Current Drug Targets (v.10, #3)

Protozoan parasites such as Trypanosoma cruzi, Leishmania spp, and Plasmodium spp have ancient histories of coevolution and complex interactions with their mammalian hosts. In order to survive against hostile environments created by the host's defense mechanisms highly adapted parasites have been selected, possessing biologic, metabolic and enzymatic features that allow them to survive and multiply within the mammalian host. Although these unique features may represent obstacles for parasite eradication, they may also be the key for development of chemotherapeutics with selective action. The development of new drugs with high efficacy and low toxicity against these protozoan parasites is of great need on account of the high toxicity of many of the available drugs and to the appearance of drug-resistant parasite strains. The diseases caused by these parasites are neglected in terms of investments for drug development because they are endemic in developing countries. Although a significant amount of basic research is carried out aiming to identify new anti-protozoan molecules, only few candidate drugs have entered clinical studies and even less became available to patients due to gaps in the pipeline of drug development. Since the identification of hits and lead compounds is a critical step towards progression in the drug development pipeline, the use of combined strategies may facilitate the identification of promising compounds and their mechanism of action. The present issue brings reviews focused on different approaches and strategies than can be employed for the development of chemotherapy for diseases caused by protozoan parasites. Cruz and collaborators also highlight the need for integrated partnerships and networks between scientists in academic institutions and industry for drug development against parasitic diseases, and the importance of initiatives such as the one aiming to produce transgenic parasites, e.g. Leishmania and Plasmodium, better suited for HTS platforms in order to accelerate the screening of antiparasitic drugs.

Current Treatment and Drug Discovery Against Leishmania spp. and Plasmodium spp.: A Review by Angela Cruz, Juliano de Toledo, Mofolusho Falade, Monica Terrao, Sumalee Kamchonwongpaisan, Dennis Kyle, Chairat Uthaipibull (178-192).
Malaria and leishmaniasis are the most prevalent tropical diseases caused by protozoan parasites. Half of world's population is at risk of malaria and more than 2 million of new cases of leishmaniasis occur annually. There are no vaccines available for these diseases and current treatments suffer from several limitations. Therefore, novel drugs for malaria and leishmaniasis are much-needed. This article reviews the agents currently in use for treatment of these diseases, their known mechanisms of action and weaknesses. We present an overview of the main strategies for drug discovery and the relevance of these parasites genomics/proteomics data for a rational search of molecular targets and matching leads. In this direction, we emphasize the importance of the highly integrated partnerships and networks between scientists in academic institutions and industry involving several countries that promise to increase the chances of success and enhance cost-effectiveness in drug discovery against these parasitic diseases. In addition, we approach the available assays for testing lead compounds in large scale and their limitations for they represent one of the bottlenecks in the pipeline for novel drug discovery. We conclude the article presenting a recent coordinated initiative (TDR Transfection Network) established to overcome some of these limitations by the generation of Plasmodium and Leishmania transgenic parasites better suited for HTS platforms.

Selection of Targets for Drug Development Against Protozoan Parasites by Walter de Azevedo Junior, Milena Pereira Soares (193-201).
Sequencing of parasite genomes opened the possibity to identify potential protein targets for drug development. Several protein targets have been found in the genome of Plasmodium falciparum, Trypanosoma cruzi, Trypanosoma brucei and Leishmania major. Bioinformatics analysis is an important tool for the identification of protein targets for drug development against parasitic diseases. In this review we comment about three protein targets, identified in parasite genomes, and discuss the main features that may guide future efforts for virtual screening initiatives.

