Abstract 1106: BRAF and EGFR fusion as a novel mechanism of resistance mechanism to Lazertinib, 3rd- generation EGFR-TKI, in EGFR-mutant NSCLC

Background: EGFR-TKI is an established first-line therapy for NSCLC with activating EGFR mutations. Osimertinib, third-generation EGFR TKI, was investigated as a first-in-class drug, but lazertinib(YH25448) was also reported as an outstanding drug which had similar efficacy as osimertinib. Nevertheless, acquired resistance is inevitably developed by third-generation EGFR-TKIs in clinic. Even though EGFR C797S mutation has been identified as a major resistant mechanism, the remaining resistance mechanisms to these TKIs are largely unknown. Methods: To explore the mechanism of resistance to lazertinib, we firstly established drug-resistant cells to lazertinib in vitro using four NSCLC cells including the patient-derived cell line (PDC), patient-derived tumor xenograft cell line (PDTC), and ATCC cell lines. To analyze the changes in global genetic alterations and gene expression of established lazertinib-resistant cell lines(YH1R), whole exome sequencing (WES) and RNA sequencing (RNA-seq) were performed. The effect of EGFR/BRAF fusion was investigated in vitro using established YH1R cell line and various patient-derived tumor xenograft samples. Western blot study was used to investigate EGFR and MAPK cascade signaling pathway. We evaluated antitumor activity of MEK inhibitor and lazertinib combination treatment in lazertinib-resistant NSCLC cells with EGFR/BRAF fusion in vitro and in vivo. Results: RNA-seq data revealed a novel EGFR/BRAF fusion transcript (EGFR Exon 19-BRAF Exon 9, in-frame fusion) in lazertinib-resistant PC9GR_YH1R. The fusion gene consisted of EGFR (Exon 1-19) portion and BRAF kinase domain (Exon 9-18). The EGFR/BRAF fusion mRNA and protein were specifically upregulated in PC9GR_YH1R compared to parental cells. Intriguingly, we detected the EGFR/BRAF fusion gene expression in patient-derived xenografts (YHIM-1045 and YHIM-1035) obtained from patients who experienced acquired resistance to lazertinib. Compared with treatment with either single agent, combination treatment of lazertinib and MEK inhibitor, trametinib or selumetinib, was significantly effective at inhibiting cell proliferation as well as EGFR signaling pathways in PC9GR_YH1R in vitro and in vivo. Conclusion: We found a novel EGFR/BRAF fusion gene, as a key driver of acquired resistant mechanism of lazertinib, in NSCLC cell lines and patient-derived xenografts with acquired resistance to lazertinib. Combination treatment of lazertinib and MEK inhibitor showed a strong antitumor