Structure elucidation of antiplasmodial sesquiterpene lactones from Vernonia staehelinoides and Oncosiphon piluliferum

Malaria continues to be a major cause of mortality and morbidity especially in Sub-Saharan Africa. The emergence and spread of drug resistant parasites has highlighted the need for new chemically diverse, effective drugs. Historically, one of the major sources of antimalarial agents and novel template compounds has been higher order plants. The widespread use of medicinal plants for the treatment of malaria in South Africa represents a diverse resource of potential antimalarial drugs. Two South African plants, Vernonia staehelinoides and Oncosiphon piluliferum, were identified as potential sources of new antimalarial drugs through a national multidisciplinary-consortium project aimed at scientifically validating South African medicinal plants for the treatment of malaria. The in vitro antiplasmodial activity of extracts of these plants warranted further investigation to identify the biologically active components. Bio-assay guided fractionation based on in vitro antiplasmodial activity against the D10 P. falciparum strain was used to identify the compounds responsible for the observed activity. Compounds were purified using silica gel column chromatography. The structures of the isolated compounds were elucidated using spectroscopic techniques. Bioassay-guided fractionation of the organic extracts of V. staehelinoides leaves identified a pair of structurally-related hirsutinolides with significant in vitro antiplasmodial activity. The compounds were found to be cytotoxic at similar concentrations but proved to be interesting scaffolds for potential structure-activity relationship studies. Three germacranolides and two eudesmanolides were identified through bioassay-guided fractionation of the organic O. piluliferum extract. Selected derivatizations were conducted in order to fully characterize the compounds. The absolute configuration of the major active germacranolide was determined using Mosher's method. The effect of the reduction of the a-methylene group of the major active germacranolide on antiplasmodial activity and cytotoxicity was also investigated. The 5 compounds and the reduction product were found to possess varying degrees of in vitro antiplasmodial activity and cytotoxicity. None was sufficiently active or selective to be a viable drug candidate but the potential for further structure-activity relationship studies exists.Dissertation (MSc (Chemistry))--University of Pretoria, 2007.Chemistryunrestricte

Antiplasmodial neolignans from Trema orientalis : identification, synthesis and analogue generation.

Includes abstract.Includes bibliographical references.Trema orientalis, a widely distributed evergreen tree with various medicinal properties including the treatment of malaria, was investigated as a potential source of new antimalarial lead compounds. Organic extracts of the young growing twigs of T. orientalis were reproducibly shown to be active against the chloroquine-sensitive (D10) and chloroquine resistant (K1) strains of Plasmodium falciparum. The 8-O-4' oxyneolignans, dadahols A and B, were identified as the major active compounds using two bioassay-guided fractionation approaches. The new accelerated “HPLC biogram” methodology allowed for early recognition of the active compounds in the complex plant extract, requiring considerably less time and material compared to the classical reiterative approach

Antiplasmodial hirsutinolides from Vernonia staehelinoides and their utilization towards a simplified pharmacophore

Please open article to read abstractThis work was financially supported by the Department of Science and Technology which awarded an innovation fund to five South African institutions (The Medical Research Council, South African National Biodiversity Institute, Council for Scientific and Industrial Research, University of Cape Town and University of Pretoria) to scientifically validate South African medicinal plants for the treatment of malaria

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

<p>Abstract</p> <p>Background</p> <p>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 <it>Dicoma anomala </it>subsp. <it>gerrardii</it>, based on its ethnomedicinal profile.</p> <p>Methods</p> <p>Standard phytochemical analysis techniques, including solvent-solvent extraction, thin-layer- and column chromatography, were used to isolate the main active constituent of <it>Dicoma anomala </it>subsp. <it>gerrardii</it>. The crystallized pure compound was identified using nuclear magnetic resonance spectroscopy, mass spectrometry and X-ray crystallography. The compound was tested <it>in vitro </it>on <it>Plasmodium falciparum </it>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 <it>in vitro </it>using the pLDH assay. The effects of the pure compound on the <it>P. falciparum </it>transcriptome were subsequently investigated by treating ring-stage parasites (alongside untreated controls), followed by oligonucleotide microarray- and data analysis.</p> <p>Results</p> <p>The main active constituent was identified as dehydrobrachylaenolide, a eudesmanolide-type sesquiterpene lactone. The compound demonstrated an <it>in vitro </it>IC₅₀of 1.865 μM against a chloroquine-sensitive strain (D10) of <it>P. falciparum</it>. 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.</p> <p>Conclusions</p> <p>The novel mode of action of dehydrobrachylaenolide, detected during these studies, will play an ongoing role in advancing anti-plasmodial drug discovery efforts.</p

