Uptake of positron emission tomography tracers in experimental bacterial infections: A comparative biodistribution study of radiolabeled FDG, thymidine, L-methionine, ⁶⁷Ga-citrate, and ¹²⁵I-HSA

The purpose of this study was to evaluate the localization of positron emission tomography (PET) tracers [2-deoxy-2-fluoro-D-glucose (FDG), thymidine, and L-methionine] in sites of bacterial infection, and to contrast this with that of other tracers. The left calf muscles of rats were infected with a suspension of Escherichia coli and the biodistribution of ¹⁸F- or -³H-FDG, ³H-thymidine, L-¹¹C- or ³H-methionine, gallium-67 citrate (⁶⁷Ga-citrate) and iodine-125 human serum albumin (¹²⁵I-HSA) was determined in these animals. ³H-FDG uptake in the infectious foci was evaluated by autoradiography of histological sections. Although ¹⁸F-FDG, ⁶⁷Ga-citrate, and ¹²⁵I-HSA showed comparatively high uptake in the infected muscle [the percentage activity of injected dose (ID) per gram of tissue normalized for rat weight in kilogram (%ID/g) x kg at 2 h postinjection was as follows: ¹⁸F-FDG, 0.184 ± 0.026 to 0.218 ± 0.046; ⁶⁷Ga-citrate, 0.221 ± 0.016; ¹²⁵I-HSA, 0.198 ± 0.019], the infected muscle to blood ratio was much higher for ¹⁸F-FDG than for ⁶⁷Ga-citrate or ¹²⁵I-HSA (¹⁸F-FDG, 10.31 ± 0.76 to 14.89 ± 2.26; ⁶⁷Ga-citrate, 1.24 ± 0.67; ¹²⁵I-HSA, 0.20 ± 0.02). The draining reactive lymph nodes also showed higher accumulation of ¹⁸F-FDG than of ⁶⁷Ga-citrate or ¹²⁵I-HSA. The uptake of ³H-thymidine and L-¹¹C- or ³H-methionine in the infected muscle was lower than that of ¹⁸F- or ³H-FDG (at 2 h postinjection, ³H-thymidine = 0.039 ± 0.005 and L-³H-methionine = 0.063 ± 0.007 (%ID/g) x kg. Autoradiographs showed that the highest ³H-FDG uptake was seen in the area of inflammatory cell infiltration surrounding the necrotic region. In conclusion, ¹⁸F-FDG, which rapidly accumulates in sites of bacterial infection and in reactive lymph nodes with a high target to background ratio, appears to be a promising infection detection agent.