Aminonaphthoquinones--a novel class of compounds with potent antimalarial activity against Plasmodium falciparum

Visible-light-induced C-H alkylation of 2-amino-1,4-naphthoquinones

Simple and practical strategies for visible-light-induced C-H alkylation of 2-amino-1,4-naphthoquinones with cyclobutanone oxime esters and hydroxamic acid derivatives have been developed under mild and redox-neutral conditions. These two reactions can be carried out at room temperature and obtain a variety of 2-amino-1,4-naphthoquinone derivatives with cyano and amide groups. Moreover, the cyanoalkylation reaction of 2-amino-1,4-naphthoquinones can proceed smoothly in the absence of photocatalysts.

α-amylase inhibition and in silico studies of novel naphtho[2,3-d]imidazole-4,9-dione linked N-acyl hydrazones

Aim: To enrich the pool of α-amylase inhibitors to manage Type 2 diabetes. Methods: Synthesis, conformational study, α-amylase inhibitory action and various in silico studies of novel N'-(arylbenzylidene)-2-(4,9-dioxo-4,9-dihydro-1H-naphtho[2,3-d]imidazol-1-yl)acetohydrazides carried out. Results: Compound H6 demonstrated the highest activity (IC50 = 0.0437 μmol mL-1) among the tested compounds. Structure-activity relationship study suggested that variable substitution at the aryl ring has a pivotal role in determining the inhibitory action of tested compounds. Docking simulations of the most active compound (H6) confirmed its interaction potential with active site residues of A. oryzae α-amylase. The root-mean-square deviation fluctuations substantiated the stability of protein-ligand complex. Absorption, distribution, metabolism and excretion prediction revealed optimal values for absorption, distribution, metabolism and excretion parameters. Conclusion: The developed molecules could be beneficial for the development of novel α-amylase inhibitors to treat Type 2 diabetes.

Bioprospection of Selected Plant Secondary Metabolites as Modulators of the Proteolytic Activity of Plasmodium falciparum Plasmepsin V

Malaria is a devastating disease, and its management is only achieved through chemotherapy. However, resistance to available medication is still a challenge; therefore, there is an urgent need for the discovery and development of therapeutics with a novel mechanism of action to counter the resistance scourge consistent with the currently available antimalarials. Recently, plasmepsin V was validated as a therapeutic target for the treatment of malaria. The pepsin-like aspartic protease anchored in the endoplasmic reticulum is responsible for the trafficking of parasite-derived proteins to the erythrocytic surface of the host cells. In this study, a small library of compounds was preliminarily screened in vitro to identify novel modulators of Plasmodium falciparum plasmepsin V (PfPMV). The results obtained revealed kaempferol, quercetin, and shikonin as possible PfPMV inhibitors, and these compounds were subsequently probed for their inhibitory potentials using in vitro and in silico methods. Kaempferol and shikonin noncompetitively and competitively inhibited the specific activity of PfPMV in vitro with IC₅₀ values of 22.4 and 43.34 μM, respectively, relative to 62.6 μM obtained for pepstatin, a known aspartic protease inhibitor. Further insight into the structure-activity relationship of the compounds through a 100 ns molecular dynamic (MD) simulation showed that all the test compounds had a significant affinity for PfPMV, with quercetin (-36.56 kcal/mol) being the most prominent metabolite displaying comparable activity to pepstatin (-35.72 kcal/mol). This observation was further supported by the compactness and flexibility of the resulting complexes where the compounds do not compromise the structural integrity of PfPMV but rather stabilized and interacted with the active site amino acid residues critical to PfPMV modulation. Considering the findings in this study, quercetin, kaempferol, and shikonin could be proposed as novel aspartic protease inhibitors worthy of further investigation in the treatment of malaria.

