Antimalarial Molecules Research Articles

First report on exploring structural requirements of alpha and beta thymidine analogs for PfTMPK inhibitory activity using in silico studies

With the emergence of multi-drug resistance of the currently available antimalarial drugs including the “magic bullet” artemisinin derivatives in the market, there is an urgent need for discovery and development of new potent antimalarial molecules. The present work deals with quantitative structure–activity relationship (QSAR) modeling, pharmacophore mapping and docking studies of a series of 35 thymidine analogs as inhibitors of Plasmodium falciparum thymidylate kinase (PfTMPK), an enzyme that catalyzes phosphorylation of thymidine monophosphate (TMP) to thymidine diphosphate (TDP). The models were validated both internally and externally and significant statistical results were obtained, indicating the robustness and reliability of the developed models. The docking study was performed using the LigandFit option of receptor–ligand interactions protocol section available in Discovery Studio 2.1 where lower RMSD values (0.6931Å) between the co-crystallized ligand and re-docked ligand assured that the ligand was bound in the same binding pocket. The QSAR, pharmacophore mapping and docking studies provide an understanding of important structural requirements or essential molecular properties, or features of molecules, and important binding interactions, and provide an important guidance for the chemist to synthesis of new molecules with improved PfTMPK inhibitory activity profile. This work revealed the importance of –NH-fragment, electrophilicity of the molecules and the number of oxygen atom towards the PfTMPK inhibitory activity of the molecules. To the best of our knowledge, this work presents the first QSAR and pharmacophore report for thymidine analogs which may serve as an efficient tool for the design and synthesis of potent molecules as PfTMPK inhibitors to address the increasing threat of multi-drug resistance against P. falciparum.

Contrasted Fitness Costs of Docking and Antibacterial Constructs in the EE and EVida3 Strains Validates Two-Phase Anopheles gambiae Genetic Transformation System

The deployment of transgenic mosquitoes carrying genes for refractoriness to malaria has long been seen as a futuristic scenario riddled with technical difficulties. The integration of anti-malarial effector genes and a gene-drive system into the mosquito genome without affecting mosquito fitness is recognized as critical to the success of this malaria control strategy. Here we conducted detailed fitness studies of two Anopheles gambiae s.s. transgenic lines recently developed using a two-phase targeted genetic transformation system. In replicated cage-invasion experiments, males and females of the EE Phase-1 docking strain and EVida3 Phase-2 strain loaded with an antimicrobial peptide (AMP) expressed upon blood-feeding, were mixed with individuals of a recently-colonized strain of the Mopti chromosomal form. The experimental design enabled us to detect initial strain reproductive success differences, assortative mating and hybrid vigor that may characterize mosquito release situations. In addition, the potential fitness costs of the unloaded Phase-1 and loaded Phase-2 genetic constructs, independent of the strains’ original genetic backgrounds, were estimated between the 1st instar larvae, pupae and adult stages over 10 generations. The Phase-1 unloaded docking cassette was found to have significantly lower allelic fitness relative to the wild type allele during larval development. However, overall genotypic fitness was comparable to the wild type allele across all stages leading to stable equilibrium in all replicates. In contrast, the Phase-2 construct expressing EVida3 disappeared from all replicates within 10 generations due to lower fitness of hemi- and homozygous larvae, suggesting costly background AMP expression and/or of the DsRed2 marker. This is the first study to effectively partition independent fitness stage-specific determinants in unloaded and loaded transgenic strains of a Phase-1–2 transformation system. Critically, the high fitness of the Phase-1 docking strain makes it the ideal model system for measuring the genetic load of novel candidate anti-malarial molecules in vivo.

Mannich reaction: A versatile and convenient approach to bioactive skeletons

This review gives an insight into the recent applications of Mannich reaction and its variants in the construction of bioactive molecules. Emphasis is given to the Mannich reaction that provides bioactive molecules and/or modifies the property of an existing bioactive molecule. The role of Mannich reaction in the construction of antimalarial, antitumour, antimicrobial, antitubercular, antiinflammatory and anticonvulsant molecules and also the significance of aminoalkyl Mannich side chain on the biological property of molecules is discussed here. Mannich reaction plays a seminal role in the sysnthesis of numerous bioactive molecules. This review potrays applications of Mannich reaction and its variants in the construction of bioactive molecules and the role of aminoalkyl Mannich side chain on the property of bioactive/therapeutic molecules.

