Abstract B138: SB2312, a novel and potent dual inhibitor of mTOR and PI3K with high target inhibition and antitumor efficacy
Authors: Hart Stefan , Williams Meredith , Chen Di Zhong , Goh Kee Chuan , Tan Yong Cheng , Amalini Chitra , Hentze Hannes , Jayaraman Ramesh , Kumar Harish , Dymock Brian , Ethirajulu Kantharaj , Wood Jeanette 
Published: June 2012

Background: Dysregulation of the PI3K signaling pathway, either through activating mutations, amplification of PI3K or deletion of PTEN has been linked to a wide range of hyperproliferative diseases. The development of small molecule modulators of key proteins in this signaling cascade has focused mainly on mTOR or PI3K. Recent investigations have shown a significant overlap in the activity of small molecule inhibitors against these two proteins. Inhibitors that are currently progressing through preclinical and early clinical development have been shown to affect both mTOR and PI3K to some extent. However, most reported inhibitors do not inhibit both of these kinases in a close dose range. Our drug discovery program has focused on equipotent inhibition of both targets. SB2312 is a novel compound with very favorable drug-like properties, giving rise to equivalent targeting of both mTOR and PI3K in cells and tumor tissue and resulting in a more efficient inhibition of this pathway compared to reference compounds currently under evaluation in clinical trials.

Methods: In vitro-IC50 for mTOR and PI3K were determined using recombinant enzymes. Inhibitory activity of SB2312 on the mTOR/PI3K signaling pathway was determined by Western-blot analysis, Surefire assays, and proliferation assays in various cancer cell lines. Efficacious doses of SB2312 or reference compounds were selected after pharmacokinetic and biomarker analysis of PC3 tumor-bearing mice, treated with single oral doses. Anti-tumor efficacy of SB2312 and reference compounds was evaluated in mouse xenograft models after continuous daily oral dosing.

Results: SB2312 is a potent and selective dual mTOR and PI3K inhibitor (IC50: 55 nM and 18 nM, respectively) and has no significant activity on a large panel of protein kinases or enzymes and receptors. Efficient and equivalent inhibition of mTOR and PI3K in human cell lines was demonstrated by blockade of the phosphorylation of S6KT389 (IC50= 70 nM) and AKTS473 (IC50= 21 nM). SB2312 inhibits the proliferation of a broad range of human tumor cell lines independently of their PI3K pathway mutation status (IC50= 112–882 nM) and is also effective against rapamycin-resistant cell lines. SB2312 has very favorable physicochemical and pharmacokinetic properties, with high oral bioavailability and tumor exposure. Efficient and sustained inhibition of PI3K and mTOR was demonstrated in tumor tissues from a PC3 tumor xenograft model, along with dose-dependent anti-tumor activity at very well tolerated doses. Tumor growth inhibition was 53%, 105%, and 117% at 11, 33, and 80 mg/kg SB2312, respectively, without any body weight loss at any of these doses.

Summary: Herein, we report SB2312, a novel, potent and selective dual mTOR/PI3K inhibitor with excellent drug-like properties. Our studies show superior and sustained blockade of the mTOR/PI3K pathway and high antitumor efficacy at well tolerated doses.

Citation Information: Mol Cancer Ther 2009;8(12 Suppl):B138.