Abstract 2061: Non-covalent EGFR T790M targeting TKIs inhibit AZD9291 resistant EGFR C797S mutants

Approximately 10-15% of non-small-cell lung cancers (NSCLC) have epidermal growth factor receptor (EGFR) mutations resulting in increased sensitivity to 1 st generation tyrosine kinase inhibitors (TKIs) such as gefitinib and erlotinib. For common mutations in EGFR, treatment with 1 st generation TKIs results in approximately 70% overall response rate, increased progression free survival, and increased quality of life compared to chemotherapy alone. However, resistance to 1 st generation TKIs typically develops within ~12 months, and approximately 55% of patients acquire a secondary mutation in EGFR Exon 20, T790M. To overcome resistance, 2 nd and 3 rd generation covalently binding TKIs targeting T790M mutations have been developed. Recent studies show that approximately 40% of acquired resistance to 2 nd and 3 rd generation TKIs can also occur via a third acquired EGFR mutation at the site of covalent binding, C797S. To date, there are no standard approved targeted therapies for treating EGFR C797S mutant NSCLC. Moreover, with proposed increased use of covalent inhibitors in the first line setting, C797S mutations are expected to become more prevalent and new strategies to overcome therapeutic resistance will be required. To this end, we have generated stable Ba/F3 and HCC827 NSCLC cell lines expressing C797S mutant EGF receptors with common mutations in EGFR including, L858R/T790M/C797S, and Ex19del/T790M/C797S. EGFR mutant cell lines expressing C797S were screened against 1 st , 2 nd , and 3 rd generation EGFR TKIs and cell viability was determined using Cell Titer Glo. Triple mutant cell lines containing T790M and C797S mutations were not sensitive to any 1 st , 2 nd , or 3 rd generation inhibitors with IC 50 values of >10µM, 7.0µM, and 7.6µM, respectively. However Ba/F3 and HCC827 cell lines transfected with EGFR triple mutants were inhibited by non-covalent inhibitors: CUDC-101, an EGFR, HER2 and HDAC inhibitor, and PKC412, a FLT3 inhibitor. In triple mutant Ba/F3 cells, CUDC-101 and PKC412 had IC 50 values of 470nM and 250nM in Ex19del/T790M/C797S EGFR mutants, respectively, and IC 50 values of 690nM and 270nM in L858R/T790M/C797S EGFR mutants. In HCC827 cells, PKC412 had IC50 values of 610nM and 520nM in L858R/T790M/C797S and Ex19del/T790M/C797S triple mutants, respectively. In addition, western blot analysis of triple mutant Ba/F3 cells showed decreased phosphorylation of EGFR in presence of 50nM PKC412 and 500nM of CUDC-101. Moreover, PKC412 partially decreased p-EGFR expression at 50nM in HCC827 triple mutant cell lines. In conclusion, unlike other EGFR TKIs, the non-covalent EGFR inhibitors CUDC-101 and PKC412 inhibit both T790M and C797S EGFR mutants at low concentrations. Currently, we are establishing additional osimertinib resistant HCC827, H1975, PC9 and H4006 cell lines for further in vitro and in vivo studies. Citation Format: Jacqulyne P. Robichaux, Monique Nilsson, John V. Heymach. Non-covalent EGFR T790M targeting TKIs inhibit AZD9291 resistant EGFR C797S mutants [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 2061. doi:10.1158/1538-7445.AM2017-2061