Abstract B71: Multiplexed kinase inhibitor beads identify multiple pathways of resistance to PI3K inhibition facilitating the rational selection of novel combination therapies in pancreatic cancer

KRAS mutational activation plays a critical role in tumorigenesis, but exactly which downstream KRAS effector pathway is critical for this role remains less clear. One of the most studied downstream pathways is the phosphatidylinositol-3-kinase (PI3K) pathway which mediates cellular metabolism, growth, and survival. We have previously shown using validated pancreatic ductal adenocarcinoma (PDAC) patient derived xenograft (PDX) mouse models that treatment with BKM120 (a pan-class 1 PI3K inhibitor, currently in Phase I/II clinical trials) resulted in tumor growth inhibition (p = 0.017) but not regression. In this study, we evaluated possible mechanisms of resistance to BKM120 therapy. We developed a novel multiplex inhibitor bead/mass spectrometry (MIB/MS) assay to measure the activation state of the kinome. We have recently shown that kinome response to kinase inhibitor therapy (i.e. kinome reprogramming) is a potential mechanism of resistance in triple negative breast cancer in response to MEK inhibition and in drug-resistant leukemia in response to imatinib. We hypothesized that kinome reprogramming may play a role in resistance to PI3K inhibition in PDAC and used MIB/MS to identify second targets that may be used in combination with PI3K inhibitors. We found kinome reprogramming in response to PI3K inhibition through both previously known as well as less studied pathways. For instance, we found MEK1 and MEK2 activation in response to BKM120 treatment. MEK activation has previously been implicated in resistance to PI3K inhibition and combined MEK and PI3K inhibition has been shown to be synergistic in PDAC. However, our results suggest that resistance to PI3K may be mediated through many more pathways than MEK alone. We found ErbB1, ErbB2 and ErbB3 activation in response to BKM120 treatment in both cell lines and PDX tumors. ErbB2 and ErbB3 activation has previously been noted in response to PI3K inhibition in breast cancer. However, no studies have evaluated this combination in PDAC. Thus we evaluated the effect of combined PI3K and pan-ErbB inhibition in a panel of 10 PDAC cell lines using BKM120 and dacomitinib (a pan-ErbB inhibitor currently in Phase III clinical trials). Combined treatment with BKM120 and dacomitinib inhibited proliferation in 10 of 10 PDAC cell lines evaluated. This combination showed impressive synergy across all cell lines with a mean combination index of 0.24 (0.00245 – 0.49). MIB/MS is a powerful unbiased approach to identify second targets for combination therapy. We identified both known and novel kinase pathways that may mediate resistance to PI3K inhibition in PDAC. Our results suggest that pan-ErbB inhibition may be a promising second target in combination with PI3K inhibition in PDAC. Combination studies in PDX models are ongoing. Pan-ErbB and PI3K inhibition in PDAC may be more effective than either single agent alone and should be considered in clinical trials. Citation Format: Christopher J. Tignanelli, Jeran Stratford, Richard Moffitt, Rachel Reuther, Gary L. Johnson, Jen Jen Yeh. Multiplexed kinase inhibitor beads identify multiple pathways of resistance to PI3K inhibition facilitating the rational selection of novel combination therapies in pancreatic cancer. [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Innovations in Research and Treatment; May 18-21, 2014; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2015;75(13 Suppl):Abstract nr B71.