RAF isoform selectivity of MEK inhibitors and rational design of a covalent ARAF-MEK inhibitor.

Aberrant activation of the RAS/RAF/mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinase pathway is a frequent cause of cancer. Allosteric MEK inhibitors block this pathway by binding RAF-MEK complexes to prevent activation of MEK by RAF. However, how MEK inhibitor potency varies across the three RAF isoforms remains poorly understood. We profiled seven allosteric MEK inhibitors and observed a notable hierarchy of sensitivity: All most potently inhibited CRAF-driven MEK activation while relatively sparing ARAF-driven activation. We identified point mutations in ARAF and CRAF proximate to the MEK inhibitor binding site that markedly altered inhibitor sensitivity. Using a rational design approach, we developed a more potent inhibitor of ARAF-driven MEK signaling, TWG-07-148. Our cryo-electron microscopy structure shows how this acrylamide-containing analog of MEK inhibitor trametinib covalently targets Cys514, a residue unique to ARAF. Our studies highlight the importance of the activating RAF isoform as a determinant of MEK inhibitor sensitivity and provide proof of concept for the development of MEK inhibitors that more effectively block ARAF-driven MEK signaling via covalent targeting of Cys514.