Plasmodium falciparum: effects of phaeanthine, a naturally-occurring bisbenzylisoquinoline alkaloid, on chloroquine-resistant and -sensitive parasites in vitro, and its influence on chloroquine activity

In vitro and in vivo antimalarial activity and cytotoxicity of extracts and fractions from the leaves, root-bark and stem-bark of Triclisia gilletii

ETHNOPHARMACOLOGICAL RELEVANCE

To evaluate the in vitro antiplasmodial activity and cytotoxicity, and the in vivo activity of extracts and fractions from the leaves, root-bark and stem-bark of Triclisia gilletii (De Wild) Staner (Menispermaceae), used in traditional medicine against malaria.

MATERIALS AND METHODS

The aqueous and 80% MeOH extracts, and a series of fractions and subfractions from the leaves, stem and root-bark of Triclisia gilletii were tested in vitro for their antiplasmodial activity against a Congolese-sensitive strain of Plasmodium falciparum, against the chloroquine and pyrimethamine-resistant K1 strain of Plasmodium falciparum, for cytotoxicity against MRC-5 cells, and in vivo in mice infected with Plasmodium berghei berghei.

RESULTS

Many samples from the three plant parts exhibited pronounced activity against the Congolese chloroquine-sensitive strain of Plasmodium falciparum with some IC50 values <0.02 µg/ml, and against the K1 strain, with some IC50 <0.25; the selectivity was higher against the Congolese strain. At oral doses of 200 and 400mg/kg body weight in infected mice, the aqueous, 80% methanol and total alkaloid extracts from the three plant parts produced more than 65% and 75% chemosuppression, respectively. The antiplasmodial activity of these three plant parts of Triclisia gilletii can at least in part be attributed to bisbenzylisoquinoline alkaloids, and supports its use for the treatment of uncomplicated malaria in traditional medicine.

Antiplasmodial and antitrypanosomal activity of Triclisia sacleuxii (Pierre) Diels

The antiplasmodial and antitrypanosomal activities of Triclisia sacleuxii (Pierre) Diels were investigated on three Plasmodium falciparum strains [FcB1, 3D7 (chloroquine-sensitive) and W2 (chloroquine-resistant) strains] and on Trypanosoma brucei Tbsf 221. Roots, stems and leaves ethanolic extracts as well as crude tertiary and quaternary alkaloids fractions were considered. Whereas the ethanolic extracts and quaternary crude alkaloids fractions exhibited no significant activity, the roots and stems tertiary alkaloid fractions revealed interesting growth inhibition against the Plasmodium FcB1 and Trypanosoma Tbsf 221 strains. The IC(50) were 1.04 and 0.89 microg/ml for roots, 2.50 and 0.91 microg/ml for stems. The leaves tertiary alkaloids fraction also showed a promising antitrypanosomal activity (IC(50): 1.85 microg/ml). Phytochemical analysis of the roots tertiary alkaloids fraction yielded four major compounds, phaeanthine, N-methylapateline, 1,2-dehydroapateline and 1,2-dehydrotelobine, which were identified on the basis of their spectroscopic data. The four compounds displayed (in vitro) antitrypanosomal activity with IC(50) of 2.68, 1.19, 1.06 and 1.11 microM, respectively. They also demonstrated antiplasmodial activity on Plasmodium falciparum 3D7, with IC(50) of 1.72, 0.93, 1.39 and 12.4 microM respectively and on the chloroquine-resistant W2 with IC(50) of 0.35, 1.10, 1.63 and 1.52 microM.

Antimalarial drug synergism and antagonism: mechanistic and clinical significance

Interactions between antimicrobial agents provide clues as to their mechanisms of action and influence the combinations chosen for therapy of infectious diseases. In the treatment of malaria, combinations of drugs, in many cases acting synergistically, are increasingly important in view of the frequency of resistance to single agents. The study of antimalarial drug interactions is therefore of great significance to both treatment and research. It is therefore worrying that the analysis of drug-interaction data is often inadequate, leading in some cases to dubious conclusions about synergism or antagonism. Furthermore, making mechanistic deductions from drug-interaction data is not straightforward and of the many reported instances of antimalarial synergism or antagonism, few have been fully explained biochemically. This review discusses recent findings on antimalarial drug interactions and some pitfalls in their analysis and interpretation. The conclusions are likely to have relevance to other antimicrobial agents.

