Q J Med 2000; 93: 391-423
© 2000 Association of Physicians
Reviews |
Impact of genomics on drug discovery and clinical medicine
1 From the Laboratory of Pharmacology, 2 Diabetes and Nutrition Unit, and 5 Department of Neurology, Université Catholique de Louvain, Brussels, Belgium, 3 Department of Oncology, University of Cambridge, Addenbrooke's Hospital, Cambridge, UK, and 4Neuroregeneration Laboratories, Harvard Medical School, McLean & Massachusetts General Hospital, Belmont MA, USA
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Summary |
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 Top Summary IntroductionGenomicsPharmacogenomicsImpact on drug discovery...Relevance to clinical medicineGenetic variations in...Gene therapyFirst genomic products in...The impact of genetic...NeuropsychiatryMigraineNeurological channelopathiesHuntington's diseaseAlzheimer's diseaseAmyotrophic lateral sclerosisParkinson's diseaseCardiologyCardiovascular risk factorsEssential hypertensionDiabetesObesityOncologyEthical issuesDiscussion and conclusionsReferences |
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Genomics, particularly high-throughput sequencing and characterization of expressed human genes, has created new opportunities for drug discovery. Knowledge of all the human genes and their functions may allow effective preventive measures, and change drug research strategy and drug discovery development processes. Pharmacogenomics is the application of genomic technologies such as gene sequencing, statistical genetics, and gene expression analysis to drugs in clinical development and on the market. It applies the large-scale systematic approaches of genomics to speed the discovery of drug response markers, whether they act at the level of the drug target, drug metabolism, or disease pathways. The potential implication of genomics and pharmacogenomics in clinical research and clinical medicine is that disease could be treated according to genetic and specific individual markers, selecting medications and dosages that are optimized for individual patients. The possibility of defining patient populations genetically may improve outcomes by predicting individual responses to drugs, and could improve safety and efficacy in therapeutic areas such as neuropsychiatry, cardiovascular medicine, endocrinology (diabetes and obesity) and oncology. Ethical questions need to be addressed and guidelines established for the use of genomics in clinical research and clinical medicine. Significant achievements are possible with an interdisciplinary approach that includes genetic, technological and therapeutic measures.
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Introduction |
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TopSummaryIntroductionGenomicsPharmacogenomicsImpact on drug discovery...Relevance to clinical medicineGenetic variations in...Gene therapyFirst genomic products in...The impact of genetic...NeuropsychiatryMigraineNeurological channelopathiesHuntington's diseaseAlzheimer's diseaseAmyotrophic lateral sclerosisParkinson's diseaseCardiologyCardiovascular risk factorsEssential hypertensionDiabetesObesityOncologyEthical issuesDiscussion and conclusionsReferences |
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Genome research centers worldwide are engaged in the Human Genome Project (HGP) with the ultimate goal of elucidating and characterizing the complete sequence of the 3x109 base pairs (bp) arranged in about 85 000 genes of the human genome. An even greater task is to determine their function and interplay. The genomic approach to mapping and sequencing the genome project has accelerated the rate of gene discovery. In 1990, 1772 human genes were identified and mapped to a specific chromosome or region of the genome. In September of 1996, this number was 3868 genes—a more than two-fold increase. As of June 1996, 62 human genes linked to human diseases had been isolated by genomic technologies and, of these, 51 (82%) were available in the public domain as clones or as DNA sequences. Moreover, biomedical research is rapidly defining the molecular mechanisms of pharmacological effects, genetic determinants of disease pathogenesis, and functionally important polymorphisms in genes that govern drug metabolism and disposition. A radical new, but complementary, approach to drug development is now emerging which promises dramatic improvements in the efficiency and speed of drug development. This approach uses the emerging technological expertise from pharmacogenetics, pharmacogenomics and functional genomics to dissect, predict and monitor the nature of the individual response to medications. Ultimately, this may lead to smaller and faster clinical studies and to individually tailored pharmacological treatments, in which patients are screened to identify which therapeutic option most suits their genetic and physiological makeup and accurately monitored for their response. This approach is likely to have radical consequences in the planning, conduct of clinical trials and medical treatment of diseases.
One important outgrowth of molecular medicine is the development of technologies for the transfer of therapeutic genes to cells in culture and tissues in vivo, with potential applications both to medical research and the practice of clinical medicine. The use of genomic databases to find new targets for drug discovery and the rapid accumulation of human gene sequences is promising for clinical medicine, if the molecular level can be translated into improved interventions. If it can, therapeutic agents with specific molecular functions can be produced, be they gene products which are deficient or abnormal in the patients, or drugs with direct transcriptional or molecular effects. Individual genetic testing, with knowledge of disease genes, will help early diagnosis and early treatment. For example, recent advances in the genetics of complex traits (for example, diabetes, coronary heart disease and Alzheimer's disease) have to some extent reshaped disease phenotypic descriptions. The techniques developed for automated sequencing and analysis of DNA may eventually allow inexpensive screening of multiple loci for polymorphisms.
Molecular genetic techniques may also translate into gene therapy. The ability to clone and manipulate genes responsible for human disease and to re-introduce functional copies of normal genes into living cells and tissues is one such therapeutic objective. The potential clinical applications of gene therapy are numerous, and a number of specific human genetic or environmentally-induced diseases that result from a lesion in a single gene have been proposed as candidates for gene therapy (Table 1
). For some of these diseases, the introduction of a functional homologue of the defective gene and the production of even small amounts of the missing gene product would have a beneficial effect; for example 10–20% production of the normal levels of factor IX can alleviate severe haemophilia B.1 At the same time, overexpression of the gene product would not be expected to have deleterious effects. Thus, these genetic disorders are candidates for gene therapy because the expression of the transduced gene need not be strictly regulated. In contrast, it is not always necessary to correct the genetic lesion in the cell type that shows the defect. In such cases, a therapeutic gene may be introduced into another cell type so that the genetically modified cells functionally replace the defective cell type.
