Background: To overcome resistance to EGFR TKI osimertinib in EGFR-mutated lung cancer is critically needed. We performed a biomarker study utilizing serially collected plasma samples in a phase I study to investigate the safety and efficacy of the combination of osimertinib and afatinib in patients who failed osimertinib (jRCTs051180008), in which a total of 13 patients were enrolled and the overall response rate and the median progression-free survival were 7.7% and 2.4 months, respectively, to access the trajectory of the mutational landscape and its association with the efficacy.

Methods: Peripheral blood samples were collected from the patients in K2 EDTA vacutainers (Becton Dickinson) before and four weeks after administration of the combination therapy and upon disease progression. Plasma was isolated immediately after the blood draw and stored at -80℃ until use. Cell-free DNA (cfDNA) was extracted from the plasma and used for next-generation sequencing library construction using AVENIO ctDNA Surveillance Kit (Roche Diagnostics) to assess genetic alterations in 197 cancer-related genes. Sequencing was performed on Nextseq500 system (Illumina) followed by mutation analyses using an Avenio Oncology Analysis Server (Roche Diagnostics).

Results: Mutations in EGFR gene were detected in 10 out of 13 cfDNA samples (77%) including T790M/cis-C797S mutations before treatment and among those harboring T790M/cis-C797S mutations, 2 patients had progressive disease (PD) and 1 had stable disease (SD), indicating little benefit from the combination therapy in this genotype. Although other mutations in genes such as TP53 and/or KRAS were detected before treatment, no relevant associations were observed between tumor response or progression-free survival. One patient who had SD harbored CNTN5 mutation at PD, suggesting the potential involvement with the acquired resistance. Early emergence of EGFR G796S mutation observed in one patient after the initiation of the treatment along with basal L858R/T790M mutations may confer early resistance to the treatment. We also observed the emergence of MET amplification in 2 patients and ERBB2 amplification in 1 patient at PD, strongly suggesting the potential mechanisms responsible for resistance.

Conclusions: This signal seeking biomarker study using cfDNA to better understand the association of mutational status and the treatment efficacy of the combined treatment was feasible and informative despite the small-scale study, supporting the advantage of further implementation.

Citation Format: Yasuhiro Koh, Satoru Miura, Jun Oyanagi, Hiroshige Yoshioka, Koichi Azuma, Hidenobu Ishii, Kayoko Kibata, Kenichi Koyama, Shunsuke Teraoka, Yuichi Ozawa, Takaaki Tokito, Toshio Shimokawa, Takayasu Kurata, Nobuyuki Yamamoto, Hiroshi Tanaka. Longitudinal cell-free DNA analysis in phase I study evaluating afatinib in combination with osimertinib in patients who failed prior osimertinib treatment [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 5153.