Antimalarial activity of thioacridone compounds related to the acronycine alkaloid

A series of thioacridone compounds that were previously shown to have DNA binding interaction, were screened for antimalarial activity. The new compounds were assessed for in vitro antimalarial activity against a chloroquine sensitive (D10) strain of the malaria parasite Plasmodium falciparum, using a lactate dehydrogenase (PfLDH) assay. In the series, the IC(50) values ranged from 0.4 to 27 microg/ml. 1-(2-Dimethylaminoethylamino)-9(10H)-thioacridone was found to be the most potent against P. falciparum (D10) with an IC(50) value of 0.4 microg/ml. This compound was also evaluated against a South African chloroquine resistant (RSA 11) P. falciparum strain and was found to have an IC(50) value of 1 microg/ml, compared with 0.16 microg/ml for chloroquine. Quantitative structure-activity relationships of this series were also investigated and a multiple linear regression r(2) of 0.58 was found for the best fit equation. The most potent compound, 1-(2-dimethylaminoethylamino)-9(10H)-thioacridone, was docked into the chloroquine binding site of PfLDH and it was found that the slightly lower activity of this compound, compared with chloroquine, is likely due to steric interference within a restricted binding pocket.     

Antimalarial activity of potential inhibitors of Plasmodium falciparum lactate dehydrogenase enzyme selected by docking studies

The Plasmodium falciparum lactate dehydrogenase enzyme (PfLDH) has been considered as a potential molecular target for antimalarials due to this parasite's dependence on glycolysis for energy production. Because the LDH enzymes found in P. vivax, P. malariae and P. ovale (pLDH) all exhibit ～90% identity to PfLDH, it would be desirable to have new anti-pLDH drugs, particularly ones that are effective against P. falciparum, the most virulent species of human malaria. Our present work used docking studies to select potential inhibitors of pLDH, which were then tested for antimalarial activity against P. falciparum in vitro and P. berghei malaria in mice. A virtual screening in DrugBank for analogs of NADH (an essential cofactor to pLDH) and computational studies were undertaken, and the potential binding of the selected compounds to the PfLDH active site was analyzed using Molegro Virtual Docker software. Fifty compounds were selected based on their similarity to NADH. The compounds with the best binding energies (itraconazole, atorvastatin and posaconazole) were tested against P. falciparum chloroquine-resistant blood parasites. All three compounds proved to be active in two immunoenzymatic assays performed in parallel using monoclonals specific to PfLDH or a histidine rich protein (HRP2). The IC(50) values for each drug in both tests were similar, were lowest for posaconazole (<5 μM) and were 40- and 100-fold less active than chloroquine. The compounds reduced P. berghei parasitemia in treated mice, in comparison to untreated controls; itraconazole was the least active compound. The results of these activity trials confirmed that molecular docking studies are an important strategy for discovering new antimalarial drugs. This approach is more practical and less expensive than discovering novel compounds that require studies on human toxicology, since these compounds are already commercially available and thus approved for human use.     

Identification and characterization of heme-interacting proteins in the malaria parasite,Plasmodium falciparum

The degradation of hemoglobin by the malaria parasite, Plasmodium falciparum, produces free ferriprotoporphyrin IX (FP) as a toxic by-product. In the presence of FP-binding drugs such as chloroquine, FP detoxification is inhibited, and the build-up of free FP is thought to be a key mechanism in parasite killing. In an effort to identify parasite proteins that might interact preferentially with FP, we have used a mass spectrometry approach. Proteins that bind to FP immobilized on agarose include P. falciparum glyceraldehyde-3-phosphate dehydrogenase (PfGAPDH), P. falciparum glutathione reductase (PfGR), and P. falciparum protein disulfide isomerase. To examine the potential consequences of FP binding, we have examined the ability of FP to inhibit the activities of GAPDH and GR from P. falciparum and other sources. FP inhibits the enzymic activity of PfGAPDH with a Ki value of 0.2 microm, whereas red blood cell GAPDH is much less sensitive. By contrast, PfGR is more resistant to FP inhibition (Ki > 25 microm) than its human counterpart. We also examined the ability of FP to inhibit the activities of the additional antioxidant enzymes, P. falciparum thioredoxin reductase, which exhibits a Ki value of 1 microm, and P. falciparum glutaredoxin, which shows more moderate sensitivity to FP. The exquisite sensitivity of PfGAPDH to FP may indicate that the glycolytic pathway of the parasite is particularly susceptible to modulation by FP stress. Inhibition of this pathway may drive flux through the pentose phosphate pathway ensuring sufficient production of reducing equivalents to counteract the oxidative stress induced by FP build-up.     

