Search For New Antimalarial Drugs Research Articles

Glucose-6-phosphate dehydrogenase–6-phosphogluconolactonase: a unique bifunctional enzyme from Plasmodium falciparum

The survival of malaria parasites in human RBCs (red blood cells) depends on the pentose phosphate pathway, both in Plasmodium falciparum and its human host. G6PD (glucose-6-phosphate dehydrogenase) deficiency, the most common human enzyme deficiency, leads to a lack of NADPH in erythrocytes, and protects from malaria. In P. falciparum, G6PD is combined with the second enzyme of the pentose phosphate pathway to create a unique bifunctional enzyme named GluPho (glucose-6-phosphate dehydrogenase-6-phosphogluconolactonase). In the present paper, we report for the first time the cloning, heterologous overexpression, purification and kinetic characterization of both enzymatic activities of full-length PfGluPho (P. falciparum GluPho), and demonstrate striking structural and functional differences with the human enzymes. Detailed kinetic analyses indicate that PfGluPho functions on the basis of a rapid equilibrium random Bi Bi mechanism, where the binding of the second substrate depends on the first substrate. We furthermore show that PfGluPho is inhibited by S-glutathionylation. The availability of recombinant PfGluPho and the major differences to hG6PD (human G6PD) facilitate studies on PfGluPho as an excellent drug target candidate in the search for new antimalarial drugs.

Refreshed approaches to the therapy of malaria - the case of natural medicine

Although the completion of genome project of the most virulent malaria parasite Plasmodium falciparum in 2002 evoked great hopes for the identification of new targets for novel drugs, these have not yet been fulfilled. This disappointment and the successful use of natural compounds emerging from traditional medicine for the chemotherapy of malaria (quinine, artemisnin) have invigorated the search for new antimalarials from natural sources. Hundreds of thousands compounds were screened and some promising leads were identified. However, the great advantage of traditional medicine is a result of demonstrable synergism between the constituents of the crude extracts. The origin and the modes of action of this synergism will be discussed. In conclusion, it is suggested that greater emphasis should be given to traditional medicine and some considerations for its controlled implementation will be mentioned.

Antiplasmodial activities of homogentisic acid derivative protein kinase inhibitors isolated from a Vanuatu marine sponge Pseudoceratina sp.

As part of our search for new antimalarial drugs in South Pacific marine sponges, we have looked for inhibitors of Pfnek-1, a specific protein kinase of Plasmodium falciparum. On the basis of promising activity in a preliminary screening, the ethanolic crude extract of a new species of Pseudoceratina collected in Vanuatu was selected for further investigation. A bioassay-guided fractionation led to the isolation of a derivative of homogentisic acid [methyl (2,4-dibromo-3,6-dihydroxyphenyl)acetate, 4a] which inhibited Pfnek-1 with an IC50 around 1.8 μM. This product was moderately active in vitro against a FcB1 P. falciparum strain (IC50 = 12 μM). From the same sponge, we isolated three known compounds [11,19-dideoxyfistularin-3 (1), 11-deoxyfistularin-3 (2) and dibromo-verongiaquinol (3)] which were inactive against Pfnek-1. Synthesis and biological evaluation of some derivatives of 4a are reported.

Testing of Natural Products and Synthetic Molecules Aiming at New Antimalarials

The search for new antimalarials, which in the past relied on animal models, is now usually performed with cultures of Plasmodium falciparum (PF) blood parasites by evaluation of parasite growth inhibition. Field isolates of PF human malaria parasite, parasite strains and clones, well characterized for their susceptibility to chloroquine and other standard antimalarials are available for the in vitro tests. The simplest method to evaluate parasite growth is the determination of parasitemias in Giemsa stained blood smears through light microscopy. Other methodologies have proven to be more precise and allow mass screening of new compounds against PF blood stages, such as: (i) measuring the incorporation of radioactive hypoxanthine by the parasites; (ii) indirect colorimetric assays in which specific parasite enzyme activities, and histidine-rich protein II (HRP2) production are measured with the help of monoclonal antibodies; (iii) the beta-haematin formation, and; (iv) assays using green fluorescent protein (GFP) in gene-expressing parasites. The advantages and disadvantages of the different in vitro screening methods, as well as the different in vivo models for antimalarial tests, are described in this review. Such tests can be used for the evaluation of medicinal plants, synthetic and hybrid molecules or drug combinations.

