Acquired Resistance to the Mutant-Selective EGFR Inhibitor AZD9291 Is Associated with Increased Dependence on RAS Signaling in Preclinical Models

Resistance to targeted EGFR inhibitorsis likely to develop in EGFR mutant lung cancers. Early identification of innate or acquired resistance mechanisms to these agents is essential to direct development of future therapies. We describe the detection of heterogeneous mechanisms of resistance within populations of EGFR mutant cells (PC9 and/or NCI-H1975) with acquired resistance to current and newly developed EGFR TKIs including AZD9291. We report the detection of NRAS mutations, including a novel E63K mutation, and a gain of copy number of WT NRAS or WT KRAS in cell populations resistant to gefitinib, afatinib, WZ4002 or AZD9291. Compared to parental cells, a number of resistant cell populations were more sensitive to inhibition by the MEK inhibitor selumetinib(AZD6244; ARRY-142886) when treated in combination with the originating EGFR inhibitor. In vitro, a combination of AZD9291 with selumetinibprevented emergence of resistance in PC9 cells and delayed resistance in NCI-H1975 cells. In vivo, concomitant dosing of AZD9291 with selumetinibcaused regression of AZD9291-resistant tumours in an EGFRm/ T790Mtransgenic model. Our data support the use of a combination of AZD9291 with a MEK inhibitorto delay or prevent resistance to AZD9291 in EGFRm and/or EGFRm/ T790Mtumours. Further, these findings suggest that NRAS modifications in tumour samples from patients who have progressed on current or EGFR inhibitorsin development may support subsequent treatment with a combination of EGFR and MEK inhibition.