2105 Graphite/metal core-shell nanocrystals as MRI contrast agents to detect vascular inflammation

Introduction

Noninvasive imaging of atherosclerosis may benefit from the characterization of plaque biological activity. Macrophages are ideal targets as they have a critical role in progression of atherosclerotic lesions. Novel graphite/metal core-shell nanocrystals (CN) show promising properties as MRI contrast agents for cellular imaging [1].

Purpose

To evaluate 1) CN uptake by macrophages in vitro, and 2) CN uptake in mouse atherosclerotic lesions in vivo.

Methods

Graphite/Metal core-shell nanocrystals

CN are composed of an Fe/Co core with a carbon graphite shell and phospholipid-polyethylene glycol molecules, with Cy5.5 attached for in vivo experiments.

In Vitro Uptake/Imaging

Mouse macrophage cells (RAW264) were incubated with ferumoxytol (Feridex), ferumoxtran-10 (Combidex), or graphite/metal core-shell nanocrystals (CN) for 24 hours, each at a concentration of 100 ugFe/ml. After incubation, 5 × 10⁶ cells from each group were scanned by MRI at 1.5 T (GE Healthcare, Milwaukee, WI) to examine cellular uptake of contrast using a standard gradient echo sequence (TR/TE = 100/10, FA = 30, Matrix = 256 × 256, slice thickness = 2.0, FOV = 12 cm).

In Vivo Uptake/Imaging

FVB mice (N = 5) underwent a carotid-ligation procedure previously shown to produce a macrophage-rich atherosclerotic lesion. Briefly, they were given high fat diet for 4 weeks and then had diabetes induced by 5 daily intraperitoneal injections of streptozotocin. After 2 weeks of diabetes, carotid ligation of the left carotid artery was performed. Two weeks post carotid ligation, mice were given CN-Cy5.5 (8 nmol of Cy5.5) via tail vein and scanned serially over 12–48 hours using the Maestro in-vivo fluorescent imaging system (CRI, Woburn, MA). After final in vivo imaging, carotid arteries were exposed and both in situ and ex vivo imaging were performed.

Results

In vitro CN uptake by macrophages was clearly seen by T2*-weighted MRI. The area of signal loss was significantly greater (p < 0.001, Figure 1) for CN than ferumoxtran-10 (Combidex; common in vivo macrophage imaging agent). However, the uptake of CN in vitro was less than ferumoxytol (Feridex; common in vitro stem cell labeling agent).

Figure 1

Contrast uptake by macrophages (T2 weighted images).

In vivo fluorescence imaging showed limited carotid signal, but both in situ and ex vivo fluorescence imaging at 12–48 hours showed high signal from the ligated left carotid, confirming in vivo CN uptake in the atherosclerotic lesions (Figure 2). No evidence of CN accumulation was seen in the non-ligated right carotid arteries.

Figure 2

Fluorescence imaging in mice.

Conclusion

The novel CN contrast agent was effectively taken up by macrophages in vitro and by macrophage-rich atherosclerotic lesions in vivo. Further development of in vivo MRI and fluorescence imaging of graphite/metal core-shell nanocrystals may allow direct noninvasive detection of vascular inflammation.