Development of a Potent Small-molecule Degrader Against Oncogenic BRAF Protein That Evades Paradoxical MAPK Activation

BRAF mutations are frequently observed in melanoma and hairy-cell leukemia. Currently approved rapidly accelerated fibrosarcoma (RAF) kinase inhibitors targeting oncogenic BRAF V600 mutations have shown remarkable efficacy in the clinic, but their therapeutic benefits are occasionally hampered by acquired resistance due to RAF dimerization-dependent reactivation of the downstream MAPK pathway, which is known as paradoxical activation. There is also a concern that paradoxical activation of the MAPK pathway may trigger secondary cancer progression. In this study, we developed chimeric compounds, proteolysis targeting chimeras (PROTACs), that target BRAF protein for degradation. CRBN(BRAF)-24, the most effective chimera, potently degraded BRAF in a ubiquitin-proteasome system (UPS)-dependent manner and inhibited the proliferation of BRAF -driven cancer cells. In BRAF wild-type cells, CRBN(BRAF)-24 induced neither BRAF degradation nor paradoxical activation of the MAPK pathway. Biochemical analysis revealed that CRBN(BRAF)-24 showed more potent and sustained suppression of MAPK signaling than a BRAF inhibitor, PLX-8394, in BRAF -driven cancer cells. Targeted degradation of BRAF by CRBN(BRAF)-24 could be a promising strategy to evade paradoxical activation of the RAF-MAPK pathway.