Current Drug Targets - Volume 7, Issue 12, 2006

This special edition of Current Drug Targets entitled “Novel targets for anti-viral treatments” was inspired by the recent advances in the HIV field. Despite intense research over more than two decades there are still several unresolved questions regarding the detailed molecular mechanism of several of the HIV proteins, their interaction with cellular proteins and the effect of such interactions. With the emergence of drug resistant and recombinant HIV strains, a renewed interest in elucidating basic molecular mechanisms of viral protein interactions with the host cells was sparked. The difficulties encountered with the development of a successful vaccine further encouraged basic research into a better understanding of the viral-host interactions in the hope of identifying novel drug targets. Recent advances in technology including the development of siRNA technology, microarray technology and increased use of advanced laser scanning confocal microscopy techniques to visualize fluorescently labelled proteins have all substantially contributed to a better understanding of many viral replication steps and hence the identification of novel drug targets, some of which may be relevant to viruses other than HIV. In this edition seven examples of the latest exciting advances of exploring novel antiviral drug targets are covered. Firstly, insights into the biology of dendritic cells and their unique role for virus dissemination to the draining lymph nodes are discussed. Development of microbicides that are capable of preventing transmission of virus at mucosal sites particularly during sexual transmission of HIV, which is greatly on the increase in developing countries, will need to include protection of dendritic cells. The discovery of C-type lectin receptors and their unique distribution on various subsets of dendritic cells may prove an exciting new target for the development of microbicides and several novel strategies are discussed by Wilkinson and Cunningham. This is followed by a discussion on the potential use of IL-7 in HIV as wells as cancer treatment. With IL-7 being currently used as adjuvant therapy in phase I/II clinical trials, the review by Sasson et al. highlights the importance of a detailed understanding of the IL-7/IL-7 receptor system in order to develop safe new therapeutics that are able to manipulate the signalling through this system resulting in beneficial outcomes not only for HIV treatment but in a number of lymphopenic conditions. Identifying cellular interactions of viral proteins has vastly increased our understanding of cellular processes and opened new potential targets for antiviral therapy which is nicely exemplified by the contributions of Carr et al. and Harrich et al. on the interaction of the HIV protein Vif with the cellular protein APOBEC3G and the interactions of the HIV protein Tat with cellular proteins such as protein kinase R, p300/CBP, P/CAF, cyclin T1, and RNA polymerase II, respectively. In addition, both Vif and Tat also interact with other HIV proteins and are involved in protease activity, virus packaging and reverse transcription, respectively. A better understanding of these and other steps of viral replication and the involvement of each and every viral protein will be essential for the future development of specifically targeted anti-HIV drugs. Another potential drug target is the genetic material of HIV, RNA itself or RNA intermediates as discussed in Fannings' review which gives an exciting insight into the co-evolution of cellular responses to signature viral RNA intermediates and the counter measures employed by viruses. Fanning further explores the potential use of the latest technologies such as RNAi and siRNA in the treatment of HIV. Advances in the lipid biology field with the discovery of specialised membrane areas, the lipid rafts, which are cholesterol rich and non-ionic detergent resistant, have also been taken into consideration for potential drug development against a variety of pathogens that rely on the presence of lipid rafts for their entry and replication as reviewed by Hawkes and Mak. Reagents that can interfere with lipid rafts may turn out to become a great addition to topical microbicide development. Last but by no means least, Rawlinson et al. review the biology of the flavivirus that causes dengue fever and dengue hemorrhagic fever which is the most common arthropod-borne viral infection. In particular, one of its non-structural proteins, NS5 is identified as an ideal drug target candidate due to its important role in two essential processes of dengue replication. These reviews are by no means the only areas of intense research but are just some examples of the exciting field devoted to discovering potential anti-viral targets. The articles in this special issue have all highlighted the importance of a better, more detailed understanding of each step of virus replication at a molecular level in order to be able to develop novel drugs in the combat of new and emerging viral diseases. I am looking forward to the results of the next 20 years in this exciting and expanding area of research.