Antimalarial Inhibitors Targeting Epigenetics or Mitochondria in Plasmodium falciparum: Recent Survey upon Synthesis and Biological Evaluation of Potential Drugs against Malaria

Despite many efforts, malaria remains among the most problematic infectious diseases worldwide, mainly due to the development of drug resistance by P. falciparum. Over the past decade, new essential pathways have been emerged to fight against malaria. Among them, epigenetic processes and mitochondrial metabolism appear to be important targets. This review will focus on recent evolutions concerning worldwide efforts to conceive, synthesize and evaluate new drug candidates interfering selectively and efficiently with these two targets and pathways. The focus will be on compounds/scaffolds that possess biological/pharmacophoric properties on DNA methyltransferases and HDAC’s for epigenetics, and on cytochrome bc1 and dihydroorotate dehydrogenase for mitochondrion.

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.

In silico, in vitro and in vivo evaluation of natural Bignoniaceous naphthoquinones in comparison with atovaquone targeting the selection of potential antimalarial candidates

The natural naphthoquinones lapachol, α- and β-lapachone are found in Bignoniaceous Brazilian plant species of the Tabebuia genus (synonym Handroanthus) and are recognized for diverse bioactivities, including as antimalarial. The aim of the present work was to perform in silico, in vitro and in vivo studies to evaluating the antimalarial potential of these three naphthoquinones in comparison with atovaquone, a synthetic antimalarial. The ADMET properties of these compounds were predicted in silico by the preADMET program. The in vitro toxicity assays were experimentally determined in immortalized and tumoral cells from different organs. In vivo acute oral toxicity was also evaluated for lapachol. Several favorable pharmacokinetics data were predicted although, as expected, high cytotoxicity was experimentally determined for β-lapachone. Lapachol was not cytotoxic or showed low cytotoxicity to all of the cells assayed (HepG2, A549, Neuro 2A, LLC-PK1, MRC-5), it was nontoxic in the acute oral test and disclosed the best parasite selectivity index in the in vitro assays against chloroquine resistant Plasmodium falciparum W2 strain. On the other hand, α- and β-lapachone were more potent than lapachol in the antiplasmodial assays but with low parasite selectivity due to their cytotoxicity. The diversity of data here reported disclosed lapachol as a promising candidate to antimalarial drug development.

Small Molecule Inhibitors Targeting the Heat Shock Protein System of Human Obligate Protozoan Parasites

Obligate protozoan parasites of the kinetoplastids and apicomplexa infect human cells to complete their life cycles. Some of the members of these groups of parasites develop in at least two systems, the human host and the insect vector. Survival under the varied physiological conditions associated with the human host and in the arthropod vectors requires the parasites to modulate their metabolic complement in order to meet the prevailing conditions. One of the key features of these parasites essential for their survival and host infectivity is timely expression of various proteins. Even more importantly is the need to keep their proteome functional by maintaining its functional capabilities in the wake of physiological changes and host immune responses. For this reason, molecular chaperones (also called heat shock proteins)-whose role is to facilitate proteostasis-play an important role in the survival of these parasites. Heat shock protein 90 (Hsp90) and Hsp70 are prominent molecular chaperones that are generally induced in response to physiological stress. Both Hsp90 and Hsp70 members are functionally regulated by nucleotides. In addition, Hsp70 and Hsp90 cooperate to facilitate folding of some key proteins implicated in cellular development. In addition, Hsp90 and Hsp70 individually interact with other accessory proteins (co-chaperones) that regulate their functions. The dependency of these proteins on nucleotide for their chaperone function presents an Achille's heel, as inhibitors that mimic ATP are amongst potential therapeutic agents targeting their function in obligate intracellular human parasites. Most of the promising small molecule inhibitors of parasitic heat shock proteins are either antibiotics or anticancer agents, whose repurposing against parasitic infections holds prospects. Both cancer cells and obligate human parasites depend upon a robust protein quality control system to ensure their survival, and hence, both employ a competent heat shock machinery to this end. Furthermore, some inhibitors that target chaperone and co-chaperone networks also offer promising prospects as antiparasitic agents. The current review highlights the progress made so far in design and application of small molecule inhibitors against obligate intracellular human parasites of the kinetoplastida and apicomplexan kingdoms.

