Abstract
OBJECTIVES: This study sought to develop an integrative positron emission tomography (PET) with magnetic resonance imaging (MRI) procedure for accurate atherosclerotic plaque phenotyping, facilitated by clinically approved and nanobody radiotracers. BACKGROUND: Noninvasive characterization of atherosclerosis remains a challenge in clinical practice. The limitations of current diagnostic methods demonstrate that, in addition to atherosclerotic plaque morphology and composition, disease activity needs to be evaluated. METHODS: We screened 3 nanobody radiotracers targeted to different biomarkers of atherosclerosis progression, namely vascular cell adhesion molecule (VCAM)-1, lectin-like oxidized low-density lipoprotein receptor (LOX)-1, and macrophage mannose receptor (MMR). The nanobodies, initially radiolabeled with copper-64 ((64)Cu), were extensively evaluated in Apoe(-/-) mice and atherosclerotic rabbits using a combination of in vivo PET/MRI readouts and ex vivo radioactivity counting, autoradiography, and histological analyses. RESULTS: The 3 nanobody radiotracers accumulated in atherosclerotic plaques and displayed short circulation times due to fast renal clearance. The MMR nanobody was selected for labeling with gallium-68 ((68)Ga), a short-lived radioisotope with high clinical relevance, and used in an ensuing atherosclerosis progression PET/MRI study. Macrophage burden was longitudinally studied by (68)Ga-MMR-PET, plaque burden by T2-weighted MRI, and neovascularization by dynamic contrast-enhanced (DCE) MRI. Additionally, inflammation and microcalcifications were evaluated by fluorine-18 ((18)F)-labeled fluorodeoxyglucose ((18)F-FDG) and (18)F-sodium fluoride ((18)F-NaF) PET, respectively. We observed an increase in all the aforementioned measures as disease progressed, and the imaging signatures correlated with histopathological features. CONCLUSIONS: We have evaluated nanobody-based radiotracers in rabbits and developed an integrative PET/MRI protocol that allows noninvasive assessment of different processes relevant to atherosclerosis progression. This approach allows the multiparametric study of atherosclerosis and can aid in early stage anti-atherosclerosis drug trials.