Abstract LB083: BRAF dimer-selective inhibitors synergize with BRAF monomer-selective inhibitors to overcome adaptive resistance and retain therapeutic index
Authors: Adamopoulos Christos , Ahmed Tamer , Ung Peter M. , Xiao Min , Aaronson Stuart , Ang Celina , Rebecca Vito , Schlessinger Avner , Herlyn Meenhard , Poulikakos Poulikos I. 
Published: June 2022

Abstract
BRAF kinase, a critical component of the RAS/MAPK signaling pathway, requires dimerization for its catalytic activity, whereas the oncoprotein BRAF(V600E) can be catalytically active as a monomer. Current clinical BRAF inhibitors selectively bind and inhibit monomeric over dimeric BRAF (i.e. “BRAF monomer-selective”) providing a high therapeutic index and are now standard practice in combination with MEK inhibitors in the treatment of patients harboring BRAF(V600E) tumors. However adaptive resistance due to RAF dimerization limits their effectiveness. Recently, RAF inhibitors that equipotently inhibit both monomeric and dimeric BRAF have been developed, but they are predicted to have lower therapeutic index due to inhibition of dimeric wild-type BRAF in normal tissues. We identified and characterized a novel class of BRAF inhibitors that preferentially bind and inhibit dimeric over monomeric BRAF(V600E) (i.e. “BRAF dimer-selective inhibitors”). Biochemical analysis using BRAF dimer-selective inhibitors revealed that the two forms of dimeric and monomeric BRAF(V600E) differ in their conformation in their active site and their strength of interaction with MEK. Further, to maximally inhibit BRAF(V600E) signaling in tumors while retaining a broad therapeutic index, we assessed the triple combination of BRAF monomer- plus BRAF dimer selective inhibitor plus a MEK inhibitor that disrupts the BRAF-MEK complex. The triple combination potently suppressed tumor growth in multiple colorectal and melanoma BRAF(V600E) cell line- and in vivo models that were resistant to the current clinical BRAF and MEK inhibitor combination. Strikingly, while the double combination treatment with the BRAF dimer-selective inhibitor and MEK inhibitor was associated with a significant gradual weight loss in vivo, the triple combination showed no weight loss or other apparent toxicities, indicating a higher Therapeutic Index. Finally, off-label use of the triple combination in patients with advanced BRAF(V600E) cancers that progressed on standard therapies achieved durable tumor control with minimal toxicities. Thus, a rationally designed combinatorial approach of conformation-selective BRAF and MEK inhibitors may be a highly effective and well tolerated therapeutic strategy for patients with BRAF(V600E) tumors. Overall, our work supports the notion that for most tumors a “3-drug” combinatorial strategy will potentially be most effective: one drug targeting MAPK signaling directly, such as a MEK inhibitor (“pathway” inhibitor), one drug targeting mutated BRAF or RAS selectively in the tumor (“therapeutic index” inhibitor, e.g. BRAF monomer-selective inhibitor, RAS(G12C) inhibitors, etc) and one drug targeting components of the feedback loop that is responsible for adaptive resistance (“feedback” inhibitor, e.g. BRAF dimer-selective inhibitors, SHP2 or SOS inhibitors, etc). Our data contribute to the development of a roadmap for the treatment of MAPK-driven tumors using drug combinations tailored to the specific driver oncoprotein.
Citation Format: Christos Adamopoulos, Tamer Ahmed, Peter M. Ung, Min Xiao, Stuart Aaronson, Celina Ang, Vito Rebecca, Avner Schlessinger, Meenhard Herlyn, Poulikos I. Poulikakos. BRAF dimer-selective inhibitors synergize with BRAF monomer-selective inhibitors to overcome adaptive resistance and retain therapeutic index [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr LB083.