Evaluation of selected South African ethnomedicinal plants as mosquito repellents against the Anopheles arabiensis mosquito in a rodent model

<p>Abstract</p> <p>Background</p> <p>This study was initiated to establish whether any South African ethnomedicinal plants (indigenous or exotic), that have been reported to be used traditionally to repel or kill mosquitoes, exhibit effective mosquito repellent properties.</p> <p>Methods</p> <p>Extracts of a selection of South African taxa were tested for repellency properties in an applicable mosquito feeding-probing assay using unfed female <it>Anopheles arabiensis</it>.</p> <p>Results</p> <p>Although a water extract of the roots of <it>Chenopodium opulifolium </it>was found to be 97% as effective as DEET after 2 mins, time lag studies revealed a substantial reduction in efficacy (to 30%) within two hours.</p> <p>Conclusions</p> <p>None of the plant extracts investigated exhibited residual repellencies >60% after three hours.</p

Screening for adulticidal bioactivity of South African plants against Anopheles arabiensis

<p>Abstract</p> <p>Background</p> <p>This study was conducted to evaluate whether a selection of South African ethnomedicinal plants included in this study displayed insecticidal properties when screened against adult stages of the mosquito.</p> <p>Methods</p> <p>381 crude extracts of 80 plant taxa in 42 families were sprayed onto ceramic tiles and screened using the cone bio-assay method for insecticide efficacy testing. Blood-fed, female <it>Anopheles arabiensis </it>mosquitoes were exposed to the treated tiles for a period of sixty minutes. Mosquito mortality was monitored for twenty-four hours.</p> <p>Results</p> <p>Of all the extracts analysed, the highest activity was observed in <it>Ptaeroxylon obliquum </it>(Ptaeroxylaceae) and <it>Pittosporum viridiflorum </it>(Pittosporaceae), a single extract from each, exhibiting more than 50% mortality. A large proportion (81.63%) of the extracts tested displayed low levels of mosquitocidal activity. The remainder of the extracts (17.85%) exhibited no bioactivity (0% mortality).</p> <p>Conclusions</p> <p>The screening results have shown that in accordance with WHO standards, none of the crude extracts tested had exhibited greater than 60% mortality against the adult stages of the malaria vector <it>Anopheles arabiensis</it>.</p

Screening of selected ethnomedicinal plants from South Africa for larvicidal activity against the mosquito <it>Anopheles arabiensis</it>

<p>Abstract</p> <p>Background</p> <p>This study was initiated to establish whether any South African ethnomedicinal plants (indigenous or exotic), that have been reported to be used traditionally to repel or kill mosquitoes, exhibit effective mosquito larvicidal properties.</p> <p>Methods</p> <p>Extracts of a selection of plant taxa sourced in South Africa were tested for larvicidal properties in an applicable assay. Thirty 3^rd instar <it>Anopheles arabiensis</it> larvae were exposed to various extract types (dichloromethane, dichloromethane/methanol) (1:1), methanol and purified water) of each species investigated. Mortality was evaluated relative to the positive control Temephos (Mostop; Agrivo), an effective emulsifiable concentrate larvicide.</p> <p>Results</p> <p>Preliminary screening of crude extracts revealed substantial variation in toxicity with 24 of the 381 samples displaying 100% larval mortality within the seven day exposure period. Four of the high activity plants were selected and subjected to bioassay guided fractionation. The results of the testing of the fractions generated identified one fraction of the plant, <it>Toddalia asiatica</it> as being very potent against the <it>An. arabiensis</it> larvae.</p> <p>Conclusion</p> <p>The present study has successfully identified a plant with superior larvicidal activity at both the crude and semi pure fractions generated through bio-assay guided fractionation. These results have initiated further research into isolating the active compound and developing a malaria vector control tool.</p