Antiplasmodial Natural Products

The potential of anti-malarial compounds derived from African medicinal plants, part I: a pharmacological evaluation of alkaloids and terpenoids

Traditional medicine caters for about 80% of the health care needs of many rural populations around the world, especially in developing countries. In addition, plant-derived compounds have played key roles in drug discovery. Malaria is currently a public health concern in many countries in the world due to factors such as chemotherapy faced by resistance, poor hygienic conditions, poorly managed vector control programmes and no approved vaccines. In this review, an attempt has been made to assess the value of African medicinal plants for drug discovery by discussing the anti-malarial virtue of the derived phytochemicals that have been tested by in vitro and in vivo assays. This survey was focused on pure compounds derived from African flora which have exhibited anti-malarial properties with activities ranging from "very active" to "weakly active". However, only the compounds which showed anti-malarial activities from "very active" to "moderately active" are discussed in this review. The activity of 278 compounds, mainly alkaloids, terpenoids, flavonoids, coumarines, phenolics, polyacetylenes, xanthones, quinones, steroids, and lignans have been discussed. The first part of this review series covers the activity of 171 compounds belonging to the alkaloid and terpenoid classes. Data available in the literature indicated that African flora hold an enormous potential for the development of phytomedicines for malaria.

Assessment of in vivo antimalarial activities of some selected medicinal plants from Turkey

Resistant infections lead to increased necessity of searching novel drugs and drug combinations. The purpose of this paper was to investigate antimalarial properties of some selected medicinal plants that have been traditionally used in Turkey for antipyretic and analgesic purposes. Lavandula stoecheas subsp. cariensis, Phlomis nissolii, Phlomis bourgaei, Phlomis leucophracta, Centaurea hierapolitana, Centaurea polyclada, Centaurea lydia, Scrophularia cryptophila, Scrophularia depauperata, Scrophularia floribunda, Rubia davisiana, and Alkanna tinctoria subsp. subleiocarpa were investigated for their in vivo antimalarial activities in mice infected with Plasmodium yoelii. Two hundred fifty to 500 mg/kg doses of plant extracts were given to mice as a single daily dose for 4 days. P. nissolii water extract, C. lydia chloroform extract, S. cryptophila ethanol extract, and C. polyclada methanol extract showed antimalarial activity with reducing parasitaemia. The chemotherapeutic effects of plant extracts ranged between 13.5% and 66.91%. The chemosuppressions exerted by combined plant extracts of P. nissolii, S. cryptophila, and C. lydia with C. polyclada methanol extract were detected as 51.25%, 57.33%, and 58.33%, respectively. Investigation of cytotoxic activities against brine shrimps revealed that methanol extract of C. polycada, chloroform extract of C. lydia, and ethanol extract of S. cryptophila showed cytotoxic activities, while water extract of P. nissolii was not active against brine shrimps.

Monoterpenes with antibacterial activities from a Cameroonian medicinal plant Canthium Multiflorum (Rubiaceae)

Investigation of the crude extract obtained from the aerial parts of Canthium multiflorum led to the isolation of a new iridoid (1) together with twelve known compounds. The structures of these compounds were elucidated by interpretation of 1D and 2D NMR spectroscopic data, accurate mass measurements and comparison with analytical data of previously known analogues. Most of the isolated compounds have been reported for the first time from C. multiflorium. The antimicrobial activities of the isolated compounds were evaluated on five different bacterial strains using agar diffusion technique. The Gram-positive bacterium Staphylococcus aureus subsp. aureus (DSM 799), and the Gram-negative bacteria Actinobacter calco-aceticus (DSM 30006), Serratia plymuthica (DSM 4540), Pseudomonas stutzeri (DSM 4166) and Escherichia coli (DSM 1116) were employed for this purpose. The new iridoid, named 6-oxo-genipin (1), demonstrated significant inhibitory activity against all microbial strains tested, especially the pathogen Staphylococcus aureus. In addition, the compounds 3, 4 and 9 exhibited antiplasmodial activity against Plasmodium falciparum strain K1 and weak cytotoxicity against L6 cell lines.

