Nitrofuran drugs as common subversive substrates of Trypanosoma cruzi lipoamide dehydrogenase and trypanothione reductase

Targeting Trypanothione Metabolism in Trypanosomatids

Infectious diseases caused by trypanosomatids, including African trypanosomiasis (sleeping sickness), Chagas disease, and different forms of leishmaniasis, are Neglected Tropical Diseases affecting millions of people worldwide, mainly in vulnerable territories of tropical and subtropical areas. In general, current treatments against these diseases are old-fashioned, showing adverse effects and loss of efficacy due to misuse or overuse, thus leading to the emergence of resistance. For these reasons, searching for new antitrypanosomatid drugs has become an urgent necessity, and different metabolic pathways have been studied as potential drug targets against these parasites. Considering that trypanosomatids possess a unique redox pathway based on the trypanothione molecule absent in the mammalian host, the key enzymes involved in trypanothione metabolism, trypanothione reductase and trypanothione synthetase, have been studied in detail as druggable targets. In this review, we summarize some of the recent findings on the molecules inhibiting these two essential enzymes for Trypanosoma and Leishmania viability.

The Existing Drug Nifuroxazide as an Antischistosomal Agent: In Vitro, In Vivo, and In Silico Studies of Macromolecular Targets

Schistosomiasis is a parasitic disease that afflicts approximately 250 million people worldwide. There is an urgent demand for new antiparasitic agents because praziquantel, the only drug available for the treatment of schistosomiasis, is not universally effective and may derail current progress toward the WHO goal of eliminating this disease as a public health problem by 2030. Nifuroxazide (NFZ), an oral nitrofuran antibiotic, has recently been explored to be repurposed for parasitic diseases. Here, in vitro, in vivo, and in silico studies were conducted to evaluate the activity of NFZ on Schistosoma mansoni. The in vitro study showed significant antiparasitic activity, with 50% effective concentration (EC50) and 90% effective concentration (EC90) values of 8.2 to 10.8 and 13.7 to 19.3 μM, respectively. NFZ also affected worm pairing and egg production and induced severe damage to the tegument of schistosomes. In vivo, a single oral dose of NFZ (400 mg/kg of body weight) to mice harboring either prepatent or patent S. mansoni infection significantly reduced the total worm burden (~40%). In patent infection, NFZ achieved a high reduction in the number of eggs (~80%), but the drug caused a low reduction in the egg burden of animals with prepatent infection. Finally, results from in silico target fishing methods predicted that serine/threonine kinases could be one of the potential targets for NFZ in S. mansoni. Overall, the present study revealed that NFZ possesses antischistosomal properties, mainly in terms of egg burden reduction in animals with patent S. mansoni infection. IMPORTANCE The increasing recognition of the burden imposed by helminthiasis, associated with the limited therapeutic arsenal, has led to initiatives and strategies to research and develop new drugs for the treatment of schistosomiasis. One of these strategies is drug repurposing, which considers low-risk compounds with potentially reduced costs and shorter time for development. In this study, nifuroxazide (NFZ) was evaluated for its anti-Schistosoma mansoni potential through in vitro, in vivo, and in silico studies. In vitro, NFZ affected worm pairing and egg production and induced severe damage to the tegument of schistosomes. In vivo, a single oral dose of NFZ (400 mg/kg) to mice harboring either prepatent or patent S. mansoni infection significantly reduced the total worm burden and egg production. In silico investigations have identified serine/threonine kinases as a molecular target for NFZ. Collectively, these results implied that NFZ might be a potential therapeutic candidate for the treatment of schistosomiasis.

Selected Aspects of the Analytical and Pharmaceutical Profiles of Nifurtimox

Nifurtimox is a nitroheterocyclic drug employed for treatment of trypanosomiases (Chagas disease and West African sleeping sickness); its use for certain cancers has also been assessed. Despite having been in the market for over 50 years, knowledge of nifurtimox is still fragmentary and incomplete. Relevant aspects of the chemistry and biology of nifurtimox are reviewed to summarize the current knowledge of this drug. These comprise its chemical synthesis and the preparation of some analogues, as well as its chemical degradation. Selected physical data and physicochemical properties are also listed, along with different approaches toward the analytical characterization of the drug, including electrochemical (polarography, cyclic voltammetry), spectroscopic (ultraviolet-visible, nuclear magnetic resonance, electron spin resonance), and single crystal X-ray diffractometry. The array of polarographic, ultraviolet-visible spectroscopic, and chromatographic methods available for the analytical determination of nifurtimox (in bulk drug, pharmaceutical formulations, and biological samples), are also presented and discussed, along with chiral chromatographic and electrophoretic alternatives for the separation of the enantiomers of the drug. Aspects of the drug likeliness of nifurtimox, its classification in the Biopharmaceutical Classification System, and available pharmaceutical formulations are detailed, whereas pharmacological, chemical, and biological aspects of its metabolism and disposition are discussed.

Efficient Oxidative Dearomatisations of Substituted Phenols Using Hypervalent Iodine (III) Reagents and Antiprotozoal Evaluation of the Resulting Cyclohexadienones against T. b. rhodesiense and P. falciparum Strain NF54

Quinones and quinols are secondary metabolites of higher plants that are associated with many biological activities. The oxidative dearomatization of phenols induced by hypervalent iodine(III) reagents has proven to be a very useful synthetic approach for the preparation of these compounds, which are also widely used in organic synthesis and medicinal chemistry. Starting from several substituted phenols and naphthols, a series of cyclohexadienone and naphthoquinone derivatives were synthesized using different hypervalent iodine(III) reagents and evaluated for their in vitro antiprotozoal activity. Antiprotozoal activity was assessed against Plasmodium falciparum NF54 and Trypanosoma brucei rhodesiense STIB900. Cytotoxicity of all compounds towards L6 cells was evaluated and the respective selectivity indices (SI) were calculated. We found that benzyl naphthoquinone 5c was the most active and selective molecule against T. brucei rhodesiense (IC₅₀ = 0.08 μM, SI = 275). Furthermore, the antiprotozoal assays revealed no specific effects. In addition, some key physicochemical parameters of the synthesised compounds were calculated.

