Abstract C59: Discovery and characterization of selective PI3Kα and PI3Kα/mTOR-dual inhibitors for targeting PI3Kα-activated tumors

Abstract Background: The PI3K pathway regulates cell growth, proliferation and survival, and is dysregulated with high frequency in human tumors. PI3K pathway activation in tumors occurs via multiple mechanisms including prevalent mutation and amplification of the PIK3CA gene (which encodes the p110 subunit of PI3Kα). Downregulation of the lipid phosphatase PTEN also activates PI3K pathway signaling in a subset of tumors, via a mechanism reported to be PI3Kβ dependent. These findings suggest that isoformselective PI3K inhibitors may have utility in distinct, genetically defined tumor populations. Downstream of PI3K, mTOR controls cell growth and proliferation through its two distinct signaling complexes: mTORC1 and mTORC2. We report on the discovery of PI3Kα and dual PI3Kα/mTOR-selective inhibitors intended to target tumors harboring activating mutations or amplifications in PIK3CA. Methods: We identified and optimized a series of PI3Kα and PI3Kα/mTOR inhibitors through high-throughput screening and medicinal chemistry techniques. Biochemical assays employed recombinant class 1 PI3K and mTOR proteins and cellular assays employed ELISA or western blot assays to monitor PI3K pathway signaling. Results: We have identified potent (biochemical IC50s < 20 nM), ATP-competitive inhibitors of PI3Kα or PI3Kα and mTOR with excellent selectivity over other PI3K isoforms and a diverse panel of protein kinases. Cellular assays in the PI3Kα-activated MCF7 cell line demonstrate that these compounds inhibit phosphorylation of targets downstream of both PI3K (Akt) and mTOR (S6 and p70S6 kinase), and also inhibit tumor cell proliferation. However, in PC3 tumor cells lacking PTEN, effects on PI3K pathway readouts are less evident. In vivo pharmacodynamic analyses following oral administration of PI3Kα/mTOR inhibitors to mice bearing xenograft tumors demonstrate dose-dependent inhibition of phosphorylation of both PI3K and mTOR effectors at well-tolerated doses. Conclusions: PI3Kα and PI3Kα/mTOR-selective small molecule inhibitors were discovered that demonstrate potent in vitro and cellular activity and good oral bioavailability. Cellular selectivity for PI3Kα mutant tumor cells versus PTEN deleted tumor cells was demonstrated. These compounds will allow an exploration of the efficacy and tolerability of selective PI3Kα and PI3Kα/mTOR inhibitors in preclinical tumor models. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):C59.