Antimalarials from nature

Antiplasmodial activity of flavonol quercetin and its analogues in Plasmodium falciparum: evidence from clinical isolates in Bangladesh and standardized parasite clones

Malaria is still a major threat in many parts of the world with resistance spreading to almost all classes of antimalarials. The limited arsenal of available antimalarial drugs emphasizes the urgent need for novel antimalarial compounds. Owing to the fact that novel leads from nature have traditionally played a pivotal role in the development of various classes of antimalarials, we investigated a set of eight naturally occurring dietary flavonoids and their analogues for their antiplasmodial activity on clinical field isolates in southeastern Bangladesh and culture-adapted chloroquine-sensitive and chloroquine-resistant parasite clones. Except for taxifolin, all the other flavonoids had 50% inhibitory concentrations below 14 μM, both in the field and laboratory-adapted parasites. Neither of the flavonoids showed any activity correlation with chloroquine. The quercetin analogue rutin (7.10 ± 10.32 μM) was the most active substance in field isolates as well as laboratory-adapted cultures (3.53 ± 13.34 μM in 3D7 and 10.38 ± 15.08 μM in K1), providing the first evidence of its activity against Plasmodium falciparum parasites. Thus, our results provide important evidence of the antimalarial activity of flavonoids in traditional use and thus warrant further investigation of these compounds as potential antiplasmodial agents.

Development predictive QSAR models for artemisinin analogues by various feature selection methods: a comparative study

Quantitative structure-activity relationship (QSAR) models were derived for 179 analogues of artemisinin, a potent antimalarial agent. The activities of these compounds were investigated by means of multiple linear regression (MLR). To select relevant descriptors, several methods including stepwise selection, successive projection algorithm and an ant colony optimization algorithm (called memorized_ACS) were employed. A wide variety of molecular descriptors belonging to various structural properties were calculated for each molecule. Two matrixes (D1 and D2) of molecular properties were built. The D1 matrix included the calculated descriptors and the D2 matrix contained the first to third orders of the calculated descriptors and the logarithm of absolute values of the calculated descriptors. For both data matrixes, significant QSAR models were obtained by the memorized_ACS algorithm. The reactive and PEOE (partial equalization of orbital electronegativity) descriptors represented the highest impact on the antimalarial activity. The PEOE descriptors belong to partial charge descriptors and the reactive descriptor is an indicator of the presence of the reactive groups in the molecule. The best MLR model has a training error of 0.71 log RA units (r (2 )= 0.81) and a prediction error of 0.48 log RA units (r (2) = 0.88).

Antimalarial natural products drug discovery in Panama

CONTEXT

Malaria is still a major public health problem. The biodiversity of the tropics is extremely rich and represents an invaluable source of novel bioactive molecules. For screening of this diversity more sensitive and economical in vitro methods are needed, Flora of Panama has been studied based on ethnomedical uses for discovering antimalarial compounds.

OBJECTIVE

This review aims to provide an overview of in vitro screening methodologies for antimalarial drug discovery and to present results of this effort in Panama during the last quarter century.

METHODS

A literature search in SciFinder and PubMed and original publications of Panamanian scientists was performed to gather all the information on antimalarial drug discovery from the Panamanian flora and in vitro screening methods.

RESULTS AND CONCLUSIONS

A variety of colorimetric, staining, fluorometric, and mass spectrometry and radioactivity-based methods have been provided. The advantages and limitations of these methods are also discussed. Plants used in ethnomedicine for symptoms of malaria by three native Panamanian groups of Amerindians, Kuna, Ngöbe Buglé and Teribes are provided. Seven most active plants with IC(50) values < 10 μg/mL were identified Talisia nervosa Radlk. (Sapindaceae), Topobea parasitica Aubl.(Melastomataceae), Monochaetum myrtoideum Naudin (Melastomataceae), Bourreria spathulata (Miers) Hemsl.(Boraginaceae), Polygonum acuminatum Kunth (Polygonaceae), Clematis campestris A. St.-Hil. (Ranunculaceae) and Terminalia triflora (Griseb.) Lillo (Combretaceae). Thirty bioactive compounds belonging to a variety of chemical classes such as spermine and isoquinoline alkaloids, glycosylflavones, phenylethanoid glycosides, ecdysteroids, quercetin arabinofuranosides, clerodane-type diterpenoids, sipandinolid, galloylquercetin derivatives, gallates, oleamide and mangiferin derivatives.

