Introduction

Malaria is still a major public health problem, especially in tropical and sub-tropical regions. It is estimated that, in 2006, 3.3 billion people were at risk of contracting malaria, which causes nearly one million deaths each year, mostly of African children aged below 5 years who are susceptible to this disease (WHO 2008). As reported recently, 406 million Indians were at risk of stable Plasmodium falciparum transmission in 2007 with an uncertainty point estimate of 101.5 million clinical cases (95% CI 31.0–187.0 million cases; Hay et al. 2010). Annually, 600 million new infections occur worldwide, and at least one million of these infections are fatal. The other hard-hit tropical areas include East Asia, China and India. Experts estimated that as many as 40% of India’s malaria cases are caused by P. falciparum (Kumar 1999). Despite the existence of an array of available anti-malarial drugs, the control of this ancient infection is increasingly threatened by the emergence of drug-resistant strains of the malaria parasite, Plasmodium (Craft 2008). There is therefore an urgent need for new chemotherapeutic compounds which are easy to administer and store and which are of low cost. One possible source for such affordable treatments lies in the use of traditional herbal remedies.

The use of plants for the treatment of malaria extends over at least three continents, including several countries in Africa, America and Asia (Phillipson et al. 1987). However, scientific data on such medicinal plants are scanty. The recognition and validation of traditional medical practices and the search for plant-derived drugs could lead to new strategies in malaria control. Since many modern drugs such as quinine and artemisinin originate from plants, it is essential that other medicinal plants which have folklore reputation for anti-malarial properties are investigated in order to establish their safety and efficacy and to determine their potential as sources of new anti-malarial drugs (Gessler et al. 1994). Here we describe the anti-malarial potential of some medicinal plants collected from the Malaiyur and Javadhu hills of South India. The ethnomedical perspectives of these plants are given below.

The genus Chrysanthemum belongs to the Asteraceae family. Earlier studies have demonstrated the potential medicinal activities of Chrysanthemum indicum which exhibited good inhibitory activity against bacteria and viruses (Liu 1991), and was used in oriental traditional medicine to treat several infectious diseases such as pneumonia, colitis, stomatitis, carbuncle and fever. Its flowers were also commonly used as tea to treat some eye diseases in Chinese traditional medicine (Matsuda et al. 2002). The flower essential oils from C. indicum exhibited effective antimicrobial activity against 15 microorganisms including three yeasts and seven clinically isolated strains using disc paper and broth microdilution methods (Shunying et al. 2005), and the alcohol extract of Chrysanthemum morifolium was effective against P. falciparum (Zhao et al. 1996). The fresh flower head organic extracts of Chrysanthemum coronarium showed moderate antibacterial activity (Urzua and Mendoza 2003) and Vernonia amygdalina (Asteraceae) ethanol extracts exhibited good in vitro antiplasmodial activity against a chloroquine (CQ)-sensitive strain of P. falciparum (Tona et al. 2004). Pyrethrins, complex esters extracted from Chrysanthemum cinerariaefolium, had insecticidal effects (Cox 2000) and in vitro activity against herpes simplex virus (Stanberry et al. 1986).

Aristolochia indica is used in traditional medicine against intestinal worms and is useful in all types of poisonous bites and stings; the crude hexane, ethyl acetate and methanol extracts of A. indica were active for adulticidal, repellent and larvicidal activity against adult and early fourth-instar larvae of Culex gelidus and Culex quinquefasciatus (Kamaraj et al. 2010b); the in vitro test of petroleum ether and chloroform extracts of Aristolochia bracteolate extracts caused 100% inhibition of schizont maturation of P. falciparum (Ahmed et al. 2010) and the root hexane extract of Holostylis reniformis (Aristolochiaceae) was the most active in vivo against Plasmodium berghei and in vitro against a chloroquine-resistant strain of P. falciparum (de Andrade-Neto et al. 2007). Cassia species have been of medical interest due to their good therapeutic value in folk medicine; the acetone, chloroform, ethyl acetate, hexane, methanol and petroleum ether extracts of the leaf and flower of Cassia auriculata are known to be most active against the fourth-instar larvae of Anopheles subpictus and Culex tritaeniorhynchus (Kamaraj et al. 2009). The hexane, chloroform, ethyl acetate, acetone and methanol extracts of the leaf and flower of C. auriculata are known to have good activity against the fourth-instar larvae of the malaria vector Anopheles stephensi and the filariasis vector C. quinquefasciatus (Kamaraj et al. 2010a), and the crude ethanol extracts of Cassia alata and Cassia occidentalis have effective in vitro anti-malarial activity using the micro-dilution test against P. falciparum (Kayembe et al. 2010).