Synthesis and Antiplasmodial Activity of 1,2,3-Triazole-Naphthoquinone Conjugates

A series of 34 1,2,3-triazole-naphthoquinone conjugates were synthesized via copper-catalyzed cycloaddition (CuAAC). They were evaluated for their in vitro antimalarial activity against chloroquine-sensitive strains of Plasmodium falciparum and against three different tumor cell lines (SKBr-3, MCF-7, HEL). The most active antimalarial compounds showed a low antiproliferative activity. Simplified analogues were also obtained and some structure–activity relationships were outlined. The best activity was obtained by compounds 3s and 3j, having IC₅₀ of 0.8 and 1.2 μM, respectively. Molecular dockings were also carried on Plasmodium falciparum enzyme dihydroorotate dehydrogenase (PfDHODH) in order to rationalize the results.

Construction of Bridged-Ring-Fused Naphthalenone Derivatives Through an Unexpected Zn(OTf)2-Catalyzed Cascade Transformation

An unexpected cascade transformation of aminonaphthoquinones with N-substituents bearing a p-methoxybenzyl ether into bridged-ring-fused naphthalenone derivatives is reported. This cascade transformation was initiated by a catalytic amount of Zn(OTf)₂ and involved with subsequent functional group migration and cyclization. The process proceeded through the cleavage of two bonds and the formation of three new bonds in one pot and was proven to be efficient and tolerant to various substituents.

Detoxification Mechanism of 8,8-Dimethyl-3-[(R-phenyl)amino]-1,4,5(8H)-naphthalentrione Derivatives by Botrytis cinerea

The effect of 8,8-dimethyl-3-[(R-phenyl)amino]-1,4,5(8H)-naphthalentrione derivatives (compounds 1⁻13) on the mycelial growth of Botrytis cinerea was evaluated. The fungitoxic effect depended on the substituent and its position in the aromatic ring. Compounds substituted with halogens in meta and/or para positions (compounds 3, 4, 5 and 7), methyl (compounds 8 and 9), methoxyl (compounds 10 and 11), or ethoxy-carbonyl groups (compound 12) presented higher antifungal activity than compound 1, which had an unsubstituted aromatic ring. In addition, compounds with halogens in the ortho position, such as compounds 2 and 6, and a substitution with an acetyl group in the para position (compound 13) were less active. The role of the ABC efflux pump Bctr B-type as a defense mechanism of B. cinerea against these naphthalentrione derivatives was analyzed. This pump could be involved in the detoxification of compounds 2, 6, and 13. On the contrary, this mechanism would not participate in the detoxification of compounds 1, 7, 9 and 12. Finally, the biotransformation of compound 7 by B. cinerea was studied. A mixture of two biotransformed products was obtained. One of them was compound 7A, which is reduced at C1 and C4, compared to compound 7. The other product of biotransformation, 7B, is oxidized at C7.

Synthesis, SAR, and Docking Studies Disclose 2-Arylfuran-1,4-naphthoquinones as In Vitro Antiplasmodial Hits

A total of 28 lapachol-related naphthoquinones with four different scaffolds were synthesized and spectroscopically characterized. In vitro antiplasmodial activity was assayed against the chloroquine-resistant Plasmodium falciparum W2 strain by the parasite lactate dehydrogenase (pLDH) method. Cytotoxicity against Hep G2A16 cell was determined by the MTT assay. All compounds disclosed higher in vitro antiplasmodial activity than lapachol. Ortho- and para-naphthoquinones with a furan ring fused to the quinonoid moiety were more potent than 2-hydroxy-3-(1′-alkenyl)-1,4-naphthoquinones, while ortho-furanonaphthoquinones were more cytotoxic. Molecular docking to Plasmodium targets Pfcyt bc1 complex and PfDHOD enzyme showed that five out of the 28 naphthoquinones disclosed favorable binding energies. Furanonaphthoquinones endowed with an aryl moiety linked to the furan ring are highlighted as new in vitro antiplasmodial lead compounds and warrant further investigation.

