Current Drug Targets - Volume 8, Issue 3, 2007

Recent progress in drug discovery has enabled advanced medicines to be developed against a wide variety of human diseases. However, several infectious diseases have been ignored by research programs in both private and public sectors. These neglected diseases, such as tuberculosis, malaria, leishmaniasis, trypanosomiasis and Chagas disease have a devastating impact on the world's poor. Neglected diseases are mainly diseases of the poor in developing countries. As a result, the pharmaceutical industry has no interest in invest on research for drugs against these diseases, since they affect mostly people with no purchasing power. Among neglected diseases tuberculosis (TB) deserves special attention due to impressive numbers of TB cases. It is estimated that approximately one-third of World's population harbour latent TB, which represents a considerable reservoir of bacilli. This present series of reviews is focused on some efforts that have been made in order to identify new targets for development of drugs against neglected diseases. It will be discussed targets such as enzymes of shikimate pathway, purine nucleoside phosphorylase, protein kinases, and InhA (Enoyl Reductase). All studies bring state-of-art research involving multidisciplinary research teams. Recent results that reveal the structural basis for inhibition of several drug targets and the validation of new targets are described. Parasitic protozoa infecting humans have a greatly impact on public health, especially in the developing countries. Several parasites have developed resistance against chemotherapeutic agents, making the research for new drugs a priority. New drugs may be obtained using Protein Kinases (PKs) as potential targets for development of drugs against a variety of diseases. The structural features that are important to design specific inhibitors against these PKs were reviewed by Canduri and collaborators. The main drugs against tuberculosis are isoniazid and rifampicin, which are used in combination with pyrazinamide and others. However, several strains have been proved to be resistant to these drugs. Mutations in the InhA structural gene and the InhA promoter region have been identified in isoniazid-resistant clinical isolates of M. tuberculosis, subsequent studies revealed that the InhA protein is the target for isoniazid. Structural studies of the InhA mutants will help in the development of a new generation of drugs against tuberculosis and they are reviewed by Oliveira and collaborators. Purine nucleoside phosphorylase (PNP) is another target for drugs against neglected disease. It catalyzes the phosphorolysis of the N-ribosidic bonds of purine nucleosides and deoxynucleosides. Silva et al review the main applications of this protein as a target for development of drugs. A set of potential targets for drug development are enzymes of shikimate pathway, which is reviewed by Ducati and collaborators. This pathway is of pivotal importance for production of a plethora of aromatic compounds in plants, bacteria, apicoplexan parasites, and fungi. The shikimate pathway is found only in microorganisms and plants, never in animals. All enzymes of this pathway have been obtained in pure form from prokaryotic and eukaryotic sources and their respective DNAs have been characterized from several organisms, since it is absent in mammals, shikimate pathway enzymes are potential targets for drug development. These enzymes have been identified in Mycobacterium tuberculosis, Plasmodium falciparum, Toxoplasm gondii, and other pathogenic agents. Several structural and activity studies will be reviewed in the present volume.

Parasitic protozoa infecting humans have a great impact on public health, especially in the developing countries. In many instances, the parasites have developed resistance against available chemotherapeutic agents, making the search for alternative drugs a priority. In line with the current interest in Protein Kinase (PK) inhibitors as potential drugs against a variety of diseases, the possibility that PKs may represent targets for novel anti-parasitic agents is being explored. Research into parasite PKs has benefited greatly from genome and EST sequencing projects, with the genomes from a few species fully sequenced (notably that from the malaria parasite Plasmodium falciparum) and several more under way, the structural features that are important to design specific inhibitors against these PKs will be reviewed in the present work.

Tuberculosis (TB) and Malaria are neglected diseases, which continue to be major causes of morbidity and mortality worldwide, killing together around 5 million people each year. Mycolic acids, the hallmark of mycobacteria, are high-molecular-weight - alkyl, β-hydroxy fatty acids. Biochemical and genetic experimental data have shown that the product of the M. tuberculosis inhA structural gene (InhA) is the primary target of isoniazid mode of action, the most prescribed anti-tubercular agent. InhA was identified as an NADH-dependent enoyl-ACP(CoA) reductase specific for long-chain enoyl thioesters and is a member of the Type II fatty acid biosynthesis system, which elongates acyl fatty acid precursors of mycolic acids. M. tuberculosis and P. falciparum enoyl reductases are targets for the development of anti-tubercular and antimalarial agents. Here we present a brief description of the mechanism of action of, and resistance to, isoniazid. In addition, data on inhibition of mycobacterial and plasmodial enoyl reductases by triclosan are presented. We also describe recent efforts to develop inhibitors of M. tuberculosis and P. falciparum enoyl reductase enzyme activity.