Solanum torvum (Solanaceae) is a plant intensively used worldwide in the traditional medicine system as poison antidote and for the treatment of fever, wounds, tooth decay, gastric ulceration, reproductive problems and arterial hypertension (Noumi et al. 1999; Noumi and Dibakto 2000; Ndebia et al. 2007). Rahuman et al. (2008) have reported the moderate larvicidal activity of the crude hexane, ethyl acetate, petroleum ether, acetone and methanol extracts of S. torvum against the early fourth-instar larvae of C. quinquefasciatus. The acetone, chloroform, ethyl acetate, hexane, methanol and petroleum ether extracts of the leaf and seed of S. torvum were active against the fourth-instar larvae of the malaria vector, A. subpictus, and the Japanese encephalitis vector, C. tritaeniorhynchus (Kamaraj et al. 2009). The complete egg hatching inhibition and larval development inhibition for leaf and seed ethyl acetate, acetone and methanol extracts of S. torvum indicated in vitro anthelmintic activity against the parasitic nematode of small ruminants Haemonchus contortus (Kamaraj et al. 2010d).

The dichloromethane and methanol crude extracts of Acanthaceae family plants, Justicia procumbens and Justicia balansae were screened for their antiplasmodial, cytotoxic, antioxidant and radical scavenging activities against P. falciparum (Charoenchai et al. 2010). Kamaraj et al. (2010b) reported the adulticidal, repellent and larvicidal activity of the crude hexane, ethyl acetate and methanol extracts of J. procumbens which were tested against adult and early fourth-instar larvae of C. gelidus and C. quinquefasciatus. Catharanthus roseus (Apocynaceae), commonly known as the tropical periwinkle, is a plant of medicinal importance due to its anti-cancer and antitumour activities which are attributed to the presence of the alkaloids vincristine and vinblastine in its leaves. The roots are used in various ways in traditional as well as folk medicine. The alkaloids present in the plants are also effective in the treatment of leukaemia, diabetes, hypertension, menorrhagia, etc. (Atal and Kapur 1977), and the leaf ethyl acetate, acetone and methanol extracts of C. roseus have demonstrated a complete inhibition against H. contortus (Kamaraj and Rahuman 2010).

Melia azedarach (Meliaceae), known as Chinaberry or Persian lilac, is a deciduous tree that is native to northwestern India and has long been recognized to elicit a variety of effects in insects, such as antifeedant, growth retardation, reduced fecundity, moulting disorders, anthelmintic, antiseptic, antipyretic and repellent activity (Schmid et al. 1998; Hammad et al. 2001; Gajmer et al. 2002; Banchio et al. 2003). M. azedarach extracts showed 100% parasitic inhibition against P. falciparum using an in vitro radioisotopic uptake technique (Ofulla et al. 1995); the leaves and seed aqueous and hydro-alcoholic extracts were evaluated for antihelmintic in vitro ovicidal and larvicidal activity against H. contortus (Kamaraj et al. 2010c), and the highest mortality was obtained with a combination of 20% ripe fruit extract with 10% ripe fruit oil against the head louse Pediculus humanus capitis (Carpinella et al. 2007). Gymnema sylvestre, a valuable medicinal plant belonging to Asclepiadaceae, is widely distributed in all parts of India and Africa. It has long been used in traditional medicine as a remedy for diabetes mellitus, stomach ache and diarrhoea. The methanol and dichloromethane extracts of the leaves of G. sylvestre showed good activity against P. falciparum (K1 chloroquine-resistant and NF54 chloroquine-sensitive strains) and low cytotoxic properties (Satdive et al. 2003; Irungu et al. 2007). The leaves of the shrub G. sylvestre contain a complex of pentacyclic triterpenes, gymnemines or gymnemic acids as natural deterrents of insects and herbivorous animals (Harborn 1985), and the leaf extracts of G. sylvestre showed active leishmanicidal activity against leishmanial parasites Leishmania major, Leishmania aethiopica and Leishmania tropica in in vitro conditions (Khanna et al. 2009).

