Labelled Stealth(R) liposomes in experimental infection: An alternative to leukocyte scintigraphy?

Indium-111 (In-111) and technetium-99m (Tc-99(m)) Stealth(R) liposomes were compared with In-111- and Tc-99(m)-labelled white blood cells (WBC) in experimental infection in a rabbit model. Preformed polyethylene glycol-coated liposomes and separated WBC were radiolabelled with either In-111-oxine or Tc-99(m)-hexamethylpropyleneamine oxime (Tc-99(m)-HMPAO). After the intravenous administration of one of the four radiopharmaceuticals to rabbits with focal Staphylococcus aureus infection, scintigraphic images were recorded at various time points post-injection and the biodistribution of the radiopharmaceuticals was determined. At 4 h post-injection, uptake of In-111-WBC in the abscess was significantly higher than that of the three other products. At later time points, In-111-WBC, In-111-liposome and Tc-99(m)-liposome uptake in the abscess were similar. In contrast, at 20 h post-injection, uptake of Tc-99(m)-WBC was significantly lower. The abscess-to-background ratios showed a similar pattern to the absolute abscess uptake: initial high values for In-111-WBC, a more gradual increase over time of the liposome preparations to the level of In-111-WBC and persistently low values for Tc-99(m)-WBC. Clearance from the blood of both labelled WBC preparations was significantly faster and splenic uptake significantly higher compared with those of the labelled liposomes. In conclusion, given the similar in vivo characteristics of labelled liposomes and labelled WBC, labelled liposomes may be an attractive replacement for labelled WBC, providing a continuously available, high-quality, Tc-99(m)-labelled radiopharmaceutical that can be prepared easily without any need to handle blood.