Sterically Stabilized Liposomes Labeled with Indium-111 to Image Focal Infection

To determine the potential of sterically stabilized liposomes to image infectious and inflammatory foci, the in vivo behavior of 111In-labeled PEGylated (coated with polyethylene glycol) liposomes was studied in a rat model. Indium-111-PEGylated lipsomes were administered intravenously to rats infected with S. aureus in the left calf muscle. The distribution of the radiolabel was studied by gamma counting of dissected tissues and gamma camera imaging for 48 hr. As a reference agent, the preparation of 111In-IgG was included in these studies. Clearance of the PEGylated liposomes from the blood compartment was similar to the clearance of 111In-IgG in this model (t1/2 approximately 20 hr). Uptake of the radiolabel in the abscess with the 111In-liposomes was twice as high as the uptake following injection of 111In-IgG (2.7%ID/g versus 1.1%ID/g at 48 hr postinjection). Tissue counting revealed that abscess-to-muscle ratios reached values up to 20 and 34 (24 and 48 p.i., respectively). As early as 1 hr postinjection, the abscess could be visualized scintigraphically. The in vivo characteristics of this liposomal formulation in this rat model indicate that sterically stabilized liposomes labeled with gamma emitters might be a valuable addition to the arsenal of radiopharmaceuticals currently used for infection imaging.