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This paper discusses the impact of genomic science on drug discovery and clinical medicine and provides examples of treatment interventions in neuropsychiatry (migraine, neurological channelopathies and neurodegenerative disorders), cardiovascular medicine, endocrinology (diabetes and obesity) and oncology.
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Genomics |
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TopSummaryIntroductionGenomicsPharmacogenomicsImpact on drug discovery...Relevance to clinical medicineGenetic variations in...Gene therapyFirst genomic products in...The impact of genetic...NeuropsychiatryMigraineNeurological channelopathiesHuntington's diseaseAlzheimer's diseaseAmyotrophic lateral sclerosisParkinson's diseaseCardiologyCardiovascular risk factorsEssential hypertensionDiabetesObesityOncologyEthical issuesDiscussion and conclusionsReferences |
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Molecular genetics reached human genetics about 1976, when the first human genes were cloned.2 Transgenic methods, ‘knock-outs’ and ‘knock-ins’ began in about 1986, and in about 1996, database searching became a fruitful way to do genomic research.3 The term ‘genome’ refers to an organism's complete set of genes and chromosomes. The term ‘genomics’ describes the scientific discipline of mapping, sequencing, and analysing genomes.4
Genome analysis may be divided into structural and functional genomics. Structural genomics is an initial phase of genome analysis, and has a clear endpoint which is the construction of high-resolution genetic, physical, and transcript maps of an organism (its complete DNA sequence). This genotypic approach focuses on understanding how genotypic variation gives rise to phenotypic variation, relying on physical and genetic maps and easily-typed DNA sequence polymorphisms. The expression approach (functional genomics) relies on the large collection of partially-sequenced cDNA clones. The benefits of the information arising from the accumulation of human gene sequences includes developing systematic ways of finding genes of interest, and their functions; hence ‘functional genomics’. The genes cloned and their corresponding DNA sequences provide the tools for comprehensive characterization of the expression patterns of this entire set of genes, and for systematic experimental investigations of the functional properties of their products. Thus, functional genomics, which represents a new phase of genome analysis, makes use of the structural genomics information. The investigation is primarily a systematic approach to elucidate the genome and its functions.
The fact that most diseases do not follow a simple inheritance patterns has led to a significant challenge in the genetic dissection of the complex traits of diseases such hypertension, Alzheimer's disease, schizophrenia and diabetes.5 Four major approaches have been developed: linkage analysis, allele-sharing methods, association studies in human populations, and polygenic analysis of experimental crosses in model organisms such as mouse and rat. The gold-standard tests for human genes should include association studies demonstrating a clear correlation between functionally relevant allelic variations and the risk of disease in humans, and transgenic studies demonstrating that gene addition or gene knockout in animals produces a phenotypic effect. If these genetic approaches are successful, they may have significant relevance on drug research and clinical medicine.
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Pharmacogenomics |
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TopSummaryIntroductionGenomicsPharmacogenomicsImpact on drug discovery...Relevance to clinical medicineGenetic variations in...Gene therapyFirst genomic products in...The impact of genetic...NeuropsychiatryMigraineNeurological channelopathiesHuntington's diseaseAlzheimer's diseaseAmyotrophic lateral sclerosisParkinson's diseaseCardiologyCardiovascular risk factorsEssential hypertensionDiabetesObesityOncologyEthical issuesDiscussion and conclusionsReferences |
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Pharmacogenomics has its roots in pharmacogenetics. Whereas pharmacogenetics is the study of the linkage between an individual's genotype and that individual's ability to metabolize a foreign compound, pharmacogenomics is quite broad in scope, and is similar to molecular medicine, aiming to detect, monitor and treat the molecular causes of disease. Pharmacogenomics involves the application of genomics technologies such as gene sequencing, statistical genetics and gene expression analysis to drugs in clinical development and trials. Since many diseases develop as a result of a network of genes failing to perform correctly, pharmacogenomics can identify the genes or loci which are involved in determining the responsiveness to a given drug. In this way, genetic characterization of patient populations is becoming an integral part of the drug discovery and development process. Pharmacogenomics may aim to capitalize on these new molecular insights to discover new therapeutic targets and interventions and to elucidate the constellation of genes that determine the efficacy and toxicity of specific medications.
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Impact on drug discovery and clinical research |
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TopSummaryIntroductionGenomicsPharmacogenomicsImpact on drug discovery...Relevance to clinical medicineGenetic variations in...Gene therapyFirst genomic products in...The impact of genetic...NeuropsychiatryMigraineNeurological channelopathiesHuntington's diseaseAlzheimer's diseaseAmyotrophic lateral sclerosisParkinson's diseaseCardiologyCardiovascular risk factorsEssential hypertensionDiabetesObesityOncologyEthical issuesDiscussion and conclusionsReferences |
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Applying pharmacogenomics in the preclinical setting, one may start screening compounds with the least variation across individuals. If the target gene is selected, the compound that works best overall against all its subtypes may be chosen. Thus, drug selection is substituted for patient selection, decreasing the uncertainties that patient stratification introduces at the FDA and in marketing, as well as the need for a genetic screening. Genomics may also be used to select out adverse effects before drugs enter the clinic. For example, the gene-expression pattern for the liver of an animal administered a drug can indicate whether gene pathways related to toxicity have been turned on. Variations in gene expression levels may prove just as useful as genetic variation in predicting drug response at any stage in the clinic and as a diagnostic. Pharmacogenetic data are vital during the development of a compound with a narrow therapeutic index or which is metabolized from a prodrug, as such information may influence decision of whether to discontinue development or design trials to study clinical responses in individual polymorphic for the relevant enzyme.
