Combined Pan-RAF and MEK Inhibition Overcomes Multiple Resistance Mechanisms to Selective RAF Inhibitors.
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.
Department of Medical Oncology, Dana-Farber Cancer Institute, Boston, Massachusetts. The Broad Institute, Cambridge, Massachusetts. Division of Cancer Therapeutics, Institute of Cancer Research, London, United Kingdom.
Steven R
SR
Whittaker
The Broad Institute, Cambridge, Massachusetts.
Glenn S
GS
Cowley
Division of Cancer Therapeutics, Institute of Cancer Research, London, United Kingdom.
Steve
S
Wagner
Department of Medical Oncology, Dana-Farber Cancer Institute, Boston, Massachusetts. The Broad Institute, Cambridge, Massachusetts.
Flora
F
Luo
The Broad Institute, Cambridge, Massachusetts.
David E
DE
Root
Department of Medical Oncology, Dana-Farber Cancer Institute, Boston, Massachusetts. The Broad Institute, Cambridge, Massachusetts. Center for Cancer Genome Discovery, Dana-Farber Cancer Institute, Boston, Massachusetts. Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts. levi_garraway@dfci.harvard.edu.
Levi A
LA
Garraway
D017209:Apoptosis / Q000187:drug effects; D045744:Cell Line, Tumor; D015179:Colorectal Neoplasms / Q000188:drug therapy* / Q000235:genetics / Q000473:pathology; D019008:Drug Resistance, Neoplasm / Q000235:genetics; D006801:Humans; D007211:Indoles / Q000008:administration & dosage; D020930:MAP Kinase Kinase Kinases / Q000037:antagonists & inhibitors* / Q000235:genetics; D020935:MAP Kinase Signaling System / Q000187:drug effects; D009154:Mutation; D047428:Protein Kinase Inhibitors / Q000008:administration & dosage; D048493:Proto-Oncogene Proteins B-raf / Q000235:genetics*; D016283:Proto-Oncogene Proteins p21(ras) / Q000235:genetics*; D013449:Sulfonamides / Q000008:administration & dosage; D048490:raf Kinases / Q000037:antagonists & inhibitors / Q000235:genetics*
D007211:Indoles; C117307:KRAS protein, human; C528407:PLX 4720; D047428:Protein Kinase Inhibitors; D013449:Sulfonamides; C482119:BRAF protein, human; D048493:Proto-Oncogene Proteins B-raf; D048490:raf Kinases; D020930:MAP Kinase Kinase Kinases; D016283:Proto-Oncogene Proteins p21(ras)