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

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Abstract
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)×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)×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.

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