular | CYP2C19 | Clinical Studies |
| Tolterodine | Reproductive and Urologic | CYP2D6 | Clinical Pharmacology, Drug Interactions, Warnings and Precautions |
| Tositumomab | Oncology | CD20 antigen | Indications and Usage, Clinical Pharmacology |
| Tramadol and acetaminophen | Analgesics | CYP2D6 | Clinical Pharmacology |
| Trastuzumab | Oncology | ERBB2 (HER2) | Indications and Usage, Precautions, Clinical Pharmacology |
| Tretinoin | Oncology | PML/RARα | Boxed Warning, Dosage and Administration, Precautions |
| Trimipramine | Psychiatry | CYP2D6 | Drug Interactions |
| Valproic acid | Psychiatry | UCD (NAGS; CPS; ASS; OTC; ASL; ARG) | Contraindications, Precautions, Adverse Reactions |
| Vemurafenib | Oncology | BRAF | Indications and Usage, Warning and Precautions, Clinical Pharmacology, Clinical Studies, Patient Counseling Information |
| Venlafaxine | Psychiatry | CYP2D6 | Drug Interactions |
| Voriconazole | Antifungals | CYP2C19 | Clinical Pharmacology, Drug Interactions |
| Warfarin (1) | Hematology | CYP2C9 | Dosage and Administration, Precautions, Clinical Pharmacology |
| Warfarin (2) | Hematology | VKORC1 | Dosage and Administration, Precautions, Clinical Pharmacology |
Adapted from the FDA. Updated June 19, 2013.12
Genetic Variants and Response to Anti-infective Drugs
More than 170 million people worldwide are chronically infected with the hepatitis C virus (HCV). Complications of this infection include liver cirrhosis, hepatic injury, and hepatic fibrosis.5 Over the previous decade, the standard of care for HCV has been 24 to 48 weeks of polyethylene glycol (PEG-interferon, or pegylated interferon) treatment, either alone or in combination with ribavirin.5 Patients who respond to this treatment often achieve a sustained virological response, which is associated with a good quality of life.5
Several independent genome-wide association studies (GWAS) demonstrated that a variant in the gene encoding interleukin-28B (IL28B) is associated with this positive treatment response.5 This genetic variant also predicted treatment response in patients with HCV who were co-infected with HIV.5 Although the package insert for PEG-interferon has been revised to include information regarding this IL28B variant, a specific recommendation with respect to screening patients before initiating treatment has not been included.12
Physicians could avoid many adverse drug reactions if they were capable of predicting an individual’s response to treatment.6 Abacavir, a nucleoside reverse transcriptase inhibitor (NRTI) used to treat AIDS, is known to cause a hypersensitivity reaction in 5% to 8% of patients.6 The reaction can include fever, rash, and gastrointestinal or respiratory symptoms, which often lead to treatment discontinuation.6HLA-B*5701, a member of the HLA gene family that codes for proteins which play a critical role in regulating immune system response, has been associated with abacavir hypersensitivity.6
In 2008, the FDA revised the product information for abacavir to include a boxed warning regarding the association of HLA-B*5701 with abacavir hypersensitivity.13 The prescribing information now recommends that patients be prescreened for the HLA-B*5701 allele prior to initiation or re-treatment with abacavir or abacavir-containing medications.13 The prescribing information also advises that abacavir treatment should not be initiated in patients who are positive for this allele except under extraordinary circumstances.13
Genetic Variants and Response to Cancer Drugs
Many early successes in pharmacogenomic research took place in oncology, where somatic genetic changes in tumors were found to have more of an effect on drug efficacy compared with variations in an individual’s germline DNA.4
Tumor cells carry the same germline genetic polymorphisms that normal cells do; however, the genetic instability of malignant cells can produce a high incidence of additional mutations, increase the copy number of inherited variants, or even repress gene expression.1,4 These genetic changes can increase copies of genes encoding DMEs or drug transporters, which can lead to alterations in the disposition of active drugs at the site of the tumor.1
Much knowledge regarding somatic genetic alterations that influence response to cancer therapies has been gathered over the past several decades. This information has led to the development of many targeted cancer treatments.1 Genetic analysis of tumors can help predict therapeutic benefits (or lack thereof) of such targeted biologics as trastuzumab (Herceptin, Genentech) for ERBB2 (HER2)-amplified breast cancers; erlotinib (e.g., Tarceva, OSI/Genentech) for epidermal growth factor receptor (EGFR)–overexpressing lung cancers; or imatinib (e.g., Gleevec, Novartis) for Philadelphia chromosome–positive chronic myelogenous leukemia (CML).4
Somatic genetic mutations in tumors can also help predict resistance to treatment, as has been observed in colorectal cancers, in which activating mutations in KRAS (V-Ki-ras2 Kirsten rat sarcoma viral oncogene homolog) are known to be a predictive marker for resistance to the EGFR-specific monoclonal antibodies cetuximab (e.g., Erbitux, Bristol-Myers Squibb/ImClone) and panitumumab (e.g., Vectibix, Amgen).4 The FDA has revised the drug labels for cetuximab and panitumumab to include this information regarding KRAS mutations.1
Much knowledge about somatic genetic alterations that influence response to cancer therapies has been gathered over the past several decades. This information has led to the development of many targeted cancer treatments.
