Antiplasmodial activity of quinones: Roles of aziridinyl substituents and the inhibition of Plasmodium falciparum glutathione reductase

Reactions of Plasmodium falciparum Type II NADH: Ubiquinone Oxidoreductase with Nonphysiological Quinoidal and Nitroaromatic Oxidants

In order to detail the antiplasmodial effects of quinones (Q) and nitroaromatic compounds (ArNO2), we investigated their reduction mechanism by Plasmodium falciparum flavoenzyme type II NADH:ubiquinone oxidoreductase (PfNDH2). The reactivity of Q and ArNO2 (n = 29) follows a common trend and exhibits a parabolic dependence on their single-electron reduction potential (E71), albeit with significantly scattered data. The reactivity of quinones with similar E71 values increases with their lipophilicity. Quinones are reduced by PfNDH2 in a two-electron way, but ArNO2 are reduced in a single-electron way. The inhibition studies using NAD+ and ADP-ribose showed that quinones oxidize the complexes of reduced enzyme with NADH and NAD+. This suggests that, as in the case of other NDH2s, quinones and the nicotinamide ring of NAD(H) bind at separate sites. A scheme of PfNDH2 catalysis is proposed, consistent with both the observed 'ping-pong' mechanism and the presence of two substrate binding sites. Molecular docking showed that Q and ArNO2 bind in a similar manner and that lipophilic quinones have a higher affinity for the binding site. One may expect that PfNDH2 can be partially responsible for the previously observed enhanced antiplasmodial activity of aziridinylbenzoquinones caused by their two-electron reduction, as well as for the redox cycling and oxidative stress-type action of ArNO2.

Pharmacomodulation of the Redox-Active Lead Plasmodione: Synthesis of Substituted 2-Benzylnaphthoquinone Derivatives, Antiplasmodial Activities, and Physicochemical Properties

Malaria remains a major global health problem that has been exacerbated by the impact of the COVID-19 pandemic on health systems. To combat this, the World Health Organization (WHO) has set a target of driving forward research into innovative treatment methods such as new drugs and vaccines. Quinones, particularly 1,4-naphthoquinones, have been identified as promising candidates for the development of antiprotozoal drugs. Herein, we report several methods for the preparation of 2-benzyl-1,4-naphthoquinones. In particular, the silver-catalyzed Kochi-Anderson radical decarboxylation is well suited for the preparation of these compounds. The antiprotozoal activity of all synthesized compounds was evaluated against Plasmodium falciparum NF54 and Trypanosoma brucei rhodesiense STIB900. Cytotoxicity towards L6 cells was also determined, and the respective selectivity indices (SI) were calculated. The synthesized compounds exhibited good antiplasmodial activity against the P. falciparum (NF54) strain, particularly (2-fluoro-5-trifluoromethylbenzyl)-menadione 2e, which showed strong efficacy and high selectivity (IC50 = 0.006 µM, SI = 7495). In addition, these compounds also displayed favorable physicochemical properties, suggesting that the benzylnaphthoquinone scaffold may be a viable option for new antiplasmodial drugs.

Analysis of the interaction of antimalarial agents with Plasmodium falciparum glutathione reductase through molecular mechanical calculations

CONTEXT

Malaria remains a significant global health challenge with emerging resistance to current treatments. Plasmodium falciparum glutathione reductase (PfGR) plays a critical role in the defense mechanisms of malaria parasites against oxidative stress. In this study, we investigate the potential of targeting PfGR with conventional antimalarials and dual drugs combining aminoquinoline derivatives with GR inhibitors, which reveal promising interactions between PfGR and studied drugs. The naphthoquinone Atovaquone demonstrated particularly high affinity and potential dual-mode binding with the enzyme active site and cavity. Furthermore, dual drugs exhibit enhanced binding affinity, suggesting their efficacy in inhibiting PfGR, where the aliphatic ester bond (linker) is essential for effective binding with the enzyme's active site. Overall, this research provides important insights into the interactions between antimalarial agents and PfGR and encourages further exploration of its role in the mechanisms of action of antimalarials, including dual drugs, to enhance antiparasitic efficacy.

