Antimalarial Development Research Articles

Targeting Host Glycolysis as a Strategy for Antimalarial Development.

Glycolysis controls cellular energy, redox balance, and biosynthesis. Antiglycolytic therapies are under investigation for treatment of obesity, cancer, aging, autoimmunity, and microbial diseases. Interrupting glycolysis is highly valued as a therapeutic strategy, because glycolytic disruption is generally tolerated in mammals. Unfortunately, anemia is a known dose-limiting side effect of these inhibitors and presents a major caveat to development of antiglycolytic therapies. We developed specific inhibitors of enolase – a critical enzyme in glycolysis – and validated their metabolic and cellular effects on human erythrocytes. Enolase inhibition increases erythrocyte susceptibility to oxidative damage and induces rapid and premature erythrocyte senescence, rather than direct hemolysis. We apply our model of red cell toxicity to address questions regarding erythrocyte glycolytic disruption in the context of Plasmodium falciparum malaria pathogenesis. Our study provides a framework for understanding red blood cell homeostasis under normal and disease states and clarifies the importance of erythrocyte reductive capacity in malaria parasite growth.

In vitro antiplasmodial evaluation of ethanolic and n-hexane extracts of Parinari curatellifolia stem bark

Parinari curatellifolia and other Parinari species are used traditionally in many parts of Africa as a remedy for malaria among other diseases. To ascertain this folkloric claim, the antiplasmodial potential of ethanol extract of Parinari curatellifolia stem bark (EEPCSB) and n-hexane extract of Parinari curatellifolia stem bark (HEPCSB) on Plasmodium falciparum was studied. Parasites were grown in a 96-well plate containing Roswell Park Memorial Institute-1640. The wells were grouped into: control (untreated), artemether-treated, EEPCSB-treated and HEPCSBtreated groups. Treatments were administered to the tune of 10, 20, 40 and 80 μg/ml. Parasitemia was observed by microscopy after 24, 48 and 72h of incubation. EEPCSB and HEPCSB elicited dose and duration-dependent reduction (p<0.05) in parasitemia when compared with the untreated group. The recorded percentage parasite inhibition by the extracts was lower (p<0.05) when compared with artemether. There was no difference (p>0.05) in plasmodium lactate dehydrogenase activity of EEPCSB-treated and artemether-treated groups. Findings from this study show that extracts of P. curatellifolia stem bark, especially EEPCSB, demonstrated excellent inhibitory activities against P. falciparum and can be a good source of compounds for the development of novel antimalarial drugs.
 Keywords: Parinari curatellifolia; Extracts; Plasmodium falciparum; Parasitemia; Antiplasmodial

Secondary metabolite profiling and characterization of diterpenes and flavones from the methanolic extract of Andrographis paniculata using HPLC-LC-MS/MS

BackgroundAndrographis paniculata is a well-known medicinal plant that contains various classes of bioactive secondary metabolites. It is widely used by the traditional medicinal healers for treatment of malaria and other diseases. There is an urgent need for screening of potent novel compounds from the methanol extract of A. paniculata. Earlier, we obtained appreciable in vitro anti-malarial activity (IC50-10.75 μg/ml) in the same plant. In current study, we developed novel analytical methods for rapid identification and characterization of diterpenes and flavones using chromatographic and spectroscopic techniques and identified major compounds that might possess anti-malarial activities.ResultsBased on the chromatographic and mass spectrometric features, we have identified a total of 74 compounds (25 compounds from positive ion mode; 49 compounds from negative ion mode). The mass spectrum data predicted andrographolide (15%) presence in the highest amount in both positive and negative ion modes. Based on the percentage purity, Andrographolide and skullcapflavone I was selected as representative class of diterpenes and flavones for fragmentation studies.ConclusionsThe result led to identification of Neoandrographolide, andrographolactone, 14-dehydroxy-11,12-didehydroandrographolide, skullcapflavone I, and 5-Hydroxy-2′,7,8-tri methoxy flavone from the methanolic extract of A. paniculata that is used in traditional medicine by tribal healers of Amarkantak region for treating malaria. These could be lead compounds for the development of novel anti-malarial drugs.

