Identification of High-Risk Plaques by MRI and Fluorescence Imaging in a Rabbit Model of Atherothrombosis

Abstract

IntroductionThe detection of atherosclerotic plaques at risk for disruption will be greatly enhanced by molecular probes that target vessel wall biomarkers. Here, we test if fluorescently-labeled Activatable Cell Penetrating Peptides (ACPPs) could differentiate stable plaques from vulnerable plaques that disrupt, forming a luminal thrombus. Additionally, we test the efficacy of a combined ACPP and MRI technique for identifying plaques at high risk of rupture.Methods and ResultsIn an atherothrombotic rabbit model, disrupted plaques were identified with in vivo MRI and co-registered in the same rabbit aorta with the in vivo uptake of ACPPs, cleaved by matrix metalloproteinases (MMPs) or thrombin. ACPP uptake, mapped ex vivo in whole aortas, was higher in disrupted compared to non-disrupted plaques. Specifically, disrupted plaques demonstrated a 4.5~5.0 fold increase in fluorescence enhancement, while non-disrupted plaques showed only a 2.2~2.5 fold signal increase. Receiver operating characteristic (ROC) analysis indicates that both ACPPs (MMP and thrombin) show high specificity (84.2% and 83.2%) and sensitivity (80.0% and 85.7%) in detecting disrupted plaques. The detection power of ACPPs was improved when combined with the MRI derived measure, outward remodeling ratio.ConclusionsOur targeted fluorescence ACPP probes distinguished disrupted plaques from stable plaques with high sensitivity and specificity. The combination of anatomic, MRI-derived predictors for disruption and ACPP uptake can further improve the power for identification of high-risk plaques and suggests future development of ACPPs with molecular MRI as a readout.

Imaging the Extracellular Matrix in Prevalent Cardiovascular Diseases

“…This contrast agent has also been shown to distinguish dietary-induced variations in MMP-related enzymatic activity within plaques in an atherosclerotic model, supporting its utility as a clinical imaging tool for in vivo detection of arterial wall remodelling [83]. Haas In a more recent study, fluorescent-labelled activatable cell penetrating peptides (ACPPs) designed to target MMPs were tested in a rabbit model of plaque rupture [84]. Fluorescence enhancement was significantly higher in ruptured plaques ex vivo, suggesting that ACPP probes can selectively differentiate unstable from stable plaques (Figure 14).…”

Monocyte and Macrophage Dynamics in the Cardiovascular System

“…The tissue accumulation of gadolinium-based MRI contrast agents is related to plaque neovascularization, permeability and, therefore, indirectly reflects the active recruitment of monocytes and accumulation of macrophages in plaques. Contrast agent uptake in the vessel wall can be assessed by measuring the extent of signal enhancement in T1-weighted MR images (53–57) and/or T1 relaxation time after injection (51,58–61). For better quantification, the temporal pattern of contrast agent uptake can be analyzed by looking at signal enhancement over time using dynamic contrast enhanced (DCE-) MRI (62).…”

Molecular imaging of the extracellular matrix in the context of atherosclerosis

“…In a different study, Hua et al evaluated a fluorescent-labeled activatable cell penetrating peptid (ACPP), cleaved by matrix metalloproteinases (MMPs) in a rabbit model. This targeted ACPP probe enabled the discrimination of ruptured and stable atherosclerotic plaques in an experimental study [135].…”