18-kDa translocator protein ligand ¹⁸F-FEMPA: Biodistribution and uptake into atherosclerotic plaques in mice

Radioligands of 18-kDa translocator protein (TSPO) expressed on activated macrophages are a potential approach for imaging of inflammation in atherosclerosis. We evaluated a novel TSPO-targeted tracer ¹⁸F-FEMPA for the detection of atherosclerotic plaque inflammation in mice.

The distribution kinetics of ¹⁸F-FEMPA was evaluated by in vivo PET/CT imaging. ¹⁸F-FEMPA uptake was compared in atherosclerotic (LDLR^−/−ApoB^100/100, n = 10) and healthy mice (C57BL/6 N, n = 7) ex vivo at twenty minutes post-injection. Biodistribution was analyzed from harvested tissue samples, and aortas were sectioned for autoradiography. Aortas of LDLR^−/−ApoB^100/100 mice showed large, macrophage-rich atherosclerotic plaques. In vivo, ¹⁸F-FEMPA showed rapid blood clearance but no difference in aortic uptake between atherosclerotic and healthy mice. In the mice studied ex vivo at 20 minutes post-injection, quantification of radioactivity in the whole aorta showed 1.3-fold higher ¹⁸F-FEMPA accumulation in atherosclerotic than healthy mice (P = .028). Autoradiography showed higher tracer uptake in plaque areas with high macrophage content as compared with areas of no macrophages (count densities 190 ± 54 vs 40 ± 13 PSL/mm², P < .001), but the uptake in the plaques was not higher than in the normal vessel wall (230 ± 78 PSL/mm²). In vitro blocking showed specific accumulation in mouse and human atherosclerotic plaques. Immunohistochemistry confirmed co-localization of TSPO and macrophages.

¹⁸F-FEMPA shows rapid blood clearance and uptake in the mouse aorta. Uptake in atherosclerotic plaques correlated with the amount of macrophages, but did not exceed that in the normal vessel wall.