Significant issues at the preclinical level usually need to be addressed. Problems of medicinal chemistry, developing drugs with the appropriate absorption, metabolism, distribution, and elimination profiles still have an empirical basis. Nonetheless, small molecule drugs directed toward targets discovered by genomics may soon account for a great majority of drugs introduced into the marketplace.
Pharmacogenomics may benefit many stages of clinical drug development. It will significantly affect trial design, primarily through improved inclusion/exclusion criteria and more effective assessment of patient responses. Genes linked with drug metabolism in preclinical studies could be genotyped in patients recruited for phase I trials. Any genotype that correlates with adverse effects could then be used to screen out relevant patients in subsequent trials. Furthermore, if efficacy data are collected during phase I trials, polymorphisms in the drug target gene could be typed in phase I participants to assess whether they are linked with side-effects or with variations in drug response. That analysis could obviously be further refined in phase II trials, enabling companies to undertake phase III trials in a subgroup of patients that responds well and exhibits fewer side-effects. The resultant drugs would be expected not only to have better efficacy, but also a better safety profile.
At the clinical level, while the disease symptoms might appear to be uniform, individual-to-individual variations in these polygenic networks may make drugs healing for certain individuals while toxic for others. Pharmacogenomics can sometimes correlate gene variations with differential responses to the same drug leads, thereby hoping to accelerate novel drug discovery dramatically, by defining specific populations that will benefit from a drug. While this approach may maximize the medical utility of existing pharmaceuticals, it could also rescue dead drugs. Several products that have failed in recent years in late stage clinical trials may on retrospective analysis be effective in subsets of patients, although at the time, there was no clear way of recognizing such subsets clinically. A study of the genetic differences between the individuals could provide an answer. Consequently, traditional approaches that focus on broad groups of patients with a diagnosis (e.g. Alzheimer's disease) may need to be much more precisely divided into subsets of patients who may have a traditionally defined disease amenable to treatment based on a particular molecular target. These pharmacogenomic developments should lead to smaller, more rapid and cost-effective trials, and ultimately to more individually focused and effective therapeutics.
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Relevance to clinical medicine |
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TopSummaryIntroductionGenomicsPharmacogenomicsImpact on drug discovery...Relevance to clinical medicineGenetic variations in...Gene therapyFirst genomic products in...The impact of genetic...NeuropsychiatryMigraineNeurological channelopathiesHuntington's diseaseAlzheimer's diseaseAmyotrophic lateral sclerosisParkinson's diseaseCardiologyCardiovascular risk factorsEssential hypertensionDiabetesObesityOncologyEthical issuesDiscussion and conclusionsReferences |
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In summary, patient segmentation and individual profiling will become increasingly important and pharmacogenomics analysis may serve to customize the use of pharmaceuticals for specific subgroups of patients. Over the past 20 years, genetic heterogeneity has been increasingly recognized as a significant source of variation in drug response. Some drugs work better in some patients than in others, and some drugs may even be highly toxic to certain patients. Pharmacogenomics can be used not only to predict drug efficacy in a particular patient, but also the likelihood of side-effects, for example those due to differences in drug metabolism. This information could be used to rescue failed or failing drugs, by repositioning them for a defined patient population. Pharmacogenomics is about spotting correlations between such responses to drugs and the genetic profiles of patients.6 It generates data that are relevant to a drug's clinical performance. Ultimately, knowledge of the genetic basis for the drug disposition and response should make it possible to select many medications and their dosages on the basis of each patient's inherited ability to metabolize, eliminate and respond to specific drugs. Therefore, for clinical management in general, genetic subtyping of the patient response, whether due to different disease subtypes or differential drug effects, should make it possible gradually to replace the current trial-and-error-based selection of the appropriate pharmacological intervention with a more informed and rational strategy. This will represent an important advance, particularly in diseases where it currently may require months or years of treatment to observe whether a positive response has occurred.
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Genetic variations in pharmacokinetic and pharmacodynamic effects |
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TopSummaryIntroductionGenomicsPharmacogenomicsImpact on drug discovery...Relevance to clinical medicineGenetic variations in...Gene therapyFirst genomic products in...The impact of genetic...NeuropsychiatryMigraineNeurological channelopathiesHuntington's diseaseAlzheimer's diseaseAmyotrophic lateral sclerosisParkinson's diseaseCardiologyCardiovascular risk factorsEssential hypertensionDiabetesObesityOncologyEthical issuesDiscussion and conclusionsReferences |
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Numerous factors, including genetics, affect drug metabolism and thus alter the bioavailability of therapeutic drugs. The best studied metabolizing enzymes are the cytochrome P450 (CYP450) isoenzymes, the N-acetyl transferase (NAT) isoenzymes, the UDP-glucuronosyl transferases, and the methyl transferases. Of these enzymes, the CYP450s are very important because they metabolize drugs into products that are readily excreted into the urine and faeces. In humans, six different forms of CYP450 (CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1 and CYP3A4) are largely responsible for eliminating drugs. The rate of metabolism by several of the cytochrome CYP450 enzyme subfamilies varies, due to genetically-determined polymorphisms in all populations studied. Recent research using phenotyping and genotyping techniques has reflected the interest and importance of these pharmacogenetic factors in determining drug responses. Some of the metabolizing enzymes such as CYP1A1, 1A2, 2A6, 2C9, 2C19, 2D6, 2E1, NAT1, NAT2 and NQO1 exhibit genetic polymorphism and alter responses to drugs (see Table 2
). These metabolic polymorphisms are determined by gender (e.g. CYP1A2) and racial/ethnic origin.7–8 Increased CYP1A activity (an enzyme catalysing a phase I oxidation reaction), coupled with slow acetylation (a phase II conjugation reaction), resulted in less myelosuppression from the active metabolites of the drug amonafide.9 Because every individual represents a combination of drug-metabolizer phenotypes, given the large number of enzymes involved in drug metabolism, it is apparent that some individuals are likely to have unusual reactions to drugs, or to combination of drugs, due to the coincident occurrence of multiple genetic defects in drug-metabolizing enzymes. Such an alignment of genotypes, particularly when coupled with polymorphisms in drug receptors, is likely to constitute part of the mechanism for the so-called ‘idiosyncratic’ drug reactions.
