Stabilization of Physical RAF/14-3‑3 Interaction by Cotylenin A as Treatment Strategy for RAS Mutant Cancers

One-third of all human cancers harbor somatic <i>RAS</i> mutations. This leads to aberrant activation of downstream signaling pathways involving the RAF kinases. Current ATP-competitive RAF inhibitors are active in cancers with somatic RAF mutations, such as BRAF^V600 mutant melanomas. However, they paradoxically promote the growth of <i>RAS</i> mutant tumors, partly due to the complex interplay between different homo- and heterodimers of A-RAF, B-RAF, and C-RAF. Based on pathway analysis and structure-guided compound identification, we describe the natural product cotylenin-A (CN-A) as stabilizer of the physical interaction of C-RAF with 14-3-3 proteins. CN-A binds to inhibitory 14-3-3 interaction sites of C-RAF, pSer233, and pSer259, but not to the activating interaction site, pSer621. While CN-A alone is inactive in <i>RAS</i> mutant cancer models, combined treatment with CN-A and an anti-EGFR antibody synergistically suppresses tumor growth <i>in vitro</i> and <i>in vivo</i>. This defines a novel pharmacologic strategy for treatment of <i>RAS</i> mutant cancers