Abstract B251: Combined anticancer effects of sphingosine kinase inhibitors and sorafenib

Purpose: The pro‐apoptotic lipid sphingosine is phosphorylated by sphingosine kinases 1 and 2 (SK1 and SK2) to generate the pro‐survival lipid sphingosine‐1‐phosphate (S1P). We previously reported that inhibition of SK activity delays tumor growth in a mouse mammary adenocarcinoma tumor model. Because SK inhibitors and the multikinase inhibitor sorafenib both suppress the MAP kinase pathway, we hypothesized that the combination may provide enhanced tumor growth inhibition. Therefore, we evaluated the effects of two SK inhibitors, ABC294640 (a SK2‐specific inhibitor) and ABC294735 (a SK1/SK2 inhibitor), alone and in combination with sorafenib on human pancreatic adenocarcinoma (Bxpc‐3) and kidney carcinoma (A‐498) cells in vitro and in vivo . Experimental Design: Effects of the compounds as single agents or in combination in cell culture were assessed using the SRB assay, and CalcuSyn software was used to calculate synergistic or additive growth inhibition. Western blotting measured effects of compounds on signaling pathways. SCID mice bearing tumors of either Bxpc‐3 or A‐498 cells were treated with the agents and tumor growth was monitored. Results: Exposure of either Bxpc‐3 or A‐498 cells to ABC29464/sorafenib or ABC294735/sorafenib combinations resulted in synergistic cytotoxicity, associated with increases in caspases 3/7 activation and apoptotic DNA fragmentation. In addition, strong decreases in ERK phosphorylation were observed in Bxpc‐3 and A‐498 cells exposed to the either sorafenib/ABC29464 or sorafenib/ABC294735 combinations. An increase in the phosphorylation of inhibitory site of c‐Raf (S259) was also observed in the drug‐treated cells. Oral administration of either ABC294640 or ABC294735 to mice led to a delay in tumor growth in both xenograft models without overt toxicity to the animals. Tumor growth delay was potentiated by co‐administration of sorafenib. Conclusions: These studies show that combination of an SK inhibitor with sorafenib causes synergistic inhibition of cell growth in vitro and potentiates antitumor activity in vivo. Thus, a foundation is established for clinical trials evaluating the efficacy of combining these signaling inhibitors. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):B251.