Imaging of a localized bacterial infection with endogenous thymidine kinase using radioisotope-labeled nucleosides

The importance of noninvasive imaging methods to bacterial infections is widely recognized. To obtain bacterial infection imaging with radioisotope-labeled nucleosides, bacterial thymidine kinase (tk) activities of Salmonella typhimurium with [ ¹²⁵I]5-iodo-1-(2'-fluoro-2'-deoxy-β-d-arabinofuranosyl)uracil ([ ¹²⁵I]FIAU) or 3'-deoxy-3'-[ ¹⁸F]fluorothymidine ([ ¹⁸F]FLT) were measured. The infection model in BALB/c mice was imaged with [ ¹²⁵I]FIAU or [ ¹⁸F]FLT using small-animal Single Photon Emission Computed Tomography (SPECT) or Positron Emission Tomography (PET), respectively.The accumulated radioactivity of [ ¹²⁵I]FIAU or [ ¹⁸F]FLT in the two strains showed a linearly increased pattern with increasing incubation time or bacterial numbers. The image clearly demonstrated a high uptake of [ ¹²⁵I]FIAU and [ ¹⁸F]FLT in the bacterial infection site. [ ¹⁸F]FLT uptake in the infection site of was 7.286±2.405, whereas that in the uninfected site was 0.519±0.561. The relative activity ratio of the infected region in relation to the uninfected region was 2.98 at 4h after an injection with [ ¹²⁵I]FIAU determined by biodistribution data.In conclusion, the bacterial tk activity was confirmed by the cellular uptake and imaging with [ ¹²⁵I]FIAU or [ ¹⁸F]FLT. Therefore, a localized bacterial infection in living mice can be monitored using radioisotope-labeled nucleosides with a nuclear medicine imaging modality.