Exploring structural requirements of aurone derivatives as antimalarials by validated DFT-based QSAR, HQSAR, and COMFA–COMSIA approach

Chloroquine resistance is nowadays a great problem in malaria. Aurone derivatives were effective against chloroquine resistant parasite. Validated density functional theory (DFT)-based chemometric modeling, hologram QSAR (HQSAR), comparative molecular field analysis (CoMFA), and comparative molecular similarity analysis (CoMSIA) studies were conducted on 35 aurone derivatives having antimalarial activity. 2D-QSAR models were developed on the training sets by Y-based ranking method. This model was validated on 50 pairs of the test and the training sets by k-Means cluster analysis method. HQSAR, CoMFA, and CoMSIA models were validated by standard techniques and each method validates the DFT-based 2D-QSAR study and in turn validates the earlier observed structural activity relationship data as well as each other. DFT-based 2D-QSAR model suggests that the increase of Mulliken charge at C14 and HOMO density located on C11 may be conducive to antimalarial activity. Ethyl group attached to C14 and the increase of the value of chemical potential may be beneficial for antimalarial activity. Methoxy fragment is important for better antimalarial activity by HQSAR study. CoMFA analysis shows a favorable steric green region is located near C14 whereas the unfavorable yellow region is far away from C14. A large blue region located near C14 indicates the positively charged groups are favorable at this position. CoMSIA steric features correlates well with the CoMFA steric features. CoMSIA study suggests the bulky hydrophobic substitution at C14 is necessary for antimalarial activity. These results may be utilized to obtain potential antimalarial molecules.

Design and evaluation of Lumefantrine – Oleic acid self nanoemulsifying ionic complex for enhanced dissolution

BackgroundLumefantrine, an antimalarial molecule has very low and variable bioavailability owing to its extremely poor solubility in water. It is recommended to be taken with milk to enhance its solubility and bioavailability. The aim of present study was to develop a Self Nanoemulsifying Delivery system (SNEDs) of lumefantrine (LF) to achieve rapid and complete dissolution independent of food-fat and surfactant in dissolution media.MethodsSolubility of LF in oil, co-solvent/co-surfactant and surfactant solution and emulsification efficiency of surfactant were analyzed to optimize the LF loaded self nanoemulsifying preconcentrate. Effect of LF-oleic acid complexation on emulsification, droplet size, zeta potential and dissolution were investigated. Effect of milk concentration and fat content on saturation solubility and dissolution of LF was investigated. Dissolution of marketed formulation and LF-SNEDs was carried out in pH 1.2 and pH 6.8 phosphate buffer.ResultsLF exhibited very high solubility in oleic acid owing to complexation between tertiary amine of LF and carboxyl group of oleic acid (OA). Cremophore EL and medium chain monoglyceride were selected surfactant and co-surfactant, respectively. Significantly smaller droplet size (37 nm), shift in zeta potential from negative to positive value, very high drug loading in lipid based system (> 10%), no precipitation after dissolution are the major distinguish characteristics contributed by LF-OA complex in the SNED system. Saturation solubility and dissolution study in milk containing media pointed the significant increment in solubility of LF in the presence of milk-food fat. LF-SNEDs showed > 90% LF release within 30 min in pH 1.2 while marketed tablet showed almost 0% drug release.ConclusionSelf nanoemulsification promoting ionic complexation between basic drug and oleic acid hold great promise in enhancing solubility of hydrophobic drugs.

