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This study evaluated a radioiodinated deoxycytidine analog, (131)I-5-iodo-2'-deoxycytidine ([(131)I]ICdR), as a novel proliferation probe and compared it with (131)I-5-iodo-2'-deoxyuridine ([(131)I]IUdR) in a NG4TL4 sarcoma-bearing mouse model. As an imaging agent, the biological characteristics of [(123)I]IUdR is not satisfactory due to its metabolic instability and short biological half-life in vivo. With [(123)I]ICdR/SPECT it was possible to clearly delineate the tumor lesion at 1h post-injection (tumor-to-muscle ratio 7.74) in tumor-bearing mice. The results of biodistribution were consistent with those observed in scintigraphic imaging. This study demonstrated that [(131)I]ICdR is a more promising SPECT probe than [(131)I]IUdR for imaging proliferation.

Objective Herpes simplex virus type 1 thymidine kinase ( HSV1-tk ) gene in combination with radiolabeled nucleoside substrates is the most widely used reporter system. This study characterized 1-(2′-deoxy-2′-[ 18 F]fluoro-β-d-arabinofuranosyl)-5-iodocytosine ( 18 F-FIAC) as a new potential positron emission tomography (PET) probe for HSV1-tk gene imaging and compared it with 2'-deoxy-2'-[ 18 F]fluoro-5-iodo-1-β-D-arabinofuranosyluracil ( 18 F-FIAU) and 2'-deoxy-2'-[ 18 F]fluoro-5-ethyl-1-β-D-arabinofuranosyluracil( 18 F-FEAU) (thymidine analogues) in an NG4TL4-WT/STK sarcoma-bearing mouse model. Methods A cellular uptake assay, biodistribution study, radioactive metabolites assay and microPET imaging of NG4TL4-WT/STK tumor-bearing mice post administration of 18 F-FIAC, 18 F-FIAU and 18 F-FEAU were conducted to characterize the biological properties of these tracers. Results Highly specific uptake of 18 F-FIAC, 18 F-FIAU and 18 F-FEAU in tk-transfected [tk(+)] cells was observed. The tk(+)-to-tk(−) cellular uptake ratio after a 2-h incubation was 66.6±25.1, 76.3±18.2 and 247.2±37.2, respectively. In biodistribution studies, 18 F-FIAC showed significant tk(+) tumor specificity (12.6; expressed as the tk(+)-to-tk(−) tumor uptake ratio at 2 h postinjection) comparable with 18 F-FIAU (15.8) but lower than 18 F-FEAU (48.0). The results of microPET imaging also revealed the highly specific accumulation of these three radioprobes in the NG4TL4-tk(+) tumor. Conclusion Our findings suggested that the cytidine analogue 18 F-FIAC is a new potential PET probe for the imaging of HSV1-tk gene expression. 18 F-FIAC may be regarded as the prodrug of 18 F-FIAU in vivo.