Imaging of Atherosclerosis in Apoliprotein E Knockout Mice: Targeting of a Folate-Conjugated Radiopharmaceutical to Activated Macrophages

Early detection of heart disease is essential for the implementation of intervention strategies that reduce the risk of cardiovascular events. Radioimaging methods that have been explored for this purpose include 18F-FDG, which measures sites of elevated metabolic activity; 99mTc-annexin A5, which reveals regions of enhanced apoptosis and thrombosis; and 99mTc-labeled anti– lectinlike oxidized low-density lipoprotein receptor 1 antibody, which detects the lectinlike oxidized low-density lipoprotein receptor 1 that is overexpressed on a variety of vasculatureassociated cells. In this study, we examine the use of a folatetargeted chelate of 99mTc, termed 99mTc-EC20, for imaging of folate receptor (FR)–expressing macrophages that accumulate in atherosclerotic plaques, internalize cholesterol-rich lipoprotein particles, and evolve into foam cells that form components of vulnerable atherosclerotic lesions. Methods: 99mTc-EC20 was injected into apoliprotein E knockout (apoE2/2) mice fed a normal or Western (high-fat) diet for 25 wk and imaged by g-scintigraphy. Treated mice were also dissected, and radioactivities in excised aortas were quantified by g-counting and imaged by autoradiography. The role of FR-expressing macrophages in uptake of 99mTc-EC20 was also examined by comparing images of apoE2/2 mice before and after treatment with clodronate liposomes to deplete tissue macrophages, comparing the sites of 99mTc-EC20 enrichment with sites of macrophage accumulation in thin sections of atherosclerotic tissues, and examining the expression of FRs on atherosclerotic plaque– derived macrophages by flow cytometry. Results: ApoE2/2 mice on Western chow exhibited significantly greater accumulation of 99mTc-EC20 in atherosclerotic lesions than their counterparts on normal chow. The aortas of apoE2/2 mice on a Western diet demonstrated greater numbers of FR-positive macrophages by flow cytometry than did those of apoE2/2 mice on a normal diet. Clodronate liposome treatment significantly reduced the accumulation of 99mTc-EC20 in atherosclerotic tissues, suggesting that macrophages or monocytes are responsible for uptake of the folate-linked radioimaging agent. Histologic and autoradiographic analysis of tissue sections demonstrated that macrophage accumulation correlated with regions of 99mTc-EC20 uptake. Conclusion: 99mTc-EC20 can be used for the imaging of atherosclerosis by selectively targeting FR-positive activated macrophages.