Modulation of the in vitro antimalarial effects of artemisinin by selected extracts: the case of olive leaf water extract

Within the framework of a larger research project [1] aimed at evaluating the possible synergistic effects in malaria treatment between artemisinins -today the most potent antimalarial agents- and a variety of plant extracts and isolated natural constituents, the specific antiplasmodial properties in vitro of a dried extract of olive leaf alone and in combination with artemisinin were evaluted against 3D7 P. falciparum strains. The extract was prepared by aqueous extraction of olive leaf and purified with membrane techniques as microfiltration, ultrafiltration, nanofiltration and reverse osmosis, both in laboratory scale and in pilot plant experimental apparatus, according to PCT/IT2006/47783 [2]. The extract was characterised by HPLC/DAD as previously reported [3] and HPLC/MS analysis, showing 20% oleuropein and 33% polyphenols. The possible synergism between artemisinin and 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 10µM (no reduction of parasitemia) or 50µM (reduction of 12% of parasitemia) concentrations of the individual compounds. The effects of artemisinin tested at different concentrations and both 10 and 50µM olive leaf extract are more than additive implying some moderate synergism between these two substances. The observed synergism by isobologram analysis, between olive leaf extract, expecially at 50µM and artemisinin might be conveniently exploited to determine the constituents responsible of the synergism and design new and/or more effective combination therapies.