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Preferred first-line treatment for patients with metastatic EGFR-mutant lung cancer is osimertinib, yet it is not known if patient outcomes may be improved by identifying and intervening upon molecular markers associated with therapeutic resistance.All patients with metastatic EGFR-mutant lung cancer treated with first-line osimertinib at Memorial Sloan Kettering Cancer Center (n=327) were identified. Available pre-treatment and post-progression tumor samples underwent targeted gene panel sequencing and mutational signature analysis using SigMA algorithm. Progression-free and overall survival were estimated using Kaplan Meier methods.Using multivariate analysis, baseline atypical EGFR (mPFS 5.8 mo, p0.001) and concurrent TP53/RB1 alterations (mPFS 10.5 mo, p=0.015) were associated with shorter progression-free survival on first-line osimertinib. Of 95 patients with post-progression biopsies, acquired resistance mechanisms were identified in 52% (off-target n=24, histological transformation n=14, on-target n=12), with MET amplification (n=9), small cell lung transformation (n=7) and acquired EGFR amplification (n=7) the most frequently identified mechanisms. While there was no difference in post-progression survival based on identified resistance (p=0.07), patients with subsequent second-line therapy tailored to post-progression biopsy results had improved post-progression survival (HR 0.09, p=0.006). Paired post-progression tumors had higher tumor mutational burden (p=0.008) and further dominant APOBEC mutational signatures (p=0.07) compared to pre-treatment samples.Patients with EGFR-mutant lung cancer treated with first-line osimertinib have improved survival with treatment adaptation based on identified mechanisms of resistance at time of progression using tissue-based genomic analysis. Further survival gains may be achieved using risk-based treatment adaptation of pre-treatment genomic alterations.