An overview of chemotherapeutic targets for antimalarial drug discovery

Sulfonamide based pyrimidine derivatives combating Plasmodium parasite by inhibiting falcipains-2 and falcipains-3 as antimalarial agents

In this report, we present the design and synthesis of a novel series of pyrimidine-tethered spirochromane-based sulfonamide derivatives aimed at combating drug resistance in malaria. The antimalarial effectiveness of these compounds was assessed in vitro. Structural validation of the synthesized compounds was conducted using mass spectrometry and NMR spectroscopy. Strong antimalarial activity against CQ-sensitive (3D7) and CQ-resistant (W2) strains of Plasmodium falciparum was demonstrated by the majority of the compounds. Notably, compounds SZ14 and SZ9 demonstrated particularly potent effects, with compound SZ14 showing IC50 values of 2.84 μM and SZ9 3.22 μM, indicating single-digit micromolar activity. The compounds exhibiting strong antimalarial activity were assessed through enzymatic tests against the cysteine protease enzymes of P. falciparum, falcipain-2 and falcipain-3. The results indicated that SZ14 and SZ9 inhibited PfFP-2 (IC50 values: 4.1 and 5.4 μM, respectively), and PfFP-3 (IC50 values: 4.9 and 6.3 μM, respectively). To confirm the compounds' specificity towards the parasite, we investigated their cytotoxicity against Vero cell lines, revealing strong selectivity indices and no significant cytotoxic effects. Additionally, in vitro hemolysis testing showed these compounds to be non-toxic to normal human blood cells. Moreover, predicted in silico ADME parameters and physiochemical characteristics demonstrated the drug-likeness of the synthetic compounds. These collective findings suggest that sulfonamide derivatives based on pyrimidine-tethered oxospirochromane could serve as templates for the future development of potential antimalarial drugs.

Emerging Drug Targets for Antimalarial Drug Discovery: Validation and Insights into Molecular Mechanisms of Function

Approximately 619,000 malaria deaths were reported in 2021, and resistance to recommended drugs, including artemisinin-combination therapies (ACTs), threatens malaria control. Treatment failure with ACTs has been found to be as high as 93% in northeastern Thailand, and parasite mutations responsible for artemisinin resistance have already been reported in some African countries. Therefore, there is an urgent need to identify alternative treatments with novel targets. In this Perspective, we discuss some promising antimalarial drug targets, including enzymes involved in proteolysis, DNA and RNA metabolism, protein synthesis, and isoprenoid metabolism. Other targets discussed are transporters, Plasmodium falciparum acetyl-coenzyme A synthetase, N-myristoyltransferase, and the cyclic guanosine monophosphate-dependent protein kinase G. We have outlined mechanistic details, where these are understood, underpinning the biological roles and hence druggability of such targets. We believe that having a clear understanding of the underlying chemical interactions is valuable to medicinal chemists in their quest to design appropriate inhibitors.

Potentials of Terpenoids as Inhibitors of Multiple Plasmodium falciparum Protein Drug Targets

PURPOSE

The resistance of parasite to readily affordable antimalarial drugs, the high cost of currently potent drugs, and the resistance of vector mosquitoes to insecticides threaten the possibility of malaria eradication in malaria endemic areas. Due to the fact that quinine and artemisinin were isolated from plants sources, researchers have been encouraged to search for new antimalarials from medicinal plants. This is especially the case in Africa where a large percentage of the population depends on medicinal plant to treat malaria and other ailments.

METHOD

In this study, we evaluated previously characterized Plasmodium-cidal compounds obtained from the African flora to identify their likely biochemical targets, for an insight into their possible antimalarial chemotherapy. Molecular docking study was first conducted, after which remarkable compounds were submitted for molecular dynamic (MD) simulations studies.

RESULTS

From a total of 38 Plasmodium-cidal compounds docked with confirmed Plasmodium falciparum protein drug targets [plasmepsin II (PMII), histo-aspartic protein (HAP) and falcipain-2 (FP)], two pentacyclic triterpene, cucurbitacin B and 3 beta-O-acetyl oleanolic acid showed high binding affinity relative to artesunate. This implies their capacity to inhibit the three selected P. falciparum target proteins, and consequently, antimalarial potential. From the MD simulations studies and binding free energy outcomes, results confirmed that the two compounds are stable in complex with the selected antimalarial targets; they also showed excellent binding affinities during the 100 ns simulation.

CONCLUSION

These results showed that cucurbitacin B and 3 beta-O-acetyl oleanolic acid are potent antimalarials and should be considered for further studies.

In Vivo Anti-Malarial Activity of the Aqueous Root Extract of Euclea divinorum Hiern (Ebenaceae) against Plasmodium berghei ANKA

Background

Drug resistance is a universal challenge to malaria control measures. As a result, the development and discovery of new chemotherapeutic agents from medicinal plants having anti-malarial traditional claims are very important. This work, therefore, attempted to evaluate the anti-malarial activity of the aqueous root extract of E. divinorum using a rodent model of malaria.

Methods

The roots of E. divinorum were extracted by hot decoction using distilled water. Anti-malarial activity of various doses (100 mg/kg, 200 mg/kg, and 600 mg/kg) of the root aqueous extract was evaluated using the 4-day suppressive test as well as curative and repository tests. Parasitemia, rectal temperature, body weight, PCV, and MST were also determined.

Results

The finding showed that there were a dose-related significant parasitemia chemo-suppression and increment in survival time as compared to the negative control (p < 0.001) in all tests. The chemo-suppression effect was higher at 400 mg/kg extract-treated groups in the 4-day suppressive test followed by the curative test. The lowest chemo-prophylaxis effect was observed in 100 mg/kg extract-treated groups in the repository test. Regarding the other parameters, the extract prevented weight loss, temperature drop, and hemolysis in all models but not in a consistent manner.

Conclusion

The current study showed that the aqueous root extract of E. divinorum possessed a varying degree of anti-malarial activity in all three tests, with greater parasitemia suppression observed in the 4-day suppressive test. The extract produced higher parasitemia chemo-suppression and longer survival time in early infections followed by established and then residual infection.

