Antimalarial Mode Of Action Research Articles

Lapatinib, Nilotinib and Lomitapide Inhibit Haemozoin Formation in Malaria Parasites.

With the continued loss of antimalarials to resistance, drug repositioning may have a role in maximising efficiency and accelerating the discovery of new antimalarial drugs. Bayesian statistics was previously used as a tool to virtually screen USFDA approved drugs for predicted β-haematin (synthetic haemozoin) inhibition and in vitro antimalarial activity. Here, we report the experimental evaluation of nine of the highest ranked drugs, confirming the accuracy of the model by showing an overall 93% hit rate. Lapatinib, nilotinib, and lomitapide showed the best activity for inhibition of β-haematin formation and parasite growth and were found to inhibit haemozoin formation in the parasite, providing mechanistic insights into their mode of antimalarial action. We then screened the USFDA approved drugs for binding to the β-haematin crystal, applying a docking method in order to evaluate its performance. The docking method correctly identified imatinib, lapatinib, nilotinib, and lomitapide. Experimental evaluation of 22 of the highest ranked purchasable drugs showed a 24% hit rate. Lapatinib and nilotinib were chosen as templates for shape and electrostatic similarity screening for lead hopping using the in-stock ChemDiv compound catalogue. The actives were novel structures worthy of future investigation. This study presents a comparison of different in silico methods to identify new haemozoin-inhibiting chemotherapeutic alternatives for malaria that proved to be useful in different ways when taking into consideration their strengths and limitations.

AntiMalarial Mode of Action (AMMA) Database: Data Selection, Verification and Chemical Space Analysis

This paper presents the effort of collecting and curating a data set of 15461 molecules tested against the malaria parasite, with robust activity and mode of action annotations. The set is compiled from in-house experimental data and the public ChEMBL database subsets. We illustrate the usefulness of the dataset by building QSAR models for antimalarial activity and QSPR models for modes of actions, as well as by the analysis of the chemical space with the Generative Topographic Mapping method. The GTM models perform well in prediction of both activity and mode of actions, on par with the classical SVM methods. The visualization of obtained maps helps to understand the distribution of molecules corresponding to different modes of action: molecules with similar targets are located close to each other on the map. Therefore, this analysis may suggest new modes of action for non-annotated or even annotated compounds. In perspective, this can be used as a tool for prediction of both antimalarial activity and target for novel, untested compounds.

Theoretical analysis of the binding of iron(III) protoporphyrin IX to 4-methoxyacetophenone thiosemicarbazone via DFT-D3, MEP, QTAIM, NCI, ELF, and LOL studies.

Thiosemicarbazones display diverse pharmacological properties, including antimalarial activities. Their pharmacological activities have been studied in depth, but little of this research has focused on their antimalarial mode of action. To elucidate this antimalarial mechanism, we investigated the nature of the interactions between iron(III) protoporphyrin IX (Fe(III)PPIX) and the thione-thiol tautomers of 4-methoxyacetophenone thiosemicarbazone (MAPTSC). Dispersion-corrected density functional theory (DFT-D3), the quantum theory of atoms in molecules (QTAIM), the noncovalent interaction (NCI) index, the electron localization function (ELF), the localized orbital locator (LOL), and thermodynamic calculations were employed in this work. Fe(III)PPIX-MAPTSC binding is expected to inhibit hemozoin formation, thereby preventing Fe(III)PPIX detoxification in plasmodia. Preliminary studies geared toward the identification of atomic binding sites in the thione-thiol tautomers of MAPTSC were carried out using molecular electrostatic potential (MEP) maps and conceptual DFT-based local reactivity indices. The thionic sulfur and the 2 N-azomethine nitrogen/thiol sulfur of, respectively, the thione and thiol tautomers of MAPTSC were identified as the most favorable nucleophilic sites for electrophilic attack. The negative values of the computed Fe(III)PPIX-MAPTSC binding energies, enthalpies, and Gibbs free energies are indicative of the existence and stability of Fe(III)PPIX-MAPTSC complexes. MAPTSC-Fe(III) coordinate bonds and strong hydrogen bonds (N-H···O) between the NH2 group in MAPTSC and the C=O group in one propionate side chain of Fe(III)PPIX are crucial to Fe(III)PPIX-MAPTSC binding. QTAIM, NCI, ELF, and LOL analyses revealed a subtle interplay of weak noncovalent interactions dominated by dispersive-like van der Waals interactions between Fe(III)PPIX and MAPTSC that stabilize the Fe(III)PPIX-MAPTSC complexes.

Substrate-analogous inhibitors exert antimalarial action by targeting the Plasmodium lactate transporter PfFNT at nanomolar scale.

