Abstract PR4: Incomplete inhibition of phosphorylation of 4EBP1 is a mechanism for KRAS driven resistance to the ATP-competitive mTOR inhibitor PP242 in colorectal cancer

Abstract

Abstract The mammalian target of rapamycin (mTOR) regulates cell growth by integrating nutrient and growth factor signaling and has been strongly implicated in cancer. But mTOR is not an oncogene, and which tumors will be resistant and sensitive to new ATP-competitive mTOR inhibitors now in clinical trials remains unknown. We screened a panel of 357 human cancer cell lines to identify common markers of resistance and sensitivity to the mTOR inhibitor PP242. Analysis by tissue of origin and genetic mutation revealed colorectal origin as the most statistically significant marker for resistance to therapy, while KRAS and PI3K mutations were the most significant genetic markers for resistance and sensitivity respectively. Notably, KRAS mutations are overrepresented in colorectal cancer (CRC) cell lines. Within CRC cell lines, those with KRAS mutations were most resistant to PP242 while those with other mutations were more sensitive; cell lines with co-mutation of PI3K and KRAS had intermediate sensitivity. Immunoblot analysis of the signaling networks downstream of mTOR revealed that the degree of cellular growth inhibition induced by PP242 was directly correlated with inhibition of phosphorylation of the translational repressor eIF4E binding protein 1 (4EBP1), but not ribosomal protein S6. We were able to recapitulate these observations using a pair of KRAS mutant isogenic cell lines differing only by a PI3K mutation. PP242 treatment of the PI3K mutant cell line increased the observed inhibition of 4EBP1 phosphorylation and improved the drug response. In an efficacy trial of PP242 in primary CRC xenograft tumors, resistance to inhibition of 4EBP1 phosphorylation was again observed in KRAS mutant tumors and these tumors responded poorly compared to KRAS WT controls. We show that, in the absence of PI3K co-mutation, KRAS mutations are associated with resistance to PP242 and that this is specifically linked to changes in the 4EBP1 phosphorylation state in both CRC cell lines and primary patient xenografts. This proffered talk is also presented as Poster A35.