Abstract B10: Acquired resistance to afatinib plus cetuximab in EGFR-mutant lung adenocarcinoma may be mediated by EGFR overexpression and overcome by the mutant-specific EGFR inhibitor, AZD9291.

EGFR mutant lung cancers are highly sensitive to first generation EGFR tyrosine kinase inhibitors (TKIs; gefitinib and erlotinib), but resistance invariably develops. In the majority of patients, disease progression is mediated by a second-site T790M mutation in EGFR. T790M-mediated acquired resistance can be overcome by the combination of the second-generation EGFR TKI, afatinib, with the anti-EGFR monoclonal antibody, cetuximab, in both preclinical models and humans. However, patients still develop acquired resistance to second-line anti-EGFR combination therapy. In order to develop therapeutic strategies for patients whose tumors progress on afatinib/cetuximab, we have modeled resistance using xenografts of PC-9/BRc1 cells (EGFR exon 19 deletion/T790M) treated chronically with the drug combination. Over the course of four months, 4 of 10 animals developed acquired resistance. Xenograft tumors still displayed features of adenocarcinoma only, and mutational analysis did not detect any common secondary hotspot mutations found in lung adenocarcinomas. Analysis of two cell lines derived from resistant tumors by array comparative genomic hybridization (aCGH) suggests that resistant tumors developed additional amplification at the EGFR locus. Fluorescence in situ hybridization (FISH) of all lines confirmed sustained amplification of EGFR in resistant cell lines, and immunoblotting of cell lysates showed increased EGFR protein expression relative to parental cells and vehicle-treated controls. Strikingly, all xenograft-derived cell lines displayed in vitro sensitivity to AZD9291, a third-generation, mutant-specific EGFR TKI. Finally, in vitro comparison of afatinib/cetuximab with AZD9291 in PC-9/BRc1 cells suggests that AZD9291 may even be more potent than afatinib/cetuximab in the setting of T790M-mediated acquired resistance to first and second-generation EGFR TKIs. Confirmatory xenograft experiments are planned, and experiments are ongoing, including with additional cell lines. Collectively, these data suggest that EGFR overexpression may be a mechanism of acquired resistance to afatinib/cetuximab combination therapy, and that resistance may still be overcome by mutant-specific EGFR small molecule inhibitors. Thus, patients with EGFR mutant lung cancers may benefit from sequential lines of therapy (erlotinib->afatinib/cetuximab->AZD9291) targeting EGFR. Further work is needed to discern specific indications for afatinib/cetuximab versus AZD9291 as therapy for patients with acquired resistance to EGFR TKIs. Citation Format: Catherine B. Meador, Hailing Jin, Elisa de Stanchina, Valentina Pirazzoli, Marc Ladanyi, Lu Wang, Xi Chen, Rosana Eisenberg, Darren Cross, William Pao. Acquired resistance to afatinib plus cetuximab in EGFR-mutant lung adenocarcinoma may be mediated by EGFR overexpression and overcome by the mutant-specific EGFR inhibitor, AZD9291. [abstract]. In: Proceedings of the AACR-IASLC Joint Conference on Molecular Origins of Lung Cancer; 2014 Jan 6-9; San Diego, CA. Philadelphia (PA): AACR; Clin Cancer Res 2014;20(2Suppl):Abstract nr B10.