Combined anticancer effects of sphingosine kinase inhibitors and sorafenib
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The pro-apoptotic lipid sphingosine is phosphorylated by sphingosine kinases 1 and 2 (SK1 and SK2) to generate the mitogenic lipid sphingosine-1-phosphate (S1P). We previously reported that inhibition of SK activity delays tumor growth in a mouse mammary adenocarcinoma model. Because SK inhibitors and the multikinase inhibitor sorafenib both suppress the MAP kinase pathway, we hypothesized that their combination may provide enhanced inhibition of tumor growth. Therefore, we evaluated the effects of two SK inhibitors, ABC294640 (a SK2-specific inhibitor) and ABC294735 (a dual 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. Exposure of either Bxpc-3 or A-498 cells to combinations of ABC294640 and sorafenib or ABC294735 and sorafenib resulted in synergistic cytotoxicity, associated with activation of caspases 3/7 and DNA fragmentation. Additionally, strong decreases in ERK phosphorylation were observed in Bxpc-3 and A-498 cells exposed to either the sorafenib/ABC294640 or the sorafenib/ABC294735 combination. 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. 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.