Genetic polymorphisms of drug-metabolising enzymes and drug transporters in the chemotherapeutic treatment of cancer

There is wide variability in the response of individuals to standard doses of drug therapy. This is an important problem in clinical practice, where it can lead to therapeutic failures or adverse drug reactions. Polymorphisms in genes coding for metabolising enzymes and drug transporters can affect drug efficacy and toxicity. Pharmacogenetics aims to identify individuals predisposed to a high risk of toxicity and low response from standard doses of anti-cancer drugs. This review focuses on the clinical significance of polymorphisms in drug-metabolising enzymes (cytochrome P450 [CYP] 2C8, CYP2C9, CYP2C19, CYP2D6, CYP3A4, CYP3A5, dihydropyrimidine dehydrogenase, uridine diphosphate glucuronosyltransferase [UGT] 1A1, glutathione S-transferase, sulfotransferase [SULT] 1A1, N-acetyltransferase [NAT], thiopurine methyltransferase [TPMT]) and drug transporters (P-glycoprotein [multidrug resistance 1], multidrug resistance protein 2 [MRP2], breast cancer resistance protein [BCRP]) in influencing efficacy and toxicity of chemotherapy.The most important example to demonstrate the influence of pharmacogenetics on anti-cancer therapy is TPMT. A decreased activity of TPMT, caused by genetic polymorphisms in the TPMT gene, causes severe toxicity with mercaptopurine. Dosage reduction is necessary for patients with heterozygous or homozygous mutation in this gene.Other polymorphisms showing the influence of pharmacogenetics in the chemotherapeutic treatment of cancer are discussed, such as UGT1A1*28. This polymorphism is associated with an increase in toxicity with irinotecan. Also, polymorphisms in the DPYD gene show a relation with fluorouracil-related toxicity; however, in most cases no clear association has been found for polymorphisms in drug-metabolising enzymes and drug transporters, and pharmacokinetics or pharmacodynamics of anti-cancer drugs. The studies discussed evaluate different regimens and tumour types and show that polymorphisms can have different, sometimes even contradictory, pharmacokinetic and pharmacodynamic effects in different tumours in response to different drugs.The clinical application of pharmacogenetics in cancer treatment will therefore require more detailed information of the different polymorphisms in drug-metabolising enzymes and drug transporters. Larger studies, in different ethnic populations, and extended with haplotype and linkage disequilibrium analysis, will be necessary for each anti-cancer drug separately.     

Tailoring drug therapy based on genotype

Polymorphisms in genes encoding drug metabolizing enzymes, drug transporters, and drug targets can influence drug effects and contribute to inter-individual differences in drug response. Genotype for drug metabolizing enzymes and drug transporters can influence drug disposition in the body (pharmacokinetics), whereas genotype for drug targets may influence sensitivity to a drug (pharmacodynamics). In some cases, response to a particular drug is contingent on genotype for both drug disposition and drug target proteins. For example, warfarin dose requirements are influenced by both cytochrome P450 2C9 (CYP2C9) and vitamin K epoxide reductase complex 1 (VKORC1) genotypes. The goal of pharmacogenetics is to maximize drug effectiveness while limiting drug toxicity, based on an individual's DNA. Over 80 drugs now contain genetic information in their FDA-approved labeling. In addition to influencing warfarin dose requirements, genotype contributes to the efficacy of clopidogrel in coronary artery disease, risk for hypersensitivity reactions to abacavir in the treatment of human immunodeficiency virus, risk for statin-induced myopathy, and responses to numerous other drugs. Genetic information is routinely integrated into decisions regarding cancer chemotherapy and treatment for human immunodeficiency virus. Clinical implementation of pharmacogenetics is becoming a reality in other therapeutic areas, such as for patients requiring dual antiplatelet therapy following coronary artery stent implantation. In the future, it is possible that individuals will be broadly genotyped so that genetic information can guide drug therapy decisions throughout their lifetime.     

