Abstract 3713: Identifying a mechanism of acquired resistance to the combined inhibition of PI3K/mTOR and MEK in colorectal carcinoma

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

Targeting PI3‐kinase (PI3K), AKT and mTOR axis in lymphoma

“…In a colorectal carcinoma cell line, combined treatment with the PI3K/mTOR inhibitor, PF‐04691502, resulted in both loss of PTEN protein as mentioned previously, as well as an acquired somatic mutation in the kinase domain of MTOR (Garza et al , ). In light of this and that MTOR mutations have been described in DLBCL, it is reasonable to presume mutations in this gene may be a common mechanism of resistance to mTOR inhibitor therapy.…”

“…In another study with findings potentially relevant to lymphoma, MYC and eIF4E network upregulation was found to mediate resistance to the combined PI3K/mTOR inhibitor Bez235 (Ilic et al , ). In colorectal and lung cancer, loss of expression of PTEN protein has been shown to be involved in resistance to PI3K inhibitors (Castel et al , ; Garza et al , ), although it remains to be seen whether this mechanism of resistance is limited to drugs which target p110‐α. Finally, acquired amplification of PIK3CA has been demonstrated as a novel mechanism of resistance in a breast cancer cell line treated with pictilisib (GDC‐0941), a drug with both alpha‐ and delta‐isoform selectivity that is currently under investigation in lymphoma (Huw et al , ).…”