Voulkensin C–E, new 11-oxocassane-type diterpenoids and a steroid glycoside from Caesalpinia volkensii stem bark and their antiplasmodial activities

A bioassay guided isolation of potential antimalarial molecules from the stem bark of Caesalpinia volkensii Harms (Fabaceae) achieved three new 11-oxocassane-type diterpenoids named voulkensin C (1), D (2) and E (3) together with one steroid glycoside named 3-O-[β-glucopyranosyl(1→2)-O-β-xylopyranosyl]-stigmasterol (4) and seven other known compounds including stigmasterol (5), β-sitosterol (6), oleanolic acid (7), 3-β-acetoxyolean-12-en-28-methyl ester (8), voucap-5-ol (9), caesadekarin C (10), deoxycaesaldekarin C (11). The structures of the new compounds were determined on the basis of extensive spectroscopic data (IR, MS, 1H and 13C NMR and 2D NMR) analyses. The polar extracts revealed moderate to good antiplasmodial activities against chloquine-sensitive (D6) and -resistant strains (W2) of Plasmodium falciparum. Whereas the pure isolates exhibited limited to moderate antiplasmodial activities with compound 4 showing the highest antiplasmodial activities (IC50 values of 4.44±0.88 and 2.74±1.10μM against D6 and W2 strains, respectively). These results suggest a possible contribution of phytochemicals from C. volkensii stem bark towards inhibition of plasmodial parasites’ growth hence potential antimalarial.

Predictive modeling of anti-malarial molecules inhibiting apicoplast formation

BackgroundMalaria is a major healthcare problem worldwide resulting in an estimated 0.65 million deaths every year. It is caused by the members of the parasite genus Plasmodium. The current therapeutic options for malaria are limited to a few classes of molecules, and are fast shrinking due to the emergence of widespread resistance to drugs in the pathogen. The recent availability of high-throughput phenotypic screen datasets for antimalarial activity offers a possibility to create computational models for bioactivity based on chemical descriptors of molecules with potential to accelerate drug discovery for malaria.ResultsIn the present study, we have used high-throughput screen datasets for the discovery of apicoplast inhibitors of the malarial pathogen as assayed from the delayed death response. We employed machine learning approach and developed computational predictive models to predict the biological activity of new antimalarial compounds. The molecules were further evaluated for common substructures using a Maximum Common Substructure (MCS) based approach.ConclusionsWe created computational models using state-of-the-art machine learning algorithms. The models were evaluated based on multiple statistical criteria. We found Random Forest based approach provides for better accuracy as assessed from ROC curve analysis. We further evaluated the active molecules using a substructure based approach to identify common substructures enriched in the active set. We argue that the computational models generated could be effectively used to screen large molecular datasets to prioritize them for phenotypic screens, drastically reducing cost while improving the hit rate.

Virtual Screening of Xanthones in Combating Malaria Targeting Plasmodium Falciparum Erythrocyte Membrane Protein 1(PfEMP1)

Malaria is the most important parasitic disease in humans, with transmission occurring in over 100 countries with a population of three billion people. It is caused by protozoan parasites of the genus Plasmodium. These parasites are transmitted from one person to another by the female anopheles mosquito. The Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) family plays a central role in antigenic variation and cytoadhesion of P. falciparum infected erythrocytes. In the present study, investigation of Xanthones as probable anti malarial molecules, was carried out targeted against Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) via molecular docking studies. The in silico effectiveness of Xanthones was studied based upon the interaction with the protein’s active site residues with less binding energy. The interacting Xanthones were further filtered to predict the bioavailability and drug likeness properties. 3, 6-dihydroxyxanthone was shown to be a better interacting ligand with low binding energy (-66.16 kcal/mol) and passed all the physicochemical parameters for drug likeness. This work encourages the development of Xanthones with some chemical modifications to augment more efficacy and better activity as anti malarial drugs.

