Quantification of Inflammation Within Rabbit Atherosclerotic Plaques Using the Macrophage-Specific CT Contrast Agent N1177: A Comparison with 18F-FDG PET/CT and Histology

Macrophages play a key role in atherosclerotic plaque rupture. The iodine-based contrast agent N1177 accumulates in macrophages, allowing for their detection with CT. In this study, we tested whether the intensity of enhancement detected with CT in the aortic wall of rabbits injected with N1177 correlated with inflammatory activity evaluated with 18F-FDG PET/CT and macrophage density on histology. Methods: Atherosclerotic plaques were induced in the aorta of New Zealand White rabbits (n = 7) by a repeated balloon injury (4 wk apart) and 4 mo of hyperlipemic diet. Noninjured rabbits, fed a chow diet, were used as controls (n = 3). A CT scan of the aorta (n = 10) was acquired in each rabbit before, during, and at 2 h after intravenous injection of N1177 (250 mg of iodine/kg). One week later, the same rabbits underwent PET/CT 3 h after injection of 18F-FDG (37 MBq/kg [1 mCi/kg]). CT enhancement was calculated as the difference in aortic wall densities between images obtained before and images obtained at 2 h after injection of N1177. Mean standardized uptake values were measured on PET axial slices of the aorta in regions of interest encompassing the vessel wall. Macrophage density was measured by immunohistology (anti-RAM-11 antibody) on corresponding aortic cross-sections. Results: N1177-enhanced CT measured stronger enhancement in the aortic wall of atherosclerotic rabbits than in control rabbits (10.0 ± 5.2 vs. 2.0 ± 2.1 Hounsfield units, respectively; P < 0.05). After the injection of 18F-FDG, PET detected higher standardized uptake values in the aortic wall of atherosclerotic rabbits than in control rabbits (0.61 ± 0.12 vs. 0.21 ± 0.02; P < 0.05). The intensity of enhancement in the aortic wall measured with CT after injection of N1177 correlated with 18F-FDG uptake on PET/CT (r = 0.61, P < 0.001) and macrophage density on immunohistology (r = 0.63, P < 0.001). Conclusion: The intensity of enhancement detected with CT in the aortic wall of rabbits injected with N1177 correlates with intense uptake of 18F-FDG measured with PET and with macrophage density on histology, suggesting a role for N1177 in noninvasive identification of high-risk atherosclerotic plaques with CT.