Targeting of vascular cell adhesion molecule-1 by 18F-labelled nanobodies for PET/CT imaging of inflamed atherosclerotic plaques

Abstract

Aims Positron emission tomography-computed tomography (PET-CT) is a highly sensitive clinical molecular imaging modality to study atherosclerotic plaque biology. Therefore, we sought to develop a new PET tracer, targeting vascular cell adhesion molecule (VCAM)-1 and validate it in a murine atherosclerotic model as a potential agent to detect atherosclerotic plaque inflammation. Methods and results The anti-VCAM-1 nanobody (Nb) (cAbVCAM-1-5) was radiolabelled with Fluorine-18 (18F), with a radiochemical purity of .98%. In vitro cell-binding studies showed specific binding of the tracer to VCAM-1 expressing cells. In vivo PET/CT imaging of ApoE2/2 mice fed aWestern diet or control mice was performed at 2h30 post-injection of [18F]-FB-cAbVCAM-1-5 or 18F-control Nb. Additionally, plaque uptake in different aorta segments was evaluated ex vivo based on extent of atherosclerosis. Atherosclerotic lesions in the aortic arch of ApoE2/2 mice, injected with [18F]-FB-anti-VCAM-1 Nb, were successfully identified using PET/CT imaging, while background signal was observed in the control groups. These results were confirmed by ex vivo analyses where uptake of [18F]-FB-cAbVCAM-1-5 in atherosclerotic lesions was significantly higher compared with control groups. Moreover, uptake increased with the increasing extent of atherosclerosis (Score 0: 0.68+0.10, Score 1: 1.18+0.36, Score 2: 1.49+0.37, Score 3: 1.48+0.38%ID/g, Spearman's r2 0.675, P , 0.0001). High lesion-to-heart, lesion-to-blood, and lesion-to-control vessel ratios were obtained (12.4+0.4, 3.3+0.4, and 3.1+0.6, respectively). Conclusion The [18F]-FB-anti-VCAM-1 Nb, cross-reactive for both mouse and human VCAM-1, allows non-invasive PET/CT imaging of VCAM-1 expression in atherosclerotic plaques in a murine model and may represent an attractive tool for imaging vulnerable atherosclerotic plaques in patients.