Pharmacogenetics of cytochromes P450 in tropical medicine

Drug response is affected by genetic and non-genetic factors, such as dietary compounds, sex, disease status and multiple drug therapy. Inherited determinants of drug disposition remain, however, the major cause of inter-individual differences due to pharmacogenetic polymorphism in drug metabolizing enzymes and transporters, or drug targets. Differences on ethnicity may have a profound impact on drug clearance, affecting the safety, efficacy and dosing regimen. In the context of tropical regions, the situation may be even more serious due to endemic infectious diseases and multiple drug therapy, which may affect drug clearance. In this review, we focus on the pharmacogenetics of the Cytochrome P450 superfamily, responsible for the highest contribution for variability among drug metabolizing enzymes, among ethnic groups from tropical settings.     

Pharmacogenetic determinants of mercaptopurine disposition in children with acute lymphoblastic leukemia

BACKGROUND: The backbone of drug therapy used in acute lymphoblastic leukemia (ALL) in children includes 6-mercaptopurine (6-MP). Intracellular metabolism of this prodrug is a key component of the therapeutic response. Many metabolizing enzymes are involved in 6-MP disposition and active 6-MP metabolites are represented by 6-thioguanine nucleotides (6-TGN) and methylated metabolites primarily methylated by the thiopurine S-methyltransferase enzyme (TPMT). The genetic polymorphism affecting TPMT activity displays an important inter-subject variability in metabolites pharmacokinetics and influences the balance between 6-MP efficacy and toxicity: patients with high 6-TGN levels are at risk of myelosuppression while patients with high levels of methylated derivates are at hepatotoxic risk. However, the genetic TPMT polymorphism does not explain all 6-MP adverse events and some severe toxicities leading to life-threatening conditions remain unexplained. Additional single nucleotide polymorphisms (SNPs) in genes encoding enzymes involved in 6-MP metabolism and 6-MP transporters may also be responsible for this inter-individual 6-MP response variability. AIM: This review presents the pharmacogenetic aspects of 6-MP metabolism in great detail. We have focused on published data on ALL treatment supporting the great potential of 6-MP pharmacogenetics to improve efficacy, tolerance, and event-free survival rates in children with ALL.     

Pharmacogenetics of the antiepileptic drugs phenytoin and lamotrigine

Patients treated with antiepileptic drugs can exhibit large interindividual variability in clinical efficacy or adverse effects. This could be partially due to genetic variants in genes coding for proteins that function as drug metabolizing enzymes, drug transporters or drug targets. The purpose of this article is to provide an overview of the current knowledge on the pharmacogenetics of two commonly prescribed antiepileptic drugs with similar mechanisms of action; phenytoin (PHT) and lamotrigine (LTG). These two drugs have been selected in order to model the pharmacogenetics of Phase I and Phase II metabolism for PHT and LTG, respectively. In light of the present evidence, patients treated with PHT could benefit from CYP2C9 and CYP2C19 genotyping/phenotyping. For those under treatment with LTG, UGT1A4 and UGT2B7 genotyping might be of clinical use and could contribute to the interindividual variability in LTG concentration to dose ratio in epileptic patients.     

The pharmacogenetics of analgesia

Genomic variations influencing response to pharmacotherapy of pain are under investigation. Candidate genes such as (opioid)-receptors, transporters and other molecules important for pharmacotherapy are discussed. Drug metabolising enzymes represent a further major target of ongoing research in order to identify associations between an individual's genetic profile and drug response (pharmacogenetics). Polymorphisms of the cytochrome P450 enzymes influence analgesic efficacy of codeine, tramadol and tricyclic antidepressants (CYP2D6). Blood levels of some NSAIDs are dependent on CYP2C9 activity, whereas opioid-receptor polymorphisms are discussed for differences in opioid mediated analgesia and side effects. Pharmacogenetics as a diagnostic tool has the potential to improve patient therapy and care, and it is hoped that pharmacogenetics will individualise drug treatment to a greater extent in the near future.     

