Synthesis and Anti-Chagas Activity Profile of a Redox-Active Lead 3-Benzylmenadione Revealed by High-Content Imaging

Chagas disease, or American trypanosomiasis, is a neglected tropical disease which is a top priority target of the World Health Organization. The disease, endemic mainly in Latin America, is caused by the protozoan Trypanosoma cruzi and has spread around the globe due to human migration. There are multiple transmission routes, including vectorial, congenital, oral, and iatrogenic. Less than 1% of patients have access to treatment, relying on two old redox-active drugs that show poor pharmacokinetics and severe adverse effects. Hence, the priorities for the next steps of R&D include (i) the discovery of novel drugs/chemical classes, (ii) filling the pipeline with drug candidates that have new mechanisms of action, and (iii) the pressing need for more research and access to new chemical entities. In the present work, we first identified a hit (4a) with a potent anti-T. cruzi activity from a library of 3-benzylmenadiones. We then designed a synthetic strategy to build a library of 49 3-(4-monoamino)benzylmenadione derivatives via reductive amination to obtain diazacyclic benz(o)ylmenadiones. Among them, we identified by high content imaging an anti-amastigote "early lead" 11b (henceforth called cruzidione) revealing optimized pharmacokinetic properties and enhanced specificity. Studies in a yeast model revealed that a cruzidione metabolite, the 3-benzoylmenadione (cruzidione oxide), enters redox cycling with the NADH-dehydrogenase, generating reactive oxygen species, as hypothesized for the early hit (4a).

Proteomic Profiling of Antimalarial Plasmodione Using 3-Benz(o)ylmenadione Affinity-Based Probes

Understanding the mechanisms of drug action in malarial parasites is crucial for the development of new drugs to combat infection and to counteract drug resistance. Proteomics is a widely used approach to study host-pathogen systems and to identify drug protein targets. Plasmodione is an antiplasmodial early-lead drug exerting potent activities against young asexual and sexual blood stages in vitro with low toxicity to host cells. To elucidate its molecular mechanisms, an affinity-based protein profiling (AfBPP) approach was applied to yeast and P. falciparum proteomes. New (pro-)AfBPP probes based on the 3-benz(o)yl-6-fluoro-menadione scaffold were synthesized. With optimized conditions of both photoaffinity labeling and click reaction steps, the AfBPP protocol was then applied to a yeast proteome, yielding 11 putative drug-protein targets. Among these, we found four proteins associated with oxidoreductase activities, the hypothesized type of targets for plasmodione and its metabolites, and other proteins associated with the mitochondria. In Plasmodium parasites, the MS analysis revealed 44 potential plasmodione targets that need to be validated in further studies. Finally, the localization of a 3-benzyl-6-fluoromenadione AfBPP probe was studied in the subcellular structures of the parasite at the trophozoite stage.

Chemoselective Synthesis and Anti-Kinetoplastidal Properties of 2,6-Diaryl-4H-tetrahydro-thiopyran-4-one S-Oxides: Their Interplay in a Cascade of Redox Reactions from Diarylideneacetones

2,6-Diaryl-4H-tetrahydro-thiopyran-4-ones and corresponding sulfoxide and sulfone derivatives were designed to lower the major toxicity of their parent anti-kinetoplatidal diarylideneacetones through a prodrug effect. Novel diastereoselective methodologies were developed and generalized from diarylideneacetones and 2,6-diaryl-4H-tetrahydro-thiopyran-4-ones to allow the introduction of a wide substitution profile and to prepare the related S-oxides. The in vitro biological activity and selectivity of diarylideneacetones, 2,6-diaryl-4H-tetrahydro-thiopyran-4-ones, and their S-sulfoxide and sulfone metabolites were evaluated against Trypanosoma brucei brucei, Trypanosoma cruzi, and various Leishmania species in comparison with their cytotoxicity against human fibroblasts hMRC-5. The data revealed that the sulfides, sulfoxides, and sulfones, in which the Michael acceptor sites are temporarily masked, are less toxic against mammal cells while the anti-trypanosomal potency was maintained against T. b. brucei, T. cruzi, L. infantum, and L. donovani, thus confirming the validity of the prodrug strategy. The mechanism of action is proposed to be due to the involvement of diarylideneacetones in cascades of redox reactions involving the trypanothione system. After Michael addition of the dithiol to the double bonds, resulting in an elongated polymer, the latter-upon S-oxidation, followed by syn-eliminations-fragments, under continuous release of reactive oxygen species and sulfenic/sulfonic species, causing the death of the trypanosomal parasites in the micromolar or submicromolar range with high selectivity indexes.

