Colin W. Wright

The evaluation of forty-three plant species for in vitro antimycobacterial activities ; isolation of active constituents from Psoralea corylifolia and Sanguinaria canadensis

Extracts from forty-three plant species were selected on account of reported traditional uses for the treatment of TB and/or leprosy. These were assayed for antimycobacterial activities. A simple in vitro screening assay was employed using two model species of mycobacteria, M. aurum and M. smegmatis. Crude methanolic extracts from three of the plants, C. mukul, P. corylifolia and S. canadensis, were found to have significant antimycobacterial activity against M. aurum only (MIC=62.5 microg/ml). Bioassay guided fractionation led to the isolation of two known benzophenanthridine alkaloids, sanguinarine (1) and chelerythrine (2), from the roots S. canadensis and the known phenolic meroterpene, bakuchiol (3) from the seeds of P. corylifolia. The fractionation of the resin of C. mukul lead to a decrease in antimycobacterial activity and hence further work was not pursued. Compound (2) was the most active against M. aurum and M. smegmatis (IC(50)=7.30 microg/ml [19.02 microM] and 29.0 microg/ml [75.56 microM], respectively). M. aurum was the most susceptible organism to all three compounds. No significant difference in antimycobacterial activity was observed when the two alkaloids were tested for activity in media of differing pH values. The activities of the pure compounds against M. aurum were comparable with those against M. bovis BCG with compound (2) being the most active (M. bovis BCG, IC(50)=14.3 microg/ml [37.3 microM]). These results support the use of these plants in traditional medicine.

Use of microdilution to assess in vitro antiamoebic activities of Brucea javanica fruits, Simarouba amara stem, and a number of quassinoids.

A microdilution technique for the assessment of in vitro activity against Entamoeba histolytica was devised and validated with metronidazole. The test was used to detect the antiamoebic activities of plant extracts prepared from the traditional remedies Brucea javanica fruits and Simarouba amara stems. The activity was associated with quassinoid-containing fractions. The 50% inhibitory concentrations for some quassinoids against amoebae were determined by using the microdilution method. These concentrations ranged from 0.019 micrograms.ml-1 for bruceantin, the most active quassinoid, to greater than 5 micrograms.ml-1 for glaucarubol, the least active compound tested. These results are discussed with reference to the known activities of these compounds against Plasmodium falciparum. Overall, the activities of the quassinoids against both protozoa are similar. The microdilution technique will be useful in the search for novel antiamoebic drugs.

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

BackgroundIn 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.MethodsNarrative review.ResultsThere 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.ConclusionsMore 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.

Synthesis and evaluation of cryptolepine analogues for their potential as new antimalarial agents

The indoloquinoline alkaloid cryptolepine 1 has potent in vitro antiplasmodial activity, but it is also a DNA intercalator with cytotoxic properties. We have shown that the antiplasmodial mechanism of 1 is likely to be due, at least in part, to a chloroquine-like action that does not depend on intercalation into DNA. A number of substituted analogues of 1 have been prepared that have potent activities against both chloroquine-sensitive and chloroquine-resistant strains of Plasmodium falciparum and also have in common with chloroquine the inhibition of beta-hematin formation in a cell-free system. Several compounds also displayed activity against Plasmodium berghei in mice, the most potent being 2,7-dibromocryptolepine 8, which suppressed parasitemia by 89% as compared to untreated infected controls at a dose of 12.5 mg kg(-1) day(-1) ip. No correlation was observed between in vitro cytotoxicity and the effect of compounds on the melting point of DNA (DeltaT(m) value) or toxicity in the mouse-malaria model.

The antimalarial and cytotoxic drug cryptolepine intercalates into DNA at cytosine-cytosine sites.