Worldwide the heterosexual route is the prevalent mode of transmission of HIV, increasing the demand for measures that block the sexual spread of HIV infection. Vaccines designed to prevent mucosal transmission of HIV should be considered a component of vaccine strategies against HIV (in addition to cytotoxic T cells required for clearance and to prevent viral dissemination) and include antibodies, which are capable of blocking HIV entry at mucosal epithelial barriers, and prevent initial infection of target cells in the mucosa. However, in the interim and in the absence of an effective vaccine, the development of microbicides, topical preparations that block the early steps of HIV infection and transmission, may represent a more viable alternative to condom use in many HIV infected regions of the world especially by empowering women. To date there has been some success with antiviral antibodies applied as a microbicide capable of preventing SIV infection in macaques [1] and reports of vaccines capable of preventing intravaginal and intrarectal inoculated SIV [2 , 3]. However, for such success in humans a much greater understanding of the mechanisms involved in the very early stages of mucosal transmission in HIV infection are required. These may lead to additional strategies to inactivate or inhibit viral uptake and replication before a potentially life threatening acute infection develops. Such measures will lead to the development of effective microbicides and vaccines that will diminish the global spread of HIV.

Interleukin-7 (IL-7) is a cytokine produced predominantly by stromal cells of the thymus and bone marrow and is essential for lymphopoiesis. This paper reviews the importance of IL-7 and its receptor (IL-7R) in T-cell genesis, peripheral survival, expansion and memory T-cell development. IL-7 is of particular importance in lymphopenic conditions. Its expression is up-regulated in a number of lymphopenic conditions including: marrow ablation prior to bone marrow transplantation, marrow suppression following chemotherapy and human immuno-deficiency virus (HIV) infection. Plasma IL-7 levels inversely correlate with CD4+ T-cell counts in these conditions. Animal models suggest that IL-7 improves immune reconstitution through increasing thymic output and, perhaps more importantly, through antigen-independent homeostatic driven proliferation in the periphery. Given the promising preliminary data on the use of IL-7 adjuvant therapy in simian immuno-deficiency virus (SIV) infected non-human primates, IL-7 has recently moved into Phase I/II clinical trials of its role as a possible adjuvant therapy for cancer and HIV infection. This paper discusses important considerations such as the possible negative impacts of IL-7 on increased viral infectivity, the induction of autoimmunity and risk of neoplastic events. Successful use of IL-7 will rely on further understanding of the regulation of the component parts of the IL-7R system. Ultimately this understanding may lead to therapeutics that manipulate and optimise signalling through the IL-7/IL-7R system.

Vif is an HIV accessory protein whose primary function is to negate the action of APOBEC3G, a naturally occurring cellular inhibitor of HIV replication. Vif acts by binding to APOBEC3G, inducing its protein degradation within infected cells and reducing its levels in progeny virions. Interventions that interfere with the Vif-APOBEC3G interaction, raise intracellular or virion associated levels of APOBEC3G, or reduce intracellular levels of Vif, all could hold promise as potential therapeutic approaches aimed at enhancing the cells innate antiviral activity. Levels of APOBEC3G might be increased or Vif levels decreased, by strategies targeting protein synthesis, protein degradation or cellular localisation and function, and properties of APOBEC3G and Vif relevant to these strategies are discussed. Recent data have suggested that Vif may have other mechanisms of action apart from the above activities against APOBEC3G, including effects against other anti-viral mechanisms independent of APOBEC3G cytidine deaminase activity. In addition to interaction with APOBEC3G, Vif may have other accessory functions, which are discussed in relation to potential therapies that may affect multiple stages of the HIV life cycle. Future development of strategies that combine enhancement of APBOEC3G functional with inhibition of multiple Vif functions may become useful tools for HIV therapy.

More than fifteen years following the description of Tat as a critical HIV gene expression regulatory protein, additional roles for Tat in HIV replication have been described, including reverse transcription. Tat achieves function through direct interaction with viral proteins, including reverse transcriptase, and numerous cellular proteins including cyclin T1, RNA polymerase II, protein kinase R (PKR), p300/CBP, and P/CAF. Despite our advanced knowledge of how Tat operates, this has not yet resulted in the discovery of effective agents capable of targeting various Tat functions. Nevertheless, Tat remains an attractive, virus-specific molecule and detailed understanding of specific protein interaction holds promise for future drug discovery.

