Abstract C111: RAS-mutant tumor-selective inhibition of dimeric RAF by a novel Type 1 RAF inhibitor

Abstract Over a third of human tumors are driven by deregulated RAS/MAPK signaling, frequently by mutations in RAS or BRAF. Current clinical RAF inhibitors bind in the Type 1.5 mode (αC-OUT/DFG-IN). They are effective in inhibiting monomeric BRAF(V600X), while inducing paradoxical RAF and MAPK activation in contexts where RAF signals as dimer, including wild-type and RAS-Mutant (RAS-MT) settings. Thus, these inhibitors are only active against BRAF(V600X), and paradoxical MAPK activation in normal cells enables vertical MAPK-targeting combinations with MEK or EGFR antagonists. These combinations improve both efficacy and tolerability, as they additively suppress MAPK in tumor cells, while they restore physiological levels of MAPK signaling in normal cells. To target cancers driven by dimeric RAF, including RAS-MT tumors, Type 2 (αC-IN/DFG-OUT) RAF inhibitors have been developed that bind both RAF monomers and dimers. These drugs inhibit RAF in all contexts, including normal cells, with minimal MAPK activation. However, Type 2 RAF inhibitors showed modest results as single agents, and their subsequent combination with MEK inhibitors led to increased efficacy, but also enhanced toxicities due to MAPK pathway suppression in normal tissue. Here, we characterized ELV-3111, a novel potent and selective Type 1 (αC-IN/DFG-IN) RAF inhibitor with broad activity in various RAS-MT models, both in cells and in vivo, similar to Type 2 RAF inhibitors. However, unlike Type 2 inhibitors, ELV-3111 induced pronounced paradoxical MAPK hyperactivation selectively in normal cells and tissues in vivo, but not in vitro. Because in vitro systems lack physiological drug clearance, and prior studies did not examine pathway responses in normal tissues, this dynamic went unnoticed with earlier, less potent and selective Type 1 RAF inhibitors. Using an array of biochemical and in-cell target engagement assays, we discovered that MAPK hyperactivation by Type 1 RAF inhibitors occurs via a RAS-dependent, allosteric mechanism that is distinct from that of Type 1,5 RAF inhibitors that we previously described. Importantly, MEKi blocked this ELV-RAFi-induced activation in normal tissues, resulting in normal levels of MAPK signaling, whereas ELV-RAFi and MEKi synergized to potently suppress MAPK signaling in RAS-MT tumors. In antitumor efficacy and tolerability studies, the combination of ELV-RAFi and MEKi potently inhibited MAPK signaling and RAS-MT tumor growth in vivo, with improved tolerability compared to either agent alone. By paradoxically activating MAPK signaling in normal tissues, Type 1 RAF inhibitor mitigates MEK inhibitor-induced toxicities, while in RAS-MT tumors, the two drugs synergize to suppress the pathway. This tumor-selective mechanism, previously exploited in BRAF-MT cancers, can now be extended to RAS-MT-driven tumors, offering a renewed therapeutic opportunity and the potential to reshape combination strategies across a broader spectrum of MAPK-driven cancers. Citation Format: Poulikos Poulikakos. RAS-mutant tumor-selective inhibition of dimeric RAF by a novel Type 1 RAF inhibitor [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 C111.