One pharmacogenomic study estimated that genetic factors could determine more than 40% of the susceptibility to cisplatin-induced cytotoxicity.7 Cisplatin is a platinating agent used to treat various types of cancers, including lung cancer and lymphoma.6 In another study, an SNP in a NRG3 gene (which has never been linked to cancer) was found to be associated with cellular sensitivity to platinum agents; this association was replicated in ovarian cancer patients who underwent platinum-based therapy.7 These findings suggest that sensitivity to the cytotoxic effects of chemotherapeutic agents could be under substantial genetic influence.7
Genetic factors can also predict tolerance to cancer treatments. For example, polymorphisms in SLCO1B1 have also been associated with methotrexate-related gastrointestinal toxicity and disposition of the cyclin-dependent kinase inhibitor flavopiridol.1
Genetic Variants and Response to Cardiovascular Drugs
The appropriate dose of warfarin can vary by as much as 10-fold among patients.4 Genetic variations in three genes— CYP2C9, VKORC1, and CYP4F2—contribute significantly to this variability.4 Approximately 550,000 genetic polymorphisms have been evaluated. The most significant effect was found for VKORC1, with a more moderate association found for the CYP2C9 variants rs4917639 (CYP2C9*2) and rs1057910 (CYP2C9*3).1
VKORC1 codes for the vitamin K epoxide reductase protein, the target enzyme for warfarin.1 Variants in VKORC1 have been significantly associated with warfarin sensitivity and reduced dose requirements.1 The primary enzyme involved in the metabolism of S-warfarin (a more potent form of warfarin), which is primarily responsible for anticoagulation, is encoded by the CYP2C9*1 gene.1,4 The CYP2C9*2 and CYP2C9*3 variants encode enzymes with decreased or nearly entirely neutralized function, respectively, decreasing warfarin metabolism.1,4 The CYP2C9*2 variant reduces the metabolism of warfarin by 30% to 50%, whereas the CYP2C9*3 variant reduces the metabolism of the drug by 90%.1,4 The decreased metabolism caused by these genetic variants leads to a longer half-life and a reduced rate of clearance for warfarin.1,4 Response to warfarin has also been found to be affected by a single SNP (rs2108622) in the CYP4F2 gene, which alters protein coding.7
The appropriate dose of warfarin can vary by as much as 10-fold among patients.4 Genetic variations in three genes—CYP2C9, VKORC1, and CYP4F2—contribute significantly to this variability.
