Abstract 5871: Discovery and characterization of firmonertinib, a novel EGFR inhibitor with broad activity against both EGFR classical and exon 20 insertion mutations

Abstract Background: Lung cancers harboring classical EGFR mutations are sensitive to approved EGFR tyrosine kinase inhibitors (TKIs) while those harboring EGFR exon 20 insertion mutations (ex20ins) show minimal sensitivity to approved EGFR TKIs and have limited treatment options. Firmonertinib is an investigational novel orally bioavailable and highly brain-penetrant irreversible EGFR TKI targeting classical and uncommon EGFR mutations including ex20ins. Firmonertinib received FDA Breakthrough Therapy Designation for first-line treatment of patients with advanced NSCLC with EGFR ex20ins mutations, and has now completed enrollment in a global phase 3 trial in these patients (NCT05607550). Here, we describe the structural and preclinical findings supporting the binding potency and anti-tumor activity of firmonertinib in EGFR ex20ins mutations. Methods & Results: Using high resolution crystal structures, we uncover a unique structural attribute that differentiates firmonertinib from other EGFR TKIs and that provides firmonertinib with superior binding to mutant EGFR proteins including those harboring ex20ins. Our biochemical and cellular assay findings are in line with the structural observations and demonstrate that firmonertinib is a potent and mutant selective EGFR inhibitor. In a large cell line panel evaluating EGFR ex20ins mutations within different regions of exon 20 (far and near loop, helical domain), all ex20ins mutations evaluated demonstrated a high sensitivity profile to firmonertinib compared to other EGFR TKIs. In addition, firmonertinib was equally highly potent against far and near loop insertion mutations which is a differentiating characteristic from some EGFR TKIs previously tested in the clinic (Elamin et al. 2022). In animal models, our findings demonstrate that firmonertinib is a highly potent inhibitor of cell proliferation in tumors harboring both near and far loop EGFR ex20ins mutations. Conclusions: Findings from this study support the structural mechanism of EGFR binding and inhibition by firmonertinib, eliciting strong anti-tumor activity in multiple in vitro and in vivo models harboring EGFR ex20ins mutations. These findings provide scientific support for the ongoing global Phase 3 study for first-line NSCLC patients with EGFR ex20ins mutations (NCT05607550). Citation Format: Huibing Luo, Qing Li, Qiufeng Liu, Huayong Zhou, Yuting Sun, Jerry Y. Hsu, Luna Musib, Marcin Kowanetz, Stuart Lutzker, Zineb Mounir. Discovery and characterization of firmonertinib, a novel EGFR inhibitor with broad activity against both EGFR classical and exon 20 insertion mutations [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 5871.