Abstract 964: Updated results of a prospective observational multicenter study on resistance mechanisms to osimertinib using circulating tumor DNA analyses in EGFR-mutated non-small cell lung cancer patients (ELUCIDATOR study)

Abstract Background: Although osimertinib (Osi) is the key standard therapy for patients (pts) with advanced non-small cell lung cancer (NSCLC) harboring sensitizing epidermal growth factor receptor (EGFR) mutations, most of cases experience the progressive disease (PD) after initial response to the treatment. Therefore, understanding the Osi resistance mechanisms and identifying the poor responders are essential to develop better treatment strategies. Thus, we conducted a prospective observational multicenter study and analyzed serially collected circulating tumor DNA (ctDNA). Here we report the results from updated analyses focusing on the association between genetic information from ctDNA and clinical outcome. Methods: Patient eligibility criteria were as follows: a definitive diagnosis of non-squamous NSCLC confirmed through biopsy or cytology, EGFR exon 19 deletion (19del) or L858R mutation, Osi planned as first-line treatment, and pts who could provide blood specimens. Paired plasma DNA samples were collected before starting Osi as baseline (BL) and at PD and analyzed to investigate Osi resistance by next-generation sequencing (NGS). Correlations between genetic information obtained from ctDNA and clinical outcome such as progression free survival (PFS) and overall survival (OS) were evaluated. The median follow-up time was 33. 1 months. Results: One hundred eighty-eight pts were enrolled between May 2019 and January 2021, and 125 (66%) were female, 96 (51%) were EGFR 19del mutation, and 109 (58%) were never smoker. Among those, 178 pts were included in the analyses, and 111 experienced PD. In consistent with previous reports, median PFS (mPFS) and median OS (mOS) were 19. 1 months and 36. 0 months, respectively. The efficacy of Osi differed between 19del and L858R in both PFS and OS, with 19del being longer outcome mPFS, 23. 3 months vs. 17. 2 months, HR: 1. 56 (95%CI: 1. 08-2. 26), mOS, 40. 3 months vs. 30. 4 months, HR: 1. 54 (95%CI: 0. 98-2. 45). Univariate analysis revealed that the presence of TP53 mutations and MET amplification (amp) at BL was a predictive factor of shorter PFS and OS. Multivariate analysis also revealed that the TP53 mutations at BL was a predictive factor of shorter PFS and OS. Plasma samples from 85 pts were available for ctDNA analysis. New adaptive mutations or amp at PD were detected in genes such as PIK3CA in 3 cases, MET amp in 4, TP53 in 4, BRIMP3 in 2, APC in 1, and BRAF in 2. Secondary EGFR C797S resistance mutation was detected in 1 case. Conclusions: TP53 mutation at BL could be predictive of poor response to Osi. MET amp and PIK3CA mutations were detected as the most common resistance mechanisms, and secondary EGFR resistance mutations were observed less frequently than previous reports. Citation Format: Mitsuo Osuga, Akihiro Tamiya, Daijiro Harada, Yasuyuki Mizumori, Shun-ichi Isa, Yoshihiko Taniguchi, Keiichi Nakamura, Tsutomu Shinohara, Hidetoshi Yanai, Katsumi Nakatomi, Masahide Oki, Masahide Mori, Tomohito Kuwako, Koji Yamazaki, Atsuhisa Tamura, Masahiko Ando, Yasuhiro Koh. Updated results of a prospective observational multicenter study on resistance mechanisms to osimertinib using circulating tumor DNA analyses in EGFR-mutated non-small cell lung cancer patients (ELUCIDATOR study) abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 964.