Selective and potent Raf inhibitors paradoxically stimulate normal cell proliferation and tumor growth.
Raf inhibitors are under clinical investigation, specifically in patients with tumor types harboring frequent activating mutations in B-Raf. Here, we show that cell lines and tumors harboring mutant B-Raf were sensitive to a novel series of Raf inhibitors (e.g., (V600E)B-Raf A375, IC(50) on cells = 2 nmol/L; ED(50) on tumor xenografts = 1.3 mg/kg). However, in cells and tumors with wild-type B-Raf, exposure to Raf inhibitors resulted in a dose-dependent and sustained activation of mitogen-activated protein kinase signaling. In some of these cell lines, Raf inhibition led to entry into the cell cycle, enhanced proliferation, and significantly stimulated tumor growth in vivo. Inhibition with structurally distinct Raf inhibitors or isoform-specific small interfering RNA knockdown of Raf showed that these effects were mediated directly through Raf. Either A-Raf or C-Raf mediated the Raf inhibitor-induced mitogen-activated protein kinase pathway activation in an inhibitor-specific manner. These paradoxical effects of Raf inhibition were seen in malignant and normal cells in vitro and in vivo. Hyperplasia of normal epithelial cells in the esophagus and the stomach was evident in mice with all efficacious Raf inhibitors (n = 8) tested. An implication of these results is that Raf inhibitors may induce unexpected normal cell and tumor tissue proliferation in patients.
Carnahan
Josette
J
Department of Hematology, Amgen, Inc., One Amgen Center Drive, Thousand Oaks, CA 91320, USA. jcarnaha@amgen.com
Beltran
Pedro J
PJ
Babij
Carol
C
Le
Quynh
Q
Rose
Mark J
MJ
Vonderfecht
Steven
S
Kim
Joseph L
JL
Smith
Adrian L
AL
Nagapudi
Karthik
K
Broome
Martin A
MA
Fernando
Manory
M
Kha
Hue
H
Belmontes
Brian
B
Radinsky
Robert
R
Kendall
Richard
R
Burgess
Teresa L
TL
D000818:Animals; D002453:Cell Cycle / Q000187:drug effects; D045744:Cell Line, Tumor; D049109:Cell Proliferation / Q000187:drug effects; D002470:Cell Survival / Q000187:drug effects; D004789:Enzyme Activation / Q000187:drug effects; D004848:Epithelium / Q000187:drug effects / Q000473:pathology; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects; D006801:Humans; D006965:Hyperplasia; D036341:Intercellular Signaling Peptides and Proteins / Q000494:pharmacology; D007527:Isoenzymes / Q000378:metabolism; D020935:MAP Kinase Signaling System / Q000187:drug effects / Q000235:genetics; D051379:Mice; D050505:Mutant Proteins / Q000378:metabolism; D009369:Neoplasms / Q000201:enzymology / Q000473:pathology*; D047428:Protein Kinase Inhibitors / Q000008:administration & dosage / Q000494:pharmacology*; D048493:Proto-Oncogene Proteins B-raf / Q000037:antagonists & inhibitors* / Q000378:metabolism; D023041:Xenograft Model Antitumor Assays
D036341:Intercellular Signaling Peptides and Proteins; D007527:Isoenzymes; D050505:Mutant Proteins; D047428:Protein Kinase Inhibitors; D048493:Proto-Oncogene Proteins B-raf