In vitro antiplasmodial activity and cytotoxicity of extracts and fractions of Vitex madiensis, medicinal plant of Gabon

Vitex madiensis Oliv. (Lamiaceae) is traditionally used to treat malaria symptoms in Haut-Ogooué, Gabon. Leaves and stem barks extracts were obtained using dichloromethane (CH(2)Cl(2)), ethyl acetate (EtOAc) and methanol (MeOH) as extraction solvents and fractionated on silica gel column. The in vitro antiplasmodial activity of CH(2)Cl(2), EtOAc and MeOH extracts and fractions was evaluated against the chloroquine-resistant FCB strain and field isolates of Plasmodium falciparum using the DELI test. The cytotoxicity of the extracts was tested on MRC-5 and THP1 cells using the tetrazolium salt MTT colorimetric assay, and the selectivity index (SI) of each extract was calculated. CH(2)Cl(2) extract, the EA1 fraction from EtOAc extract of stem barks and cyclohexane (L(cycl)), dichloromethane (L(DM)) and butanol (L(but)) fractions from MeOH/H(2)O extract of leaves exhibited the highest in vitro antiplasmodial activity on FCB strain and field isolates (IC(50) from 0.53 to 4.87 μg/ml) with high selectivity index (of 20.15-1800). These data support the use of V. madiensis in malaria treatment along with continued investigations within traditional medicines in the search of new antimalarial agents. The EA1, C(6)H(12) and CH(2)Cl(2) fractions could be selected for future investigation or/and for the treatment of malaria symptoms after standardization.

New Antifungal Cholestane and Aldehyde Derivatives from the Red Alga Laurencia papillosa

The chloroform/methanol extract of the red alga, Laurencia papillosa, collected from the Red Sea in Saudi Arabia, was found to contain two cholestane derivatives: 3α, 6α-dihydroxy-5β-cholestan-12-one (1) and the known, 6β-hydroxycholest-4-en-3-one (2), which was isolated separately in a pure form for the first time. In addition to these compounds, a new aldehyde derivative, ( E)-2-{( E) tridec-2-en-2-yl} heptadec-2-enal (3), was isolated. The structures of all compounds were established based on extensive spectroscopic (1D and 2D NMR, UV, IR) and mass spectrometric studies. All compounds, except 2, were tested for their antifungal activity. Significant activities were associated with 1 and 3 against Candida albicans, Aspergillus fumigatus, and A. flavus.

Furfuran lignans and a flavone from Artemisia gorgonum Webb and their in vitro activity against Plasmodium falciparum

The chemical composition of the aerial parts of the Cape Verdean endemic shrub Artemisia gorgonum Webb (Asteraceae) was careful investigated, which led to the isolation and identification of six known furfuran lignans: eudesmin (1), magnolin (2), epimagnolin A (3), aschantin (4), kobusin (5), sesamin (6) and a flavone: artemetin (7). Compounds 1-7 were evaluated in vitro for their cytotoxicity in a screening panel consisting of various mammalian tumor cell lines, for their antimalarial activity against chloroquine-resistant Plasmodium falciparum (FcB1 strain) and for their cytotoxicity against murine normal cells (CFU-GM). While no promising cytotoxicity against human tumor cells were noticed, marginal potency and selectivity was found for compounds 1-5 against murine colon 38. Besides, compounds 2-7 showed mild antiplasmodial activities, 6 and 7 being the most active compounds (IC(50) 3.37 and 3.50 μg/ml respectively) without noticeable toxicity on mammalian normal cells. This is the first report of antiplasmodial activity for furfuran lignans and the first isolation of 1-7 from Artemisia gorgonum.

Antiplasmodial and antiproliferative pseudoguaianolides of Athroisma proteiforme from the Madagascar Dry Forest

Investigation of extracts from the plant Athroisma proteiforme (Humbert) Mattf. (Asteraceae) for antimalarial activity led to the isolation of the five new sesquiterpene lactones 1-5 together with centaureidin (6). The structures of the new compounds were deduced from analyses of physical and spectroscopic data, and the absolute configuration of compound 1 was confirmed by an X-ray crystallographic study. Athrolides C (3) and D (4) both showed antiplasmodial activities with IC50 values of 6.6 (3) and 7.2 μM (4) against the HB3 strain and 5.5 (3) and 4.2 μM (4) against the Dd2 strain of the malarial parasite Plasmodium falciparum. The isolates 1-6 also showed antiproliferative activity against A2780 human ovarian cancer cells, with IC50 values ranging from 0.4 to 2.5 μM.

In vitro anti-plasmodial activity of Dicoma anomala subsp. gerrardii (Asteraceae): identification of its main active constituent, structure-activity relationship studies and gene expression profiling

Background

Anti-malarial drug resistance threatens to undermine efforts to eliminate this deadly disease. The resulting omnipresent requirement for drugs with novel modes of action prompted a national consortium initiative to discover new anti-plasmodial agents from South African medicinal plants. One of the plants selected for investigation was Dicoma anomala subsp. gerrardii, based on its ethnomedicinal profile.