Antiparasitic activity of menadione (vitamin K3) against Schistosoma mansoni in BABL/c mice

Schistosomiasis is one of the neglected tropical diseases affecting nearly quarter of a billion people in economically challenged tropical and subtropical countries of the world. Praziquantel (PZQ) is the only drug currently available to treat this parasitic disease in spite being ineffective against juvenile worms and concerns about developing resistance to treat reinfections. Our earlier in vitro viability studies demonstrated significant antiparasitic activity of menadione (MEN) (vitamin K₃) against Schistosoma mansoni adult worms. To gain insight into plausible mechanism of antischistosomal activity of MEN, its effect on superoxide anion levels in adult worms were studied in vitro which showed significant increases in both female and male worms. Further confirmation of the deleterious morphological changes in their teguments and organelles were obtained by ultrastructural analysis. Genotoxic and cytotoxic studies in male Swiss mice indicated that MEN was well tolerated at the oral dose of 500mg/kg using the criteria of MNPCE frequency and PCE/RBC ratio in the bone marrow of infected animals. The in vivo antiparasitic activity of MEN was conducted in female BALB/c mice infected with S. mansoni and significant reductions (P<0.001) in total worm burden were observed at single oral doses of 40 and 400mg/kg (48.57 and 61.90%, respectively). Additionally, MEN significantly reduced (P<0.001) the number of eggs in the liver of infected mice by 53.57 and 58.76%, respectively. Similarly, histological analysis of the livers showed a significant reduction (P<0.001) in the diameter of the granulomas. Since MEN is already in use globally as an over-the-counter drug for a variety of common ailments and a dietary supplement with a safety record in par with similar products when used in recommended doses, the above antiparasitic results which compare reasonably well with PZQ, make a compelling case for considering MEN to treat S. mansoni infection in humans.

Factors Affecting the Formation of 2:1 Host:Guest Inclusion Complexes of 2-[(R-Phenyl)amine]-1,4-naphthalenediones (PAN) in β- and γ-Cyclodextrins

The molecular hosts cyclodextrins form inclusion complexes with a wide variety of guests, resulting in complexes with various host:guest stoichiometries. In the case of a series of 19 1,4-naphthoquinolines as guests with either β- or γ-cyclodextrin studied using electrospray mass spectroscopy, in most cases only 1:1 complexes were observed, with 2:1 host:guest complexes observed in just 6 out of 38 host:guest combinations. It is shown that these higher-order complexes were observed only in the case of small (or no) electronically withdrawing substituents, and were much less likely in the case of the larger γ-cyclodextrin host. The size and electronic properties of the substituents involved shows that both steric and electronic factors must be taken into account in predicting which cyclodextrin host:guest stoichiometries will be stable enough to form (or once formed, be robust enough to be observed in the ESI-MS experiments). It is clear that the prediction of host-guest stoichiometry for a specific host-guest pair is complicated, and involves a subtle interplay of both electronic and steric factors. However, there are definite trends, which can be used to help predict host:guest stoichiometry for a given host-guest pair.

Antifungal and Cytotoxic Assessment of Lapachol Derivatives Produced by Fungal Biotransformation

In the screening for biological active compounds, the biotransformation processes catalyzed by filamentous fungi are useful because they can provide information about the possible appearance of toxic metabolites after oral administration and also generate new leads. In this paper, biotransformation of lapachol (1) by three fungal strains, Mucor circinelloides NRRL3631, Botrytis cinerea UCA992 and Botrytis cinerea 2100, has been investigated for the first time. Lapachol (1) was biotransformed into avicequinone-A (2) by M. circinelloides, 3′-hydroxylapachol (3) by B. cinerea, and into dehydro-α-lapachone (4) by both fungi. All these compounds were evaluated for their cytotoxic activities. The metabolite 2 displayed non-selective cytotoxicity against tumor and normal cell lines, 3 did not show cytotoxicity against the same cells, while 4 showed higher cytotoxicity against cancer cell lines than lapachol (1). The transformation of 1 into harmless and reactive metabolites evidences the importance of the evaluation of drug metabolism in the drug discovery process. Antifungal potential of lapachol (1) and its metabolites 2 and 4 against B. cinerea has also been evaluated. Dehydro-α-lapachone (4) has been shown to be less toxic to fungal growth than lapachol (1), which indicates a detoxification mechanism of the phytopathogen.

