Development of a multimodal imaging probe for vulnerable plaque detection.

Objectives We have reported a new vulnerable plaque imaging probe, ¹¹¹In-labeled liposome, previously [Ogawa et al., JNM., 55(1):115-20, 2014.]. The liposome was modified with phosphatidylserine (PS), since macrophages recognize PS to phagocytize apoptotic cells in the plaque. In this study, we developed a multimodal imaging probe for SPECT, optical imaging and MRI.

Methods PS liposomes were prepared by lipid-film hydration method (DSPC: DSPS: chol: PEG2000= 1:1:1:0.05). The prepared size was 200 nm. The liposomes were radiolabeled by encapsulating ¹¹¹In-NTA using active-loading method. For optical imaging, ICG (a near-infrared fluorescent molecule) was encapsulated in the liposome by a freeze-and-thaw method. For MRI, Gd-DTPA was encapsulated in the same manner as ICG. For in vitro studies, the liposomes were incubated with mouse peritoneum derived macrophages. The in vivo studies were done in apoE-/- mice and WHHL rabbits using SPECT, handy NIR camera and 3T MRI.

Results The in vitro uptake was 10.5 % dose/mg protein, which was evaluated by radioactivity measurement. The fluorescence of ICG was quenched by liposome capsulation, and was activated after the disruption of the liposome membrane. The in vitro microscopic observation showed fluorescence activation in macrophages. Similarly, Gd-DTPA labeled liposome behaved as an activatable probe, that is, the MRI signal was enlarged by the disruption of the liposome membrane. In in vivo studies, we could detect the atherosclerotic plaques in apoE-/- mice, and the radioactive and fluorescence images were well matched with Oil-Red O staining. SPECT imaging, real-time fluorescence imaging and MRI were successful in WHHL rabbits.

Conclusions The atherosclerotic plaques were successfully imaged by the multimodal system. Since ICG- and Gd- liposome worked as activatable probes, the background signal could be suppressed to low levels. This multimodal imaging probe can be utilized for quantitative imaging by SPECT, easy-to-use imaging by fluorescence camera, and precise analysis by MRI.