Febrifugine derivative antimalarial activity: quantum mechanical predictors

Plasmodium falciparum resistant strain development has encouraged the search for new antimalarial drugs. Febrifugine is a natural substance with high activity against P. falciparum presenting strong emetic property and liver toxicity, which prevent it from being used as a clinical drug. The search for analogues that could have a better clinical performance is a current topic. We aim to investigate the theoretical electronic structure by means of febrifugine derivative family semi-empirical molecular orbital calculations, seeking the electronic indexes that could help the design of new efficient derivatives. The theoretical results show there is a clustering in well-defined ranges of several electronic indexes of the most selective molecules. The model proposed for achieving high selectivity was tested with success.

Ferredoxin-NADP+ Reductase from Plasmodium falciparum Undergoes NADP+-dependent Dimerization and Inactivation: Functional and Crystallographic Analysis

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

Antimalarial potential of xestoquinone, a protein kinase inhibitor isolated from a Vanuatu marine sponge Xestospongia sp.

As part of our search for new antimalarial drugs, we have screened for inhibitors of Pfnek-1, a protein kinase of Plasmodium falciparum, in south Pacific marine sponges. On the basis of a preliminary screening, the ethanolic crude extract of a new species of Xestospongia collected in Vanuatu was selected for its promising activity. A bioassay-guided fractionation led us to isolate xestoquinone which inhibits Pfnek-1 with an IC 50 around 1 μM. Among a small panel of plasmodial protein kinases, xestoquinone showed modest protein kinase inhibitory activity toward PfPK5 and no activity toward PfPK7 and PfGSK-3. Xestoquinone showed in vitro antiplasmodial activity against a FCB1 P. falciparum strain with an IC 50 of 3 μM and a weak selectivity index (SI 7). Xestoquinone exhibited a weak in vivo activity at 5 mg/kg in Plasmodium berghei NK65 infected mice and was toxic at higher doses.

Antimalarial activity of Sida acuta Burm. f. (Malvaceae) and Pterocarpus erinaceus Poir. (Fabaceae)

Among strategies to combat malaria, the search for new antimalarial drugs appears to be a priority. Sheering for new antimalarial activities, four plants of the traditional medicine of Burkina Faso: Combretum micranthum, Khaya senegalensis, Pterocarpus erinaceus and Sida acuta, were tested in vitro on fresh clinical isolates of Plasmodium falciparum. The screening showed that Sida acuta has a significant activity (IC 50<5 μg/ml), and Pterocarpus erinaceus has a moderate activity (5 μg/ml<IC 50<50 μg/ml). Further chemical screening showed that the activity of the most active plant, Sida acuta, was related to its alkaloid contents.