Methods

Standard phytochemical analysis techniques, including solvent-solvent extraction, thin-layer- and column chromatography, were used to isolate the main active constituent of Dicoma anomala subsp. gerrardii. The crystallized pure compound was identified using nuclear magnetic resonance spectroscopy, mass spectrometry and X-ray crystallography. The compound was tested in vitro on Plasmodium falciparum cultures using the parasite lactate dehydrogenase (pLDH) assay and was found to have anti-malarial activity. To determine the functional groups responsible for the activity, a small collection of synthetic analogues was generated - the aim being to vary features proposed as likely to be related to the anti-malarial activity and to quantify the effect of the modifications in vitro using the pLDH assay. The effects of the pure compound on the P. falciparum transcriptome were subsequently investigated by treating ring-stage parasites (alongside untreated controls), followed by oligonucleotide microarray- and data analysis.

Results

The main active constituent was identified as dehydrobrachylaenolide, a eudesmanolide-type sesquiterpene lactone. The compound demonstrated an in vitro IC₅₀ of 1.865 μM against a chloroquine-sensitive strain (D10) of P. falciparum. Synthetic analogues of the compound confirmed an absolute requirement that the α-methylene lactone be present in the eudesmanolide before significant anti-malarial activity was observed. This feature is absent in the artemisinins and suggests a different mode of action. Microarray data analysis identified 572 unique genes that were differentially expressed as a result of the treatment and gene ontology analysis identified various biological processes and molecular functions that were significantly affected. Comparison of the dehydrobrachylaenolide treatment transcriptional dataset with a published artesunate (also a sesquiterpene lactone) dataset revealed little overlap. These results strengthen the notion that the isolated compound and the artemisinins have differentiated modes of action.

Conclusions

The novel mode of action of dehydrobrachylaenolide, detected during these studies, will play an ongoing role in advancing anti-plasmodial drug discovery efforts.

Opioid δ₁ receptor antagonist 7-benzylidenenaltrexone as an effective resistance reverser for chloroquine-resistant Plasmodium chabaudi

We evaluated antimalarial and/or chloroquine-resistance reversing effects of five opioid receptor antagonists. Although none of the evaluated compounds showed antimalarial effects, some of them, especially the δ(1) receptor antagonist, 7-benzylidenenaltrexone (BNTX) exhibited potent chloroquine-resistance reversing effects in Plasmodium chabaudi.

In Vitro Antiplasmodial Activity of Sesquiterpene Lactones from Ambrosia tenuifolia

The in vitro antiplasmodial activity of Ambrosia tenuifolia organic extract and its isolated sesquiterpene lactones, psilostachyin and peruvin, has been evaluated against Plasmodium falciparum F32 and W2 strains. The cytotoxicity of both compounds was determined on lymphoid cells, and their corresponding selectivity indexes (SIs) were calculated. Peruvin was the most active compound on F32 strain of P. falciparum with a 50% inhibitory concentration value (IC₅₀) of 0.3 μg/mL (1.1 μM) whereas psilostachyin showed activity on both strains (IC₅₀ = 0.6 (2.1 μM) and 1.8 μg/mL (6.4 μM)). Fifty percent cytotoxic concentration (CC₅₀) values (48 h) were 6.8 μg/mL (24.3 μM) and 10.0 μg/mL (37.9 μM) for psilostachyin and peruvin, respectively.

Annonaceae: bio-resource for tomorrow's drug discovery

One of the rich sources of lead compounds is the Angiosperms. Many of these lead compounds are useful medicines naturally, whereas others have been used as the basis for synthetic agents. These are potent and effective compounds, which have been obtained from plants, including anti-cancer (cytotoxic) agents, anti-malaria (anti-protozoal) agents, and anti-bacterial agents. Today, the number of plant families that have been extensively studied is relatively very few and the vast majorities have not been studied at all. The Annonaceae is the largest family in the order Magnoliales. It includes tropical trees, bushes, and climbers, which are often used as traditional remedies in Southeast Asia. Members of the Annonaceae have the particularity to elaborate a broad spectrum of natural products that have displayed anti-bacterial, anti-fungal, and anti-protozoal effects and have been used for the treatment of medical conditions, such as skin diseases, intestinal worms, inflammation of the eyes, HIV, and cancer. These special effects and the vast range of variation in potent compounds make the Annonaceae unique from other similar families in the Magnoliales and the Angiosperms in general. This paper attempts to summarize some important information and discusses a series of hypotheses about the effects of Annonaceae compounds.

Expanding the Antimalarial Drug Arsenal-Now, But How?

