Abstract 1963: Combination of rapamycin with sorafenib as a therapeutic strategy against colorectal cancers with mutational activation of KRAS and PIK3CA
Authors: Gulhati Pat , Zaytseva Yekaterina , Weiss Heidi , Gao Tianyan , Dong Jianli , Evers B Mark 
Published: August 2014

Abstract
Both PI3K/Akt/mTOR and Ras/Raf/ERK (MAPK) signaling pathways are constitutively activated and serve critical functions in the growth of colorectal cancer (CRC). Rapamycin inhibits the kinase activity of mTORC1 but leads to feedback activation of Akt, resulting in cell survival and chemoresistance. Sorafenib is a multikinase inhibitor that inhibits Raf kinase and subsequently MAPK signaling. Recent studies suggest that inhibition of mTORC1 with rapamycin leads to feedback induction of MAPK signaling through a S6K-PI3K-Ras feedback loop. Therefore, we investigated whether combination of rapamycin with sorafenib has therapeutic benefit in CRC. METHODS. HCT116 and DLD-1 human CRC cells (mutant K-RAS and PIK3CA) were treated with either rapamycin (50 nM) or sorafenib (5 µM) alone or in combination. Effects on cell proliferation (measured by cell counting), apoptosis (measured by detecting levels of histone-associated DNA fragments in mono- and oligonucleosomes) and subcutaneous growth in athymic nude mice were analyzed. RESULTS. (i) pAktSer473 and pERKThr202/Tyr204 are overexpressed together in stage IV CRC tissues and in matched liver metastasis tissues from the same patient (Spearman's correlation=0.77; p&lt;0.0001). (ii) Inhibition of mTORC1 (using rapamycin or targeted shRNA-mediated inhibition of mTORC1 proteins, mTOR and Raptor) leads to strong feedback activation of MAPK signaling. (iii) Combined treatment with rapamycin and sorafenib prevents feedback induction of both PI3K/Akt and MAPK signaling pathways. (iv) Rapamycin or sorafenib significantly decreased proliferation in cells with mutational activation of KRAS and PIK3CA; combinatorial treatment significantly potentiated inhibition of proliferation compared to single agent treatment. (v) Combination treatment significantly increased induction of apoptosis compared to single agent treatment in cells with mutational activation of KRAS and PIK3CA. (vi) Rapamycin or sorafenib significantly decreased growth of HCT116 xenografts while combinatorial treatment significantly potentiated growth inhibition compared to single agent treatment. CONCLUSIONS. Combined treatment with rapamycin and sorafenib potentiates attenuation of growth and proliferation compared to monotherapy in CRCs with mutational activation of both KRAS and PIK3CA. This effect may be due to reduced feedback activation of PI3K/Akt and MAPK signaling. CRC is a heterogeneous disease with multiple aberrant signaling pathways driving the oncogenic behavior of tumors. Simultaneous inhibition of parallel signaling pathways may be more effective than single pathway inhibition in CRC therapies.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1963. doi:10.1158/1538-7445.AM2011-1963