Antiplasmodial, antimalarial activities and toxicity of African medicinal plants: a systematic review of literature

BackgroundMalaria still constitutes a major public health menace, especially in tropical and subtropical countries. Close to half a million people mainly children in Africa, die every year from the disease. With the rising resistance to frontline drugs (artemisinin-based combinations), there is a need to accelerate the discovery and development of newer anti-malarial drugs. A systematic review was conducted to identify the African medicinal plants with significant antiplasmodial and/or anti-malarial activity, toxicity, as wells as assessing the variation in their activity between study designs (in vitro and in vivo).MethodsKey health-related databases including Google Scholar, PubMed, PubMed Central, and Science Direct were searched for relevant literature on the antiplasmodial and anti-malarial activities of African medicinal plants.ResultsIn total, 200 research articles were identified, a majority of which were studies conducted in Nigeria. The selected research articles constituted 722 independent experiments evaluating 502 plant species. Of the 722 studies, 81.9%, 12.4%, and 5.5% were in vitro, in vivo, and combined in vitro and in vivo, respectively. The most frequently investigated plant species were Azadirachta indica, Zanthoxylum chalybeum, Picrilima nitida, and Nauclea latifolia meanwhile Fabaceae, Euphorbiaceae, Annonaceae, Rubiaceae, Rutaceae, Meliaceae, and Lamiaceae were the most frequently investigated plant families. Overall, 248 (34.3%), 241 (33.4%), and 233 (32.3%) of the studies reported very good, good, and moderate activity, respectively. Alchornea cordifolia, Flueggea virosa, Cryptolepis sanguinolenta, Zanthoxylum chalybeum, and Maytenus senegalensis gave consistently very good activity across the different studies. In all, only 31 (4.3%) of studies involved pure compounds and these had significantly (p = 0.044) higher antiplasmodial activity relative to crude extracts. Out of the 198 plant species tested for toxicity, 52 (26.3%) demonstrated some degree of toxicity, with toxicity most frequently reported with Azadirachta indica and Vernonia amygdalina. These species were equally the most frequently inactive plants reported. The leaves were the most frequently reported toxic part of plants used. Furthermore, toxicity was observed to decrease with increasing antiplasmodial activity.ConclusionsAlthough there are many indigenous plants with considerable antiplasmodial and anti-malarial activity, the progress in the development of new anti-malarial drugs from African medicinal plants is still slothful, with only one clinical trial with Cochlospermum planchonii (Bixaceae) conducted to date. There is, therefore, the need to scale up anti-malarial drug discovery in the African region.

Structural analyses of the malaria parasite aminoacyl-tRNA synthetases provide new avenues for antimalarial drug discovery.

Malaria is a parasitic illness caused by the genus Plasmodium from the apicomplexan phylum. Five plasmodial species of P. falciparum (Pf), P. knowlesi, P. malariae, P. ovale, and P. vivax (Pv) are responsible for causing malaria in humans. According to the World Malaria Report 2020, there were 229 million cases and ~ 0.04 million deaths of which 67% were in children below 5 years of age. While more than 3 billion people are at risk of malaria infection globally, antimalarial drugs are their only option for treatment. Antimalarial drug resistance keeps arising periodically and thus threatens the main line of malaria treatment, emphasizing the need to find new alternatives. The availability of whole genomes of P. falciparum and P. vivax has allowed targeting their unexplored plasmodial enzymes for inhibitor development with a focus on multistage targets that are crucial for parasite viability in both the blood and liver stages. Over the past decades, aminoacyl-tRNA synthetases (aaRSs) have been explored as anti-bacterial and anti-fungal drug targets, and more recently (since 2009) aaRSs are also the focus of antimalarial drug targeting. Here, we dissect the structure-based knowledge of the most advanced three aaRSs-lysyl- (KRS), prolyl- (PRS), and phenylalanyl- (FRS) synthetases in terms of development of antimalarial drugs. These examples showcase the promising potential of this family of enzymes to provide druggable targets that stall protein synthesis upon inhibition and thereby kill malaria parasites selectively.

