Synergistic interactions of saponins and monoterpenes in HeLa cells, Cos7 cells and in erythrocytes

In phytomedicine complex extracts consisting of phenolics, monoterpenes or saponins are traditionally used. It is often impossible to attribute the biological activity of an extract to one or few compounds. As an explanation of the superior activity of extracts, a synergistic effect of combinations of active compounds has been suggested. Since lipophilic monoterpenes or saponins targeting the biomembrane usually accompany polar polyphenols in phytomedical preparations, we decided to investigate their effect as single substances and in combination to gain further insight into potential synergistic effects of herbal medicine. Combinations of the monoterpenes α-pinene, thymol and menthol with the monodesmosidic saponins digitonin, aescin, glycyrrhizic acid and Quillaja saponin demonstrated strong synergistic activity. The IC50 of haemolysis was lowered by a factor of 10–100 from 316 μg/ml to 2 μg/ml for aescin, 157 μg/ml to 11 μg/ml for Quillaja saponins and 20 μg/ml to 3 μg/ml for digitonin when combined with thymol. A similar significant synergistic cytotoxicity occurred both in HeLa and Cos7 cells by combining the α-pinene, thymol and menthol with the saponins. The IC50 of glycyrrhizic acid was lowered by a factor 100 from around 300 μg/ml to around 1–10 μg/ml and the IC50 of aescin, digitonin and Quillaja saponins about the factor 10. Monoterpenes and monodesmosidic saponins have a common target, the biomembrane, which is present in all animal, fungal and bacterial cells. Disturbance of membrane fluidity and permeability is the mode of action. This activity is non-specific which makes it extremely difficult for bacteria and fungi to develop resistance. This explains the overall success of these molecules as defence chemicals in the plant kingdom. The synergistic effect of combinations of saponins with monoterpenes opens a complete new field of possible applications in medicine to overcome resistance in multidrug resistant microbial and human cell.