Efforts Made to Eliminate Drug-Resistant Malaria and Its Challenges

Since 2000, a good deal of progress has been made in malaria control. However, there is still an unacceptably high burden of the disease and numerous challenges limiting advancement towards its elimination and ultimate eradication. Among the challenges is the antimalarial drug resistance, which has been documented for almost all antimalarial drugs in current use. As a result, the malaria research community is working on the modification of existing treatments as well as the discovery and development of new drugs to counter the resistance challenges. To this effect, many products are in the pipeline and expected to be marketed soon. In addition to drug and vaccine development, mass drug administration (MDA) is under scientific scrutiny as an important strategy for effective utilization of the developed products. This review discusses the challenges related to malaria elimination, ongoing approaches to tackle the impact of drug-resistant malaria, and upcoming antimalarial drugs.

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 growing resistance to the current antimalarial drugs in the absence of a vaccine can be effectively tackled by identifying new metabolic pathways that are essential to the survival of the malaria parasite and developing new drugs against them. Triterpenes and steroids are the most abundant group of natural products with a great variety of biological activities. However, lanosterol is not known to possess any significant biological activity. In this study the binding and interactions of a dinitrophenyl hydrazine (DNP) derivative of lanosterol, LAN (a derivative that incorporates a substantially polar moiety into the steroid) with P. falciparum transketolase was studied by molecular docking and MD simulation with the view to exploit the DNP derivative as a lead in antimalarial chemotherapy development considering that the P. falciparum transketolase (PfTk) is a novel target in antimalarial chemotherapy. The enzyme catalyses the production of ribose sugars needed for nucleic acid synthesis; it lacks a three-dimensional (3D) structure necessary for docking because it is difficult to obtain a crystalline form. A homology model of PfTk was constructed using Saccharomyces cerevisiae transketolase (protein data bank ID of 1TRK) as the template. The compound was observed to have Free Energy of Binding higher than that of the cofactor of the protein (Thiamine Pyrophosphate, TPP) and a synthetic analog (SUBTPP) used as reference compounds after MD Simulation. The compound was synthesized in a two-step, one-pot reaction, utilizing a non-acidic and mild oxidant to oxidize the lanosterol in order to avoid the rearrangement that accompanies the oxidation of sterols using acidic oxidants. The LAN was characterized using IR spectroscopy and NMR experiments and tested in-vivo for its antimalarial chemo suppression using a murine model with Chloroquine as a standard. The LAN at a concentration of 25 mg/kg was found to have a comparable activity with Chloroquine at 10 mg/kg and no mortality was observed among the test animals 24 days post drug administration showing that the compound indeed has potential as an antimalarial agent and a likely inhibitor of PfTk considering that there is a strong agreement between the in-silico results and biological study.

SUPPLEMENTARY INFORMATION

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

Current progress in antimalarial pharmacotherapy and multi-target drug discovery

Discovery and development of antimalarial drugs have long been dominated by single-target therapy. Continuous effort has been made to explore and identify different targets in malaria parasite crucial for the malaria treatment. The single-target drug therapy was initially successful, but it was later supplanted by combination therapy with multiple drugs to overcome drug resistance. Emergence of resistant strains even against the combination therapy has warranted a review of current antimalarial pharmacotherapy. This has led to the development of the new concept of covalent biotherapy, in which two or more pharmacophores are chemically bound to produce hybrid antimalarial drugs with multi-target functionalities. Herein, the review initially details the current pharmacotherapy for malaria as well as the conventional and novel targets of importance identified in the malaria parasite. Then, the rationale of multi-targeted therapy for malaria, approaches taken to develop the multi-target antimalarial hybrids, and the examples of hybrid molecules are comprehensively enumerated and discussed.

Nano-facilitated drug delivery strategies in the treatment of plasmodium infection

Malaria, one of the major infectious disease-causing sizeable morbidity, mortality and economic loss worldwide. The main drawback for the failure to eradicate malaria is the spread of multiple drug resistance to the majority of currently available chemotherapy. At present nanotechnology offers an advanced opportunity in the delivery of drugs and vaccines to the desired targeted site in the body following oral and systemic administration. It confers the major advantages like improving drug pharmacokinetic profiles, reduce dose frequency and reduction in drug toxicity. Hence, Nano-based drug delivery system can provide a promising prospect in the way of malaria treatment. This paper is a review of recent researches highlighting includes nanocarriers loaded antimalarial drugs for better therapeutic efficacy and future perspective in the treatment of malaria.

Antiplasmodial Anthraquinones from Medicinal Plants: The Chemistry and Possible Mode of Actions

Malaria killed nearly half a million people in 2015, and 70% of this victims were young children. Malarial chemotherapy makes use of several drugs, each with its own pharmacological limitations, and with parasite resistance being the most challenging. People of low income nations often rely on traditional medicine as a treatment due to limited access to modern healthcare services. Despite uncertainties present in the outcome of traditional medicine, ethnomedicine approach has yielded important lead candidates. The investigation of medicinal plants utilized in the malaria endemic region yielded many antiplasmodial compounds with anthraquinone moiety. This paper describes natural anthraquinones extracted from medicinal plants utilized in traditional medicine for the treatment of malaria. In addition, the insight on structure-activity relationship and their mode of actions are also elaborated.

Pyrazole-pyrazoline as promising novel antimalarial agents: A mechanistic study

A series of pyrazole-pyrazoline substituted with benzenesulfonamide were synthesized and evaluated for their antimalarial activity in vitro and in vivo. The compounds were active against both chloroquine (CQ) sensitive (3D7) and CQ resistant (RKL-9) strains of Plasmodium falciparum. Seven compounds (7e, 7i, 7j, 7l, 7m, 7o and 7p) exhibiting EC50 less than 2 μM. A mechanistic study of compound 7o revealed that these compound act through the inhibition of β-hematin. The study indicated that these compounds can serve as lead compounds for further development of potent antimalarial drugs.