Escitalopram for the treatment of major depressive disorder in youth

Genomic variations influencing response to pharmacotherapy of pain are under investigation. Candidate genes such as (opioid)-receptors, transporters and other molecules important for pharmacotherapy are discussed. Drug metabolising enzymes represent a further major target of ongoing research in order to identify associations between an individual's genetic profile and drug response (pharmacogenetics). Polymorphisms of the cytochrome P450 enzymes influence analgesic efficacy of codeine, tramadol and tricyclic antidepressants (CYP2D6). Blood levels of some NSAIDs are dependent on CYP2C9 activity, whereas opioid-receptor polymorphisms are discussed for differences in opioid mediated analgesia and side effects. Pharmacogenetics as a diagnostic tool has the potential to improve patient therapy and care, and it is hoped that pharmacogenetics will individualise drug treatment to a greater extent in the near future.     

Pharmacogenetics and drug therapy in psychiatry--the role of the CYP2D6 polymorphism

The importance of pharmacogenetics in medicine is growing with the identification of genetic variability by faster screening methods using automatic sequencers. A particularly interesting finding is that apart from environmental and psychological factors, drug response may be influenced by several biological factors as a result of genetic determinants leading to interindividual variability. Several mutations in genes coding for enzymes of the drug metabolizing system, as well as for neurotransmitter receptors or degrading enzymes and monoamine transport proteins, have been identified and investigated in psychiatry. But, despite the fact that some genetic polymorphisms of enzymes (mainly cytochrome P450 2D6) are well known, the application of pharmacogenetics as a therapeutic tool for improving patient care is rare. This review has three parts. In the first an overview is given of CYP450 characteristics and the genetic polymorphisms of interest to psychiatry. In the second the clinical implications of the CYP2D6 polymorphism are reviewed and in the third part other aspects on pharmacogenetic research in psychiatry are discussed. The aim of our review is to promote the application of pharmacogenetics in everyday clinical practice.     

基因多态性对巯嘌呤类药物作用影响的研究进展

基因多态性是影响巯嘌呤类药物在体内代谢,最终造成药物疗效和毒性差异的主要原因。近年来研究发现,单一药物代谢酶或转运体的基因多态性与巯嘌呤类药物临床不良反应不完全相关,而多基因分析可能更好地解释患者对该药的不耐受原因。本文介绍了多种与巯嘌呤类药物有关的酶基因多态性的研究情况,同时就其代谢特点及与该药敏感性的关系进行综述。     

Pharmacogenetics of analgesics: toward the individualization of prescription

The use of analgesics is based on the empiric administration of a given drug with clinical monitoring for efficacy and toxicity. However, individual responses to drugs are influenced by a combination of pharmacokinetic and pharmacodynamic factors that can sometimes be regulated by genetic factors. Whereas polymorphic drug-metabolizing enzymes and drug transporters may affect the pharmacokinetics of drugs, polymorphic drug targets and disease-related pathways may influence the pharmacodynamic action of drugs. After a usual dose, variations in drug toxicity and inefficacy can be observed depending on the polymorphism, the analgesic considered and the presence or absence of active metabolites. For opioids, the most studied being morphine, mutations in the ABCB1 gene, coding for P-glycoprotein (P-gp), and in the micro-opioid receptor reduce morphine potency. Cytochrome P450 (CYP) 2D6 mutations influence the analgesic effect of codeine and tramadol, and polymorphism of CYP2C9 is potentially linked to an increase in nonsteroidal anti-inflammatory drug-induced adverse events. Furthermore, drug interactions can mimic genetic deficiency and contribute to the variability in response to analgesics. This review summarizes the available data on the pharmacokinetic and pharmacodynamic consequences of known polymorphisms of drug-metabolizing enzymes, drug transporters, drug targets and other nonopioid biological systems on central and peripheral analgesics.     