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Although no evidence to date suggests the CYP3A4 isoenzyme exhibits genetic polymorphism, in recent years there has been much discussion about the 3A4 system because of life-threatening arrhythmic side-effects that can occur as result of enzyme inhibition and accumulation of the antihistamines terfenadine, astemizole and cisapride.10–12 Terfenadine has been removed from the market because of its serious cardiovascular drug interactions. Concerning CYP2C9, recent data suggest that patients who require low doses of warfarin (
1.5 mg/day) carry point mutations (alleles CYP2C9*2 and CYP2C9*3) at the gene coding for CYP2C9 (which could occur at a frequency of 21% in the general population). These patients metabolized warfarin poorly, and responded to small doses of the drug with greater lengthening of the prothrombin time and higher international normalized ratio (INR) values than did carriers of the wild-type allele CYP2C9*1.13 Genetically determined high-responders to warfarin had bleeding complications four times more commonly than did a control group stabilized on larger doses of the drug. Knowledge of carriage of the hyper-responsiveness alleles of CYP2C9*2 and CYP2C9*3 might help the clinician to decide against the use of warfarin (in favour of other coumarin derivatives such as phenoprocoumon and acenocoumarol, the metabolism of which is less influenced by CYP2C9), particularly in high-risk elderly patients. In addition to variation in drug metabolism or pharmacokinetics, the genetic variations in receptor function (and thereby pharmacodynamic effects) are important. Subtle differences in the sequences of receptor subtypes for dopamine, serotonin and catecholamines may result in individual differences in behavior and drug responses.6 Overall, a highly complex picture emerges in which genetic variation in both pharmacodynamic and pharmacokinetic factors contributes to drug responses. Some patients do not respond to a given drug because it is not processed efficiently; other patients do not respond because the disease gene defects or its pathway is not targeted by the drug.
Great progress has been made in understanding the molecular genetics of acetylation as well as the clinical consequences of being a rapid or slow acetylator. Inborn errors (several different alleles) at the hepatic arylamine N-acetyltransferase-2 (NAT2) locus are responsible for the traditional acetylator polymorphism.14 Rapid and slow acetylators reflect the genetically determined variation in the elimination of xenobiotics, as well as in NAT2 activity in the liver and other tissues.15 The human NAT2 gene contains an 870 bp intronless protein-coding region.16 To date, one allele with a code for fast acetylation (wild-type) and several mutated alleles with codes for impaired acetylation activity have been discovered.17–18 Of all the NAT2 allelic variants that had been identified, three (NAT*5, NAT*6 and NAT*7) account for majority of the slow NAT2 acetylator genotype in White subjects.18 N-acetylation status seems to be associated with several kinds of diseases, such as colon cancer, rheumatoid arthritis, and systemic lupus erythematosus.19–21 The independent genetic feature as a rate of acetylation was shown to be related to the immunological system dysfunction. It may be one of the factors that makes an individual susceptible to the development of an atopic disease, and one study showed that up to 80% of individuals with chronic allergic rhinitis had a slow acetylation phenotype.22 A recent study which assessed the influence of NAT2 polymorphism on the risk of development of atopic disease also suggests that the risk of development of atopic diseases was five-fold greater for homozygous slow acetylators compared to healthy subjects, and that slow acetylation genotype may be an important factor of individual susceptibility to atopic diseases.23 This group of patients may also be at increased risk of adverse reactions after using drugs which are mainly metabolized by acetylation reaction. Among them, the mechanism of hypersensitivity to sulfonamides typical for slow acetylators seems to be of particular importance.
Studies have revealed variant alleles at the NAT1 locus as well. The consequences of pharmacogenetic variation in these enzymes include altered kinetics of specific drug substrates, drug–drug interactions resulting from altered kinetics, and idiosyncratic adverse drug reactions. The latter have been extensively investigated for the arylamine-containing sulfonamide antimicrobial drugs. Individual differences in multiple metabolic pathways can increase the likelihood of covalent binding of reactive metabolites of the drugs to cell macromolecules with resultant cytotoxicity and immune response to neoantigens. This can result clinically in an idiosyncratic hypersensitivity reaction manifested by fever, skin rash and variable toxicity to organs including liver, bone marrow, kidney, lung, heart and thyroid.
