Targeting P-Selectin by Gallium-68–Labeled Fucoidan Positron Emission Tomography for Noninvasive Characterization of Vulnerable Plaques

Abstract

Objective-Nuclear imaging of active plaques still remains challenging. Advanced atherosclerotic plaques have a strong expression of P-selectin by the endothelium overlying active atherosclerotic plaques, but not on the endothelium overlying inactive fibrous plaques. We proposed a new approach for noninvasive in vivo characterization of P-selectin on active plaques based on 68 Ga-Fucoidan, which is a polysaccharidic ligand of P-selectin with a nanomolar affinity. Approach and Results-68 Ga-Fucoidan was tested for its potential to discriminate vulnerable plaques on apolipoprotein E-deficient mice receiving a high cholesterol diet by positron emission tomography and in correlation with 17.6T MRI. Furthermore, 68 Ga-Fucoidan was evaluated on endothelial cells in vitro and ex vivo on active plaques using autoradiography. The cellular uptake rate was increased ≈2-fold by lipopolysaccharide induction. Interestingly, on autoradiography, more intensive tracer accumulation at active plaques with thin fibrous caps and high-density foam cells were observed in comparison with a weaker focal uptake in inactive fibrous plaque segments (R=1.7±0.3; P<0.05) and fatty streaks (R=2.4±0.4; P<0.01). Strong uptake of radiotracer colocalized with increased P-selectin expression and high-density macrophage. Focal vascular uptake (mean of target to background ratio=5.1±0.8) of 68 Ga-Fucoidan was detected in all apolipoprotein E-deficient mice. Anatomic structures of plaque were confirmed by 17.6T MRI. The autoradiography showed a good agreement of 68 Ga-Fucoidan uptake with positron emission tomography. Conclusions-Our data suggest that 68 Ga-Fucoidan represents a versatile imaging biomarker for P-selectin with the potential to specifically detect P-selectin expression using positron emission tomography and to discriminate vulnerable plaques in vivo. pivotal role in recruiting leukocytes to the sites of injury. 14-16This interaction is mediated by P-selectin glycoprotein ligand 1, expressed by monocytes, neutrophils, and platelets. 17,18 Most active atherosclerotic lesions remain undetected until plaque rupture and thrombosis occur. A previous study found that inflamed atherosclerotic plaques have a strong expression of P-selectin on the overlying endothelium, but much less in normal arterial endothelium or in endothelium overlying inactive fibrous plaques.19 Consequently, P-selectin is thought to be an effective biomarker for assessing the bioactivity of active plaques.Fucoidan is a synthetic sialyl-lewis X mimic, which is the natural ligand of P-selectin and is found on leukocytes. 20,21 It is mainly derived from brown seaweed with an efficient binding of P-selectin as well. 22 In previous studies, a high specific affinity of fucoidan for P-selectin was confirmed. 22We developed a novel PET tracer by an efficient introduction of the positron emitter 68 Ga into the Fucoidan moiety. The 68 Ga-Fucoidan obtained this way was used to validate the P-selectin expression in vivo on an established apolipoprotein E-deficient (apoE −/− ) m...

Exploring the nature of atheroma and cardiovascular inflammationin vivousing positron emission tomography (PET)

“…Other agents such as gallium-68 ( 68 Ga) octreotide have been shown to identify vascular inflammation possibly more specifically than 18 F-FDG. Hypoxia within the plaque can be imaged with 18 F-fluoromisonidazole and resulting angiogenesis with 18 F-RGD peptides. Active calcification such as that found in unstable atheromatous plaques can be imaged with 18 F-NaF.…”

“…Hypoxia within the plaque can be imaged with 18 F-fluoromisonidazole and resulting angiogenesis with 18 F-RGD peptides. Active calcification such as that found in unstable atheromatous plaques can be imaged with 18 F-NaF. PET imaging enables us to understand the mechanisms by which cardiovascular disease, including atheroma, leads to morbidity and death and thus increases the chance of finding new and effective treatments.…”

“…The most commonly used radiopharmaceutical for PET imaging is 18-fludeoxyglucose ( 18 F-FDG), but it is a non-specific agent and relies on the fact that cancer cells have a higher uptake of glucose than do normal tissue. There is also increased uptake of 18 F-FDG in other cell types including both lymphocytes and macrophages. Therefore, when reading an 18 F-FDG PET study, care must be taken not to assume that focal uptake of 18 F-FDG is a site of cancer when it could be owing, for example, to inflammation such as can be seen in atheroma in a major artery.…”

“…There is also increased uptake of 18 F-FDG in other cell types including both lymphocytes and macrophages. Therefore, when reading an 18 F-FDG PET study, care must be taken not to assume that focal uptake of 18 F-FDG is a site of cancer when it could be owing, for example, to inflammation such as can be seen in atheroma in a major artery. The use of fusion imaging with PET-CT allows for better localization and should ensure correct localization ( Figure 1).…”

“…The use of fusion imaging with PET-CT allows for better localization and should ensure correct localization ( Figure 1). There has been increasing use of 18 F-FDG imaging in inflammation, and recently published European and North American guidelines have recommended that 18 F-FDG PET can be used in cardiovascular inflammation. 1 In oncology, the non-specific nature of 18 F-FDG has led to the development of a toolbox of different PET radiopharmaceuticals to look at a range of different aspects of cancer cell function and metabolism, such as carbon-11 ( 11 C) methionine to look at amino acid metabolism, 18 F-fluorolevothymidine to look at DNA synthesis in cancer cells and 18 F-fluoromisonidazole (FMISO) to look at hypoxia.…”