Optimisation of 2-(N-phenyl carboxamide) triazolopyrimidine antimalarials with moderate to slow acting erythrocytic stage activity

Malaria is a devastating parasitic disease caused by parasites from the genus Plasmodium. Therapeutic resistance has been reported against all clinically available antimalarials, threatening our ability to control the disease and therefore there is an ongoing need for the development of novel antimalarials. Towards this goal, we identified the 2-(N-phenyl carboxamide) triazolopyrimidine class from a high throughput screen of the Janssen Jumpstarter library against the asexual stages of the P. falciparum parasite. Here we describe the structure activity relationship of the identified class and the optimisation of asexual stage activity while maintaining selectivity against the human HepG2 cell line. The most potent analogues from this study were shown to exhibit equipotent activity against P. falciparum multidrug resistant strains and P. knowlesi asexual parasites. Asexual stage phenotyping studies determined the triazolopyrimidine class arrests parasites at the trophozoite stage, but it is likely these parasites are still metabolically active until the second asexual cycle, and thus have a moderate to slow onset of action. Non-NADPH dependent degradation of the central carboxamide and low aqueous solubility was observed in in vitro ADME profiling. A significant challenge remains to correct these liabilities for further advancement of the 2-(N-phenyl carboxamide) triazolopyrimidine scaffold as a potential moderate to slow acting partner in a curative or prophylactic antimalarial treatment.

Antiplasmodial effects of Allium sativum extract on haematological Parameters of Albino Wistar rats infected with Plasmodium berghei

This study aimed at evaluating the antiplasmodial effects of Allium sativum (bulbs) on haematological parameters of albino rats infected with Plasmodium berghei, divided the animal model into groups, viz: the experimental group (with five different concentrations of 100, 300, 500, 800 and 1000mg/kg), the normal-control (non-inoculated), negative-control (inoculated, but untreated) and positive control (inoculated and treated with 10mg/kg Combisunate). Allium sativum ethanolic extracts were administered and monitored for four days before the treatment. Afterwards, the average parasitaemia was calculated and the average percentage parasite inhibition (suppressive effect) was obtained. Result showed that the parasitemia level for the treated groups decreased progressively for the five-day period. This is indicative in the mean number of the percentage parasitized red cells of 1000mg/kg doses as 9.055±2.06 on the first day post inoculation and 0.30±0.04 on the fifth day. The decrease is also observed in the 100, 300, 500 and 800mg/kg groups. Except the untreated group which showed a progressive increase in parasitemia level with average percentage parasitized red cells as 11.33±1.97 on the first day post inoculation and 18.15±1.49 on the fifth day. The haematological result showed a significant (p<0.05) decrease in values of RBC, PCV, Hb, and neutrophils in the inoculated groups, especially the untreated group. As compared to the treated groups, these parameters showed progressive increase as concentrations increased. Conclusively, it can be inferred that the bulbs of Allium sativum have antiplasmodial potentials and can therefore be purified for development of antimalarial drugs.

2-Phenoxy-3-Trichloromethylquinoxalines Are Antiplasmodial Derivatives with Activity against the Apicoplast of Plasmodium falciparum

The malaria parasite harbors a relict plastid called the apicoplast. Although not photosynthetic, the apicoplast retains unusual, non-mammalian metabolic pathways that are essential to the parasite, opening up a new perspective for the development of novel antimalarials which display a new mechanism of action. Based on the previous antiplasmodial hit-molecules identified in the 2-trichloromethylquinoxaline series, we report herein a structure–activity relationship (SAR) study at position two of the quinoxaline ring by synthesizing 20 new compounds. The biological evaluation highlighted a hit compound (3i) with a potent PfK1 EC50 value of 0.2 µM and a HepG2 CC50 value of 32 µM (Selectivity index = 160). Nitro-containing (3i) was not genotoxic, both in the Ames test and in vitro comet assay. Activity cliffs were observed when the 2-CCl3 group was replaced, showing that it played a key role in the antiplasmodial activity. Investigation of the mechanism of action showed that 3i presents a drug response by targeting the apicoplast and a quick-killing mechanism acting on another target site.