Synthesis and Photochemical Properties of Fluorescent Metabolites Generated from Fluorinated Benzoylmenadiones in Living Cells

This work describes the reactivity and properties of fluorinated derivatives (F-PD and F-PDO) of plasmodione (PD) and its metabolite, the plasmodione oxide (PDO). Introduction of a fluorine atom on the 2-methyl group markedly alters the redox properties of the 1,4-naphthoquinone electrophore, making the compound highly oxidizing and particularly photoreactive. A fruitful set of analytical methods (electrochemistry, absorption and emission spectrophotometry, and HRMS-ESI) have been used to highlight the products resulting from UV photoirradiation in the absence or presence of selected nucleophiles. With F-PDO and in the absence of nucleophile, photoreduction generates a highly reactive ortho-quinone methide (o-QM) capable of leading to the formation of a homodimer. In the presence of thiol nucleophiles such as β-mercaptoethanol, which was used as a model, o-QMs are continuously regenerated in sequential photoredox reactions generating mono- or disulfanylation products as well as various unreported sulfanyl products. Besides, these photoreduced adducts derived from F-PDO are characterized by a bright yellowish emission due to an excited-state intramolecular proton transfer (ESIPT) process between the dihydronapthoquinone and benzoyl units. In order to evidence the possibility of an intramolecular coupling of the o-QM intermediate, a synthetic route to the corresponding anthrones is described. Tautomerization of the targeted anthrones occurs and affords highly fluorescent stable hydroxyl-anthraquinones. Although probable to explain the intense visible fluorescence emission also observed in tobacco BY-2 cells used as a cellular model, these coupling products have never been observed during the photochemical reactions performed in this study. Our data suggest that the observed ESIPT-induced fluorescence most likely corresponds to the generation of alkylated products through reduction species, as demonstrated with the β-mercaptoethanol model. In conclusion, F-PDO thus acts as a novel (pro)-fluorescent probe for monitoring redox processes and protein alkylation in living cells.

Evaluation of ferrocenyl-containing γ-hydroxy-γ-lactam-derived tetramates as potential antiplasmodials

A series of ferrocenyl-containing γ-hydroxy-γ-lactam tetramates were prepared in 2-3 steps through ring opening-ring closure (RORC) process of γ-ylidene-tetronate derivatives in the presence of ferrocenyl alkylamines. The compounds were screened in vitro for their antiplasmodial activity against chloroquine-sensitive (3D7) and chloroquine-resistant (W2) clones of P. falciparum, displaying activity in the range of 0.12-100 μM, with generally good resistance index. The most active ferrocene in these series exhibited IC₅₀ equal to 0.09 μM (3D7) and 0.12 μM (W2). The low cytotoxicity of the ferrocenyl-containing γ-hydroxy-γ-lactam tetramates against Human Umbilical Vein Endothelial (HUVEC) cell line demonstrated selective antiparasitic activity. The redox properties of these ferrocene-derived tetramates were studied and physico-biochemical studies evidenced that these derivatives can exert potent antimalarial activities via a mechanism distinct from ferroquine.

The parasitophorous vacuole nutrient channel is critical for drug access in malaria parasites and modulates the artemisinin resistance fitness cost

Intraerythrocytic malaria parasites proliferate bounded by a parasitophorous vacuolar membrane (PVM). The PVM contains nutrient permeable channels (NPCs) conductive to small molecules, but their relevance for parasite growth for individual metabolites is largely untested. Here we show that growth-relevant levels of major carbon and energy sources pass through the NPCs. Moreover, we find that NPCs are a gate for several antimalarial drugs, highlighting their permeability properties as a critical factor for drug design. Looking into NPC-dependent amino acid transport, we find that amino acid shortage is a reason for the fitness cost in artemisinin-resistant (ART^R) parasites and provide evidence that NPC upregulation to increase amino acids acquisition is a mechanism of ART^R parasites in vitro and in human infections to compensate this fitness cost. Hence, the NPCs are important for nutrient and drug access and reveal amino acid deprivation as a critical constraint in ART^R parasites.

Bioinspired Photoredox Benzylation of Quinones

3-Benzylmenadiones were obtained in good yield by using a blue-light-induced photoredox process in the presence of Fe(III), oxygen, and γ-terpinene acting as a hydrogen-atom transfer agent. This methodology is compatible with a wide variety of diversely substituted 1,4-naphthoquinones as well as various cheap, readily available benzyl bromides with excellent functional group tolerance. The benzylation mechanism was investigated and supports a three-step radical cascade with the key involvement of the photogenerated superoxide anion radical.

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.

A Class of Valuable (Pro-)Activity-Based Protein Profiling Probes: Application to the Redox-Active Antiplasmodial Agent, Plasmodione

