Synergistic antiplasmodial activity of artemisinin and olive leaf decoction: the role of the constituents of the phytocomplex

Within the framework of a larger research project aiming at evaluating the possible synergistic effects in malaria treatment between artemisinins and a variety of plant extracts, the antiplasmodial properties of a dried decoction of olive leaf and its principal constituents, namely phenylpropanoids and flavonoids were tested. Both the extract and constituents were assayed alone and in combination with artemisinin against 3D7 P. falciparum strains. The extract was prepared by aqueous extraction of olive leaf and purified with membrane techniques according to PCT/IT2006/47783 and it was characterised by HPLC/DAD as previously reported. Polyphenols represented 33% of olive leaf being oleuropein the principal constituent (20%). The synergism between artemisinin and the decoction or the single constituents on the parasite growth was investigated adding artemisinin, at sublethal doses, ranging from 0.625 to 40 nM, either in the presence (or in the absence) of low concentrations of the decoction or individual compounds. The effects of artemisinin tested at different concentrations olive leaf extract (10 and 50µg/ml) were synergistic by isobologram analysis. Oleuropein had a moderate activity but only additive effects were found in the presence of artemisinin. Among the other tested compounds verbascoside was the most active (IC50=63.73µg/ml). The observed synergisms of the phytocomplex were exploited to determine the constituents responsible for the synergism in an attempt to design new and/or more effective combination therapies.