Unique thiol metabolism in trypanosomatids: Redox homeostasis and drug resistance

Trypanosomatids are mainly responsible for heterogeneous parasitic diseases: Leishmaniasis, Sleeping sickness, and Chagas disease and control of these diseases implicates serious challenges due to the emergence of drug resistance. Redox-active biomolecules are the endogenous substances in organisms, which play important role in the regulation of redox homeostasis. The redox-active substances like glutathione, trypanothione, cysteine, cysteine persulfides, etc., and other inorganic intermediates (hydrogen peroxide, nitric oxide) are very useful as defence mechanism. In the present review, the suitability of trypanothione and other essential thiol molecules of trypanosomatids as drug targets are described in Leishmania and Trypanosoma. We have explored the role of tryparedoxin, tryparedoxin peroxidase, ascorbate peroxidase, superoxide dismutase, and glutaredoxins in the anti-oxidant mechanism and drug resistance. Up-regulation of some proteins in trypanothione metabolism helps the parasites in survival against drug pressure (sodium stibogluconate, Amphotericin B, etc.) and oxidative stress. These molecules accept electrons from the reduced trypanothione and donate their electrons to other proteins, and these proteins reduce toxic molecules, neutralize reactive oxygen, or nitrogen species; and help parasites to cope with oxidative stress. Thus, a better understanding of the role of these molecules in drug resistance and redox homeostasis will help to target metabolic pathway proteins to combat Leishmaniasis and trypanosomiases.

Pyridazino-pyrrolo-quinoxalinium salts as highly potent and selective leishmanicidal agents targeting trypanothione reductase

Fifteen pyridazino-pyrrolo-quinoxalinium salts were synthesized and tested for their antiprotozoal activity against Leishmania infantum amastigotes. Eleven of them turned out to be leishmanicidal, with EC₅₀ values in the nanomolar range, and displayed low toxicity against the human THP-1 cell line. Selectivity indices for these compounds range from 10 to more than 1000. Compounds 3b and 3f behave as potent inhibitors of the oxidoreductase activity of the essential enzyme trypanothione disulfide reductase (TryR). Interestingly, binding of 3f is not affected by high trypanothione concentrations, as revealed by the noncompetitive pattern of inhibition observed when tested in the presence of increasing concentrations of this substrate. Furthermore, when analyzed at varying NADPH concentrations, the characteristic pattern of hyperbolic uncompetitive inhibition supports the view that binding of NADPH to TryR is a prerequisite for inhibitor-protein association. Similar to other TryR uncompetitive inhibitors for NADPH, 3f is responsible for TryR-dependent reduction of cytochrome c in a reaction that is typically inhibited by superoxide dismutase.

Single- and Two-Electron Reduction of Nitroaromatic Compounds by Flavoenzymes: Mechanisms and Implications for Cytotoxicity

Nitroaromatic compounds (ArNO₂) maintain their importance in relation to industrial processes, environmental pollution, and pharmaceutical application. The manifestation of toxicity/therapeutic action of nitroaromatics may involve their single- or two-electron reduction performed by various flavoenzymes and/or their physiological redox partners, metalloproteins. The pivotal and still incompletely resolved questions in this area are the identification and characterization of the specific enzymes that are involved in the bioreduction of ArNO₂ and the establishment of their contribution to cytotoxic/therapeutic action of nitroaromatics. This review addresses the following topics: (i) the intrinsic redox properties of ArNO₂, in particular, the energetics of their single- and two-electron reduction in aqueous medium; (ii) the mechanisms and structure-activity relationships of reduction in ArNO₂ by flavoenzymes of different groups, dehydrogenases-electrontransferases (NADPH:cytochrome P-450 reductase, ferredoxin:NADP(H) oxidoreductase and their analogs), mammalian NAD(P)H:quinone oxidoreductase, bacterial nitroreductases, and disulfide reductases of different origin (glutathione, trypanothione, and thioredoxin reductases, lipoamide dehydrogenase), and (iii) the relationships between the enzymatic reactivity of compounds and their activity in mammalian cells, bacteria, and parasites.

Plasmodium falciparum Ferredoxin-NADP+ Reductase-Catalyzed Redox Cycling of Plasmodione Generates Both Predicted Key Drug Metabolites: Implication for Antimalarial Drug Development

Plasmodione (PD) is a potent antimalarial redox-active 3-benzyl-menadione acting at low nanomolar range concentrations on different malaria parasite stages. The specific bioactivation of PD was proposed to occur via a cascade of redox reactions starting from one-electron reduction and then benzylic oxidation, leading to the generation of several key metabolites including corresponding benzylic alcohol (PD-bzol, for PD benzhydrol) and 3-benzoylmenadione (PDO, for PD oxide). In this study, we showed that the benzylic oxidation of PD is closely related to the formation of a benzylic semiquinone radical, which can be produced under two conditions: UV photoirradiation or catalysis by Plasmodium falciparum apicoplast ferredoxin-NADP⁺ reductase (PfFNR) redox cycling in the presence of oxygen and the parent PD. Electrochemical properties of both PD metabolites were investigated in DMSO and in water. The single-electron reduction potential values of PD, PD-bzol, PDO, and a series of 3-benzoylmenadiones were determined according to ascorbate oxidation kinetics. These compounds possess enhanced reactivity toward PfFNR as compared with model quinones. Optimal conditions were set up to obtain the best conversion of the starting PD to the corresponding metabolites. UV irradiation of PD in isopropanol under positive oxygen pressure led to an isolated yield of 31% PDO through the transient semiquinone species formed in a cascade of reactions. In the presence of PfFNR, PDO and PD-bzol could be observed during long lasting redox cycling of PD continuously fueled by NADPH regenerated by an enzymatic system. Finally, we observed and quantified the effect of PD on the production of oxidative stress in the apicoplast of transgenic 3D7^{[Api-roGFP2-hGrx1]} P. falciparum parasites by using the described genetically encoded glutathione redox sensor hGrx1-roGFP2 methodology. The observed fast reactive oxygen species (ROS) pulse released in the apicoplast is proposed to be mediated by PD redox cycling catalyzed by PfFNR.

