EGFR T790M and C797S Mutations as Mechanisms of Acquired Resistance to Dacomitinib

Introduction: Dacomitinib was superior to gefitinib in terms of progression-free survival in patients with EGFR-mutant lung cancer in a recent ARCHER 1050 trial. However, despite a marked initial response, lung cancers eventually acquire resistance to these inhibitors. This study aimed to elucidate the mechanisms of acquired resistance to dacomitinib in vitro. Methods: Dacomitinib-resistant clones were established by exposure to fixed concentrations of dacomitinib by using N-ethyl- N-nitrosourea (ENU) mutagenesis or by chronic exposure to increasing concentrations of dacomitinib without ENU. EGFR secondary mutations were analyzed by Sanger sequencing. Time to resistance in each clone was compared according to the mutational status. EGFR Del19, L858R, and G719A mutations were introduced into Ba/F3 cells by using retroviral vectors. Results: Chronic exposure to dacomitinib without ENU induced T790M in Ba/F3 cells expressing Del19. ENU mutagenesis resulted in 171 dacomitinib-resistant clones. Among these clones, 90% acquired T790M. However, C797S occurred in 11% of L858R-mutant clones (four of 35) and in 24% of G719A-mutant clones (12 of 38) established by using low-dose dacomitinib. Time to resistance was not significantly different between T790M- and C797S-mutant clones in both of L858R clones (p = 0.93) and G719A clones (p = 0.86). Cells expressing Del19 that acquired T790M were sensitive to osimertinib, whereas cells with L858R plus C797S mutations were sensitive to gefitinib or erlotinib. Conclusions: These in vitro data demonstrate that dacomitinib can directly induce T790M or C797S secondary mutations. Our data suggest the importance of analyzing these secondary mutations because appropriate selection of EGFR inhibitors could overcome acquired resistance to dacomitinib in a subset of lung cancers. (C) 2018 International Association for the Study of Lung Cancer. Published by Elsevier Inc. All rights reserved.