Stage-specific activity of potential antimalarial compounds measured in vitro by flow cytometry in comparison to optical microscopy and hypoxanthine uptake

The evaluation of new antimalarial agents using older methods of monitoring sensitivity to antimalarial drugs are laborious and poorly suited to discriminate stage-specific activity. We used flow cytometry to study the effect of established antimalarial compounds, cysteine protease inhibitors, and a quinolone against asexual stages of Plasmodium falciparum. Cultured P. falciparum parasites were treated for 48 h with different drug concentrations and the parasitemia was determined by flow cytometry methods after DNA staining with propidium iodide. P. falciparum erythrocytic life cycle stages were readily distinguished by flow cytometry. Activities of established and new antimalarial compounds measured by flow cytometry were equivalent to results obtained with microscopy and metabolite uptake assays. The antimalarial activity of all compounds was higher against P. falciparum trophozoite stages. Advantages of flow cytometry analysis over traditional assays included higher throughput for data collection, insight into the stage-specificity of antimalarial activity avoiding use of radioactive isotopes.     

Double-drug development against antioxidant enzymes from Plasmodium falciparum

New drugs against malaria are urgently and continuously needed. Plasmodium parasites are exposed to higher fluxes of reactive oxygen species and need high activities of intracellular antioxidant systems. A most important antioxidative system consists of (di)thiols which are recycled by disulfide reductases (DR), namely both glutathione reductases (GR) of the malarial parasite Plasmodium falciparum and man, and the thioredoxin reductase (TrxR) of P. falciparum. The aim of our interdisciplinary research is to substantiate DR inhibitors as antimalarial agents. Such compounds are active per se but, in addition, they can reverse thiol-based resistance against other drugs in parasites. Reversal of drug resistance by DR inhibitors is currently investigated for the commonly used antimalarial drug chloroquine (CQ). Our recent strategy is based on the synthesis of inhibitors of the glutathione reductases from parasite and host erythrocyte. With the expectation of a synergistic or additive effect, double-headed prodrugs were designed to be directed against two different and essential functions of the malarial parasite P. falciparum, namely glutathione regeneration and heme detoxification. The prodrugs were prepared by linking bioreversibly a GR inhibitor to a 4-aminoquinoline moiety which is known to concentrate in the acidic food vacuole of parasites. Drug-enzyme interaction was correlated with antiparasitic action in vitro on strains resistant towards CQ and in vivo in Plasmodium berghei-infected mice as well as absence of cytotoxicity towards human cells. Because TrxR of P. falciparum was recently shown to be responsible for the residual glutathione disulfide-reducing capacity observed after GR inhibition in P. falciparum, future development of antimalarial drug-candidates that act by perturbing the redox equilibrium of parasites is based on the design of new double-drugs based on TrxR inhibitors as potential antimalarial drug candidates.     

