Abstract 3940: Small molecule profiling uncovers the landscape uncovers the landscape of combinational PI3K inhibitor responses in HNSCC
Authors: Michmerhuizen Nicole Lynn , Matovina Chloe , Leonard Elizabeth , Harris Micah , Heenan Caitlin , Herbst Gabrielle , Foltin Susan K. , Kulkarni Aditi , Carey Thomas E. , Bradford Carol R. , Jiang Hui , Brenner Chad 
Published: August 2018

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 shown limited clinical efficacy as monotherapies. To identify factors that might predict sensitivity and resistance to PI3K inhibitors, we tested a panel of more than 20 patient-derived HNSCC cell lines and observed varying sensitivity to PI3K inhibitors. While all cell lines were much more sensitive than fibroblasts to these drugs, responses varied widely; for pan-PI3K inhibitor BKM120, IC50 values ranged from 0.4 to 7.4 uM across the OSCC cell line panel. Responses to BKM120 not correlate with genetic or phenotypic features, including: PIK3CA mutation status copy number, RNA expression, p110a protein levels, and AKT phosphorylation at baseline and after inhibitor treatment.
In order to characterize potential mechanisms of PI3K inhibitor resistance in HNSCC, we have also developed and optimized an unbiased, high-throughput screening approach. We have used this assay to test ~1400 inhibitors as monotherapies and in combination with PI3K inhibitors HS-173 and BKM120 in several HNSCC models. Our initial screening data suggested that the combination of PI3K inhibitor and EGFR/HER2 inhibitor afatinib is synergistic in a subset of cell lines. After testing this combination in additional models, we identified some cell lines that were responsive to the combination of HS-173 and afatinib, but not HS-173 and either EGFR inhibitor gefitinib or HER2 inhibitor CP-724714. In Detroit 562 cells, combination treatment with 0.25 uM HS-173 and 2.5 uM afatinib resulted in 35% more trypan positive cells than either drug alone. This also induced significantly more trypan positivity than combinations of 0.25 uM HS-173 with 5 uM gefitinib or 5 uM CP-724714. Likewise, the same HS-173 + afatinib treatment resulted in 35% or greater increases in Annexin V positivity compared to gefitinib and CP-724714 combinations. Western blot analysis demonstrated that these doses of gefitinib, CP-724714 and afatinib all inhibited target after 20 minute treatment in this cell line.
Future studies will also examine additional factors that may predict PI3K inhibitor responses and will seek to better understand promising combination treatments and advance them for effective use in HNSCC patients.
Citation Format: Nicole Lynn Michmerhuizen, Chloe Matovina, Elizabeth Leonard, Micah Harris, Caitlin Heenan, Gabrielle Herbst, Susan K. Foltin, Aditi Kulkarni, Thomas E. Carey, Carol R. Bradford, Hui Jiang, Chad Brenner. Small molecule profiling uncovers the landscape uncovers the landscape of combinational PI3K inhibitor responses in HNSCC [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3940.