Abstract A077: BGB3245-mediated RAF dimer disruption combined with MEK inhibition suppresses ERK reactivation in BRAF-mutant and ERK-dependent glioblastoma models

Abstract Background: RAF inhibitors have significantly advanced the therapeutic landscape for patients with BRAFV600-mutant cancers; however, their clinical efficacy is constrained by emergent reactivation of ERK signaling through a variety of mechanisms, many involving increased RAF dimerization. This reactivation limits the durability of responses and contributes to therapeutic resistance. Methods: BGB3245, a novel RAF dimer inhibitor, was evaluated in a range of preclinical glioma models including neurosphere and adherent lines with a variety of MAPK pathway activating mutations (BRAF-V600E, NF1), those without clear ERK dependence (EGFR amp., PTEN, CDKN2A/B), and BRAF-V600E lines resistant to first generation BRAF inhibitors. BGB3245 was evaluated alongside two mechanistically distinct BRAF inhibitors with different selectivity profiles: tovorafenib (DAY101), a type II pan-RAF inhibitor that targets both monomeric and dimeric forms of wild-type and mutant BRAF and CRAF proteins, and plixorafenib (PLX8394), a mutant-specific BRAF paradox breaker affecting monomeric and dimer-inducing mutations. Proliferation was evaluated by Incucyte Live Cell Imager (S3) and synergy by SynergyFinder. Protein activity was assessed via immunoblot following treatment. Efficacy was evaluated in vivo in a patient-derived xenograft (PDX, BRAF-D594G, Class III BRAF mutant) and in orthotopic PDX models with a primary endpoint of survival and secondary endpoints of tumor volume and proliferation. Results: BGB3245 demonstrates robust inhibition of ERK-signaling and proliferation in BRAF-mutant models, with higher potency in BRAF-V600E mutant lines as compared to type I inhibitor resistant lines or BRAF-WT lines. To evaluate the benefits of enhanced vertical inhibition strategies, BGB3245 and other RAF inhibitors were evaluated as monotherapy or in combination with the MEK inhibitor mirdametinib. Combination studies with mirdametinib revealed profound phospho-ERK inhibition and anti-proliferative effects from BGB3245 as compared to other RAF inhibitors across multiple glioma lines, including models with acquired BRAF inhibitor resistance. The combination of BGB3245 and mirdametinib showed synergy across all tested models and induced apoptosis in some lines, while sparing normal human astrocyte growth. In heterotopic PDX models, the combination significantly reduced tumor growth and inhibited downstream ERK phosphorylation compared to vehicle or single-agent treatment. Orthotopic intracranial PDX studies showed limited therapeutic efficacy, indicating suboptimal brain penetration of BGB3245. Conclusion: These findings support dual pan-RAF and MEK inhibition as a rational strategy for GBM subtypes addicted to ERK signaling, particularly in BRAF-mutant gliomas. Pan-RAF inhibitors may overcome some forms of resistance, though brain-penetrant RAF-dimer inhibitors are needed for optimal clinical translation. Citation Format: Abiola A. Ayanlaja, Kriti Lalwani, Shreya Jagtap, Isaiah Hanson, Andrew Baker, Christine A. Pratilas, Karisa C. Schreck. BGB3245-mediated RAF dimer disruption combined with MEK inhibition suppresses ERK reactivation in BRAF-mutant and ERK-dependent glioblastoma models [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2025 Oct 22-26; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2025;24(10 Suppl):Abstract nr A077.