Combined Pan-RAF and MEK Inhibition Overcomes Multiple Resistance Mechanisms to Selective RAF Inhibitors

Mol Cancer Ther. 2015 Dec;14(12):2700-11. doi: 10.1158/1535-7163.MCT-15-0136-T. Epub 2015 Sep 8.


RAF and MEK inhibitors are effective in BRAF-mutant melanoma but not in BRAF-mutant colorectal cancer. To gain additional insights into this difference, we performed a genome-scale pooled shRNA enhancer screen in a BRAF-mutant, RAF inhibitor-resistant colorectal cancer cell line exposed to the selective RAF inhibitor PLX4720. We identified multiple genes along the receptor tyrosine kinase (RTK)/mitogen-activated protein kinase (MAPK) signaling axis that, when suppressed, either genetically or pharmacologically, sensitized cells to the selective RAF inhibitor through sustained inhibition of MAPK signaling. Strikingly, CRAF was a key mediator of resistance that could be overcome by the use of pan-RAF inhibitors in combination with a MEK inhibitor. Furthermore, the combination of pan-RAF and MEK inhibitors displayed strong synergy in melanoma and colorectal cancer cell lines with RAS-activating events such as RTK activation, KRAS mutation, or NF1 loss-of-function mutations. Combinations of selective RAF inhibitors, such as PLX4720 or dabrafenib, with MEK inhibitors did not incur such profound synergy, suggesting that inhibition of CRAF by pan-RAF inhibitors plays a key role in determining cellular response. Importantly, in contrast to the modest activity seen with single-agent treatment, dual pan-RAF and MEK inhibition results in the induction of apoptosis, greatly enhancing efficacy. Notably, combined pan-RAF and MEK inhibition can overcome intrinsic and acquired resistance to single-agent RAF/MEK inhibition, supporting dual pan-RAF and MEK inhibition as a novel therapeutic strategy for BRAF- and KRAS-mutant cancers.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Apoptosis / drug effects
  • Cell Line, Tumor
  • Colorectal Neoplasms / drug therapy*
  • Colorectal Neoplasms / genetics
  • Colorectal Neoplasms / pathology
  • Drug Resistance, Neoplasm / genetics
  • Humans
  • Indoles / administration & dosage
  • MAP Kinase Kinase Kinases / antagonists & inhibitors*
  • MAP Kinase Kinase Kinases / genetics
  • MAP Kinase Signaling System / drug effects
  • Mutation
  • Protein Kinase Inhibitors / administration & dosage
  • Proto-Oncogene Proteins B-raf / genetics*
  • Proto-Oncogene Proteins p21(ras) / genetics*
  • Sulfonamides / administration & dosage
  • raf Kinases / antagonists & inhibitors
  • raf Kinases / genetics*


  • Indoles
  • KRAS protein, human
  • PLX 4720
  • Protein Kinase Inhibitors
  • Sulfonamides
  • BRAF protein, human
  • Proto-Oncogene Proteins B-raf
  • raf Kinases
  • MAP Kinase Kinase Kinases
  • Proto-Oncogene Proteins p21(ras)