Prospective Observational Study of Treatment Resistance-related Gene Screening Using Plasma Circulating Tumor DNA in Third-generation EGFR-TKI Osimertinib Therapy (Elucidator)

Osimertinib is a third-generation epidermal growth factor receptor (EGFR)-tyrosine kinase inhibitor (TKI) that potently and selectively inhibits EGFR activating and EGFR T790M resistance mutations. Osimertinib was found to be more effective than first-generation EGFR-TKIs in patients with previously untreated advanced non-small-cell lung cancer (NSCLC) harboring EGFR-positive mutations in a prior phase III trial. Osimertinib is, therefore, one of the most important standard therapies for EGFR mutation-positive patients. However, there are few reports about osimertinib resistance mechanisms in first-line EGFR-TKI therapy. Understanding first-line osimertinib resistance mechanisms is essential for future therapeutic strategies in patients with NSCLC with EGFR-positive mutations. To clarify the resistance mechanisms of first-line osimertinib, we proposed to analyze circulating tumor (ct) deoxyribonucleic acid (DNA) by the ultra-sensitive next-generation sequencing method.We aim to collect ctDNA samples from patients with the following key inclusion criteria: histologically or cytologically proven NSCLC, activating EGFR mutation-positive, planned treatment with first-line osimertinib, and written informed consent. Patients with comorbidities, who are deemed unsuitable for participation by an attending physician, would be excluded. We plan to enroll 180 cases and estimate a final analysis of 120 cases following registration and 2-year observation. ctDNA samples are collected at osimertinib treatment initiation, 3 and 12 months later, and disease progression. The key primary endpoint is to clarify the incidence and ratio of osimertinib resistance. The key secondary endpoint is to examine how the quantity of osimertinib resistance-associated mutations detected in ctDNA at treatment initiation influences disease progression.