Effects of phosphorylation of immunomodulatory agent FTY720 (fingolimod) on antiproliferative activity against breast and colon cancer cells

FTY720 (fingolimod), a novel immunosuppressant, was found to become biologically activated by phosphorylation into FTY720-1-phosphate (FTV720-P), which is a high-affinity agonist for sphingosine-1-phosphate (sphingosine-1-P)-receptors. FTY720 has also been reported to have a strong antitumor activity. The association between the phosphorylation of FTY720 and the growth inhibition of FTY720 against cancer cells are still not completely understood. In this study, we investigated the effects of FTY720, sphingosine, and their related compounds on the proliferation of human breast cancer cell lines (MCF-7, MDA-MB-231 and Sk-Br-3) and human colon cancer cell lines (HCT-116 and SW620). Non-phosphorylated FTY720, sphingosine and an FTY720 derivative, ISP-I-55, showed significant growth inhibition against these cells, with IC50 values of 5-20 mu M at 48 h postdrug treatment. We confirmed that FTY720 induces the activation of a major mitogen-activated protein kinase, JNK, without the activation of p38 and down-regulation of phospho-ERK in MCF-7 breast cancer cells. In contrast, the phosphorylated derivatives, FTY720-P and sphingosine-1-P, as well as a phosphinane FTY720 derivative, cFTY720-P, did not inhibit the growth of the cells in the concentration range of 5-50 mu M, whereas FTY720-P and sphingosine-1-P slightly induced the growth of MCF-7 cells. Combining FTY720 with dimethylsphingosine, a sphingosine kinase inhibitor, augmented the inhibitory effect of FTY720. These results indicate that the antiproliferative activity, of FTY720 does not result from its phosphorylation, either endogenous or exogenous.