Noninvasive detection of atherosclerotic lesions by ^99mTc-based immunoscintigraphic targeting of proliferating smooth muscle cells

Background: Mouse/human chimeric antibody Z2D3 identifies an antigen produced exclusively by proliferating smooth muscle cells of human atheroma, and also cross reacts with experimentally induced atherosclerotic lesions in rabbits. Fab' fragments of Z2D3 antibody were labeled with ^99mTc using glucaric acid as a weak transchelator. The potential role of ^99mTc- labeled Z2D3 scintigraphy was explored for noninvasive imaging of experimental atherosclerotic lesions. Methods and results: ^99mTc-Z2D3 Fab' was utilized for noninvasive imaging in four rabbits with experimentally induced atherosclerotic lesions and in one control rabbit. In addition, ^99mTc-labeled nonspecific 103D2 Fab' was used for comparison in four other rabbits with atherosclerotic lesions. The atherosclerotic lesions were induced by balloon de-endothelialization of the infradiaphragmatic abdominal aorta and 12 weeks of hyperlipidemic diet. An aliquot of 15 mCi (550 mBq) of ^99mTc pertechnetate was incubated with 6.25 mg of glucaric acid for 30 min followed by incubation of ^99mTc glucarate with 375 μg of Z2D3 Fab' or 103D2 Fab' for an additional 30 mi. Instant thin-layer chromatography demonstrated almost complete radiolabeling. ^99mTc-Z2D3 was administered IV and gamma imaging was performed at the time of injection, 3, 6, 9, and 12 h, followed by ex vivo imaging of the excised aorta, and biodistribution was performed. Unequivocal visualization of atherosclerotic lesions was possible in all four animals at 9 to 12 h with Z2D3 Fab'. Quantitative uptake, as represented by mean lesion-to-liver count density ratio, was 0.6±0.05. Imaging with nonspecific 103D2 Fab' did not show any localization in the abdominal aorta (lesion-to-liver ratio, 0.45±0.02, p=0.02). Ex vivo lesion- to-normal aortic segment ration was 4.3±0.9 for A2D3 and 1.04±0.08 for nonspecific 103D2 Fab' (p=0.01). Biodistribution studies demonstrated 0.03±0.003% injected Z2D3 dose per gram in the atherosclerotic lesions as compared with 0.01±0.003% in the nondenuded thoracic aorta of atherosclerotic rabbits (p=0.008). However, only 0.008±0.002% of the mean injected dose per gram was obtained in the atherosclerotic lesions (p=0.001) as compared with 0.005±0.003% in the normal aortic segments with 103D2. No Z2D3 uptake in normal rabbits was observed in either the in vivo or ex vivo images. Conclusions: The present study demonstrates that ^99mTc-based immunoimaging of the vascular lesions may be feasible by the use of smaller antibody fragments. Earlier visualization is possible at the expense of a lower absolute antibody uptake in the lesions as compared to the use of intact antibody or larger fragments with longer circulating time.