Determination of Antiplasmodial Activity and Binding Affinity of Selected Natural Products towards PfTrxR and PfGR

In our study, the binding affinities of selected natural products towards PfTrxR, PfGR, human TrxR and human GR were determined using a mass spectrometry based ligand binding assay. The in vitro antimalarial activity and cytotoxicity of these ligands were also determined. Catharanthine, 11-(OH)-coronaridine, hernagine, vobasine and hispolone displayed antiplasmodial activity against PfK1 (IC50 = 0.996–3.63 μg/mL).

The antimalarial natural product symplostatin 4 is a nanomolar inhibitor of the food vacuole falcipains

The marine natural product symplostatin 4 (Sym4) has been recognized as a potent antimalarial agent. However, its mode of action and, in particular, direct targets have to date remained elusive. We report a chemical synthesis of Sym4 and show that Sym4-treatment of P. falciparum-infected red blood cells (RBCs) results in the generation of a swollen food vacuole phenotype and a reduction of parasitemia at nanomolar concentrations. We furthermore demonstrate that Sym4 is a nanomolar inhibitor of the P. falciparum falcipains in infected RBCs, suggesting inhibition of the hemoglobin degradation pathway as Sym4's mode of action. Finally, we reveal a critical influence of the unusual methyl-methoxypyrrolinone (mmp) group of Sym4 for potent inhibition, indicating that Sym4 derivatives with such a mmp moiety might represent viable lead structures for the development of antimalarial falcipain inhibitors.

Macrocyclic spermidine alkaloids from Androya decaryi L. Perrier

Three new spermidine alkaloids and two known compounds were isolated from the leaves of Androya decaryi. Their structures were elucidated on the basis of their spectroscopic data (NMR and mass spectrometry), by X-Ray diffraction and by comparison with literature values. Evaluation of the in vitro antiplamosdial properties of the isolated compounds revealed they did not possess any significant activity.

Medicinal plants: a source of anti-parasitic secondary metabolites

This review summarizes human infections caused by endoparasites, including protozoa, nematodes, trematodes, and cestodes, which affect more than 30% of the human population, and medicinal plants of potential use in their treatment. Because vaccinations do not work in most instances and the parasites have sometimes become resistant to the available synthetic therapeutics, it is important to search for alternative sources of anti-parasitic drugs. Plants produce a high diversity of secondary metabolites with interesting biological activities, such as cytotoxic, anti-parasitic and anti-microbial properties. These drugs often interfere with central targets in parasites, such as DNA (intercalation, alkylation), membrane integrity, microtubules and neuronal signal transduction. Plant extracts and isolated secondary metabolites which can inhibit protozoan parasites, such as Plasmodium, Trypanosoma, Leishmania, Trichomonas and intestinal worms are discussed. The identified plants and compounds offer a chance to develop new drugs against parasitic diseases. Most of them need to be tested in more detail, especially in animal models and if successful, in clinical trials.

Anti-plasmodial activity of Dicoma tomentosa (Asteraceae) and identification of urospermal A-15-O-acetate as the main active compound

Background

Natural products could play an important role in the challenge to discover new anti-malarial drugs. In a previous study, Dicoma tomentosa (Asteraceae) was selected for its promising anti-plasmodial activity after a preliminary screening of several plants traditionally used in Burkina Faso to treat malaria. The aim of the present study was to further investigate the anti-plasmodial properties of this plant and to isolate the active anti-plasmodial compounds.

Methods

Eight crude extracts obtained from D. tomentosa whole plant were tested in vitro against two Plasmodium falciparum strains (3D7 and W2) using the p-LDH assay (colorimetric method). The Peters’ four-days suppressive test model (Plasmodium berghei-infected mice) was used to evaluate the in vivo anti-plasmodial activity. An in vitro bioguided fractionation was undertaken on a dichloromethane extract, using preparative HPLC and TLC techniques. The identity of the pure compound was assessed using UV, MS and NMR spectroscopic analysis. In vitro cytotoxicity against WI38 human fibroblasts (WST-1 assay) and haemolytic activity were also evaluated for extracts and pure compounds in order to check selectivity.

