Saponins: Current Progress and Perspectives

Saponins are gaining increasing attention as natural products because of their structural characteristics combining a lipophilic part (triterpene or steroid type) and a hydrophilic part (oligosaccharide moieties) in one molecule. They are thereby able to interact with biological membranes and induce a potential toxic or specific biological response, which is often of pharmacological interest. The wide structural diversity of saponins results in a large variety of biological effects, such as immunoadjuvant, cytotoxic, antitumor, antiinflammatory, antimicrobial properties and so on, just to mention a few. This reflects the increasing number of publications, as indicated in [Fig. 1] showing the numbers of saponin papers over the last 25 years (data from MedLine). Some examples of the current research on saponins can be read in this special edition of Planta Medica. Several articles focus on the isolation, structure elucidation and biological evaluation of previously undescribed saponins in Gledistia sinensis, Maesa argentea, Hernaria hirsuta, Allium fistulosum. As amphiphilic compounds, they possess several properties including foaming and pore forming abilities. The results of one study suggest that the cancer cell death induced by α-hederin mainly depends on its interaction with membrane cholesterol and consequent pore formation. A nice review reports saponin interactions with model membrane systems as a Langmuir monolayer. The pore forming property might represent an interesting mechanism for drug delivery into cells. This opens up promising applications in cancer combined chemotherapy using reduced doses, limiting the systemic toxicity. Another research group has shown the synergy of pro-apoptotic and anti-invasive effects of mitoxantrone in combination with a purified saponin fraction from Lysimachia ciliata in prostate cancer cells. A promising strategy to overcome the limitations of therapeutic antibodies and to improve their therapeutic values in cancer therapy has been investigated with a rituxumab-saporin immunotoxin co-administered with purified saponins from Saponaria officinalis. This combination significantly enhanced cytotoxicity in B-lymphocytes tumor cells originated from Burkittʼs lymphoma. Such beneficial effects of combined therapy were also shown by co-application of glycyrrhizic acid and gentamycin on vancomycin-resistant enterococci. The interest of acetylated saponins from Caryophyllaceae was underlined in a study showing their strong ability to inhibit the efflux of P-glycoprotein in an intestinal barrier model comprised of Caco-2 cells without affecting the cells. Therefore, the use of these surface-active agents in oral dosage forms may represent a promising strategy to enhance the bioavailability of poorly permeable drugs. Finally a nice review provides an overview on the steroid saponins from Ruscus ssp. from a chemical, pharmacological, and quality control point of view.