The number of available and effective antimalarial drugs is quickly dwindling. This is mainly because a number of drug resistance-associated mutations in malaria parasite genes, such as crt, mdr1, dhfr/dhps, and others, have led to widespread resistance to all known classes of antimalarial compounds. Unfortunately, malaria parasites have started to exhibit some level of resistance in Southeast Asia even to the most recently introduced class of drugs, artemisinins. While there is much need, the antimalarial drug development pipeline remains woefully thin, with little chemical diversity, and there is currently no alternative to the precious artemisinins. It is difficult to predict where the next generation of antimalarial drugs will come from; however, there are six major approaches: (i) re-optimizing the use of existing antimalarials by either replacement/rotation or combination approach; (ii) repurposing drugs that are currently used to treat other infections or diseases; (iii) chemically modifying existing antimalarial compounds; (iv) exploring natural sources; (v) large-scale screening of diverse chemical libraries; and (vi) through parasite genome-based ("targeted") discoveries. When any newly discovered effective antimalarial treatment is used by the populus, we must maintain constant vigilance for both parasite-specific and human-related factors that are likely to hamper its success. This article is neither comprehensive nor conclusive. Our purpose is to provide an overview of antimalarial drug resistance, associated parasite genetic factors (1. Introduction; 2. Emergence of artemisinin resistance in P. falciparum), and the antimalarial drug development pipeline (3. Overview of the global pipeline of antimalarial drugs), and highlight some examples of the aforementioned approaches to future antimalarial treatment. These approaches can be categorized into "short term" (4. Feasible options for now) and "long term" (5. Next generation of antimalarial treatment-Approaches and candidates). However, these two categories are interrelated, and the approaches in both should be implemented in parallel with focus on developing a successful, long-lasting antimalarial chemotherapy.

In vitro antiplasmodial activity and cytotoxicity of extracts of selected medicinal plants used by traditional healers of Western cameroon

Medicinal plants play a key role in malaria control in Africa, especially in remote areas where health facilities are limited. In order to assess their acclaimed potentials, eleven extracts were prepared from seven selected plants commonly used in Western Cameroon, and tested both for their antiplasmodial activity and cytotoxicity. The antiplasmodial activity was assessed using Lactate Dehydrogenase Assay (pLDH) and the cytotoxicity estimated on LLC-MK2 monkey kidney epithelial cells. Seven extracts from five different plants were significantly active, with very weak or no cytotoxicity. The Dacryodes edulis leaves showed the highest activity (IC₅₀ of 6.45 μg/mL on 3D7 and 8.2 μg/mL on DD2) followed by the leaves of Vernonia amygdalina (IC₅₀ of 8.72 and 11.27 μg/mL on 3D7 and DD2 resp.) and roots of V. amygdalina (IC₅₀ of 8.72 μg/mL on 3D7), Coula edulis leaves (IC₅₀ of 13.80 μg/mL and 5.79 μg/mL on 3D7 and DD2 resp.), Eucalyptus globulus leaves (IC₅₀ of 16.80 μg/mL and 26.45 μg/mL on 3D7 and DD2) and Cuviera longiflora stem bark (IC₅₀ of 20.24 μg/mL and 13.91 μg/mL on 3D7 and DD2). These findings justify the use of five of the seven plants in malaria treatment by traditional healers of Western Cameroon.

How can natural products serve as a viable source of lead compounds for the development of new/novel anti-malarials?

Malaria continues to be an enormous global health challenge, with millions of new infections and deaths reported annually. This is partly due to the development of resistance by the malaria parasite to the majority of established anti-malarial drugs, a situation that continues to hamper attempts at controlling the disease. This has spurred intensive drug discovery endeavours geared towards identifying novel, highly active anti-malarial drugs, and the identification of quality leads from natural sources would greatly augment these efforts. The current reality is that other than compounds that have their foundation in historic natural products, there are no other compounds in drug discovery as part of lead optimization projects and preclinical development or further that have originated from a natural product start-point in recent years. This paper briefly presents both classical as well as some more modern, but underutilized, approaches that have been applied outside the field of malaria, and which could be considered in enhancing the potential of natural products to provide or inspire the development of anti-malarial lead compounds.

Whole plant extracts versus single compounds for the treatment of malaria: synergy and positive interactions

Background

In traditional medicine whole plants or mixtures of plants are used rather than isolated compounds. There is evidence that crude plant extracts often have greater in vitro or/and in vivo antiplasmodial activity than isolated constituents at an equivalent dose. The aim of this paper is to review positive interactions between components of whole plant extracts, which may explain this.

Methods

Narrative review.

Results

There is evidence for several different types of positive interactions between different components of medicinal plants used in the treatment of malaria. Pharmacodynamic synergy has been demonstrated between the Cinchona alkaloids and between various plant extracts traditionally combined. Pharmacokinetic interactions occur, for example between constituents of Artemisia annua tea so that its artemisinin is more rapidly absorbed than the pure drug. Some plant extracts may have an immunomodulatory effect as well as a direct antiplasmodial effect. Several extracts contain multidrug resistance inhibitors, although none of these has been tested clinically in malaria. Some plant constituents are added mainly to attenuate the side-effects of others, for example ginger to prevent nausea.

Conclusions

More clinical research is needed on all types of interaction between plant constituents. This could include clinical trials of combinations of pure compounds (such as artemisinin + curcumin + piperine) and of combinations of herbal remedies (such as Artemisia annua leaves + Curcuma longa root + Piper nigum seeds). The former may enhance the activity of existing pharmaceutical preparations, and the latter may improve the effectiveness of existing herbal remedies for use in remote areas where modern drugs are unavailable.

