Tumor-selective dimeric and monomeric RAF targeting with a next-generation Type 1 RAF inhibitor

Therapeutically silencing the RAS/MAPK signaling cascade, an oncogenic driver in more than one-third of human cancers, is constrained by a fundamental trade-off: potent pathway inhibition in tumors versus dose-limiting toxicities in normal tissues. BRAF-mutant (BRAF-MUT) cancers are a notable exception, where current clinical RAF inhibitors (RAFis) (Type 1.5 - alphaC-OUT/DFG-IN) selectively inhibit monomeric BRAF(V600X), while paradoxically activating MAPK signaling pathway in settings where RAF signals as a dimer, including wild-type and RAS-Mutant (RAS-MUT) contexts. While this paradoxical activation limits the broader applicability to BRAF-MUT tumors, it has been therapeutically exploited in vertical MAPK-targeting combinations with MEK or EGFR inhibitors, enhancing antitumor efficacy while restoring physiological MAPK signaling in normal tissues, achieving an improved therapeutic window and enhancing tolerability.To target dimeric RAF-driven tumors, including RAS-MUT tumors, Type 2 (alphaC-IN/DFG-OUT) RAFis were developed to engage both RAF monomers and dimers. However, as single agents, Type 2 RAFis showed only modest activity. Combining them with MEK inhibitors improved efficacy but also exacerbated toxicities due to MAPK pathway suppression in normal tissues, limiting dosing and ultimately constraining therapeutic benefit.Here, we characterized ELV-3111, a next-generation, highly potent and selective Type 1 RAFi with broad activity across BRAF class I/II/III, CRAF, and RAS-MUT models, including contexts resistant to current MAPK-targeted therapies. Unlike Type 2 RAFis, ELV-3111 induces robust paradoxical MAPK hyperactivation selectively in normal tissues - a phenomenon we successfully modeled in cells. Using complementary biochemical and live-cell assays, alongside molecular dynamics simulations, we demonstrate that this MAPK hyperactivation occurs via a RAS-dependent allosteric mechanism distinct from the paradoxical activation described for Type 1.5 RAFis. This unique property can be therapeutically exploited. Combining ELV-3111 with a MEK inhibitor overcomes the therapeutic ceiling of MAPK pathway targeting by creating a pharmacologically complementary interaction: additive suppression in tumors, where both agents inhibit MAPK signaling, and opposing effects in normal tissues, where MEK inhibition counteracts RAFi-driven hyperactivation. This configuration produced profound and durable regressions across RAS- and BRAF-MUT models, including a RAS-MUT model refractory to current therapies, while maintaining favorable tolerability. This tumor-selective mechanism, previously exploited in BRAF-MUT cancers, can now be extended to RAS-MUT and other dimeric RAF-driven tumors, offering a renewed therapeutic opportunity and the potential to reshape combination strategies across a broader spectrum of MAPK-driven cancers.