Purine nucleoside phosphorylase (PNP) catalyzes the reversible phosphorolysis of nucleosides and deoxynucleosides, generating ribose 1-phosphate and the purine base, which is an important step of purine catabolism pathway. The lack of such an activity in humans, owing to a genetic disorder, causes T-cell impairment, and thus drugs that inhibit human PNP activity have the potential of being utilized as modulators of the immunological system to treat leukemia, autoimmune diseases, and rejection in organ transplantation. Besides, the purine salvage pathway is the only possible way for apicomplexan parasites to obtain the building blocks for RNA and DNA synthesis, which makes PNP from these parasites an attractive target for drug development against diseases such as malaria. Hence, a number of research groups have made efforts to elucidate the mechanism of action of PNP based on structural and kinetic studies. It is conceivable that the mechanism may be different for PNPs from diverse sources, and influenced by the oligomeric state of the enzyme in solution. Furthermore, distinct transition state structures can make possible the rational design of specific inhibitors for human and apicomplexan enzymes. Here, we review the current status of these research efforts to elucidate the mechanism of PNP-catalyzed chemical reaction, focusing on the mammalian and Plamodium falciparum enzymes, targets for drug development against, respectively, T-Cell- and Apicomplexan parasites-mediated diseases.

The aetiological agent of tuberculosis (TB), Mycobacterium tuberculosis, is responsible for millions of deaths annually. The increasing prevalence of the disease, the emergence of multidrug-resistant strains, and the devastating effect of human immunodeficiency virus co-infection have led to an urgent need for the development of new and more efficient antimycobacterial drugs. Since the shikimate pathway is present and essential in algae, higher plants, bacteria, and fungi, but absent from mammals, the gene products of the common pathway might represent attractive targets for the development of new antimycobacterial agents. In this review we describe studies on shikimate pathway enzymes, including enzyme kinetics and structural data. We have focused on mycobacterial shikimate pathway enzymes as potential targets for the development of new anti-TB agents.

The increase in incidence of infectious diseases worldwide, particularly in developing countries, is worrying. Each year, 14 million people are killed by infectious diseases, mainly HIV/AIDS, respiratory infections, malaria and tuberculosis. Despite the great burden in the poor countries, drug discovery to treat tropical diseases has come to a standstill. There is no interest by the pharmaceutical industry in drug development against the major diseases of the poor countries, since the financial return cannot be guaranteed. This has created an urgent need for new therapeutics to neglected diseases. A possible approach has been the exploitation of the inhibition of unique targets, vital to the pathogen such as the shikimate pathway enzymes, which are present in bacteria, fungi and apicomplexan parasites but are absent in mammals. The chorismate synthase (CS) catalyses the seventh step in this pathway, the conversion of 5- enolpyruvylshikimate-3-phosphate to chorismate. The strict requirement for a reduced flavin mononucleotide and the anti 1,4 elimination are both unusual aspects which make CS reaction unique among flavin-dependent enzymes, representing an important target for the chemotherapeutic agents development. In this review we present the main biochemical features of CS from bacterial and fungal sources and their difference from the apicomplexan CS. The CS mechanisms proposed are discussed and compared with structural data. The CS structures of some organisms are compared and their distinct features analyzed. Some known CS inhibitors are presented and the main characteristics are discussed. The structural and kinetics data reviewed here can be useful for the design of inhibitors.

EPSP synthase (EPSPS) is an essential enzyme in the shikimate pathway, transferring the enolpyruvyl group of phosphoenolpyruvate to shikimate-3-phosphate to form 5-enolpyruvyl-3-shikimate phosphate and inorganic phosphate. This enzyme is composed of two domains, which are formed by three copies of βα βα ββ-folding units; in between there are two crossover chain segments hinging the nearly topologically symmetrical domains together and allowing conformational changes necessary for substrate conversion. The reaction is ordered with shikimate-3-phosphate binding first, followed by phosphoenolpyruvate, and then by the subsequent release of phosphate and EPSP. N-[phosphomethyl]glycine (glyphosate) is the commercial inhibitor of this enzyme. Apparently, the binding of shikimate-3-phosphate is necessary for glyphosate binding, since it induces the closure of the two domains to form the active site in the interdomain cleft. However, it is somehow controversial whether binding of shikimate-3-phosphate alone is enough to induce the complete conversion to the closed state. The phosphoenolpyruvate binding site seems to be located mainly on the C-terminal domain, while the binding site of shikimate-3-phosphate is located primarily in the N-terminal domain residues. However, recent results demonstrate that the active site of the enzyme undergoes structural changes upon inhibitor binding on a scale that cannot be predicted by conventional computational methods. Studies of molecular docking based on the interaction of known EPSPS structures with (R)- phosphonate TI analogue reveal that more experimental data on the structure and dynamics of various EPSPS-ligand complexes are needed to more effectively apply structure-based drug design of this enzyme in the future.