A call for using natural compounds in the development of new antimalarial treatments - an introduction

Natural compounds, mostly from plants, have been the mainstay of traditional medicine for thousands of years. They have also been the source of lead compounds for modern medicine, but the extent of mining of natural compounds for such leads decreased during the second half of the 20^th century. The advantage of natural compounds for the development of drugs derives from their innate affinity for biological receptors. Natural compounds have provided the best anti-malarials known to date. Recent surveys have identified many extracts of various organisms (mostly plants) as having antiplasmodial activity. Huge libraries of fractionated natural compounds have been screened with impressive hit rates. Importantly, many cases are known where the crude biological extract is more efficient pharmacologically than the most active purified compound from this extract. This could be due to synergism with other compounds present in the extract, that as such have no pharmacological activity. Indeed, such compounds are best screened by cell-based assay where all potential targets in the cell are probed and possible synergies identified. Traditional medicine uses crude extracts. These have often been shown to provide many concoctions that deal better with the overall disease condition than with the causative agent itself. Traditional medicines are used by ~80 % of Africans as a first response to ailment. Many of the traditional medicines have demonstrable anti-plasmodial activities. It is suggested that rigorous evaluation of traditional medicines involving controlled clinical trials in parallel with agronomical development for more reproducible levels of active compounds could improve the availability of drugs at an acceptable cost and a source of income in malaria endemic countries.

Antiplasmodial Natural Products

Malaria is a human infectious disease that is caused by four species of Plasmodium. It is responsible for more than 1 million deaths per year. Natural products contain a great variety of chemical structures and have been screened for antiplasmodial activity as potential sources of new antimalarial drugs. This review highlights studies on natural products with antimalarial and antiplasmodial activity reported in the literature from January 2009 to November 2010. A total of 360 antiplasmodial natural products comprised of terpenes, including iridoids, sesquiterpenes, diterpenes, terpenoid benzoquinones, steroids, quassinoids, limonoids, curcubitacins, and lanostanes; flavonoids; alkaloids; peptides; phenylalkanoids; xanthones; naphthopyrones; polyketides, including halenaquinones, peroxides, polyacetylenes, and resorcylic acids; depsidones; benzophenones; macrolides; and miscellaneous compounds, including halogenated compounds and chromenes are listed in this review.

In vitro antiplasmodial activity and cytotoxicity of nine plants traditionally used in Gabon

AIM OF THE STUDY

As part of a project to identify new compounds active on malarial parasites, we tested the in vitro antiplasmodial activity of nine plants traditionally used to treat malaria symptoms in Haut-Ogooué Province, South-East Gabon.

MATERIALS AND METHODS

Dichloromethane and methanolic extracts of each plant were tested for their antiplasmodial activity on two chloroquine-resistant strains of Plasmodium falciparum (FCB and W2), based on lactate dehydrogenase activity. Cytotoxicity was assessed with the MTT test on MRC-5 human diploid embryonic lung cells.

RESULTS

The methanolic extract of Staudtia gabonensis and the dichloromethane extract of Adhatoda latibracteata showed high antiplasmodial activity (IC₅₀<1 μg/ml) and low cytotoxicity, with selectivity indexes of about 58.25 and 16.43, respectively. The methanolic extract of Monodora myristica and the dichloromethane extract of Afromomum giganteum also showed promising activity (1<IC₅₀<10 μg/ml) and low cytotoxicity, with selectivity indexes about 15.70 and 12.48, respectively. Dichloromethane extracts of Monodora myristica and Leonotis Africana showed moderate activity (10<IC₅₀<40 μg/ml), with selectivity indexes about 6.07 and 28.89, respectively. Both extracts of Culcasia lancifolia had IC₅₀ values of 10-40 μg/ml but high cytotoxicity (selectivity indexes <2.77). The methanolic extract of Dorstenia klaineana had moderate antiplasmodial activity (IC₅₀ around 17 μg/ml) but strong cytotoxicity (0.43 μg/ml), giving a selectivity index of about 0.03.

CONCLUSIONS

Most extracts of nine selected plants traditionally used to treat malaria in Gabon had interesting antiplasmodial activity in vitro. This supports continued investigations of traditional medicines in the search for new antimalarial agents. The compounds responsible for the observed antiplasmodial effects are under investigation.

Antiplasmodial activity of aporphine alkaloids and sesquiterpene lactones from Liriodendron tulipifera L

AIM OF THE STUDY

The objective of this study was to isolate and characterize the active constituents of the traditionally used antimalarial plant Liriodendron tulipifera by antiplasmodial-assay guided fractionation.

MATERIALS AND METHODS

Bark and leaves were extracted with solvents of increasing polarity. Fractions were generated using flash chromatography, counter current chromatography and preparative HPLC and subjected to in vitro antiplasmodial and cytotoxicity assays. Active fractions were subjected to further fractionation until pure compounds were isolated, for which the IC(50) values were calculated.

