Abstract 6389: Unveiling the potent anti-tumor activity and underlying mechanism of action of the novel pan-RAF inhibitor exarafenib in BRAF-mutated NSCLC

Abstract Somatic BRAF alterations occur in up to 5% of non-small cell lung cancer (NSCLC) cases and comprise three mechanistically distinct molecular subtypes: Class I alterations (e.g., V600E) function as constitutively active monomers, Class II alterations that signal as RAS-independent mutant BRAF homodimers, and Class III alterations that activate MAPK signaling via RAS-dependent mutant BRAF/wild-type RAF heterodimers. While BRAF V600E (Class I) mutations are effectively targeted by approved therapies that are active against these oncogenic BRAF monomers, there are no established targeted treatments for both Class II and III BRAF mutations that operate as BRAF dimers in NSCLC. Analysis of the GuardantINFORM™ database revealed that these untreatable Class II and III BRAF mutations comprise 65% of BRAF-mutant NSCLC cases, with Class II mutations associated with significantly worse survival compared to Class I (median survival 514 vs 818 days, HR: 1.4, P=0.0395). Here we evaluate exarafenib, a novel type 2 pan-RAF inhibitor designed to target both monomeric and dimeric forms of RAF proteins by binding the αC-helix-IN and DFG-out conformations. Through comprehensive mechanistic studies, we demonstrate that while exarafenib induces RAF dimerization, it maintains potent inhibition of both monomeric and dimeric RAF proteins with high selectivity for RAF kinases, leading to effective suppression of downstream MAPK pathway signaling. In preclinical studies, exarafenib showed robust anti-tumor activity in BRAF-mutant NSCLC cell lines and patient-derived xenograft models, with favorable pharmacokinetic and pharmacodynamic properties. Clinical case studies revealed promising responses, including a partial response and sustained disease control for over two years in a patient with BRAF G469S mutation, and a significant initial response lasting approximately 32 weeks in a patient with TMEM106B-BRAF fusion. Through analysis of acquired resistance models, we identified RAS activation as a key resistance mechanism and biomarker that was accompanied by sustained MAPK pathway signaling. Based on this mechanism, we evaluated combination strategies and found that co-targeting MEK with binimetinib provided synergistic anti-tumor effects both in vitro and in vivo, while maintaining tolerability. These mechanistic insights provide strong biological rationale for the ongoing Phase 1 trial (KN-8701; NCT04913285) evaluating exarafenib as monotherapy and in combination with binimetinib in BRAF-altered solid tumors including NSCLC and NRAS-mutant melanoma, offering a potential therapeutic strategy for patients that currently lack effective targeted treatment options. Citation Format: Tadashi Manabe, Tim Sen Wang, Paul Severson, Nichol Miller, Catherine Lee, Elifnur Yay Donderici, Nicole Zhang, Yu-ting Chou, Daniel L. Kerr, Wei Wu, Kathryn B. Grandinetti, Liliana Soroceanu, Robert J. Pelham, Eric S. Martin, Eric A. Murphy, Vishesh Khanna, Joel W. Neal, Christopher T. Chen, Shumei Kato, Richard Williams, Trever Bivona. Unveiling the potent anti-tumor activity and underlying mechanism of action of the novel pan-RAF inhibitor exarafenib in BRAF-mutated NSCLC [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 6389.