Consideration of the genetic characteristics leads to population segmentation into groups, the slow metabolizers (having a slow metabolism) and fast metabolizers (having a normal metabolism). For example, in some Asian populations the incidence of poor metabolizers of the gastrointestinal drug omeprazole (due to polymorphism in CYP2C19) is 15–23%, compared to 2.5–6% in Caucasians.24 In individuals with a poor-metabolizer genotype for CYP2C19, the therapeutic efficacy of omeprazole (a proton-pump inhibitor widely used as acid inhibitory agent for the treatment of upper gastrointestinal diseases and metabolized by CYP2C19) may be increased.25 In patients with a poor-metabolizer phenotype or genotype of CYP2C19, the area under the plasma concentration-time curve of omeprazole is markedly increased, and the clinical effect of omeprazole is greater. Acid secretion in patients with a poor metabolizer status of CYP2C19 who are undergoing an omeprazole therapy is therefore assumed to be more strongly inhibited than those with the extensive metabolizer status. Cure rates for Helicobacter pylori were noted to be 28.6%, 60% and 100% in the rapid-, intermediate-, and poor-metabolizer groups, respectively. The results of the genotyping test for CYP2C19 seem to predict the cure of Helicobacter pylori infection and peptic ulcer in patients who receive dual therapy with omeprazole and amoxicillin. A recent study designed to determine whether the effects of omeprazole on intragastric pH depends on CYP2C19 genotype status confirmed that after omeprazole administration, significant differences in mean intragastric pH values and plasma levels of gastrin, omeprazole and its metabolites were observed among the three groups of volunteers (homozygous extensive metabolizers, heterozygous extensive metabolizers and poor metabolizers), whereas no significant differences in these parameters were observed with the placebo administration.26 Both the individual omeprazole AUC and mean intragastric pH values were greater in the poor metabolizer group compared with those in the homozygous extensive metabolizer and heterozygous extensive metabolizer groups. The results confirmed that the effects of omeprazole on intragastric pH significantly depends on CYP2C19 genotype status, and also suggest that the genotyping test of CYP2C19 may be useful for an optimal prescription of omeprazole.
Low metabolic activity of the CYP2D6 enzymes is inherited as an autosomal recessive gene and although CYP2D6 represents only about 1.5% of the total liver enzymes, it is involved in the metabolism of a number of commonly used drugs.7 There are now more than 20 identified variant CYP2D6 alleles which contribute to the variation in CYP2D6 metabolism. The most common allelic variations associated with poor-metabolizers in Caucasians are CYP2D6*4 (75%), *3 (5%) and the gene deletion *5 (15%).27 For drugs in which CYP2D6 plays a predominant role in metabolism, poor-metabolizers will have high plasma concentrations and report the most severe adverse reactions.28 Studies in Caucasian extensive-metabolizers and poor-metabolizers have uniformly demonstrated a 2- to 5-fold difference in the capacity to metabolize CYP2D6 substrates, such as antidepressants and neuroleptics.29 On the other hand, non-Westerners (Asians and Indians) may require lower doses of several classes of psychotropics that are metabolized by CYP2D6 (e.g. conventional neuroleptics and tricyclic antidepressants) than do Westerners.30 The poor-metabolizers lack this enzyme as a result of an autosomal recessively transmitted defect in its expression. When drugs are converted to an active metabolite by 2D6 (e.g. conversion of codeine to morphine), the drug may be ineffective in poor-metabolizers. Although significant interactions between 2D6-metabolized drugs with the well-known inducers rifampin and antiepileptics have been described, specific inducers of 2D6 have yet to be clearly identified. Administration of dextromethorphan followed by measurement of O-demethylated metabolite excretion in urine is an accurate and non-invasive way of phenotyping individuals as either extensive-metabolizers or poor-metabolizers for 2D6 activity.
Many opioid analgesics are activated by CYP2D6, rendering the 2–10% of the population who are homozygous for non-functional CYP2D6 mutant alleles relatively resistant to opioid analgesic effects.31 It is thus not surprising that there is remarkable interindividual variability in the adequacy of pain relief when uniform doses of codeine are used.
Nothing is known about any particular advantage or disadvantage of any CYP2D6 variant. Because CYP2D6 occurs not only in the liver but also in the brain, it might affect some personality traits: this might affect fitness and thereby frequency of the variants.32–33
Thiopurine methyltransferase (TPMT) is a cytosolic enzyme that catalyses the S-methylation of aromatic and heterocyclic sulfhydryl compounds, including the thiopurine drugs 6-mercaptopurine (6-MP) and 6-thioguanine.34 Thiopurines are used to treat patients with neoplasia and autoimmune disease as well as recipients of transplanted organs. The TPMT genetic polymorphism may represent a striking example of the potential clinical importance of pharmacogenetic variation in expression of a drug-metabolizing enzyme.35 Individuals with genetically very low levels of TPMT activity are at a greatly increased risk for potentially life-threatening toxicity when exposed to standard doses of thiopurines, while those with very high levels of this enzyme activity may be undertreated with the same dosages of these drugs.36,37 Recent genetic data suggest that the active gene for the TPMT enzyme is ~34 kb in length, consists of 10 exons and has been localized to chromosome band 6p22.3.38 The wild-type allele for high TPMT activity has been designated TPMT*1, and to date eight variants for very low TPMT activity have been reported.38,39 The most common of these in Caucasians, TPMT*3A, represents 55–70% of all variant alleles for very low activity.39 TPMT*3A contains two point mutations, G460
A and A719G, resulting in Ala154Thr and Tyr240Cys amino acid substitutions, respectively.38 However, because of the clinical significance of inherited variation in levels of TPMT activity, characterization of as many variant alleles responsible for very low TPMT activity as possible will be necessary so that DNA-based diagnostic tests can be compared with the phenotypic test presently used to individualize therapy with thiopurine drugs. The ultimate aim is to minimize toxicity and improve the therapeutic efficacy of this important class of pharmacotherapeutic treatments.
Besides helping delineate such biological differences, genetic markers could also be used in selecting patients for clinical trials either for screening out individuals with genotypes that react adversely or for selecting patients who are more likely to respond well. The advent of DNA chip technology presents the opportunity not only to rapidly genotype individuals to provide information on polymorphic drug metabolism genes, but also to identify genes differentially expressed in response to a drug. Affymetrix is already marketing a CYP2C6/CYP2C19 genechip for identifying potential poor drug metabolizers. The recent development of a simple mouthwash method for obtaining genomic DNA clinical studies appears promising.40 Using this cheap and simple-to-perform approach, subjects were successfully genotyped by PCR-based assays for polymorphisms in the CYP1A1, CYP2E1 and NQO1 genes, confirming that the quality of DNA isolated from mouthwash samples was sufficient to support PCR amplification reliably. This mouthwash procedure may be suitable for large community-based studies of genetic susceptibility to disease in which samples can be collected by the participants themselves. However, there should be agreed standards for the reproducibility and robustness of such systems.