RESULTS AND DISCUSSION

Six known aporphine alkaloids, asimilobine (1), norushinsunine (2), norglaucine (3), liriodenine (4), anonaine (5) and oxoglaucine (6) were found to be responsible for the antiplasmodial activity of the bark. Leaves yielded two known sesquiterpene lactones, peroxyferolide (7) and lipiferolide (8) with antiplasmodial activity. The antiplasmodial activity of (2) (IC(50)=29.6 μg/mL), (3) (IC(50)=22.0 μg/mL), (6) (IC(50)=9.1 μg/mL), (7) (IC(50)=6.2 μg/mL) and (8) (IC(50)=1.8 μg/mL) are reported for the first time.

CONCLUSION

This work supports the historical use of Liriodendron tulipifera as an antimalarial remedy of the United States and characterizes its antiplasmodial constituents.

Incorporation of basic side chains into cryptolepine scaffold: structure-antimalarial activity relationships and mechanistic studies

The synthesis of cryptolepine derivatives containing basic side-chains at the C-11 position and their evaluations for antiplasmodial and cytotoxicity properties are reported. Propyl, butyl, and cycloalkyl diamine side chains significantly increased activity against chloroquine-resistant Plasmodium falciparum strains while reducing cytotoxicity when compared with the parent compound. Localization studies inside parasite blood stages by fluorescence microscopy showed that these derivatives accumulate inside the nucleus, indicating that the incorporation of a basic side chain is not sufficient enough to promote selective accumulation in the acidic digestive vacuole of the parasite. Most of the compounds within this series showed the ability to bind to a double-stranded DNA duplex as well to monomeric hematin, suggesting that these are possible targets associated with the observed antimalarial activity. Overall, these novel cryptolepine analogues with substantially improved antiplasmodial activity and selectivity index provide a promising starting point for development of potent and highly selective agents against drug-resistant malaria parasites.

Antiplasmodial volatile extracts from Cleistopholis patens Engler & Diels and Uvariastrum pierreanum Engl. (Engl. & Diels) (Annonaceae) growing in Cameroon

In a search for alternative treatment for malaria, plant-derived essential oils extracted from the stem barks and leaves of Cleistopholis patens and Uvariastrum pierreanum (Annonaceae) were evaluated in vitro for antiplasmodial activity against the W2 strain of Plasmodium falciparum. The oils were obtained from 500 g each of stem barks and leaves, respectively, by hydrodistillation, using a Clevenger-type apparatus with the following yields: 0.23% and 0.19% for C. patens and 0.1% and 0.3% for U. pierreanum (w/w relative to dried material weight). Analysis of 10% (v/v) oil in hexane by gas chromatography and mass spectrometry identified only terpenoids in the oils, with over 81% sesquiterpene hydrocarbons in C. patens extracts and U. pierreanum stem bark oil, while the leaf oil from the latter species was found to contain a majority of monoterpenes. For C. patens, the major components were α-copaene, δ-cadinene, and germacrene D for the stem bark oil and β-caryophyllene, germacrene D, and germacrene B for the leaf oil. The stem bark oil of U. pierreanum was found to contain mainly β-bisabolene and α-bisabolol, while α- and β-pinenes were more abundant in the leaf extract. Concentrations of oils obtained by diluting 1-mg/mL stock solutions were tested against P. falciparum in culture. The oils were active, with IC(50) values of 9.19 and 15.19 μg/mL for the stem bark and leaf oils, respectively, of C. patens and 6.08 and 13.96 μg/mL, respectively, for those from U. pierreanum. These results indicate that essential oils may offer a promising alternative for the development of new antimalarials.

Aplidiopsamine A, an antiplasmodial alkaloid from the temperate Australian ascidian, Aplidiopsis confluata

A polyaromatic alkaloid, aplidiopsamine A was isolated from the temperate Australian ascidian, Aplidiopsis confluata, and its structure was determined from interpretation of mass, 1D and 2D NMR spectra. Aplidiopsamine A is the first alkaloid to possess the tricyclic aromatic substructure 3H-pyrrolo[2,3-c]quinoline conjugated to an adenine. Aplidiopsamine A exhibited significant inhibition of growth of chloroquine resistant and sensitive strains of the malaria parasite, Plasmodium falciparum, and minimal toxicity toward human cells.