Resistance against all available antimalarial drugs calls for novel compounds that hit unexploited targets in the parasite. Here, we show that the recently discovered Plasmodium falciparum lactate/proton symporter, PfFNT, is a valid druggable target, and describe a new class of fluoroalkyl vinylogous acids that potently block PfFNT and kill cultured parasites. The original compound, MMV007839, is derived from the malaria box collection of potent antimalarials with unknown targets and contains a unique internal prodrug principle that reversibly switches between a lipophilic transport form and a polar, substrate-analogous active form. Resistance selection of cultured P. falciparum parasites with sub-lethal concentrations of MMV007839 produced a single nucleotide exchange in the PfFNT gene; this, and functional characterization of the resulting PfFNT G107S validated PfFNT as a novel antimalarial target. From quantitative structure function relations we established the compound binding mode and the pharmacophore. The pharmacophore largely circumvents the resistance mutation and provides the basis for a medicinal chemistry program that targets lactate and proton transport as a new mode of antimalarial action.

Antimalarial activity of novel imidazoisoquinolinone derivatives correlates with heme binding affinity

A series of novel imidazoisoquinolinone derivatives were synthesized and evaluated for in vitro antimalarial efficacy against chloroquine sensitive GHA strain of Plasmodium falciparum. Compounds 2, 4, 6, 9, and 17 revealed moderate to good activities in the micromolar range. Binding interaction between these active compounds and heme were determined and correlated with antimalarial activity. A good correlation (r = 0.98) was observed between antimalarial activity and the heme dissociation constants (Kd). These suggest that antimalarial mode of action of this class of compounds appears to be similar to that of chloroquine and involves the inhibition of hemozoin formation.

Glutathione Reductase-Catalyzed Cascade of Redox Reactions To Bioactivate Potent Antimalarial 1,4-Naphthoquinones – A New Strategy to Combat Malarial Parasites

Our work on targeting redox equilibria of malarial parasites propagating in red blood cells has led to the selection of six 1,4-naphthoquinones, which are active at nanomolar concentrations against the human pathogen Plasmodium falciparum in culture and against Plasmodium berghei in infected mice. With respect to safety, the compounds do not trigger hemolysis or other signs of toxicity in mice. Concerning the antimalarial mode of action, we propose that the lead benzyl naphthoquinones are initially oxidized at the benzylic chain to benzoyl naphthoquinones in a heme-catalyzed reaction within the digestive acidic vesicles of the parasite. The major putative benzoyl metabolites were then found to function as redox cyclers: (i) in their oxidized form, the benzoyl metabolites are reduced by NADPH in glutathione reductase-catalyzed reactions within the cytosols of infected red blood cells; (ii) in their reduced forms, these benzoyl metabolites can convert methemoglobin, the major nutrient of the parasite, to indigestible hemoglobin. Studies on a fluorinated suicide-substrate indicate as well that the glutathione reductase-catalyzed bioactivation of naphthoquinones is essential for the observed antimalarial activity. In conclusion, the antimalarial naphthoquinones are suggested to perturb the major redox equilibria of the targeted infected red blood cells, which might be removed by macrophages. This results in development arrest and death of the malaria parasite at the trophozoite stage.

Cryptolepine and development of new antimalarial agents

Natural product-derived drugs exemplified by quinine, isolated from South American Cinchona species and artemisinin discovered in China are of immense importance for the treatment of malaria. Although malaria parasites resistant to artemisinin have not yet been found in malaria patients, the need for new antimalarial agents remains. The burden of malaria is heaviest in Africa where over a million children die of the disease each year, but although many medicinal plants are used in African traditional medicines for the treatment of malaria, none has yet yielded and effective antimalarial compound. Cryptolepis sanguinolenta (Periplocaceae), is a West African climbing shrub used traditionally for the treatment of malaria. Cryptolepine, an indoloquinoline alkaloid is the major constituent of the roots of C. sanguinolenta and has potent activity against malaria parasites in vitro, but it is cytotoxic on account of its abilities to inhibit topoisomerase II and to intercalate into DNA and it is also toxic to mice in vivo. However, the antimalarial mode of action of cryptolepine appears to involve a quinine-like mechanism (inhibition of ?-haematin formation) that is independent of interactions with DNA. This opens up the possibility that it may be feasible to prepare analogues of cryptolepine that do not interact with DNA (and hence may be less cytotoxic) but that retain or have enhanced antiplasmodial activities. Using various synthetic routes, a number of substituted cryptolepine analogues have been made and evaluated for their potential as leads to new antimalarial agents. Compounds were assessed for in-vitro activities against chloroquine-sensitive and chloroquine-resistant strains of Plasmodium falciparum and for cytotoxic activities. Some compounds have been assessed for their abilities to interact with DNA and a number of compounds has also been assessed for antimalarial activity against P. berghei in mice. Several cryptolepine analogues were found to have potent antiplasmodial activities (IC50 < 0.1 µM) against chloroquine-resistant P. falciparum (IC50 for chloroquine = 0.44 µM), but they are generally of similar cytotoxicity to cryptolepine. However, while cryptolepine is toxic to mice, a number of analogues have been shown to have promising antimalarial activities in the mouse-malaria model with no apparent toxicity. In view of this, derivatives of cryptolepine appear to be worthy of further investigation as potential antimalarial agents. In this lecture, the current status of antimalarial cryptolepine research will be reviewed and the potential of cryptolepine as a lead to new antimalarial agents will be discussed.