In 2007, the FDA modified the drug label for warfarin to include these findings, stating that VKORC1 and CYP2C9 geno-types might be useful in determining the optimal initial dose of warfarin.7 In 2010, the warfarin label was further updated to include a table with dosage range recommendations for patients with different combinations of CYP2C9 and VKORC1 genotypes.5,7 Several warfarin dosing algorithms that incorporate both genetic and nongenetic parameters have also been developed.5,7 A study published in 2012 reported that using patient genotype data for warfarin dosing reduced the risk of hospitalization by nearly one-third.4
Clopidogrel is an inactive prodrug, which is activated in vivo by several CYP450 enzymes, some of which are encoded by CYP2C19.4 Variants in CYP2C19 can reduce enzymatic function, lowering conversion of the clopidogrel prodrug to its active form and thus reducing efficacy.4 This effect was originally described in two studies: the Trial to Assess Improvement in Therapeutic Outcomes by Optimizing Platelet Inhibition with Prasugrel–Thrombolysis in Myocardial Infarction (TRITON– TIMI) 38, and a French registry study of patients with acute myocardial infarction (MI).4 TRITON–TIMI 38 also found that ABCB1 and CYP2C19 variants were significant independent predictors of cardiovascular (CV) death, MI, and stroke.4 However, these effects were not observed in two more recent studies: Clopidogrel in Unstable Angina to Prevent Recurrent Events (CURE) and the Atrial Fibrillation Clopidogrel Trial with Irbesartan for Prevention of Vascular Events (ACTIVE).4
Nonetheless, most investigators have concluded that the CYP2C19 genotype influences platelet response to clopidogrel in the setting of percutaneous coronary intervention.4 However, sufficient consensus recommending routine genetic testing in clinical practice has not emerged.4 In 2010, the FDA added a boxed warning to the clopidogrel product information, describing patients with the CYP2C19 polymorphism as being at higher risk of experiencing reduced efficacy with clopidogrel treatment.1,4
Statins, which are among the top 10 drugs prescribed in the U.S., are relatively safe, with the most common side effect being dose-dependent skeletal muscle toxicity.7 This toxicity ranges in severity from incipient myopathy, to myopathy, to rhabdomyolysis, which is life-threatening but rare (1 in 100,000).7
The first GWAS to investigate statin-induced myopathy examined approximately 300,000 markers in 85 subjects with definite or incipient myopathy along with 90 controls, all of whom had taken a daily dose of simvastatin 80 mg as part of a clinical trial (n = 12,000).7 In this study, genetic variants of SLCO1B1, a gene that encodes drug transporter proteins, were found to be significantly associated with the risk of myopathy.1,7 This finding has since been replicated in several independent studies, including the Heart Protection Study, the Statin Response Examined by Genetic Haplotype Markers (STRENGTH) study, and a retrospective case– control study.6 In 2011, the FDA updated the simvastatin product information, advising against initiation of the 80-mg dose and cautioning against continuation of an 80-mg dose unless the patient has been taking it without muscle problems for more than 1 year.7
Genetic Variants and Response to Opioid Drugs
Codeine is a prodrug that is converted to its active metabolite, morphine, by CYP450 enzymes that are encoded by the CYP2D6 gene.1,3 There are 80 CYP2D6 variants.1,3 The CYP2D6*3, *4, *5, *6, and *7 alleles are reported to account for the majority of decreased CYP2D6 enzyme activity, whereas the CYP2D6*9 and CYP2D6*10 alleles have been associated with decreased drug-metabolizing activity in intermediate metabolizers (IMs).3 By contrast, the duplication of the CYP2D6 gene in ultra-rapid metabolizers (UMs) is associated with higher morphine plasma concentrations and area-under-the-curve (AUC) concentrations compared with those observed in homozygous normal/extensive metabolizers (EMs).3 One study evaluated oxycodone, which is structurally similar to codeine and is also metabolized by CYP2D6 drug-metabolizing enzymes (DMEs).3 This study showed that poor metabolizers (PMs) had a two-fold to 20-fold decrease, and UMs had a 1.5-fold to 6-fold increase, in analgesic effects compared with EMs.3
With respect to adverse effects, although UMs experience greater analgesic effects, they are also at an increased risk of toxicity.3 In addition, although PMs might not experience the expected analgesic effects, they can still experience side effects (e.g., sedation, headaches, dizziness, and dry mouth), which may be associated with the codeine prodrug rather than its metabolites.3 The AmpliChip CYP450 microarray (Roche/Affymetrix), which can detect 33 CYP2D6 alleles, is the only approved test for CYP2D6 polymorphisms in the U.S.3
