OA10.07 Resistance Mechanisms of Osimertinib in Chinese Non-Small Cell Lung Cancer Patients: Analysis from AURA17 Trial

Osimertinib is approved for metastatic NSCLC patients with EGFR T790M mutation after progression from TKI therapy. Despite impressive tumor responses, drug resistance usually develops. The resistance mechanisms of osimertinib are emerging but studies with large cohorts of Chinese patients and association with clinical outcomes are lacking. Here we report a biomarker study of osimertinib using plasma samples from 107 Chinese patients who had progressed by 24 months after LSFD (Oct. 2017) of AURA17 (NCT02442349), the 2nd-line pivotal trial in China. Serial plasma cell-free DNA (cfDNA) were collected from baseline until progressive disease (PD) by investigator assessment. Capture-based 75-gene NGS panel with unique molecular index (UMI) system was used to identify resistance mechanisms to osimertinib by comparing paired cfDNA at baseline and PD. Droplet digital PCR (ddPCR) was used to dynamically monitor EGFR mutation changes (L858R, Ex19Del, T790M and C797S) during treatment course. Association of cfDNA biomarkers based on valid test results with objective response rate (ORR), progression-free survival (PFS) and overall survival (OS) from DCO3 (Mar. 20, 2018) was analyzed. The 107 patients were with ORR of 68.2%, median PFS of 8.2 months, and median OS of 21.5 months. Eight-two had detectable EGFR sensitizing mutations (L858R or Ex19Del) in their PD cfDNA samples. Among them, 15 had acquired EGFR C797S, all in cis with T790M, and with no enrichment for L858R or Ex19Del (6 and 9, respectively). The median time of C797S detection from plasma was 2.8 (1.2-8.4) months prior to PD. EGFR L718Q, I744T, C775Y, G796S/D, T854I mutations, or amplification were found in 11 patients. Aberrations in bypass tracks including AKT2, ALK, DDR2, ERBB2/3, HRAS, JAK1/2, KRAS, MET, NTRK1, PIK3CA, RIT1, etc. were observed in 45 patients. Clearance of EGFR sensitizing mutations at weeks 3 of treatment was associated with favorable ORR (78.7% vs. 33.3%), PFS (9.6 vs. 4.0 months, p<0.001) and OS (21.5 vs. 11.7 months, p<0.001). Clearance of EGFR sensitizing mutations at weeks 6 of treatment was also associated with favorable ORR (80.0% vs. 36.8%) and PFS (8.3 vs. 4.2 months, p<0.001). Presence of T790M at PD was correlated with longer PFS (12.3 vs. 5.5 months, p<0.001) and OS (21.3 vs. 13.2 months, p=0.045). Acquired or enriched TP53 alterations at PD were associated with worse PFS (4.2 vs. 8.3 months, p=0.008). Our study revealed diverse resistance mechanisms to osimertinib in Chinese NSCLC patients and urged for new drug discovery or combination strategies to overcome this clinical challenge.