Prediction of potential antimalarial targets of artemisinin based on protein information from whole genome of Plasmodium falciparum

On the basis of the genomic data and protein pathway information about Plasmodium falciparum clone 3D7 from the NCBI taxonomy database and the KEGG database, eight key protein enzymes in the signal pathways were selected to perform molecular docking with artemisinin. The binding modes obtained from the molecular docking suggested that purine nucleoside phosphorylase (pfPNP), peptide deformylase (pfPDF), and ribose 5-phosphate isomerase (pfRpiA) may be involved in the antimalarial mode of action of artemisinin. Artemisinin exhibited its antimalarial activity probably by interfering with the metabolic pathways of purine, pyrimidine, methionine, glyoxylate and dicarboxylate, or pentose phosphate.

Reactions of Artemisinin and Arteether with Acid: Implications for Stability and Mode of Antimalarial Action

The currently accepted mechanism of trioxane antimalarial action involves generation of free radicals within or near susceptible sites probably arising from the production of distonic radical anions. An alternative mechanistic proposal involving the ionic scission of the peroxide group and consequent generation of a carbocation at C-4 has been suggested to account for antimalarial activity. We have investigated this latter mechanism using DFT (B3LYP/6-31+G* level) and established the preferred Lewis acid protonation sites (artemisinin O5a>>O4a approximately O3a>O2a>O1a; arteether O4a>or=O3a>O5b>>O2a>O1a; Figure 3) and the consequent decomposition pathways and hydrolysis sites. In neither molecule is protonation likely to occur on the peroxide bond O1-O2 and therefore lead to scission. Therefore, the alternative radical pathway remains the likeliest explanation for antimalarial action.

Eosin B as a Novel Antimalarial Agent for Drug-Resistant Plasmodium falciparum

4',5'-Dibromo-2',7'-dinitrofluorescein, a red dye commonly referred to as eosin B, inhibits Toxoplasma gondii in both enzymatic and cell culture studies with a 50% inhibitory concentration (IC(50)) of 180 microM. As a non-active-site inhibitor of the bifunctional T. gondii dihydrofolate reductase-thymidylate synthase (DHFR-TS), eosin B offers a novel mechanism for inhibition of the parasitic folate biosynthesis pathway. In the present study, eosin B was further evaluated as a potential antiparasitic compound through in vitro and cell culture testing of its effects on Plasmodium falciparum. Our data revealed that eosin B is a highly selective, potent inhibitor of a variety of drug-resistant malarial strains, with an average IC(50) of 124 nM. Furthermore, there is no indication of cross-resistance with other clinically utilized compounds, suggesting that eosin B is acting via a novel mechanism. The antimalarial mode of action appears to be multifaceted and includes extensive damage to membranes, the alteration of intracellular organelles, and enzymatic inhibition not only of DHFR-TS but also of glutathione reductase and thioredoxin reductase. In addition, preliminary studies suggest that eosin B is also acting as a redox cycling compound. Overall, our data suggest that eosin B is an effective lead compound for the development of new, more effective antimalarial drugs.

QSAR of Antimalarial Cyclic Peroxy Ketals II: Exploration of Pharmacophoric Site Using AM1 Calculations

A series of antimalarial cyclic peroxy ketals (n=20) have been subjected to energy minimization using AM1 method, and Wang–Ford charges of the non-hydrogen common atoms (Figure 1), obtained from molecular electrostatic potential surface of the energy minimized geometries, have been used to model the antimalarial activity against P. falciparum. It is found that the difference in charges between the peroxy oxygens contribute positively to the activity, and this is in good agreement with the mode of antimalarial action of the peroxy compounds involving breakage of the peroxy bridge by the haem-iron within the parasite. It is hypothesized that difference in charges between two peroxy oxygens may facilitate the bond breakage. It is further found that the activity increases with increase in negative charge of the methoxy carbon of the common fragment of the molecule. This is related with possible secondary electronic interaction with the positively charged side chains of the histidine rich protein of P. falciparum. Attempt was made to incorporate steric and indicator parameters which emerged as important contributors from previous Hansch analysis. The present results support the previous observations that bulky phenyl ring substituents and a seven-member carbocylic ring attached to the peroxy bridge-containing ring are conducive to the activity.