Unraveling the antitrypanosomal mechanism of benznidazole and related 2-nitroimidazoles: From prodrug activation to DNA damage

Nitroheterocycles represent an important class of compound used to treat trypanosomiasis. They often function as prodrugs and can undergo type I nitroreductase (NTR1)-mediated activation before promoting their antiparasitic activities although the nature of these downstream effects has yet to be determined. Here, we show that in an NTR1-dependent process, benznidazole promotes DNA damage in the nuclear genome of Trypanosoma brucei, providing the first direct link between activation of this prodrug and a downstream trypanocidal mechanism. Phenotypic and protein expression studies revealed that components of the trypanosome's homologous recombination (HR) repair pathway (TbMRE11, γH2A, TbRAD51) cooperate to resolve the benznidazole-induced damage, indicating that the prodrug-induced lesions are most likely double stand DNA breaks, while the sequence/recruitment kinetics of these factors parallels that in other eukaryotes HR systems. When extended to other NTR1-activated 2-nitroimidazoles, some were shown to promote DNA damage. Intriguingly, the lesions induced by these required TbMRE11 and TbCSB activities to fix leading us to postulate that TbCSB may operate in systems other than the transcription-coupled nucleotide excision repair pathway. Understanding how existing trypanosomal drugs work will aid future drug design and help unlock novel reactions/pathways that could be exploited as targets for therapeutic intervention.

Identification of a Protective Leishmania Antigen Dihydrolipoyl Dehydrogenase and Its Responding CD4+ T Cells at Clonal Level

There is currently no clinically effective vaccine against cutaneous leishmaniasis because of poor understanding of the Ags that elicit protective CD4⁺ T cell immunity. In this study, we identified a naturally processed peptide (DLD_63-79) that is derived from Leishmania dihydrolipoyl dehydrogenase (DLD) protein. DLD is conserved in all pathogenic Leishmania species, is expressed by both the promastigote and amastigote stages of the parasite, and elicits strong CD4⁺ T cell responses in mice infected with L. major We generated I-A^b-DLD_63-79 tetramer and identified DLD-specific CD4⁺ T cells at clonal level. Following L. major infection, DLD_63-79-specific CD4⁺ T cells massively expanded and produced effector cytokines (IFN-γ and TNF). This was followed by a gradual contraction, stable maintenance following lesion resolution, and display of memory (recall) response following secondary challenge. Vaccination with rDLD protein induced strong protection in mice against virulent L. major challenge. Identification of Ags that elicit protective immunity and their responding Ag-specific T cells are critical steps necessary for developing effective vaccines and vaccination strategies against infectious agents, including protozoan parasites.

Design, synthesis and antitrypanosomal activity of heteroaryl-based 1,2,4-triazole and 1,3,4-oxadiazole derivatives

Two series of novel 1,2,4-triazol-3-yl-thioacetamide 3a-b and 4a-b and 5-pyrazin-2-yl-3H-[1,3,4]oxadiazole-2-thiones 9a-h were designed and synthesized. The compounds prepared have been identified using ¹H NMR, ¹³C NMR and elemental analyses. The synthesized compounds 3a, 3b, 4a, 4b, 9a, 9b, 9d-e and 9f have been evaluated with α-difluoromethylornithine (DFMO) as a control drug for their in vitro antitrypanosomal activity against Trypanosoma brucei. Results showed that 3b was the most active compound in general and also more potent than control DFMO. 3b was 8 folds more potent than the reference with IC₅₀ of 0.79 μM and IC₉₀ of 1.35 μM, respectively compared to DFMO (IC₅₀ = 6.10 μM and IC₉₀ of 8.66 μM). The tested compounds showed moderate cytotoxicity with selectivity indices ranging from 12 (9d) to 102 (3b) against L6 cells. Docking study was performed into ten of T. brucei enzymes which have been identified as potential/valid targets for most of the antitrypanosomal agents. The results of the docking study revealed high binding scores toward many of the selected enzymes. A good correlation was observed only between log (IC₅₀) of antitrypanosomal activity of the new compounds and their calculated Ki values against TryR enzyme (R² = 0.726). Compound 3b, the most active as antitrypanosomal agents exhibited similar binding orientation and interaction to those of WP6 against TryR enzyme. However, in a next round of work, a complementary studies will be carried out to clarify the mechanism of action of these compounds.

Syntheses and Biological Activities of triazole-based Sulfonamides

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The triazole and sulfonamide compounds are known as biologically active agents that were employed for medicinal applications. These compounds were obtained in different forms by a variety of techniques to vast ranges of applications. The broad biological properties of these compounds have encouraged researchers to design and synthesize triazole-based sulfonamide derivatives as compounds with potential biological activity. In this review, we summarized the synthetic procedures of triazole-based sulfonamide compounds together with their biological activities during the last two decades.

Current and Future Prospects of Nitro-compounds as Drugs for Trypanosomiasis and Leishmaniasis

Interest in nitroheterocyclic drugs for the treatment of infectious diseases has undergone a resurgence in recent years. Here we review the current status of monocyclic and bicyclic nitroheterocyclic compounds as existing or potential new treatments for visceral leishmaniasis, Chagas' disease and human African trypanosomiasis. Both monocyclic (nifurtimox, benznidazole and fexinidazole) and bicyclic (pretomanid (PA-824) and delamanid (OPC-67683)) nitro-compounds are prodrugs, requiring enzymatic activation to exert their parasite toxicity. Current understanding of the nitroreductases involved in activation and possible mechanisms by which parasites develop resistance is discussed along with a description of the pharmacokinetic / pharmacodynamic behaviour and chemical structure-activity relationships of drugs and experimental compounds.

Investigating the mode of action of the redox-active antimalarial drug plasmodione using the yeast model

Malaria is caused by protozoan parasites and remains a major public health issue in subtropical areas. Plasmodione (3-[4-(trifluoromethyl)benzyl]-menadione) is a novel early lead compound displaying fast-acting antimalarial activity. Treatment with this redox active compound disrupts the redox balance of parasite-infected red blood cells. In vitro, the benzoyl analogue of plasmodione can act as a subversive substrate of the parasite flavoprotein NADPH-dependent glutathione reductase, initiating a redox cycling process producing ROS. Whether this is also true in vivo remains to be investigated. Here, we used the yeast model to investigate the mode of action of plasmodione and uncover enzymes and pathways involved in its activity. We showed that plasmodione is a potent inhibitor of yeast respiratory growth, that in drug-treated cells, the ROS-sensitive aconitase was impaired and that cells with a lower oxidative stress defence were highly sensitive to the drug, indicating that plasmodione may act via an oxidative stress. We found that the mitochondrial respiratory chain flavoprotein NADH-dehydrogenases play a key role in plasmodione activity. Plasmodione and metabolites act as substrates of these enzymes, the reaction resulting in ROS production. This in turn would damage ROS-sensitive enzymes leading to growth arrest. Our data further suggest that plasmodione is a pro-drug whose activity is mainly mediated by its benzhydrol and benzoyl metabolites. Our results in yeast are coherent with existing data obtained in vitro and in Plasmodium falciparum, and provide additional hypotheses that should be investigated in parasites.