Annona squamosa, commonly known as custard apple, is a native of West Indies and is cultivated throughout India mainly for its edible fruit. The major constituents identified in the members of the Annonaceae are, typically, acetogenins (Zeng et al. 1996; Liu et al. 1999; Bermejo et al. 2005) that exhibited active in vitro anti-malarial activities (Rakotomanga et al. 2004). de Mesquita et al. (2007) have reported the antiplasmodial activity of the hexane and ethanolic extracts of the leaves of Annona crassiflora, Duguetia furfuracea and Xylopia emarginata (Annonaceae) in terms of significant inhibition rates of P. falciparum (FcB1 strain) growth. The ethyl acetate and methanol extract of the leaves of A. squamosa showed moderate anti-malarial activity against CQ-sensitive (3D7) and CQ-resistant (Dd2 and INDO) strains of P. falciparum (Bagavan et al. 2011a); the methanolic extracts of the leaves of A. squamosa showed high antiplasmodial activity against the chloroquine-sensitive strain 3D7 and chloroquine resistant-strain Dd2 of P. falciparum (El Tahir et al. 1999); the hexane, chloroform, ethyl acetate, acetone and methanol extracts of the bark of A. squamosa have been reported as very active agents against the fourth-instar larvae of the malaria vector A. stephensi and the lymphatic filariasis vector C. quinquefasciatus (Kamaraj et al. 2010a), and the root extracts of Annona senegalensis showed good activity against the chloroquine-resistant strain of P. falciparum (Fall et al. 2003).

Dolichos biflorus (Fabaceae) is a branched, sub-erect and downing herb native to most parts of India. It is a fast-growing annual vine with trifoliate leaves and brown, flat, curved pods filled with seeds (Sastri 1969). In Ayurveda, the seed is used in the treatment of piles, pain, constipation, wounds, urinary calculi, cough, edoema, asthma, etc. The soup prepared from seeds is also beneficial in the treatment of enlarged liver and spleen. The seeds of D. biflorus have been reported to show antilithiatic (Garimella et al. 2001), antihepatotoxic (Laskar et al. 1998) and hypolipidemic activities (Muthu et al. 2005). The adulticidal, repellent and larvicidal activity of crude hexane, ethyl acetate and methanol extracts of D. biflorus has been found to be moderate against the adult and early fourth-instar larvae of C. gelidus and C. quinquefasciatus (Kamaraj et al. 2010b).

Ocimum gratissimum (Labiatae) is an erect small shrub, about 1 m in height, growing abundantly in India and Nigeria in deciduous forests and the savannah. It is commonly found around village huts and gardens and is cultivated in West Africa for its medicinal uses and as food flavouring (Iwu 1993). The leaf ethyl acetate extract of O. gratissimum is endowed with effective in vitro anti-malarial activity against NF54 (CQ sensitive) and K1 (CQ resistant) strains of P. falciparum (Abiodun et al. 2011); the hexane, chloroform, ethyl acetate, acetone and methanol extracts of Ocimum sanctum possess moderate activity against the chloroquine-sensitive (3D7) strain of P. falciparum (Bagavan et al. 2011b), and the aqueous extract of the leaves of O. gratissimum was screened for antidiarrhoeal effects (Offiah and Chikwendu 1999). Ocimum bacilicum and Ocimum suave extracts showed good in vitro anti-malarial activity against the chloroquine-resistant strain of P. falciparum (Nguta et al. 2010). The essential oil from O. gratissimum showed complete growth inhibition of P. berghei (Tchoumbougnang et al. 2005), and leaf decoctions of O. gratissimum were found to be moderately active against Plasmodium yoelii (Agomo et al. 1992).

Tribulus terrestris (Zygophyllaceae) is an annual plant distributed in the warm regions of India, Africa, Europe, America and Australia (Kostova et al. 2002). It is used in folk medicine as tonic, aphrodisiac, analgesic, astringent, stomachic, anti-hypertensive, diuretic, lithontriptic and urinary anti-infective (Ody 2000). Acetone extracts of the leaves and seeds from T. terrestris showed moderate activity against the third-instar larvae of Anopheles culicifacies, A. stephensi, C. quinquefasciatus and Aedes aegypti (Singh et al. 2008), and the alcoholic, methanolic, acetonitrilic, and hexanic extracts of T. terrestris showed effective in vitro antibacterial activity against Staphylococcus aureus, Escherichia coli, Proteus vulgaris, Pseudomonas aeruginosa and Klebsiella (Hussain et al. 2009).

