Current Pharmaceutical Design - Volume 16, Issue 2, 2010

Long before the term pharmacogenetics was coined, it was recognized that patients respond very differently to treatment with a drug. Indeed, the question how variability of drug response can be explained is as old as the science of pharmacology. The concept of interindividual differences in drug response was proposed as early as 1909 by Garrod in his book The Inborn Errors of Metabolism [1]. Only since the late 50s of the last century it was shown that variations in DNA may play a role in variable drug response. In 1957, Motulsky was the first to describe that some American soldiers of African descent were sensitive to the antimalarial drug primaquine [2]. It caused hemolytic anemia in a subgroup of primaquine users which could be ascribed to a deficiency of the enzyme glucose-6-phosphate dehydrogenase (abbreviated G6PD), a metabolic enzyme involved in the pentose phosphate pathway, and especially important in red blood cell metabolism. Moreover, it was recognized that some patients experienced muscle-relaxation for hours, sometimes necessitating artificial ventilation, instead of minutes after the administration of the muscle-relaxant suxamethonium. A deficiency of the enzyme pseudocholinesterase, involved in the degradation of the drug, causes this extraordinary drug response to suxamethonium [3]. In 2003 the International Human Genome Sequencing Consortium declared that the Human Genome Project had been completed, raising expectations of clinical application in the near future. Pharmacogenetics is one of the first clinical applications of the postgenomic era. It promises personalized medicine rather than the established “one size fits all” approach to drugs and dosages. The expected reduction in “trial and error drug treatment” should ultimately lead to more efficient and safer drug therapy [4]. Today, the concept of pharmacogenetics, namely that variation in drug response is related to genetic variation, is widely recognized. Commercially available pharmacogenetic tests have been approved by the Food and Drug Administration (FDA). These commercial tests detect variations in the genes coding for enzymes involved in drug metabolism, for example cytochrome P450 CYP2C19 and CYP2D6 or UDP-glucuronosyltransferase or in drug targets such as Vitamin K epoxide reductase complex, subunit 1 (abbreviated VKORC1) which is the drug target for the cumarins such as warfarin. However, it appears that the application of these and other pharmacogenetic tests in patient care remains limited. More generally, the implementation of pharmacogenetics in routine clinical practice presents significant challenges [4]. Providing scientific evidence for improved outcome in patient care by pharmacogenetic testing is one such challenge. In addition, for most pharmacogenetic tests (such as tests for genetic variants of cytochrome P450 enzymes) a detailed knowledge of pharmacology is a prerequisite for application in clinical practice, and clinicians might find it difficult to interpret the clinical value of pharmacogenetic test results. Guidelines that link the result of a pharmacogenetic test to therapeutic recommendations might help to overcome these problems, and have become available only recently [5,6]. Major developments in pharmacogenetics are seen in several clinical areas. In the present special issue the current status of pharmacogenetics and future perspectives in the field of psychiatry, inflammatory bowel disease, oncology, rheumatology, renal transplantation and anticoagulant therapy are described. In addition, the role of pharmacogenetics in phase I and II drug metabolism and in drug transporters is discussed. Finally, valuable papers on laboratory techniques, pharmacogenetics and drug design and development, and ethics of pharmacogenetic testing are included. A variety of internationally recognized experts have contributed to this special issue and compiled an excellent overview of the dynamic and promising field of pharmacogenetics.

