SPECT and Fluorescence Imaging of Vulnerable Atherosclerotic Plaque with a Vascular Cell Adhesion Molecule-1 Single-chain Antibody Fragment

Objectives Early detection and evaluation of vulnerable atherosclerotic plaque is important for risk stratification and timely intervention. Vascular cell adhesion molecule 1 (VCAM-1) is crucial to vulnerable plaque formation because of its assistance in adhesion and recruitment of inflammatory cells to atherosclerotic lesions. We labeled a single-chain variable fragment (scFv) of VCAM-1 with 99mTechnetium (99mTc) and fluorescent markers to investigate its potential utility in detecting vulnerable plaques in animal models of atherosclerosis.

Methods We used 6-hydrazinonicotinamide and N-hydroxysuccinimide ester to label VCAM-1scFv with 99mTc and cyanine5 (CY5). Apolipoprotein E gene-deficient mice and New Zealand White rabbits with induced atherosclerosis were evaluated. Histopathology and western blot examinations confirmed atherosclerotic plaque and elevated VCAM-1 expression in their aortas. In vivo biodistribution of 99mTc-scFv-VCAM1 was studied on model mice. Abdominal organs of experimental and control mice were removed after CY5-scFv-VCAM-1administration for aortic fluorescence imaging. Rabbits underwent SPECT imaging after injection of 99mTc-scFv-VCAM-1, and autoradiography (ARG) of the aortas was performed to confirm the tracer uptake.

Results The radiochemical purity of 99mTc-scFv-VCAM-1 was 98.72 ± 1.04% (n=5) and its specific activity was 7.8 MBq/μg. Biodistribution studies indicated that it was cleared predominantly by the kidneys. In fluorescence imaging, a stronger signal from CY5-scFv-VCAM-1 in the aorta was observed in atherosclerotic mice than that in the controls, and the target-to-background (T/B) ratios of the two groups exhibited statistical significance at both 3 and 6 h (P<0.05). SPECT imaging with 99mTc-scFv-VCAM-1 showed the tracer uptake in the abdominal aorta and aortic arch of atherosclerotic animals (Figure 1), and there was statistical significance of T/B ratios between experimental and control animals at 2 and 4 h (P<0.05 for both). ARG confirmed higher uptake in the aortas of atherosclerotic rabbits, with significantly different uptake ratios of aortic arch/descending aorta between the experimental animals and controls (P<0.05).

Conclusions SPECT and fluorescence imaging results showed the feasibility and effectiveness of detecting vulnerable plaque with scFv of VCAM-1, indicating its potential for early diagnosis and evaluation of atherosclerosis. This work was supported by the National Natural Science Foundation of China (No. 81271623).

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