Abstract

Multiple myeloma (MM) remains an incurable neoplasia, despite recent development of several novel therapies. As part of our efforts to identify new compounds with anti-MM activity, we evaluated the class of avicins, which are triterpenoid saponins that have been shown to induce apoptosis of neoplastic cells by affecting mitochondrial function independently of membrane-bound death receptors. Because we have previously shown that mitochondria constitute key regulators of MM cell responsiveness to diverse anti-tumor agents, (e.g. the proteasome inhibitor bortezomib), we evaluated the in vitro anti-MM effects of this class of compounds. Our vitro drug-sensitivity studies showed that Avicin D and Avicin G, the main members of this class of compounds, are active against a broad panel of MM cell lines and primary tumor cells, including tumor cells resistant to conventional (e.g. dexamethasone, alkylating agents, anthracyclines) or novel (e.g. thalidomide, immunomodulatory thalidomide derivatives, proteasome inhibitor PS-341[bortezomib], Apo2L/TRAIL) anti-MM agents. Using MTT survival assays, we confirmed that the IC50 values for both Avicins were highly concordant and were less than 250 nM for the overwhelming majority of MM cell lines tested. Importantly, this potent in vitro anti-MM activity was triggered by concentrations of Avicins which had minimal, if any, effect on the viability of normal hematopoietic cells or bone marrow stromal cells. Furthermore these IC50 values were comparable with the in vitro activity of this agent among the most Avicin-sensitive tumor models that have been previously tested. This potent anti-MM effect was not inhibited by transfection of MM cells with construct for constitutively active Akt. Although cytokine- or cell adhesion-mediated interactions of the local bone marrow (BM) microenvironment (e.g. BM stromal cells) protects MM cells from conventional therapies (e.g. dexamethasone or cytotoxic chemotherapy), avicins were able to overcome this protective effect in co-culture models of MM cells with BM stromal cells and sensitized MM cells to cytotoxic chemotherapy-induced cell death. Using hierarchical clustering analyses and relevance network algorithms, we compared the patterns of MM cell sensitivity to Avicin D vs Avicin G vs. other anti-cancer drugs and found that the pattern of dose-response relationships of the 2 main members of this class of compounds are very similar to each other, but clearly distinct from the patterns of sensitivity or resistance to other drugs, either conventional or investigational. This further supports the notion that the anti-MM properties of Avicins are mediated by molecular mechanisms distinct from those of currently available anti-MM drugs, and also suggests that Avicins may have anti-tumor activity even against subgroups of MM which may be resistant to other novel therapies that are currently in clinical development. These results have provided the framework for ongoing in vivo studies of anti-tumor activity of these agents, to evaluate the feasibility of future clinical trials of Avicins to improve patient outcome in MM.

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