More than fifty years of pharmacogenetic research have produced many examples of the impact of inherited variability in the response to psychotropic drugs. These successes, however, have as yet failed to translate into broadly applicable strategies for the improvement of individual drug treatment in psychiatry. One important argument against the widespread adoption of pharmacogenetics as a clinical tool is the lack of evidence showing its impact on medical decision making and on risk benefit ratio for the patients. The individual drug metabolizing capacity is assessed by genotyping drug metabolizing enzymes. The potential implications of information gained from genotyping are dose adjustments according to genotype. However, even when the consequences of genotype on pharmacokinetics are significant and well known, as in the case of many tricyclic antidepressants and several SSRIs, there is still considerable controversy on whether adjustment of dosage driven by genetic information may improve therapeutic efficacy, and/or adverse events is prevented, to an extent of any practical importance in clinical practice. Different types of pharmacogenetic studies may improve our understanding of the functional consequence of a genetic variant in the clinical setting. The use of intermediate phenotypes instead of broad outcome parameters such as drug response or remission might improve our knowledge on what exactly happens if an individual with a specific genotype takes a certain drug. Here, we review the potential impact of an integrated approach, including the assessment of intermediate phenotypes for the effect of genetic polymorphism, the monitoring of therapy progress, and response prediction in depression.

Thiopurines are widely used in the treatment of inflammatory bowel disease (IBD). However, in clinical practice azathioprine (AZA) or 6-mercaptopurine (6-MP) are not effective in one-third of patients and up to one-fifth of patients discontinue thiopurine therapy due to adverse reactions. The observed interindividual differences in therapeutic response and toxicity to thiopurines are explained to a large extent by the variable formation of active metabolites, which is at least partly caused by genetic polymorphisms of the genes encoding crucial enzymes in thiopurine metabolism. In this in-depth review we discuss the genetic polymorphisms of genes encoding for glutathione S-tranferases, xanthine oxidase, thiopurine S-methyltransferase, inosine triphosphate pyrophosphatase, hypoxanthine phosphoribosyltransferase, inosine monophosphate dehydrogenase and multidrug resistance proteins. Pharmacogenetic knowledge in this field has increased dramatically and is still rapidly increasing, but the translation into practical guidelines with tailored advices will cost much effort in the near future.

Pharmacogenetics is a rapidly developing field, especially in oncology. In the most ideal situation pharmacogenetics will allow oncologists to individualize therapy based on patients’ individual germline genetic test results. This can help to improve efficacy, reduce toxicity and predict non-responders in a way that alternative therapy can be chosen or individual dose adjustments can be made. Multiple pathways have been studied extensively of which a brief review is presented here. Increased 5FU toxicity is associated with variations in the DPYD gene, TYMS gene and MTHFR gene. Furthermore variations in the UGT1A gene and the ABCB1 gene influence irinotecan metabolism and disposition. Other genetic changes result in reduced DNA repair capacity related to platinum efficacy or reduced cytochrome P450 2D6 activity related to tamoxifen efficacy. Despite the extensive number of pharmacogenetic studies and promising results, it is still unclear when and how pretreatment genetic screening should be implemented in oncology. Future prospective studies should focus on the effect of pharmacogenetics on patient outcome and combine this with cost effectiveness evaluations. Thus supplying us with predictive models helping in deciding when pretreatment genetic screening is useful.

Over the last decades important progress is being made regarding disease modifying anti-rheumatic drugs (DMARDs) in the treatment of rheumatoid arthritis (RA). Nevertheless, a substantial part of the patients fail to achieve a good response and/or experience toxicity, which limits further treatment leading to progression of inflammation and destruction of joints. These high interindividual differences in drug response gave rise to the need for prognostic markers in order to individualize and optimize therapy with these antirheumatic agents. Besides demographic and clinical factors, studies in the research field of pharmacogenetics have reported potential markers associated with clinical response on treatment with methotrexate and TNF inhibitors. However, publicized conflicting results and underlying interpretation difficulties inhibit drawing definitive conclusions. Presently, clinical implementation of pharmacogenetics as an important step for individualizing drug therapy in RA is not feasible yet. Replication and prospective validation in large patient cohorts are required before pharmacogenetics can be used in clinical practice. This review provides the current state of art in genotyping RA patients as a potential guide for clinical decision making.