Isoquinolines from the roots of Thalictrum flavum L. and their evaluation as antiparasitic compounds

Alkaloids from Thalictrum flavum L. (Ranuculaceae) growing in the Loire valley (France) were isolated and evaluated for their antiplasmodial and leishmanicidal activities. Berberine was identified as a major component but its analogue, pseudoberberine, was isolated for the first time from this plant. As far as bisbenzylisoquinolines are concerned, thalfoetidine was also isolated and, besides, its nor- derivative, northalfoetidine, was identified as a new compound. Previously isolated alkaloids from Thalictrum species such as northalidasine, northalrugosidine, thaligosidine, thalicberine, thaliglucinone, preocoteine, O-methylcassythine and armepavine were newly described in the roots of T. flavum. Tertiary isoquinolines, and particularly bisbenzylisoquinolines, were found to be leishmanicidal against L. major. Thalfoetidine appeared as the most potent but its new nor- derivative northalfoetidine, as well as northalidasine, were of particular interest due to the fact that their potential leishmanicidal activity was not associated to a strong cytotoxicity.

Unusual antimalarial meroditerpenes from tropical red macroalgae

Three antimalarial meroditerpenes have been isolated from two Fijian red macroalgae. The absolute stereochemistry of callophycolide A (1), a unique macrolide from Callophycus serratus, was determined using a combination of Mosher's ester analysis, circular dichroism analysis with a dimolybdenum tetraacetate complex, and conformational analysis using NOEs. In addition, two known tocopherols, β-tocopherylhydroquinone (4) and δ-tocopherylhydroquinone (5), were isolated from Amphiroa crassa. By oxidizing 5 to the corresponding δ-tocopherylquinone (6), antimalarial activity against the human malaria parasite Plasmodium falciparum was increased by more than 20-fold.

Evaluation of 13 selected medicinal plants from Burkina Faso for their antiplasmodial properties

AIM OF THE STUDY

The aim of this study was to evaluate the antiplasmodial properties of 13 plants used against malaria in traditional medicine in Burkina Faso.

MATERIALS AND METHODS

In vitro antiplasmodial activity of dichloromethane, methanol and aqueous crude extracts obtained from vegetal samples collected in Burkina Faso was first evaluated on the Plasmodium falciparum 3D7 chloroquine-sensitive strain using a colorimetric method.

RESULTS

Thirteen extracts obtained from 8 different species were found to exhibit antiplasmodial activity (IC(50)<50 microg/ml). Five species demonstrated a moderate activity (15 microg/ml<IC(50)<50 microg/ml): Boswellia dalzielii (leaves), Waltheria indica (roots and aerial parts), Bergia suffruticosa (whole plant), Vitellaria paradoxa (bark) and Jatropha gossypiifolia (leaves). The best results were obtained with extracts from the Dicoma tomentosa whole plant, from Psorospermum senegalense leaves and from Gardenia sokotensis leaves. These extracts found to display promising antiplasmodial activity, with IC(50) values ranging from 7.0 to 14.0 microg/ml. The most active plant extracts were then tested for in vitro activity on the Plasmodium falciparum W2 chloroquine-resistant strain and also for in vitro cytotoxicity on normal human fibroblasts (WI-38) in order to determine the selectivity index.

CONCLUSIONS

Dicoma tomentosa (Asteraceae) and Psorospermum senegalense (Clusiaceae) appeared to be the best candidates for further investigation of their antiplasmodial properties, reported for the first time by this study.

Antimalarial bromotyrosine derivatives from the Australian marine sponge Hyattella sp

A drug discovery program aimed at identifying new antimalarial leads from a prefractionated natural product library has resulted in the identification of a new bromotyrosine alkaloid, psammaplysin G (1), along with the previously isolated compound, psammaplysin F (2). When tested against two different strains of the parasite Plasmodium falciparum (Dd2 and 3D7), 2 displayed IC(50) values of 1.4 and 0.87 microM, respectively, while 1 showed 98% inhibition at 40 microM against the chloroquine-resistant (Dd2) strain of P. falciparum.

Antimalarial compounds isolated from plants used in traditional medicine

Objectives

This review covers the compounds with antiplasmodial activity isolated from plants published from 2005 to the end of 2008, organized according to their phytochemical classes. Details are given for substances with IC50 values ≤ 11 μM.

Key findings

Malaria is a major parasitic disease in many tropical and subtropical regions and is responsible for more than 1 million deaths each year in Africa. The rapid spread of resistance encourages the search for new active compounds. Nature and particularly plants used in traditional medicine are a potential source of new antimalarial drugs as they contain molecules with a great variety of structures and pharmacological activities.

Summary

A large number of antimalarial compounds with a wide variety of structures have been isolated from plants and can play a role in the development of new antimalarial drugs. Ethnopharmacological approaches appear to be a promising way to find plant metabolites that could be used as templates for designing new derivatives with improved properties.

