We hypothesize that many cases of both intrinsic and acquired resistance to single agent targeted cancer drugs occur due to the activation of compensatory signaling pathways. Combination therapies targeting both the primary and compensatory pathways may be able to produce more robust and durable responses. To identify these compensatory pathways and rationally construct combination therapies, we performed a synthetic lethal screen of 420 targeted small molecule inhibitor combinations on 17 cancer cell lines, yielding over 5500 combinatorial tests. We identified both novel and expected synergistic combinations. Among the expected combinations was the combination of inhibitors of EGF Receptor with inhibitors of PI3 Kinase. Because this combination is currently being evaluated in clinical trials, we have focused on examination of the cellular and molecular mechanism(s) of the synthetic lethality. We found that inhibition of PI3 Kinase led to increased ERK activation, and conversely that inhibition of MEK resulted in increased signaling through the PI3 Kinase pathway. Flow cytometric analysis revealed that this synergistic cytotoxicity is both due to a robust apoptotic response to combination treatment measured by caspase 3 cleavage and also a decrease in proliferation upon combination treatment measured by BrdU incorporation. Using Reverse Phase Protein Arrays, we identified phosphorylation of ribosomal protein S6 as a molecular biomarker of synergistic cytotoxicity with these agents. To determine whether this biomarker reflects a mechanistic node in the signaling network, we tested pharmacological inhibition of 70 kD ribosomal protein S6 kinase (p70S6K) and found only modest cytotoxicity, but also robust activation of ERK1 and 2. This increase could be blocked by the addition of a pharmacological inhibitor of the EGF Receptor, suggesting a negative feedback loop between p70S6K and the EGF Receptor. Combined treatment with p70S6K and EGF Receptor inhibitors resulted in synergistic cytotoxicity. Currently, we are identifying the node between EGF Receptor and p70S6K signaling that could be responsible for this synthetic lethality. Additionally, we are conducting xenograft studies using inhibitors of this pathway that are currently in clinical trials, to test the efficacy of this drug combination in vivo.

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 2933. doi:1538-7445.AM2012-2933

©2012 American Association for Cancer Research
2012