Abstract 3713: Identifying a mechanism of acquired resistance to the combined inhibition of PI3K/mTOR and MEK in colorectal carcinoma
Authors: Garza Scott J. , Lira Paul , Huang Stephen D. , Cheng Hengmiao , Dann Stephen G. , VanArsdale Todd L. , Fantin Valeria , Christensen James , Kan Julie L. 
Published: May 2015

Abstract
The phosphatidylinositol 3-kinase/mechanistic target of rapamycin (PI3K/mTOR) pathway and the mitogen-activated protein kinase (MAPK) signaling cascades are central facilitators of cellular growth, proliferation, survival and motility within a variety of tumor lineages. To date, several ongoing oncology clinical trials are currently evaluating the benefit of combining PI3K with MEK inhibition. While this dual combination is well founded pre-clinically, investigating potential mechanisms of resistance and identifying novel biological signatures of sensitivity is of great clinical importance. We sought to establish a model of acquired resistance by evaluating the co-treatment of a colorectal carcinoma model with inhibitors of PI3K/mTOR inhibitor (PF-04691502) and MEK (PD-0325901). We can associate the onset of acquired resistance in this model to re-engagement of signaling within the PI3K/mTOR pathway correlative to a loss of PTEN protein expression , relative to parental cells. In addition, next-generation sequencing (NSG) comparing parental versus resistant cells identified a unique somatic “gatekeeper” mutation located in the kinase domain of mTOR. Furthermore, a cell proliferation screen evaluating potential cross resistance was analyzed using a PI3K/mTOR inhibitor with a chemical scaffold distinct from PF-502 (PF-05212384). Treatment of PF-384 as a single agent (IC50 ∼150nM) circumvented the acquired resistance derived from the combination of PF-502+ PD-901 (IC50: ∼2uM and ∼334nM respectfully). Inhibition of growth observed with PF-384 correlated with rescuing of PI3K/mTOR signaling as measured by substrate phosphorylation readouts. Finally, co-crystal structure modeling of mTOR was used to evaluate the potential interaction of PF-502 within the ATP binding pocket and we confirm that this interaction is hindered due to the point mutation identified at mTOR. Furthermore, co-crystal modeling of mTOR with PF-384 illustrates an independence and lack of PF-384 binding to the mutated site and thus substantiates a mechanistic explanation for overcoming PF-502 acquired resistance. In summary, we have identified and characterized a novel mechanism of resistance to the combined treatment of a Pyrimidinone class PI3K/mTOR inhibitor in addition to a MEK inhibitor, and have shown that rescuing of this resistance can be achieved through the use of a Triazine class PI3K/mTOR clinical candidate.
Citation Format: Scott J. Garza, Paul Lira, Stephen D. Huang, Hengmiao Cheng, Stephen G. Dann, Todd L. VanArsdale, Valeria Fantin, James Christensen, Julie LC Kan. Identifying a mechanism of acquired resistance to the combined inhibition of PI3K/mTOR and MEK in colorectal carcinoma. [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 3713. doi:10.1158/1538-7445.AM2014-3713