Synthesis of triazol derivatives of lupeol with potential antimalarial activity

The goal of this project is synthesize and characterization of derivatives of lupeol and evaluated antimalarial activity. Historically, plants are important source of antimalarial medicines, highlighting quinine ( 1 ) (Figure 1), an important alkaloid from the Cinchona calisaya bark. This compound was an important model for cloroquine synthesis, a drug that was widely used in malaria treatment. In addition, one of the principal medicines used today is artemisinine, isolated from the Chinese plant Artemisia annua L ( 2 ) (Figure 1), and their semi synthetic derivatives (artesunate, artemeter, arteter). However, the malaria parasite has already shown resistance to most of these current drugs and the search for new candidates is essential. Lupeol ( 3 ) (Figura 1) is a compound that occurs in many plant species and discloses antimalarial, antiinflamatoryl and antitumoral activities. Considering its potential as a lead antimalarial molecule, we focused our work in the synthesis of new lupeol derivatives with increased antimalarial activity(scheme 1).

Short synthesis and antimalarial activity of fagaronine

Herein, we report a new synthesis of fagaronine 1, inspired by the synthesis reported by Luo for nornitidine. The in vitro biological activity of fagaronine against malaria on several chloroquine-sensitive and resistant Plasmodium falciparum strains was confirmed, and the selectivity index compared to mammalian cells was calculated. Fagaronine was found to have very good antimalarial activity in vivo, comparable to the activity of the reference compound chloroquine. Therefore, fagaronine appears to be a good potential lead for the design of new antimalarial molecules.

QSAR studies on imidazopyridazine derivatives as PfPK7 inhibitors

Quantitative structure–activity relationship (QSAR) studies were carried out on a series of 35 recently synthesised imidazopyridazine derivatives to investigate the structural requirements of their inhibitory activity against malarial kinase, Plasmodium falciparum protein kinase 7 (PfPK7). Partial least square (PLS) methodology coupled with various feature selection methods, viz. stepwise (SW), genetic algorithm (GA) and simulated annealing, was applied to derive QSAR models which were further validated for statistical significance and predictive ability by internal and external validation. The statistically significant best 2D-QSAR model having correlation coefficient r 2 = 0.9190 and cross-validated squared correlation coefficient q 2 = 0.8575 with external predictive ability of pred_r 2 = 0.7212 was developed by SW-PLS with the descriptors like StNE-index, T_N_O_2, T_N_O_7, YcompDipole and SaasCE-index. Molecular field analysis was used to construct the best 3D-QSAR model using GA-PLS method, showing good correlative and predictive capabilities in terms of q 2 = 0.7494 and pred_r 2 = 0.8841. The information rendered by 2D- and 3D-QSAR models may lead to a better understanding of structural requirements of PfPK7 inhibitors and also aid in designing novel potent antimalarial molecules.

Trioxaferroquines as New Hybrid Antimalarial Drugs

The synthesis, characterization, and antimalarial evaluation of a new series of potential antimalarial molecules, named trioxaferroquines, are reported. Trioxaferroquines are hybrid antimalarial drugs containing a 1,2,4-trioxane covalently linked to ferroquine (Fq), a synthetic ferrocenylquinoline derivative currently under clinical development. The aim was to combine, within a single structure, an iron(II) species, a 1,2,4-trioxane, as in artemisinin, and a substituted quinoline, as in chloroquine.

Synthesis of novel α-pyranochalcones and pyrazoline derivatives as Plasmodium falciparum growth inhibitors

Both the lack of a credible malaria vaccine and the emergence and spread of parasites resistant to most of the clinically used antimalarial drugs and drug combination have aroused an imperative need to develop new drugs against malaria. In present work, α-pyranochalcones and pyrazoline analogs were synthesized to discover chemically diverse antimalarial leads. Compounds were tested for antimalarial activity by evaluation of the growth of malaria parasite in culture using the microtiter plate based SYBR-Green-I assay. The (E)-3-(3-(2,3,4-trimethoxyphenyl)-acryloyl)-2H-chromen-2-one (Ga6) turned out to be the most potent analog of the series, showing IC50 of 3.1μg/ml against chloroquine-sensitive (3D7) strain and IC50 of 1.1μg/ml against chloroquine-resistant field isolate (RKL9) of Plasmodium falciparum. Cytotoxicity study of the most potent compounds was also performed against HeLa cell line using the MTT assay. All the tested compounds showed high therapeutic indices suggesting that they were selective in their action against the malaria parasite. Furthermore, docking of Ga6 into active site of falcipain enzyme revealed its predicted interactions with active site residues. This is the first instance wherein chromeno-pyrazolines have been found to be active antimalarial agents. Further exploration and optimization of this new lead could provide novel, antimalarial molecules which can ward off issues of cross-resistance to drugs like chloroquine.