A new validated target for antimalarial chemotherapy

The constant search for new antimalarials is dictated by the emerging resistance of malaria parasites to available drugs. Whereas herbal medicine continues to provide promising lead compounds, the identification of specific targets in Plasmodium spp. results from thorough biochemical studies. Henri Vial et al. have been targeting the phospholipid metabolism of malaria parasites as a feasible chemotherapeutic approach, which is described in Antimalarial Chemotherapy: Mechanisms of Action, Resistance, and New Directions in Drug Discovery (Rosenthal, P.J., ed., Humana Press, 2001). After deciphering various pathways which provide phospholipids for the fivefold increase of membrane material in the growing intracellular parasite, Vial et al. discovered that choline uptake was essential for the synthesis of the major phospholipid phosphatidylcholine. Therefore, they aimed at inhibiting choline uptake and synthesized >400 compounds. They found that the most active compounds which have the least toxicity to host cells should have two bulky (200–350 A3) quaternary ammonium moieties (N-methylpyrrolidinium or tripropylammonium) separated by several methylenes. Activity increased with chain length up to 21 methylenes. From these compounds, G25 [1,16 hexamethylene bis (N-methylpyrrolidinium) dichloride] was selected for further studies because of its ease of synthesis, low production cost, and potent antimalarial activity in vitro (Plasmodium falciparum and Plasmodium vivax) and in vivo (Plasmodium vinckei and Plasmodium chabaudi). G25 has cytotoxic activity and exerts its maximal effect in vitro within 5 h of exposure.Recently, Vial's group reported that oral or intramuscular administration of G25 can cure Aotus monkeys infected with P. falciparum or Plasmodium cynomolgi, although the therapeutic index (∼30) was much lower in vivo than in vitro. The close association between the IC50 for inhibition of parasite growth and inhibition of choline uptake insinuates the mode of action, whereas the moderate effect of G25 on cholinergic receptors explains the therapeutic index. Using an analog of G25, N,N,N′,N′-tetramethyl-N,N′-di{2-N[2-(p-3H-benzoyl)benzoyl]-aminoethyl} -1,12-dodecanediaminium dibromide, VB5. Wengelnik et al. have shown that VB5 accumulates to high levels in Plasmodium-infected erythrocytes, suggesting that accumulation involves dissolution in the membrane fraction where the drug targets are localized [1xA class of potent antimalarials and their specific accumulation in infected erythrocytes. Wengelnik, K. et al. Science. 2002; 295: 1311–1314Crossref | PubMed | Scopus (141)See all References][1]. Surprisingly, no accumulation occurred in lymphocytes. To remedy the poor oral absorption of G25, Wengelnik et al. synthesized bioisoesters of the quaternary ammonium and non-ionic prodrugs. These compounds displayed better absorption and lower toxicity. Further development of such lead compounds could result in even more potent antimalarial drugs.

Inhibition of P. falciparum by steroids isolated from Solanum nudum.

Resistance to antimalarials has been widely reported and constitutes a major problem around the world. In Colombia, resistance of P. falciparum to chloroquine has been reported as 47%-97%, to amodiaquine 3%-7% and to sulfadoxine-pyrimethamine as 9%-13%. The search for new antimalarials is a priority and with this aim we studied the in vitro antimalarial activity of plants used by traditional healers. Incorporation of (3)H-hypoxanthine by the strain FCB-2 of P. falciparum was used to measure the degree of inhibition produced by the steroids SN-1 tumacone A (C(29)H(44)O(5)), SN-2 tumacone B (C(27)H(42)O(4)), SN-3 tumacoside A (C(35)H(54)O(10)), and SN-4 tumacoside B (C(33)H(52)O(9)). All compounds were obtained from the dried stems and leaves of Solanum nudum. The mean growth inhibition of P. falciparum was 71%, 56%, 21% and 12% with each of the compounds SN-1, SN-2, SN-3 and SN-4. These results constitute an important discovery since they may account for the antimalarial properties of extracts of Solanum nudum by a sensitive method. Future work should include study of the in vivo anti-malarial effect of these extracts.

The search for new antimalarial drugs from plants used to treat fever and malaria or plants ramdomly selected: a review.

In this review we discuss the ongoing situation of human malaria in the Brazilian Amazon, where it is endemic causing over 610,000 new acute cases yearly, a number which is on the increase. This is partly a result of drug resistant parasites and new antimalarial drugs are urgently needed. The approaches we have used in the search of new drugs during decades are now reviewed and include ethnopharmocology, plants randomly selected, extracts or isolated substances from plants shown to be active against the blood stage parasites in our previous studies. Emphasis is given on the medicinal plant Bidens pilosa, proven to be active against the parasite blood stages in tests using freshly prepared plant extracts. The anti-sporozoite activity of one plant used in the Brazilian endemic area to prevent malaria is also described, the so called "Indian beer" (Ampelozizyphus amazonicus, Rhamnaceae). Freshly prepared extracts from the roots of this plant were totally inactive against blood stage parasites, but active against sporozoites of Plasmodium gallinaceum or the primary exoerythrocytic stages reducing tissue parasitism in inoculated chickens. This result will be of practical importance if confirmed in mammalian malaria. Problems and perspectives in the search for antimalarial drugs are discussed as well as the toxicological and clinical trials to validate some of the active plants for public health use in Brazil.

Herbicide thwarts growth of malarial parasite

A surprising compound may be a promising lead candidate in the search for new antimalarial drugs. It's the herbicide glyphosate, the active ingredient in Monsanto's Roundup. A team of researchers h...