Results

The best in vitro anti-plasmodial results were obtained with the dichloromethane, diethylether, ethylacetate and methanol extracts, which exhibited a high activity (IC₅₀ ≤ 5 μg/ml). Hot water and hydroethanolic extracts also showed a good activity (IC₅₀ ≤ 15 μg/ml), which confirmed the traditional use and the promising anti-malarial potential of the plant. The activity was also confirmed in vivo for all tested extracts. However, most of the active extracts also exhibited cytotoxic activity, but no extract was found to display any haemolytic activity. The bioguided fractionation process allowed to isolate and identify a sesquiterpene lactone (urospermal A-15-O-acetate) as the major anti-plasmodial compound of the plant (IC₅₀ < 1 μg/ml against both 3D7 and W2 strains). This was also found to be the main cytotoxic compound (SI = 3.3). While this melampolide has already been described in the plant, this paper is the first report on the biological properties of this compound.

Conclusions

The present study highlighted the very promising anti-plasmodial activity of D. tomentosa and enabled to identify its main active compound, urospermal A-15-O-acetate. The high anti-plasmodial activity of this compound merits further study about its anti-plasmodial mechanism of action. The active extracts of D. tomentosa, as well as urospermal A 15-O-acetate, displayed only a moderate selectivity, and further studies are needed to assess the safety of the use of the plant by the local population.

Antiplasmodial activity of the andiroba (Carapa guianensis Aubl., Meliaceae) oil and its limonoid-rich fraction

ETHNOPHARMACOLOGICAL RELEVANCE

From seeds of Carapa guianensis the Amazon native people extracts the andiroba oil, which is traditionally used as febrifuge, anti-malarial, insecticidal and repellant. The non-saponifiable fraction separated from the oil is rich in limonoids, which assigns its pharmacological effects.

MATERIALS AND METHODS

The andiroba oil and its limonoid-rich fraction were submitted to in vitro antiplasmodial bioassay using W(2) and Dd(2) strains of Plasmodium falciparum. The acute toxicity of andiroba oil was evaluated. The limonoid-rich fraction was subjected to fractionation and identified its major constituents.

RESULTS

Andiroba oil and its limonoid-rich fraction inhibited the growth of W(2) clone in 100%, between 24 and 72 h, at concentrations of 8.2 μg/mL and 3.1 μg/mL, respectively. Under the same conditions, the parasitaemia of Dd(2) clone provoked by the andiroba oil showed inhibition of 31% (IC(50) >82 μg/mL) with a time-dependent relationship of 24h and inhibition of 88% (IC(50) 8.4 μg/mL) after 72 h, while for the limonoid-rich fraction the inhibition of Dd(2) clone was 56% (IC(50) 2.8μg/mL) at 24h and 82% (IC(50) 0.4 μg/mL) after 72 h. Andiroba oil in acute toxicity test with a fixed dose (LD(50) >2000 mg/kg) was not toxic The limonoids identified in the oil were gedunin, 6α-acetoxygedunin, 7-deacetoxy-7-oxogedunin, 7-deacetylgedunin, 1,2-dihydro-3β-hydroxy-7-deacetoxy-7-oxogedunin and andirobin. Gedunin and derivatives has been reputed as anti-malarials.

CONCLUSION

The results support the traditional use of andiroba oil as antiplasmodial, which additionally proved not to be toxic in bioassays conducted with mice.

Synthesis and antimalarial activity of new haemanthamine-type derivatives

Thirty one derivatives were prepared from the natural alkaloids haemanthamine (1), haemanthidine (2) and 11-hydroxyvittatine (3). They were evaluated for their in vitro antimalarial activity against chloroquine-sensitive strains of Plasmodium falciparum and some structure-activity relationships were outlined. For haemanthamine derivatives having a methoxy group at C-3, the presence of a free hydroxyl group at C-11 is important for the activity. The double bond at C-1-C-2 plays also an important role to achieve good inhibitory activity. Compound 35 with two nicotinate groups at C-3 and at C-11 was the most active compound with a IC(50) = 0.8 ± 0.06 μM.

Biomimetic synthesis of the antimalarial flindersial alkaloids

A biomimetic strategy for the synthesis of the antimalarial flindersial alkaloids is described. Flinderoles A, B, and C, desmethylflinderole C, isoborreverine, and dimethylisoborreverine were all synthesized in three steps from tryptamine. The key step is an acid-promoted dimerization of the natural product borrerine. This approach is thought to mirror the biosynthesis of these compounds.