Abstract 4092: Targeting compensatory mechanisms of resistance to phosphatidylinositol 3-kinase inhibitors in head and neck squamous cell carcinoma

Abstract

Abstract Recent sequencing studies of head and neck squamous cell carcinomas (HNSCCs) have identified the phosphatidylinositol 3-kinase (PI3K) pathway as the most frequently mutated, oncogenic pathway in this cancer type. Despite the frequency of activating mutations or amplification in PIK3CA (the gene encoding the catalytic subunit of PI3K), targeted inhibitors of PI3K have not shown clinical efficacy as monotherapies. We tested a panel of more than 20 patient-derived oral cavity squamous cell carcinoma (OCSCC) cell lines, which were profiled by Nimblegen V3 exome sequencing, and observed resistance to PI3K inhibitors despite PIK3CA copy number amplification and/or mutation. Of six inhibitors tested, only alpha-isoform selective and pan-PI3K agents were somewhat effective; these inhibitors, despite on-target and downstream activity, did not cause an appreciable reduction in cell viability when administered at submicromolar concentrations. We hypothesized that other oncogenic pathways might still be functional in the presence of PI3K inhibitors and might serve as mediators of this resistance. For example, our group and others have evaluated co-dependence on the Ras-MEK-ERK pathway in OCSCC, showing that this serves as a compensatory mechanism in some models. In order to more fully characterize other novel mechanisms of resistance, we have also developed a high-throughput screening approach that utilizes a resazurin cell viability assay. This screen serves as an unbiased means of testing ~1400 inhibitors as monotherapies and in combination with alpha-isoform selective PI3K inhibitor HS-173 and pan-PI3K inhibitor BKM120 in several OCSCC models. Further validation of “hits” from this screen, which are drugs that are effective in combination but not as monotherapies, may identify additional resistance mechanisms and lead to combination therapies to improve HNSCC patient prognosis. Citation Format: Nicole L. Michmerhuizen, Elizabeth Leonard, Susan K. Foltin, Aditi Kulkarni, Thomas E. Carey, Carol R. Bradford, Hui Jiang, Chad Brenner. Targeting compensatory mechanisms of resistance to phosphatidylinositol 3-kinase inhibitors in head and neck squamous cell carcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4092. doi:10.1158/1538-7445.AM2017-4092