| dc.description.abstract | EGFR-mutant lung cancers are highly sensitive to EGFR tyrosine kinase inhibitors (TKIs; erlotinib/gefitinib/afatinib), but tumors develop drug resistance within 9-16 months. Resistance to gefitinib/erlotinib commonly occurs via a second-site EGFR mutation, T790M. Two strategies to overcome T790M+ resistance are mutant-specific EGFR TKIs, such as AZD9291, and dual inhibition of EGFR with afatinib plus the anti-EGFR antibody, cetuximab (A+C). Unfortunately, ‘second-line’ acquired resistance to A+C and AZD9291, after ‘first-line’ acquired resistance to erlotinib/gefitinib/afatinib, also occurs. To prevent/delay resistance to AZD9291, the combination of AZD9291 plus selumetinib (MEK1/2 inhibitor; AZD6244/ARRY-142886) is also currently being tested in a Phase I clinical trial (NCT02143466). The effects of sequential and combination treatment with various anti-EGFR agents on tumor evolution and drug resistance are largely unknown. In these studies, we modeled drug resistance pre-clinically to: 1. Assess the heterogeneity of mechanisms of first-line resistance to erlotinib and afatinib 2. Determine the optimum order of treatment with A+C vs. AZD9291 in the setting of T790M+ EGFR-mutant lung tumors 3. Elucidate mechanisms of first- and second-line acquired resistance to AZD9291 and 4. Elucidate mechanisms of resistance to AZD9291 plus selumetinib. Next-generation sequencing of genomic DNA from cell line models of resistance to erlotinib/afatinib revealed multiple potentially functional genomic changes within a given pool of resistant cells (including T790M). We also found that AZD9291 is more potent than A+C at inhibiting cell growth in the setting of T790M+ resistance to erlotinib. A+C-resistant cell lines remain sensitive to AZD9291, but AZD9291-resistant cell lines are cross-resistant to A+C. Resistance to AZD9291 is associated with dysregulation of MAPK signaling and can be overcome by addition of the MEK 1/2 inhibitor, selumetinib. Finally, AZD9291 plus selumetinib-resistant cell lines display increased baseline phospho-MEK/ERK and are sensitive to in vitro treatment with an ERK inhibitor, SCH772984 or alternative MEK inhibitor, trametinib. These studies provide a more comprehensive understanding of how EGFR-mutant tumors undergo rewiring of their signaling circuitry in response to single-agent EGFR- and combined EGFR+MEK-inhibition. This work, emphasizing a mechanistic understanding of the effects of therapies on tumor evolution, provides a framework for future clinical trials testing different treatment sequences. | |
