In Vivo Molecular Imaging of Atherosclerotic Lesions in ApoE-/- Mice Using VCAM-1-Specific, 99mTc-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-99m-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-99m-radiolabeled B2702p derivatives might allow the molecular imaging of VCAM-1 expression in an experimental model of atherosclerosis. Methods: 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-99m-B2702p and Tc-99m-B2702p1-Tc-99m-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-99m-B2702p1. The animals were then euthanized, and the biodistribution of Tc-99m-labeled peptides was evaluated by gamma-well counting of excised organs. Expression of VCAM-1 in the ligated and contralateral carotid arteries was evaluated by immunohistology. Results: 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-99m-B2702p1 exhibited the most favorable properties. By gamma-well counting, there was a significant 2.0-fold increase in the Tc-99m-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 Tc-99m-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-99m-82702p1- than in Tc-99m-B2702p-injected mice (1.2 +/- 0.1 vs. 1.0 +/- 0.0, respectively, P < 0.05). Finally, a significantly higher Tc-99m-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/mm(2)/injected dose, respectively, P < 0.05). Conclusion: Tc-99m-B2702p1 is a potentially useful radiotracer for the in vivo molecular imaging of VCAM-1 expression in atherosclerotic plaques.