Dihydrolipoamide dehydrogenase from Leishmania donovani: New insights through biochemical characterization

Dihydrolipoamide dehydrogenase (DLDH) regulates many crucial metabolic pathways as a multi-enzyme complex. Leishmania donovani dihydrolipoamide dehydrogenase (LdDLDH) has two variants present on two different chromosomes with very less sequence similarities. In the current study, we cloned both the variants in pET28a (+) vector and expressed in Rosetta-gami (DE3) E. coli strain. Expressed proteins were finally purified from pellets using Ni-NTA affinity chromatography. Purified enzymes were biochemically characterized and different kinetic parameters were studied. Both the variants showed maximum activity in pH range of 7.0-8.0 and temperature 50±5°C in the physiological direction. The estimated K_m for dihydrolipoamide (DLA) and NAD⁺ were 2.7±0.48mM and 171.23±11.59μM respectively for variant 1 (LdBPK291950.1). In the case of variant 2 (LdBPK323510.1), K_m values for DLA and NAD⁺ were found to be 829.85±37μM and 226±1.56μM respectively. The variant 2 was more efficient in terms of activity. While both the forms of the enzymes showed diaphorase activity, variant 1 was found to be better. Sequence dissimilarities of both forms were analyzed for biological insights.

A Redox-Active Fluorescent pH Indicator for Detecting Plasmodium falciparum Strains with Reduced Responsiveness to Quinoline Antimalarial Drugs

Mutational changes in the Plasmodium falciparum chloroquine resistance transporter (PfCRT) have been associated with differential responses to a wide spectrum of biologically active compounds including current and former quinoline and quinoline-like antimalarial drugs. PfCRT confers altered drug responsiveness by acting as a transport system, expelling drugs from the parasite's digestive vacuole where these drugs exert, at least part of, their antiplasmodial activity. To preserve the efficacy of these invaluable drugs, novel functional tools are required for epidemiological surveys of parasite strains carrying mutant PfCRT variants and for drug development programs aimed at inhibiting or circumventing the action of PfCRT. Here we report the synthesis and characterization of a pH-sensitive fluorescent chloroquine analogue consisting of 7-chloro-N-{2-[(propan-2-yl)amino]ethyl}quinolin-4-amine functionalized with the fluorochrome 7-nitrobenzofurazan (NBD) (henceforth termed Fluo-CQ). In the parasite, Fluo-CQ accumulates in the digestive vacuole, giving rise to a strong fluorescence signal but only in parasites carrying the wild type PfCRT. In parasites carrying the mutant PfCRT, Fluo-CQ does not accumulate. The differential handling of the fluorescent probe, combined with live cell imaging, provides a diagnostic tool for quick detection of those P. falciparum strains that carry a PfCRT variant associated with altered responsiveness to quinoline and quinoline-like antimalarial drugs. In contrast to the accumulation studies, chloroquine (CQ)-resistant parasites were observed cross-resistant to Fluo-CQ when the chemical probe was tested in various CQ-sensitive and -resistant parasite strains. NBD derivatives were found to act as redox cyclers of two essential targets, using a coupled assay based on methemoglobin and the NADPH-dependent glutathione reductase (GRs) from P. falciparum. This redox activity is proposed to contribute to the dual action of Fluo-CQ on redox equilibrium and methemoglobin reduction via PfCRT-mediated drug efflux in the cytosol and then continuous redox-dependent shuttling between food vacuole and cytosol. Taking into account these physicochemical characteristics, a model was proposed to explain Fluo-CQ antimalarial effects involving the contribution of PfCRT-mediated transport, methemoglobin reduction, hematin binding, and NBD reduction activity catalyzed by PfGR in CQ-resistant versus CQ-sensitive parasites. Therefore, introduction of NBD fluorophore in drugs is not inert and should be taken into account in drug transport and imaging studies.

Putative Role of the Aldo-Keto Reductase from Trypanosoma cruzi in Benznidazole Metabolism

Benznidazole (Bz), the drug used for treatment of Chagas' disease (caused by the protozoan Trypanosoma cruzi), is activated by a parasitic NADH-dependent type I nitroreductase (NTR I). However, several studies have shown that other enzymes are involved. The aim of this study was to evaluate whether the aldo-keto reductase from T. cruzi (TcAKR), a NADPH-dependent oxido-reductase previously described by our group, uses Bz as the substrate. We demonstrated that both recombinant and native TcAKR enzymes reduce Bz by using NADPH, but not NADH, as a cofactor. TcAKR-overexpressing epimastigotes showed higher NADPH-dependent Bz reductase activity and a 50% inhibitory concentration (IC50) value for Bz 1.8-fold higher than that of the controls, suggesting that TcAKR is involved in Bz detoxification instead of activation. To understand the role of TcAKR in Bz metabolism, we studied TcAKR expression and NADPH/NADH-dependent Bz reductase activities in two T. cruzi strains with differential susceptibility to Bz: CL Brener and Nicaragua. Taking into account the results obtained with TcAKR-overexpressing epimastigotes, we expected the more resistant strain, Nicaragua, to have higher TcAKR levels than CL Brener. However, the results were the opposite. CL Brener showed 2-fold higher TcAKR expression and 5.7-fold higher NADPH-Bz reduction than the Nicaragua strain. In addition, NADH-dependent Bz reductase activity, characteristic of NTR I, was also higher in CL Brener than in Nicaragua. We conclude that although TcAKR uses Bz as the substrate, TcAKR activity is not a determinant of Bz resistance in wild-type strains and may be overcome by other enzymes involved in Bz activation, such as NADPH- and NADH-dependent reductases.

