Inhibition of tumor cell growth, proliferation and migration by X-387, a novel active-site inhibitor of mTOR

The mammalian target of rapamycin (mTOR), is deregulated in about 50% of human malignancies and exists in two complexes: mTORC1 and mTORC2. Rapalogs partially inhibit mTORC1 through allosteric binding to mTORC1 and their efficacy is modest as a cancer therapy. A few mTOR kinase inhibitors that inhibit both mTORC1 and mTORC2 have been reported to possess potent anticancer activities. Herein, we designed and synthesized a series of pyrazolopyrimidine derivatives targeting mTOR kinase domain and X-387 was identified as a promising lead. X-387 selectively inhibited mTOR in an ATP-competitive manner while sparing a panel of kinases from the PIKK family. X-387 blocked mTORC1 and mTORC2-mediacted signaling pathway in cell lines with activated mTOR signaling and in rapamycin-resistant cells. Specifically, X-387 inhibited phosphorylation of AKT at T308, which is thought to be a target of PDK1 but not mTOR. Such activity was not due to inhibition of PI3K since X-387 did not inhibit translocation of AKT to the cell membrane. X-387 induced autophagy as observed for other mTOR inhibitors, while induced autophagy is pro-survival since concurrent inhibition of autophagy by 3-MA reinforced the antiproliferative activity of mTOR inhibitors. X-387 also inhibited cell motility, which is associated with decrease in activity of small GTPases such as RhoA, Rac1 and Cdc42. Taken together, X-387 is a promising compound lead targeting mTOR and with a wide spectrum anticancer activity among tumor cell lines. The data also underscores the complexity of the mTOR signaling pathways which are far from being understood.

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