A search for natural bioactive compounds in Bolivia through a multidisciplinary approach. Part V. Evaluation of the antimalarial activity of plants used by the Tacana Indians

One hundred and twenty-five extracts of 122 different plant species traditionally used by the Tacana, a native community living in lowland forest at the base of the last foothills of the Cordillera Oriental of the Bolivian Andes, were screened for antimalarial activity in vitro on Plasmodium falciparum chloroquine resistant (D2) and sensitive strains (F32), and were evaluated in vivo on rodent malaria Plasmodium berghei. Five ethanolic stembark extracts showed marked activity either in vitro or in vivo, and only one of them, Bowdichia virgilioides being traditionally used against malaria, was active in vitro (IC50=1 microg/ml on both strains) and in vivo (51% at 100 mg/kg). Other active extracts were from Caesalpinia pluviosa bark displaying activity in vitro against chloroquine resistant strain (IC50 8.3 microg/ml), traditionally used against dysentery; two Lauraceae bark extracts, Nectandra aff. hihua and Licaria canella respectively used for construction purposes and against stomach ache, both displaying activity in vitro against P. falciparum sensible and resistant strains (IC50 around 4 microg/ml); finally, the bark of a strongly aromatic Burseraceae, Protium glabrescens exuding an anti-inflammatory and analgesic resin, was active in vivo only (61% at 100 mg/kg). Results are discussed in relation with Tacana traditional medicine.

In vitro antimalarial activity of limonoids from Khaya grandifoliola C.D.C. (Meliaceae)

The crude extract from the bark and seeds of Khaya grandifoliola was active in vitro against Plasmodium falciparum with an IC50 value of 13.23 microg/ml. The extract was purified to obtain seven limonoids--methylangolensate (1), 6-methylhydroxyangolensate (2), gedunin (3), 7-deacetylkhivorin (5), 1-deacetylkhivorin (6), swietenolide (7), 6-acetylswietenolide (8)--and one flavonoid, catechin (4). Five limonoids (1, 3, 5, 6, 8) were active with IC50 values between 1.25 and 9.63 microg/ml. Catechin was practically devoid of activity. The most active limonoid, gedunin, exhibited an additive effect when combined with chloroquine.

Medicinal plants and the control of protozoal disease, with particular reference to malaria

Malaria and other protozoal diseases continue to pose serious health problems world-wide. Resistance of the malaria parasites, Plasmodium spp., to drugs such as chloroquine (and, more lately, quinine) occurs with increasing frequency and underlies the necessity to develop new agents for malaria chemotherapy; in the case of diseases caused by species of Leishmania and Trypanosoma there has always been a marked paucity of effective drugs, particularly those with a wide safety margin and minimal or no undesirable side effects. Novel drugs, are required to help alleviate morbidity and mortality and to contribute to the world-wide control of these diseases, in part by helping to reduce the reservoirs of infection. Reliance upon plants for the treatment of disease is high in the developing world and such plants offer a source of new molecules. Research centered upon Plasmodium has produced a number of findings which now prompt the formulation of important questions which may influence and focus the direction of phytotherapy research in the future.

Bisbenzylisoquinolines as modulators of chloroquine resistance in Plasmodium falciparum and multidrug resistance in tumor cells

Ten naturally occurring bisbenzylisoquinolines (BBIQ) and two dihydro derivatives belonging to five BBIQ subgroups were evaluated in vitro for their ability to inhibit Plasmodium falciparum growth and, in drug combination, to reverse the resistance to chloroquine of strain FcB1. The same alkaloids were also assessed in vitro for their potentiating activity against vinblastine with the multidrug-resistant clone CCRF-CEM/VLB, established from lymphoblastic acute leukemia. Three of the BBIQ tested had 50% inhibitory concentrations of less than 1 microM. The most potent antimalarial agent was cocsoline (50% inhibitory concentration, 0.22 microM). Regarding the chloroquine-potentiating effect, fangchinoline exhibited the highest biological activity whereas the remaining compounds displayed either antagonistic or slight synergistic effects. Against the multidrug-resistant cancer cell line, fangchinoline was also by far the most active compound. Although there were clear differences between the activities of tested alkaloids, no relevant structure-activity relationship could be established. Nevertheless, fangchinoline appears to be a new biochemical tool able to help in the comprehension of the mechanism of both chloroquine resistance in P. falciparum and multidrug resistance in tumor cells.