A REVIEW ON THE CHEMISTRY AND PHARMACOLOGY OF Rennellia elliptica Korth

Rennellia elliptica, popularly dubbed as Malaysian Ginseng, is widely used in traditional medicine among the local Jakun community in Endau-Rompin State Park, Pahang, Malaysia. The decoction of the roots is traditionally taken for treatment of body aches, as postpartum tonic, as aphrodisiac and for the treatment of jaundice. In the effort of searching new botanical drugs and drug candidates from tropical rainforest, the team from this laboratory had conducted a sizeable phytochemical and biological screening program of tropical plant at Endau Rompin State Park, Pahang with the help from the indigenous people. R. elliptica showed strong antiplasmodial activity in vitro with the IC50 value of 4.04µg/mL. The comprehensive study on the root extract of R. elliptica in this laboratory yielded seventeen compounds from four different classes, including 2 new pyranoanthraquinones, one new anthraquinone, eleven known anthraquinones, one lactone triterpenoid, one coumarin and one phenolic acid. The chemical profile of the root extract was established using HPLC and the selected marker compounds were used as external standards and quantified using standard calibration curve. Nordamnacanthal 5, damnacanthal 7, 2-formyl-3-hydroxy-9,10-anthraquinone 6, 2-methyl-3-hydroxy-9,10-anthraquinone 11 and 1,2-dimethoxy-6-methyl-9,10-anthraquinone 3 were determined at 3.57, 10.32, 4.47, 12.18 and 4.09 µg/g, respectively. Owing to the toxicity of dichloromethane, the extraction of the desired marker compounds was attempted using accelerated solvent extraction and soxhlet extraction using ethanol and water at different compositions. R. elliptica root extract and the isolated anthraquinones showed potential antiplasmodial activity, and the active compounds were probed for their mode of action. In addition, the dichloromethane root extract of R. elliptica and the selected anthraquinones were screened for anticancer, antioxidant, and α-glucosidase inhibitory activities as well as toxicity study in vitro.  The review summarizes the findings on Rennellia elliptica which includes phytochemistry, toxicity and its biological activities. The chemotaxonomic significance of Rennellia elliptica is also discussed

Diarylpyrazole Ligated Dihydropyrimidine Hybrids as Potent Non-Classical Antifolates and Their Efficacy Against Plasmodium falciparum

A series of diarylpyrazole clubbed dihydropyrimidine derivatives (J1-J30) was synthesized under microwave-assisted heating conditions by employing Biginelli reaction methodology and utilizing triethylammonium acetate both as a catalyst and as reaction medium, leading towards a greener reaction pathway. The synthesized entities were screened for their antimalarial efficacy against a Plasmodium falciparum strain in vitro. The active entities (J9, J15, J21, J25, and J27) obtained out of the in vitro screening were further evaluated for their enzyme inhibitory potency against the Pf-DHFR enzyme in vitro as well as in silico using Glide. Furthermore, the active scaffolds were tested for their cytotoxicity against Vero cells, proving their nontoxic behavior and selectivity. The ADME parameters were also evaluated and predicted in silico, indicating good oral bioavailability of the compounds.

Anti-plasmodial and Chloroquine Resistance Suppressive Effects of Embelin

BACKGROUND

Emergence of chloroquine (CQ) resistance among different strains of Plasmodium falciparum is the worst catastrophe that has ever perplexed the dedicated efforts to eradicate malaria. This urged the scientists to search for new alternatives or sensitizers to augment its antiplasmodium effect.

MATERIALS AND METHOD

In this experiment, the potential of embelin, isolated from Embelia ribes, to inhibit the growth and sensitize CQ action was screened using SYBRE-green-I based drug sensitivity and isobologram assays, respectively. Its effect on red blood cells stability was screened to assess its safety. To explore its molecular mechanism, its effect on plasmodial Hemozoin and the in vitro β-hematin formation was screened as well. Furthermore, its anti-oxidant activity was measured using the conventional in vitro tests and its molecular characters were obtained using Molispiration program.

RESULTS

The results showed that its anti-plasmodial effect was weaker than CQ but synergism was obtained when they were combined at ratios lower than 5:5 CQ/embelin. Furthermore, β-hematin formation was inhibited by embelin without showing any synergism after mixing with CQ.

CONCLUSION

Overall, embelin is not ideal to be suggested as a conventional antiplasmodium but it has a potential to ameliorate CQ resistance. Furthermore, its action is not related to its impact on hemozoin formation. Further, investigations are recommended to illustrate its detailed mechanism of action. Abbreviation used: CQ-DV-PBS-HEPES: Chloroquine-Digestive vacuole-Phosphate-buffer-saline-4-(2-hydroxyethyl-1-piperazin-ethan-sulphoni-acid), EDTA: Ethylen-diamin-tetra-acetic-acid, g.m.wt: Gram molecular weight, cMCM: Complete-malaria-culture-medium, Hct: Hematocrite, PRBCs: Parasitized-redblood-cells, nRBCs: Normal-red-blood-cells, RT: Room temperature, IC: Inhibitory concentration, FIC: Fractional inhibitory concentration, iCM: Incomplete-culturemedium, BSA: Bovin serum albumin, MTT: 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide, DPPH: 2,2-diphenyl-1- picrylhydrazy, BHT: Butylatedhydroxyl-toleuen, PSA: Polar surface area, ClogP: Log partition coefficient (octanol/water), GPCR: G-protein-coupled-receptors, DMSO: Dimethylsulphoxide, NaOH: Sodium hydroxide.

Microwave Assisted Synthesis of Pyrimidines in Ionic Liquid and Their Potency as Non-Classical Malarial Antifolates

Synthesis of pyrazole-linked triazolo-pyrimidine hybrids was achieved by employing Biginelli-type reaction methodology in an ionic liquid (triethylammonium acetate) under microwave irradiation. This method proved to be highly efficient and the ionic liquid employed was found recyclable for up to five consecutive cycles. The synthesized molecules were further screened for their antimalarial efficacy screening out the active scaffolds J15, J18, J21, J24, J27, and J30. The active molecules were evaluated in an enzyme inhibition study against the active Plasmodium falciparum dihydrofolate reductase (Pf-DHFR), computationally as well as in vitro, demonstrating their potency as DHFR inhibitors. The active entities were also investigated for their oral bioavailability by predicting ADME properties in silico, indicating good bioavailability.

