Current Medicinal Chemistry - Volume 21, Issue 15, 2014

Glycolysis is an important metabolic pathway for most organisms, including protozoan parasites. Many of these primitive eukaryotes have streamlined their metabolism, favoring glycolysis for generating ATP in the glucose-rich environments in which they reside. Therefore, the enzymes involved in hexose metabolism could prove to be attractive targets for therapeutic development. This hypothesis is supported by a number of chemical and genetic validation studies. Additionally, the peculiar biochemistry of many of the components, along with limited protein sequence identity emphasizes the likelihood of developing compounds that selectively inhibit the parasite enzymes. In this review, we examine the status of target validation at the genetic and/or chemical levels from the protozoan parasites. While the proteins from some species have been interrogated to the point that well-defined lead compounds have been identified with activities against both enzyme and parasite growth, progress in other systems has to date been limited.

Glycosomes are peroxisome-related organelles found in all kinetoplastid protists, including the human pathogenic species of the family Trypanosomatidae: Trypanosoma brucei, Trypanosoma cruzi and Leishmania spp. Glycosomes are unique in containing the majority of the glycolytic/gluconeogenic enzymes, but they also possess enzymes of several other important catabolic and anabolic pathways. The different metabolic processes are connected by shared cofactors and some metabolic intermediates, and their relative importance differs between the parasites or their distinct lifecycle stages, dependent on the environmental conditions encountered. By genetic or chemical means, a variety of glycosomal enzymes participating in different processes have been validated as drug targets. For several of these enzymes, as well as others that are likely crucial for proliferation, viability or virulence of the parasites, inhibitors have been obtained by different approaches such as compound libraries screening or design and synthesis. The efficacy and selectivity of some initially obtained inhibitors of parasite enzymes were further optimized by structure-activity relationship analysis, using available protein crystal structures. Several of the inhibitors cause growth inhibition of the clinically relevant stages of one or more parasitic trypanosomatid species and in some cases exert therapeutic effects in infected animals. The integrity of glycosomes and proper compartmentalization of at least several matrix enzymes is also crucial for the viability of the parasites. Therefore, proteins involved in the assembly of the organelles and transmembrane passage of substrates and products of glycosomal metabolism offer also promise as drug targets. Natural products with trypanocidal activity by affecting glycosomal integrity have been reported.

Infections caused by protozoan parasites are one of the most important public health problems in developing countries. One approach to design new drugs for these parasitic diseases relies on metabolic and molecular features which are ideally absent in mammalian hosts. Out of them, nutrient transporters play an important role since they were subjected to millions of years of adaptation to parasitism, in which this protozoan replaced many biosynthetic routes for transport systems. Here we address the current knowledge of trypanosomatids transport systems and the molecules related to such processes, including a description of permeases involved in drug uptake, and also those responsible of drug resistance. The latter process produces, in many cases, the treatment failure due to the loss of the transporter function, as is the case of eflornithine, as well as by increasing the extrusion of drugs, in which usually ABC-type transporters are involved. All these aspects and the perspectives on this topic are briefly updated in this review.

Atoxyl, the first medicinal drug against human African trypanosomiasis (HAT), also known as sleeping sickness, was applied more than 100 years ago. Ever since, the search for more effective, more specific and less toxic drugs continued, leading to a set of compounds currently in use against this devastating disease. Unfortunately, none of these medicines fulfill modern pharmaceutical requirements and may be considered as therapeutic ultima ratio due to the many, often severe side effects. Starting with a historic overview on drug development against HAT, we present a selection of trypanosome specific pathways and enzymes considered as highly potent druggable targets. In addition, we describe cellular mechanisms the parasite uses for differentiation and cell density regulation and present our considerations how interference with these steps, elementary for life cycle progression and infection, may lead to new aspects of drug development. Finally we refer to our recent work about CNS infection that offers novel insights in how trypanosomes hide in an immune privileged area to establish a chronic state of the disease, thereby considering new ways for drug application. Depressingly, HAT specific drug development has failed over the last 30 years to produce better suited medicine. However, unraveling of parasite-specific pathways and cellular behavior together with the ability to produce high resolution structures of essential parasite proteins by X-ray crystallography, leads us to the optimistic view that development of an ultimate drug to eradicate sleeping sickness from the globe might just be around the corner.

Plasmodium falciparum is responsible for the most severe form of human malaria. P. vivax, in contrast, is the most widespread malaria parasite with an enormous impact on health and economy, since the infection is characterized by high rates of relapses. Due to the mild course of malaria tertiana and complicated in vitro culturing conditions of P. vivax, most of the research on malaria parasites has focused on P. falciparum so far. The redox metabolism of P. falciparum is a promising target for novel antimalarial drugs, since maintaining a redox equilibrium is of fundamental importance for the parasite. P. falciparum contains a cytosolic glutathione and thioredoxin system, as well as redox systems in the apicoplast and the mitochondrion. In contrast to P. falciparum, little is known about the redox processes in P. vivax so far. This review summarizes the current knowledge of the redox metabolism in malaria parasites and provides a detailed in silico comparison of the known and mostly well characterized redox enzymes from P. falciparum and the largely unknown redox proteins from P. vivax. Known antimalarials at least partially mediating their antiparasitic activity by influencing the redox balance of Plasmodium, including dehydroepiandrosterone, Mannich bases, methylene blue, and naphthoquinones, are discussed. Furthermore, we present novel inhibitors identified via screening of a compound library from the Leibniz Institute for Natural Product Research and Infection Biology, Jena that are active against the redox-related enzymes thioredoxin reductase, glutathione reductase, glutathione-S-transferase, and glucose-6-phosphate dehydrogenase 6- phosphoglucono- lactonase from P. falciparum.