Viruses have many strategies for negotiating entry into cells and harnessing the cellular machinery for their propagation [1, 2]. The diversity of strategies is however bound by the central process of translating protein from RNA. The co-evolution of cellular responses to signature viral RNA intermediates and the counter measures employed by viruses, highlight the vulnerability of this aspect of viral replication and the potential of viral RNA as a drug target. In this review we will discuss novel efforts to target the RNA intermediates of the HIV life cycle.

Lipid rafts are defined as specialized, dynamic microdomains that can be found in plasma membrane, and they are enriched with cholesterol and sphingolipids. Since lipid rafts' first debut in the mid 1990's, their existence, function and biological relevance have been a subject of intense scrutiny within the scientific community. Throughout this debate, we have learned a great deal regarding how cargos (both pathogens and cellular factors) are transported into and out of the cell through raft-dependent or raft-independent pathways. It is now apparent that a number of toxins, bacterial-, and viral-pathogens are able to exploit cholesterol and/or lipid rafts to gain a foot hold in their target hosts. The objective of this review is to describe our current appreciation on how selected pathogens utilise cholesterol and/or lipid rafts to support their propagation and to speculate on how some of these observations can be explored for the development of novel strategies that target plasma membrane lipids to control the spread of these viral- and bacterial-pathogens.

Dengue fever (DF)/dengue haemorrhagic fever (DHF) is the most common arthropod-borne viral infection, where it is now estimated that 2.5-3 billion people world-wide are at risk of infection. Currently there is no available treatment, in the form of vaccine or drug, making eradication of the mosquito vector the only viable control measure, which has proved costly and of limited success. There are a number of different vaccines undergoing testing, but whilst a dengue vaccine is clearly desirable, there are several issues which make live-attenuated vaccines problematic. These include the phenomenon of antibody-dependent enhancement (ADE) and the possibility of recombination of attenuated vaccine strains with wild-type flavivirus members reverting vaccines to a virulent form. Until we gain a better understanding of these issues and their associated risks, the safety of any live dengue vaccine cannot be assured. It therefore may be safer and more feasible for therapeutic-based approaches to be developed as an alternative to live vaccines. As our understanding of dengue molecular biology expands, new potential targets for drugs are emerging. One of the most promising is the dengue non-structural protein 5 (NS5), the largest and most highly conserved of the dengue proteins. This review examines the unique properties of NS5, including its functions, interactions, subcellular localisation and regulation, and looks at ways in which some of these may be exploited in our quest for effective drugs.

