Abstract 349: PI3K inhibitor GDC-0941 synergizes with mTOR inhibitors by blocking negative feedback and causes distinct effects on signaling and apoptosis compared to a dual PI3K/mTOR inhibitor

Abstract

Abstract Background: The PI3K/Akt/mTOR pathway is deregulated in many cancers. and activation of Akt leads to the stimulation of cell survival and proliferation pathways. Akt activation also regulates mTOR to control cell growth in response to growth factors and nutrients. The PI3K pathway is therefore an attractive target for cancer therapy. Multiple targeted inhibitors of PI3K, mTOR and PI3K/mTOR are currently in clinical trials, whileTORC1 inhibitors including rapamycin and analogs are approved. In this study we assessed the consequence of the combination of a PI3K inhibitor, GDC-0941, plus a TORC1 or TORC1/2 inhibitor compared to dual PI3K/mTOR inhibitor, in cancer cell lines with different genetic backgrounds. Method: We analyzed the effects of PI3K, TORC1 or TORC1/2 inhibition alone or in combination on the viability, cell cycle profile and apoptosis of cancer cell lines with varying PI3K, PTEN, K-ras, and LKB status. Effects on signaling were analyzed by measuring the phosphorylation of signaling proteins by reverse phase protein array and other techniques. Effects on viability and apoptosis were assessed, along with a calculation of synergy for combinations. Results: Inhibition of PI3K with GDC-0941 or a dual PI3K/mTOR inhibitor inhibited proliferation of cancer cell lines and in certain tumor lines induced apoptosis. Treatment also resulted in the inactivation of downstream markers including pAKT, pPRAS40 and pS6. TORC1/2 inhibition resulted in a potent inhibition of pS6, however it had differential effects on phospho-AKT and survival proteins pBad and pGSK3b, likely due to the release of the negative feedback regulation. The signaling effects demonstrated by TORC1/2 inhibition also differed from TORC1 inhibition by rapamycin. Inhibition of mTOR resulted in a cytostatic response in all lines tested. Combination of PI3K and mTOR inhibitors (either TORC1 or TORC1/2) caused a significant decrease in cell viability, enhanced apoptosis and caused a greater, sustained knockdown of the signaling pathway. However, treatment with a dual PI3K/mTOR inhibitor led to slight but consistently lower levels of apoptosis than the combination of the separate agents. There were also differences in phosphoprotein status of some of the pathway components. Conclusion: Inhibiting multiple nodes within the same signaling pathway is synergistic and can be an effective strategy to enhance potency in cancer cells. However, combinations of selective targeted agents do not necessarily cause identical effects as a single molecule that inhibit multiple targets simultaneously. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 349.