Bromine substituted aminonaphthoquinones: synthesis, characterization, DFT and metal ion binding studies

Bromine substituted aminonaphthoquinones – chemosensors for metal ions.

Chemical Constituents of Dendrobium venustum and their Antimalarial and Anti-herpetic Properties

A MeOH extract from the whole plant Dendrobium venustum exhibited significant antimalarial and anti-herpetic activities. Bioassay-guided isolation of the plant extract resulted in the isolation of seven known phenolic compounds. Densiflorol B (3) and phoyunnanin E (6) showed the strongest antimalarial activity and a high selectivity index, whereas gigantol (2), batatasin III (5) and phoyunnanin C (7) exhibited moderate activity. Compounds 2 and 5 also showed weak activity against the Herpes simplex virus. This study is the first report on the chemical and biological activities of D. venustum.

Cytotoxicity of lapachol metabolites produced by probiotics

Probiotics are currently added to a variety of functional foods to provide health benefits to the host and are commonly used by patients with gastrointestinal complaints or diseases. The therapeutic effects of lapachol continue to inspire studies to obtain derivatives with improved bioactivity and lower unwanted effects. Therefore, the general goal of this study was to show that probiotics are able to convert lapachol and are important to assess the effects of bacterial metabolism on drug performance and toxicity. The microbial transformations of lapachol were carried out by Bifidobacterium sp. and Lactobacillus acidophilus and different metabolites were produced in mixed and isolated cultures. The cytotoxic activities against breast cancer and normal fibroblast cell lines of the isolated metabolites (4α-hydroxy-2,2-dimethyl-5-oxo-2,3,4,4α,5,9β-hexahydroindeno[1,2-β]pyran-9β-carboxilic acid, a new metabolite produced by mixed culture and dehydro-α-lapachone produced by isolated cultures) were assessed and compared with those of lapachol. The new metabolite displayed a lower activity against a breast cancer cell line (IC50 = 532.7 μmol l(-1) ) than lapachol (IC50 = 72.3 μmol l(-1) ), while dehydro-α-lapachone (IC50 = 10.4 μmol l(-1) ) displayed a higher activity than lapachol. The present study is the first to demonstrate that probiotics are capable of converting lapachol into the most effective cytotoxic compound against a breast cancer cell line.

SIGNIFICANCE AND IMPACT OF THE STUDY

Probiotics have been used in dairy products to promote human health and have the ability to metabolize drugs and other xenobiotics. Naphthoquinones, such as lapachol, are considered privileged scaffolds due to their high propensity to interact with biological targets. The present study is the first to demonstrate that probiotics are capable of converting lapachol into the most effective cytotoxic compound against a breast cancer cell line. The developed approach highlights the importance of probiotics to assess the effects of bacterial metabolism on drug performance and toxicity.

tert-ButylN-{3-[(3-chloro-1,4-dioxo-1,4-dihydronaphthalen-2-yl)amino]propyl}carbamate

In the title compound, C₁₈H₂₁ClN₂O₄, the mol­ecular sytructure is stabilized by two intra­molecular N—H⋯O hydrogen bonds. In the crystal, mol­ecules are linked by pairs of C—H⋯O hydrogen bonds, forming inversion dimers with graph-set motif R₂²(10). N—H⋯O hydrogen bonds further link the dimers into C(10) chains along [010].

