Integrin αvβ3 has been considered as a promising biomarker for vulnerable atherosclerotic plaques, and it is highly expressed by those instability-associated factors, such as macrophages, vessel endothelial cells, and smooth muscle cells. Our previous study successfully showed that the 64Cu-labeled divalent (containing two RGD motifs) cystine knot peptide, 64Cu-NOTA-3-4A, had high binding affinity and specificity in targeting vulnerable carotid atherosclerotic plaques with increased αvβ3 levels. Therefore, considering that 68Ga has excellent nuclear physical properties for positron emission tomography (PET), this study aimed to investigate the feasibility of using 68Ga-NOTA-3-4A for PET study of vulnerable atherosclerotic plaques. The vulnerable carotid atherosclerotic plaques were induced and maintained in ApoE-/- mice through carotid artery ligation and a high-fat diet. Divalent knottin peptide 3-4A was synthesized through solid-phase peptide synthesis chemistry and radiolabeled with 68Ga after being conjugated with 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA). The probe stability was analyzed in phosphate buffered saline (PBS) buffer and mouse serum. ApoE-/- mice with atherosclerotic plaques ( n = 4) were imaged by PET/CT at 1 and 2 h after the tail vein injection of 68Ga-NOTA-3-4A. The targeting specificity was determined by coinjection of 68Ga-NOTA-3-4A and nonradioactive c(RGDyK) peptide. The carotid artery tissues were removed, and immunofluorescent staining was performed to evaluate αvβ3 integrin expression. It was found that 68Ga-NOTA-3-4A displayed high stability in both PBS buffer and mouse serum. Small animal PET/CT images and quantification analysis indicated the quick and high plaque uptake of 68Ga-NOTA-3-4A (6.67 ± 1.44 and 2.97 ± 0.46%ID/g at 1 and 2 h, respectively). The plaque-to-normal artery ratio was 15.88 and 9.90 at 1 and 2 h, respectively. Furthermore, the plaque accumulation of 68Ga-NOTA-3-4A was significantly inhibited via coinjection of c(RGDyK). Finally, immunostaining identified integrin αvβ3 expressed by macrophages, vessel endothelial cells, and smooth muscle cells. In summary, 68Ga-NOTA-3-4A has high potential to be a promising PET tracer for imaging vulnerable atherosclerotic plaques.
Keywords: 68Ga; PET; atherosclerotic plaque; cystine knot peptide; divalent; integrin αvβ3.