BackgroundThe Quinine tree (Rauvolfia caffra) is used as a medicinal plant among traditional communities in many countries to manage tumors and other diseases associated with oxidative stress. To validate indigenous knowledge and possibly position this herb for technology uptake and utilization, we established the level of antioxidant activity in R. caffra, and probed for the presence of associated phytochemicals.MethodsAntioxidant activity was determined on 1,1-diphenyl-2-picrylhydrazyl (DPPH) while major phytochemicals were identified by multiple tests on methanol fractions.ResultsR. caffra showed promise as a cure, with antioxidant activity comparable to the commercially used drug quercetin (R. caffra = 79.7% ±1.9; quercetin = 82.6% ± 2.0). However, we found two phytochemicals with possible antagonistic effect: co-occurrence of alkaloids and saponins significantly reduced antioxidant activity (alkaloids only = 63%; alkaloids plus saponins = 15%; steroids, terpenoids and cardiac glycosides = 82%), thus alkaloids and saponins should be exclusive to each other in drug formulations.ConclusionsAntagonistic relationship among phytochemicals would affect the efficacy of crude extracts as used in traditional medicine. Unlike in herbal medicine, use of modern biotechnology in extraction, purification and design of optimal combinations will ensure efficient drug formulations with optimum bioactivity and minimum toxicity. Metabolic pathway engineering under a controlled environment may optimize availability of desired compounds.
Female Anopheles mosquitoes transmit Plasmodium parasites that cause human malaria. Currently, vector control is the most widely deployed approach to reduce mosquito population and hence disease transmission. This relies on use of insecticide-based interventions including Long-lasting Insecticide-treated Nets (LLINs) and Indoor Residual Spraying (IRS) where scale-up has contributed to a dramatic decline in malaria deaths and morbidity over the past decade. Challenges to their effective use include the emergence and spread of insecticide resistance by malaria vector populations coupled with the inability to curb outdoor transmission. Under these situations, use of larvicides through larval source management (LSM) can complement these existing measures. The need to minimize environmental impact and effect on non-target organisms has spurred interest in the development of eco-friendly larvicides of natural origin. Here, we review literature published in the last five years to highlight compounds of natural origin found to exhibit larvicidal activity against malaria mosquitoes. Specifically, the larvicidal activity of different classes of compounds is discussed including their effect on non-target organisms. Additionally, we provide suggestions for future research into mosquito larvicides including the use of chemical synthesis to improve the bioactivity of known natural compounds.
BACKGROUND
Extracts of the invasive weed Parthenium hysterophorus (Asteraceae) have been shown to possess larvicidal activity against a wide range of disease vectors. However, the phytochemicals responsible for the larvicidal activity from this plant remain unidentified. Here, we isolated the major sesquiterpene lactone, parthenin (1) from the plant and synthesized two derivatives [ethylene glycol (2) and azide (3) derivatives] targeting the α,β‐unsaturated carbonyl group, previously known to account for its biological activity such as toxicity towards cells and microorganism. All three compounds were screened for larvicidal activity against the African malaria vector Anopheles gambiae.
RESULTS
The larval mortality of ethylene glycol derivative (2) and 2α‐azidocoronopilin (3) were approximately two–four‐fold higher than that of parthenin (1) and neem oil with LC50 values of 37 and 66 mg L−1, respectively. Parthenin (1) and the positive control, neem oil, had comparable median lethal concentration (LC50) values of 154 and 121 mg L−1, respectively. In assays with binary combinations of the three compounds, larvicidal activity followed the order: parthenin (1) + 2α‐azidocoronopilin (3) (LC50 = 14 mg L−1) > parthenin (1) + ethylene glycol derivative (2) (LC50 = 109 mg L−1), > blend of 2α‐azidocoronopilin (3) and ethylene glycol derivative (2) (LC50 = 200 mg L−1).
CONCLUSION
Structural modification of parthenin (1) through addition of hydroxyl groups increases its larvicidal effects. These findings advance the use of structural modification approach in the development of lead chemical molecules for potential exploitation in larval source management.
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