In the present study, we describe the in vitro antiplasmodial activity of the methanolic extracts of the above mentioned 13 medicinal plant extracts tested against the chloroquine-sensitive 3D7 and chloroquine-resistant INDO strains of P. falciparum together with in vitro cytotoxicity against mammalian HeLa cell line.

Materials and methods

Collection of plant materials

The bark of A. squamosa, leaf and flower of A. indica, leaf of C. auriculata, C. roseus, C. indicum, D. biflorus, G. sylvestre, J. procumbens, M. azedarach, O. gratissimum, O. tenuiflorum, T. terrestris and leaf and seed of S. torvum were selected in the present study based upon their medicinal uses (Table 1) and biological activities (Table 2). The main target of this study was to investigate the antiplasmodial activities of 13 methanol plant extracts from Tamil Nadu, India. The plant materials were collected from Malaiyur Hills, Dharmapuri district (11°53′28″ N, 078°30′26″ E, altitude 959 m), and Javadhu Hills, Tiruvannamalai district (12°36′10′′N, 078°53'07′′E, altitude 705 m), Tamil Nadu, South India, in May 2010, and taxonomic identification was made by Dr. C. Hema, Department of Botany, Arignar Anna Government Arts College for Women, Walajapet, Vellore, India. The voucher specimens were numbered and kept in our research laboratory for further reference.

Table 1 Details of medicinal plants tested for antiplasmodial activity and their medicinal properties
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Table 2 Biological activity of selected medicinal plants
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Preparation of plant extracts

The plant materials (leaf, bark, flower and seed) were air-dried for 10–25 days in the shade at environmental temperatures (27–37°C daytime), and the leaf (600 g), bark (700 g), flower (450 g) and seed (500 g) were powdered mechanically using a commercial electrical stainless steel blender and extracted with methanol (3,600 mL, Qualigens) in a Soxhlet apparatus (boiling point range 60–80°C) for 8 h. The extract was concentrated under a reduced pressure of 22–26 mmHg at 45°C, and the residue obtained was stored at 4°C.

In vitro cultivation of P. falciparum

CQ-sensitive strain 3D7 and CQ-resistant strain INDO of P. falciparum were used in in vitro blood stage culture to test the anti-malarial efficacy of different plant extracts. The culture was maintained at the Malaria Research Laboratory, International Centre for Genetic Engineering and Biotechnology, New Delhi, India. P. falciparum culture was maintained according to the method described by Trager and Jensen (1976), with minor modifications. P. falciparum (3D7) cultures were maintained in fresh O+ve human erythrocytes suspended at 4% hematocrit in RPMI 1640 (Sigma) containing 0.2% sodium bicarbonate, 0.5% albumax, 45 μg/L hypoxanthine and 50 μg/l gentamycin and incubated at 37°C under a gas mixture of 5% O2, 5% CO2 and 90% N2. Every day, infected erythrocytes were transferred into a fresh complete medium to propagate the culture. For P. falciparum (INDO strain) in culture medium, albumax was replaced by 10% pooled human serum.

Drug dilutions

Stock solutions of CQ were prepared in water (milli-Q grade). Artemisinin, and test extracts were prepared in dimethyl sulfoxide (DMSO). All stocks were then diluted with culture medium to achieve the required concentrations (in all cases except CQ, the final solution contained 0.4% DMSO, which was found to be non-toxic to the parasite). Drugs and test compounds were then placed in 96-well flat-bottom tissue culture-grade plates.

Assay for antiplasmodial activity

The methanol extracts of experimental plants were evaluated for their anti-malarial activity against P. falciparum strains 3D7 and INDO. For drug screening, SYBR green I-based fluorescence assay was used as described (Smilkstein et al. 2004). Sorbitol-synchronized parasites were incubated under normal culture conditions at 2% hematocrit and 1% parasitemia in the absence or presence of increasing concentrations of plant extracts. CQ and artemisinin were used as positive controls, while 0.4% DMSO was used as the negative control. After 48 h of incubation, 100 μl of SYBR Green I solution {0.2 μl of 10,000 X SYBR Green I (Invitrogen)/mL} in lysis buffer [Tris (20 mM; pH 7.5), EDTA (5 mM), saponin (0.008%; w/v) and Triton X-100 (0.08%; v/v)] was added to each well and mixed gently twice with a multi-channel pipette and incubated in the dark at 37°C for 1 h. Fluorescence was measured with a Victor fluorescence multi-well plate reader (Perkin Elmer) with excitation and emission wavelength bands centred at 485 and 530 nm, respectively. The fluorescence counts were plotted against the drug concentration and the 50% inhibitory concentration (IC50) was determined by an analysis of dose–response curves. Results were validated microscopically by the examination of giemsa-stained smears of extract-treated parasite cultures (Bagavan et al. 2011a).