Identification of 3,4-Dihydro-2H,6H-pyrimido[1,2-c][1,3]benzothiazin-6-imine Derivatives as Novel Selective Inhibitors of Plasmodium falciparum Dihydroorotate Dehydrogenase.

Plasmodium falciparum’s resistance to available antimalarial drugs highlights the need for the development of novel drugs. Pyrimidine de novo biosynthesis is a validated drug target for the prevention and treatment of malaria infection. P. falciparum dihydroorotate dehydrogenase (PfDHODH) catalyzes the oxidation of dihydroorotate to orotate and utilize ubiquinone as an electron acceptor in the fourth step of pyrimidine de novo biosynthesis. PfDHODH is targeted by the inhibitor DSM265, which binds to a hydrophobic pocket located at the N-terminus where ubiquinone binds, which is known to be structurally divergent from the mammalian orthologue. In this study, we screened 40,400 compounds from the Kyoto University chemical library against recombinant PfDHODH. These studies led to the identification of 3,4-dihydro-2H,6H-pyrimido[1,2-c][1,3]benzothiazin-6-imine and its derivatives as a new class of PfDHODH inhibitor. Moreover, the hit compounds identified in this study are selective for PfDHODH without inhibition of the human enzymes. Finally, this new scaffold of PfDHODH inhibitors showed growth inhibition activity against P. falciparum 3D7 with low toxicity to three human cell lines, providing a new starting point for antimalarial drug development.

Cover Feature: Chemoproteomics for Plasmodium Parasite Drug Target Discovery (ChemBioChem 16/2021)

Understanding drugs’ mechanism of action is integral to successful antimalarial drug development. Recent advances in chemoproteomic methods have created an avenue to globally profile drug targets in the Plasmodium parasites that cause malaria. Some of these strategies even allow direct measurement in living intracellular parasites. The cover feature highlights the achievement of probing the drug–protein interactome in an intact Plasmodium-infected erythrocyte. More information can be found in the review article by E. R. Derbyshire et al.

Structure based design, stability study and synthesis of the dinitrophenylhydrazone derivative of the oxidation product of lanosterol as a potential P. falciparum transketolase inhibitor and in-vivo antimalarial study.

The online version contains supplementary material available at 10.1007/s40203-021-00097-8.

High Throughput Screening to Identify Selective and Nonpeptidomimetic Proteasome Inhibitors As Antimalarials.

The Ubiquitin Proteasome System is the main proteolytic pathway in eukaryotic cells, playing a role in key cellular processes. The essentiality of the Plasmodium falciparum proteasome is well validated, underlying its potential as an antimalarial target, but selective compounds are required to avoid cytotoxic effects in humans. Almost 550000 compounds were tested for the inhibition of the chymotrypsin-like activity of the P. falciparum proteasome using a Proteasome-GLO luminescence assay. Hits were confirmed in an orthogonal enzyme assay using Rho110-labeled peptides, and selectivity was assessed against the human proteasome. Four nonpeptidomimetic chemical families with some selectivity for the P. falciparum proteasome were identified and characterized in assays of proteasome trypsin and caspase activities and in parasite growth inhibition assays. Target engagement studies were performed, validating our approach. Hits identified are good starting points for the development of new antimalarial drugs and as tools to better understand proteasome function in P. falciparum.

CAMP-Dependent Signaling Pathways as Potential Targets for Inhibition of Plasmodium falciparum Blood Stages.