Structures of P. falciparum PfPK5 test the CDK regulation paradigm and suggest mechanisms of small molecule inhibition

Plasmodium falciparum cell cycle regulators are promising targets for antimalarial drug design. We have determined the structure of PfPK5, the first structure of a P. falciparum protein kinase and the first of a cyclin-dependent kinase (CDK) not derived from humans. The fold and the mechanism of inactivation of monomeric CDKs are highly conserved across evolution. ATP-competitive CDK inhibitors have been developed as potential leads for cancer therapeutics. These studies have identified regions of the CDK active site that can be exploited to achieve significant gains in inhibitor potency and selectivity. We have cocrystallized PfPK5 with three inhibitors that target such regions. The sequence differences between PfPK5 and human CDKs within these inhibitor binding sites suggest that selective inhibition is an attainable goal. Such compounds will be useful tools for P. falciparum cell cycle studies, and will provide lead compounds for antimalarial drug development.     

High-throughput screening for small-molecule inhibitors of plasmodium falciparum glucose-6-phosphate dehydrogenase 6-phosphogluconolactonase

Plasmodium falciparum causes severe malaria infections in millions of people every year. The parasite is developing resistance to the most common antimalarial drugs, which creates an urgent need for new therapeutics. A promising and attractive target for antimalarial drug design is the bifunctional enzyme glucose-6-phosphate dehydrogenase-6-phosphogluconolactonase (PfGluPho) of P. falciparum, which catalyzes the key step in the parasites' pentose phosphate pathway. In this study, we describe the development of a high-throughput screening assay to identify small-molecule inhibitors of recombinant PfGluPho. The optimized assay was used to screen three small-molecule compound libraries-namely, LOPAC (Sigma-Aldrich, 1280 compounds), Spectrum (MicroSource Discovery Systems, 1969 compounds), and DIVERSet (ChemBridge, 49 971 compounds). These pilot screens identified 899 compounds that inhibited PfGluPho activity by at least 50%. Selected compounds were further studied to determine IC(50) values in an orthogonal assay, the type of inhibition and reversibility, and effects on P. falciparum growth. Screening results and follow-up studies for selected PfGluPho inhibitors are presented. Our high-throughput screening assay may provide the basis to identify novel and urgently needed antimalarial drugs.     

Novel oxazine skeletons as potential antiplasmodial active ingredients: Synthesis, in vitro and in vivo biology of some oxazine entities produced via cyclization of novel chalcone intermediates

A novel series of 6-(2-chloroquinolin-3-yl)-4-substituted-phenyl-6H-1,3-oxazin-2-amines were synthesized and evaluated for in vitro antimalarial efficacy against chloroquine sensitive (MRC-02) as well as chloroquine resistant (RKL9) strains of Plasmodium falciparum. The activity tested was at nanomolar concentration. β-Hematin formation inhibition activity (BHIA(50)) of oxazines were determined and correlated with antimalarial activity. A reasonably good correlation (r?=?0.49 and 0.51, respectively) was observed between antimalarial activity (IC(50)) and BHIA(50). This suggests that antimalarial mode of action of these compounds seems to be similar to that of chloroquine and involves the inhibition of hemozoin formation. Some of the compounds were showing better antimalarial activity than chloroquine against resistant strain of P. falciparum and were also found to be active in the in vivo experiment.     

Synthesis and in vitro evaluation of new chloroquine-chalcone hybrids against chloroquine-resistant strain of Plasmodium falciparum

The control of malaria has been complicated with increasing resistance of malarial parasite against existing antimalarials. Herein, we report the synthesis of a new series of chloroquine-chalcone based hybrids (8-22) and their antimalarial efficacy against both chloroquine-susceptible (3D7) and chloroquine-resistant (K1) strains of Plasmodium falciparum. Most of the compounds showed enhanced antimalarial activity as compared to chloroquine in chloroquine-resistant (K1) strain of Plasmodium falciparum. Furthermore, to unfold the mechanism of action of these synthesized hybrid molecules, we carried out hemin dependent studies, in which three compounds were found to be active.     