The FDA has revised the drug label for codeine to include information about CYP2D6 polymorphisms.12 However, there are currently no formal recommendations or requirements regarding genetic testing prior to initiating codeine treatment.3
Genetic Variants and Response to Proton Pump Inhibitors
The proton pump inhibitors (PPIs) omeprazole and esomeprazole (Prilosec and Nexium, AstraZeneca), lansoprazole (Prevacid, Takeda), and pantoprazole (Protonix, Pfizer) are metabolized primarily by enzymes encoded by CYP2C19 and, to some extent, by CYP3A.3 PPIs are unique, in that pharmacokinetic variability also affects drug pharmacodynamics.3 For example, with omeprazole, the AUC time curve is 7-fold to 14-fold higher in PMs than in homozygous EMs.3 Consequently, patients who are CYP2C19 PMs have a higher intragastric pH compared with EMs.3
A higher intragastric pH has been shown to increase antibiotic concentrations and to improve antibiotic bioavailability and stability.3 The various Helicobacter pylori cure rates that have been observed for PPIs can be explained to some extent by the different effects of CYP2C19 genetic polymorphisms on PPI pharmacokinetics and pharmacodynamics.3 In one study, Japanese patients with confirmed H. pylori infection received dual therapy with omeprazole and amoxicillin for several weeks.3 Because of increased antibiotic concentrations and bio-availability resulting from higher intragastric pH for omeprazole PMs, H. pylori cure rates were observed to be 28.6%, 60%, and 100%, respectively, in homozygous EMs, heterozygous EMs, and PMs.3 These results have been consistently observed in a majority of studies, which have concluded that the efficacy of omeprazole is influenced by the CYP2C19 genotype.3 However, some studies have yielded conflicting data, because they failed to identify a correlation between omeprazole treatment, H. pylori cure rates, and the CYP2C19 genotype.3
Studies examining PPI dosing strategies based on the CYP2C19 genotype are currently lacking.3 This could be because such strategies may be of limited use due to the large therapeutic window, the many dual-therapy or triple-therapy regimens that are available, and the low incidence of clinically significant adverse effects for PPIs.3 Although it has been suggested that higher PPI doses be considered when treating homozygous EMs, this recommendation has not yet been adopted in clinical practice.3 However, an FDA-approved test is commercially available (AmpliChip CYP450) for CYP2C19 genotyping.3
The FDA has revised the clinical pharmacology and drug-interaction sections in the prescribing information for some PPIs, such as esomeprazole, pantoprazole, and rabeprazole (e.g., Aciphex, Eisai/Janssen), to mention the effect of CP2C19 polymorphisms on drug metabolism; however, no formal recommendations for genetic testing are included.3
Genetic Variants and Response to Psychotropic Drugs
Evidence accumulated over the previous 15 years indicates that genetic factors contribute substantially to drug treatment response in schizophrenic patients.8 Most findings indicate that genetic polymorphisms that code for dopamine receptors and 5-hydroxytryptamine (5-HT, or serotonin) receptor neurotransmission are linked to symptom improvement.8
The primary mechanism of antipsychotic drugs involves antagonist action at dopamine D2 receptors in the brain.8 Polymorphisms in the DRD2, DRD3, and DRD4 genes coding for dopamine D2, D3, and D4, receptors, respectively, have been widely investigated with respect to symptom response.8 The association of the D2-receptor gene with drug response has been confirmed in a systematic review; however, a link with the D3 receptor gene remains weak and inconsistent.8 There have been no significant findings indicating an influence of SNPs in the dopamine D1 or D4 receptor genes with treatment response.8 The dopamine transporter gene, DAT, has been positively associated with clozapine response but negatively with risperidone.8
Other genes that have been studied include HTR1A and HTT, which code for serotonin receptors and transporters.8 The 5-HT2A receptor is a major target that differentiates second-generation (atypical) antipsychotic drugs from older medications.8 Although an SNP in the 5-HT1A receptor gene was found to affect antipsychotic treatment response, a review of studies of SNPs in the 5-HT2A receptor gene concluded that there is only a weak association with antipsychotic response as well as with psychosis itself.8 A study of the indel sequence in the promoter region of the serotonin transporter (HTT) gene has been associated with an antipsychotic response in some but not all studies.8 However, these findings have not been consistently replicated, so there is still little solid evidence supporting a correlation between pharmacogenetic associations and treatment response in patients with schizophrenia.8 Therefore, the clinical utility of genetic screening in treating schizophrenia has not yet been established.2