In trypanosomatids, redox homeostasis is centered on trypanothione (N1,N8-bis(glutathionyl)spermidine, T(SH)2), a low molecular weight thiol that is distinctive for this taxonomic family and not present in the mammalian host. Thus, the study of the metabolism of T(SH)2 is interesting as a potential therapeutic target. In this review, we summarize the existing evidence about the metabolism of thiols in Trypanosoma cruzi, focused on those proteins that can be considered the best candidates for selective therapy. Herein, we examine the biosynthetic pathway of T(SH)2, identifying three key points that are susceptible to attack pharmacologically: the activity of the trypanothione reductase (TR), the function of glutamate-cysteine ligase (GCL) and polyamine transport in T. cruzi. TR has been widely studied and is a good example for the development of the medicinal chemistry of antichagasic compounds. Conversely, GCL and the polyamine uptake system are high flow points in the reductive metabolism of the parasite. However, very little is known at the molecular level about these two systems. Therefore, their potential as targets for drug development is discussed, and it is suggested that research should focus on the production of alternative drugs for Chagas' disease treatment.

Selenium (Se) is an essential trace element for several organisms and is present in proteins as selenocysteine (Sec or U), an amino acid that is chemically distinct from serine and cysteine by a single atom (Se instead of O or S, respectively). Sec is incorporated into selenoproteins at an in-frame UGA codon specified by an mRNA stem-loop structure called the selenocysteine incorporating sequence (SECIS) presented in selenoprotein mRNA and specific selenocysteine synthesis and incorporation machinery. Selenoproteins are presented in all domains but are not found in all organisms. Although several functions have been attributed to this class, the majority of the proteins are involved in oxidative stress defense. Here, we discuss the kinetoplastid selenocysteine pathway and how selenium supplementation is able to alter the infection course of trypanosomatids in detail. These organisms possess the canonical elements required for selenoprotein production such as phosphoseryl tRNA kinase (PSTK), selenocysteine synthase (SepSecS), selenophosphase synthase (SelD or SPS), and elongation factor EFSec (SelB), whereas other important factors presented in mammal cells, such as SECIS binding protein 2 (SBP) and SecP 43, are absent. The selenoproteome of trypanosomatids is small, as is the selenoproteome of others parasites, which is in contrast to the large number of selenoproteins found in bacteria, aquatic organisms and higher eukaryotes. Trypanosoma and Leishmania are sensitive to auranofin, a potent selenoprotein inhibitor; however, the probable drug mechanism is not related to selenoproteins in kinetoplastids. Selenium supplementation decreases the parasitemia of various Trypanosome infections and reduces important parameters associated with diseases such as anemia and parasite-induced organ damage. New experiments are necessary to determine how selenium acts, but evidence suggests that immune response modulation and increased host defense against oxidative stress contribute to control of the parasite infection.

Many of the deadliest neglected tropical diseases are caused by protozoan and helminthic parasites. These organisms have evolved several enzymes to exploit their host’s metabolic resources and evade immune responses. Because these essential proteins are absent in humans, they are targets for antiparasitic drug development. Despite decades of investigation, no therapy has been successful in the eradication of these diseases, so new approaches are desired. Chemically stable analogues of the transition states of enzymatic reactions are often potent inhibitors, and several examples of clinically effective compounds are known for other diseases. The design of transition-state analogues is aided by structural models of the transition state, which are obtained by complementing experimental measurement of kinetic isotope effects with theoretical calculations. Such transition-state-guided inhibitor design has been demonstrated for human, bovine, malarial, and trypanosomal enzymes of the purine salvage pathway, including purine nucleoside phosphorylase, nucleoside hydrolases, and adenosine deaminase. Cysteine proteases, trans-sialidase, 1-deoxy-d-xylulose-5-phosphate reductoisomerase, and trypanothione synthetase are presented as additional candidates for application of transition-state analysis with the goal of identifying new leads for the treatment of parasitic neglected tropical diseases.

Helminths that are the causative agents of numerous neglected tropical diseases continue to be a major problem for human global health. In the absence of vaccines, control relies solely on pharmacoprophylaxis and pharmacotherapy to reduce transmission and to relieve symptoms. There are only a few drugs available and resistance in helminths of lifestock has been observed to the same drugs that are also used to treat humans. Clearly there is an urgent need to find novel antiparasitic compounds. Not only are helminths confronted with their own metabolically derived toxic and redox-active byproducts but also with the production of reactive oxygen species (ROS) by the host immune system, adding to the overall oxidative burden of the parasite. Antioxidant enzymes of helminths have been identified as essential proteins, some of them biochemically distinct to their host counterpart and thus appealing drug targets. In this review we have selected a few enzymatic antioxidants of helminths that are thought to be druggable.

Infections caused by the methicillin-resistant Staphylococcus aureus (MRSA) are today a major burden in nosocomial disease control. The global trend shows an alarming increase of MRSA infections as well as multi-drug resistance (MDR). The problem is exacerbated by the fact that infections with community-associated (CA) MRSA strains showing increased virulence and fitness add to infections with multi-drug resistant hospital-associated (HA) MRSA. The toxicity of pathogens and limited effectiveness of available treatment have led to high mortality rates and vast expenses caused by prolonged hospitalization and usage of additional antibiotics. Recently approved drugs still have classical targets and upcoming resistance can be expected. In a new approach by targeting co-factor syntheses of bacteria, the drug target and the affected pathways are uncoupled. This novel strategy is based on the thought of a classical pro-drug which has to be metabolized before becoming toxic for the bacterium as a dysfunctional co-factor, named suicide drug. Ideally these metabolizing pathways are solely present in the bacterium and absent in the human host, such as vitamin biosyntheses. This mini-review discusses current ways of MRSA infection treatment using new approaches including suicide drugs targeting co-factor biosyntheses.