METHODS

The drugs were tested as PfGR potential inhibitors via molecular docking on AutoDock 4, which was performed based on the preoptimized structures in HF/3-21G-PCM level of theory on ORCA 5. Drug-receptor systems with the most promising binding affinities were then studied with a molecular dynamic's simulation on AMBER 16. The molecular dynamics simulations were performed with a 100 ns NPT ensemble employing GAFF2 forcefield in the temperature of 310 K, integration time step of 2 fs, and non-bond cutoff distance of 6.0 Å.

Oxidative Stress in Malaria: Potential Benefits of Antioxidant Therapy

Malaria is an infectious disease and a serious public health problem in the world, with 3.3 billion people in endemic areas in 100 countries and about 200 million new cases each year, resulting in almost 1 million deaths in 2018. Although studies look for strategies to eradicate malaria, it is necessary to know more about its pathophysiology to understand the underlying mechanisms involved, particularly the redox balance, to guarantee success in combating this disease. In this review, we addressed the involvement of oxidative stress in malaria and the potential benefits of antioxidant supplementation as an adjuvant antimalarial therapy.

Effect of Artemisinin on the Redox System of NADPH/FNR/Ferredoxin from Malaria Parasites

FNR and ferredoxin constitute a redox cascade, which provides reducing power in the plastid of malaria parasites. Recently, mutation of ferredoxin (D97Y) was reported to be strongly related to the parasite’s resistance to the front-line antimalarial drug artemisinin. In order to gain insight into the mechanism for the resistance, we studied the effect of dihydroartemisinin (DHA), the active compound of artemisinin, on the redox cascade of NADPH/FNR/ferredoxin in in vitro reconstituted systems. DHA partially inhibited the diaphorase activity of FNR by decreasing the affinity of FNR for NADPH. The activity of the electron transfer from FNR to wild-type or D97Y mutant ferredoxin was not significantly affected by DHA. An in silico docking analysis indicated possible binding of DHA molecule in the binding cavity of 2′5′ADP, a competitive inhibitor for NADPH, on FNR. We previously showed that the D97Y mutant of ferredoxin binds to FNR more strongly than wild-type ferredoxin, and ferredoxin and FNR are generally known to be involved in the oxidative stress response. Thus, these results suggest that ferredoxin is not a direct target of artemisinin, but its mutation may be involved in the protective response against the oxidative stress caused by artemisinin.

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.

Synthesis and Evaluation of 2-Hydroxy-1,4-naphthoquinone Derivatives as Potent Antimalarial Agents

A series of 3-substituted-2-hydroxy-1,4-naphthoquinone derivatives with a variety of side chains were successfully synthesized by Mannich reaction of 2-hydroxy-1,4-naphthoquinone (lawsone) with selected amines and aldehydes. All substances (1-16) were evaluated for in-vitro antimalarial activity against strains of Plasmodium falciparum by microculture radioisotope technique. Bioassay data revealed that ten derivatives (1-8, 11 and 13) displayed significantly good activity with values of IC₅₀ ranging from 0.77 to 4.05 µg/mL. The best biological profile (IC₅₀ = 0.77 µg/mL) was observed in compound 1, possessing a n-butyl substituted aminomethyl group. Experimental results support the potential use of our active Mannich components as promising antimalarial agents in the fight against malaria infections and multidrug resistance problems.

Reactions of Plasmodium falciparum Ferredoxin:NADP+ Oxidoreductase with Redox Cycling Xenobiotics: A Mechanistic Study

