Abstract 2661: Bilateral blockade of MEK- and PI3K-driven pathways is effective in the treatment of KAS mutant mucinous colorectal cancer cells

Purpose. A significant proportion of sporadic colorectal cancers (CRCs) (10-15%) are of the mucinous subtype (>50% of the tumor). Mucinous colorectal adenocarcinomas (MCAs) are clinically, morphologically and molecularly distinct from nonmucinous CRCs as they show a different spectrum of genetic alterations (higher KRAS mutations, MSI-H, lower p53, high MUC2) and exhibit more aggressive behavior (more prone to peritoneal dissemination and lymph node involvement). MCAs are associated with poorer response to chemotherapy and have limited treatment options. The purpose of this study was to test the effectiveness of combinatorial targeting of two KRAS-associated parallel pathways in an attempt to counter adaptive resistance associated with single pathway inhibition. Methods. Two KRAS mutant MCA cell lines LS174T and RW7213 were treated with two small molecule inhibitors, GDC0973 and GDC0941, to block mitogen-activated extracellular regulated kinase kinase (MEK) and phosphoinositide 3-kinase (PI3K) respectively. Adaptive resistance of these two cell lines to the PI3K inhibitor (PI3Ki) was examined by protein phospho-receptor tyrosine kinase (RTK) arrays, western blots and immunocytochemistry. Results. Bioinformatic analysis of the colon cancer datasets in the cancer genome atlas (TCGA) showed increased mutation rates in mucinous over nonmucinous CRCs for effectors of RAS-RAF-MEK-ERK (80% versus 41.9%) and PI3K-AKT-mTOR (60% versus 21.8%) pathways. Both MCA cell lines were sensitive to MEK inhibitor (MEKi) treatment. In contrast, these cell lines, though initially sensitive to PI3Ki, later became resistant. Concomitant with the development of resistance were a) increase in phosphorylated RTKs including IR, IGF-1R, EGFR, ErbB2 b) increase in phosphorylated ERK and AKT, c) nuclear translocation of the transcription factor FOXO3A and d) a decrease in proapoptotic protein BIM. Combinatorial treatment of PI3Ki and MEKi synergistically reduced viability of MCA tumor cells with attendant decrease in phosphorylated ERK and AKT and increase in BIM. Conclusions. Our data suggest cellular dependence (‘addiction’) of KRAS-mutant mucinous CRC cells on hyperactivation of MEK and PI3K pathways. Interestingly, PI3K single agent inhibition initially triggers pathway downregulation and reduced tumor cell viability but later leads to development of adaptive resistance. Our studies suggest this resistance involves dynamic rewiring of signaling circuits mediated through FOXO3A nuclear localization, relief of RTK inhibition, MEK-ERK activation and BIM reduction. Adaptive resistance however, can be overcome by co-targeting of PI3K and MEK. Our studies thus provide a rational basis for MEKi and PI3Ki combination therapy for KRAS mutant CRCs. Citation Format: Murali R. Kuracha, Peter Thomas, Brian W. Loggie, Venkatesh Govindarajan. Bilateral blockade of MEK- and PI3K-driven pathways is effective in the treatment of KAS mutant mucinous colorectal cancer cells. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2661. doi:10.1158/1538-7445.AM2015-2661