The pharmacodynamics of drug action may be subject to genetic variation with respect to the sensitivity of the drug's target to its action (e.g. due to subtle conformational variations at the drug's binding site). Most drugs interact with specific target proteins to exert their pharmacological effects, such as receptors, enzymes, or proteins involved in signal transduction, cell cycle control, or many other cellular events. Molecular studies have shown that many of the genes encoding these drug targets exhibit genetic polymorphism, which in many cases alters their sensitivity to specific pharmacological treatments. Such examples include polymorphisms in ß-adrenergic receptors and their sensitivity to ß-agonists in asthmatics, sulfonylurea receptor and responsiveness to sulfonylurea hypoglycaemic agents, and 5-hydroxytryptamine receptor and response to neuroleptics such as clozapine.41,42 In addition, genetic polymorphisms that underlie disease pathogenesis can also be major determinants of drug efficacy, such as mutations in the apolipoprotein E (ApoE) gene and responsiveness of patients with AD to tacrine therapy.43 Finally, the risk of adverse drug effects has been linked to genetic polymorphism that predispose to toxicity, such as dopamine D3 receptor polymorphism and the risk of drug-induced tardive dyskinesia, potassium channel mutations and drug-induced dysrhythmias and polymorphism in the ryanodine receptor and anesthesia-induced malignant hyperthermia.44
Bronchodilator responsiveness to ß2-adrenergic receptor agonists in patients with asthma varies considerably and several missense mutations in the coding region of the ß2-adrenergic receptor gene have been identified.45,46 Among the general population (including patients with asthma), ß2-adrenergic receptor alleles are distributed in the following approximate proportions: homozygous Arg (Arg16/Arg16), 15%; heterozygous (Arg16/Gly16) 38%; homozygous Gly 16 (Gly 16/Gly 16), 45%; homozygous Gln27 (Gln27/Gln27), 26%; heterozygous (Gln27/Glu27), 49%; and homozygous Glu27 (Glu27/Glu27), 22%.45–48 The Gly6 allele has been associated with enhanced agonist-promoted ß2-receptor down-regulation, whereas the Glu27 allele showed minimal down-regulation compared with the Arg16 and Gln27 alleles.49,50 Although asthma is primarily an inflammatory disease of the airways, mutations in the ß2-adrenergic receptor may be risk factors in certain asthma phenotypes.45 They may also be factors in determining responsiveness to ß2-agonists.46 In a study of 269 children with asthma, a glycine/arginine polymorphism at amino acid 16 was noted to be associated with a difference in responsiveness to albuterol.51 Individuals homozygous for the arginine variant were over five times more likely to respond to albuterol than individuals homozygous for the glycine variant. In another recent study determined to assess whether genetic polymorphisms of the ß2-adrenergic receptor gene affect the relationship between albuterol, plasma concentrations and the forced expiratory volume in 1 s (FEV1) in patients with moderate asthma, it was reported that the albuterol-evoked FEV1 was higher and the response was more rapid in Arg16 homozygotes compared with the cohort of carriers of the Gly16 variant (maximal percentage increase in FEV1 was 18% vs. 4.9%, p<0.03).41 The results of this study confirm that ß2-adrenergic receptor gene polymorphism is a major determinant of bronchodilator response to albuterol and that future pharmacodynamic studies of ß2-agonists should include determination of ß2-adrenergic receptor genotype.
The variation in cytochrome drug-metabolizing genes that correlates with patients' adverse response or non-response in clinical trials need to be considered. This information could be used to stratify clinical trials, leading to higher efficacy and limiting adverse reactions. Ultimately, detailed information about each patient's genetic variants relevant to drug treatments might eliminate the use of ineffective or even dangerous treatments. Prognosis of patients will be more informed, because more precise information on the aetiology of the illness, its pathophysiology and the effectiveness of therapeutic interventions will be available. Thus, the incorporation of pharmacogenetic information into trials as early as possible is recommended and appears very useful for effective drug development.
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Gene therapy |
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TopSummaryIntroductionGenomicsPharmacogenomicsImpact on drug discovery...Relevance to clinical medicineGenetic variations in...Gene therapyFirst genomic products in...The impact of genetic...NeuropsychiatryMigraineNeurological channelopathiesHuntington's diseaseAlzheimer's diseaseAmyotrophic lateral sclerosisParkinson's diseaseCardiologyCardiovascular risk factorsEssential hypertensionDiabetesObesityOncologyEthical issuesDiscussion and conclusionsReferences |
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Gene therapy in the broadest sense is the introduction of foreign genetic material into a cell with therapeutic intent. For deficiency states such as haemophilia, the aim is to add a normal gene to that cell to complement an abnormal counterpart. It has been estimated that 1 : 100 children have a serious genetic defect, and the possibility now exists of introducing engineered genes, with the protein product supplementing the defective gene. Ideally, the treatment should include replacing the defective gene with a function gene for a complete elimination of the disease-provoking gene.
The primary problem in gene therapy is the method of gene delivery. Successful gene therapy depends on the availability of reliable methods for delivering a gene into the nuclei of selected target cells and subsequently ensuring the regulation of gene expression. Genes can be delivered (transfected) into cells by various systems.52 These fall into two main categories: gene delivery using recombinant viral vectors, and physical gene delivery. Somatic transfer of single genes is being attempted at the present time for terminal diseases such as cystic fibrosis and Duchenne muscular dystrophy, for which there is no effective treatment.53,54 Understanding polygenic diseases represents a more difficult challenge, and another problem is presented by diseases such as diabetes mellitus in which feedback control of gene expression is important. The problem with some of the delivery systems is the size of the DNA that can be introduced. In the course of a typical infection, viruses insert their genetic material into cells of the victim, where this added genetic code directs the synthesis of various molecules needed to make new viral particles. Although natural viruses can be destructive, it is possible to tame and convert some of them so that they can carry a therapeutic gene and quietly deposit it inside a cell without causing damage. All ‘gene therapy’ that is reported in the literature is gene addition rather than replacement. Gene replacement is straightforward in yeast and is feasible in mammalian cell lines, but remains a long way from effective clinical applications.
