title: PET imaging of colorectal cancer in xenograft-bearing mice by use of an 18F-labeled T84.66 anti-carcinoembryonic antigen diabody.
abstract: In this study, we investigated the 18F-labeled anti-carcinoembryonic antigen (CEA) T84.66 diabody, a genetically engineered noncovalent dimer of single-chain variable fragments, for small-animal PET imaging of CEA expression in xenograft-bearing mice.
  
  
  METHODS
  18F labeling of the anti-CEA T84.66 diabody (molecular mass, 55 kDa) was achieved with N-succinimidyl-4-18F-fluorobenzoate (18F-SFB). The biodistribution of the 18F-fluorobenzyl-T84.66 diabody (18F-FB-T84.66 diabody) was evaluated in athymic nude mice bearing subcutaneous LS 174T human colon carcinoma and C6 rat glioma tumors. Serial small-animal PET imaging studies were performed to further evaluate in vivo targeting efficacy and pharmacokinetics.
  
  
  RESULTS
  Radiolabeling required 35 +/- 5 (mean +/- SD) min starting from 18F-SFB, and the tracer 18F-FB-T84.66 diabody was synthesized with a specific activity of 1.83 +/- 1.71 TBq/mmol. The decay-corrected radiochemical yield was 1.40% +/- 0.16% (n = 4), and the radiochemical purity was greater than 98%. The radioimmunoreactivity was 57.1% +/- 2.0%. The 18F-FB-T84.66 diabody showed rapid and high tumor uptake and fast clearance from the circulation in the LS 174T xenograft model, as evidenced by both small-animal PET imaging and biodistribution studies. High-contrast small-animal PET images were obtained as early as 1 h after injection of the 18F-FB-T84.66 diabody, and only a background level of activity accumulation was found in CEA-negative C6 tumors. The tracer exhibited predominantly renal clearance, with some activity in the liver and spleen at early time points.
  
  
  CONCLUSIONS
  The 18F-labeled diabody represents a new class of tumor-specific probes for PET that are based on targeting cell surface antigen expression. The 18F-FB-T84.66 diabody can be used for high-contrast small-animal PET imaging of CEA-positive tumor xenografts. It may be translated to the clinic for PET of CEA-positive malignancies.
authors.affiliation: Molecular Imaging Program at Stanford, Department of Radiology, Stanford University School of Medicine, Stanford, California 94305-5484, USA.
authors.forename: Weibo
authors.initials: W
authors.lastname: Cai
authors.forename: Tove
authors.initials: T
authors.lastname: Olafsen
authors.forename: Xianzhong
authors.initials: X
authors.lastname: Zhang
authors.forename: Qizhen
authors.initials: Q
authors.lastname: Cao
authors.forename: Sanjiv S
authors.initials: SS
authors.lastname: Gambhir
authors.forename: Lawrence E
authors.initials: LE
authors.lastname: Williams
authors.forename: Anna M
authors.initials: AM
authors.lastname: Wu
authors.forename: Xiaoyuan
authors.initials: X
authors.lastname: Chen
mesh_terms: D000818:Animals; D000911:Antibodies, Monoclonal; D002272:Carcinoembryonic Antigen / Q000276:immunology*; D045744:Cell Line, Tumor; D015179:Colorectal Neoplasms / Q000000981:diagnostic imaging*; D005260:Female; D005462:Fluorine Radioisotopes; D006207:Half-Life; D006801:Humans; D007091:Image Processing, Computer-Assisted; D007140:Immunoglobulin Fab Fragments; D007457:Iodine Radioisotopes; D007553:Isotope Labeling; D051379:Mice; D008819:Mice, Nude; D009368:Neoplasm Transplantation; D049268:Positron-Emission Tomography; D019275:Radiopharmaceuticals; D051381:Rats; D014018:Tissue Distribution; D014183:Transplantation, Heterologous
chemical_list: D000911:Antibodies, Monoclonal; D002272:Carcinoembryonic Antigen; D005462:Fluorine Radioisotopes; D007140:Immunoglobulin Fab Fragments; D007457:Iodine Radioisotopes; D019275:Radiopharmaceuticals