Impact of T790M Mutation Status on Later-Line Osimertinib Treatment in Non-Small Cell Lung Cancer Patients

Background: Osimertinib is a third-generation epidermal growth factor receptor tyrosine kinase inhibitor (EGFR-TKI) designed to overcome acquired T790M resistance mutations in non-small cell lung cancer (NSCLC). However, the efficacy of osimertinib in patients without acquired T790M mutations has not been well studied. This study aimed to evaluate the efficacy of osimertinib in patients treated with first- and second-generation EGFR-TKIs followed by later-line osimertinib treatment. Patients: The clinical data and survival outcomes of 172 patients with advanced NSCLC treated with osimertinib following frontline EGFR-TKIs at Chang Gung Memorial Hospital from 2014 to 2018 were retrospectively reviewed. T790M mutations were detected using tissue sequencing and/or liquid biopsy. Results: A total of 172 EGFR-mutated NSCLC patients treated with frontline EGFR-TKI therapy followed by later-line osimertinib were enrolled in the current study and divided into three groups based on the T790M status (positive, negative, or unknown T790M). Patients with NSCLC harboring acquired T790M mutation treated with osimertinib had the best objective response rate (ORR) (52.6%, 25.0%, and 32.0%, p = 0.044), disease control rate (DCR) (79.3%, 41.7%, and 68.0%, p = 0.011), and progression-free survival (PFS, median PFS, 12.6, 3.1, 10.4 months, p = 0.001) among the three groups (positive, negative, and unknown T790M, respectively). However, a marked difference was found between positive and negative T790M mutations but not between positive and unknown T790M mutations. Univariate analysis was performed to identify potential prognostic factors for PFS in 172 patients treated with osimertinib. Lung metastasis (p < 0.001), brain metastasis (p < 0.009), number of metastatic sites (p < 0.001), PFS with frontline EGFR-TKIs (p = 0.03), and T790M status (p = 0.006) were identified as prognostic factors for PFS with osimertinib. Multivariate analysis showed that lung metastasis (p < 0.001) and PFS with frontline EGFR-TKIs and T790M status were independent prognostic factors. Conclusion: This study confirmed the greater efficacy of later-line osimertinib for NSCLC with acquired T790M mutation than for NSCLC without acquired T790M mutation. Detection of the T790M mutation after frontline treatment (first- and second-generation EGFR-TKI) is crucial for prolonging the survival of NSCLC patients harboring EGFR mutation. Osimertinib may be considered an option for NSCLC with unknown T790M mutations, as a certain subpopulation may benefit from osimertinib.