Plasmodione (PD) is a potent antimalarial redox-active drug acting at low nM range concentrations on different malaria parasite stages. In this study, in order to determine the precise PD protein interactome in parasites, we developed a class of (pro-)activity-based protein profiling probes (ABPP) as precursors of photoreactive benzophenone-like probes based on the skeleton of PD metabolites (PDO) generated in a cascade of redox reactions. Under UV-photoirradiation, we clearly demonstrate that benzylic oxidation of 3-benzylmenadione 11 produces the 3-benzoylmenadione probe 7, allowing investigation of the proof-of-concept of the ABPP strategy with 3-benzoylmenadiones 7-10. The synthesized 3-benzoylmenadiones, probe 7 with an alkyne group or probe 9 with -NO₂ in para position of the benzoyl chain, were found to be the most efficient photoreactive and clickable probes. In the presence of various H-donor partners, the UV-irradiation of the photoreactive ABPP probes generates different adducts, the expected "benzophenone-like" adducts (pathway 1) in addition to "benzoxanthone" adducts (via two other pathways, 2 and 3). Using both human and Plasmodium falciparum glutathione reductases, three protein ligand binding sites were identified following photolabeling with probes 7 or 9. The photoreduction of 3-benzoylmenadiones (PDO and probe 9) promoting the formation of both the corresponding benzoxanthone and the derived enone could be replaced by the glutathione reductase-catalyzed reduction step. In particular, the electrophilic character of the benzoxanthone was evidenced by its ability to alkylate heme, as a relevant event supporting the antimalarial mode of action of PD. This work provides a proof-of-principle that (pro-)ABPP probes can generate benzophenone-like metabolites enabling optimized activity-based protein profiling conditions that will be instrumental to analyze the interactome of early lead antiplasmodial 3-benzylmenadiones displaying an original and innovative mode of action.

A role for the succinate dehydrogenase in the mode of action of the redox-active antimalarial drug, plasmodione

Malaria, caused by protozoan parasites, is a major public health issue in subtropical countries. An arsenal of antimalarial treatments is available, however, resistance is spreading, calling for the development of new antimalarial compounds. The new lead antimalarial drug plasmodione is a redox-active compound that impairs the redox balance of parasites leading to cell death. Based on extensive in vitro assays, a model of its mode of action was drawn, involving the generation of active plasmodione metabolites that act as subversive substrates of flavoproteins, initiating a redox cycling process producing reactive oxygen species. We showed that, in yeast, the mitochondrial respiratory chain NADH-dehydrogenases are the main redox-cycling target enzymes. Furthermore, our data supported the proposal that plasmodione is a pro-drug acting via its benzhydrol and benzoyl metabolites. Here, we selected plasmodione-resistant yeast mutants to further decipher plasmodione mode of action. Of the eleven mutants analysed, nine harboured a mutation in the FAD binding subunit of succinate dehydrogenase (SDH). The analysis of the SDH mutations points towards a specific role for SDH-bound FAD in plasmodione bioactivation, possibly in the first step of the process, highlighting a novel property of SDH.

Repurposing Auranofin and Evaluation of a New Gold(I) Compound for the Search of Treatment of Human and Cattle Parasitic Diseases: From Protozoa to Helminth Infections

Neglected parasitic diseases remain a major public health issue worldwide, especially in tropical and subtropical areas. Human parasite diversity is very large, ranging from protozoa to worms. In most cases, more effective and new drugs are urgently needed. Previous studies indicated that the gold(I) drug auranofin (Ridaura^®) is effective against several parasites. Among new gold(I) complexes, the phosphole-containing gold(I) complex {1-phenyl-2,5-di(2-pyridyl)phosphole}AuCl (abbreviated as GoPI) is an irreversible inhibitor of both purified human glutathione and thioredoxin reductases. GoPI-sugar is a novel 1-thio-β-d-glucopyranose 2,3,4,6-tetraacetato-S-derivative that is a chimera of the structures of GoPI and auranofin, designed to improve stability and bioavailability of GoPI. These metal-ligand complexes are of particular interest because of their combined abilities to irreversibly target the essential dithiol/selenol catalytic pair of selenium-dependent thioredoxin reductase activity, and to kill cells from breast and brain tumors. In this work, screening of various parasites—protozoans, trematodes, and nematodes—was undertaken to determine the in vitro killing activity of GoPI-sugar compared to auranofin. GoPI-sugar was found to efficiently kill intramacrophagic Leishmania donovani amastigotes and adult filarial and trematode worms.

Selenium Status in Elderly People: Longevity and Age-Related Diseases

BACKGROUND

Selenium (Se) is a trace element active in selenoproteins, which can regulate oxidative stress. It is generally perceived as an import factor for maintaining health in the elderly.

METHODS

The goal of this review is to discuss selenium concentration in biological samples, primarily serum or plasma, as a function of age and its relation with longevity. The elemental level in various age-related diseases is reviewed.

CONCLUSION

Highest selenium values were observed in healthy adults, while in an elderly population significantly lower concentrations were reported. Variables responsible for contradictory findings are mentioned. Risk and benefits of Se-supplementation still remain under debate.