A major cause of morbidity and mortality in contemporary medicine is the outcome of interindividual differences in therapeutic response, drug interactions, and adverse drug reactions. The scope of the problem will only increase because the population ages and grows fatter, the rates of chronic illness rise and the use of multiple medications for extended periods becomes increasingly more common. Negative outcomes to pharmacologic treatment are partly related to genetic and environmental factors influencing pharmacokinetic and pharmacodynamic phases. Interindividual differences in plasma concentrations in patients treated with the same doses if the same drugs have been demonstrated. Variability might also be at the level of receptors, transporters, etc. Despite the growing body of experimental and pharmacological research results, the clinical consequences of genetically inherited differences in drug metabolism are poorly established. Meanwhile, knowledge of pharmacogenetics of receptors and other pharmacodynamic targets is also increasing. The sources of variability in the elimination of a drug are genetic (polymorphisms) and environmental (food intake, drug interactions, etc.). Additionally, the potential influence of physiological factors (gender, age, etc.) and the involvement of homeostatic mechanisms, including endogenous metabolism, must be taken into consideration. Such variability can be manifested interindividually (between individuals) or interethnically (between populations). The evaluation of the relevance for patients of the specific factors requires well-defined clinical studies. The use of phenotyping and genotyping and measurements of drugs and drug metabolites with simultaneous evaluation of response and side effects in prospective studies may give clues to the clinical significance of these effects. Nevertheless, physicians today need to be aware of pharmacogenetic factors, which could influence drug therapy, and should take into account this possibility as a potential cause of undesired effects or therapeutic failure [1]. It would be highly desirable that when a new drug is marketed the knowledge of interindividual and interethnic differences in drug response due to pharmacogenetic factors should be known and considered in order to optimize treatment and improve the cost/benefit ratio [2]. There is a growing consensus on the potential use of pharmacogenetics in clinical practice, and hopes have been expressed for their potential in the development of global health. However, there are two major concerns that may lead to widening the “biotechnological gap” between the developing and the industrialized world: first the unaffordability of some current biotechnological methods for poorer countries, and second the unpredictability of categorizing patients by ethnicity (which is a construct with a substantial social substrate) makes it necessary to analyze multiple described alleles for individual clinical cases. As of today, conventional PCR methodology is available, and automatization using chips is a reality. However, in the near future the development of a variety of methods may decrease the direct cost, so that it will be easier to implement pharmacogenetic analyses as a routine in the clinic [3]. The sequencing and haplotype mapping of the human genome have in the context of increasingly admixtured populations made it feasible and desirable to genotype the most frequent known functional alleles in a given clinical situation [3]. For example, CYP2C9 and CYP2D6 allele frequencies vary between populations and geographical areas, so that in admixtured populations (e.g., in North, Central, and South America) it would be very useful to have genotyping methods that would allow the phenotype of a given patient or healthy volunteer to be predicted. In the present Hot-Topic Issue, two papers deal with this problem (“Ethnic Differences in Pharmacogenetically Relevant Genes”, by R M Engen et al., and “Pharmacogenomics in the Americas: The Impact of Genetic Admixture” by G. Suarez-Kurtz and S. D. J. Pena). An individualized pharmacogenetic approach would allow recommended dosages to be adjusted for each location, considering the growing body of knowledge emerging from pharmacogenetics. Another important aspect to consider as a source of variability is the potential involvement of pharmacogenetic mechanisms in endogenous functioning. Several decades ago, the potential involvement was described of varying allele frequency of one of the most relevant drug metabolizing enzymes, CYP2D6, in psychological factors [4,5]. Since then, although several works have addressed the topic, it remains unclear. In the present issue A. Serretti et al. add another view with potential links to pharmacodynamics in their review “Serotonin transporter gene variants and behaviour”. There have been calls for the use of biotechnologies and pharmacogenetics for global health by improving the drug-treatment of the most important diseases. We cover here the use of pharmacogenetics in relevant clinical problems: psychiatric disorders, by Dorado et al. (“CYP2D6: clinical use during treatment with antipsychotic drugs”) and Chagon et al. (“Susceptibility genes for the side effect of antipsychotics on body weight and obesity”), respiratory diseases by Tantisira et al. (“The pharmacogenetics of asthma therapy”), and infectious diseases by Marques et al. (“Pharmacogenetics and tropical pharmacology”). Considering the rapid growth of knowledge and relevance of critically sensitive issues such as informed consent, and the use and storage of DNA [6], we devoted the last review by Paul et al. to “Ethical aspects of pharmacogenetics”.

There is great heterogeneity in the way humans respond to medications, often requiring empirical strategies to define the appropriate drug therapy for each patient. Genetic polymorphisms in drug metabolizing enzymes, transporters, receptors, and other drug targets provide putative markers for predicting which patients will experience extreme toxicity and treatment failure. Both quantitative (allele frequency) and qualitative (specific allele) differences for polymorphic genes have been observed between different population groups. For example, the frequency of mutations in thiopurine methyltransferase is lower in Chinese than Caucasian populations. In addition, the predominant mutation responsible for deficient enzyme activity differs between the two populations (TPMT*3C versus TPMT*3A). Understanding the influence of ethnicity on pharmacogenomics will allow for comprehensive strategies for using the genome to optimize therapy for patients throughout the world.

