Abstract 18681: Molecular Imaging of Angiogenesis in Atherosclerosis Using a Caspase 3 Targeted PET Tracer ¹⁸F-CP18

Abstract

Background: Rupture of unstable coronary artery plaque results in luminal thrombosis and myocardial infarction. Smooth muscle migration, neovascularization and apoptosis are believed to constitute key steps that precede plaque rupture. Identification of plaque with enhanced apoptosis may therefore allow differentation of stable from high-risk plaque. We have developed [¹⁸F]CP18, a PET tracer that is cleaved by caspase 3, a terminal enzyme in the apoptosis cascade. We sought to test the ability of this tracer to detect plaque apoptosis in an ApoE double knockout mouse model of atherosclerosis.

Methods: ApoE knockout mice (n=6) were fed a high fat western diet for 30 weeks at the end of which 250µCi of ¹⁸F-CP18 was injected intravenously. Mice were sacrificed 60 min post-injection and the aorta was excised. Ex-vivo autoradiography was performed using either whole aorta or 10 µm-thick transverse sections of the aorta mounted on glass slides. Apoptotic activity and plaque inflammation in tissue sections from similar locations in the aorta were visually assessed using TUNEL staining and CD68 antibody staining respectively. Lipid deposition was visualized by oil red O staining with an H&E counterstain.

Results: In the ApoE KO mice fed a high fat diet, atherosclerotic plaque visualized by Oil Red O was present in the aortic root and aortic arch. Qualitative analysis of ex-vivo autoradiography of the aorta showed preferential uptake of ¹⁸F-CP18 by aortic plaque at these same locations. Tunel staining and CD68 staining co-localized with plaques that exhibited increased ¹⁸F-CP18 uptake. In contrast, in ApoE mice fed a regular diet and in wild-type mice fed the high-fat diet, there were fewer plaques in the aorta, with minimal tracer uptake and negligible histochemical evidence of macrophage influx and apoptotic activity.

Conclusion: We were able to label a caspase 3 substrate CP18 with ¹⁸F by click chemistry. Preliminary studies demonstrate that uptake of this tracer in aortic plaque strongly correlates with high levels of apoptotic activity by TUNEL staining and macrophage entry detected by CD68 staining. ¹⁸F-CP18 may be a valuable imaging agent to detect plaque apoptosis and may allow early detection of hig-risk plaque.