Antiplasmodial Activity of Nitroaromatic Compounds: Correlation with Their Reduction Potential and Inhibitory Action on Plasmodium falciparum Glutathione Reductase

With the aim to clarify the mechanism(s) of action of nitroaromatic compounds against the malaria parasite Plasmodium falciparum, we examined the single-electron reduction by P. falciparum ferredoxin:NADP⁺ oxidoreductase (PfFNR) of a series of nitrofurans and nitrobenzenes (n = 23), and their ability to inhibit P. falciparum glutathione reductase (PfGR). The reactivity of nitroaromatics in PfFNR-catalyzed reactions increased with their single-electron reduction midpoint potential (E¹₇). Nitroaromatic compounds acted as non- or uncompetitive inhibitors towards PfGR with respect to NADPH and glutathione substrates. Using multiparameter regression analysis, we found that the in vitro activity of these compounds against P. falciparum strain FcB1 increased with their E¹₇ values, octanol/water distribution coefficients at pH 7.0 (log D), and their activity as PfGR inhibitors. Our data demonstrate that both factors, the ease of reductive activation and the inhibition of PfGR, are important in the antiplasmodial in vitro activity of nitroaromatics. To the best of our knowledge, this is the first quantitative demonstration of this kind of relationship. No correlation between antiplasmodial activity and ability to inhibit human erythrocyte GR was detected in tested nitroaromatics. Our data suggest that the efficacy of prooxidant antiparasitic agents may be achieved through their combined action, namely inhibition of antioxidant NADPH:disulfide reductases, and the rapid reduction by single-electron transferring dehydrogenases-electrontransferases.

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.

Physicochemical Properties Govern the Activity of Potent Antiviral Flavones

Ladanein (i.e., 5,6,7-trihydroxylated flavone) was demonstrated to act as a powerful virucidal agent toward a broad range of enveloped virus particles. Fe(III) coordination and pH are indeed among the key parameters that might favor both bioactivation of the flavone and consequent host cell entry inhibition. In this present work, the impact of fluorinated groups on the physicochemical and antiviral properties of the flavone was investigated, thus allowing a deeper understanding of the antiviral mode of action. The improved synthesis of ladanein allowed accessing a broad range of analogues, some of them being significantly more active than the former ladanein lead compound. We first determined the acido-basic properties of this homogenous series of compounds and then investigated their electrochemical behavior. Fe(III) coordination properties (stability, spectral behavior, and kinetics) of ladanein and its analogues were then examined (quasiphysiological conditions) and provided key information of their stability and reactivity. Using the determined physicochemical parameters, the critical impact of the iron complexation and medium acidity was confirmed on hepatitis C virus (HCV) particles (pre)treated with ladanein. Finally, a preliminary structure-HCV entry inhibition relationship study evidenced the superior antiviral activity of the ladanein analogues bearing an electron-withdrawing group in para position (FCF ₃ > FOCF ₃ > FFCF ₃ > FF > FOMe) on the B cycle in comparison with the parent ladanein itself.

Iron(iii) coordination properties of ladanein, a flavone lead with a broad-spectrum antiviral activity

The Fe(iii) complexation properties of ladanein, a potent antiviral flavone, and related analogues (negletein and salvigenin), have been studied in solution under quasi-physiological conditions using physico-chemical tools and provided important insights into their stability/reactivity in solution.

Synthesis of plasmodione metabolites and13C-enriched plasmodione as chemical tools for drug metabolism investigation

A 10-step synthesis of the antimalarial lead,¹³C₁₈-enriched plasmodione, and of seven putative metabolites is described.

A high susceptibility to redox imbalance of the transmissible stages of Plasmodium falciparum revealed with a luciferase-based mature gametocyte assay

The goal to prevent Plasmodium falciparum transmission from humans to mosquitoes requires the identification of targetable metabolic processes in the mature (stage V) gametocytes, the sexual stages circulating in the bloodstream. This task is complicated by the apparently low metabolism of these cells, which renders them refractory to most antimalarial inhibitors and constrains the development of specific and sensitive cell-based assays. Here, we identify and functionally characterize the regulatory regions of the P. falciparum gene PF3D7_1234700, encoding a CPW-WPC protein and named here Upregulated in Late Gametocytes (ULG8), which we have leveraged to express reporter genes in mature male and female gametocytes. Using transgenic parasites containing a pfULG8-luciferase cassette, we investigated the susceptibility of stage V gametocytes to compounds specifically affecting redox metabolism. Our results reveal a high sensitivity of mature gametocytes to the glutathione reductase inhibitor and redox cycler drug methylene blue (MB). Using isobologram analysis, we find that a concomitant inhibition of the parasite enzyme glucose-6-phosphate dehydrogenase-6-phosphogluconolactonase, a key component of NADPH synthesis, potently synergizes MB activity. These data suggest that redox metabolism and detoxification activity play an unsuspected yet vital role in stage V gametocytes, rendering these cells exquisitely sensitive to decreases in NADPH concentration.

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.