Effect of Momordica charantia L. in mice infected with Plasmodium berghei

According to the World Health Organization malaria is one of the major public health problems in Brazil and all over developing countries, where 80% of the population use traditional medicine to solve their primary medical problems. Both treatment and control of this parasitosis have become difficult, because of parasite strains that are resistant to conventional drugs, such as chloroquine. That makes the search for new antimalarial drugs not only important but urgent. We aimed therefore at evaluating the effects of Momordica charantia L. (Cucurbitaceae) in mice infected with Plasmodium berghei. We used aquose and ethanotic extracts in a dose of 1000 mg/kg of body weight, orally, for five consecutive days (i.e. from day 2 to day 6 postinfection). We then followed up the parasitaemia during the course of infection. Although the population use this plant as an antimalarial, in our experimental conditions, M. charantia extracts have not shown such activity.

Plasmodium falciparum and Plasmodium vivax infections in the owl monkey (Aotus trivirgatus). I. The courses of untreated infections.

This study, the first of three designed to determine the feasibility of using owl monkeys infected with human plasmodia in the search for new, more broadly active antimalarial drugs, dealt with the characteristics of untreated infections with eight strains of Plasmodium falciparum and two strains of P. vivax. Such infections, induced by standardized inocula of these strains in 1,733 monkeys, all Aotus trivirgatus griseimembra, were followed from day of inoculation to death of self-cure. The virulence of the various strains differed strikingly. Incidences of fatal reactions, ranging from 24.4--89.4% and 8.1--45.8%, respectively, in infections with strains of P. falciparum and P. vivax, were closely related to the rate at which parasitemia evolved, the height of parasitemia in the primary attack, and/or the time period over which a high parasite level was sustained. Antemortem symptom complexes and gross tissue and organ reactions in infections with P. falciparum varied with survival time, but within that boundary, were the same for infections with all eight strains of this plasmodium. Morbidity in both fatal and self-limited infections with both plasmodial species was related to height of parasitemia; however, at comparable parasite levels, symptoms exhibited in infections with P. vivax were more severe than in infections with P. falciparum. Overall, the characteristics of infections with these plasmodia in owl monkeys were remarkably similar to those of human infections. With respect to biological features, infections with P. falciparum and P. vivax in this simian host appear to have much to offer in the search for new antimalarial drugs.

ChemInform Abstract: ANTIFOLATE STUDIES. ACTIVITIES OF 40 POTENTIAL ANTIMALARIAL COMPOUNDS AGAINST SENSITIVE AND CHLORGUANIDE TRIAZINE RESISTANT STRAINS OF FOLATE‐REQUIRING BACTERIA AND ESCHERICHIA COLI

AbstractIn Tabellen werden ca. 40 "chemisch unterschiedlich aufgebaute Verbindungen, z.B. der Strukturen (I)‐(VII), aufgeführt.

Search for New Antimalarials. Part VIII. Synthesis of Some I,5-Diarylbiguanides

Search for New Antimalarials. Part VIII. Synthesis of Some I,5-Diarylbiguanides

Search for New Antimalarials. Part VII. Synthesis of Some New 1, 2-Dihydro-sym-triazines

Search for New Antimalarials. Part VII. Synthesis of Some New 1, 2-Dihydro-sym-triazines

CHLOROQUINE FOR TREATMENT OF ACUTE ATTACKS OF VIVAX MALARIA

During the recent war an intensive search for new antimalarial drugs was conducted in the course of which thousands of chemical substances were investigated under the auspices of the Office of Scientific Research and Development. Studies were designed to find drugs which might have one or more of the following properties: (a) true causal prophylaxis, (b) ability to effect complete cure following treatment, (c) greater efficacy than quinacrine and quinine in suppression and (d) greater efficacy than quinine and quinacrine in the treatment of acute clinical attacks. The present report is concerned with chloroquine (SN 7618): 7-chloro-4-(4-diethylamino-1-methylbutylamino) quinoline, which is one of a series of new active antimalarial agents. Detailed studies of the antimalarial properties, pharmacology and toxicity of this drug have been carried out by numerous investigators, and undoubtedly the results of these studies will be generally available in the near future.1This paper is concerned principally with