Cytotoxic activity on HeLa cells using MTT assay

The cytotoxic effects of extracts on host cells was assessed by functional assay as described (Mosmann 1983) using HeLa cells cultured in RPMI containing 10% fetal bovine serum, 0.21% sodium bicarbonate (Sigma) and 50 μg/mL gentamycin (complete medium). Briefly, cells (104 cells/200 μl/well) were seeded into 96-well flat-bottom tissue culture plates in complete medium. Drug solutions were added after 24 h of seeding and incubated for 48 h in a humidified atmosphere at 37°C and 5% CO2. DMSO (as a positive inhibitor) was added at 10%. Twenty microliters of a stock solution of MTT (5 mg/mL in 1X phosphate-buffered saline) was added to each well, gently mixed and incubated for another 4 h. After spinning the plate at 1,500 rpm for 5 min, the supernatant was removed and 100 μl of DMSO (stop agent) was added to each well. The formation of formazon was read on a microtiter plate reader (Versa max tunable multi-well plate reader) at 570 nm. The 50% cytotoxic concentration (TC50) of drug was determined by an analysis of dose–response curves. Therapeutic index was calculated as a ratio of TC50 HeLa/IC50 3D7.

Results

Thirteen plant methanol extracts were selected for antiplasmodial activity on the 3D7 CQ-sensitive and CQ-resistant INDO strains of P. falciparum. Promising antiplasmodial activity was found in the leaf extracts from four plants: A. indica , C. auriculata , C. indicum and D. biflorus at 50% inhibitory concentration (IC50 3D7, 10, 14, 20 and 20 μg/mL), respectively. Moderate activity was found in the bark methanol extracts of A. squamosa and leaf extracts of O. gratissimum, O. tenuiflorum, S. torvum, J. procumbens and T. terrestris (IC50 3D7, 30, 32, 31, 63 and 40 μg/mL, respectively), whereas the methanolic extracts of S. torvum (seed) and G. sylvestre (leaf) displayed poor activity (IC50 3D7, 89 and 100 μg/mL respectively). On the other hand, the methanolic extracts A. indica (flower) and C. roseus (leaf) were inactive up to 100 μg/mL. Plant extracts which showed IC50 below 20 μg/mL (A. indica, C. auriculata, C. indicum and D. biflorus) on further analysis against the CQ-resistant strain INDO of P. falciparum exhibited a similar potency with IC50 of 17, 24, 19 and 24 μg/mL, respectively (Table 3), giving resistance indices in the range of 0.9–1.7. As shown in Table 3, the leaf methanol extracts of A. indica and C. auriculata (Fig. 1) more active than the other plant extracts tested against P. falciparum. Furthermore, they were also non-toxic to mammalian cell line HeLa up to a concentration of 100 μg/mL. The growth inhibition profiles and the IC50 values of different plant extracts against 3D7 strain of P. falciparum are shown in Fig. 2a–d.

Table 3 Antimalarial activities and cytotoxicity of methanol extract of different plants against P. falciparum
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Fig. 1
figure 1

a A. indica L. and b C. auriculata L., the most active plants against P. falciparum

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Fig. 2
figure 2

Dose-dependent growth inhibition curve of P. falciparum (3D7) by methanolic extracts of a leaf of C. indicum (P1), A. indica (P2), C. auriculata (P3) and seed of S. torvum (P4), b leaf of J. procumbens (P5), flower of A. indica (P6), leaf of S. torvum (P7) and C. roseus (P8), c leaf of M. azedarach (P9), G. sylvestre (P10), bark of A. squamosa (P11) and leaf of D. biflorus (P12), (d) leaf of Ocimum tenuiflorum (P13), O. gratissimum (P14) and T. terrestris (P15). Keys in each panel indicate extract ID (normal text) and IC50 μg/ml (subscript)

