Multispectral Near-Infrared Fluorescence Molecular Imaging of Matrix Metalloproteinases in a Human Carotid Plaque Using a Matrix-Degrading Metalloproteinase–Sensitive Activatable Fluorescent Probe

Abstract

F ormation of unstable atherosclerotic plaque in the internal carotid artery carries a high risk for emboli and subsequent cerebral ischemic events. The fibrous cap of such a plaque may become thin and rupture as a result of the depletion of matrix components through the activation of proteolytic enzymes such as matrix-degrading proteinases. Enhanced matrix breakdown has been attributed primarily to a family of matrix-degrading metalloproteinases (MMPs) that are highly concentrated in atherosclerotic plaques by inflammatory cells (eg, macrophages, foam cells), smooth muscle cells and endothelial cells. 1 Elevated serum MMP-9 concentration is associated with carotid plaque instability and the presence of infiltrated macrophages. 2 Furthermore, analysis of the presence of MMP-9 protein by ELISA within excised carotid plaques revealed high MMP-9 protein mass in calcified segments at or near the carotid bifurcation and in segments with intraplaque hemorrhage. Gelatin zymography showed an increased gelatinase activity of MMP-9 in these segments. 3 These data favor the important role of MMP-9 in the pathogenesis of plaque instability. We analyzed the topographic distribution of MMPs within an excised human carotid plaque by applying multispectral near-infrared fluorescence (NIRF) imaging (IVIS Spectrum, Caliper Life Sciences, Hopkinton, Mass).A surgical endarterectomy was performed on a 74-year-old women with a left-sided, symptomatic, Ͼ70% carotid stenosis. Immediately after endarterectomy, the plaque was placed in PBS and transported to the NIRF system. The plaque was then stretched out and fixed on a silicon plate with 25G needles. A PBS NIRF image was generated from both the intraluminal and extraluminal side of the plaque to determine the level of autofluorescence (background) (Figure 1). After (Figure 1). Compared with the autofluorescence signal obtained without incubation with MMPSense, a 6-and 7-fold increase of the total NIRF signal was observed on the intraluminal and extraluminal side, respectively, after incubation with MMPSense 680. The NIRF signal was not homogeneously distributed across the plaque surface but rather resulted in the identification of areas with high NIRF intensity (denominated as "hot spots") and areas with relatively low NIRF intensity (so-called "cold spots") ( Figure 1). Because MMP-9 activity has been positively correlated with plaque instability as described above, we next determined the contribution of MMP-9 to the NIRF signals as shown in Figure 1. To this end, in situ zymography was performed on tissue sections from excised hot and cold spots as indicated in Figure 1 by the framed areas. After we had used dye-quenched gelatin as Figure 2. Hot spot identified using NIRF is characterized by increased gelatinase activity compared with coldspot. In situ zymography was performed on cryosections (7 m) of cold (A) and hot spot (B). To this end, sections were incubated with DQ-gelatin (50 g/mL, EnzCheck Gelatinase/Collagenase Assay Kit, Invitrogen-Molecular Probes, Breda, the Netherla...

A Molecular Imaging Primer: Modalities, Imaging Agents, and Applications

“…Through this imaging strategy, Van Fluorescence optical imaging can be used for a variety of molecular imaging applications in living subjects. Some specific examples include the study of matrix metalloproteinases activities using activatable imaging agents (35,447), detection of human epidermal growth factor receptor 2 (HER2/neu)-tumors in animal models using antibodies conjugated to rhodamine green (229), monitoring tumor growth and metastases using GFP-expressing tumors (161), and NIR activatable fluorescent imaging agents for imaging Alzheimer's disease that only "switch on" when bound to amyloid-beta (Abeta) protein (348).…”

Concept of Vulnerable/Unstable Plaque

“…74 The first application of this technology was recently reported in carotid plaques examined ex vivo with a MMP-sensitive fluorescent probe, where hot spots corresponding to proteolytic activity of MMP-9 were identified at the origin of the internal carotid artery and at the level of the common carotid bulb. 75 …”

18F-sodium fluoride positron emission tomography assessed microcalcifications in culprit and non-culprit human carotid plaques

“…These processes have become targets of various molecular imaging techniques, as they potentially allow non-invasive risk stratification of individual patients with carotid artery stenosis. 9,10 Recently, several studies have shown the feasibility of 18 F-sodium fluoride ( 18 F-NaF) positron emission tomography (PET) for imaging of atherosclerotic plaques. 11-13 18 F-NaF predominantly binds to areas of microcalcification within the plaque.…”