In vitro antiplasmodial activity and identification, using tandem LC-MS, of alkaloids from Aspidosperma excelsum, a plant used to treat malaria in Amazonia

Abstract

Ethnopharmacological relevance: Aspidosperma excelsum Benth. (Apocynaceae), a native tree in the Brazilian Amazonia, is traditionally used to treat various diseases, including malaria.

Aim of study: To investigate the chemical constitution, antiplasmodial activity and cytotoxicity of samples obtained from A. excelsum trunk bark by different procedures aiming to evaluate their potential as an antimalarial phytomedicine.

Materials and methods: A hydroethanolic extract and alkaloid extracts were prepared and assayed for antiplasmodial activity and cytotoxicity against chloroquine-resistant Plasmodium falciparum (W2 strain) and HepG2 cells, respectively. Taking into account the known occurrence and antimalarial activity of Aspidosperma monoterpene indole alkaloids (MIA), acid-base extractions were carried out and the fractions were assayed for antiplasmodial activity and cytotoxicity. All the samples were analysed by hyphenated chromatographic techniques, such as UPLC-DAD-ESI-MS/MS and HRMS (HPLC-MS MicroTOF), comparing their chemical composition to the literature data.

Results: The hydroethanolic extract disclosed a moderate in vitro activity against chloroquine-resistant Plasmodium falciparum (W2 strain) with IC₅₀ 23.68 ± 3.08 µg/mL), low cytotoxicity to HepG2 cells (> 250 µg/mL) and good SI (> 10.56). A total of 20 known monoterpene indole alkaloids were identified, seven of which are here firstly described for A. excelsum. Known highly active alkaloids, namely demethylaspidospermine, aspidocarpine, and ochrolifuanine are present in active alkaloid fractions and might contribute to their observed antiplasmodial effect. An alkaloid fraction (Ae-Alk2), obtained directly from trunk bark by extraction with dil. aqueous HCl, pointed out for its activity (IC₅₀ 8.75±2.26 µg/mL, CC₅₀ 185.14±1.97 µg/mL, SI 21.16) and should be highlighted as the most promising out of the assayed samples.

Conclusion: The present results represent a preliminary support to the alleged antimalarial use of A. excelsum trunk bark and allowed to highlight alkaloid fractions as promising phytomedicines.

Keywords: Anti-malarial activity; Apocynaceae; HPLC-MS MicroTOF; Indole-alkaloids; Plasmodium falciparum; UPLC-ESI-MS/MS.