We review the role of signaling pathways in regulation of the key processes of merozoite egress and red blood cell invasion by Plasmodium falciparum and, in particular, the importance of the second messengers, cAMP and Ca2+, and cyclic nucleotide dependent kinases. cAMP-dependent protein kinase (PKA) is comprised of cAMP-binding regulatory, and catalytic subunits. The less well conserved cAMP-binding pockets should make cAMP analogs attractive drug leads, but this approach is compromised by the poor membrane permeability of cyclic nucleotides. We discuss how the conserved nature of ATP-binding pockets makes ATP analogs inherently prone to off-target effects and how ATP analogs and genetic manipulation can be useful research tools to examine this. We suggest that targeting PKA interaction partners as well as substrates, or developing inhibitors based on PKA interaction sites or phosphorylation sites in PKA substrates, may provide viable alternative approaches for the development of anti-malarial drugs. Proximity of PKA to a substrate is necessary for substrate phosphorylation, but the P. falciparum genome encodes few recognizable A-kinase anchor proteins (AKAPs), suggesting the importance of PKA-regulatory subunit myristylation and membrane association in determining substrate preference. We also discuss how Pf14-3-3 assembles a phosphorylation-dependent signaling complex that includes PKA and calcium dependent protein kinase 1 (CDPK1) and how this complex may be critical for merozoite invasion, and a target to block parasite growth. We compare altered phosphorylation levels in intracellular and egressed merozoites to identify potential PKA substrates. Finally, as host PKA may have a critical role in supporting intracellular parasite development, we discuss its role at other stages of the life cycle, as well as in other apicomplexan infections. Throughout our review we propose possible new directions for the therapeutic exploitation of cAMP-PKA-signaling in malaria and other diseases caused by apicomplexan parasites.

Antimalarial properties and molecular docking analysis of compounds from Dioscorea bulbifera L. as new antimalarial agent candidates

BackgroundAt present, the emergence and spread of antimalarial drug resistance has become a significant problem worldwide. There has been a challenge in searching for natural products for the development of novel antimalarial drugs. Therefore, this study aims to evaluate compounds from Dioscorea bulbifera responsible for antimalarial properties and investigate potential interactions of the compounds with Plasmodium falciparum lactate dehydrogenase (PfLDH), an essential glycolytic enzyme in the parasite’s life cycle.MethodsAn in vitro study of antimalarial activity against chloroquine (CQ)-resistant Plasmodium falciparum (K1 strain) and CQ-sensitive P. falciparum (3D7 strain) was performed using the 3H-hypoxanthine uptake inhibition method. The cytotoxic effects of the pure compounds were tested against Vero cells using a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The interactions of the compounds with the PfLDH active site were additionally investigated using a molecular docking method.ResultsQuercetin (6) exhibited the highest antimalarial activity against the P. falciparum K1 and 3D7 strains, with IC50 values of 28.47 and 50.99 μM, respectively. 2,4,3′,5′-Tetrahydroxybibenzyl (9), 3,5-dimethoxyquercetin (4) and quercetin-3-O-β-D-galactopyranoside (14) also possessed antimalarial effects against these two strains of P. falciparum. Most pure compounds were nontoxic against Vero cells at a concentration of 80 μg/ml, except for compound 9, which had a cytotoxic effect with a CC50 value of 16.71 μM. The molecular docking results indicated that 9 exhibited the best binding affinity to the PfLDH enzyme in terms of low binding energy (− 8.91 kcal/mol) and formed strong hydrogen bond interactions with GLY29, GLY32, THR97, GLY99, PHE100, THR101 and ASN140, amino acids as active sites. In addition, 6 also possessed remarkable binding affinity (− 8.53 kcal/mol) to PfLDH by interacting with GLY29, ILE31, ASP53, ILE54, THR97 and THR101.ConclusionQuercetin is a major active compound responsible for the antimalarial activity of D. bulbifera and is an inhibitor of PfLDH. These findings provide more evidence to support the traditional use of D. bulbifera for malaria treatment. Structural models of its interactions at the PfLDH active site are plausibly useful for the future design of antimalarial agents.

Heme Detoxification in the Malaria Parasite: A Target for Antimalarial Drug Development.

