In Vitro | In vitro activity: PF-4708671 is a piperazinyl-pyrimidine analogue and the first S6K1-specific inhibitor. PF-4708671 does not significantly inhibit the activity of the closely related S6K2 isoform or a number of other AGC kinases (Akt1, Akt2, PKA, PKCα, PKCϵ, PRK2, ROCK2, RSK1, RSK2 or SGK1) . PF-4708671 prevents the S6K1-mediated phosphorylation of S6 protein in response to IGF-1 (insulin-like growth factor 1), while having no effect upon the PMA-induced phosphorylation of substrates of the highly related RSK (p90 ribosomal S6 kinase) and MSK (mitogen- and stress-activated kinase) kinases. PF-4708671 is also found to induce phosphorylation of the T-loop and hydrophobic motif of S6K1, an effect that is dependent upon mTORC1 (mTOR complex 1). PF-4708671 does not affect activity of mTORC1. Kinase Assay: PF-4708671 is a cell-permeable inhibitor of p70 ribosomal S6 kinase (S6K1 isoform) with Ki/IC50 of 20 nM/160 nM in cell-free assays, 400-fold greater selectivity for S6K1 than S6K2, and 4- and >20-fold selectivity for S6K1 than MSK1 and RSK1/2, respectively. Cell Assay: The effects of OSI-906 or the combination of OSI-906 and PF-4708671 on cell proliferation is determined with XTT and clonogenic assays (GEO, HT29, SW480, and HCT116 cells are used). XTT assays are performed using the Cell Proliferation Kit II (XTT). For clonogenic assays, cells (1×103cells/well) are seeded on a 6-well plate and subsequently treated with drugs (OSI-906 5 μM, PF-4708671 10 μM). After 1 week of incubation, cells are stained with 1% crystal violet, and the number of colonies is counted and recorded. |