title: The HSP90 inhibitor XL888 overcomes BRAF inhibitor resistance mediated through diverse mechanisms.
abstract: OBJECTIVE
  The clinical use of BRAF inhibitors is being hampered by the acquisition of drug resistance. This study shows the potential therapeutic use of the HSP90 inhibitor (XL888) in six different models of vemurafenib resistance.
  
  
  METHODS
  The ability of XL888 to inhibit growth and to induce apoptosis and tumor regression of vemurafenib-resistant melanoma cell lines was shown in vitro and in vivo. A novel mass spectrometry-based pharmacodynamic assay was developed to measure intratumoral HSP70 levels following HSP90 inhibition in melanoma cell lines, xenografts, and melanoma biopsies. Mechanistic studies were carried out to determine the mechanism of XL888-induced apoptosis.
  
  
  RESULTS
  XL888 potently inhibited cell growth, induced apoptosis, and prevented the growth of vemurafenib-resistant melanoma cell lines in 3-dimensional cell culture, long-term colony formation assays, and human melanoma mouse xenografts. The reversal of the resistance phenotype was associated with the degradation of PDGFRβ, COT, IGFR1, CRAF, ARAF, S6, cyclin D1, and AKT, which in turn led to the nuclear accumulation of FOXO3a, an increase in BIM (Bcl-2 interacting mediator of cell death) expression, and the downregulation of Mcl-1. In most resistance models, XL888 treatment increased BIM expression, decreased Mcl-1 expression, and induced apoptosis more effectively than dual mitogen-activated protein-extracellular signal-regulated kinase/phosphoinositide 3-kinase (MEK/PI3K) inhibition.
  
  
  CONCLUSIONS
  HSP90 inhibition may be a highly effective strategy at managing the diverse array of resistance mechanisms being reported to BRAF inhibitors and appears to be more effective at restoring BIM expression and downregulating Mcl-1 expression than combined MEK/PI3K inhibitor therapy.
authors.affiliation: The Department of Molecular Oncology, The Moffitt Cancer Center & Research Institute, Tampa, Florida, USA.
authors.forename: Kim H T
authors.initials: KH
authors.lastname: Paraiso
authors.forename: H Eirik
authors.initials: HE
authors.lastname: Haarberg
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Wood
authors.forename: Vito W
authors.initials: VW
authors.lastname: Rebecca
authors.forename: Y Ann
authors.initials: YA
authors.lastname: Chen
authors.forename: Yun
authors.initials: Y
authors.lastname: Xiang
authors.forename: Antoni
authors.initials: A
authors.lastname: Ribas
authors.forename: Roger S
authors.initials: RS
authors.lastname: Lo
authors.forename: Jeffrey S
authors.initials: JS
authors.lastname: Weber
authors.forename: Vernon K
authors.initials: VK
authors.lastname: Sondak
authors.forename: Jobin K
authors.initials: JK
authors.lastname: John
authors.forename: Amod A
authors.initials: AA
authors.lastname: Sarnaik
authors.forename: John M
authors.initials: JM
authors.lastname: Koomen
authors.forename: Keiran S M
authors.initials: KS
authors.lastname: Smalley
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000187:drug effects; D051017:Apoptosis Regulatory Proteins / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D053961:Azabicyclo Compounds / Q000494:pharmacology*; D000072224:Bcl-2-Like Protein 11; D015153:Blotting, Western; D045744:Cell Line, Tumor; D049109:Cell Proliferation / Q000187:drug effects; D003114:Colony-Forming Units Assay; D019008:Drug Resistance, Neoplasm / Q000187:drug effects*; D048049:Extracellular Signal-Regulated MAP Kinases / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D005434:Flow Cytometry; D005455:Fluorescent Antibody Technique; D000071316:Forkhead Box Protein O3; D051858:Forkhead Transcription Factors / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D018841:HSP90 Heat-Shock Proteins / Q000037:antagonists & inhibitors* / Q000378:metabolism; D006801:Humans; D007124:Immunoenzyme Techniques; D007211:Indoles / Q000009:adverse effects; D008545:Melanoma / Q000188:drug therapy* / Q000378:metabolism / Q000473:pathology; D008565:Membrane Proteins / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D051379:Mice; D008807:Mice, Inbred BALB C; D064549:Myeloid Cell Leukemia Sequence 1 Protein; D019869:Phosphatidylinositol 3-Kinases / Q000235:genetics / Q000378:metabolism; D000081082:Phosphoinositide-3 Kinase Inhibitors; D010795:Phthalic Acids / Q000494:pharmacology*; D011446:Prospective Studies; D047428:Protein Kinase Inhibitors / Q000494:pharmacology; D011518:Proto-Oncogene Proteins / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D048493:Proto-Oncogene Proteins B-raf / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism; D051057:Proto-Oncogene Proteins c-akt / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D012333:RNA, Messenger / Q000235:genetics; D060888:Real-Time Polymerase Chain Reaction; D020133:Reverse Transcriptase Polymerase Chain Reaction; D015398:Signal Transduction / Q000187:drug effects*; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization; D013449:Sulfonamides / Q000009:adverse effects; D000077484:Vemurafenib
chemical_list: D051017:Apoptosis Regulatory Proteins; D053961:Azabicyclo Compounds; D000072224:Bcl-2-Like Protein 11; D048049:Extracellular Signal-Regulated MAP Kinases; D000071316:Forkhead Box Protein O3; D051858:Forkhead Transcription Factors; D018841:HSP90 Heat-Shock Proteins; D007211:Indoles; D008565:Membrane Proteins; D064549:Myeloid Cell Leukemia Sequence 1 Protein; D019869:Phosphatidylinositol 3-Kinases; D000081082:Phosphoinositide-3 Kinase Inhibitors; D010795:Phthalic Acids; D047428:Protein Kinase Inhibitors; D011518:Proto-Oncogene Proteins; D048493:Proto-Oncogene Proteins B-raf; D051057:Proto-Oncogene Proteins c-akt; D019253:Proto-Oncogene Proteins c-bcl-2; D012333:RNA, Messenger; D013449:Sulfonamides; D000077484:Vemurafenib; C000606775:BCL2L11 protein, human; C000606776:Bcl2l11 protein, mouse; C499517:FOXO3 protein, human; C579094:Mcl1 protein, mouse; C559121:XL 888; C482119:BRAF protein, human