In Vivo Molecular Imaging of Atherosclerotic Lesions in ApoE-/- Mice Using VCAM-1-Specific, ^sup 99m^Tc-Labeled Peptidic Sequences

Vascular cell adhesion molecule 1 (VCAM-1) plays a major role in the chronic inflammatory processes involved in vulnerable atherosclerotic plaque development. We previously showed that the ...Tc-labeled major histocompatibility complex 1-derived peptide B2702p bound specifically to VCAM-1 and allowed the ex vivo imaging of atherosclerotic lesions in Watanabe heritable hyperlipidemic rabbits. However, B2702p target-to-background ratio was suboptimal for the in vivo imaging of VCAM-1 expression in atherosclerotic lesions. To improve the target-to-background ratio, 20 derivatives of B2702p (B2702p1-B2702p20) were synthesized using the alanine scan methodology. We hypothesized that ...Tc-radiolabeled B2702p derivatives might allow the molecular imaging of VCAM-1 expression in an experimental model of atherosclerosis. A mouse model of focal atherosclerotic plaque development induced by left carotid artery ligation in apolipoprotein E double-knockout (ApoE-/-) mice was used (n = 82). ...Tc-B2702p and ...Tc-B2702p1-...Tc-B2702p20 were injected intravenously in anesthetized animals 3 wk after the ligation. Whole-body planar imaging was performed for 3 h. SPECT imaging of 6 additional ligated ApoE-/- mice was also performed with ...Tc-B2702p1. The animals were then euthanized, and the biodistribution of ...Tc-labeled peptides was evaluated by ...-well counting of excised organs. Expression of VCAM-1 in the ligated and contralateral carotid arteries was evaluated by immunohistology. Robust VCAM-1 immunostaining was observed in the left carotid atherosclerotic lesions as a consequence of artery ligation, whereas no VCAM-1 expression was detected in the contralateral carotid artery. Among all evaluated peptides, ...Tc-B2702p1 exhibited the most favorable properties. By ...-well counting, there was a significant 2.0-fold increase in the ...Tc-B2702p1 left-to-right carotid artery activity ratio (2.6 ± 0.6) and a 3.4-fold increase in the left carotid-to-blood activity ratio (1.4 ± 0.4) in comparison to 99mTc-B2702p (1.3 ± 0.2 and 0.4 ± 0.1, respectively, P < 0.05 for both comparisons). Similarly, planar image quantification indicated a higher left-to-right carotid activity ratio in ...Tc-B2702p1- than in ...Tc-B2702p-injected mice (1.2 ± 0.1 vs. 1.0 ± 0.0, respectively, P < 0.05). Finally, a significantly higher 99mTc-B2702p1 activity in the left than in the right carotid artery was observed by SPECT imaging (2.2 ± 0.4 vs. 1.4 ± 0.3 cpm/mm2/injected dose, respectively, P