Abstract 2925: Mechanism-based combinatorial treatment of KRAS mutant colorectal cancer with MEK and PI3K pathway inhibitors
Authors: Clarke Paul A. , Roe Toby , Workman Paul 
Published: May 2015

Abstract
Treatment of metastatic colorectal cancer remains a considerable hurdle as typically &lt;20% of patients respond to available targeted drugs. The most advanced treatment options for these patients are anti-EGFR therapies. However, it is now accepted that tumors with KRAS mutations will not respond to these therapies. Genomic aberrations, including KRAS mutation, frequently activate the MAPK and PI3K pathways in cancer, and combined inhibition of these pathways are now being trialed widely in the clinic. We used a panel of 28 colorectal cancer cell lines to explore combinations of PI3K (PI3Ki) and MEK inhibitors (MEKi). As expected BRAF mutant tumors were sensitive to allosteric MEKi. KRAS/BRAF mutant tumors were resistant to PI3K pathway inhibitors compared to wild-type cells, while tumors with PIK3CA and KRAS/BRAF mutations had intermediate sensitivity. PI3Ki induced a robust G1 cell cycle arrest in sensitive cells and a potent autophagic response in less sensitive lines. MEKi induced a cytostatic response. Combination of MEKi with PI3K pathway inhibitors of different chemotype and selectivity demonstrated that class I PI3Ki were potent inducers of synergy with MEK inhibition. Combination of the MEKi with mTOR inhibitors was at best additive, while class I PI3K/mTOR inhibitors were borderline synergistic/additive with MEKi. Normal non-transformed colon cell lines did not show synergy, which was in part due to a lack of response to the MEKi. We selected a combination of GDC0941 (class I PI3Ki) with GDC0973 (MEKi) to explore the mechanism behind the synergistic interaction. A potent combinatorial effect was observed at the level of FOXO-regulated transcription. Regulators of apoptosis were affected both at the transcriptional and post-translational level by the combination, and this resulted in switching from cytostasis/autophagy to an apoptotic response. Gene knockout and a siRNA screen targeting apoptotic regulators identified a dual requirement for BIM and BBC3 BH3-proteins that acted through activation of BAX and abrogation of MCL1/BCL-XL to drive apoptosis. Continuous exposure to the combination resulted in acquired resistance, through resistance to the MEKi. This resistant line retained sensitivity to an ERK inhibitor and a synergistic interaction when combined with GDC0941. Overall, our data suggest that combination of MEK and class I PI3Ki may be a treatment option for the particularly hard to treat KRAS mutant colorectal cancers. Resistance to targeted agents is major challenge in clinic and may be overcome by combinatorial treatments. Intriguingly, our observations indicate that, as with single agent treatments, cancer cells can also evolve resistance to combinatorial regimes. This suggests that combined treatment with two targeted agents may not be sufficient to overcome resistance and that combinations of multiple (&gt;2) agents or intermittent/alternating treatment strategies will be required.
Citation Format: Paul A. Clarke, Toby Roe, Paul Workman. Mechanism-based combinatorial treatment of KRAS mutant colorectal cancer with MEK and PI3K pathway inhibitors. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2925. doi:10.1158/1538-7445.AM2014-2925