Toxicity mechanisms of voacangine isolated from Tabernaemontana cymosa Jacq., on Aedes aegypti L. mosquito larvae

Abstract

The Aedes aegypti L. mosquito is considered the most important vector of arboviruses in the world. The phenomenon of resistance to insecticides is a difficult barrier to overcome for government health entities around the planet. This problem increases the concentrations of insecticides in the environment, causing environmental contamination and threats to human health. Plants have been used to combat pests for centuries and are an ecological source for searching for molecules with larvicidal activity. In this work, 65 ethanol-soluble extracts of 56 plants from the Colombian Caribbean region were evaluated as potential larvicides against the Aedes aegypti mosquito, as well as for their toxic effects on nontarget organisms. High larvicidal activity was found for 16 ethanol plant extracts; however, the most potent activity against larvae was obtained for five plant extracts, Annona squamosa, Annona cherimolia, Annona muricata, Tabernaemontana cymosa and Mammea americana, with LC50 (LCL – UCL) values of 58 (24 –142), 65 (33 – 127), 85 (42 – 170), 25 (23 – 27) and 39 (34 – 43) µg/mL, respectively. The T. cymosa seed extract was selected for bioguided fractionation due to its great larvicidal activity. Five indole alkaloids were isolated and characterized from the active fraction of T. cymosa using Liquid Chromatography and Nuclear Magnetic Resonance (NMR), respectively. Voacangine showed an LC50 of 5.1 µg/mL, indicating high larvicidal potency and low risk for non target organisms due to its selectivity (>40) against the model Caenorhabditis elegans. We also report the characterization of a new indole alkaloid from T. cymosa. Alkaloids are a group of secondary metabolites that have been extensively studied for the discovery of new drugs due to their properties on the central nervous system and their anti-inflammatory, antioxidant and anticancer activities. In the larvicidal fraction, 10 indole alkaloids were identified, and computational tools were used to evaluate their potential biological activities in humans. Consequently, molecular docking was performed using 951 human targets involved in different diseases. The results were analyzed through tools included in the KEGG and STRING databases, and relevant physiological functions associated with the alkaloids were found. Most of the alkaloids showed affinity for the same type of proteins, forming stable complexes with affinity energies of less than −8.0 kcal/mol. However, the 5-oxocoronaridine molecule proved to be the most active molecule binding human proteins (mean binding energy affinity = −9.2 kcal/mol). Gene ontology analysis of the interactions between the affected proteins pointed to the PI3K/Akt signaling pathway. /mTOR as the main target. On the other hand, all alkaloids showed good affinity for AChE from A. aegypti, but only voacangine had larvicidal potential. Moreover, the alkaloid voacangine caused a significant increase in lipid peroxidation in the larvae when compared to the control when tested at its diagnostic concentration. Our study demonstrated the potential of the Colombian Caribbean flora as a host for bioactive plants against the A. aegypti mosquito, with potential use in controlled environments. The data showed that the mechanism of action of voacangine involves oxidative stress and likely other biochemical processes linked to the central nervous system of the larva, causing the death of the insect without major adverse effects on human targets. Finally, 5-oxocoronaridine, voacangine-7- hydroxyndolenine and voacrisitine are potential ligands for key human proteins involved in cellular proliferation, making them promising leads for the development of new treatments against cancer pathologies.