Metal-free synthesis of N-fused heterocyclic iodides via C-H functionalization mediated by tert-butylhydroperoxide

Direct, regioselective and metal-free synthesis of fused N-heterocyclic iodides is reported. This regioselective C-H functionalization is mediated by tert-butylhydroperoxide (TBHP), via dual activation of molecular iodine and a heterocyclic substrate, resulting in the in situ generation of electrophilic iodine species (I(+)), and free radical(s) (t)BuO˙ or (t)BuOO˙, driving the iodination reaction.

Synthetic compounds from an in house library as inhibitors of falcipain-2 from Plasmodium falciparum

Falcipain-2 (FP-2) is a key cysteine protease from the malaria parasite Plasmodium falciparum. Many previous studies have identified FP-2 inhibitors; however, none has yet met the criteria for an antimalarial drug candidate. In this work, we assayed an in-house library of non-peptidic organic compounds, including (E)-chalcones, (E)-N'-benzylidene-benzohydrazides and alkyl-esters of gallic acid, and assessed the activity toward FP-2 and their mechanisms of inhibition. The (E)-chalcones 48, 54 and 66 showed the lowest IC50 values (8.5 ± 0.8 µM, 9.5 ± 0.2 µM and 4.9 ± 1.3 µM, respectively). The best inhibitor (compound 66) demonstrated non-competitive inhibition, and using mass spectrometry and fluorescence spectroscopy assays, we suggest a potential allosteric site for the interaction of this compound, located between the catalytic site and the hemoglobin binding arm in FP-2. We combined structural biology tools and mass spectrometry to characterize the inhibition mechanisms of novel compounds targeting FP-2.

Disulfide linkages in Plasmodium falciparum plasmepsin-i are essential elements for its processing activity and multi-milligram recombinant production yield

Plasmodium falciparum plasmepsin-I (PM-I) has been considered a potential drug target for the parasite that causes fatal malaria in human. Determination of PM-I structures for rational design of its inhibitors is hindered by the difficulty in obtaining large quantity of soluble enzyme. Nearly all attempts for its heterologous expression in Escherichia coli result in the production of insoluble proteins in both semi-pro-PM-I and its truncated form, and thus require protein refolding. Moreover, the yields of purified, soluble PM-I from all reported studies are very limited. Exclusion of truncated semi-pro-PM-I expression in E. coli C41(DE3) is herein reported. We also show that the low preparation yield of purified semi-pro-PM-I with autoprocessing ability is mainly a result of structural instability of the refolded enzyme in acidic conditions due to incomplete formation of disulfide linkages. Upon formation of at least one of the two natural disulfide bonds, nearly all of the refolded semi-pro-PM-I could be activated to its mature form. A significantly improved yield of 10 mg of semi-pro-PM-I per liter of culture, which resulted in 6-8 mg of the mature PM-I, was routinely obtained using this strategy.

Role of traditional antimalarial plants in the battle against the global malaria burden

Malaria continues to be a major global public health problem with 3.3 billion people at risk in 106 endemic countries. Globally, over 1000 plants have been used as potential antimalarials in resource-poor settings due to fragile health-care systems and lack of accessibility and affordability of artemisinin-based combination therapies (ACTs). Although many believe that the use of medicinal plants that have folklore reputations for antimalarial properties is relatively safe, many herbs may be potentially toxic due to their intrinsic adverse side effects. Therefore, herbal-derived remedies require powerful and deep assessment of their pharmacological qualities to establish their mode of action, safety, quality, and efficacy. In addition, the evolution of drug resistance also demands new antimalarial agents. This can be achieved by forming a vibrant antimalarial discovery pipeline among all stakeholders, including traditional healers, ethnobotanists, scientists, entomologists, pharmacists, and research institutions, for the isolation and characterization of the bioactive compounds with the ultimate objective of finding novel modes of action antimalarial compounds that can be used to fight against drug-resistant malarial parasites.

Quantitative proteomics of Trypanosoma cruzi during metacyclogenesis

Trypanosoma cruzi is the etiologic agent of Chagas disease, which is estimated to affect over eight million people around the world. Trypanosoma cruzi has a complex life cycle, involving insect and mammalian hosts and four distinct developmental stages: epimastigotes, metacyclic trypomastigotes, amastigotes, and bloodstream trypomastigotes. Metacyclogenesis is the process by which T. cruzi epimastigotes differentiate into metacyclic trypomastigotes and acquire infectivity, and involves differential gene expression associated with acquisition of virulence. In T. cruzi, gene expression regulation is achieved mainly posttranscriptionally. Therefore, proteomics-based approaches are extremely useful for gaining a better understanding of the changes that occur in the stage-regulated gene expression program of the parasite at the molecular level. Here, we performed an in-depth quantitative MS-based proteomic study of T. cruzi metacyclogenesis and quantified almost 3000 proteins expressed during the process of differentiation. To the best of our knowledge, this work is the most comprehensive quantitative proteomics study of different cell populations of T. cruzi available so far. We identified relevant proteins and pathways involved in the parasite's differentiation and infectivity acquisition, opening new perspectives for further studies that could, ultimately, lead to the identification of new targets for chemotherapy.

Novel antimalarial drug targets: hope for new antimalarial drugs

Malaria is a major global threat, that results in more than 2 million deaths each year. The treatment of malaria is becoming extremely difficult due to the emergence of drug-resistant parasites, the absence of an effective vaccine, and the spread of insecticide-resistant vectors. Thus, malarial therapy needs new chemotherapeutic approaches leading to the search for new drug targets. Here, we discuss different approaches to identifying novel antimalarial drug targets. We have also given due attention to the existing validated targets with a view to develop novel, rationally designed lead molecules. Some of the important parasite proteins are claimed to be the targets; however, further in vitro or in vivo structure-function studies of such proteins are crucial to validate these proteins as suitable targets. The interactome analysis among apicoplast, mitochondrion and genomic DNA will also be useful in identifying vital pathways or proteins regulating critical pathways for parasite growth and survival, and could be attractive targets. Molecules responsible for parasite invasion to host erythrocytes and ion channels of infected erythrocytes, essential for intra-erythrocyte survival and stage progression of parasites are also becoming attractive targets. This review will discuss and highlight the current understanding regarding the potential antimalarial drug targets, which could be utilized to develop novel antimalarials.