Synthesis and evaluation of 1,4-naphthoquinone ether derivatives as SmTGR inhibitors and new anti-schistosomal drugs

Investigations regarding the chemistry and mechanism of action of 2-methyl-1,4-naphthoquinone (or menadione) derivatives revealed 3-phenoxymethyl menadiones as a novel anti-schistosomal chemical series. These newly synthesized compounds (1-7) and their difluoromethylmenadione counterparts (8, 9) were found to be potent and specific inhibitors of Schistosoma mansoni thioredoxin-glutathione reductase (SmTGR), which has been identified as a potential target for anti-schistosomal drugs. The compounds were also tested in enzymic assays using both human flavoenzymes, i.e. glutathione reductase (hGR) and selenium-dependent human thioredoxin reductase (hTrxR), to evaluate the specificity of the inhibition. Structure-activity relationships as well as physico- and electro-chemical studies showed a high potential for the 3-phenoxymethyl menadiones to inhibit SmTGR selectively compared to hGR and hTrxR enzymes, in particular those bearing an α-fluorophenol methyl ether moiety, which improves anti-schistosomal action. Furthermore, the (substituted phenoxy)methyl menadione derivative (7) displayed time-dependent SmTGR inactivation, correlating with unproductive NADPH-dependent redox cycling of SmTGR, and potent anti-schistosomal action in worms cultured ex vivo. In contrast, the difluoromethylmenadione analog 9, which inactivates SmTGR through an irreversible non-consuming NADPH-dependent process, has little killing effect in worms cultured ex vivo. Despite ex vivo activity, none of the compounds tested was active in vivo, suggesting that the limited bioavailability may compromise compound activity. Therefore, future studies will be directed toward improving pharmacokinetic properties and bioavailability.

Antimalarial NADPH-Consuming Redox-Cyclers As Superior Glucose-6-Phosphate Dehydrogenase Deficiency Copycats

AIMS

Early phagocytosis of glucose-6-phosphate dehydrogenase (G6PD)-deficient erythrocytes parasitized by Plasmodium falciparum were shown to protect G6PD-deficient populations from severe malaria. Here, we investigated the mechanism of a novel antimalarial series, namely 3-[substituted-benzyl]-menadiones, to understand whether these NADPH-consuming redox-cyclers, which induce oxidative stress, mimic the natural protection of G6PD deficiency.

RESULTS

We demonstrated that the key benzoylmenadione metabolite of the lead compound acts as an efficient redox-cycler in NADPH-dependent methaemoglobin reduction, leading to the continuous formation of reactive oxygen species, ferrylhaemoglobin, and subsequent haemichrome precipitation. Structure-activity relationships evidenced that both drug metabolites and haemoglobin catabolites contribute to potentiate drug effects and inhibit parasite development. Disruption of redox homeostasis by the lead benzylmenadione was specifically induced in Plasmodium falciparum parasitized erythrocytes and not in non-infected cells, and was visualized via changes in the glutathione redox potential of living parasite cytosols. Furthermore, the redox-cycler shows additive and synergistic effects in combination with compounds affecting the NADPH flux in vivo.

INNOVATION

The lead benzylmenadione 1c is the first example of a novel redox-active agent that mimics the behavior of a falciparum parasite developing inside a G6PD-deficient red blood cell (RBC) giving rise to malaria protection, and it exerts specific additive effects that are inhibitory to parasite development, without harm for non-infected G6PD-sufficient or -deficient RBCs.

CONCLUSION

This strategy offers an innovative perspective for the development of future antimalarial drugs for G6PD-sufficient and -deficient populations.

Antiglioma activity of GoPI-sugar, a novel gold(I)-phosphole inhibitor: chemical synthesis, mechanistic studies, and effectiveness in vivo

Glioblastoma, an aggressive brain tumor, has a poor prognosis and a high risk of recurrence. An improved chemotherapeutic approach is required to complement radiation therapy. Gold(I) complexes bearing phosphole ligands are promising agents in the treatment of cancer and disturb the redox balance and proliferation of cancer cells by inhibiting disulfide reductases. Here, we report on the antitumor properties of the gold(I) complex 1-phenyl-bis(2-pyridyl)phosphole gold chloride thio-β-d-glucose tetraacetate (GoPI-sugar), which exhibits antiproliferative effects on human (NCH82, NCH89) and rat (C6) glioma cell lines. Compared to carmustine (BCNU), an established nitrosourea compound for the treatment of glioblastomas that inhibits the proliferation of these glioma cell lines with an IC50 of 430μM, GoPI-sugar is more effective by two orders of magnitude. Moreover, GoPI-sugar inhibits malignant glioma growth in vivo in a C6 glioma rat model and significantly reduces tumor volume while being well tolerated. Both the gold(I) chloro- and thiosugar-substituted phospholes interact with DNA albeit more weakly for the latter. Furthermore, GoPI-sugar irreversibly and potently inhibits thioredoxin reductase (IC50 4.3nM) and human glutathione reductase (IC50 88.5nM). However, treatment with GoPI-sugar did not significantly alter redox parameters in the brain tissue of treated animals. This might be due to compensatory upregulation of redox-related enzymes but might also indicate that the antiproliferative effects of GoPI-sugar in vivo are rather based on DNA interaction and inhibition of topoisomerase I than on the disturbance of redox equilibrium. Since GoPI-sugar is highly effective against glioblastomas and well tolerated, it represents a most promising lead for drug development. This article is part of a Special Issue entitled: Thiol-Based Redox Processes.