Flavins and flavoproteins: applications in medicine

The potential of flavoproteins as targets of pharmacological treatments is immense. In this review we present an overview of the current research progress on medical interventions based on flavoproteins with a special emphasis on cancer, infectious diseases, and neurological disorders.

Indazoles: a new top seed structure in the search of efficient drugs against Trypanosoma cruzi

For years, Chagas disease treatment has been limited to only two drugs of highly questionable and controversial use (Nifurtimox® and Benznidazole®). In the search of effective drugs, many efforts have been made, but only a few structures have emerged as actual candidates. Heading into this, the multitarget-directed approach appears as the best choice. In this framework, indazoles were shown to be potent Trypanosoma cruzi growth inhibitors, being able to lead both the formation of reactive oxygen species and the inhibition of trypanothione reductase. Herein, we discuss the main structural factors that rule the anti-T. cruzi properties of indazoles, and how they would be involved in the biological properties as well as in the action mechanisms, attempting to make parallels between the old paradigms and current evidences in order to outline what could be the next steps to follow in regard to the future drug design for Chagas disease treatment.

The antimalarial activities of methylene blue and the 1,4-naphthoquinone 3-[4-(trifluoromethyl)benzyl]-menadione are not due to inhibition of the mitochondrial electron transport chain

Methylene blue and a series of recently developed 1,4-naphthoquinones, including 3-[4-(substituted)benzyl]-menadiones, are potent antimalarial agents in vitro and in vivo. The activity of these structurally diverse compounds against the human malaria parasite Plasmodium falciparum might involve their peculiar redox properties. According to the current theory, redox-active methylene blue and 3-[4-(trifluoromethyl)benzyl]-menadione are "subversive substrates." These agents are thought to shuttle electrons from reduced flavoproteins to acceptors such as hemoglobin-associated or free Fe(III)-protoporphyrin IX. The reduction of Fe(III)-protoporphyrin IX could subsequently prevent essential hemoglobin digestion and heme detoxification in the parasite. Alternatively, owing to their structures and redox properties, methylene blue and 1,4-naphthoquinones might also affect the mitochondrial electron transport chain. Here, we tested the latter hypothesis using an established system of transgenic P. falciparum cell lines and the antimalarial agents atovaquone and chloroquine as controls. In contrast to atovaquone, methylene blue and 3-[4-(trifluoromethyl)benzyl]-menadione do not inhibit the mitochondrial electron transport chain. A systematic comparison of the morphologies of drug-treated parasites furthermore suggests that the three drugs do not share a mechanism of action. Our findings support the idea that methylene blue and 3-[4-(trifluoromethyl)benzyl]-menadione exert their antimalarial activity as redox-active subversive substrates.

Evaluating 5-nitrofurans as trypanocidal agents

The nitroheterocycle nifurtimox, as part of a nifurtimox-eflornithine combination therapy, represents one of a limited number of treatments targeting Trypanosoma brucei, the causative agent of human African trypanosomiasis. The mode of action of this prodrug involves an initial activation reaction catalyzed by a type I nitroreductase (NTR), an enzyme found predominantly in prokaryotes, leading to the formation of a cytotoxic unsaturated open-chain nitrile metabolite. Here, we evaluate the trypanocidal activities of a library of other 5-nitrofurans against the bloodstream form of T. brucei as a preliminary step in the identification of additional nitroaromatic compounds that can potentially partner with eflornithine. Biochemical screening against the purified enzyme revealed that all 5-nitrofurans were effective substrates for T. brucei NTR (TbNTR), with the preferred compounds having apparent kcat/Km values approximately 50-fold greater than those of nifurtimox. For several compounds, in vitro reduction by this nitroreductase yielded products characterized by mass spectrometry as either unsaturated or saturated open-chain nitriles. When tested against the bloodstream form of T. brucei, many of the derivatives displayed significant growth-inhibitory properties, with the most potent compounds generating 50% inhibitory concentrations (IC50s) around 200 nM. The antiparasitic activities of the most potent agents were demonstrated to be NTR dependent, as parasites having reduced levels of the enzyme displayed resistance to the compounds, while parasites overexpressing TbNTR showed hypersensitivity. We conclude that other members of the 5-nitrofuran class of nitroheterocycles have the potential to treat human African trypanosomiasis, perhaps as an alternative partner prodrug to nifurtimox, in the next generation of eflornithine-based combinational therapies.

Quantitative proteomics of Trypanosoma cruzi during metacyclogenesis

Trypanosoma cruzi is the etiologic agent of Chagas disease, which is estimated to affect over eight million people around the world. Trypanosoma cruzi has a complex life cycle, involving insect and mammalian hosts and four distinct developmental stages: epimastigotes, metacyclic trypomastigotes, amastigotes, and bloodstream trypomastigotes. Metacyclogenesis is the process by which T. cruzi epimastigotes differentiate into metacyclic trypomastigotes and acquire infectivity, and involves differential gene expression associated with acquisition of virulence. In T. cruzi, gene expression regulation is achieved mainly posttranscriptionally. Therefore, proteomics-based approaches are extremely useful for gaining a better understanding of the changes that occur in the stage-regulated gene expression program of the parasite at the molecular level. Here, we performed an in-depth quantitative MS-based proteomic study of T. cruzi metacyclogenesis and quantified almost 3000 proteins expressed during the process of differentiation. To the best of our knowledge, this work is the most comprehensive quantitative proteomics study of different cell populations of T. cruzi available so far. We identified relevant proteins and pathways involved in the parasite's differentiation and infectivity acquisition, opening new perspectives for further studies that could, ultimately, lead to the identification of new targets for chemotherapy.

Synthesis and biological evaluation of 1,4-naphthoquinones and quinoline-5,8-diones as antimalarial and schistosomicidal agents

Improving the solubility of polysubstituted 1,4-naphthoquinone derivatives was achieved by introducing nitrogen in two different positions of the naphthoquinone core, at C-5 and at C-8 of menadione through a two-step, straightforward synthesis based on the regioselective hetero-Diels-Alder reaction. The antimalarial and the antischistosomal activities of these polysubstituted aza-1,4-naphthoquinone derivatives were evaluated and led to the selection of distinct compounds for antimalarial versus antischistosomal action. The Ag(II)-assisted oxidative radical decarboxylation of the phenyl acetic acids using AgNO(3) and ammonium peroxodisulfate was modified to generate the 3-picolinyl-menadione with improved pharmacokinetic parameters, high antimalarial effects and capacity to inhibit the formation of β-hematin.

