Lung cancers with acquired resistance to EGFR inhibitors occasionally harbor <i>BRAF</i> gene mutations but lack mutations in <i>KRAS, NRAS,</i> or <i>MEK1</i>

<jats:p> Acquired resistance to EGF receptor (EGFR) tyrosine kinase inhibitors (TKIs) is inevitable in metastatic <jats:italic>EGFR</jats:italic> -mutant lung cancers. Here, we modeled disease progression using <jats:italic>EGFR</jats:italic> -mutant human tumor cell lines. Although five of six models displayed alterations already found in humans, one harbored an unexpected secondary <jats:italic>NRAS</jats:italic> Q61K mutation; resistant cells were sensitive to concurrent EGFR and MEK inhibition but to neither alone. Prompted by this finding and because <jats:italic>RAS</jats:italic> / <jats:italic>RAF</jats:italic> / <jats:italic>MEK</jats:italic> mutations are known mediators of acquired resistance in other solid tumors (colon cancers, gastrointestinal stromal tumors, and melanomas) responsive to targeted therapies, we analyzed the frequency of secondary <jats:italic>KRAS/NRAS/BRAF/MEK1</jats:italic> gene mutations in the largest collection to date of lung cancers with acquired resistance to EGFR TKIs. No recurrent <jats:italic>NRAS</jats:italic> , <jats:italic>KRAS,</jats:italic> or <jats:italic>MEK1</jats:italic> mutations were found in 212, 195, or 146 patient samples, respectively, but 2 of 195 (1%) were found to have mutations in BRAF (G469A and V600E). Ectopic expression of mutant <jats:italic>NRAS</jats:italic> or <jats:italic>BRAF</jats:italic> in drug-sensitive <jats:italic>EGFR</jats:italic> -mutant cells conferred resistance to EGFR TKIs that was overcome by addition of a MEK inhibitor. Collectively, these positive and negative results provide deeper insight into mechanisms of acquired resistance to EGFR TKIs in lung cancer and inform ongoing clinical trials designed to overcome resistance. In the context of emerging knowledge about mechanisms of acquired resistance to targeted therapies in various cancers, our data highlight the notion that, even though solid tumors share common signaling cascades, mediators of acquired resistance must be elucidated for each disease separately in the context of treatment. </jats:p>