Abstract 3761: Efficacy of the ATP-competitive mTOR inhibitor AZD8055 in PTEN-wild type cancer cells

Abstract

Abstract Although the PI3K/Akt/mTOR pathway is frequently hyperactivated in human cancers due to the loss of function of the PTEN tumor suppressor, a proportion of cancer cells still carries wild type PTEN and thus the PI3K/Akt/mTOR pathway in these cells may remain unaltered. While it has been widely accepted that these cancer cells are less susceptible to the allosteric mTOR inhibitor rapamycin, it is unclear whether this is also the case for an ATP-competitive mTOR inhibitor. To address this question we evaluated the effects of the mTOR kinase inhibitor AZD8055 in a panel of wild type PTEN expressing cancer cells (6 cell lines of 4 tumor types). The phosphorylated Akt (S473) level was undetectable in most of these cells, suggesting the regular mTOR complex 1 (mTORC1) pathway and low mTOR complex 2 (mTORC2) activity. Cell proliferation was determined by WST-1 (MTT) assay and clonogenic survival was tested by colony formation assay. AZD8055 suppressed cell proliferation at the nanomolar level in a dose dependent manner, with IC50 values in a range of 6∼35 nM for different cell types. In contrast, rapamycin exhibited limited anti-proliferative effect, with less than 50% of inhibition observed even with higher doses in most cell lines. Similarly, clonogenic capacity was impaired by AZD8055 to a greater extent than was the case for rapamycin. To detect the mTOR pathway alterations by drug treatment, two cell lines with different expression levels of mTORC1 downstream pathways were selected: murine mammary tumor 4T1 cells (low phosphorylated S6 and high phosphorylated 4E-BP1) and human breast cancer MDA-MB-231 cells (normal phosphorylated S6 and moderate phosphorylated 4E-BP1). While the phospho-S6K (T389) and phospho-S6 (S235/236) pathway was potently inhibited by both agents, AZD8055 displayed more striking inhibition of the hierarchical 4E-BP1 phosphorylation (T37/46, T70 and S65) than rapamycin in both cells. In addition, both rapamycin and AZD8055 slightly increased Akt phosphorylation (T308 or S473), suggesting that the activation of the feedback loop controlled by S6K was functional. However, the PI3K inhibitor wortmannin did not sensitize cells to either agent, implying such feedback-dependent activation is dispensable for attenuating the drug response. Taken together, these findings indicate that AZD8055 significantly inhibits growth and survival of cancer cells that have wild type PTEN and normal PI3K/Akt/mTOR signaling, an observation mainly associated with the comprehensive activation of 4E-BP1. These data provide in vitro evidence in support of broadening the application of ATP-competitive mTOR inhibitors in cancer treatment. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3761. doi:1538-7445.AM2012-3761