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Discussion

Plants have contributed in a big way towards modern medicine and have always remained a primary subject for drug discovery. In our quest for novel drugs against malaria, we focused on the rich flora of South Indian medicinal plants. In a similar study, El Tahir et al. (1999) reported that the methanolic extract of A. squamosa leaves showed a high antiplasmodial activity with IC50 values of 2 and 30 μg/mL against the CQ-sensitive strain 3D7 and CQ-resistant strain Dd2 of P. falciparum, respectively. The leaf ethyl acetate extracts of A. squamosa displayed a moderate activity against P. falciparum (IC50 3D7, 33 μg/mL) (Bagavan et al. 2011a). The other plant extracts from the same family Annonaceae like Desmopsis panamensis, Pseudomalmea boyacana, Rollinia exsucca and Rollinia pittieri showed a good antiplasmodial activity (IC50 < 10 μg/mL) against the CQ-sensitive (F32) and CQ-resistant (W2) P. falciparum (Osorio et al. 2007). The whole plant petroleum ether/chloroform (1:1) extract of Aristolochia bracteolata produced a 100% inhibition of parasite growth at a concentration of ≤ 50 μg/ml (Ahmed et al. 2010). The extracts from H. reniformis (Aristolochiaceae) were found to be active both in vivo and in vitro. The hexane extract of the roots was the most active, causing a 67% reduction of parasitemia in vivo against P. berghei (de Andrade-Neto et al. 2007).

Traoré et al. (2008) have reported that the extracts of C. alata (leaf) showed a significant effect against P. falciparum (IC50 = 80.11 μg/mL) and P. berghei (ED50 = 112.78 mg/kg). The methanol extract of the root bark of Cassia singueana displayed an anti-malarial activity against P. berghei in an in vivo rodent model (ED50 of 847±30 mg/kg) (Adzu et al. 2003). Tona et al. (2001) found that the dichloromethane extract of C. occidentalis (200 mg/kg of body wt) showed a 60% growth suppression of P. berghei ANKA strain. Kayembe et al. (2010) isolated 20 quinones from C. alata, C. occidentalis, Garcinia kola and Ocimum basilicum and investigated the in vitro antiplasmodial activity against P. falciparum. Six of the 20 isolated quinones were found to be active with an IC50 value of below 1 μg/ml; the other quinones were bearing a moderate activity with IC50 values between 5 and 20 μg/ml.

In the present study, the leaf methanol extracts of C. roseus and G. sylvestre showed a weak antiplasmodial activity (IC50= >100 and 100 μg/mL, respectively) and the leaf methanol extract of C. indicum and D. biflorus showed a high antiplasmodial activity against the CQ-sensitive (3D7) strain of P. falciparum (IC50 = 20 μg/mL).

Plant extracts from Asteraceae family also have been reported as possessing anti-malarial properties. Four non-toxic diterpenes isolated from Aspilia pruliseta were found to possess a moderate activity (IC50, 14 to 23 μM) against the chloroquine-sensitive (D6) and chloroquine-resistant (W2) strains of P. falciparum (Sebisubi et al. 2010). The ethanolic extracts of Artemisia annua and A. absinthium showed an antiplasmodial activity against the multidrug resistant and sensitive strains of P. falciparum. A. annua and A. absinthium when examined in an in vivo rodent model displayed 94% and 83% reduction in parasitemia, respectively, at 200 mg/kg for 4 days. (Ramazani et al. 2010a; Malebo et al. 2009). Baccharis dracunculifolia extract showed a good antiplasmodial activity against P. falciparum with an IC50 value of 20 μg/mL (da Silva Filho et al. 2009).

Valadeau et al. (2009) have reported that the ethanolic extracts of Cestrum racemosum (Solanaceae), Hyptis lacustris (Lamiaceae) and Calea montana (Asteraceae) plant species displayed a good activity against the P. falciparum CQ-resistant strain (IC50 < 10 μg/ml). An in vitro screening of the dichloromethane extracts of Xanthium brasilicum (Asteraceae) and a bioactivity-guided fractionation resulted in the isolation of 8-epixanthatin 1beta, 5beta-epoxide compound having an activity against Trypanosoma brucei rhodesiense, Trypanosoma cruzi, Leishmania donovani and P. falciparum (IC50 0.09, 2.95, 0.16 and 1.71 μg/mL, respectively) (Nour et al. 2009). Samoylenko et al. (2009) demonstrated a strong in vitro anti-malarial activity in the dichloromethane and methanol extracts of Albizia schimperiana (Leguminosae). Thus, CQ-susceptible (D6) and -resistant (W2) strains of P. falciparum showed IC50 values ranging from 120 to 270 ng/mL.

