Homo sapiens

Resistance to targeted EGFR inhibitors is inevitable in metastatic EGFR mutant lung cancers. Early identification of innate or acquired resistance mechanisms to these agents is essential to direct development of future therapies. Here, we describe the detection of heterogeneous mechanisms of resistance within populations of EGFR mutant cells (PC9 and/or NCI-H1975) with acquired resistance to first, second or third generation EGFR inhibitors. We report the detection of NRAS mutations, including a novel E63K mutation, in 5/-17, and a gain of copy number of WT NRAS in 3/-17, cell populations resistant to AZD9291. In addition, gain of KRAS was detected in populations resistant to gefitinib, afatinib or WZ4002. Compared to parental cells, 19 resistant cell populations were more sensitive to selumetinib inhibition when treated in combination with the originating EGFR inhibitor. The presence of an NRAS modification within a population correlated with increased sensitivity to selumetinib. In vitro, a combination of AZD9291 with selumetinib prevented emergence of resistance in PC9 cells and delayed resistance in NCI-H1975 cells. In vivo, concomitant dosing of AZD9291 with selumetinib caused regression of 2/-33 AZD9291-resistant tumours in an EGFRm+/T790M transgenic model. Our data support the use of a combination of AZD9291 with a MEK inhibitor to delay or prevent resistance to AZD9291 in EGFRm+ and/or EGFRm+/T790M tumours. These findings further suggest that NRAS modifications in tumour samples from patients who have progressed on first or third generation EGFR inhibitors may provide a marker for further treatment with a combination of EGFR and MEK inhibition.