Novel 3-nitro-1H-1,2,4-triazole-based amides and sulfonamides as potential antitrypanosomal agents

A series of novel 3-nitro-1H-1,2,4-triazole-based (and in some cases 2-nitro-1H-imidazole-based) amides and sulfonamides were characterized for their in vitro antitrypanosomal and antileishmanial activities as well as mammalian toxicity. Out of 36 compounds tested, 29 (mostly 3-nitro-1H-1,2,4-triazoles) displayed significant activity against Trypanosoma cruzi intracellular amastigotes (IC(50) ranging from 28 nM to 3.72 μM) without concomitant toxicity to L6 host cells (selectivity 66-2782). Twenty-three of these active compounds were more potent (up to 58-fold) than the reference drug benznidazole, tested in parallel. In addition, nine nitrotriazoles which were moderately active (0.5 μM ≤ IC(50) < 6.0 μM) against Trypanosoma brucei rhodesiense trypomastigotes were 5-31-fold more active against bloodstream-form Trypanosoma brucei brucei trypomastigotes engineered to overexpress reduced nicotinamide adenine dinucleotide dependent nitroreductase. Finally, three nitrotriazoles displayed a moderate activity against the axenic form of Leishmania donovani . Therefore, 3-nitro-1H-1,2,4-triazole-based amides and sulfonamides are potent antitrypanosomal agents.

Caco-2 cells cytotoxicity of nifuroxazide derivatives with potential activity against Methicillin-resistant Staphylococcus aureus (MRSA)

It is important to determine the toxicity of compounds and co-solvents that are used in cell monolayer permeability studies to increase confidence in the results obtained from these in vitro experiments. This study was designed to evaluate the cytotoxicity of new nifuroxazide derivatives with potential activity against Methicillin-resistant Staphylococcus aureus (MRSA) in Caco-2 cells to select analogues for further in vitro permeability analyses. In this study, nitrofurantoin and nifuroxazide, in addition to 6 furanic and 6 thiophenic nifuroxazide derivatives were tested at 2, 4, 6, 8 and 10 μg/mL. In vitro cytotoxicity assays were performed according to the MTT (methyl tetrazolium) assay protocol described in ISO 10993-5. The viability of treated Caco-2 cells was greater than 83% for all tested nitrofurantoin concentrations, while those treated with nifuroxazide at 2, 4 and 6 μg/mL had viabilities greater than 70%. Treatment with the nifuroxazide analogues resulted in viability values greater than 70% at 2 and 4 μg/mL with the exception of the thiophenic methyl-substituted derivative, which resulted in cell viabilities below 70% at all tested concentrations. Caco-2 cells demonstrated reasonable viability for all nifuroxazide derivatives, except the thiophenic methyl-substituted compound. The former were selected for further permeability studies using Caco-2 cells.

The bacterial redox signaller pyocyanin as an antiplasmodial agent: comparisons with its thioanalog methylene blue

The quorum sensor and signalling molecule pyocyanin (PYO) contributes significantly to the pathophysiology of Pseudomonas aeruginosa infections. Comparison to phenothiazine drugs suggests that the antimalarial compound methylene blue (MB) can be regarded as a sulfur analog of PYO. This working hypothesis would explain why the synthetic drug MB behaves as a compound shaped in biological evolution. Here we report on redox-associated biological and biochemical properties of PYO in direct comparison to its synthetic analog MB. We quantitatively describe the reactivity of both compounds toward cellular reductants, the reactivity of their reduced leuco-forms towards O2, and their interactions with FAD-containing disulfide reductases. Furthermore, the interaction of PYO with human glutathione reductase was studied in structural detail by x-ray crystallography, showing that a single PYO molecule binds to the intersubunit cavity of the enzyme. Like MB, also PYO was also found to be active against blood schizonts of the malaria parasite P. falciparum in vitro. Furthermore, both compounds were active against the disease transmitting gametocyte forms of the parasites, which was systematically studied in vitro. As shown for mice, PYO is too toxic to be used as a drug. It may, however, have antimalarial activity in numerous human patients with concomitant Pseudomonas infections. MB, in contrast to PYO, is well tolerated and represents a promising agent for MB-based combination therapies against malaria. Current and future clinical studies can be guided by the comparisons between MB and PYO reported here. Additionally, it is of interest to study if and to what extent the protection from malaria in patients with cystic fibrosis or with severe wound infections is based on PYO produced by Pseudomonas species.

Novel 3-nitro-1H-1,2,4-triazole-based aliphatic and aromatic amines as anti-chagasic agents

A series of novel 2-nitro-1H-imidazole- and 3-nitro-1H-1,2,4-triazole-based aromatic and aliphatic amines were screened for antitrypanosomal activity and mammalian cytotoxicity by the Drugs for Neglected Diseases initiative (DNDi). Out of 42 compounds tested, 18 3-nitro-1,2,4-triazoles and one 2-nitroimidazole displayed significant growth inhibitory properties against T. cruzi amastigotes (IC(50) ranging from 40 nM to 1.97 μM), without concomitant toxicity toward the host cells (L6 cells), having selectivity indices (SI) 44-1320. Most (16) of these active compounds were up to 33.8-fold more potent than the reference drug benznidazole, tested in parallel. Five novel 3-nitro-1,2,4-triazoles were active against bloodstream-form (BSF) T. b. rhodesiense trypomastigotes (IC(50) at nM levels and SI 220-993). An NADH-dependent nitroreductase (TbNTR) plays a role in the antiparasitic activity because BSF T. b. brucei trypomastigotes with elevated TbNTR levels were hypersensitive to tested compounds. Therefore, a novel class of affordable 3-nitro-1,2,4-triazole-based compounds with antitrypanosomal activity has been identified.

