Triterpenoids as inhibitors of erythrocytic and liver stages of Plasmodium infections

Abstract

Bioassay-guided fractionation of the methanol extract of Momordica balsamina led to the isolation of two new cucurbitane-type triterpenoids, balsaminol F (1) and balsaminoside B (2), along with the known glycosylated cucurbitacins, cucurbita-5,24-diene-3β,23(R)-diol-7-O-β-d-glucopyranoside (3) and kuguaglycoside A (4). Compound 1 was acylated yielding two new triesters, triacetylbalsaminol F (5) and tribenzoylbalsaminol F (6). The structures were elucidated based on spectroscopic methods including 2D-NMR experiments (COSY, HMQC, HMBC and NOESY). Compounds 1–6, were evaluated for their antimalarial activity against the erythrocytic stages of the Plasmodium falciparum chloroquine-sensitive strain 3D7 and the chloroquine-resistant clone Dd2. Assessment of compounds (1–3 and 5, 6) activity against the liver stage of Plasmodium berghei was also performed, measuring the luminescence intensity in Huh-7 cells infected with a firefly luciferase-expressing P. berghei line, PbGFP-Luccon. Active compounds were shown to inhibit the parasite’s intracellular development rather than its ability to invade hepatic cells. Toxicity of compounds (1–3 and 5, 6) was assessed on the same cell line and on mouse primary hepatocytes through the fluorescence measurement of cell confluency. Furthermore, toxicity of compounds 1–6 towards human cells was also investigated in the MCF-7 breast cancer cell line, showing that they were not toxic or exhibited weak toxicity. In blood stages of P. falciparum, compounds 1–5 displayed antimalarial activity, revealing triacetylbalsaminol F (5) the highest antiplasmodial effects (IC50 values: 0.4μM, 3D7; 0.2μM, Dd2). The highest antiplasmodial activity against the liver stages of P.berghei was also displayed by compound 5, with high inhibitory activity and no toxicity.