In vivo imaging of herpes simplex virus type 1 thymidine kinase gene expression: early kinetics of radiolabelled FIAU.
Previous studies have shown that the herpes simplex virus type 1 thymidine kinase gene (HSV1-tk), in combination with appropriate radiolabelled substrates (e.g. [I*]-2'-fluoro-2'-deoxy-5-iodo-1-beta-D-arabinofuranosyluracil, I*-FIAU, where the asterisk indicates that any of the various radioactive iodine isotopes can be used), can be used as a reporter gene for in vivo monitoring of gene transfer and expression. The aim of our study was to examine the early kinetics of I*-FIAU and the possibility of utilising iodine-123-labelled FIAU for imaging of gene expression. CMS-5 fibrosarcoma cells were transduced in vitro with the retroviral vector STK containing the HSV1-tk gene. BALB/c mice were inoculated subcutaneously with HSV1-tk(+) and tk(-) cells into both flanks. FAU (2'-fluoro-2'-deoxy-1-beta-D-arabinofuranosyluracil was radioiodinated (123I, 125I) using the iodogen method. High-performance liquid chromatography purification resulted in high specific activity and radiochemical purity for both tracers ([123I]FIAU and [125I]FIAU). Biodistribution studies and gamma camera imaging were performed at 0.5, 1, 2 and 4 h p.i. In addition, the genomic DNA of the tumours was isolated for measurement of the activity accumulation resulting from the [125I]FIAU incorporation. Biodistribution studies 0.5 h p.i. showed tumour/blood and tumour/muscle ratios of 3.8 and 7.2, respectively, for the HSV1-tk(+) tumours, and 0.6 and 1.2, respectively, for negative control tumours. Fast renal elimination of the tracer from the body resulted in rapidly increasing tumour/blood and tumour/muscle ratios which reached values of 32 and 88 at 4 h p.i., respectively. Tracer clearance from blood was bi-exponential, with an initial half-life of 0.6 h followed by a half-life of 4.6 h. The tracer half-life in herpes simplex viral thymidine kinase-expressing tumours was 35.7 h. The highest activity accumulation (20.3%+/-5.7% ID/g) in HSV1-tk(+) tumours was observed 1 h p.i. At that time, about 46% of the total activity found in HSV1-tk(+) tumours was incorporated into genomic DNA. Planar gamma camera imaging showed a distinct tracer accumulation as early as 0.5 h p.i., with an increase in contrast over time. These results suggest that sufficient tumour/background ratios for in vivo imaging of HSV1-tk expression with [123I]FIAU are reached as early as 1 h p.i.
Haubner
R
R
Department of Nuclear Medicine, Technische Universität München, Munich, Germany. R.Haubner@lrz.tum.de
Avril
N
N
Hantzopoulos
P A
PA
Gansbacher
B
B
Schwaiger
M
M
D000818:Animals; D001086:Arabinofuranosyluracil / Q000031:analogs & derivatives* / Q000493:pharmacokinetics; D002851:Chromatography, High Pressure Liquid; D005354:Fibrosarcoma / Q000000981:diagnostic imaging / Q000235:genetics* / Q000378:metabolism; D015870:Gene Expression; D018014:Gene Transfer Techniques; D017930:Genes, Reporter; D015316:Genetic Therapy; D018259:Herpesvirus 1, Human / Q000201:enzymology / Q000235:genetics*; D007457:Iodine Radioisotopes / Q000493:pharmacokinetics*; D051379:Mice; D008807:Mice, Inbred BALB C; D009368:Neoplasm Transplantation; D011877:Radionuclide Imaging; D013937:Thymidine Kinase / Q000235:genetics*; D014018:Tissue Distribution; D014161:Transduction, Genetic
D007457:Iodine Radioisotopes; D001086:Arabinofuranosyluracil; C043457:fialuridine; D013937:Thymidine Kinase