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.

A new process of multicomponent Povarov reaction–aerobic dehydrogenation: synthesis of polysubstituted quinolines

A new domino process of three-component Povarov reaction and aerobic dehydrogenation was developed toward the synthesis of polysubstituted quinolines. 2,4-Disubstituted-8-nitroquinolines, which are important precursors for the synthesis of antimalarial primaquine drug-like molecules, were prepared conveniently by this method in moderate to good yields. Brönsted acid (HClO 4)-modified montmorillonite was found to be a crucial catalyst in promoting the procedure.

Antimalarial Activity of Simalikalactone E, a New Quassinoid from Quassia amara L. (Simaroubaceae)

We report the isolation and identification of a new quassinoid named simalikalactone E (SkE), extracted from a widely used Amazonian antimalarial remedy made out of Quassia amara L. (Simaroubaceae) leaves. This new molecule inhibited the growth of Plasmodium falciparum cultured in vitro by 50%, in the concentration range from 24 to 68 nM, independently of the strain sensitivity to chloroquine. We also showed that this compound was able to decrease gametocytemia with a 50% inhibitory concentration sevenfold lower than that of primaquine. SkE was found to be less toxic than simalikalactone D (SkD), another antimalarial quassinoid from Q. amara, and its cytotoxicity on mammalian cells was dependent on the cell line, displaying a good selectivity index when tested on nontumorogenic cells. In vivo, SkE inhibited murine malaria growth of Plasmodium vinckei petteri by 50% at 1 and 0.5 mg/kg of body weight/day, by the oral or intraperitoneal routes, respectively. The contribution of quassinoids as a source of antimalarial molecules needs therefore to be reconsidered.

Marine Antimalarials

Malaria is an infectious disease causing at least 1 million deaths per year, and, unfortunately, the chemical entities available to treat malaria are still too limited. In this review we highlight the contribution of marine chemistry in the field of antimalarial research by reporting the most important results obtained until the beginning of 2009, with particular emphasis on recent discoveries. About 60 secondary metabolites produced by marine organisms have been grouped into three structural types and discussed in terms of their reported antimalarial activities. The major groups of metabolites include isonitrile derivatives, alkaloids and endoperoxide derivatives. The following discussion evidences that antimalarial marine molecules can efficiently integrate the panel of lead compounds isolated from terrestrial sources with new chemical backbones and, sometimes, with unique functional groups.

Mimicking direct protein–protein and solvent-mediated interactions in the CDP-methylerythritol kinase homodimer: a pharmacophore-directed virtual screening approach

The 2C-methylerythritol 4-phosphate (MEP) pathway for the biosynthesis of isopentenyl pyrophosphate and its isomer dimethylallyl pyrophosphate, which are the precursors of isoprenoids, is present in plants, in the malaria parasite Plasmodium falciparum and in most eubacteria, including pathogenic agents. However, the MEP pathway is absent from fungi and animals, which have exclusively the mevalonic acid pathway. Given the characteristics of the MEP pathway, its enzymes represent potential targets for the generation of selective antibacterial, antimalarial and herbicidal molecules. We have focussed on the enzyme 4-(cytidine 5'-diphospho)-2-C-methyl-D-erythritol kinase (CMK), which catalyses the fourth reaction step of the MEP pathway. A molecular dynamics simulation was carried out on the CMK dimer complex, and protein-protein interactions analysed, considering also water-mediated interactions between monomers. In order to find small molecules that bind to CMK and disrupt dimer formation, interactions observed in the dynamics trajectory were used to model a pharmacophore used in database searches. Using an intensity-fading matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry approach, one compound was found to interact with CMK. The data presented here indicate that a virtual screening approach can be used to identify candidate molecules that disrupt the CMK-CMK complex. This strategy can contribute to speeding up the discovery of new antimalarial, antibacterial, and herbicidal compounds.