Identification and characterization of small molecule inhibitors of Plasmodium falciparum dihydroorotate dehydrogenase

Plasmodium falciparum causes the most deadly form of malaria and accounts for over one million deaths annually. The malaria parasite is unable to salvage pyrimidines and relies on de novo biosynthesis for survival. Dihydroorotate dehydrogenase (DHOD), a mitochondrially localized flavoenzyme, catalyzes the rate-limiting step of this pathway and is therefore an attractive antimalarial chemotherapeutic target. Using a target-based high throughput screen, we have identified a series of potent, species-specific inhibitors of P. falciparum DHOD (pfDHOD) that are also efficacious against three cultured strains (3D7, HB3, and Dd2) of P. falciparum. The primary antimalarial mechanism of action of these compounds was confirmed to be inhibition of pfDHOD through a secondary assay with transgenic malaria parasites, and the structural basis for enzyme inhibition was explored through in silico structure-based docking and site-directed mutagenesis. Compound-mediated cytotoxicity was not observed with human dermal fibroblasts or renal epithelial cells. These data validate pfDHOD as an antimalarial drug target and provide chemical scaffolds with which to begin medicinal chemistry efforts.     

New hybrid trifluoromethylquinolines as antiplasmodium agents

Malaria remains a major public health problem worldwide, and it is responsible for high rates of morbidity and mortality. Resistance to current antimalarial drugs has been identified, and new drugs are urgently needed. In this study, we designed and synthesized seventeen novel quinolines based on the structures of mefloquine ((2,8-bis(trifluoromethyl)quinolin-4-yl)(piperidin-2-yl)methanol) and amodiaquine (4-((7-chloroquinolin-4-yl)amino)-2-((diethylamino)methyl)phenol) using ring bioisosteric replacement and molecular hybridization of the functional groups. The compounds were evaluated in vitro against Plasmodium falciparum and in vivo in mice infected with P. berghei. All derivatives presented anti-P. falciparum activity with IC₅₀ values ranging from 0.083 to 33.0?µM. The compound with the best anti-P. falciparum activity was N-(5-methyl-4H-1,2,4-triazol-3-yl)-2,8-bis(trifluoromethyl)quinolin-4-amine (12) which showed an IC₅₀ of 0.083?µM. The three most active compounds were selected for antimalarial activity tests against P. berghei-infected mice. Compound 12 was the most active on the 5th day after infection, reducing parasitemia by 66%, which is consistent with its in vitro activity. This is an important result as 12, a simpler molecule than mefloquine, does not contain the stereogenic center, and consequently, its synthesis in the laboratory is easier and less expensive. This system proved promising for the design of potential antimalarial compounds.     

Antimalarial activity and mechanisms of action of two novel 4-aminoquinolines against chloroquine-resistant parasites

Chloroquine (CQ) is a cost effective antimalarial drug with a relatively good safety profile (or therapeutic index). However, CQ is no longer used alone to treat patients with Plasmodium falciparum due to the emergence and spread of CQ-resistant strains, also reported for P. vivax. Despite CQ resistance, novel drug candidates based on the structure of CQ continue to be considered, as in the present work. One CQ analog was synthesized as monoquinoline (MAQ) and compared with a previously synthesized bisquinoline (BAQ), both tested against P. falciparum in vitro and against P. berghei in mice, then evaluated in vitro for their cytotoxicity and ability to inhibit hemozoin formation. Their interactions with residues present in the NADH binding site of P falciparum lactate dehydrogenase were evaluated using docking analysis software. Both compounds were active in the nanomolar range evaluated through the HRPII and hypoxanthine tests. MAQ and BAQ derivatives were not toxic, and both compounds significantly inhibited hemozoin formation, in a dose-dependent manner. MAQ had a higher selectivity index than BAQ and both compounds were weak PfLDH inhibitors, a result previously reported also for CQ. Taken together, the two CQ analogues represent promising molecules which seem to act in a crucial point for the parasite, inhibiting hemozoin formation.     