Prospects and limits of pharmacogenetics: the thiopurine methyl transferase (TPMT) experience

Thiopurine drug metabolism is a quintessential case of pharmacogenetics. A wealth of experimental and clinical data on polymorphisms in the thiopurine metabolizing enzyme thiopurine methyl transferase (TPMT) has been generated in the past decade. Pharmacogenetic testing prior to thiopurine treatment is already being practiced to some extent in the clinical context, and it is likely that it will be among the first pharmacogenetic tests applied on a regular basis. We analyzed the published TPMT data and identified some lessons to be learned for the future implementation of pharmacogenetics for thiopurines as well as in other fields. These include the need for comprehensive and unbiased data on allele frequencies relevant to a broad range of populations worldwide. The nature and frequency of TPMT gene polymorphisms in some ethnic groups is still a matter of speculation, as the vast majority of studies on TPMT allele distribution are limited to only a small subset of alleles and populations. Secondly, an appreciation of the limits of pharmacogenetics is warranted, as pharmacogenetic testing can help in avoiding some, but by far not all adverse effects of drug therapy. An analysis of six clinical studies correlating adverse thiopurine effects and TPMT genotype revealed that an average of 78% of adverse drug reactions were not associated with TPMT polymorphisms. Pharmacogenetic testing will thus not eliminate the need for careful clinical monitoring of adverse drug reactions. Finally, a careful approach toward dose increases for patients with high enzyme activity is necessary, as TPMT-mediated methylation of thiopurines generates a possibly hepatotoxic byproduct.     

Cancer treatment and pharmacogenetics of cytochrome P450 enzymes

Summary For the treatment of cancer, the window between drug toxicity and suboptimal therapy is often narrow. Interindividual variation in drug metabolism therefore complicates therapy. Genetic polymorphisms in phase I and phase II enzymes may explain part of the observed interindividual variation in pharmacokinetics and pharmacodynamics of anticancer drugs. The cytochrome P450 superfamily is involved in many drug metabolizing reactions. Information on variant alleles for the different isoenzymes of this family, encoding proteins with decreased enzymatic activity, is rapidly growing. The ultimate goal of ongoing research on these enzymes would be to enable pharmacogenetic screening prior to anticancer therapy. At this moment, potential clinically relevant application of CYP450 pharmacogenetics for anticancer therapy may be found for CYP1A2 and flutamide, CYP2A6 and tegafur, CYP2B6 and cyclophosphamide, CYP2C8 and paclitaxel, CYP2D6 and tamoxifen, and CYP3A5. For this latter enzyme, the drugs of interest still need to be identified.     

Role of pharmacogenetics in variable response to drugs: focus on opioids

Pharmacogenetics is the study of the role of genetics in inter-individual variability to drug response and therapy. This review provides a comprehensive report of the present status of pharmacogenetic studies for opioid drugs. Opioid analgesics are widely used clinically for pain management, and inter-patient variability with opioid therapy is often reported. Information on genetic polymorphisms in enzymes, receptors and transporters related to opioid disposition (pharmacokinetics) and pharmacology (pharmacodynamics) is discussed. Pharmacogenetics of enzymes, including the cytochrome P450s and uridine diphosphoglucuronosyltransferases, opioid receptors and the ABC family of transporters, is reviewed. The role of genetic variability in clinical opioid therapy is examined, and relevant clinical trials cited. The present status of opioid pharmacogenetics, promises and challenges and future directions are discussed.     

Genetic Polymorphism of Drug Metabolizing Enzymes: New Mutations in CYP2D6 and CYP2A6 Genes in Japanese

Genetic polymorphism of drug metabolizing enzymes, particularly cytochrome P450 (CYP), is an important cause of adverse drug reactions. Multiple gene mutations in CYP have been shown to be phenotype. The occurrence of genetic polymorphism has been seen in genes for CYP1A1, CYP2A6, CYP2C9, CYP2C19, CYP2D6, CYP2E1 and CYP3A5. This review discusses the molecular mechanism of two genetic polymorphisms, debrisoquine/sparteine (CYP2D6) coumarin (CYP2A6) polymorphisms. In addition, elucidation of gene mutations of CYP2D6 and CYP2A6 in Japanese will be discussed.     

