Abstract 937: Longitudinal circulating-tumor DNA profiling ofEGFR-mutated non-small cell lung cancer patients treated with EGFR-tyrosine kinase inhibitors

Background: In EGFR-mutant non-small cell lung cancer (NSCLC), progression disease (PD) under 1st-generation EGFR tyrosine kinase inhibitors (EGFR-TKIs) is driven by the EGFR T790M mutation in about 50% of cases. This mutation is targeted with osimertinib, a specific and potent inhibitor that also showed superior efficacy to that of 1st-generation EGFR-TKIs in the 1st-line treatment EGFR+ NSCLC. Strategy: We performed a longitudinal circulating-tumor DNA (ctDNA) analysis to evaluate 1) the dynamics of sensitizing or resistant mutations over time and 2) the genomic alterations associated with EGFR-TKI resistance, in NSCLC patients treated with either 1st-generation EGFR-TKIs (n=24) or with osimertinib (n=20). ctDNA sequencing was performed using InVisionSeq™, an amplicon-based NGS (36-gene panel) assay. Results: Sensitivity was 80% and 100% for the detection of EGFR-activating mutations and the EGFR T790M mutation, respectively, at PD in ctDNA vs tissue biopsies. The type of progression appears to influence the detection of EGFR mutations in plasma. Patients (pts) with exclusively brain or thoracic PD had significantly lower allelic fractions (AFs) of EGFR-activating mutations than pts with systemic PD (P=0.006). AFs of both EGFR-activating and EGFR T790M mutations were highest in patients with progressive metastases in liver or bone. High cell-free DNA load was associated with the presence of TP53 mutations regardless of the time under treatment (n=248 samples). The total number of mutations detected in ctDNA was higher in pts treated with osimertinib vs. 1st-generation EGFR-TKIs (P=0.004), suggesting a potential increase of tumor heterogeneity over time. We observed that the AFs of mutant ctDNA were correlated with response to treatment. Complete clearance of EGFR T790M (AF detection limit: 0.01%) was observed in pts who presented a partial response to osimertinib (n=17), of these, EGFR-activating mutations were found in 4/17 pts (AF:0.05-1.03%). PD was evidenced in 36/44 pts. Emergence of mechanisms of resistance, before the confirmation of clinical PD, was evidenced in 5 and 3 pts under erlotinib and osimertinib, respectively. PD to 1st-generation EGFR-TKIs was mainly driven by EGFR T790M (86%), followed by mutations in PIK3CA or PTEN. We detected the emergence of heterogeneous potential mechanisms of resistance to osimertinib in 3/9 cases, including mutations in EGFR, KRAS, BRAF as well as HER2 and MET amplification. We did not identify genomic drivers of resistance in 16/36 pts, despite evidence of clinical PD. These pts had either exclusive brain or thoracic progression. ctDNA WES will be performed in these samples. Conclusion: Our results suggest that ctDNA sequencing might be a complementary, noninvasive tool to monitor response to treatment and heterogeneous mechanisms of resistance in NSCLC pts treated with EGFR inhibitors. Citation Format: Sandra Ortiz-Cuaran, Aurelie Swalduz, Camille Leonce, Solene Marteau, Severine Martinez, Gilles Clapisson, Virginie Avrillon, Luc Odier, Lionel Falchero, Pierre Fournel, Emma Green, Clive Morris, Maurice Perol, Pierre Saintingy. Longitudinal circulating-tumor DNA profiling of EGFR-mutated non-small cell lung cancer patients treated with EGFR-tyrosine kinase inhibitors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 937.