Simple, fast, and accurate fluorometric method to determine drug susceptibility of Plasmodium falciparum in 24-well suspension cultures

An in vitro test which quantifies drug inhibition of Plasmodium falciparum replication by measuring the fluorescence intensity of Hoechst 33258 dye bound to DNA is described. The procedure does not require expensive reagents or equipment and can be completed in less than 10 min. The assay was highly accurate and sensitive: cultures with as few as 0.4% schizont-infected erythrocytes could reliably be analyzed. The method was not biased by the actual parasite stage used; i.e., the amount of fluorescence detected in a sample of a culture of mature schizonts equaled the amount detected with the ring form culture derived from these schizonts. Even the presence of large proportions of free merozoites, which are easily neglected in microscopic estimates, did not bias the results. Furthermore, measurement of the chloroquine susceptibility of the multidrug-resistant K1 strain and the chloroquine-susceptible NF54 strain showed that the method is most suitable for quantifying the drug resistance of P. falciparum.

Isobolographic analysis of interactions: an update on applications and utility

Isobolographic analysis provides a fundamental basis for assessing whether biological responses induced by mixtures of agents are greater, equal or smaller than would have been expected on the basis of the individual activities of the component agents and the concept of dose additivity. Limited in its direct application to binary mixtures, isobolographic analysis provides a conceptual framework and an unambiguous terminology, as well as an algebraic paradigm for the analysis of the interaction of ternary and higher order mixtures. A library of examples generously illustrated graphically is provided to facilitate the understanding of the methodology and serve as a guide for investigators who are unfamiliar with the approach. Also discussed are the theoretical derivation of the isobologram, the representation of various dosage combinations, the derivation of the principle of dose additivity, supra-additivity, infra-additivity, antagonism, the methods for probit analysis of mixture potency, effect addition and the consequences of peak effect coincidence in time or lack thereof, and the role of isobolographic analysis in the various aspects of dose-response surface methodology.

A matter of some sensitivity

The development of sensitive chromatographic and spectroscopic techniques for the isolation and structure determination of natural products has greatly facilitated phytochemical investigations. Chemical investigations of herbarium material have resulted in the isolation of indole, quinoline and isoquinoline alkaloids from a wide number of plants. Examples of novel natural products from higher plants are given and include alkaloids, terpenoids, phenolics and quinones. Some plants investigated have not yielded the types of constituents which would have been predicted from them. Plant tissue cultures provide alternative sources of biologically active compounds and examples investigated include Cinchona, Ailanthus, Brucea and Artemisia for antiprotozoal compounds and Datura for tropane alkaloids. Biological tests are useful for bioassay-guided fractionation of plant extracts and examples of the isolation of a series of natural products with antiprotozoal and cytotoxic activities are given. Chemical and biological investigations into the traditional medicine Dragon's blood (Croton lechleri) from S. America and a Chinese prescription for the treatment of atopic eczema are described. The use of radio-ligand binding assays for the detection of a wide range of biological activities is discussed. Sensitivity of chemical and biological techniques has greatly improved prospects for finding new drug entities from plants and for investigating traditional medicines. Basic phytochemical investigations should continue to be encouraged especially in view of the rapid loss of plant species.

Natural products as drugs

Higher plants, many of which are threatened with extinction, are used as sources of pharmaceuticals and as ingredients of traditional medicines and are of value in new drug discovery. Artemisinin, taxol and camptothecin are examples of natural products which are undergoing clinical and commercial development. Several natural products isolated from plants used in traditional medicine have potent antiplasmodial action in vitro and represent potential sources of new antimalarial drugs. Plant biotechnology offers the possibility of improved production methods of cultivated medicinal plants as well as alternative approaches to the production of natural products for the preparation of pharmaceuticals.

In vitro antiplasmodial, antiamoebic, and cytotoxic activities of a series of bisbenzylisoquinoline alkaloids

Twenty-four bisbenzylisoquinoline alkaloids were screened for antiplasmoidal, antiamoebic, and cytotoxic activities by use of in vitro microtests. Eight of the alkaloids had antiplasmodial activity, with a 50% inhibitory concentration (IC50) of less than 1 microM against a multidrug-resistant strain of Plasmodium falciparum (chloroquine had an IC50 of 0.2 microM). The three alkaloids most active against Entamoeba histolytica, aromoline, isotrilobine, and insularine, had IC50s of 5 to 11.1 microM (metronidazole had an IC50 of 1.87 microM). None of the 24 bisbenzylisoquinoline alkaloids exhibited significant cytotoxicity against the KB cell line, the most toxic being berbamine, with an IC50 of 17.8 microM (the IC50 of podophyllotoxin was 0.008 microM). Bisbenzylisoquinoline alkaloids merit further investigation as potential novel antimalarial agents.