Tuberculosis (TB) remains the leading cause of mortality due to a bacterial pathogen, Mycobacterium tuberculosis. However, no new classes of drugs for TB have been developed in the past 30 years. Therefore there is an urgent need to develop faster acting and effective new antitubercular agents, preferably belonging to new structural classes, to better combat TB, including MDR-TB, to shorten the duration of current treatment to improve patient compliance, and to provide effective treatment of latent tuberculosis infection. The enzymes in the shikimate pathway are potential targets for development of a new generation of antitubercular drugs. The shikimate pathway has been shown by disruption of aroK gene to be essential for the Mycobacterium tuberculosis. The shikimate kinase (SK) catalyses the phosphorylation of the 3-hydroxyl group of shikimic acid (shikimate) using ATP as a co-substrate. SK belongs to family of nucleoside monophosphate (NMP) kinases. The enzyme is an α/β protein consisting of a central sheet of five parallel β-strands flanked by α- helices. The shikimate kinases are composed of three domains: Core domain, Lid domain and Shikimate-binding domain. The Lid and Shikimate-binding domains are responsible for large conformational changes during catalysis. More recently, the precise interactions between SK and substrate have been elucidated, showing the binding of shikimate with three charged residues conserved among the SK sequences. The elucidation of interactions between MtSK and their substrates is crucial for the development of a new generation of drugs against tuberculosis through rational drug design.

In recent years, RNA interference (RNAi) is one of the most important discoveries. RNAi is an evolutionarily conserved mechanism for silencing gene expression by targeted degradation of mRNA. Short double-stranded RNAs, known as small interfering RNAs (siRNA), are incorporated into an RNA-induced silencing complex that directs degradation of RNA containing a homologous sequence. siRNA has been shown to work in mammalian cells, and can inhibit viral infection and control tumor cell growth in vitro. Recently, it has been shown that intravenous injection of siRNA or of plasmids expressing sequences processed to siRNA can protect mice from autoimmune and viral hepatitis. In this review, we have discussed about the discovery of RNAi and siRNA, mechanism of siRNA mediated gene silencing, mediated gene silencing in mammalian cells, vectored delivery of siRNA, pharmaceutical potentiality of siRNA from mice to human. We have also discussed about promise and hurdles of siRNA or RNAi that could provide an exciting new therapeutic modality for treating infection, cancer, neurodegenerative disease, antiviral diseases (like viral hepatitis and HIV-1), huntington's disease, hematological disease, pain research and therapy, sarcoma research and therapy and many other illness in details. It will be a tool for stem cell biology research and now, it is a therapeutic target for gene-silencing.

Currently, low density lipoprotein cholesterol (LDL-C) levels are the main, if not the only, lipid target in the effort to reduce cardiovascular disease (CVD) morbidity and mortality. Several primary and secondary CVD prevention trials with statins shaped current guidelines and provided detailed targets across a range of CVD risk categories. These targets can be attained using effective statins or combination therapy. However, the net benefit in CVD risk reduction may be improved if we address other lipid risk factors. High density lipoprotein cholesterol (HDL-C) emerges from epidemiological studies as the most promising target. This review links the increase in HDL-C levels with clinical benefit from “old” (e.g. sustained release niacin) and new treatment options. Synthetically produced recombined apolipoprotein A-I Milano administered intravenously seems to have a marked effect in reducing the atheroma burden. The anti-cholesterol ester transfer protein (CETP) vaccine (CETi-1) produces auto-antibodies against CETP thus increasing the cholesterol ester content in HDL particles. CETP inhibitors (e.g. JTT-705 and torcetrapib) seem to be the most promising regimen to increase HDL-C levels. Torcetrapib (already in phase IIIa studies) can substantially increase HDL-C levels (up to 106%), alone or in combination with atorvastatin. HDL-C strategies, in combination with effective statins, are a new drug target aimed at a further reduction in CVD morbidity and mortality compared with statin monotherapy.

“Due to a misunderstanding, the names of some co-authors of the article ”Histone deacetylase inhibitors as potent modulators of cellular contacts”, published in the journal Current Drug Targets, Volume 7, issue 6, June 2006, pg 773-787 were not printed correctly in the article. The complete and correct list of authors is as follows: Vinken, M., Papeleu, P., Rogiers, V. and Vanhaecke, T.”