Ferredoxin:NADP⁺ oxidoreductase from Plasmodium falciparum (PfFNR) catalyzes the NADPH-dependent reduction of ferredoxin (PfFd), which provides redox equivalents for the biosynthesis of isoprenoids and fatty acids in the apicoplast. Like other flavin-dependent electrontransferases, PfFNR is a potential source of free radicals of quinones and other redox cycling compounds. We report here a kinetic study of the reduction of quinones, nitroaromatic compounds and aromatic N-oxides by PfFNR. We show that all these groups of compounds are reduced in a single-electron pathway, their reactivity increasing with the increase in their single-electron reduction midpoint potential (E¹₇). The reactivity of nitroaromatics is lower than that of quinones and aromatic N-oxides, which is in line with the differences in their electron self-exchange rate constants. Quinone reduction proceeds via a ping-pong mechanism. During the reoxidation of reduced FAD by quinones, the oxidation of FADH^. to FAD is the possible rate-limiting step. The calculated electron transfer distances in the reaction of PfFNR with various electron acceptors are similar to those of Anabaena FNR, thus demonstrating their similar “intrinsic” reactivity. Ferredoxin stimulated quinone- and nitro-reductase reactions of PfFNR, evidently providing an additional reduction pathway via reduced PfFd. Based on the available data, PfFNR and possibly PfFd may play a central role in the reductive activation of quinones, nitroaromatics and aromatic N-oxides in P. falciparum, contributing to their antiplasmodial action.

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.

Distinct activation mechanisms trigger the trypanocidal activity of DNA damaging prodrugs

Quinone-based compounds have been exploited to treat infectious diseases and cancer, with such chemicals often functioning as inhibitors of key metabolic pathways or as prodrugs. Here, we screened an aziridinyl 1,4-benzoquinone (ABQ) library against the causative agents of trypanosomiasis, and cutaneous leishmaniasis, identifying several potent structures that exhibited EC50 values of <100 nM. However, these compounds also displayed significant toxicity towards mammalian cells indicating that they are not suitable therapies for systemic infections. Using anti-T. brucei ABQs as chemical probes, we demonstrated that these exhibit different trypanocidal modes of action. Many functioned as type I nitroreductase (TbNTR) or cytochrome P450 reductase (TbCPR) dependent prodrugs that, following activation, generate metabolites which promote DNA damage, specifically interstrand crosslinks (ICLs). Trypanosomes lacking TbSNM1, a nuclease that specifically repairs ICLs, are hypersensitive to most ABQ prodrugs, a phenotype exacerbated in cells also engineered to express elevated levels of TbNTR or TbCPR. In contrast, ABQs that contain substituent groups on the biologically active aziridine do not function as TbNTR or TbCPR-activated prodrugs and do not promote DNA damage. By unravelling how ABQs mediate their activities, features that facilitate the desired anti-parasitic growth inhibitory effects could be incorporated into new, safer compounds targeting these neglected tropical diseases.

Millipedes as food for humans: their nutritional and possible antimalarial value-a first report

The first record of millipedes (Diplopoda) being regularly used for food by humans (the Bobo people of Burkina Faso) is given, including information on how the millipedes are prepared. The species in question are Tymbodesmus falcatus (Karsch, 1881) and Sphenodesmus sheribongensis (Schiøtz, 1966) (Gomphodesmidae) and an unidentified species of Spirostreptidae. New information on the nutritional value of millipedes is provided; unsaturated fatty acids, calcium, and iron contents are particularly high. The millipedes' defensive secretions, hydrogen cyanide and benzoquinones, present a severe challenge for the spread of millipedes as an everyday food source. On the other hand, the possibility that benzoquinones may act as insect-repellents, as known from studies on nonhuman primates, and that sublethal cyanide ingestion may enhance human innate resistance to malaria, suggests promising ethnomedical perspectives to our findings.

Oxidative stress in malaria

Malaria is a significant public health problem in more than 100 countries and causes an estimated 200 million new infections every year. Despite the significant effort to eradicate this dangerous disease, lack of complete knowledge of its physiopathology compromises the success in this enterprise. In this paper we review oxidative stress mechanisms involved in the disease and discuss the potential benefits of antioxidant supplementation as an adjuvant antimalarial strategy.

Redox-active antiparasitic drugs

SIGNIFICANCE

Parasitic diseases affect hundreds of millions of people worldwide and represent major health problems. Treatment is becoming extremely difficult due to the emergence of drug resistance, the absence of effective vaccines, and the spread of insecticide-resistant vectors. Thus, identification of affordable and readily available drugs against resistant parasites is of global demand.