Leishmania-macrophage interactions: insights into the redox biology

Leishmaniasis is a neglected tropical disease that affects about 350 million individuals worldwide. The protozoan parasite has a relatively simple life cycle with two principal stages: the flagellated mobile promastigote living in the gut of the sandfly vector and the intracellular amastigote within phagolysosomal vesicles of the vertebrate host macrophage. This review presents a state-of-the-art overview of the redox biology at the parasite-macrophage interface. Although Leishmania species are susceptible in vitro to exogenous superoxide radical, hydrogen peroxide, nitric oxide, and peroxynitrite, they manage to survive the endogenous oxidative burst during phagocytosis and the subsequent elevated nitric oxide production in the macrophage. The parasite adopts various defense mechanisms to cope with oxidative stress: the lipophosphoglycan membrane decreases superoxide radical production by inhibiting NADPH oxidase assembly and the parasite also protects itself by expressing antioxidant enzymes and proteins. Some of these enzymes could be considered potential drug targets because they are not expressed in mammals. In respect to antileishmanial therapy, the effects of current drugs on parasite-macrophage redox biology and its involvement in the development of drug resistance and treatment failure are presented.

Nifurtimox activation by trypanosomal type I nitroreductases generates cytotoxic nitrile metabolites

The prodrug nifurtimox has been used for more than 40 years to treat Chagas disease and forms part of a recently approved combinational therapy that targets West African trypanosomiasis. Despite this, its mode of action is poorly understood. Detection of reactive oxygen and nitrogen intermediates in nifurtimox-treated extracts led to the proposal that this drug induces oxidative stress in the target cell. Here, we outline an alternative mechanism involving reductive activation by a eukaryotic type I nitroreductase. Several enzymes proposed to metabolize nifurtimox, including prostaglandin F2α synthase and cytochrome P450 reductase, were overexpressed in bloodstream-form Trypanosoma brucei. Only cells with elevated levels of the nitroreductase displayed altered susceptibility to this nitrofuran, implying a key role in drug action. Reduction of nifurtimox by this enzyme was shown to be insensitive to oxygen and yields a product characterized by LC/MS as an unsaturated open-chain nitrile. This metabolite was shown to inhibit both parasite and mammalian cell growth at equivalent concentrations, in marked contrast to the parental prodrug. These experiments indicate that the basis for the selectivity of nifurtimox against T. brucei lies in the expression of a parasite-encoded type I nitroreductase.

Identification, cloning and characterization of an aldo-keto reductase from Trypanosoma cruzi with quinone oxido-reductase activity

Drugs currently used for treatment of Trypanosoma cruzi infection, the ethiological agent of Chagas' disease, have shown side effects and variable efficiency. With the aim to describe parasite enzymes involved in the mechanisms of action of trypanocidal drugs and since it has been reported that reductases are crucial in their metabolism, we attempted to identify novel NADPH-dependent oxido-reductases from T. cruzi. The percolation of a soluble fraction of epimastigote lysates through a Cibacron Blue-Sepharose column followed by elution by NADPH yielded a predominant protein with an apparent molecular weight of 32 kDa. This protein was identified by MALDI-TOF as an aldo-keto reductase (AKR) and hence denominated TcAKR. TcAKR was mainly localized in the cytosol and was also present in trypomastigote and amastigote lysates. The recombinant TcAKR (recTcAKR) showed NADPH-dependent reductase activity with the AKR substrates 4-nitrobenzaldehyde and 2-dihydroxyacetone. The saturation curves for both substrates were consistent with the Michaelis-Menten model. We also tested whether recTcAKR may reduce naphthoquinones (NQ), since many of these compounds have displayed important trypanocidal activity. recTcAKR reduced o-NQ (1,2-naphthoquinone, 9,10-phenanthrenquinone and beta-lapachone) with concomitant generation of free radicals but did not exhibit affinity for p-NQ (5-hydroxy-1,4-naphthoquinone, 2-hydroxy-1,4-naphthoquinone, alpha-lapachone and menadione). The substrate saturation curve with o-NQ fitted to a sigmoidal curve, suggesting that recTcAKR presents a cooperative behavior. In addition, three peaks assigned to monomers, dimers and tetramers were obtained when recTcAKR was submitted to a Superose 12 gel chromatography column. TcAKR is the first member of the AKR family described in T. cruzi. Our results indicate that this enzyme may participate in the mechanisms of action of trypanocidal drugs.

ESR and electrochemical study of 1,2-disubstituted 5-nitroindazolin-3-ones and 2-substituted 3-alkoxy-5-nitro-2H-indazoles: reactivity and free radical production capacity in the presence of biological systems

Two families of 5-nitroindazole derivatives were electrochemically studied in an aprotic solvent using cyclic voltammetry (CV) technique. The produced nitro-anion radical species were characterized using electron spin resonance spectroscopy (ESR). Also, we examined the interaction between the radical species generated from nitroindazole derivatives and glutathione (GSH). Moreover, the capacity of intraparasite and intramammals-free radical production, through ESR spectroscopy, was performed.

Trypanocidal drugs: mechanisms, resistance and new targets

The protozoan parasitesTrypanosoma bruceiandTrypanosoma cruziare the causative agents of African trypanosomiasis and Chagas disease, respectively. These are debilitating infections that exert a considerable health burden on some of the poorest people on the planet. Treatment of trypanosome infections is dependent on a small number of drugs that have limited efficacy and can cause severe side effects. Here, we review the properties of these drugs and describe new findings on their modes of action and the mechanisms by which resistance can arise. We further outline how a greater understanding of parasite biology is being exploited in the search for novel chemotherapeutic agents. This effort is being facilitated by new research networks that involve academic and biotechnology/pharmaceutical organisations, supported by public–private partnerships, and are bringing a new dynamism and purpose to the search for trypanocidal agents.

2,3-diphenyl-1,4-naphthoquinone: a potential chemotherapeutic agent against Trypanosoma cruzi

Chagas disease, caused by Trypanosoma cruzi, is a widespread infection in Latin America. Currently, only 2 partially effective and highly toxic drugs, i.e., benznidazole and nifurtimox, are available for the treatment of this disease, and several efforts are underway in the search for better chemotherapeutic agents. Here, we have determined the trypanocidal activity of 2,3-diphenyl-1 ,4-naphthoquinone (DPNQ), a novel quinone derivative. In vitro, DPNQ was highly cytotoxic at a low, micromolar concentration (LD50 = 2.5 microM) against epimastigote, cell-derived trypomastigote, and intracellular amastigote forms of T. cruzi, but not against mammalian cells (LD50 = 130 microM). In vivo studies on the murine model of Chagas disease revealed that DPNQ-treated animals (3 doses of 10 mg/kg/day) showed a significant delay in parasitemia peak and higher (up to 60%) survival rate 70 days post-infection, when compared with the control group (infected, untreated). We also observed a 2-fold decrease in parasitemia between the control group (infected, untreated) and the treated group (infected, treated). No apparent drug toxicity effects were noticed in the control group (uninfected, treated). In addition, we determined that DPNQ is the first competitive inhibitor of T. cruzi lipoamide dehydrogenase (TcLipDH) thus far described. Our results indicate that DPNQ is a promising chemotherapeutic agent against T. cruzi.