Plasmodium serine hydroxymethyltransferase as a potential anti-malarial target: inhibition studies using improved methods for enzyme production and assay

Background

There is an urgent need for the discovery of new anti-malarial drugs. Thus, it is essential to explore different potential new targets that are unique to the parasite or that are required for its viability in order to develop new interventions for treating the disease. Plasmodium serine hydroxymethyltransferase (SHMT), an enzyme in the dTMP synthesis cycle, is a potential target for such new drugs, but convenient methods for producing and assaying the enzyme are still lacking, hampering the ability to screen inhibitors.

Methods

Production of recombinant Plasmodium falciparum SHMT (PfSHMT) and Plasmodium vivax SHMT (PvSHMT), using auto-induction media, were compared to those using the conventional Luria Bertani medium with isopropyl thio-β-D-galactoside (LB-IPTG) induction media. Plasmodium SHMT activity, kinetic parameters, and response to inhibitors were measured spectrophotometrically by coupling the reaction to that of 5,10-methylenetetrahydrofolate dehydrogenase (MTHFD). The identity of the intermediate formed upon inactivation of Plasmodium SHMTs by thiosemicarbazide was investigated by spectrophotometry, high performance liquid chromatography (HPLC), and liquid chromatography-mass spectrometry (LC-MS). The active site environment of Plasmodium SHMT was probed based on changes in the fluorescence emission spectrum upon addition of amino acids and folate.

Results

Auto-induction media resulted in a two to three-fold higher yield of Pf- and PvSHMT (7.38 and 29.29 mg/L) compared to that produced in cells induced in LB-IPTG media. A convenient spectrophotometric activity assay coupling Plasmodium SHMT and MTHFD gave similar kinetic parameters to those previously obtained from the anaerobic assay coupling SHMT and 5,10-methylenetetrahydrofolate reductase (MTHFR); thus demonstrating the validity of the new assay procedure. The improved method was adopted to screen for Plasmodium SHMT inhibitors, of which some were originally designed as inhibitors of malarial dihydrofolate reductase. Plasmodium SHMT was slowly inactivated by thiosemicarbazide and formed a covalent intermediate, PLP-thiosemicarbazone.

Conclusions

Auto-induction media offers a cost-effective method for the production of Plasmodium SHMTs and should be applicable for other Plasmodium enzymes. The SHMT-MTHFD coupled assay is equivalent to the SHMT-MTHFR coupled assay, but is more convenient for inhibitor screening and other studies of the enzyme. In addition to inhibitors of malarial SHMT, the development of species-specific, anti-SHMT inhibitors is plausible due to the presence of differential active sites on the Plasmodium enzymes.

Formation of catalytically active cross-species heterodimers of thymidylate synthase from Plasmodium falciparum and Plasmodium vivax

Thymidylate synthase (TS) of Plasmodium dihydrofolate reductase-thymidylate synthase (DHFR-TS) functions as a homodimeric enzyme with two active sites located near the subunit interface. The dimerization is essential for catalysis, since the active site of each subunit contains amino acid residues contributed from the other TS domain. In P. falciparum DHFR-TS, it has been shown that the active sites require Cys-490 from one domain and Arg-470 donated from the other domain. Mutants of these two series can complement one another giving rise to active enzyme. Here, the potential to form cross-species heterodimers between P. falciparum and P. vivax TS has been explored. Formation of cross-species heterodimer was tested by co-transformation of TS-inactive Cys-490 mutants of P. falciparum or P. vivax with corresponding TS-inactive Arg-486 mutants of P. vivax or P. falciparum into thymidine-requiring Escherichia coli. Active heterodimers were detected by subunit complementation and 6-[(3)H]-FdUMP binding assays. All combinations of the mutants tested, except for (Pf)R470A+(Pv)C506Y, were able to form catalytically active cross-species heterodimers. The single active site formed by (Pf)R470D+(Pv)C506Y and (Pv)R486D+(Pf)C490A pairs of cross-species heterodimers has k(cat) and K(m) values similar to those of intra-species heterodimers of P. falciparum and P. vivax. This is the first report to demonstrate that the TS subunit interface between Plasmodium species is sufficiently conserved to allow formation of fully active cross-species heterodimer.

Anti-Plasmodium activity of imidazolium and triazolium salts

We have previously reported that tetrazolium salts were both potent and specific inhibitors of Plasmodium replication, and that they appear to interact with a parasite component that is both essential and conserved. The use of tetrazolium salts in vivo is limited by the potential reduction of the tetrazolium ring to form an inactive, neutral acyclic formazan. To address this issue imidazolium and triazolium salts were synthesized and evaluated as Plasmodium inhibitors. Many of the imidazolium and triazolium salts were highly potent with active concentrations in the nanomolar range in Plasmodium falciparum cultures, and specific to Plasmodium with highly favorable therapeutic ratios. The results corroborate our hypothesis that an electron-deficient core is required so that the compound may thereby interact with a negatively charged moiety on the parasite merozoite; the side groups in the compound then form favorable interactions with adjacent parasite components and thereby determine both the potency and selectivity of the compound.

Structure-based approach to pharmacophore identification, in silico screening, and three-dimensional quantitative structure-activity relationship studies for inhibitors of Trypanosoma cruzi dihydrofolate reductase function

We have employed a structure-based three-dimensional quantitative structure-activity relationship (3D-QSAR) approach to predict the biochemical activity for inhibitors of T. cruzi dihydrofolate reductase-thymidylate synthase (DHFR-TS). Crystal structures of complexes of the enzyme with eight different inhibitors of the DHFR activity together with the structure in the substrate-free state (DHFR domain) were used to validate and refine docking poses of ligands that constitute likely active conformations. Structural information from these complexes formed the basis for the structure-based alignment used as input for the QSAR study. Contrary to indirect ligand-based approaches the strategy described here employs a direct receptor-based approach. The goal is to generate a library of selective lead inhibitors for further development as antiparasitic agents. 3D-QSAR models were obtained for T. cruzi DHFR-TS (30 inhibitors in learning set) and human DHFR (36 inhibitors in learning set) that show a very good agreement between experimental and predicted enzyme inhibition data. For crossvalidation of the QSAR model(s), we have used the 10% leave-one-out method. The derived 3D-QSAR models were tested against a few selected compounds (a small test set of six inhibitors for each enzyme) with known activity, which were not part of the learning set, and the quality of prediction of the initial 3D-QSAR models demonstrated that such studies are feasible. Further refinement of the models through integration of additional activity data and optimization of reliable docking poses is expected to lead to an improved predictive ability.