RECENT ADVANCES

Susceptibility of many parasites to oxidative stress is a well-known phenomenon. Therefore, generation of reactive oxygen species (ROS) or inhibition of endogenous antioxidant enzymes would be a novel therapeutic approach to develop antiparasitic drugs. This article highlights the unique metabolic pathways along with redox enzymes of unicellular (Plasmodium falciparum, Trypanosoma cruzi, Trypanosoma brucei, Leishmania donovani, Entamoeba histolytica, and Trichomonas vaginalis) and multicellular parasites (Schistosoma mansoni), which could be utilized to promote ROS-mediated toxicity.

CRITICAL ISSUES

Enzymes involved in various vital redox reactions could be potential targets for drug development.

FUTURE DIRECTIONS

The identification of redox-active antiparasitic drugs along with their mode of action will help researchers around the world in designing novel drugs in the future.

Redox reactions of the FAD-containing apoptosis-inducing factor (AIF) with quinoidal xenobiotics: a mechanistic study

Mitochondrial apoptosis-inducing factor (AIF) is a FAD-containing protein that under certain conditions translocates to the nucleus and causes a programmed cell death, apoptosis. The apoptogenic action of AIF is redox controlled as the NADH-reduced AIF dimer has lower affinity for DNA than the oxidized monomer. To gain further insights into the mechanism of AIF, we investigated its interaction with a series of quinone oxidants, including a number of anticancer quinones. Our data indicate that the NADH:quinone oxidoreduction catalyzed by AIF follows a "ping-pong" scheme, with the reductive half-reaction being rate-limiting and the FADH(-)-NAD(+) charge-transfer complex serving as an electron donor. AIF is equally reactive toward benzo- and naphthoquinones, but may discriminate structures with a higher number of aromatic rings. The reactivity of quinones is mainly defined by their one-electron reduction potential, whereas the size and nature of the substituents play a minor role. AIF is unlikely to significantly contribute to bioreductive activation of low-potential quinoidal anticancer quinones. However, high-potential quinones, e.g. a toxic natural compound naphthazarin, maintain AIF in the oxidized state when a significant excess of NADH is present. Thus, these compounds may prevent the accumulation of the reduced form of AIF in vivo, and enhance AIF-mediated apoptosis.

A partial convergence in action of methylene blue and artemisinins: antagonism with chloroquine, a reversal with verapamil, and an insight into the antimalarial activity of chloroquine

Artemisinins rapidly oxidize leucomethylene blue (LMB) to methylene blue (MB); they also oxidize dihydroflavins such as the reduced conjugates RFH₂ of riboflavin (RF), and FADH₂ of the cofactor flavin adenine dinucleotide (FAD), to the corresponding flavins. Like the artemisinins, MB oxidizes FADH₂, but unlike artemisinins, it also oxidizes NAD(P)H. Like MB, artemisinins are implicated in the perturbation of redox balance in the malaria parasite by interfering with parasite flavoenzyme disulfide reductases. The oxidation of LMB by artemisinin is inhibited by chloroquine (CQ), an inhibition that is abruptly reversed by verapamil (VP). CQ also inhibits artemisinin-mediated oxidation of RFH₂ generated from N-benzyl-1,4-dihydronicotinamide (BNAH)-RF, or FADH₂ generated from NADPH or NADPH-Fre, an effect that is also modulated by verapamil. The inhibition likely proceeds by the association of LMB or dihydroflavin with CQ, possibly involving donor-acceptor or π complexes that hinder oxidation by artemisinin. VP competitively associates with CQ, liberating LMB or dihydroflavin from their respective CQ complexes. The observations explain the antagonism between CQ-MB and CQ-artemisinins in vitro, and are reconcilable with CQ perturbing intraparasitic redox homeostasis. They further suggest that a VP-CQ complex is a means by which VP reverses CQ resistance, wherein such a complex is not accessible to the putative CQ-resistance transporter (PfCRT).