In this review we focus on the impact of genetic admixture on pharmacogenomics in the American continent, where five centuries of intermarriage between Amerindians, European and Africans, resulted in the extensive population heterogeneity observed nowadays. We compare two alternative views of human genomic variation, one stressing populations and the other stressing individuals, and discuss their important and far-reaching consequences to implementation of pharmacogenetics/genomics in practice, especially when dealing with admixed populations. We conclude that a variable mosaic genome paradigm, which envisages the genome of any particular individual as a unique mosaic of variable haplotype blocks - has considerably higher explanation and predictive power for the populations of the Americas. We then move to the more formal pharmacogenomics arena to examine the pharmacogenetic/pharmacogenomic diversity in the Americas and review the challenges and advantages of admixed populations for pharmacogenomic studies. Because interethnic admixture is either common or increasing at a fast pace in many, if not most populations, extrapolation on a global scale of pharmacogenomic data from well-defined ethnic groups is plagued with uncertainty. Intra-ethnic diversity adds complexity to the scientific appraisal, regulatory decisions and, eventually, prescribing of drugs purportedly targeted to a given “race” or ethnicity. Pharmacogenetics/ genomics has the potential to benefit people worldwide and to reduce the health disparities between developing and developed nations. This goal is unlikely to be achieved by relinquishing the notion of personalized drug therapy tailored to individual genetic characteristics - the original promise of pharmacogenetics - in favor of a model (pharmacogenomic?) of population-based drug development and prescription, with all its potential pitfalls, especially when extended to admixed populations in developing or developed nations.

The serotonin system modulates affective, cognitive and behavioral processes. A key molecular structure of this system, the serotonin transporter (SERT) gene, has been associated with many human behaviors, both normal and pathological. This article aim is a comprehensive overview of the human behavioral features influenced by SERT gene variants and to suggest some comprehensive hypotheses. In particular, the SERTPR insertion/deletion polymorphism has been related to hippocampal volume and amygdala response and it has been found to influence anxiety-related personality traits and anxiety disorders; in mood disorders it showed some influences on age at onset, periodicity, illness recurrence, rapid cycling, antidepressants response and depressive reaction to stressful life events. Psychosomatic disorders, suicide, alcoholism, smoking, eating disorders, attention deficit hyperactivity disorders and autism have been also found to be related to SERTPR variants. SERT gene variants seem therefore to modulate a wide range of aspects in both normal and affected individuals, many of which are possibly due to indirect correlations between such human features.

CYP2D6 is described as the most relevant enzyme in the metabolism of many antipsychotic drugs. Its contribution to the interindividual differences in drug response is reviewed here highlighting its role in the kinetics of antipsychotic drugs and the occurrence of drug interactions. The activity of CYP2D6 is inherited as a monogenetic trait and the CYP2D6 gene appears highly polymorphic in humans. The polymorphic alleles may lead to altered activity of the CYP enzymes causing absent, decreased (poor), or increased (ultrarapid) metabolism that in turn influence the disposition of the antipsychotic drugs. Antipsychotic drug biotransformation is mainly determined by genetic factors mediating CYP2D6 gene polymorphism, however the importance of environmental factors (dietary, smoking, diseases, etc.) is also recognized. Additionally, the potential interaction between CYP2D6 and the endogenous metabolism must be taken into consideration. The present review summarizes the relevance of physiological and environmental factors in CYP2D6 hydroxylation capacity, the inhibition of CYP2D6 activity during treatment, the use of drug/metabolite ratio as a tool to evaluate CYP2D6 hydroxylation capacity in a patient, and the relevance of CYP2D6 for drug plasma concentration and for QTc interval lengthening during treatment with antipsychotic drugs.