Whole-genome analysis reveals molecular innovations and evolutionary transitions in chromalveolate species

The chromalveolates form a highly diverse and fascinating assemblage of organisms, ranging from obligatory parasites such as Plasmodium to free-living ciliates and algae such as kelps, diatoms, and dinoflagellates. Many of the species in this monophyletic grouping are of major medical, ecological, and economical importance. Nevertheless, their genome evolution is much less well studied than that of higher plants, animals, or fungi. In the current study, we have analyzed and compared 12 chromalveolate species for which whole-sequence information is available and provide a detailed picture on gene loss and gene gain in the different lineages. As expected, many gene loss and gain events can be directly correlated with the lifestyle and specific adaptations of the organisms studied. For instance, in the obligate intracellular Apicomplexa we observed massive loss of genes that play a role in general basic processes such as amino acid, carbohydrate, and lipid metabolism, reflecting the transition of a free-living to an obligate intracellular lifestyle. In contrast, many gene families show species-specific expansions, such as those in the plant pathogen oomycete Phytophthora that are involved in degrading the plant cell wall polysaccharides to facilitate the pathogen invasion process. In general, chromalveolates show a tremendous difference in genome structure and evolution and in the number of genes they have lost or gained either through duplication or horizontal gene transfer.

Plasmepsins as potential targets for new antimalarial therapy

Malaria is one of the major diseases in the world. Due to the rapid spread of parasite resistance to available antimalarial drugs there is an urgent need for new antimalarials with novel mechanisms of action. Several promising targets for drug intervention have been revealed in recent years. This review addresses the parasitic aspartic proteases termed plasmepsins (Plms) that are involved in the hemoglobin catabolism that occurs during the erythrocytic stage of the malarial parasite life cycle. Four Plasmodium species are responsible for human malaria; P. vivax, P. ovale, P. malariae, and P. falciparum. This review focuses on inhibitors of the haemoglobin-degrading plasmepsins of the most lethal species, P. falciparum; Plm I, Plm II, Plm IV, and histo-aspartic protease (HAP). Previously, Plm II has attracted the most attention. With the identification and characterization of new plasmepsins and the results from recent plasmepsin knockout studies, it now seems clear that in order to achieve high-antiparasitic activities in P. falciparum-infected erythrocytes it is necessary to inhibit several of the haemoglobin-degrading plasmepsins. Herein we summarize the structure-activity relationships of the Plm I, II, IV, and HAP inhibitors. These inhibitors represent all classes which, to the best of our knowledge, have been disclosed in journal articles to date. The 3D structures of inhibitor/plasmepsin II complexes available in the protein data bank are briefly discussed and compared.

Immobilization of Malarial (Plasmodium falciparum) Dihydrofolate Reductase for the Selection of Tight-Binding Inhibitors from Combinatorial Library

A simple procedure for selection of tight-binding inhibitors of mutant dihydrofolate reductases from Plasmodium falciparum (PfDHFRs) based on preferential binding to the enzyme immobilized on a Sepharose column has been described. PfDHFRs with a cysteine residue at the C-terminal have been prepared in order to immobilize to a thiopropyl-Sepharose gel via S-S linkage. The amount of immobilized DHFRs was estimated to be 4-5 mg/g of dried gel, and the activities of bound DHFRs were comparable to that of free enzymes. The prepared immobilized enzyme has been used for the selection of tight-binding inhibitors from combinatorial libraries, based on the affinities of each ligand with the enzyme. Free ligands were then identified and analyzed quantitatively by high-performance liquid chromatography-mass spectrometry, and the components with high binding affinity of the library could thus be realized. Results could be confirmed by quantitative analysis of the bound ligands released from the enzyme by guanidine hydrochloride treatment.

Artemisinin effectiveness in erythrocytes is reduced by heme and heme-containing proteins

Artemisinin loses its antimalarial activity on prolonged exposure to erythrocytes, especially alpha-thalassemic erythrocytes. In this report, we show that the major artemisinin-inactivating factor in cytosol of normal erythrocytes was heat-labile but a heat-stable factor from alpha-thalassemic cells also played a significant role in reducing artemisinin effectiveness, which was shown to be heme released from hemoglobin (Hb). Studies of fractionated lysate from genetically normal erythrocytes revealed that the protein fraction with molecular weight greater than 100 kDa was capable of reducing artemisinin effectiveness more readily than lower molecular weight fraction. Catalase and Hb A, but not selenoprotein glutathione peroxidase, were capable of reducing artemisinin effectiveness. Hemin (ferriprotoporphyrin IX) also reduced artemisinin effectiveness in a concentration- and time-dependent manner. It is concluded that heme and heme-containing proteins in erythrocyte are largely responsible for reducing artemisinin effectiveness and may contribute to resistance of Plasmodium falciparum infecting alpha-thalassemic erythrocytes observed in vitro.

Therapeutic targets: progress of their exploration and investigation of their characteristics

Modern drug discovery is primarily based on the search and subsequent testing of drug candidates acting on a preselected therapeutic target. Progress in genomics, protein structure, proteomics, and disease mechanisms has led to a growing interest in and effort for finding new targets and more effective exploration of existing targets. The number of reported targets of marketed and investigational drugs has significantly increased in the past 8 years. There are 1535 targets collected in the therapeutic target database compared with approximately 500 targets reported in a 1996 review. Knowledge of these targets is helpful for molecular dissection of the mechanism of action of drugs and for predicting features that guide new drug design and the search for new targets. This article summarizes the progress of target exploration and investigates the characteristics of the currently explored targets to analyze their sequence, structure, family representation, pathway association, tissue distribution, and genome location features for finding clues useful for searching for new targets. Possible "rules" to guide the search for druggable proteins and the feasibility of using a statistical learning method for predicting druggable proteins directly from their sequences are discussed.

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.

