A spatial-trapping mechanism of RAF by RAF/MEK glue enables full-dose combination with a pan-RAF inhibitor and drives potent, RAS-mutant tumor-selective MAPK and growth inhibition

The therapeutic benefits of MAPK pathway inhibitors depend on achieving more potent pathway inhibition in tumors over normal tissues, maintaining a broad therapeutic index. In BRAF-mutant cancers, selective BRAF inhibitors (BRAFi) achieve this balance, and their combination with MEK inhibitors (MEKi) at full doses resulted in significant clinical benefit. In contrast, next-generation Type 2 dimeric RAF, also known as pan-RAF inhibitors (pan-RAFi), which target wild-type BRAF and CRAF and demonstrate single-agent activity in RAS-mutant (RAS-MUT) tumors, have failed to achieve comparable therapeutic impact in combination with MEKi, owing to toxicity-driven dose reductions that constrain clinical efficacy. In preclinical and clinical studies, these combinations have reached a therapeutic ceiling, yielding predominantly stable disease and only rare tumor regressions. We show that dose-limiting toxicity in pan-RAFi + MEKi combinations stems from relief of negative feedback by MEK inhibition, which amplifies RAF activation and pan-RAFi engagement in normal tissues, oversuppressing MAPK signaling and limiting the therapeutic index. RAF/MEK glues constitute a distinct category of MEK inhibitors that stabilize RAF-MEK complexes and functionally suppress RAF. Although structural studies have captured these glues bound to both active and inactive RAF conformations, we show that their inhibitory activity stems from a spatial-trapping mechanism: we found that MEK is constitutively cytosolic, RAF/MEK glues sequester RAF in the cytosol, blocking its membrane recruitment and dimerization, steps essential for activation. In line with this mechanism, the RAF/MEK glue avutometinib, when combined with a pan-RAFi, was well tolerated at full dose and, critically, drove tumor regressions across multiple RAS-MUT models, achieving a 90% ORR compared with 0% using a conventional pan-RAFi + MEKi regimen in an insensitive RAS-MUT model. Tumor regressions corresponded with deeper MAPK pathway suppression by both pharmacodynamic and transcriptional metrics, without increased toxicity. Together, these findings reveal an unrecognized mechanism of RAF inactivation by RAF/MEK glues and show that substituting the MEKi with a RAF/MEK glue can overcome the therapeutic ceiling of current MAPK-targeting regimens, shifting outcomes from mostly stable disease to frequent tumor regressions. More broadly, this work establishes drug-induced proximity as a means to increase tumor selectivity by reprogramming the spatial and biochemical state of wild-type signaling effectors, thereby widening the therapeutic window for oncogenic pathway inhibition and providing a new paradigm for precision oncology.