Modern Approaches in the Search for New Lead Antiparasitic Compounds from Higher Plants by Emerson Queiroz, Jean-Luc Wolfender, Kurt Hostettmann (202-211).
Higher plants represent a rich source of new molecules with pharmacological properties, which are lead compounds for the development of new drugs. During the last decades, the renewed interest in investigating natural products has led to the introduction of several important drugs, such as the anticancer drugs vinblastine and taxol, or the antimalarial agent artemisinin. Success in natural products research is conditioned by careful plant selection, based on various criteria such as chemotaxonomic data, information from traditional medicine, field observation, or even random collection. One main strategy in the isolation of new lead compounds consists of so-called bioactivity-guided isolation, in which pharmacological or biological assays are used to target the isolation of bioactive compounds. One major drawback of this strategy is the frequent isolation of known metabolites. Therefore, metabolite profiling using hyphenated techniques such as LC/UV, LC/MS and more recently LC/NMR rapidly provides plenty of structural information, leading to a partial or a complete on-line de novo structure determination of the natural products of interest. The combination of metabolite profiling and LC/bioassays gives the possibility to distinguish between already known bioactive compounds and new molecules directly in crude plant extracts (dereplication). Thus, the tedious isolation of compounds of low interest can be avoided and a targeted isolation of new bioactive products. Some examples of rapid localization of bioactive compounds, based on post-chromatographic bioautographic testing of LC/NMR microfractions and subsequent on-line identification will be illustrated in this review. Possibilities and limitations of LC/UV/NMR/MS and LC/bioassay as well as future developments expected in this field will be discussed.

Approaches for the Development of New Anti-Trypanosoma cruzi Agents by Diogo Rodrigo Magalhaes Moreira, Ana Lima Leite, Ricardo Ribeiro Santos, Milena Soares (212-231).
The recent highlights on the biochemical pathways of Trypanosoma cruzi have allowed a significant improvement in the development of new strategies for drug design and also in the understanding of the mechanisms of action of new trypanocidal agents. Several biochemical pathways of fundamental importance and validated targets (e.g. cysteine protease cruzain, trypanothione reductase, trans-sialidase) of T. cruzi have proved usefulness for drug development in many examples of new candidates to anti-T. cruzi drugs. This review will focus on some approaches used for the design of new potential trypanocidal agents, exploring modern concepts of medicinal chemistry such as bioisosterism, molecular hybridization, bioinspired design in lead compounds, as well as the complexation of transition metals with bioactive ligands. The examples discussed in this article may serve as lessons for the antitrypanosomal drug design.

Bioinformatics Tools for Screening of Antiparasitic Drugs by Walter Filgueira de Azevedo Junior, Raquel Dias, Luis Fernando Macedo Timmers, Ivani Pauli, Rafael Caceres, Milena Pereira Soares (232-239).
Drug development has become the Holy Grail of many structural bionformatics groups. The explosion of information about protein structures, ligand-binding affinity, parasite genome projects, and biological activity of millions of molecules opened the possibility to correlate this scattered information in order to generate reliable computational models to predict the likelihood of being able to modulate a target with a small-molecule drug. Computational methods have shown their potential in drug discovery and development allied with in vitro and in vivo methodologies. The present review discusses the main bioinformatics tools available for drug discovery and development.

Genomic Databases and the Search of Protein Targets for Protozoan Parasites by Luis Fernando Timmers, Ivani Pauli, Guy Barcellos, Kelen Rocha, Rafael Caceres, Walter de Azevedo Jr., Milena Soares (240-245).
The development of databases devoted to biological information opened the possibility to integrate, query and analyze biological data obtained from several sources that otherwise would be scattered through the web. Several issues arise in the handling of biological information, mainly due to the diversity of biological subject matter and the complexity of biological approaches towards phenomena of the living world. The integration of genomic data, three-dimensional structures of proteins, biological activity, and drugs availability allows a system approach to the study of the biology. Here we review the current status of these research efforts to develop genomic databases for protozoan parasites, such as the apicomplexan parasites, Trypanosoma cruzi and Leishmania spp. These databases may help in the discovery and development of new drugs against parasite-mediated diseases.

Electron microscopy may be useful in chemotherapy studies at distinct levels, such as the identification of subcellular targets in the parasites and the elucidation of the ultimate drug mechanism of action, inferred by the alterations induced by antiparasitic compounds. In this review we present data obtained by electron microscopy approaches of different parasitic protozoa, such as Trypanosoma cruzi, Leishmania spp., Giardia lamblia and trichomonads, under the action of drugs, demonstrating that the cell architecture organization is only determined in detail at the ultrastructural level. The transmission electron microscopy may shed light (i.e. electrons) not only on the affected compartment, but also on the manner it is altered, which may indicate presumable target metabolic pathways as well as the actual toxic or lethal effects of a drug. Cytochemical and analytical techniques can provide valuable information on the composition of the altered cell compartment, permitting the bona fide identification of the drug target and a detailed understanding of the mechanism underneath its effect. Scanning electron microscopy permits the recognition of the drug-induced alterations on parasite surface and topography. Such observations may reveal cytokinetic dysfunctions or membrane lesions not detected by other approaches. In this context, electron microscopy techniques comprise valuable tools in chemotherapy studies.

