Altered splenic uptake in the biodistribution of 99MTc-stannous fluoride colloid in rats

Full text: Radiolabelled white cells have found extensive application in nuclear medicine practice, with use in detecting sites of infection and inflammation. In 99MTc-stannous flu- oride (SnF2) colloid labelled leucocyte scans high liver and spleen activity is visualised, limiting the potential of this technique. This is thought to be due to free or unphagocytosed 99MTc-SnF2 colloid in the preparation. Labelling white cells with 99MTc-SnF2 depends upon phagocytic engulfment of this colloid. Non-phagocy-tosed colloid may be a cause of increased splenic activity. Our aims were to assess if it were possible to reduce liver/spleen uptake of 99MTc-SnF2 colloid in a rat model, by administering unlabelled SnF2 colloid before injection of radiocolloid, in a modification of the standard British Pharmacopoeial test. The biodistribution of 99MTc-SnF2 colloid alone gave 85.4% uptake in the liver and 7.6% in the spleen. A pre-injection of 26.5μg cold SnF2 and then 99MTc-SnF2 colloid resulted in decreased spleen uptake by 43%. A higher (52μg) pre-injection dose gave a greater decrease in splenic uptake by 53%. When ex vivo opsonised 99MTc-SnF2 colloid was administered, both liver and spleen uptake was not reduced. From these results, a pre-injection of unlabelled SnF2 substantially depletes the circulating opsonin concentration, thereby decreasing the opsonisation rate of unopsonised 99MTc-SnF2 colloid. The limiting factor is likely to be the amount of opsonisation in the plasma rather than phagocytic capacity of the macrophages of liver and spleen. In conclusion these results suggest that a pre-injection of cold stannous fluoride may reduce splenic uptake in 99MTc-SnF2 colloid labelled white cell scans. Copyright (2003) The Australian and New Zealand Society of Nuclear Medicine Inc