Synthesis, Antimalarial Activity, and Intracellular Targets of MEFAS, a New Hybrid Compound Derived from Mefloquine and Artesunate

A new synthetic antimalarial drug, a salt derived from two antimalarial molecules, mefloquine (MQ) and artesunate (AS), here named MEFAS, has been tested for its pharmacological activity. Combinations of AS plus MQ hydrochloride are currently being used in areas with drug-resistant Plasmodium falciparum parasites; although AS clears parasitemia in shorter time periods than any other antimalarial drug, it does not cure infected patients; in addition, MQ causes side effects and is rather expensive, important problems considering that malaria affects mostly populations in poor countries. Here, we show that MEFAS is more effective than the combination of AS and MQ, tested in parallel at different mass proportions, against P. falciparum (chloroquine-resistant clone W2 and chloroquine-sensitive clone 3D7) in vitro and in mice infected with Plasmodium berghei, promoting cure of this infection. MEFAS tested against HepG2 hepatoma cells exhibited lower toxicity than the antimalarials AS and MQ alone or combined. Possible targets of MEFAS have been studied by confocal microscopy using fluorescent probes (Fluo-4 AM and BCECF-AM) in P. falciparum synchronous culture of W2-infected red blood cells. Dynamic images show that MEFAS exhibited intracellular action increasing cytoplasmic Ca(2+) at 1.0 ng/ml. This effect was also observed in the presence of tapsigargin, an inhibitor of SERCA, suggesting an intracellular target distinct from the endoplasmic reticulum. Trophozoites loaded with BCECF-AM, when treated with MEFAS, were still able to mobilize protons from the digestive vacuole (DV), altering the pH gradient. However, in the presence of bafilomycin A1, an inhibitor of the H(+) pump from acidic compartments of eukaryotic cells, MEFAS had no action on the DV. In conclusion, the endoplasmic reticulum and DV are intracellular targets for MEFAS in Plasmodium sp., suggesting two modes of action of this new salt. Our data support MEFAS as a candidate for treating human malaria.

In vitro activity of ferroquine (SSR 97193) against Plasmodium falciparum isolates from the Thai-Burmese border.

BackgroundOn the borders of Thailand, Plasmodium falciparum has become resistant to nearly all available drugs, and there is an urgent need to find new antimalarial drugs or drug combinations. Ferroquine (SSR97193) is a new 4-aminoquinoline antimalarial active against chloroquine resistant and sensitive P. falciparum strains in vivo and in vitro. This antimalarial organic iron complex (a ferrocenyl group has been associated with chloroquine) is meant to use the affinity of Plasmodium for iron to increase the probability for encountering the anti-malarial molecule.The aim of the present study was to investigate the activity of ferroquine against P. falciparum isolates from an area with a known high multi-drug resistance rate.MethodsParasite isolates were obtained from patients with acute falciparum malaria attending the clinics of SMRU. In vitro cultures of these isolates were set-up in the SMRU-laboratory on pre-dosed drug plates, and grown in culture for 42 hours. Parasite growth was assessed by the double-site enzyme-linked pLDH immunodetection (DELI) assay.ResultsSixty-five P. falciparum isolates were successfully grown in culture. The ferroquine mean IC50 (95% CI) was 9.3 nM (95% C.I.: 8.7 – 10.0). The mean IC50 value for the principal metabolite of ferroquin, SR97213A, was 37.0 nM (95% C.I.: 34.3 – 39.9), which is four times less active than ferroquine. The isolates in this study were highly multi-drug resistant but ferroquine was more active than chloroquine, quinine, mefloquine and piperaquine. Only artesunate was more active than ferroquine. Weak but significant correlations were found between ferroquine and its principal metabolite (r2 = 0.4288), chloroquine (r2 = 0.1107) and lumefantrine (r2 = 0.2364).ConclusionThe results presented in this study demonstrate that the new ferroquine compound SSR97193 has high anti-malarial activity in vitro against multi-drug resistant P. falciparum.