Patient variability in clinical response to the calcineurin inhibitors (CNIs) cyclosporine A and tacrolimus partly results from differences in CNI exposure. For tacrolimus drug interactions and genetic variability relate to tacrolimus exposure. Patients carrying the CYP3A5*1 allele have an increased tacrolimus metabolism, hence lower drug exposure. Adjusting the tacrolimus dose to this genotype is a tool to optimize therapy from a pharmacokinetic perspective. In contrast, no genetic variants have been found to clearly relate to cyclosporine A exposure. Despite therapeutic drug monitoring aimed at individualizing CNI therapy, patients still suffer from acute or chronic rejection and CNI toxicity. To further optimize CNI therapy future research may incorporate genetic polymorphisms in proteins involved in CNI pharmacodynamics (i.e. drug target). Proteins potentially relevant for drug response are calcineurin and the CNI binding proteins immunophilins. Moreover, since the expression of the nuclear factor of activated T-cells (NFAT) is reduced after calcineurin inhibition, genetic polymorphisms in the genes encoding NFAT may also be interesting candidates for studying inter-patient differences in CNI efficacy and toxicity. In addition, the existence of isoforms and differences in tissue distribution of the calcineurin protein could potentially explain variable drug response. At present, the focus has been on the metabolism of CNIs and not on variability in the drug target. Therefore, future improvements in CNI therapy are likely to occur from a systems pharmacology approach taking into account genetic markers for both CNI pharmacokinetics and pharmacodynamics.

The identification of the genes encoding CYP2C9, the principal metabolizing enzyme of the coumarins, and VKORC1, the molecular target for coumarins, has strongly stimulated the research on pharmacogenetics of vitamin K antagonists, also designated as coumarins. From 1999 to 2004 a number of observational studies firmly established associations between being carrier of the CYP2C9*2 and especially the CYP2C9*3 allele and reduced coumarin dose requirements and increased risks of overanticoagulation and even major bleeding compared to CYP2C9 wild type patients. The identification of the VKORC1 gene in 2004 gave rise to more observational studies, which mostly indicated a larger contribution of variants of these gene to the interindividual variability in dose requirements. However, whereas overanticoagulation in the initial period of therapy appears to be associated with VKORC1 as well as CYP2C9 genotype, the CYP2C9 genotype could be a more important predictor for major bleeding and retarded stabilisation. The recent discovery that only one single nucleotide polymorphism in the VKORC1 gene, the -1639G>A polymorphism, is representative for VKORC1 activity and the recent conclusion from a genome-wide scan that VKORC1 and CYP2C9 are the only genes with relevant effects on coumarin response, seem to be definitive demarcations of the genetic information which could be needed for improvement of the existing coumarin dosing algorithms. The observational studies from the last decade provided valuable insights into the effects of genetic factors on variability in coumarin response. During the forthcoming years randomized clinical trials are needed to evaluate whether this genetic information will improve the benefit-risk ratio of coumarins.

Genetic variation in the receptors and other intracellular targets that mediate the pharmacodynamic effects of drugs can affect therapeutic outcomes. However, at present greater knowledge is available concerning the extent of gene variation in drug metabolizing enzymes that determine drug pharmacokinetics and, in turn, drug efficacy and toxicity. Information on the incidence of polymorphisms in the cytochrome P450 (CYP) genes that mediate phase I biotransformation is increasing, although the level of detail in the case of phase II conjugation enzymes, such as the UDP-glucuronosyltransferases (UGTs) and N-acetyltransferases (NATs), is not as extensive. It is now apparent that defective alleles that encode variant CYPs, UGTs, NATs and other biotransformation enzymes can influence the outcome of therapy. Diminished rates of drug clearance can increase the incidence of toxicity from many drugs, but may also enhance efficacy, as in the case of the proton-pump inhibitor omeprazole, that maintains therapeutic serum concentrations in individuals that carry null alleles for CYP2C19. Variant alleles of UGT1A1 are less capable of conjugating and eliminating SN-38, the active form of the topoisomerase inhibitor irinotecan, and defective alleles for NAT2 are responsible for the well-described acetylation polymorphism that leads to impaired clearance of isoniazid and other agents. This review focuses on reports that relate pharmacogenetic variation in phase I and phase II enzymes to the safety and toxicity of drug therapy and highlights a number of themes that have emerged recently that may be developed to streamline therapy for individuals.

