Abstract 588: Optimization of Radiolabeled Liposomes for in vivo Atherosclerosis Imaging by SPECT

Abstract

Objectives: Molecular imaging enables non-invasive in vivo detection of vulnerable plaques. Last year, we reported an atherosclerosis imaging agent, 111 In-labeled liposome. The liposome was modified with phosphatidylserine (PS), since macrophages recognize PS to phagocytize apoptotic cells in the plaque. PS modification was successful and we could visualize atherosclerotic plaques by SPECT. However, the too quick blood clearance seemed to have reduced PS-liposome accumulation to the plaque. Therefore, in this study, we PEGylated the liposome to retard the blood clearance. Methods: PS liposomes were prepared by lipid-film hydration method. The liposomes, sizes in 100 or 200 nm, were PEGylated with PEG2000 or PEG5000 in 1 or 5% w/w. The liposomes were radiolabeled by encapsulating 111 In-NTA using active-loading method. For in vitro uptake study, the liposomes were incubated with mouse peritoneum macrophages. The in vivo biodistribution studies were done in ddY mice. En face autoradiograms were obtained with ApoE-/- mice 2 hr after the liposome injection. For SPECT imaging, WHHL rabbits were used. Results: The in vitro uptake levels to macrophages were decreased by PEGylation. 5% PEG5000 decreased the uptake to the half of non-PEGylated one, but 1% PEG2000 affected less than 5%. In vivo results showed slower blood clearance in PEGylated liposomes (12 %dose/g for 5%PEG5000, 2.0 %dose/g for 1%PEG2000, and 1.0 %dose/g for non-PEGylated liposome at 60 min after the injection). PS modification improved target-to-nontarget ratios, and 5%PEG2000 added 200 nm liposome showed the highest uptake to the region. The aorta was successfully visualized by SPECT at 48hr after the 111 In-labeled 5%PEG2000-PS200 liposome injection. Conclusions: PEGlylation prolonged the blood clearance, but decreased the uptake to macrophages. However, the effect was larger for blood clearance and decrease in macrophage uptake was acceptable for in vivo imaging.