Circulating Tumor DNA T790M Testing as a Predictor of Osimertinib Efficacy in Epidermal Growth Factor Receptor Mutant Non-small Cell Lung Cancer: A Single Center Experience

Background: The main acquired resistance mechanism to first and second-generation epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs) in EGFR mutant non-small cell lung cancer (NSCLC) is the propagation of T790M clones, which can be detected in circulating tumor DNA (ctDNA). Objectives: To analyze osimertinib outcomes according to T790M testing method. Methods: The study comprised 33 consecutive patients with advanced EGFR mutant NSCLC who were diagnosed with a T790M mutation after progression on first- or second-generation EGFR TKIs and treated with osimertinib. The patients were divided into groups A (diagnosed by tumor testing) and B (by ctDNA testing). Osimertinib outcomes were compared between the groups. Results: Objective response rate with osimertinib comprised 54% and 62% in groups A and B, respectively (P = 0.58). Median progression-free survival (PFS) with osimertinib was 8.9 months (95% confidence interval [95%Cl] 1.8-17.5) and 9.1 months (95%Cl 5.3-12.6) in groups A and B, respectively (log-rank test 0.12, P = 0.73). Median overall survival (OS) was 13.8 months (95%Cl 4.9-25.5) and 13.8 months (95%Cl 7.7-27.7) in groups A and B, respectively (log-rank test 0.09, P = 0.75). T790M testing technique did not affect PFS (hazard ratio [HR] 1.16, 95 /0C1 0.50-2.69, P = 0.73) or OS (HR = 1.16, 95%Cl 0.45-3.01, P = 0.76). The proportion of patients diagnosed by ctDNA grew from 56% in 2015 to 67% in 2016-2017. Conclusions: Our study provides a ctDNA validation for the purpose of T790M testing in EGFR mutant NSCLC.