The U.S. Department of Health and Human Services’ Agency for Healthcare Research and Quality (AHRQ) has also undertaken a systematic review of published studies to determine whether CYP450 polymorphisms correlate with positive outcomes in adults taking selective serotonin reuptake inhibitors (SSRIs).2 The AHRQ concluded that the data did not demonstrate a clear correlation between CYP450 polymorphisms and SSRI drug levels, efficacy, or tolerability.2
Companion Diagnostic Tests
Although genetic research began in the 1950s, tests for genetic variants that influence drug response have become available only relatively recently.9 The FDA has revised numerous drug labels to include information about pharmacogenetic biomarkers; however, genetic screening is still not required to initiate treatment with most of these drugs.9 As evidence for gene–drug associations continues to emerge, changes in drug labels for both approved and new agents are likely to continue.1,4
Several companies manufacture FDA-approved pharmacogenetic screening tests. The first test of this type available for clinical use was the AmpliChip CYP450, which was introduced to the market in 2005.10 This test identifies CYP2D6 and CYP2C19 variants that code for DMEs and predict whether a patient exhibits a PM, IM, EM, or UM phenotype.10 This information helps to characterize how an individual will metabolize approximately 25% of prescribed drugs.10 Another available test is the Affymetrix DMET (DMEs and transporters) chip.10 This product evaluates polymorphisms in CYP1A2, CYP2C9, CYP2C19, CYP2D6, and CYP3A4, A5, and A7 genes.10
The PHARMAChip, made by Progenika Biopharma SA, is even more versatile; it enables identification of 85 of the most relevant pharmacogenetic polymorphisms with a sensitivity and specificity greater than 99.9%.10 The PHARMAChip detects polymorphisms in CYP450 enzymes and in genes that code for drug receptors, transporters, and other targets.10 The test also identifies whether a patient is likely to exhibit a PM, IM, EM, or UM phenotype, based on identified genetic variants, thereby enabling the determination of the type and dose of drugs that might best be used to treat the patient.10 In 2013, two new companion diagnostic devices were approved for therapies used in non–small-cell lung cancer: the Therascreen Kit (Boehringer Ingelheim/Qiagen) for use with afatinib (Gilotrif) and the Cobas EGFR Mutation Test (Roche) for use with erlotinib.
The FDA has revised many drug labels to include information about pharmacogenetic biomarkers, but genetic screening is still not required to initiate treatment.
Companion tests are also now being used in clinical trials for drugs that are in development.5 In July 2011, the FDA published draft guidelines requiring companion diagnostic tests to be approved simultaneously with their accompanying therapies if they are necessary for the safe and effective use of a medication.5 The FDA approved the clinical trial design for bucindolol, a beta-blocker and mild vasodilator, in a genotype-defined population with heart failure.4 This trial highlights the value of pharmacogenetic information for pharmaceutical companies, because drugs that don’t achieve clinical endpoints required for FDA approval in the general population may successfully do so in genotype-defined subpopulations.4
In 2003, the FDA also initiated a voluntary data exchange program through which companies could choose to submit genomic data with their New Drug Applications (NDAs), which many drug companies now do.1
Despite these advances, surveys have shown that although 30% to 50% of drugs currently in development have an accompanying biomarker program, only 10% of them are expected to launch with a companion diagnostic test in the next 5 to 10 years.5 Nonetheless, the growing frequency with which companion diagnostic tests accompany new agents is expected to encourage the clinical utilization of pharmacogenetic testing in an increasing number of therapeutic areas.1
Conclusion
Pharmacogenomic studies conducted in recent decades have provided an over whelming amount of evidence regarding the influence of inter-individual genetic variations on drug response.5 As genotyping technology becomes more advanced, cost-effective, and widely accessible, data from future studies will undoubtedly reveal many more important gene–drug associations.8 Once the clinical utility of predictive pharmacogenetic testing becomes more widely established, genotyping is expected to be an indispensible tool in predicting and improving drug response, with the ultimate goal of personalizing patient drug treatment to provide better therapeutic outcomes.8
References
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