Screening for small molecule modulators of Hsp70 chaperone activity using protein aggregation suppression assays: inhibition of the plasmodial chaperone PfHsp70-1

Plasmodium falciparum heat shock protein 70 (PfHsp70-1) is thought to play an essential role in parasite survival and virulence in the human host, making it a potential antimalarial drug target. A malate dehydrogenase based aggregation suppression assay was adapted for the screening of small molecule modulators of Hsp70. A number of small molecules of natural (marine prenylated alkaloids and terrestrial plant naphthoquinones) and related synthetic origin were screened for their effects on the protein aggregation suppression activity of purified recombinant PfHsp70-1. Five compounds (malonganenone A-C, lapachol and bromo-β-lapachona) were found to inhibit the chaperone activity of PfHsp70-1 in a concentration dependent manner, with lapachol preferentially inhibiting PfHsp70-1 compared to another control Hsp70. Using growth inhibition assays on P. falciparum infected erythrocytes, all of the compounds, except for malonganenone B, were found to inhibit parasite growth with IC50 values in the low micromolar range. Overall, this study has identified two novel classes of small molecule inhibitors of PfHsp70-1, one representing a new class of antiplasmodial compounds (malonganenones). In addition to demonstrating the validity of PfHsp70-1 as a possible drug target, the compounds reported in this study will be potentially useful as molecular probes for fundamental studies on Hsp70 chaperone function.

Facile Synthesis of Imidazoisoindolones and Quinoxalinediones from 2,3-diamino-1,4-naphthoquinone

In aqueous acetic acid, 2,3-diamino-1,4-naphthoquinone reacted with phthalic anhydride and 4,5,6,7-tetrabromophthalic anhydride to give naphtho[2’,3’:4,5]imidazo[2,1- a]isoindole-6,11,13-triones. In the same solvent, the reaction of diaminonaphthoquinone with 2,2-dihydroxy-1 H-indene-1,3(2 H)-dione gave 11 H-benzo[ g]indeno [1,2- b]quinoxaline-6,11,13-trione. Under the same conditions, diaminonaphthoquinone reacted with succinic anhydride and with maleic anhydride, the corresponding 4-(3-amino-1,4-dioxo-1,4-dihydronaphthalene-2-ylamino)-4-oxobutanoic acid and 3,5,10-trioxo-3,4,5,10-tetrahydrobenzo[ g]-quinoxaline-2(1 H)-ylidene-acetic acid derivatives were obtained. The reaction of 2,3-diamino-1,4-naphthoquinone with ( E)-1,2-dibenzoylethylene and 1,4-diphenylbut-2-yne-1,4-dione in aqueous acetic acid afforded dihydrobenzo[ g]quinoxaline-5,10-diones.

Comprehensive evaluation of a novel nuclear factor-kappaB inhibitor, quinoclamine, by transcriptomic analysis

BACKGROUND AND PURPOSE

The transcription factor nuclear factor-kappaB (NF-kappaB) has been linked to the cell growth, apoptosis and cell cycle progression. NF-kappaB blockade induces apoptosis of cancer cells. Therefore, NF-kappaB is suggested as a potential therapeutic target for cancer. Here, we have evaluated the anti-cancer potential of a novel NF-kappaB inhibitor, quinoclamine (2-amino-3-chloro-1,4-naphthoquinone).

EXPERIMENTAL APPROACH

In a large-scale screening test, we found that quinoclamine was a novel NF-kappaB inhibitor. The global transcriptional profiling of quinoclamine in HepG2 cells was therefore analysed by transcriptomic tools in this study.

KEY RESULTS

Quinoclamine suppressed endogenous NF-kappaB activity in HepG2 cells through the inhibition of IkappaB-alpha phosphorylation and p65 translocation. Quinoclamine also inhibited induced NF-kappaB activities in lung and breast cancer cell lines. Quinoclamine-regulated genes interacted with NF-kappaB or its downstream genes by network analysis. Quinoclamine affected the expression levels of genes involved in cell cycle or apoptosis, suggesting that quinoclamine exhibited anti-cancer potential. Furthermore, quinoclamine down-regulated the expressions of UDP glucuronosyltransferase genes involved in phase II drug metabolism, suggesting that quinoclamine might interfere with drug metabolism by slowing down the excretion of drugs.

CONCLUSION AND IMPLICATIONS

This study provides a comprehensive evaluation of quinoclamine by transcriptomic analysis. Our findings suggest that quinoclamine is a novel NF-kappaB inhibitor with anti-cancer potential.

