title: 18F-FEAU as a radiotracer for herpes simplex virus thymidine kinase gene expression: in-vitro comparison with other PET tracers.
abstract: OBJECTIVE
  The herpes simplex virus thymidine kinase (HSVtk) gene has frequently been applied as a reporter gene for monitoring transgene expression in animal models. In clinical gene therapy protocols, however, extremely low expression levels of the transferred gene are generally observed. Consequently, sensitive and selective radiotracers for imaging are required. This study describes the in-vitro evaluation of 2'-[18F]fluoro-5-ethyl-1-beta-D-arabinofuranosyluracil (18F-FEAU) as a candidate tracer for HSVtk imaging with positron emission tomography (PET).
  
  
  METHODS
  In cellular accumulation experiments, the potential of 18F-FEAU as a PET tracer for HSVtk was compared to the known acyclic guanosine derivatives 9-[(3-[18F]fluoro-1-hydroxy-2-propoxy)methyl]guanine (18F-FHPG) and 9-[4-[18F]fluoro-3-(hydroxymethyl)butyl]guanine (18F-FHBG), and the thymidine derivatives 3'-deoxy-3'-[18F]fluorothymidine (18F-FLT), 2'-deoxy-2'-[18F]fluoro-5-methyl-1-beta-D-arabinofuranosyluracil (18F-FMAU) and 2'-deoxy-2'-[18F]fluoro-5-iodo-1-beta-D-arabinofuranosyluracil (18F-FIAU). For this purpose, C6 control cells and HSVtk-expressing C6tk cells were incubated with the different tracers for various periods of time and cellular uptake and initial uptake rates were analysed. The initial rate of tracer uptake was determined from the slope of the plot of tracer uptake versus incubation time.
  
  
  RESULTS
  After 2 h of tracer incubation, the C6tk/C6 accumulation ratio was 1.6 for 18F-FLT, 2.4 for F-FMAU, 5.5 for 18F-FHPG, 10.3 for 18F-FIAU, 40.8 for 18F-FHBG and 84.5 for 18F-FEAU. The initial tracer uptake rate in C6tk cells was in the order FLT>FMAU>FEAU>FIAU>FHBG>FHPG, whereas the initial tracer uptake rate in C6 control cells was FLT>FMAU>FIAU>FEAU approximately = FHBG approximately = FHPG. The highest HSVtk specific uptake was observed for FEAU.
  
  
  CONCLUSIONS
  This study indicates that the high uptake rate of FEAU together with its high selectivity make this tracer an excellent candidate as a PET tracer for HSVtk gene expression.
authors.affiliation: Department of Nuclear Medicine and Molecular Imaging, Medical Center Groningen, University of Groningen, The Netherlands.
authors.forename: Anne Rixt
authors.initials: AR
authors.lastname: Buursma
authors.forename: Vera
authors.initials: V
authors.lastname: Rutgers
authors.forename: Geke A P
authors.initials: GA
authors.lastname: Hospers
authors.forename: Nanno H
authors.initials: NH
authors.lastname: Mulder
authors.forename: Willem
authors.initials: W
authors.lastname: Vaalburg
authors.forename: Erik F J
authors.initials: EF
authors.lastname: de Vries
mesh_terms: D000818:Animals; D001086:Arabinofuranosyluracil / Q000031:analogs & derivatives* / Q000493:pharmacokinetics; D045744:Cell Line, Tumor; D005462:Fluorine Radioisotopes / Q000493:pharmacokinetics; D020869:Gene Expression Profiling / Q000379:methods*; D005910:Glioma / Q000000981:diagnostic imaging* / Q000235:genetics / Q000378:metabolism*; D008657:Metabolic Clearance Rate; D049268:Positron-Emission Tomography; D019275:Radiopharmaceuticals / Q000493:pharmacokinetics; D051381:Rats; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D013937:Thymidine Kinase / Q000235:genetics* / Q000378:metabolism*; D014162:Transfection; D014764:Viral Proteins / Q000235:genetics* / Q000378:metabolism*
chemical_list: D005462:Fluorine Radioisotopes; D019275:Radiopharmaceuticals; D011994:Recombinant Proteins; D014764:Viral Proteins; D001086:Arabinofuranosyluracil; C054103:2'-fluoro-5-ethylarabinosyluracil; C099241:thymidine kinase, Canid herpesvirus 1; D013937:Thymidine Kinase