Optimization of culture conditions for production of antimalarial menisporopsin A by the seed fungusMenisporopsis theobromaeBCC 4162

AIMS

The aim of this work was to optimize the production of a novel antimaralial menisporopsin A by the seed fungus Menisporopsis theobromae BCC 4162.

METHODS AND RESULTS

Fungal cultures were grown in shake flasks at 25 degrees C in the basal medium with varying carbon and nitrogen sources, aeration rates and initial pH levels. The optimal carbon and nitrogen sources that improved the production of menisporopsin A were 1% fructose and 2.5% meat extract respectively. The production was further enhanced when the culture incubated on a shaker at 200 rev min(-1) with an initial pH of 8. The yield of menisporopsin A cultured under the optimized conditions was increased from 348.30 (obtained from basal medium) to 889.02 mg l(-1), and the cultivation time was reduced from 28 to only 4 days. As a result, the productivity of menisporopsin A was greatly enhanced to 222.26 mg l(-1) day(-1) which is 18-fold higher than that of basal conditions. Larger scale production in a fermenter was also achieved, yielding menisporopsin A at a maximal level of 594.32 mg l(-1) in 4 days.

CONCLUSIONS

The optimized culture conditions for menisporopsin A production by M. theobromae BCC 4162 was the cultivation under shaking or agitation at 25 degrees C in fructose-meat extract medium with an initial pH of 8.

SIGNIFICANCE AND IMPACT OF THE STUDY

The production of menisporopsin A in a fermenter with a relatively short incubation period could be valuable for further utilization for chemical structure modification and derivatization.

Synthesis of three classes of rhodacyanine dyes and evaluation of their in vitro and in vivo antimalarial activity

Selected members of three classes of rhodacyanine dyes, [0,0]-, [1,0]-, and [0,0,0]-rhodacyanines, were synthesized and their in vitro antimalarial activities against Plasmodium falciparum K1 (chloroquine-resistant strain) as well as their in vivo activities against P. berghei in mice were determined. The novel [0,0,0]-rhodacynines, 3e and 3h, possessing a benzothiazole moiety, were shown to have highly promising antimalarial activities in vivo. Moreover, the [0,0,0]-rhodacyanines were found to be orally bioavailable.

Plasmodium falciparum pyruvate kinase as a novel target for antimalarial drug-screening

BACKGROUND

Global travellers are increasingly at risk of contracting malaria. The increasing occurrence of drug-resistance in many endemic areas emphasizes the need for novel drug targets for antimalarial-screening. In this study, the use of pyruvate kinase as a drug-target is evaluated. The functional validation of a gene encoding pyruvate kinase (designated PK1) has previously been reported. However, alternative copies of this enzyme encoded by Plasmodium falciparum could also circumvent the role of PK1. A survey of genome data revealed a putative ORF seemingly coding for another pyruvate kinase (designated PK2).

METHODS

The expression of PK1 and PK2 in in vitro cultures were investigated by RT-PCR. Biocomputational analysis was carried out to identify structural differences between the P. falciparum pyruvate kinases and the corresponding enzymes from its human host.

RESULTS

Both PK1 and PK2 were indeed actively transcribed during the intraerythrocytic stages, suggesting the involvement of both enzymes during infection. A comparison of amino acid residues at the effector binding sites of PK1 and PK2, to those of the human pyruvate kinases revealed some significant differences that could serve as targets for selective inhibitors to be designed against parasitic pyruvate kinases.

CONCLUSION

Experimental evidence for the expression of both PK1 and PK2 during the blood stages of malaria infection was provided. Interestingly, phylogenetic analysis revealed that the "PK2" type of enzyme appears to be confined to Apicomplexans, an important observation with respect to the assessment of PK2 as a drug-target.

Toxoplasma gondii is capable of exogenous folate transport. A likely expansion of the BT1 family of transmembrane proteins

Folates are key elements in eukaryotic biosynthetic processes. The protozoan parasite Toxoplasma gondii possesses the enzymes necessary for de novo folate synthesis and has been suggested to lack alternative mechanisms for folate acquisition. In this paper, we present a different view by providing evidence that Toxoplasma is capable of salvaging exogenous folates. By monitoring uptake of radiolabeled folates by parasites in axenic conditions, our studies revealed a common folate transporter that has a high affinity for folic acid. Transport of this compound across the parasite plasma membrane is rapid, biphasic, temperature dependent, bi-directional, concentration dependent and specific. In addition, morphological evidence demonstrates that fluorescent methotrexate, a folate analog, is internalized by Toxoplasma and shows localization reminiscent to the mitochondrion. The presence of putative folate transporter genes in the Toxoplasma genome, which are homologous to the BT1 family of proteins, suggests that Toxoplasma may encode proteins involved in folate transport. Interestingly, genome analysis suggests that the BT1 family of proteins exists not only in Toxoplasma, but in other Apicomplexan parasites as well. Altogether, our results not only have implications for current therapeutic regimens against T. gondii, but they also allude that the folate transport mechanism may represent a novel Apicomplexan target for the development of new drugs.

Structural and active site analysis of plasmepsins of Plasmodium falciparum: Potential anti-malarial targets

Comparative protein modeling, active site analysis and binding site specificity for the homologous series of plasmepsins (PM's), present in food vacuole of Plasmodium falciparum, are carried out. Four loops (L1, L2, L3 and L4), which show maximum structural deviations irrespective of type of inhibitor, have been identified. Comparison of the crystal structures of ligand complexes reveal that residues belonging to these loops have negligible coulomb and VDW interactions with the inhibitor but play major role in determining the openness of the binding cavity. The coulomb and VDW interactions between the PMII subsite pockets and inhibitors, which play a major role in determining the inhibition constants, are delineated. Besides small displacements, the catalytic residues D32 of PMII undergoes rotation around the Cgamma-Cbeta single bond to assist catalysis whereas side chain conformational deviations are not observed in D214 on plasmepsin activation. The mutant S79D of PMII (and the corresponding residues of PMI and PMIV) which helps in recognizing and cleaving substrates containing lysine at P1 position is surrounded by highly polar atmosphere stabilized by lysine. However, in PMIII significantly lower polar atmosphere around the mutant A78S/A78D is observed. Large buried side chain area of residues located at M15 and I289 of PMII (and corresponding residues of PMI and PMIV) corroborates well with increase in specificity constant for hydrophobic substrates.