Antimalarial effect of bis-pyridinium salts, N,N'-hexamethylenebis(4-carbamoyl-1-alkylpyridinium bromide)

The in vitro antimalarial activity of bis-pyridinium salts, N,N'-hexamethylenebis(4-carbamoyl-1-decylpyridinium bromide) and their derivatives, against the Plasmodium falciparum FCR-3 strain (ATCC 30932, chloroquine-sensitive) was evaluated. All test compounds exhibited antimalarial activity over a concentration range of 3.5microM to 10nM. The chain length of the N1-alkyl moiety was found to be very beneficial in terms of antimalarial activity, and in this series of compounds, the most appropriate N1-alkyl chain length was found to be eight.     

Ferredoxin-NADP+ reductase from Plasmodium falciparum undergoes NADP+-dependent dimerization and inactivation: functional and crystallographic analysis

The completion of the Plasmodium falciparum genome sequence has recently promoted the search for new antimalarial drugs. More specifically, metabolic pathways of the apicoplast, a key organelle for survival of the parasite, have been recognized as potential targets for the development of specific new antimalarial agents. As most apicomplexan parasites, P. falciparum displays a plant-type ferredoxin-NADP(+) reductase, yielding reduced ferredoxin for essential biosynthetic pathways in the apicoplast. Here we report a molecular, kinetic and ligand binding characterization of the recombinant ferredoxin-NADP(+) reductase from P. falciparum, in the light of current data available for plant ferredoxin-NADP(+) reductases. In parallel with the functional characterization, we describe the crystal structures of P. falciparum ferredoxin-NADP(+) reductase in free form and in complex with 2'-phospho-AMP (at 2.4 and 2.7 A resolution, respectively). The enzyme displays structural properties likely to be unique to plasmodial reductases. In particular, the two crystal structures highlight a covalent dimer, which relies on the oxidation of residue Cys99 in two opposing subunits, and a helix-coil transition that occurs in the NADP-binding domain, triggered by 2'-phospho-AMP binding. Studies in solution show that NADP(+), as well as 2'-phospho-AMP, promotes the formation of the disulfide-stabilized dimer. The isolated dimer is essentially inactive, but full activity is recovered upon disulfide reduction. The occurrence of residues unique to the plasmodial enzyme, and the discovery of specific conformational properties, highlight the NADP-binding domain of P. falciparum ferredoxin-NADP(+) reductase as particularly suited for the rational development of antimalarial compounds.     

Antiplasmodial activity of novel keto-enamine chalcone-chloroquine based hybrid pharmacophores

A series of novel keto-enamine chalcone-chloroquine based hybrids were synthesized following new methodology developed in our laboratory. The synthesized compounds were screened against chloroquine sensitive strain (3D7) of Plasmodium falciparum in an in vitro model. Some of the compounds were showing comparable antimalarial activity at par with chloroquine. Compounds with significant in vitro antimalarial activity were then evaluated for their in vivo efficacy in Swiss mice against Plasmodium yoelii (chloroquine resistant N-67 strain), wherein compounds 25 and 27 each showed an in vivo suppression of 99.9% parasitaemia on day 4. Biochemical studies reveal that inhibition of hemozoin formation is the primary mechanism of action of these analogues.     

The antimalarial effect of iron chelators: studies in animal models and in humans with mild falciparum malaria.

In this study we explore the antimalarial effects of 3-hydroxypyridin-4-ones (CP compounds), a family of bidentate orally effective iron chelators in experimental animal systems in vivo and in vitro, and examine whether the iron chelator deferoxamine (DF) is active against human infection with P. falciparum. There was direct relation between lipid solubility of the CP compounds, which would facilitate membrane transit, and their in vivo antimalarial action, suggesting direct intracellular iron chelation as the most likely explantation for the antimalarial effect of iron chelators. Results of the double-blind, placebo controlled trial of DF in humans with asymptomatic parasitemia provided unequivocal evidence that this iron-chelating agent has antimalarial activity. Depriving the parasite of a metabolically important source of iron may represent a novel approach to antimalarial drug development. DF is a relatively ineffective intraerythrocytic chelator, and our data indicate that other orally effective iron chelators may have superior antimalarial activity in vivo. A systematic screening of available iron chelating drugs may result in the identification of potentially useful antimalarial compounds.     