Changing ideas on chloroquine in Plasmodium falciparum.

For 40 years scientists have hotly debated the questions of how chloroquine kills malarial parasites and how resistance to this once first-line antimalarial drug has evolved. While an end to these debates is not in sight, as a result of the complexity of the subject, new findings have come forward that give the discussion a new direction. In this paper we will summarize current knowledge on chloroquine's antimalarial mode of action and the genesis of the resistant phenotype in the human malarial parasite Plasmodium falciparum, with special emphasis on the most recent developments in this field.

Chloroquine - some open questions on its antimalarial mode of action and resistance.

During the digestion of its host cell hemoglobin, large amounts of toxic ferriprotoporphyrin IX (FPIX) are generated in the intraerythrocytic malaria parasite. FPIX is detoxified either by being polymerized into hemozoin inside the food vacuole, or through its degradation by glutathione in the cytosol. Chloroquine is able to complex with FPIX, thus inhibiting both processes and thereby generating receptors for its own uptake. These leads to the accumulation of FPIX in the membrane fraction of infected cells that results in membrane permeabilization and disruption of cation homeostasis and concluded in parasite death. Several unresolved questions, such as the site of FPIX:chloroquine complex formation, the role of pH gradient in drug accumulation and resistance, the role of Pgh-1 in resistance, the mode of action of reversers and the involvement of proteins and their mutants in resistance, are discussed. Copyright 1999 Harcourt Publishers Ltd.

Inhibition of glutathione-dependent degradation of heme by chloroquine and amodiaquine as a possible basis for their antimalarial mode of action

We propose here a new and detailed model for the antimalarial action of chloroquine (CQ), based on the its ability to inhibit degradation of heme by glutathione. Heme, which is toxic to the malaria parasite, is formed when the intraerythrocytic malaria parasite ingests and digests inside its food vacuole its host cell cytosol, which consists mainly of hemoglobin. The parasite protects itself against the toxicity of heme by polymerizing some of it to insoluble hemozoin (HZ). We show here that in Plasmodium falciparum at the trophozoite stage only ca. 30% of the heme is converted into hemozoin. We suggest that nonpolymerized heme exits the food vacuole and is subsequently degraded by glutathione, as has been shown before for uninfected erythrocytes. Marginal amounts of free heme could be detected in the membrane fraction of infected cells but nowhere else. It is well established that CQ and amodiaquine (AQ) accumulate in the parasite’s food vacuole and inhibit heme polymerization, thereby increasing its efflux out of the food vacuole. We found that these drugs competitively inhibit the degradation of heme by glutathione, thus allowing heme to accumulate in membranes. Incubation of intact infected cells with CQ and AQ results in a marked increase in membrane-associated heme in a dose- and time-dependent manner, and a relationship exists between membrane heme levels and the extent of parasite killing. Heme has been shown to disrupt the barrier properties of membranes and to upset ion homeostasis in CQ-treated malaria-infected cells. In agreement with the predictions of our model, increasing the cellular levels of glutathione leads to increased resistance to CQ, whereas decreasing them results in enhanced sensitivity to the drug. These results insinuate a novel mechanism of drug resistance.

Orally-administered silver sulfadiazine: chemotherapy and toxicology in CF-1 mice; Plasmodium berghei (Malaria) and Pseudomonas aeruginosa.

Silver sulfadiazine when administered orally and subcutaneously to CF-1 mice in doses not exceeding 1,050 mg/kg proved to have minimal toxicity. No pathology or abnormal reactions were seen in CF-1 mice after receiving 1,050 mg/kg orally and subcutaneously once a day for 30 days. Silver sulfadiazine in doses of 1,050 mg/kg, once a day for 5 days cured mice of Plasmodium berghei even after splenectomy. Parasitemia was reduced to zero in 1-3 days and antimalarial activity was not inhibited significantly with doses of 313 mg/kg/day of PABA, thereby indicating that silver sulfadiazine's antimalarial mode of action is different from that of the sulfonamides. Doses of 1,050 mg/kg/day had significant activity against systemic infections of Pseudomonas aeruginosa.