A philosophy of anti-infectives as a guide in the search for new drugs for tuberculosis

How we develop antibiotics is shaped by how we view infectious disease. Given the urgent need for new chemotherapeutics for tuberculosis and other infectious diseases, it is timely to reconsider a view of infectious disease that is strongly supported by contemporary evidence but that has rarely been applied in antibiotic development. This view recognizes the importance of nonreplicating bacteria in persistent infections, acknowledges the heterogeneity and stringency of chemical environments encountered by the pathogen in the host, and emphasizes metabolic adaptation of the host and the pathogen during their competition. For example, efforts in our lab are guided by the perspective that Mycobacterium tuberculosis (Mtb) has co-evolved with the human immune response, with the result that Mtb turns host-imposed metabolic adversity to its own advantage. We seek chemotherapeutics that turn Mtb's adversity to the host's advantage.

Redox-active dinitrodiphenylthioethers against Leishmania: synthesis, structure-activity relationships and mechanism of action studies

BTB 06237 (2-[(2,4-dichloro-5-methylphenyl)sulfanyl]-1,3-dinitro-5-(trifluoromethyl) benzene), a compound previously identified through QSAR pharmacophore development and a virtual screen of the Maybridge database, possesses potent and selective activity against Leishmania parasites. In the present study, several analogs of BTB 06237 were synthesized and analyzed for activity against Leishmania axenic amastigotes, their ability to reduce the level of parasitemia in peritoneal macrophages, and their ability to generate reactive oxygen species (ROS) in L. donovani promastigotes. It was found that an aromatic ring must be present in the position occupied by the 2,4-dichloro-5-methylphenyl group in the lead compound, but changing the functional groups generally has little effect on the antileishmanial potency. Alterations to the 1,3-dinitro-5-(trifluoromethyl)benzene ring have more influence on antiparasitic activity with two aromatic nitro groups and a third electron-withdrawing group being required. This structural requirement corresponds with redox potential, the ability to generate ROS in the parasites, and dissipation of the mitochondrial membrane potential. Finally, we used this collection of data to design a new antileishmanial compound with strong activity in vitro and improved properties as an antileishmanial candidate.

Corroborative cobalt and zinc model compounds of alpha-amino-beta-carboxymuconic-epsilon-semialdehyde decarboxylase (ACMSD)

We have synthesised and characterised a series of new Co(II) complexes (1-4, 6, 7) and one new Zn(II) complex (5) employing N(3)- and N(3)O-donor ligands [biap: N,N-bis(2-ethyl-5-methyl-imidazol-4-ylmethyl)amino-propane, KBPZG: potassium N,N-bis(3,5-dimethylpyrazolylmethyl) glycinate, KBPZA: potassium N,N-bis(3,5-dimethylpyrazolylmethyl) alaninate, KB(i)PrPZG: potassium N,N-bis(3,5-di-iso-propylpyrazolylmethyl) glycinate, and KB((t)BuM)PZG: potassium N,N-bis(3-methyl-5-tert-butyl-pyrazolylmethyl)glycinate] as structural models of the metalloenzyme alpha-amino-beta-carboxymuconic-epsilon-semialdehyde decarboxylase (ACMSD). These complexes were characterised by several techniques including X-ray crystallographic analysis, X-band EPR, and mass spectrometry (ESI-MS). The crystal structures of 1, 2, 6,7 revealed that they exist as mononuclear Co(II) complexes with trigonal-bipyramidal geometry in the solid state. Compounds 3 and 5 form infinite polymeric chains of Co(II) or Zn(II) complexes, respectively, linked by the pendant carboxylate arms of the BPZG(-) ligand. By comparing the degree of distortion in the penta-coordinate complexes, defined by the Addison-parameter tau, with the value determined for the five-coordinate centres found in the active site of ACMSD, it could be seen that complexes 5 and 7 are very good matches for the geometry of the zinc(II) centre in monomer A of the native enzyme. All complexes could be seen as model compounds for the active site of the enzyme ACMSD, where the Co(II) complexes reflected the structural flexibility found in case of two histidine (His177 and His228) residues found in the active site of the enzyme.

Differential gene expression in Trypanosoma cruzi populations susceptible and resistant to benznidazole

Differential gene expression in three pairs of Trypanosoma cruzi populations or clones susceptible or resistant to benznidazole (BZ) was investigated by differential display (DD) and representation of differential expression (RDE). GenBank searches of 14 genes selected by DD showed that four sequences corresponded to different hypothetical proteins and the others were very similar to T. cruzi genes encoding mucin (TcMUC), dihydrolipoamide dehydrogenase (TcLipDH), the hexose transporter (TcHT), or a ribosomal protein. Sequence analysis was performed on 34 clones obtained by RDE; approximately half of these clones encoded 14 different hypothetical proteins and the other half encoded proteins involved with stress response, antioxidant defence, metabolism, transporter proteins, surface proteins, ribosomal proteins and others. The mRNA levels of eight T. cruzi genes obtained by RDE and DD were analysed by northern blotting to confirm the differential expression of these sequences. For six of the eight genes, TcLipDH, TcHT, TcFeSOD-A (iron superoxide dismutase-A), TcHSP70, TcHSP100 (heat shock protein) and Tc52 (thiol-transferase), mRNA levels in the drug-resistant T. cruzi population were at least twice those in the susceptible population. Further analysis of TcHSP70 showed that although the levels of TcHSP70 mRNA were four-fold higher in T. cruzi BZ-resistant population, no corresponding increase was observed in the levels of TcHSP70 protein expression. The results suggest that TcHSP70 is not directly associated with the T. cruzi drug resistance phenotype.