title: 3'-Deoxy-3'-18F-fluorothymidine PET predicts response to (V600E)BRAF-targeted therapy in preclinical models of colorectal cancer.
abstract: Selective inhibition of oncogenic targets and associated signaling pathways forms the basis of personalized cancer medicine. The clinical success of (V600E)BRAF inhibition in melanoma, coupled with the emergence of acquired resistance, underscores the importance of rigorously validating quantitative biomarkers of treatment response in this and similar settings. Because constitutive activation of BRAF leads to proliferation in tumors, we explored 3'-deoxy-3'-(18)F-fluorothymidine ((18)F-FLT) PET to noninvasively quantify changes in tumor proliferation that are associated with pharmacologic inhibition of (V600E)BRAF downstream effectors and that precede changes in tumor volume.
  
  
  METHODS
  Human colorectal cancer (CRC) cell lines expressing (V600E)BRAF were used to explore relationships between upregulation of p27 and phosphorylation of BRAF downstream effectors on small-molecule (V600E)BRAF inhibitor exposure. Athymic nude mice bearing (V600E)BRAF-expressing human CRC cell line xenografts were treated with a small-molecule (V600E)BRAF inhibitor (or vehicle) daily for 10 d. Predictive (18)F-FLT PET was conducted before changes in tumor volume occurred. Correlations were evaluated among PET, inhibition of phosphorylated MEK (p-MEK) and phosphorylated-ERK (p-ERK) by Western blot, tumor proliferation by histology, and small-molecule exposure by matrix-assisted laser desorption/ionization (MALDI) imaging mass spectrometry (IMS).
  
  
  RESULTS
  Treatment of CRC cell lines with PLX4720 reduced proliferation associated with target inhibition and upregulation of p27. In vivo, PLX4720 treatment reduced (18)F-FLT uptake, but not (18)F-FDG uptake, in Lim2405 xenografts before quantifiable differences in xenograft volume. Reduced (18)F-FLT PET reflected a modest, yet significant, reduction of Ki67 immunoreactivity, inhibition of p-MEK and p-ERK, and elevated tumor cell p27 protein levels. Both (18)F-FLT PET and (18)F-FDG PET accurately reflected a lack of response in HT-29 xenografts, which MALDI imaging mass spectrometry suggested may have stemmed from limited PLX4720 exposure.
  
  
  CONCLUSIONS
  We used preclinical models of CRC to demonstrate (18)F-FLT PET as a sensitive predictor of response to (V600E)BRAF inhibitors. Because (18)F-FLT PET predicted reduced proliferation associated with attenuation of BRAF downstream effectors, yet (18)F-FDG PET did not, these data suggest that (18)F-FLT PET may represent an alternative to (18)F-FDG PET for quantifying clinical responses to BRAF inhibitors.
authors: Vanderbilt University Institute of Imaging Science, Vanderbilt University, Nashville, Tennessee 37232-2310, USA.
authors: Eliot T
authors: ET
authors: McKinley
authors: R Adam
authors: RA
authors: Smith
authors: Ping
authors: P
authors: Zhao
authors: Allie
authors: A
authors: Fu
authors: Samir A
authors: SA
authors: Saleh
authors: Md Imam
authors: MI
authors: Uddin
authors: M Kay
authors: MK
authors: Washington
authors: Robert J
authors: RJ
authors: Coffey
authors: H Charles
authors: HC
authors: Manning
mesh_terms: D000818:Animals; D045744:Cell Line, Tumor; D015179:Colorectal Neoplasms / Q000000981:diagnostic imaging* / Q000235:genetics / Q000378:metabolism / Q000628:therapy*; D015224:Dideoxynucleosides; D019008:Drug Resistance, Neoplasm; D005462:Fluorine Radioisotopes; D006801:Humans; D007211:Indoles / Q000627:therapeutic use; D020935:MAP Kinase Signaling System; D051379:Mice; D008819:Mice, Nude; D058990:Molecular Targeted Therapy; D050505:Mutant Proteins / Q000037:antagonists & inhibitors* / Q000235:genetics*; D049268:Positron-Emission Tomography; D011237:Predictive Value of Tests; D047428:Protein Kinase Inhibitors / Q000627:therapeutic use; D048493:Proto-Oncogene Proteins B-raf / Q000037:antagonists & inhibitors* / Q000235:genetics*; D019275:Radiopharmaceuticals; D013449:Sulfonamides / Q000627:therapeutic use; D023041:Xenograft Model Antitumor Assays
chemical_list: D015224:Dideoxynucleosides; D005462:Fluorine Radioisotopes; D007211:Indoles; D050505:Mutant Proteins; C528407:PLX 4720; D047428:Protein Kinase Inhibitors; D019275:Radiopharmaceuticals; D013449:Sulfonamides; C482119:BRAF protein, human; D048493:Proto-Oncogene Proteins B-raf; C002854:alovudine