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Abstract 2956: Mechanisms of resistance for osimertinib for patients withEGFR-mutant lung cancer: MD Anderson Cancer Center single institution experience with osimertinib resistance

PURPOSE: Osimertinib was initially approved for treating epidermal growth factor receptor (EGFR) T790M positive non-small cell lung cancer (NSCLC), and recently is moving forward as the first-line treatment for EGFR sensitizing mutation positive NSCLC. However, the types and frequencies of resistance mechanisms to osimertinib have been incompletely described, and currently there are no guidelines for managing these patients. We evaluated clinical and genetic characteristics of patients who received osimertinib for EGFR-mutant NSCLC. We also provide treatment experience after progression on osimertinib. PATIENTS AND METHODS: Using the MD Anderson Lung Cancer Moon Shot GEMINI database, we identified patients treated with osimertinib and performed clinical outcome analysis. Molecular profiling analysis was performed at the time of progression when available. RESULTS: Eighty two patients were identified. Median PFS on osimertinib was 8.8 months (95% CI, 6.7 to 10.9). Fifty three of the 82 patients had disease progression on osimertinib. Among them, 30 had molecular profiling upon progression. Other than two cases with germline T790M, 11 cases preserved and 17 cases lost T790M mutation. In T790M preserved cases, tertiary mutations of C797S (5 cases) and L792H (2 cases) in EGFR gene, as well as MET amplification (4 cases), were the most common mechanisms of resistance. In T790M lost cases, 12 cases were without known resistance mechanisms. Upon progression, osimertinib was continued in 34 of 51 patients and 17 (50%) of them received local consolidation radiation. The osimertinib continued patients had longer PFS2 compared to the patients who discontinued on osimertinib upon first progression. CONCLUSION: T790M loss is common (61%) in osimertinib resistance cases. In T790M-preserved cases, tertiary mutation within EGFR was the most common mechanism of resistance. MET amplification was observed in both T790M preserved and loss cases. In T790M loss cases, the mechanisms of resistance were mostly EGFR independent, highlighting the importance of understanding T790M-negative resistance mechanisms in the osimertinib era. Citation Format: Xiuning Le, Marcelo Vailati Negrao, Monique Nilsson, Jacqulyne Robichaux, Emily Roarty, Waree Rinsurongkawong, Bonnie Glisson, Jianjun Zhang, John V. Heymach. Mechanisms of resistance for osimertinib for patients with EGFR -mutant lung cancer: MD Anderson Cancer Center single institution experience with osimertinib resistance [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2956.