Abstract #2015: Hellebrin and hellebrigenin derivatives from helleborus species display in vitro anti-tumor activity through sodium pump targeting

Several plant genera (notably Asclepiadacea, Apocynaceae, Ranunculaceae and Scrophulariaceae ) are recognized to contain cardiotonic steroids (comprising cardenolides and bufadienolides) which are the natural ligands and inhibitors of the sodium pump (Na + /K + -ATPase). Cardiotonic steroids are also extensively found in animal species and occur mainly in toads ( Bufo genera). Na + /K + -ATPase should be considered an important target for the development of anti-cancer drugs as it serves as a versatile signal transducer, it is a key player in cell adhesion and its aberrant expression and activity are implicated in the development and progression of different cancers. By binding to Na + /K + -ATPase, cardiotonic steroids elicit marked effects on cancer cells. Regretfully however, their narrow therapeutic index has hitherto prevented their development as anti-cancer drugs. Preliminary data have indicated that bufadienolides may have a larger therapeutic window than that displayed by cardenolides. The bufadienolide class of cardiotonic steroids remains largely unexplored despite their structural diversity and natural abundance. Plants of the family Ranunculaceae (mainly Helleborus species) are known to contain several bufadienolides such as: hellebrin, helleborein and helleborin. In order to further explore the potential of bufadienolides, hellebrin was first isolated from Helleborus sp. with a good overall yield. Starting from this natural product, hellebrigenin was obtained by bioconversion using a glycolytic enzyme. Positions 19 of hellebrin and 3 and 19 of hellebrigenin were then explored chemically to generate novel hemi-synthetic derivatives of these two natural products. The selection of a lead compound from the series was guided by results from the MTT cytotoxicity assay on human and rodent cancer cell lines and a Na + /K + -ATPase inhibition assay, and by the determination of maximum tolerated dose in vivo in rodents. Evaluated derivatives showed a remarkable correlation between cytotoxic activity towards cancer cells and their inhibition of Na + /K + -ATPase. The IC 50 values for cytotoxicity and Na + /K + -ATPase inhibition for the most potent compounds were in nM range. Furthermore, investigated compounds displayed marked selectivity towards cancer compared to normal human cell lines. Our investigations further showed that the bufadienolide derivatives could be less toxic suggesting the potential for a larger therapeutic window. In addition, good oral bioavailability was demonstrated for certain of these derivatives indicating that they could be potentially developed as oral agents in cancer patients. Structure-activity relationship analyses showed hellebrigenin derivatives to have greater potential for development and mechanism of action deciphering. Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 2015.