Activity of osimetrinib/AZD9291 in pretreated patients (pts) with epidermal growth factor receptor mutant (EGFRmt) non-small cell lung cancer (NSCLC), according to EGFR mutations detected in circulating plasma DNA (ctDNA)

e20608 Background: Osimertinib is an irreversible EGFR tyrosine kinase inhibitor (TKI), selective for EGFR TKI-sensitizing mutations and the T790M resistance mutation. We sought to determine the activity of osimertinib after progression on EGFR TKIs in pts with T790-negative ctDNA. Methods: A multicenter phase II study ( clinicaltrials.gov NCT02771314) of osimertinib (80 mg daily) was conducted in pts with metastatic EGFRmt NSCLC, who had progressed after previous treatment with EGFR TKIs. Serial serum and/or plasma samples were drawn for ctDNA analysis at enrollment, 1 month and every 3 months of treatment, until disease progression. Efficacy outcomes in pts without the T790M mutation in ctDNA at baseline are reported. Results: Thirty-seven NSCLC patients with undetectable baseline T790M in the plasma have been enrolled. Median age was 67 years, 21.6% were male, and histology was adenocarcinoma in 100%. More frequent adverse events (grade 1/2) included diarrhea (12.5%), fatigue (12.5%), anorexia (12.5%) and acneiform rash (10.4%). The overall response rate (ORR) was 40.5% (95% CI, 24.7-56.4%) and the disease stabilization rate 37.8%; the median progression-free survival (PFS) was 8.9 months (range, 1.6-30), and the estimated median overall survival (OS) 26 months (range, 2.2-30). At enrollment, EGFR mutations del19 and L858R were detected in ctDNA in 9 and 3 pts, respectively. After one month of treatment with osimertinib, EGFR mutations in ctDNA were not detectable in 4/9 and 2/3 of pts with del19 and L858R at baseline, respectively. Pts without detectable EGFRm ctDNA at baseline remained negative throughout the study. Efficacy according to baseline ctDNA status was as follows: Clinical trial information: NCT02771314. Conclusions: Osimertinib was effective in EGFR TKI pretreated pts without EGFR T790M mutation in plasma. Pts with detectable del19 or L858R mutations in ctDNA before treatment had worse clinical outcomes, despite the elimination of EGFRmt ctDNA.[Table: see text]