Effect of inducers of DT-diaphorase on the haemolytic activity and nephrotoxicity of 2-amino-1,4-naphthoquinone in rats

Reduction of naphthoquinones by DT-diaphorase is often described as a detoxification reaction. This is true for some naphthoquinone derivatives, such as alkyl and di-alkyl naphthoquinones, but the situation with other substances, such as 2-hydroxy-1,4-naphthoquinone, is more complex. In the present study, the effect of several substances that are known to increase tissue activities of DT-diaphorase on the toxicity of 2-amino-1,4-naphthoquinone has been investigated. Like 2-hydroxy-1,4-naphthoquinone, the 2-amino-derivative was found to cause both haemolytic anaemia and renal tubular necrosis in rats. Again like 2-hydroxy-1,4-naphthoquinone, the severity of the haemolysis induced by the 2-amino derivative was increased in animals pre-treated with inducers of DT-diaphorase, but the degree of nephrotoxicity was decreased. With these substances, therefore, DT-diaphorase both activates and detoxifies the quinone, depending on the target organ. It is not possible to generalize with regard to the effects of modulation of tissue levels of DT-diaphorase on naphthoquinone toxicity in vivo, since this may change not only the severity of the toxic effects, but also the target organ specificity. In evaluating the possible therapeutic applications of such compounds, the possibility of toxic effects upon the blood and kidney must be borne in mind. In man, renal damage by compounds such as 2-hydroxy- and 2-amino-1,4-naphthoquinone may be a particular problem, because of the low level of DT-diaphorase in human liver.

Antimalarial activity of phenazines from lapachol, β-lapachone and its derivatives against Plasmodium falciparum in vitro and Plasmodium berghei in vivo

The antimalarial activity of benzo[a]phenazines synthesized from 1,2-naphthoquinone, lapachol, beta-lapachone and several derivatives have been tested against Plasmodium falciparum in vitro using isolates of parasites with various susceptibilities to chloroquine and/or mefloquine. Parasite growth in the presence of the test drugs was measured by incorporation of [(3)H]-hipoxanthine in comparison to controls with no drugs, always testing in parallel chloroquine, a standard antimalarial. Among seven benzophenazines tested, four had significant in vitro activities; important, the parasites resistant to chloroquine were more susceptible to the active phenazines in vitro. The doses of phenazines causing 50% inhibition of parasite growth varied from 1.67 to 9.44 microM. The two most active ones were also tested in vivo against Plasmodium berghei in mice, in parallel with lapachol and beta-lapachone. The 3-sulfonic acid-beta-lapachone-derived phenazine was the most active causing up to 98% inhibition of parasitaemia in long term treatment (7 doses) subcutaneously, whereas the phenazine from 3-bromo-beta-lapachone was inactive. Thus, these simple phenazines, containing polar (-Br,-I) and ionizable (-SO(3)H, -OH) groups, easily synthesized from cheap, natural or synthetic precursors (lapachol and beta-lapachone), at rather low cost, provide prototypes for development of new antimalarials aiming the chloroquine resistant parasites.

Novel antiplasmodial agents

The morbidity and mortality associated with malaria have spurred efforts to find novel antimalarial agents with improved potency and selectivity. Leads for agents continue to be obtained from natural sources (plants and microorganisms) and chemical syntheses. Screening of commercial or specialized databases have also yielded promising leads. The structural diversity of compounds with good (micromolar and lower) activity point to the considerable tolerance for different structural elements in the "antimalarial pharmacophore." It may also be a reflection of the varied targets present in the plasmodia. The challenge in malaria chemotherapy is to find safe and selective agents whose potencies will not be compromised by plasmodial resistance. Modification of potential leads should also aim at improving "drug-like" character, viz. to ensure acceptable oral bioavailability. A review of the literature shows that there is a growing trend towards the development of target-specific antimalarial agents (for example, agents inhibiting plasmodial farnesyl transferase, cyclin dependent kinases, proteases, choline transport). An increasing number of reports focus on the development of chemosensitizers, agents that are capable of reversing plasmodial resistance.