Lipophilic antifolate trimetrexate is a potent inhibitor of Trypanosoma cruzi: prospect for chemotherapy of Chagas' disease

Trypanosoma cruzi, a protozoan parasite, is the causative agent for Chagas' disease, which poses serious public health problem in Latin America. The two drugs available for the treatment of this disease are effective only against recent infections and are toxic. Dihydrofolate reductase (DHFR) has a proven track record as a drug target. The lipophilic antifolate trimetrexate (TMQ), which is an FDA-approved drug for the treatment of Pneumocystis carinii infection in AIDS patients, is a potent inhibitor of T. cruzi DHFR activity, with an inhibitory constant of 6.6 nM. The compound is also highly effective in killing T. cruzi parasites. The 50 and 90% lethal dose values against the trypomastigote are 19 and 36 nM, and the corresponding values for the amastigote form are 26 and 72 nM, respectively. However, as TMQ is also a good inhibitor of human DHFR, further improvement of the selectivity of this drug would be preferable. Identification of a novel antifolate selective against T. cruzi would open up new therapeutic avenues for treatment of Chagas' disease.

Pharmacological properties of a new antimalarial bisthiazolium salt, T3, and a corresponding prodrug, TE3

A new approach to malarial chemotherapy based on quaternary ammonium that targets membrane biogenesis during intraerythrocytic Plasmodium falciparum development has recently been developed. To increase the bioavailability, nonionic chemically modified prodrugs were synthesized. In this paper, the pharmacological properties of a bisthiazolium salt (T3) and its bioprecursor (TE3) were studied. Their antimalarial activities were determined in vitro against the growth of P. falciparum and in vivo against the growth of P. vinckei in mice. Pharmacokinetic evaluations were performed after T3 (1.3 and 3 mg/kg of body weight administered intravenously; 6.4 mg/kg administered intraperitoneally) and TE3 (1.5 and 3 mg/kg administered intravenously; 12 mg/kg administered orally) administrations to rats. After intraperitoneal administration, very low doses offer protection in a murine model of malaria (50% efficient dose [ED50] of 0.2 to 0.25 mg/kg). After oral administration, the ED50 values were 13 and 5 mg/kg for T3 and TE3, respectively. Both compounds exerted antimalarial activity in the low nanomolar range. After TE3 administration, rapid prodrug-drug conversion occurred; the mean values of the pharmacokinetic parameters for T3 were as follows: total clearance, 1 liter/h/kg; steady-state volume of distribution, 14.8 liters/kg; and elimination half-life, 12 h. After intravenous administration, T3 plasma concentrations increased in proportion to the dose. The absolute bioavailability was 72% after intraperitoneal administration (T3); it was 15% after oral administration (TE3). T3 plasma concentrations (8 nM) 24 h following oral administration of TE3 were higher than the 50% inhibitory concentrations for the most chloroquine-resistant strains of P. falciparum (6.3 nM).

Subunit complementation of thymidylate synthase in Plasmodium falciparum bifunctional dihydrofolate reductase-thymidylate synthase

Thymidylate synthase of Plasmodium falciparum dihydrofolate reductase-thymidylate synthase (PfDHFR-TS) functions as a dimeric enzyme with extensive contact between the two TS domains. Structural data of PfDHFR-TS shows that the formation of the two TS active sites involves contribution of the amino acid residues from both TS domains. Arg-470 donated from the adjoining domain is shown to hydrogen-bond to dUMP, while Cys-490 is a key nucleophile for TS catalysis by attacking C-6 of dUMP. However, mutants of the two series could complement one another, giving rise to active enzyme. By means of subunit complementation assay using Arg-470 and Cys-490 mutants, it is shown that co-transformants of both TS-inactive Arg-470 and Cys-490 mutants can complement the growth of thymidine auxotroph chi2913RecA(DE3) by formation of a functional TS heterodimer contributing from both Arg-470 and Cys-490 mutant subunits. 6-[3H]-FdUMP thymidylate synthase activity assay further elaborate the essence of restoration of TS activity. The TS k(cat) value of the R470D+C490A heterodimer is decreased by half from that of the wild-type PfDHFR-TS. However, the Km values for dUMP and CH2H4folate of the R470D+C490A heterodimer are similar to those of wild-type enzyme, indicating that the catalytic efficiency of the functional TS from the R470D+C490A heterodimer is similar to the wild-type TS enzyme in P. falciparum DHFR-TS.

Plasmodium falciparum hypoxanthine guanine phosphoribosyltransferase. Stability studies on the product-activated enzyme

Hypoxanthine guanine phosphoribosyltransferases (HGPRTs) catalyze the conversion of 6-oxopurine bases to their respective nucleotides, the phosphoribosyl group being derived from phosphoribosyl pyrophosphate. Recombinant Plasmodium falciparum HGPRT, on purification, has negligible activity, and previous reports have shown that high activities can be achieved upon incubation of recombinant enzyme with the substrates hypoxanthine and phosphoribosyl pyrophosphate [Keough DT, Ng AL, Winzor DJ, Emmerson BT & de Jersey J (1999) Mol Biochem Parasitol98, 29-41; Sujay Subbayya IN & Balaram H (2000) Biochem Biophys Res Commun279, 433-437]. In this report, we show that activation is effected by the product, Inosine monophosphate (IMP), and not by the substrates. Studies carried out on Plasmodium falciparum HGPRT and on a temperature-sensitive mutant, L44F, show that the enzymes are destabilized in the presence of the substrates and the product, IMP. These stability studies suggest that the active, product-bound form of the enzyme is less stable than the ligand-free, unactivated enzyme. Equilibrium isothermal-unfolding studies indicate that the active form is destabilized by 2-3 kcal x mol(-1) compared with the unactivated state. This presents a unique example of an enzyme that attains its active conformation of lower stability by product binding. This property of ligand-mediated activation is not seen with recombinant human HGPRT, which is highly active in the unliganded state. The reversibility between highly active and weakly active states suggests a novel mechanism for the regulation of enzyme activity in P. falciparum.