Receptor-targeted iron oxide nanoparticles for molecular MR imaging of inflamed atherosclerotic plaques

Abstract

In a number of literature reports iron oxide nanoparticles have been investigated for use in imaging atherosclerotic plaques and found to accumulate in plaques via uptake by macrophages, which are critical in the process of atheroma initiation, propagation, and rupture. However, the uptake of these agents is non-specific; thus the labeling efficiency for plaques in vivo is not ideal. We have developed targeted agents to improve the efficiency for labeling macrophage-laden plaques. These probes are based on iron oxide nanoparticles coated with dextran sulfate, a ligand of macrophage scavenger receptor type A (SR-A). We have sulfated dextran-coated iron oxide nanoparticles (DIO) with sulfur trioxide, thereby targeting our nanoparticle imaging agents to SR-A. The sulfated DIO (SDIO) remained mono-dispersed and had an average hydrodynamic diameter of 62 nm, an r(1) relaxivity of 18.1 mM(-1) s(-1), and an r(2) relaxivity of 95.8 mM(-1) s(-1) (37 °C, 1.4 T). Cell studies confirmed that these nanoparticles were nontoxic and specifically targeted to macrophages. In vivo MRI after intravenous injection of the contrast agent into an atherosclerotic mouse injury model showed substantial signal loss on the injured carotid at 4 and 24 h post-injection of SDIO. No discernable signal decrease was seen at the control carotid and only mild signal loss was observed for the injured carotid post-injection of non-sulfated DIO, indicating preferential uptake of the SDIO particles at the site of atherosclerotic plaque. These results indicate that SDIO can facilitate MRI detection and diagnosis of vulnerable plaques in atherosclerosis.

Fig. 1

(a) TEM, and (b) DLS of dextran sulfate coated iron oxide nanoparticles (SDIO).

Fig. 2

Mean T₂ values of cell lysates incubated for 2 h with SDIO () or DIO () of different iron concentrations.

Fig. 3

Competitive uptake of SDIO ([Fe] = 5.0 × 10⁻⁵ M) and dextran sulfate (), or dextran () by P388D1 cells.

Fig. 4

Cell viability of P388D1 after 4- (), or 24-h () incubation with different concentrations of SDIO.

Fig. 5

Signal change in MRI over time after SDIO (a) or DIO (b) injection. The ligated carotid artery is denoted by the yellow arrow and the control carotid artery is denoted by the red arrow. Circles indicate the ROI measures used to derive the contrast ratio (CR) metric. To facilitate comparison, the magnified images in carotid areas were first rotated in plane so all time points in the orientation. They were then zero-filled by a factor of 2 and then smoothed with a 2×2 Gaussian filter. (Scale bar = 10 mm, both scale bar and intensity bar apply to the whole animal images only. L = left, R = right, green box shows magnification area, white arrow denotes the trachea)

Fig. 6

Contrast Ratio between SDIO and DIO particle accumulation in vivo.

Scheme 1

Sulfation of dextran coated iron oxide nanoparticles.