Comparison of 131I-tetracycline and 67Ga-citrate as abscess localizing agents

Previous studies have shown that radiolabeled tetracyclines tend to accumulate in infarcts and necrotic tumors. These results suggested that radiolabeled tetracyclines might also accumulate in necrotic abscesses or areas of inflammation. In order to develop a better abscess scanning agent, we compared the efficiency of 131I-tetracycline with 67Ga-citrate in labeling experimentally induced staphylococcal aureus abscesses in rats 24 and 72 hours after injection. In addition to evaluating 131I-tetracycline as an abscess scanning agent, we hoped to obtain data which might clarify the controversy regarding early versus late gallium scanning in suspected infection. 131I-tetracycline was chosen over 99mTc-tetracycline because the longer half-life of 131I would allow 72 hour imaging. Absolute concentrations of gallium in the abscess contents and in the surrounding areas of inflammation were significantly greater than the concentration of 131I-tetracycline at both 24 and 72 hours. With the exception of blood, muscle, and bone, the abscess-to-tissue activity ratios for gallium and 131I-tetracycline were similar; however, the ratio of gallium activity in the inflammed tissue to other tissues was greater than that of 131I-tetracycline for every tissue examined at both time periods. The data suggest that 131I-tetracycline has little potential as a general abscess scanning agent. The gallium tissue concentrations and tissue ratios suggest that abscesses which can be imaged at 72 hours can probably be imaged at 24 hours, thus allowing earlier initiation of appropriate therapy. Because of the higher lesion-to-blood ratio at 72 hours, a 72-hour scan would appear to be indicated before a scan is interpreted as normal.