Antipsychotic-induced body weight gain is becoming a major health concern since the increasing use of this medication in different mental disorders with a high prevalence in different populations. The percentage of patients gaining weight following antipsychotic medication can reach up to 80% according to the antipsychotic used, with around 30% developing obesity. The origins of this adverse effect of antipsychotics are probably multifactorial with the environment (food and exercise habits, medication) and the genetics coming into play. We have compiled the available genetic results on the antipsychotic-induced body weight gain and obesity. Candidate genes analysis showed that six genes have been associated with this adverse effect of antipsychotics. Among these, the associations with the serotonin receptor 2C and the leptin genes have been observed in more then one study. Thirteen other genes, mainly antipsychotic known receptors, have shown negative results. To find informative gene variations, we have also compared the effects of some polymorphisms of the serotonin receptor 2C and 2A in mental disorders, for antipsychotic therapeutic effect, for antipsychotic neuronal side effects, and for obesity. We have found results for six polymorphisms in each of the two genes. When association was observed for more then one phenotype, the same genotype or allele was generally involved identifying those sensitive to environmental pressures and to genetic background. Animal transgenic models of knockout or overexpressed genes of antipsychotic receptors have been evaluated for changes in obesity-related phenotypes. Seventeen out of the twenty-three antipsychotic receptors with transgenic models showed some effects on obesity-related phenotypes. Ten of these receptors have not been tested yet for antipsychotic-induced body weight gain, while the others have been tested only once with negative results, or is already associated to the effect such as the serotonin receptor 2C. Finally, pharmacogenomic approaches have allowed to detect more then 300 possible candidate genes for antipsychotic-induced body weight gain.

Asthma affects an estimated 300 million individuals worldwide, resulting in substantial morbidity, mortality, and health care utilization. The response to the three major classes of asthma therapy, β-agonists, leukotriene antagonists, and inhaled corticosteroids, demonstrates wide inter-individual variability, with a significant number of non-responders. In addition, both asthma itself and the intermediate phenotypes of asthma that are measured in response to therapy, including the forced expiratory volume at one second (FEV1), are highly heritable. Both of these facts indicate that a significant portion of the therapeutic response to asthma may be determined by genetic factors. This review summarizes the asthma pharmacogenetics literature as it pertains to human studies, focusing on asthma pharmacogenetic phenotypes and human genetic association studies that have been published for response to each of the three major classes of asthma therapy. Of the four major classes of pharmacogenetic response, there is now evidence that genetic factors influencing the pharmacokinetics, the pharmacodynamics, and the disease modification of asthma therapies may form the basis of credible pharmacogenetic associations. Altogether, the available data indicate that an individual's likelihood of responding to a given therapy is influenced by genetics. Therefore, genetic testing may play a significant role in the care for individuals with asthma in the forseeable future.

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.

Individualized medicine, methodologically rooted in pharmacogenetics and pharmacogenomics, is now venturing into clinical application. Prescribing the right drug in the right dose to the right patient according to specific health needs and individual characteristics is a core mission of individualized medicine. The intrinsic values of this mission are so self-evident that - at first glance - the ethical and social issues raised by individualized medicine seem to be negligible. However, the translation of pharmacogenetics and pharmacogenomics into clinical routine not only requires the collection and evaluation of large amounts of individual genetic data, but also heralds the need for further clinical studies on the applicability of genotype-related pharmacotherapy. Both requirements raise a set of specific normative issues. We argue that ethical and social questions of the desirability and applicability of individualized medicine should be integrated in a reconstructive approach to biomedical ethics, which is guided by criteria of social accountability. As a first step, we analyse the ethical and social issues of individualized medicine in the transition to clinical practice, using social accountability heuristically and as an evolutionary approach. Since the pharmacogenetics and pharmacogenomics of neuropsychiatric disorders are among the most advanced fields of individualized medicine, as a second step we use depressive disorders to elucidate the specific, crucial ethical and social questions involved in assessing patients' situations, disease entities and phenotypes to relate them to genetic variations for the purpose of individualized drug regimens.

Due to an overlook the names of some co-authors of the article “Cancer Cell Permeability-Glycoprotein as a Target of MDR Reverters: Possible Role of Novel Dihydropyridine Derivatives“, published in the journal Current Drug Targets, Volume 7, issue 8, August 2006, were missed in the article. The complete list of authors is as follows: F. Fusi, S. Saponara, M. Valoti, S. Dragoni, and P.D’Elia, T. Sgaragli, D. Alderighi, M. Kawase, A. Shah, N. Motohashi and G. Sgaragli. Also Fig. (1) (page No. 950) and Fig. (2) (957) of the article were incorrectly printed .The correct figures are given below.