Testing of Natural Products and Synthetic Molecules Aiming at New Antimalarials by Antoniana Krettli, Joseph Adebayo, Luisa Krettli (261-270).
The search for new antimalarials, which in the past relied on animal models, is now usually performed with cultures of Plasmodium falciparum (PF) blood parasites by evaluation of parasite growth inhibition. Field isolates of PF human malaria parasite, parasite strains and clones, well characterized for their susceptibility to chloroquine and other standard antimalarials are available for the in vitro tests. The simplest method to evaluate parasite growth is the determination of parasitemias in Giemsa stained blood smears through light microscopy. Other methodologies have proven to be more precise and allow mass screening of new compounds against PF blood stages, such as: (i) measuring the incorporation of radioactive hypoxanthine by the parasites; (ii) indirect colorimetric assays in which specific parasite enzyme activities, and histidine-rich protein II (HRP2) production are measured with the help of monoclonal antibodies; (iii) the and#946;- haematin formation, and; (iv) assays using green fluorescent protein (GFP) in gene-expressing parasites. The advantages and disadvantages of the different in vitro screening methods, as well as the different in vivo models for antimalarial tests, are described in this review. Such tests can be used for the evaluation of medicinal plants, synthetic and hybrid molecules or drug combinations.

Protein-Drug Interaction Studies for Development of Drugs Against Plasmodium falciparum by Walter de Azevedo Jr., Rafael Caceres, Ivani Pauli, Luis Fernando Timmers, Guy Barcellos, Kelen Rocha, Milena Soares (271-278).
The study of protein-drug interaction is of pivotal importance to understand the structural features essential for ligand affinity. The explosion of information about protein structures has paved the way to develop structure-based virtual screening approaches. Parasitic protein kinases have been pointed out as potential targets for antiparasitic development. The identification of protein kinases in the Plasmodium falciparum genome has opened the possibility to test new families of inhibitors as potential antimalarial drugs. In addition, other key enzymes which play roles in biosynthetic pathways, such as enoyl reductase and chorismate synthase, can be valuable targets for drug development. This review is focused on these protein targets that may help to materialize new generations of antimalarial drugs.

The development of antimalarial drugs involving novel mechanisms of action is of imminent importance. Several potential drug candidates of synthetic and natural origin as well as their combination therapies are currently being evaluated for their efficacy against drug-resistant strains of the parasite. Various plasmodial targets/pathways, such as the Purine salvage pathway, Pyrimidine biosynthesis pathway and also the processes in the apicoplast, have been identified and are being utilized for the discovery and development of novel antimalarial therapies. This article provides an overview of the latest developments in terms of cell and molecular biology that will improve the knowledge related to drug-resistant malaria and to new molecular targets.

Early Toxicity Screening and Selection of Lead Compounds for Parasitic Diseases by Renata Nogueira, Jose Oliveira-Costa, Matheus de Sa, Ricardo dos Santos, Milena Soares (291-298).
Despite many advances made in disease mechanisms knowledge and drug discovery and development processes, the election of promising lead compounds continues to be a challenge. Efficient techniques are required for lead selection of hit compounds selected through in vitro pharmacological studies, in order to generate precise low cost throughput data with minimal amount of compound to support the right decision making. In this context, the selection of lead compounds with physicochemical parameters that will benefit orally bioavailable drugs are crucial for patients compliance and cost effectiveness, as well as for successful pharmacology. A concept based in Lipinski's rules point out the importance of analyzing these informations in early stages. A hepatocyte screening system may provide data on many processes such as drug-drug interaction, metabolite formation, drug toxicity and ADME profile of a hit. Drug-induced liver injury is the most frequent reason for the withdrawal of an approved drug from the market and hepatocytes have a central role in the metabolism of xenobiotics. Cytotoxicity screening assays can also give some information about toxicity early drug discovery process. A set of goals in lead compound selection must be shared between all areas involved so the chances of success can be improved in translational research.