Two general strategies have been developed for gene therapy: the in vivo approach and the ex vivo approach. These two approaches each have potential advantages and disadvantages which render them appropriate under different conditions. The in vivo approach is conceptually and technically more direct, involving the introduction of a gene directly into the tissues of an affected individual. In principle, it does not depend on the success of cell culture or subsequent survival of transplanted cells. The ex vivo approach, on the other hand, is technically more demanding. First, a suitable cell type is harvested from a donor and grown in tissue culture. Since mature neurons and glia are notoriously difficult to grow and genetically manipulate in culture, alternative cell types such as fibroblasts and myoblasts have been used.55 Next the gene is introduced into the cells in vivo and cells expressing the transgene are amplified. The genetically altered cells are then harvested and reimplanted into an affected host. This approach is labor-intensive and time-consuming, and it requires the growth of suitable cells in vivo and their subsequent survival after implantation. However, one advantage of the ex vivo approach is that it does not require a highly efficient method for gene transfer, because genetically altered cells may be amplified in vivo prior to implantation.
Gene therapy is today a robust scientific discipline with several new reagents which are being released for specific clinical applications. Transgenic technology/transgenic animal models are continuously helping to set the stage for somatic gene therapy in humans. Encouraging results have been reported from long-term animal studies of gene transfer and expression by direct injection of vector into brain, muscle and liver.56,57 These data have led to an increased interest in adeno-associated virus and expanded its use in human gene therapy trials. To date, clinical gene therapy has been attempted in only two central nervous system disorders, namely brain tumours and Canavan disease (an autosomal recessive leukodystrophy associated with spongiform degeneration of the brain and is characterized by mutations in the aspartoacylase gene, resulting in loss of enzyme activity).58,59 Most early phase I clinical studies that have been performed were trials not designed to demonstrate efficacy at all, but instead to assess the safety of transferring cloned genes into humans. The results suggest that these approaches were safe; remarkably little morbidity and no deaths have been noted. Therefore with the advance of the engineering of new vectors including the adenoviruses, adeno-associated viruses and lentiviruses which promise to greatly enhance the efficiency of in vivo gene delivery and to simultaneously reduce the immunogenicity of both vectors and transgenes, prospects for the clinical application of gene therapy appear good. Obviously, the cloning and sequencing of large numbers of new human genes and a better understanding of the genetic bases of human diseases have greatly increased the scope of diseases that may be amenable to treatment by gene therapy.
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First genomic products in clinical trials |
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TopSummaryIntroductionGenomicsPharmacogenomicsImpact on drug discovery...Relevance to clinical medicineGenetic variations in...Gene therapyFirst genomic products in...The impact of genetic...NeuropsychiatryMigraineNeurological channelopathiesHuntington's diseaseAlzheimer's diseaseAmyotrophic lateral sclerosisParkinson's diseaseCardiologyCardiovascular risk factorsEssential hypertensionDiabetesObesityOncologyEthical issuesDiscussion and conclusionsReferences |
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The number of clinical trials involving human gene therapy has dramatically increased since the initiation of the first approved trial in the US to treat adenosine deaminase deficiency (ADA, a hereditary deficiency of an enzyme essential to the immune system) in 1990.60 Since then, >2100 patients have been enrolled in trials worldwide, with >1700 in the US.61 The majority of active trials involve gene therapy for malignancy (68%), AIDS (18%) and cystic fibrosis (8%).61 The key vectors used remain retroviruses (56%), but this high percentage is decreasing.
On 18 December 1997, Human Genome Sciences (HGS) announced the filing of its first investigational new drug (IND) application and was preparing to begin clinical testing of a chemokine called myeloid progenitor inhibitor factor-1 (MPIF-1) for the treatment of cancer patients to allow more potent doses of chemotherapy. According to HGS, MPIF-1 is the first genomics-derived therapeutic product to enter clinical trials. Among more than 50 candidate proteins tested in an extensive range of high-throughput cellular assays, MPIF-1 showed the greatest selectivity, the least inflammatory properties, and the best protective activity against a wide range of chemotherapeutics in over 100 primary human cell lines. MPIF-1 moved into animal testing with excellent results. In mouse models, it reduced the severity of neutropenia, prolonged stem-cell survival, and rapidly reverted white blood cell counts to normal following successive rounds of chemotherapy. For the development of MPIF-1, HGS will be the sole sponsor of the phase I/IIa trial, and on completion, first Schering-Plough and then SmithKline will have an option to co-develop the protein in later trials.
Clinical trials are currently addressing a very broad range of potential delivery systems and disease targets. Of the 313 clinical studies listed in the public database maintained by the US National Institute of Health, 70% are involved in the treatment of cancer. This preponderance of cancer-related trials may be surprising if one considers gene therapy as a treatment for genetic diseases, but in the broader context, gene therapy could be considered as another form of drug delivery, and this accounts for the wide variety of applications of this approach. Therefore, clinical gene therapy applications include treatments aimed at a diverse list of disorders such as arthritis, HIV infection, several types of cancers and extremely rare genetic diseases. Often, the number of patients enrolled in these trials is small (fewer than 20 patients) and this is mainly because of the necessity for ex vivo manipulation of the individual's patient's cells.