Discovery and biochemical characterization of Plasmodium thioredoxin reductase inhibitors from an antimalarial set

Plasmodium falciparum is the most prevalent and deadly species of the human malaria parasites, and thioredoxin reductase (TrxR) is an enzyme involved in the redox response to oxidative stress. Essential for P. falciparum survival, the enzyme has been highlighted as a promising target for novel antimalarial drugs. Here we report the discovery and characterization of seven molecules from an antimalarial set of 13533 compounds through single-target TrxR biochemical screens. We have produced high-purity, full-length, recombinant native enzyme from four Plasmodium species, and thioredoxin substrates from P. falciparum and Rattus norvegicus. The enzymes were screened using a unique, high-throughput, in vitro native substrate assay, and we have observed selectivity between the Plasmodium species and the mammalian form of the enzyme. This has indicated differences in their biomolecular profiles and has provided valuable insights into the biochemical mechanisms of action of compounds with proven antimalarial activity.     

Synthesis and antimalarial evaluation of novel isocryptolepine derivatives

A series of mono- and di-substituted analogues of isocryptolepine have been synthesized and evaluated for in vitro antimalarial activity against chloroquine sensitive (3D7) and resistant (W2mef) Plasmodium falciparum and for cytotoxicity (3T3 cells). Di-halogenated compounds were the most potent derivatives and 8-bromo-2-chloroisocryptolepine displayed the highest selectivity index (106; the ratio of cytotoxicity (IC(50)=9005 nM) to antimalarial activity (IC(50)=85 nM)). Our evaluation of novel isocryptolepine compounds has demonstrated that di-halogenated derivatives are promising antimalarial lead compounds.     

Aryl aryl methyl thio arenes prevent multidrug-resistant malaria in mouse by promoting oxidative stress in parasites

We have synthesized a new series of aryl aryl methyl thio arenes (AAMTAs) and evaluated antimalarial activity in vitro and in vivo against drug-resistant malaria. These compounds interact with free heme, inhibit hemozoin formation, and prevent Plasmodium falciparum growth in vitro in a concentration-dependent manner. These compounds concentration dependently promote oxidative stress in Plasmodium falciparum as evident from the generation of intraparasitic oxidants, protein carbonyls, and lipid peroxidation products. Furthermore, AAMTAs deplete intraparasite GSH levels, which is essential for antioxidant defense and survival during intraerythrocytic stages. These compounds displayed potent antimalarial activity not only in vitro but also in vivo against multidrug-resistant Plasmodium yoelii dose dependently in a mouse model. The mixtures of enantiomers of AAMTAs containing 3-pyridyl rings were found to be more efficient in providing antimalarial activity. Efforts have been made to synthesize achiral AAMTAs 17-23 and among them, compound 18 showed significant antimalarial activity in vivo.     

Antimalarial histone deacetylase inhibitors containing cinnamate or NSAID components

Malaria is the most lethal parasite-mediated tropical infectious disease, killing 1-2 million people each year. An emerging drug target is the enzyme Plasmodium falciparum histone deacetylase 1 (PfHDAC1). We report 26 compounds designed to bind the zinc and exterior surface around the entrance to the active site of PfHDAC1, 16 displaying potent in vitro antimalarial activity (IC(50)<100 nM) against P. falciparum. Selected compounds were shown to cause hyperacetylation of P. falciparum histones and be >10-fold more cytotoxic towards P. falciparum than a normal human cell type (NFF). Twenty-two inhibitors feature cinnamic acid derivatives or non-steroidal anti-inflammatory drugs (NSAIDs) as HDAC-binding components. A homology model of PfHDAC1 enzyme gives new insights to interactions likely made by some of these inhibitors. Results support PfHDAC1 as a promising new antimalarial drug target.