title: Preliminary validation of varicella zoster virus thymidine kinase as a novel reporter gene for PET.
abstract: BACKGROUND
  Imaging of gene expression with positron emission tomography (PET) has emerged as a powerful tool for biomedical research during the last decade. The prototypical herpes simplex virus type 1 thymidine kinase (HSV1-TK) PET reporter gene (PRG) is widely used and many other PRGs have also been validated. We investigated varicella zoster virus thymidine kinase (VZV-tk) as new PRG with radiolabeled bicyclic nucleoside analogues (BCNAs) as PET tracers.
  
  
  METHODS
  The uptake and washout of four different radiolabeled BCNAs was evaluated in cells expressing VZV-tk after lentiviral vector (LV) transduction and in control cells. Metabolism of the tracers was assayed by high pressure liquid chromatography (HPLC). Mice bearing VZV-TK expressing xenografts were imaged with PET.
  
  
  RESULTS
  High uptake in VZV-tk expressing cells was seen for 3 of the 4 tracers tested. The uptake of the tracers could be blocked by the presence of excess thymidine in the incubation solution. Cellular retention was variable, with one tracer showing an acceptable half-life of ~1 hour. The amount of intracellular tracer correlated with the titer of LV used to transduce the cells. VZV-TK dependent conversion into metabolites was shown by HPLC. No specific accumulation was observed in cells expressing a fusion protein containing an HSV1-TK moiety. VZV-tk expression in xenografts resulted in a 60% increase in uptake in vivo as measured with PET.
  
  
  CONCLUSIONS
  We have validated the combination of VZV-tk and radiolabeled BCNAs as new PRG/PRP system. Further optimization of the PRPs and the PRG are warranted to increase the signal.
authors.affiliation: Division of Nuclear Medicine, University Hospital Leuven, Leuven, Belgium. christophe.deroose@uzleuven.be
authors.forename: Christophe M
authors.initials: CM
authors.lastname: Deroose
authors.forename: Satish K
authors.initials: SK
authors.lastname: Chitneni
authors.forename: Rik
authors.initials: R
authors.lastname: Gijsbers
authors.forename: Peter
authors.initials: P
authors.lastname: Vermaelen
authors.forename: Abdelilah
authors.initials: A
authors.lastname: Ibrahimi
authors.forename: Jan
authors.initials: J
authors.lastname: Balzarini
authors.forename: Veerle
authors.initials: V
authors.lastname: Baekelandt
authors.forename: Alfons
authors.initials: A
authors.lastname: Verbruggen
authors.forename: Johan
authors.initials: J
authors.lastname: Nuyts
authors.forename: Zeger
authors.initials: Z
authors.lastname: Debyser
authors.forename: Guy M
authors.initials: GM
authors.lastname: Bormans
authors.forename: Luc
authors.initials: L
authors.lastname: Mortelmans
mesh_terms: D000818:Animals; D001692:Biological Transport; D045744:Cell Line, Tumor; D002471:Cell Transformation, Neoplastic; D005462:Fluorine Radioisotopes; D017930:Genes, Reporter / Q000235:genetics*; D005822:Genetic Vectors / Q000235:genetics; D057809:HEK293 Cells; D014645:Herpesvirus 3, Human / Q000201:enzymology* / Q000235:genetics*; D006801:Humans; D042541:Intracellular Space / Q000378:metabolism; D007700:Kinetics; D016086:Lentivirus / Q000235:genetics / Q000502:physiology; D008297:Male; D051379:Mice; D009705:Nucleosides / Q000737:chemistry / Q000378:metabolism; D049268:Positron-Emission Tomography / Q000379:methods*; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging / Q000235:genetics / Q000473:pathology; D015203:Reproducibility of Results; D013937:Thymidine Kinase / Q000235:genetics*; D014161:Transduction, Genetic; D019562:Viral Load
chemical_list: D005462:Fluorine Radioisotopes; D009705:Nucleosides; D013937:Thymidine Kinase