Cryptolepine, a naturally occurring indoloquinoline alkaloid used as an antimalarial drug in Central and Western Africa, has been found to bind to DNA in a formerly unknown intercalation mode. Evidence from competition dialysis assays demonstrates that cryptolepine is able to bind CG-rich sequences containing nonalternating CC sites. Here we show that cryptolepine interacts with the CC sites of the DNA fragment d(CCTAGG)2 in a base-stacking intercalation mode. This is the first DNA intercalator complex, from ∼90 solved by X-ray crystallography, to bind a nonalternating (pyrimidine-pyrimidine) DNA sequence. The asymmetry of the drug induces a perfect stacking with the asymmetric site, allowing for the stability of the complex in the absence of hydrogen bonding interactions. The crystal structure of this antimalarial drug–DNA complex provides evidence for the first nonalternating intercalation and, as such, provides a basis for the design of new anticancer or antimalarial drugs.

A review of antimycobacterial natural products

Tuberculosis is a chronic infectious disease caused by several species of mycobacteria. Due to multi‐drug resistant strains of mycobacteria and to a high prevalence of tuberculosis in patients who have acquired human immunodeficiency syndrome (AIDS), the number of patients infected with the disease is increasing worldwide. Thus there is an urgent need for new effective antimycobacterial agents to replace those currently in use. In this instance, the plant kingdom is undoubtedly a valuable source for new anti‐tuberculosis agents. The present review article reports the findings from an extensive literature search of all plants that have been assessed for antimycobacterial/antitubercular activity over the past 20–30 years. An attempt has been made to summarize the information in order to highlight those promising plant species which are worthy of further investigation as leads for drug development. Over 350 plant species from a wide range of families and origins, containing various chemical classes of compounds, have been screened for such activity. A review of the relevant in vitro assays using different species of pathogenic and non‐pathogenic mycobacteria is also included. Copyright

Can ethnopharmacology contribute to the development of antimalarial agents

The resistance of Plasmodium falciparum, the cause of tertian malaria, to synthetic antimalarials, together with the resistance of the vector mosquitoes to insecticides, has resulted in a resurgence in the use of quinine and a search for new antimalarial agents. In recent years, artemisinin, isolated from Artemisia annua which is used in Chinese traditional medicine for the treatment of malaria, has proved to be effective in the treatment of cerebral malaria due to chloroquine-resistant strains of P. falciparum. The development of in vitro tests utilising P. falciparum obtained from malaria patients means that it is possible to use bioassay guided fractionation of active extracts in order to isolate active principles. A number of laboratories throughout the world are currently investigating plants used in traditional medicine for their active constituents. Some of their results will be described and in particular two aspects of our investigations with species of Simaroubaceae and Menispermaceae will be discussed. There is every possibility that such approaches which use leads from Ethnopharmacology will result in the development of new antimalarial agents. It is vitally important to those populations relying on traditional medicines for the treatment of malaria that the safety and efficacy of such medicines be established, their active principles determined and that reproducible dosage forms be prepared and made available for use.

Antimalarial Activity of Cryptolepine and Some Other Anhydronium Bases

Eight naturally occurring anhydronium bases and the synthetic quaternary compound Nb‐methylharmalane were tested against Plasmodium falciparum (strain K1) in vitro. Cryptolepine was found to have similar activity to that of chloroquine but alstonine, 5,6‐dihydroflavopereirine, matadine, Nb‐methylharmalane, melinonine F, normelinonine F, strychnoxanthine and serpentine were found to have little activity. Cryptolepine, given orally to mice infected with Plasmodium berghei berghei was found to have moderate antimalarial activity; parasitaemia was suppressed by 80% at 50 mg/kg/day.

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.

Parasitological and microbiological evaluation of Mixe Indian medicinal plants (Mexico)

Medicinal plants are an important health resource in many regions of the Americas and are of particular importance to many Indian communities. Based on a recent ethnobotanical study in Mexico, we investigated the activity of 29 plant extracts against Entamoeba histolytica, three bacteria (Bacillus subtilis, Escherichia coli, and Micrococcus luteus) and two fungi (Cladosporium cucumerinum and Penicillium oxalicum). After separation of these extracts between CH2Cl2 and H2O the resulting phases were also evaluated.