Esmaeili et al. (2009) reported the in vitro antiplasmodial activity of the methanol extracts (IC50, 4.7–26.6 μg/ml) of Buxus hyrcana (aerial part), Erodium oxyrrhnchum (aerial part), Glycyrrhiza glabra (aerial part) and Ferula oopoda (roots) against the P. falciparum K1 and 3D7 strains. The chloroformic fraction of G. glabra at 10 mg/kg suppressed 86% parasitemia of an in vivo rodent model of P. berghei. The bark of Avicennia marina (Acanthaceae) extract exhibited an in vitro antiplasmodial activity against P. falciparum with a IC50 value of 49.63 μg ml−1 (Ravikumar et al. 2010). The in vitro anti-malarial activity of M. azedarach (Meliaceae) extracts against P. falciparum exhibited an IC50 of 300 μg/ml (Ofulla et al. 1995). Maneerat et al. (2008) found that the seeds of Chisocheton siamensis (Meliaceae), as anti-malarial, have IC50 values ranging from 2.06 to 6.31 μg/ml against P. falciparum. Lee et al. (2008) reported the in vitro anti-malarial activity of anthothecol (IC50, 1.4 μM) and limonoid (IC50, 0.17 μM) isolated from Khaya anthotheca (Meliaceae). The ethyl acetate extract of leaves of O. gratissimum displayed a promising antiplasmodial activity (IC50, 1.8–1.93 μg/mL) against the P. falciparum K1 strain and a low cytotoxicity (TC50 > 10 μg/mL) (Abiodun et al. 2011). The leaf hexane, chloroform, ethyl acetate, acetone and methanol extracts of O. sanctum showed a moderate antiplasmodial activity against the P. falciparum 3D7 strain (IC50 = 51, 30, 36, 36 and l45 μg/ml, respectively) (Bagavan et al. 2011b).

The five glycoalkaloids, namely, chaconine and solanine from Solanum tuberosum sprouts, solamargine and solasonine from Solanum nigrum fruit and tomatine from Lycopersicon esculentum fruit, were isolated and studied in vivo on the rodent model of P. yoelii 17XL. Chaconine, tomatine, solamargine, solasonine and solanine at a dose of 7.50 mg/kg showed 71%, 65%, 64%, 57% and 41%, respectively, suppression of parasitemia (Chen et al. 2010). The hydro-alcoholic extract of Solanum surattense was found to be active against P. falciparum K1 (CQ-resistant strain) and CY27 (CQ-sensitive strain) (IC50 = 50 and 40 μg/ml, respectively) (Ramazani et al. 2010b). Steroids isolated from Solanum nudum (Solanaceae) were found to be inhibitory to the asexual blood stages of the P. falciparum strain 7G8 with an IC50 between 20 and 87 μM (López et al. 2010). The alkaloids harmine, harmaline, vasicinone and deoxyvasicinone have been isolated from the seeds of Peganum harmala (Zygophyllaceae). Of these, harmine and harmaline showed a moderate in vitro antiplasmodial activity against P. falciparum (Astulla et al. 2008).

It would be of interest and necessary to pursue further pharmacological and phytochemical studies regarding these active plants. In our own future studies, we plan to focus on the evaluation of the in vivo activity of these plants against a mouse model of P. berghei, on the isolation and identification of active compounds through a bio-guided fractionation and on the activity and toxicity of these active isolated compounds. From these additional results, we will also consider the possibility of local valorization of active and non-toxic plants in standardized improved traditional medicine.

This preliminary study confirms the antiplasmodial activities of some plants already in medicinal usage, further justifying their use in South India as traditional medicine. The methanol extract of A. indica was more active against the chloroquine-resistant strain than the chloroquine-sensitive strain. This study indicated that A. indica, C. auriculata, C. indicum and D. biflorus have an important antiplasmodial activity and significant therapeutic index, making them good candidates for further pharmacological studies. The presence of promising anti-malarial activities in other members of the respective families to which these plants belong suggests the possibility of shared pharmacophoric metabolites with fine-tuned molecular architectures in individual species.