Squalene: A natural triterpene for use in disease management and therapy

Squalene is a natural lipid belonging to the terpenoid family and a precursor of cholesterol biosynthesis. It is synthesized in humans and also in a wide array of organisms and substances, from sharks to olives and even bran, among others. Because of its significant dietary benefits, biocompatibility, inertness, and other advantageous properties, squalene is extensively used as an excipient in pharmaceutical formulations for disease management and therapy. In addition, squalene acts as a protective agent and has been shown to decrease chemotherapy-induced side-effects. Moreover, squalene alone exhibits chemopreventive activity. Although it is a weak inhibitor of tumor cell proliferation, it contributes either directly or indirectly to the treatment of cancer due to its potentiation effect. In addition, squalene enhances the immune response to various associated antigens, and it is therefore being investigated for vaccine delivery applications. Since this triterpene is well absorbed orally, it has been used to improve the oral delivery of therapeutic molecules. All of these qualities have rendered squalene a potentially interesting excipient for pharmaceutical applications, especially for the delivery of vaccines, drugs, genes, and other biological substances. This paper is the first review of its kind and offers greater insight into squalene's direct or indirect contribution to disease management and therapy.

Antimalarial activity of ultra-short peptides

Ultra-short peptides 1-9 were designed and synthesized with phenylalanine, ornithine and proline amino acid residues and their effect on antimalarial activity was analyzed. On the basis of the IC₅₀ data for these compounds, the effects of nature, polarity, and amino acid sequence on Plasmodium berghei schizont cultures were analyzed too. Tetrapeptides Phe-Orn-Phe-Orn (4) and Lys-Phe-Phe-Orn (5) showed a very important activity with IC₅₀ values of 3.31 and 2.57 μM, respectively. These two tetrapeptides are candidates for subsequent in vivo assays and SARS investigations.

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 IC(50) around 1.8 muM. This product was moderately active in vitro against a FcB1 P. falciparum strain (IC(50) = 12 muM). 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.

A Protocol for HPLC-based Activity Profiling for Natural Products with Activities against Tropical Parasites

HPLC based activity profiling is an effective strategy to accelerate the discovery of new hits and leads from nature. It conveniently combines the superior separation power of HPLC micro scale compound separation with miniaturized biological screening methods, and on-line and off-line spectroscopy (PDA, MSn, HR-MS, NMR) for structure elucidation. We here describe a protocol for the discovery of natural products with antimalarial, antileishmanial and antitrypanosomal activity, from extracts libraries in 96-well format. Analytical gradient HPLC on a 3 × 150 mm column of 350 μg of extract, and collection of one-minute fractions into 96 deep-well microtiter plates, parallel evaporation of the micro-fractions, and a suitable dilution scheme permitted parallel activity profiling against three parasites from a single HPLC injection. The protocol was validated with extracts and positive controls such as Artemisia annua.

Isoflavonoids from Erythrina poeppigiana: evaluation of their binding affinity for the estrogen receptor

Five new isoflavones, named 5,4'-dihydroxy-7-methoxy-3'-(3-methylbuten-2-yl)isoflavone (1), 5,2',4'-trihydroxy-7-methoxy-5'-(3-methylbuten-2-yl)isoflavone (2), 5,4'-dihydroxy-7-methoxy-3'-(3-methyl-2-hydroxybuten-3-yl)isoflavone (3), 3'-formyl-5,4'-dihydroxy-7-methoxyisoflavone (4), and 5-hydroxy-3''-hydroxy-2'',2''-dimethyldihydropyrano[5'',6'':3',4']isoflavone (5), as well as six known compounds, wighteone (6), 3'-isoprenylgenistein (7), isolupabigenin (8), alpinumisoflavone (9), erypoegin D (10), and crystacarpin (11), were isolated from Erythrina poeppigiana. The structures of the isolated compounds were elucidated on the basis of chemical and spectroscopic analysis. The affinity of these compounds for the estrogen receptors ERalpha and ERbeta was evaluated using a receptor binding assay. While isoprenyl and dimethylpyrano substituents in ring A reduced the affinity of binding to ERbeta ca. 100-fold compared to genistein, the isoprenyl substituent in ring B was better accommodated, allowing 7 to bind with ca. 10-fold lower affinity than genistein.

Synthesis and antimicrobial activities of pyrano[3,2-c]quinoline, pyrimido [5′,4′:5,6]pyrano[3,2-c]quinoline and [1,2,4]triazolo[2″,3″:1′,6′]pyrimido [5′,4′:5,6]pyrano[3,2-c]quinoline derivatives

The synthesis of novel 7-(4-chlorophenyl)-6-hydroxy-3-methoxy-10-methyl-8,9-dihydro-7 H-pyrimido[5′, 4′: 5, 6]pyrano-[3,2- c]quinoline and [1,2,4]triazolo[2″,3″: 1′, 6′]pyrimido[5′, 4′: 5,6]pyrano[3,2- c]quinoline derivatives has been reported. The key intermediate 2-amino-4-(4-chlorophenyl)-5-hydroxy-8-methoxy-4H-pyrano[3, 2- c]-quinoline-3-carbonitrile was obtained by treating 4-hydroxy-7-methoxyquinolin-2(1 H)-one with various substituted α-cyanocinnamonitrile in ethanolic piperidine solution. Antimicrobial activity was shown for most of the synthesised compounds.