Over the last century, malaria deaths have decreased by more than 85%. Nonetheless, there were 405 000 deaths in 2018, mostly resulting from Plasmodium falciparum infection. In the 21st century, much of the advance has arisen from the deployment of insecticide-treated bed nets and artemisinin combination therapy. However, over the past few decades parasites with a delayed artemisinin clearance phenotype have appeared in Southeast Asia, threatening further gains. The effort to find new drugs is thus urgent. A prominent process in blood stage malaria parasites, which we contend remains a viable drug target, is hemozoin formation. This crystalline material consisting of heme can be readily seen when parasites are viewed microscopically. The process of its formation in the parasite, however, is still not fully understood.In early work, we recognized hemozoin formation as a biomineralization process. We have subsequently investigated the kinetics of synthetic hemozoin (β-hematin) crystallization catalyzed at lipid-aqueous interfaces under biomimetic conditions. This led us to the use of neutral detergent-based high-throughput screening (HTS) for inhibitors of β-hematin formation. A good hit rate against malaria parasites was obtained. Simultaneously, we developed a pyridine-based assay which proved successful in measuring the concentrations of hematin not converted to β-hematin.The pyridine assay was adapted to determine the effects of chloroquine and other clinical antimalarials on hemozoin formation in the cell. This permitted the determination of the dose-dependent amounts of exchangeable heme and hemozoin in P. falciparum for the first time. These studies have shown that hemozoin inhibitors cause a dose-dependent increase in exchangeable heme, correlated with decreased parasite survival. Electron spectroscopic imaging (ESI) showed a relocation of heme iron into the parasite cytoplasm, while electron microscopy provided evidence of the disruption of hemozoin crystals. This cellular assay was subsequently extended to top-ranked hits from a wide range of scaffolds found by HTS. Intriguingly, the amounts of exchangeable heme at the parasite growth IC50 values of these scaffolds showed substantial variation. The amount of exchangeable heme was found to be correlated with the amount of inhibitor accumulated in the parasitized red blood cell. This suggests that heme-inhibitor complexes, rather than free heme, lead to parasite death. This was supported by ESI using a Br-containing compound which showed the colocalization of Fe and Br as well as by confocal Raman microscopy which confirmed the presence of a complex in the parasite. Current evidence indicates that inhibitors block hemozoin formation by surface adsorption. Indeed, we have successfully introduced molecular docking with hemozoin to find new inhibitors. It follows that the resulting increase in free heme leads to the formation of the parasiticidal heme-inhibitor complex. We have reported crystal structures of heme-drug complexes for several aryl methanol antimalarials in nonaqueous media. These form coordination complexes but most other inhibitors interact noncovalently, and the determination of their structures remains a major challenge.It is our view that key future developments will include improved assays to measure cellular heme levels, better in silico approaches for predicting β-hematin inhibition, and a concerted effort to determine the structure and properties of heme-inhibitor complexes.

Some conditions apply: Systems for studying Plasmodium falciparum protein function

Malaria, caused by infection with Plasmodium parasites, remains a significant global health concern. For decades, genetic intractability and limited tools hindered our ability to study essential proteins and pathways in Plasmodium falciparum, the parasite associated with the most severe malaria cases. However, recent years have seen major leaps forward in the ability to genetically manipulate P. falciparum parasites and conditionally control protein expression/function. The conditional knockdown systems used in P. falciparum target all 3 components of the central dogma, allowing researchers to conditionally control gene expression, translation, and protein function. Here, we review some of the common knockdown systems that have been adapted or developed for use in P. falciparum. Much of the work done using conditional knockdown approaches has been performed in asexual, blood-stage parasites, but we also highlight their uses in other parts of the life cycle and discuss new ways of applying these systems outside of the intraerythrocytic stages. With the use of these tools, the field’s understanding of parasite biology is ever increasing, and promising new pathways for antimalarial drug development are being discovered.

Potent Antimalarials with Development Potential Identified by Structure-Guided Computational Optimization of a Pyrrole-Based Dihydroorotate Dehydrogenase Inhibitor Series.

Dihydroorotate dehydrogenase (DHODH) has been clinically validated as a target for the development of new antimalarials. Experience with clinical candidate triazolopyrimidine DSM265 (1) suggested that DHODH inhibitors have great potential for use in prophylaxis, which represents an unmet need in the malaria drug discovery portfolio for endemic countries, particularly in areas of high transmission in Africa. We describe a structure-based computationally driven lead optimization program of a pyrrole-based series of DHODH inhibitors, leading to the discovery of two candidates for potential advancement to preclinical development. These compounds have improved physicochemical properties over prior series frontrunners and they show no time-dependent CYP inhibition, characteristic of earlier compounds. Frontrunners have potent antimalarial activity in vitro against blood and liver schizont stages and show good efficacy in Plasmodium falciparum SCID mouse models. They are equally active against P. falciparum and Plasmodium vivax field isolates and are selective for Plasmodium DHODHs versus mammalian enzymes.