1,4-naphthoquinones and other NADPH-dependent glutathione reductase-catalyzed redox cyclers as antimalarial agents

The homodimeric flavoenzyme glutathione reductase catalyzes NADPH-dependent glutathione disulfide reduction. This reaction is important for keeping the redox homeostasis in human cells and in the human pathogen Plasmodium falciparum. Different types of NADPH-dependent disulfide reductase inhibitors were designed in various chemical series to evaluate the impact of each inhibition mode on the propagation of the parasites. Against malaria parasites in cultures the most potent and specific effects were observed for redox-active agents acting as subversive substrates for both glutathione reductases of the Plasmodium-infected red blood cells. In their oxidized form, these redox-active compounds are reduced by NADPH-dependent flavoenzyme-catalyzed reactions in the cytosol of infected erythrocytes. In their reduced forms, these compounds can reduce molecular oxygen to reactive oxygen species, or reduce oxidants like methemoglobin, the major nutrient of the parasite, to indigestible hemoglobin. Furthermore, studies on a fluorinated suicide-substrate of the human glutathione reductase indicate that the glutathione reductase-catalyzed bioactivation of 3-benzylnaphthoquinones to the corresponding reduced 3-benzoyl metabolites is essential for the observed antimalarial activity. In conclusion, the antimalarial lead naphthoquinones are suggested to perturb the major redox equilibria of the targeted cells. These effects result in developmental arrest of the parasite and contribute to the removal of the parasitized erythrocytes by macrophages.

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.

Interactions of the antimalarial drug methylene blue with methemoglobin and heme targets in Plasmodium falciparum: a physico-biochemical study

AIMS

Resistance of Plasmodium falciparum to drugs has led to renewed interest of redox-active methylene blue (MB) for which no resistance has been reported so far. Moreover, MB displays unique interactions with glutathione reductase (GR). However, the mechanisms of action/interaction with potential targets of MB are yet to be elucidated. Our physico-biochemical study on MB and relevant hematin-containing targets was performed under quasi-physiological conditions.

RESULTS

The water deprotonation of the Fe(III)protoporphyrin dimer, the major building block of β-hematin, was studied. At pH 6, the predominant dimer possesses water coordinated to both metals. Below pH 6, spontaneous precipitation of β-hematin occurred reminiscent of hemozoin biomineralization at pH 5.0-5.5 in the food vacuole of the malarial parasite. MB also forms dimers (K(Dim)=6800 M(-1)) and firmly binds to hematin in a 2:1 hematin:MB sandwich complex (K(D)=3.16 μM). MB bioactivation catalyzed by GR induces efficient methemoglobin(Fe(III)) [metHb(Fe(III))] reduction to hemoglobin(Fe(II)). The reduction rate, mediated by leucomethylene blue (LMB), was determined (k(metHb)(red)=991 M(-1)·s(-1)) in an assay coupled to the GR/reduced form of nicotinamide adenine dinucleotide phosphate system.

INNOVATION AND CONCLUSION

Our work provides new insights into the understanding of (i) how MB interacts with hematin-containing targets, (ii) other relevant MB properties in corroboration with the distribution of the three major LMB species as a function of pH, and (iii) how this redox-active cycler induces efficient catalytic reduction of metHb(Fe(III)) to hemoglobin(Fe(II)) mediated by oxidoreductases. These physico-biochemical parameters of MB open promising perspectives for the interpretation of the pharmacology and pathophysiology of malaria and possibly new routes for antimalarial drug development.

A plant-derived flavonoid inhibits entry of all HCV genotypes into human hepatocytes

BACKGROUND & AIMS

Interferon-based therapies for hepatitis C virus (HCV) infection are limited by side effects and incomplete response rates, particularly among transplant recipients. We screened a library of plant-derived small molecules to identify HCV inhibitors with novel mechanisms.

METHODS

We isolated phenolic compounds from Marrubium peregrinum L (Lamiaceae). Replication of HCV RNA, virus production, and cell entry were monitored using replicons and infectious HCV. Inhibition of HCV was measured in hepatoma cells and primary human hepatocytes using luciferase reporter gene assays, core enzyme-linked immunosorbent assays, or infectivity titration. We tested the bioavailability of the compound in mice.

RESULTS

We identified a flavonoid, ladanein (BJ486K), with unreported antiviral activity and established its oral bioavailability in mice. Natural and synthetic BJ486K inhibited a post-attachment entry step, but not RNA replication or assembly; its inhibitory concentration 50% was 2.5 μm. BJ486K was effective against all major HCV genotypes, including a variant that is resistant to an entry inhibitor; it prevented infection of primary human hepatocytes. Combined administration of BJ486K and cyclosporine A had a synergistic effect in inhibition of HCV infection.

CONCLUSIONS

BJ486K has oral bioavailability and interferes with entry of HCV into cultured human hepatocytes. It synergizes with cyclosporine A to inhibit HCV infection. Its inhibitory effects are independent of HCV genotype, including a variant that is resistant to an entry inhibitor against scavenger receptor class B type I. Flavonoid derivatives therefore might be developed as components of combination therapies because they are potent, broadly active inhibitors of HCV entry that could prevent graft reinfection after liver transplantation.