Analysis and its application for prevention of side-effects of drugs and for evaluation of drug responsiveness

The response of patients to drugs can be affected by genetic polymorphisms/defects in drug metabolizing enzymes, transporters, and receptors. Genetic polymorphisms/defects are generated by mutation of coding regions and/or noncoding regions of target genes, such as single-point mutations, deletions/insertions, variation in the number of tandem repeats, etc. If a genetic defect in a patient which affects drug response were known, it would be possible to optimize medications individually. The author developed two improved methods for detecting CYP2C19(*)2 and CYP2C19(*)3. Using the methods, the type of CYP2C19 gene was examined in 80 inpatients, and the medication status of patients with the mutation was examined focusing on dosage and side effects. The author also examined polymorphisms of the serotonin transporter/biosynthetic or metabolizing enzymes in depressive patients treated with fluvoxamine, a selective serotonin reuptake inhibitor, and the relationship between clinical efficacy and polymorphisms was investigated. As a result, patients with the S/S genotype of 5-HTTLPR were found to experience better clinical efficacy.     

Therapeutic failure: importance of genes?

Drug management can be a difficult task in certain situations because of the variable response observed from one patient to another. Genetic factors affecting the pharmacokinetics and pharmacodynamics of drug reactions could explain the interindividual variability in drug response. Pharmacogenetic analysis provides insight into the molecular mechanisms involved in drug response, with the ultimate goal of achieving optimal drug efficacy and safety. Numerous polymorphisms have been described in genes encoding drug-metabolising enzymes, transporters, and receptors. For some drugs, the impact on drug bioavailability and effect has been elucidated. We review here the molecular basis of interindividual variation in drug response and the methods used to identify individual risk of drug failure or toxicity. Clinical applications, concerning enzymes metabolising drugs (cytochrome P4502D6, thiopurine S-methyltransferase and N-acetyltransferase) provide an illustrative demonstration of the usefulness of pharmacogenetic tests in improving patient management. Clinical validation of these tests and new technologies (real-time PCR, DNA chips) should, in the future promote pharmacogenetics in clinical practice and may be lead to more individualized drug therapy.     

Review article: interactions between genotype and response to therapy in inflammatory bowel diseases

BACKGROUND: More than half of the patients with inflammatory bowel diseases are candidates for immunosuppressive therapy. However, even the most effective drugs used in inflammatory bowel disease are only successful in about two-thirds of patients. Adverse events limit their use in a further substantial proportion of patients. Recent research has focussed on the possibility of predicting a drugs' efficacy and/or toxicity by identifying polymorphic variants in the genes encoding enzymes involved in metabolic pathways. AIM: To highlight recent advances and limitations in the field of pharmacogenetics in inflammatory bowel disease. RESULTS: Recent pharmacogenetic studies have mainly focussed on immunosuppressive agents including corticosteroids, azathioprine, methotrexate and infliximab. Several polymorphic genes encoding enzymes involved in the metabolism of these drugs have been identified including the inosine triphosphate pyrophosphatase in thiopurine therapy, the methylene tetrahydrofolate reductase in methotrexate therapy and polymorphisms in apoptosis genes in infliximab therapy. However, at the present time, genotyping for the variants of the thiopurine methyltransferase gene, an enzyme important for the metabolism of the thiopurine drugs, is the only useful test in clinical practice. CONCLUSIONS: Although the field of pharmacogenetics in inflammatory bowel disease is promising most new targets have so far failed to translate into clinical practice. Future pharmaceutical trials should include pharmacogenetic research to test appropriate candidate genes in a prospective manner.