Abstract 5734: Designing RAF and MEK inhibitor combinations based on their biochemical properties to effectively target MAPK-driven cancers

Abstract Oncogenic alterations of the RAS/RAF/MEK/ERK signaling pathway drives growth of over 40% of human tumors, frequently due to activating mutations of components of the pathway, such as RAS or BRAF. Targeting the pathway using BRAF inhibitors (BRAFi) in combination with MEK inhibitors (MEKi) is now the standard of care for metastatic melanoma patients harboring the BRAF(V600E) mutation. However, effective targeted therapies are lacking for most cancers driven by other pathway alterations, including those with RAS activating mutations. The effectiveness of current clinical BRAFi relies on the fact that they bind selectively and inhibit the mutationally activated monomeric form of the BRAF(V600E) kinase in tumors but not wild-type BRAF in normal tissue where BRAF signals in its dimeric form. Yet, the development of adaptive resistance, frequently due to RAF dimerization, limits BRAFi therapeutic effectiveness. Another class of BRAFi equipotent for both monomeric and dimeric BRAF have been developed, but are predicted to have lower therapeutic index due to inhibition of dimeric wild-type BRAF in normal tissues as well. Recently, we identified and characterized a novel class of BRAFi that preferentially binds and inhibits dimeric BRAF over monomeric BRAF(V600E) (i.e. RAF dimer-selective inhibitors). Furthermore, we rationally designed a combinatorial approach utilizing a BRAF monomer-selective plus a BRAF dimer-selective inhibitor plus a MEK inhibitor, that potently disrupts the BRAF/MEK complex, to maximally inhibit BRAF(V600E) signaling in tumors while retaining a broad therapeutic index. This triple combination potently suppressed tumor growth of multiple BRAF(V600E) colorectal cancer and melanoma models resistant to the current clinical BRAFi and MEKi combinations both in vitro and in vivo. To design effective therapeutic approaches for RAS-mutated tumors, we also assessed the effectiveness of RAF and MEK inhibitors belonging to diverse biochemical and structural classes, either alone or in combination in RAS-mutant tumor models and normal cells with wild-type RAS signaling. We found that RAF dimer-selective inhibitors potently antagonized MAPK signaling and growth in various RAS mutant cancer models as compared to normal cells. We found further that RAF and MEK inhibitor combinations belonging to certain biochemical classes resulted in more potent MAPK suppression in RAS-mutant tumor as compared to wild-type RAS normal cells (“therapeutic synergy”) and should be prioritized for clinical testing. In contrast, those combinations which are more potent in normal over tumor cells (“adverse synergy”) are unlikely to be clinically successful. Thus, rationally designed combinations of selected RAFi and MEKi, based on their distinct biochemical properties, can uncover new therapeutic opportunities tailored for more effective targeting of BRAF or RAS mutant tumors. Citation Format: Ana Orive-Ramos, Christos Adamopoulos, Beau Baars, Jason Lam, Ankita Punetha, Jason Huang, Vasileios I. Petrou, Stuart A. Aaronson, Poulikos I. Poulikakos. Designing RAF and MEK inhibitor combinations based on their biochemical properties to effectively target MAPK-driven cancers. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5734.