Abstract 151: Thrombosis Prone Plaques Exhibit Inflammation Prior to Rupture: Intravascular Detection via Macrophage-targeted Optical Imaging

Abstract

Introduction Inflammation modulates atherogenesis and the destabilization of plaques. However, the role of macrophages in plaque rupture is incompletely understood. Here we investigated the in vivo spatial distribution of plaque macrophages prior to triggered plaque rupture, using intravascular molecular imaging and a near-infrared fluorescence (NIRF) macrophage targeted agent, CLIO-CyAm7. We hypothesized that CLIO-CyAm7 would illuminate macrophages in vivo and preferentially localize to plaques that are vulnerable to plaque rupture. Methods Atherosclerosis was induced in rabbits (n=14) using a 12-week hyperlipidemic diet with alternating 1% high cholesterol and normal chow, concomitant with aortic balloon injury at 2 weeks. Rabbits were injected with 2.5mg/kg (n=8) or 5mg/kg (n=6) of CLIO-CyAm7 at 24 or 48 hours, respectively, prior to thrombosis triggering using Russell’s Viper Venom and histamine over 48 hours. The 2.5mg/kg subgroup was imaged in vivo using NIRF imaging and intravascular ultrasound (IVUS), immediately prior to thrombosis triggering. After sacrifice, ex vivo imaging, fluorescence microscopy (FM), Carstairs and Ram-11 immunostaining were performed. Results FM identified significantly higher CLIO-CyAm7 signal in areas of atherosclerosis compared to normal segments of the aorta (2.5mg/kg and 5.0mg/kg, p=0.03 and p=0.03). This signal was primarily localized at the intimal-luminal border of atheroma, with some penetration in the media and adventitia. CLIO-CyAm7 colocalized with a subset of superficial macrophages. Carstairs staining identified fibrin-rich thrombi, indicating plaque rupture in 6 of 14 rabbits (43%). There was increased CLIO-CyAm7 in areas underlying thrombus and particularly at the shoulder of ruptured plaques. Plaque macrophages were detectable via in vivo intravascular NIRF imaging. Areas of atherosclerosis, determined by IVUS, showed significantly higher NIRF peak signal-to-noise ratio than normal segments of the aorta (40.2±16.9 and 5.9±1.9, p=0.02). Conclusion Macrophages are consistently present in plaques that undergo triggered rupture, and can be detected via CLIO-CyAm7 intravascular NIRF molecular imaging. In vivo macrophage imaging may identify thrombosis-prone plaques.