Design, Synthesis, and Structural Analysis of Cladosporin-Based Inhibitors of Malaria Parasites.

Here we have described a systematic structure activity relationship (SAR) of a set of compounds inspired from cladosporin, a tool compound that targets parasite (Plasmodium falciparum) lysyl tRNA synthetase (KRS). Four sets of analogues, synthesized based on point changes in the chemical scaffold of cladosporin and other logical modifications and hybridizations, were assessed using high throughput enzymatic and parasitic assays along with in vitro pharmacokinetics. Co-crystallization of the most potent compound in our series (CL-2) with PfKRS revealed its structural basis of enzymatic binding and potency. Further, we report that CL-2 has performed better than cladosporin in terms of metabolic stability. It thus represents a new lead for further optimization toward the development of antimalarial drugs. Collectively, along with a lead compound, the series offers insights on how even the slightest chemical modification might play an important role in enhancing or decreasing the potency of a chemical scaffold.

Absence of Association between Methylene Blue Reduced Susceptibility and Polymorphisms in 12 Genes Involved in Antimalarial Drug Resistance in African Plasmodium falciparum.

Half the human population is exposed to malaria. Plasmodium falciparum antimalarial drug resistance monitoring and development of new drugs are major issues related to the control of malaria. Methylene blue (MB), the oldest synthetic antimalarial, is again a promising drug after the break of its use as an antimalarial drug for more than 80 years and a potential partner for triple combination. Very few data are available on the involvement of polymorphisms on genes known to be associated with standard antimalarial drugs and parasite in vitro susceptibility to MB (cross-resistance). In this context, MB susceptibility was evaluated against 482 isolates of imported malaria from Africa by HRP2-based ELISA chemosusceptibility assay. A total of 12 genes involved in antimalarial drug resistance (Pfcrt, Pfdhfr, Pfmdr1, Pfmdr5, Pfmdr6, PfK13, Pfubq, Pfcarl, Pfugt, Pfact, Pfcoronin, and copy number of Pfpm2) were sequenced by Sanger method and quantitative PCR. On the Pfmdr1 gene, the mutation 86Y combined with 184F led to more susceptible isolates to MB (8.0 nM vs. 11.6 nM, p = 0.03). Concerning Pfmdr6, the isolates bearing 12 Asn repetitions were more susceptible to MB (4.6 nM vs. 11.6 nM, p = 0.005). None of the polymorphisms previously described as involved in antimalarial drug resistance was shown to be associated with reduced susceptibility to MB. Some genes (particularly PfK13, Pfugt, Pfact, Pfpm2) did not present enough genetic variability to draw conclusions about their involvement in reduced susceptibility to MB. None of the polymorphisms analyzed by multiple correspondence analysis (MCA) had an impact on the MB susceptibility of the samples successfully included in the analysis. It seems that there is no in vitro cross-resistance between MB and commonly used antimalarial drugs.

Advancement of chimeric hybrid drugs to cure malaria infection: An overview with special emphasis on endoperoxide pharmacophores.

Emergence and spread of Plasmodium falciparum resistant to artemisinin-based combination therapy has led to a situation of haste in the scientific and pharmaceutical communities. Sincere efforts are redirected towards finding alternative chemotherapeutic agents that are capable of combating multidrug-resistant parasite strains. Extensive research yielded the concept of "Chimeric Bitherapy (CB)" which involves the linking of two molecules with individual pharmacological activity and exhibit dual mode of action into a single hybrid molecule. Current research in this field seems to endorse hybrid molecules as the next-generation antimalarial drugs and are more effective compared to the multi-component drugs because of the lower occurrence of drug-drug adverse effects. This review is an attempt to congregate complete survey on endoperoxide based hybrid antiplasmodial molecules that will give glimpse on the future directions for successful development and discovery of useful antimalarial hybrid drugs.