A physico-biochemical study on potential redox-cyclers as antimalarial and anti-schistosomal drugs

The role of redox enzymes in establishing a microenvironment for parasite development is well characterized. Mimicking human glucose-6-phosphate dehydrogenase and glutathione reductase (GR) deficiencies by redox-cycling compounds thus represents a challenge to the design of new preclinical antiparasitic drug candidates. Schistosomes and malarial parasites feed on hemoglobin. Heme, the toxic prosthetic group of the protein, is not digested and represents a challenge to the redox metabolism of the parasites. Here, we report on old and new redox-cycling compounds--whose antiparasitic activities are related to their interference with (met)hemoglobin degradation and hematin crystallization. Three key-assays allowed probing and differentiating the mechanisms of drug actions. Inhibition of β-hematin was first compared to the heme binding as a possible mode of action. All tested ligands interact with the hematin π-π dimer with K(D) similar to those measured for the major antiparasitic drugs. No correlation between a high affinity for hematin and the capacity to prevent β-hematin formation was however deduced. Inhibition of β-hematin formation is consequently not the result of a single process but results from redox processes following electron transfers from the drugs to iron(III)-containing targets. The third experiment highlighted that several redox-active compounds (in their reduced forms) are able to efficiently reduce methemoglobin to hemoglobin in a GR/NADPH-coupled assay. A correlation between methemoglobin reduction and inhibition of β-hematin was shown, demonstrating that both processes are closely related. The ability of our redox-cyclers to trigger methemoglobin reduction therefore constitutes a critical step to understand the mechanism of action of our drug candidates.

Antimalarial versus cytotoxic properties of dual drugs derived from 4-aminoquinolines and Mannich bases: interaction with DNA

The synthesis and biological evaluation of new organic and organometallic dual drugs designed as potential antimalarial agents are reported. A series of 4-aminoquinoline-based Mannich bases with variations in the aliphatic amino side chain were prepared via a three-steps synthesis. These compounds were also tested against chloroquine-susceptible and chloroquine-resistant strains of Plasmodium falciparum and assayed for their ability to inhibit the formation of beta-hematin in vitro using a colorimetric beta-hematin inhibition assay. Several compounds showed a marked antimalarial activity, with IC(50) and IC(90) values in the low nM range but also a high cytotoxicity against mammalian cells, in particular a highly drug-resistant glioblastoma cell line. The newly designed compounds revealed high DNA binding properties, especially for the GC-rich domains. Altogether, these dual drugs seem to be more appropriate to be developed as antiproliferative agents against mammalian cancer cells than Plasmodium parasites.

Antimalarial activities of ferroquine conjugates with either glutathione reductase inhibitors or glutathione depletors via a hydrolyzable amide linker

Based on the prodrug concept as well as the combination of two different classes of antimalarial agents, we designed and synthesized two series of ferrocenic antimalarial dual molecules consisting of a ferroquine analogue conjugated with a glutathione reductase inhibitor (or a glutathione depletor) through a cleavable amide bond in order to target two essential pathways in the malarial parasites. The results showed no enhancement of the antimalarial activity of the dual molecules but evidenced a unique mode of action of ferroquine and ferrocenyl analogues distinct of those of chloroquine and nonferrocenic 4-aminoquinoline analogues.

Unsaturated Mannich bases active against multidrug-resistant Trypanosoma brucei brucei strains

A series of unsaturated Mannich bases possessing two electrophilic sites was recently identified as irreversible inhibitors of trypanothione reductase from Trypanosoma cruzi. New derivatives were synthesized by modifying the substitution pattern on the aromatic ring and by incorporating the melamine motif of melarsoprol. Their affinity to P2 transporter and their trypanocidal properties have been studied using three strains expressing various purine transporters. While the melamine derivatives showed some affinity to the P2 transporter, unsaturated Mannich bases without the melamine motif showed excellent potencies against pentamidine-resistant strains of T. brucei brucei suggesting alternative drug uptake routes. The Michael acceptor properties of the three most active compounds towards glutathione correlated with the observed trypanocidal activities.

Thioredoxin glutathione reductase from Schistosoma mansoni: an essential parasite enzyme and a key drug target

BACKGROUND

Schistosomiasis--infection with helminth parasites in the genus Schistosoma, including S. mansoni--is a widespread, devastating tropical disease affecting more than 200 million people. No vaccine is available, and praziquantel, the only drug extensively utilized, is currently administered more than 100 million people yearly. Because praziquantel resistance may develop it is essential to identify novel drug targets. Our goal was to investigate the potential of a unique, selenium-containing parasite enzyme thioredoxin glutathione reductase (TGR) as a drug target.