During the last decade, a greater focus has been given to impact of genetic variation in membrane transporters on the pharmacokinetics and toxicity of numerous therapeutic drugs. While the majority of transporter-related pharmacogenetic research has been in regards to classic genes encoding the outward-directed ATP-binding cassette (ABC) transporters, such as ABCB1 (Pglycoprotein), ABCC2 (MRP2), and ABCG2 (BCRP), more studies have been conducted in recent years evaluating genes encoding solute carriers (SLC) that mediate the cellular uptake of drugs, such as SLCO1B1 (OATP1B1) and SLC22A1 (OCT1). The distribution of ABC and SLC transporters in tissues key to pharmacokinetics, such as intestine (absorption), blood-brain-barrier (distribution), liver (metabolism), and kidneys (excretion), strongly suggests that genetic variation associated with changes in protein expression or function of these transporters may have a substantial impact on systemic drug exposure and toxicity. In this current article, we will review recent advances in understanding the contribution of critical ABC and SLC transporters to interindividual pharmacokinetic and dynamic variability of substrate drugs.

With the exponential increase in publications on DNA markers explaining and/or predicting response to drug therapy, the potential of pharmacogenetic testing of individual patients to optimize drug treatment is expanding. For the identification of pharmacogenetic markers, several techniques can be used. The specific method usually depends on the requirements of the study, ranging from determining one or two single nucleotide polymorphisms (SNPs) in one to ten patients, one or two SNPs in thousands of patients, several thousands of SNPs in an individual patient, or thousands of SNPs in thousands of patients. In this review we identify and evaluate the information present in the literature on genotyping assays that are currently used for pharmacogenetic analyses, and discuss the advantages and disadvantages of these techniques.

The detailed knowledge of the human genome has not fulfilled its promise as yet. It seems fair to say that we are far from treating existing diseases by therapeutic interventions developed on the basis of genetic knowledge. However, pharmacogenetics has shown to be useful in improving our understanding of pharmacotherapy. Industry is starting to embed this knowledge in the design of innovative drugs and there are three important areas of interest: safety, efficacy and target identification. Application of pharmacogenetics e.g. in patient selection are leading to the direction of more personalised medicine. The future will bring more of such applications. However, current knowledge also leads to a more integrated approach of pharmacogenetics as part of systems biology, providing an even more complete image of reality surrounding disease and therapy, including for example environmental factors and behaviour. In addition, collaborative efforts with academic partners are very much welcomed by the pharmaceutical industry and are expected to have a synergistic effect on progression in this field.

In genomics research, pathways that lead to disease and the role of drugs in these pathways are being unravelled at a high rate. In this paper ethical and social challenges related to pharmacogenomics research are discussed as well as clinical applications. In research, ethical thinking evolves due to the fast pace of research. Genome-wide association studies trying to identify genes that contribute a small risk to common diseases can only be performed on an international scale. Meanwhile, it is becoming more and more clear that genomic information is hard to hide. Thus the traditional promise in research that privacy will be protected appears to be less realistic. Nowadays, adequate information (veracity) and protection against potential risks of discrimination based on predictive medical information is required. A new balance needs to be found. In the clinic, different ethical and social challenges become apparent. The promise to improve diagnosis, treatment and prevention is genuine, but many potentially useful applications do not reach the bedside. There is a need for both translation and for assessment of evidence if “do good and do not harm” is to be taken seriously. In addition, sustainable use of pharmacogenetic knowledge holds promises for developed and developing countries but these promises will only materialize if evidence is built, translated into guidelines, incorporated into education, implemented in pharmacy databases, and evaluated. While translational research in health care progresses slowly, direct-to-consumer testing is being implemented rapidly. International validated quality criteria should apply both to health care and to this commercial field.