Abstract Body: Background: Atherosclerotic plaque inflammation, driven by activated macrophages, is a major contributor to plaque rupture and acute coronary syndromes. Somatostatin receptor subtype 2 (SSTR2), expressed on activated macrophages, represents an ideal target for molecular imaging, and has been imaged in larger arteries using clinical PET imaging. This study evaluates the potential of a novel near-infrared fluorescence (NIRF) somatostatin-based probe to image macrophage-driven plaque inflammation, with new potential applicability to human coronary sized arteries.
Methods: The binding specificity of the NIRF probe was evaluated in vitro using human macrophages with or without activation by lipopolysaccharide (LPS, 100 ng/mL) and imaged at 1, 6, and 24 hours post-incubation. The pharmacokinetics and biodistribution of the probe were assessed in C57BL/6J mice via fluorescence reflectance imaging. Specificity was validated through competitive inhibition with excess unlabeled somatostatin. The probe’s plaque-targeting efficacy was tested in atherosclerotic ApoE-/- mouse model (N=6) using intravital fluorescence microscopy (IVM), followed by FM and immunohistochemical detection of SSTR2 and macrophage CD68 expression
Results: The probe demonstrated significant and specific uptake by LPS-stiumlated macrophages, with enhanced NIRF signal intensity on microscopy compared to controls (Fig 1A and B p <0.05). In vivo biodistribution studies revealed rapid clearance, with a calculated blood half-life of 15.79 minutes (Fig 1C), and preferential accumulation in macrophage-rich organs such as the liver and spleen. Competitive inhibition with excess unlabeled somatostatin resulted in a significant reduction in probe uptake in hepatic tissue (p<0.01),. In carotid plaques of ApoE-/- mice (Fig 1D), the probe showed substantial accumulation in macrophage-rich regions on IVM, with NIRF signal peaking at 60 minutes post-injection (Fig 1E; normalized intensity ratio: 4.41, p<0.05). Histological analysis (Fig 1F) demonstrated robust colocalization of the SMT probe signal with SSTR2 (Fig 1G; r=0.71, p=0.001) and CD68 (Fig 1G; r=0.54, p=0.002).
Conclusions: This somatostatin-based NIRF probe shows rapid pharmacokinetics, high specificity for SSTR2+ macrophages, and strong efficacy in imaging inflamed plaques. It lays the groundwork for intravascular NIRF imaging to detect high-risk plaques, now under clinical evaluation