Reversible EGFR tyrosine kinase inhibitors (TKIs) such as erlotinib and gefitinib offer significant clinical benefit to patients with EGFR mutation positive NSCLC compared to chemotherapy alone. In nearly all cases resistance emerges after approximately a year on therapy, often due to the EGFR T790M (50-60% of cases). Irreversible pan-HER TKIs, including dacomitnib and afatinib were developed in part to address the emergence of T790M mutations. Both molecules showed promise, but the T790M mutation is still a dominant resistance mechanism and the number and severity of adverse events in the clinic underscore the unmet need for better targeted treatments for NSCLC patients. Third generation TKIs—AZ9291 and CO-1686—were designed against activating EGFR mutations in order to spare the wild type pathway and developed for efficacy in the T790M setting. However, resistance to the new class of EGFR TKIs is inevitable, prompting us to investigate the impact of erlotinib resistance mechanisms on AZ9291 and ask what new mechanisms are involved. NSCLC cell lines harboring activating EGFR mutations with (H1975) or without T790M (HCC4006 and HCC827) were exposed to erlotinib, dacomitnib, or AZ9291 at IC 75 or greater for three months to generate resistance. The PTEN null line H1650 was found to be resistant to all three TKIs without long-term drug exposure, implicating constitutive PIK3 pathway signaling as one mechanism of resistance. In addition, AZ9291 resistance in H1975 resulted in an epithelial to mesenchymal transition (EMT) at the phenotypic level and a corresponding increase in vimentin staining, with at least a 1000-fold increase in IC 50. Preliminary results indicate both acquired and de novo mechanisms account for AZ9291 resistance in H1975. AZ9291 resistance in HCC827 resulted in resistance to erlotinib and dacomitnib, implicating either additional mutations in EGFR or in downstream genes. Expression of E-cadherin was markedly increased in the AZ9291 resistant HCC827 line. An analysis of EGFR mutation status, PTEN status, Met expression, and EMT gene expression and methylation signatures will be presented, and the possible implications for patients will be discussed.

Citation Format: Sarah M. Paul, Xioafen Ye, Zora Modrusan, Somasekar Seshagiri, Robert Yauch, David S. Shames. A comparison of resistance mechanisms to 1st, 2nd, and 3rd generation EGFR TKIs in NSCLC. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Drug Sensitivity and Resistance: Improving Cancer Therapy; Jun 18-21, 2014; Orlando, FL. Philadelphia (PA): AACR; Clin Cancer Res 2015;21(4 Suppl): Abstract nr B25.

©2015 American Association for Cancer Research.
2015