METHODS AND FINDINGS

Using RNA interference we found that TGR is essential for parasite survival; after silencing of TGR expression, in vitro parasites died within 4 d. We also found that auranofin is an efficient inhibitor of pure TGR (Ki = 10 nM), able to kill parasites rapidly in culture at physiological concentrations (5 microM), and able to partially cure infected mice (worm burden reductions of ~60%). Furthermore, two previously used antischistosomal compounds inhibited TGR activity, suggesting that TGR is a key target during therapy with those compounds.

CONCLUSIONS

Collectively, our results indicate that parasite TGR meets all the major criteria to be a key target for antischistosomal chemotherapy. To our knowledge this is the first validation of a Schistosoma drug target using a convergence of both genetic and biochemical approaches.

A Fluoro Analogue of the Menadione Derivative 6-[2‘-(3‘-Methyl)-1‘,4‘-naphthoquinolyl]hexanoic Acid Is a Suicide Substrate of Glutathione Reductase. Crystal Structure of the Alkylated Human Enzyme†

Glutathione reductase is an important housekeeping enzyme for redox homeostasis both in human cells and in the causative agent of tropical malaria, Plasmodium falciparum. Glutathione reductase inhibitors were shown to have anticancer and antimalarial activity per se and to contribute to the reversal of drug resistance. The development of menadione chemistry has led to the selection of 6-[2'-(3'-methyl)-1',4'-naphthoquinolyl]hexanoic acid, called M(5), as a potent reversible and uncompetitive inhibitor of both human and P. falciparum glutathione reductases. Here we describe the synthesis and kinetic characterization of a fluoromethyl-M(5) analogue that acts as a mechanism-based inhibitor of both enzymes. In the course of enzymatic catalysis, the suicide substrate is activated by one- or two-electron reduction, and then a highly reactive quinone methide is generated upon elimination of the fluorine. Accordingly the human enzyme was found to be irreversibly inactivated with a k(inact) value of 0.4 +/- 0.2 min(-1). The crystal structure of the alkylated enzyme was solved at 1.7 A resolution. It showed the inhibitor to bind covalently to the active site Cys58 and to interact noncovalently with His467', Arg347, Arg37, and Tyr114. On the basis of the crystal structure of the inactivated human enzyme and stopped-flow kinetic studies with two- and four-electron-reduced forms of the unreacted P. falciparum enzyme, a mechanism is proposed which explains naphthoquinone reduction at the flavin of glutathione reductase.

Specific inhibitors of Plasmodium falciparum thioredoxin reductase as potential antimalarial agents

Plasmodium falciparum thioredoxin reductase (PfTrxR: NADPH+Trx(S)2+H+<-->NADP++Trx(SH)2) is a high Mr flavin-dependent TrxR that reduces thioredoxin (Trx) via a CysXXXXCys pair located penultimately to the C-terminal Gly. In this respect, PfTrxR differs significantly from its human counterpart which bears a Cys-Sec redox pair at the same position. PfTrxR is essentially involved in antioxidant defense and redox regulation of the parasite and has been previously validated by knock-out studies as a potential drug target for malaria chemotherapy. Moreover, human TrxR is present in most cancer cells at levels tenfold higher than in normal cells. Here we report the discovery of a series of potent inhibitors of PfTrxR. The three most promising inhibitors, 3(IC50(PfTrxR)=2 microM and IC50(hTrxR)=50 microM), 7(IC50(PfTrxR)=2 microM and IC50(hTrxR)=140 microM), and 11(IC50(PfTrxR)=0.5 microM and IC50(hTrxR)=4 microM) were selective for the parasite enzyme. Detailed mechanistic characterization of the effects of these compounds on the PfTrxR-catalyzed reaction showed clear uncompetitive inhibition with respect to both substrate and cofactor. For the most specific PfTrxR inhibitor 7, an alkylation mechanism study based on a thiol conjugation model was performed. Furthermore, all three compounds were active in the lower micromolar range on the chloroquine-resistant P. falciparum strain K1 in vitro.

Irreversible Inactivation of Trypanothione Reductase by Unsaturated Mannich Bases: A Divinyl Ketone as Key Intermediate

Trypanothione reductase is a flavoenzyme unique to trypanosomatid parasites. Here we show that unsaturated Mannich bases irreversibly inactivate trypanothione reductase from Trypanosoma cruzi, the causative agent of Chagas' disease. The inhibitory potency of the compounds strongly increased upon storage of the DMSO stock solutions. HPLC, NMR, and mass spectrometry data of potential intermediates revealed a divinyl ketone as the active compound inactivating the enzyme. ESI- and MALDI-TOF mass spectrometry of trypanothione reductase modified by the Mannich base or the divinyl ketone showed specific alkylation of the active site Cys52 by a 5-(2'chlorophenyl)-3-oxo-4-pentenyl substituent. The reaction mechanism and the site of alkylation differ from those in Plasmodium falciparum thioredoxin reductase where the C-terminal redox active dithiol is modified. After deamination, unsaturated Mannich bases are highly reactive in polycondensation with trypanothione. Interaction of these compounds with both trypanothione and trypanothione reductase could account for their potent trypanocidal effect against Trypanosoma brucei.