One of the more encouraging results in recent reports comes from the use of injections of DNA encoding vascular endothelial growth factors to promote angiogenesis in tissues affected by vascular insufficiency.62
One of the most exciting applications of the use of viral vectors is adeno-associated vector injection into muscle. A clinical trial was recently initiated to inject factor IX expressing adeno-associated vectors into the muscles of patients with haemophilia B, and similar approaches have been suggested for retinitis pigmentosa, familial hypercholesterolemia, and muscular dystrophy.
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The impact of genetic advances on clinical medicine |
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TopSummaryIntroductionGenomicsPharmacogenomicsImpact on drug discovery...Relevance to clinical medicineGenetic variations in...Gene therapyFirst genomic products in...The impact of genetic...NeuropsychiatryMigraineNeurological channelopathiesHuntington's diseaseAlzheimer's diseaseAmyotrophic lateral sclerosisParkinson's diseaseCardiologyCardiovascular risk factorsEssential hypertensionDiabetesObesityOncologyEthical issuesDiscussion and conclusionsReferences |
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These genetic advances have greatly enhanced understanding of disease mechanisms and have begun to explain why the clinical course of common disorders such as diabetes is so variable. In future, presymptomatic population-based genetic testing for common late-onset disorders such as Alzheimer's disease may become widespread and bring important health benefits.63 Genotyping may become part of routine investigations to help clinicians tailor drug treatment effectively. Soon medical prescriptions may be personalized to our genotype, along with specific neutraceutical foods. Some vaccines will be delivered through foods such as raw potatoes or bananas.64
However, there are issues which need to be better understood. Even in the simple Mendelian disorders, the relation between the DNA sequence of a gene and the corresponding phenotype is far from clear. In late-onset conditions, such as coronary heart disease and diabetes, where genetic, social, biological and environmental factors interact over time, predicting the clinical importance in a given patient of several different mutations of low penetrance genes is very difficult.65 Whether testing will inevitably become widespread as more tests become available is uncertain. Much depends on the severity of the disease and the scope for effective treatment or prevention. Readiness to undergo testing also depends on how testing is offered and on personal, social, and psychological factors. Rigorous assessment of the benefits and costs, both economic and psychosocial is essential, not least because information from genetic screening tests carry implications for families as well as individuals.
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Neuropsychiatry |
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TopSummaryIntroductionGenomicsPharmacogenomicsImpact on drug discovery...Relevance to clinical medicineGenetic variations in...Gene therapyFirst genomic products in...The impact of genetic...NeuropsychiatryMigraineNeurological channelopathiesHuntington's diseaseAlzheimer's diseaseAmyotrophic lateral sclerosisParkinson's diseaseCardiologyCardiovascular risk factorsEssential hypertensionDiabetesObesityOncologyEthical issuesDiscussion and conclusionsReferences |
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Psychiatrists must usually rely on complex clinical symptoms and diagnostic schemes that although highly reliable, have no obvious biological criteria. Thus, the question of whether our modern definitions of clinical syndromes (considered as phenotypes) accurately reflect underlying genetic substrates (genotypes) remains. Genetic analysis of some psychiatric disorders might be improved by the identification of basic phenotypes for which a more homogeneous aetiology might be expected. The identification of these phenotypes could be achieved by two complementary strategies: the description of the affected subjects and the identification of vulnerability traits in non-affected relatives of affected individuals. By studying the core symptoms of an illness, the clinical phenotype will be more aetiologically homogeneous. The extent to which genetic mapping is simplified by restriction of the phenotype redefinition can be assessed by measuring the recurrence risk for a relative of an affected person, divided by the risk in the general population.5
Factors such as genetic polymorphisms, age at onset, disease severity and family history can be helpful in the identification of homogeneous subtypes. Early onset is associated with increased familial risk in schizophrenia, bipolar affective disorder, major depressive disorder and obsessive-compulsive disorder.66–68 The study of associated symptoms and co-morbid conditions has also proved helpful in the identification of subgroups. Increased familial risk can also provide a key in the identification of subgroups that have a genetic basis. In schizophrenia, a study of genetic polymorphism for drug metabolism (CYP2D6) and tardive dyskinesia suggests that heterozygous carriers of 2D6 mutated alleles may show an increased susceptibility to developing dyskinesia.69 In Alzheimer's disease, point mutations in the gene encoding the amyloid precursor protein (chromosome 21), the gene encoding presenilin 1 (chromosome 14) and the gene encoding presenilin 2 (chromosome 1) were identified only after early-onset familial cases that showed an autosomal dominant pattern of inheritance were recognized. Furthermore, subdivision according to age at onset and mode of inheritance has been particularly useful in the clarification of genetic heterogeneity in dementias of the Alzheimer type.
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Migraine |
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TopSummaryIntroductionGenomicsPharmacogenomicsImpact on drug discovery...Relevance to clinical medicineGenetic variations in...Gene therapyFirst genomic products in...The impact of genetic...NeuropsychiatryMigraineNeurological channelopathiesHuntington's diseaseAlzheimer's diseaseAmyotrophic lateral sclerosisParkinson's diseaseCardiologyCardiovascular risk factorsEssential hypertensionDiabetesObesityOncologyEthical issuesDiscussion and conclusionsReferences |
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Molecular genetics offers a novel approach to the understanding and management of migraine, since the disorder is known to have a strong genetic component. In a recent study, a Nocardia corallina-I (NcoI) polymorphism in the gene encoding dopamine D2 receptor was evaluated in a group of 250 unrelated individuals.70 The major findings of the study was that susceptibility to migraine with aura is modified by dopamine D2 NcoI alleles. However, it is also clear that since not all individuals with the dopamine D2 NcoI A1/A1 genotype suffer from migraine with aura, multiple additional genes are involved in the pathogenesis of migraine.
A gene for familial hemiplegic migraine (