Molecular Imaging of Inflammation in a Mouse Model of Atherosclerosis Using a Zirconium-89-Labeled Probe

Background: Beyond clinical atherosclerosis imaging of vessel stenosis and plaque morphology, early detection of inflamed atherosclerotic lesions by molecular imaging could improve risk assessment and clinical management in high-risk patients. To identify inflamed atherosclerotic lesions by molecular imaging in vivo, we studied the specificity of our radiotracer based on maleylated (Mal) human serum albumin (HSA), which targets key features of unstable atherosclerotic lesions.

Materials and methods: Mal-HSA was radiolabeled with a positron-emitting metal ion, zirconium-89 (⁸⁹Zr⁴⁺). The targeting potential of this probe was compared with unspecific ⁸⁹Zr-HSA and ¹⁸F-FDG in an experimental model of atherosclerosis (Apoe^-/- mice, n=22), and compared with wild-type (WT) mice (C57BL/6J, n=21) as controls.

Results: PET/MRI, gamma counter measurements, and autoradiography showed the accumulation of ⁸⁹Zr-Mal-HSA in the atherosclerotic lesions of Apoe^-/- mice. The maximum standardized uptake values (SUV_max) for ⁸⁹Zr-Mal-HSA at 16 and 20 weeks were 26% and 20% higher (P<0.05) in Apoe^-/- mice than in control WT mice, whereas no difference in SUV_max was observed for ¹⁸F-FDG in the same animals. ⁸⁹Zr-Mal-HSA uptake in the aorta, as evaluated by a gamma counter 48 h postinjection, was 32% higher (P<0.01) for Apoe^-/- mice than in WT mice, and the aorta-to-blood ratio was 8-fold higher (P<0.001) for ⁸⁹Zr-Mal-HSA compared with unspecific ⁸⁹Zr-HSA. HSA-based probes were mainly distributed to the liver, spleen, kidneys, bone, and lymph nodes. The phosphor imaging autoradiography (PI-ARG) results corroborated the PET and gamma counter measurements, showing higher accumulation of ⁸⁹Zr-Mal-HSA in the aortas of Apoe^-/- mice than in WT mice (9.4±1.4 vs 0.8±0.3%; P<0.001).

Conclusion: ⁸⁹Zr radiolabeling of Mal-HSA probes resulted in detectable activity in atherosclerotic lesions in aortas of Apoe^-/- mice, as demonstrated by quantitative in vivo PET/MRI. ⁸⁹Zr-Mal-HSA appears to be a promising diagnostic tool for the early identification of macrophage-rich areas of inflammation in atherosclerosis.

Keywords: atherosclerosis; human serum albumin; macrophages; molecular imaging; positron emission tomography; zirconium.