Our main results are that 68Ga-NODAGA-RGD arterial wall uptake was higher in patients with previous clinically documented ASCVD and correlated with prior cardiovascular or cerebrovascular event and with progressive atherosclerotic plaque burden. Our study would suggest that 68Ga-NODAGA-RGD holds promise as a non-invasive marker of disease activity in atherosclerosis, providing information on key features of high-risk atheroma: inflammation and angiogenesis.
Both inflammation and angiogenesis processes are associated with atheroma progression, plaque rupture and clinical events. The necrotic core in culprit plaques forms as result of increasing inflammation (20). In response to pro-atherogenic stimuli, activated monocytes infiltrated within the intima and differentiate into pro-inflammatory macrophages (21). While progressing, atherosclerotic plaques will develop a lipid-rich or necrotic core, resulting from the apoptosis of the resident pro-inflammatory macrophages (20). Pro-inflammatory macrophage activities within the atherosclerotic plaque lead to the weakening of the protective fibrous cap, mediated by the matrix metalloproteinases, that degrade the extracellular matrix components, predisposing it to rupture (22). Angiogenesis is believed to occur in response to hypoxic conditions within the necrotic core. Indeed, increasing wall thickness during atherosclerosis lead to a reduction of the intravascular oxygen amount, a situation further exacerbated by the increased oxygen consumption of high metabolic activated inflammatory cells within the atherosclerotic plaque (23). Vascular endothelial growth factor further modulates the activation state of the adventitial vasa vasorum endothelial cells to a highly migratory and proliferative state, resulting in neovessels formation towards the base of the plaque (24). Neovessels, arising from the adventitial vasa vasorum, grow into the base of progressive atherosclerotic lesions and provide an alternative entry pathway for monocytes and immune cells. The plaque neovessels are fragile and leaky, giving rise to local extravasation of plasma proteins and erythrocytes (25). Plaque hemorrhage itself results in a pro-inflammatory response, plaque destabilization and clinical events (3–4).
The use of the PET technique to visualize inflammation in vivo in atherosclerosis in large arteries has been performed with success using tracers such as 18F-FDG, DOTA-derived somatostatin analogs, or 68Ga-Pentixafor (18, 26–28). A non-invasive imaging technique that can inform about the activity of two adverse pathological processes, namely inflammation and angiogenesis, might therefore be even more accurate in identifying patients with active high-risk atheroma and potentially predicting risk of rupture. Hence, over the past decade, pre-clinical studies using RGD-based tracers have shown interesting results. In vivo imaging with a small animal PET/CT demonstrated 18F-galacto-RGD PET signal corresponding to the advanced calcified plaques of the aortic arch region of hypercholesterolemic mice (11). Another study on atherosclerotic mice showed accumulation of 68Ga-DOTA-RGD into aortic plaques (9). The role of a single photon emission computed tomography 99mTc-RGD-based probe in detection of inflammation in mouse models of carotid arteries remodeling has been demonstrated (8). More recently, Su et al. demonstrated a 18F-labeled RGD preferentially binds to aortic plaque in an ApoE knock out mouse model of atherosclerosis, and Golestani et al. demonstrated a good correlation between 18F-RGD-K5 uptake and intraplaque neovessels density in carotid endarterectomy specimens (7, 10).
The discussion about the assessment of the atherosclerotic inflammatory activity as a marker of plaque vulnerability relies among others on data showing that non-obstructive coronary artery disease is responsible for most acute coronary syndromes (29, 30). Moreover, some data from catheterization laboratories have shown a high proportion of significant stenosis (> 70% reduction of the coronary lumen) of culprit lesions in patients presenting with ST-segment elevation myocardial infarction (31). In the same line, it has been demonstrated that acute coronary event resulting from obstructive coronary plaque with prior inducible ischemia have better outcome in comparison to acute coronary event from non-obstructive coronary plaque without prior inducible ischemia (32). The protective adaptative changes resulting from the myocardial preconditioning could explain these different outcomes depending on the severity of the artery lumen stenosis and the presence of inducible ischemia prior acute coronary event (33). Nevertheless, these observations further confirm that other criteria above the solely angiographic evaluation of the coronary plaque stenosis should be considered. For this purpose, 68Ga-NODAGA-RGD PET/CT may aid our pathophysiological understanding of this important condition and help to identify patients at increased risk of adverse cardiovascular events.
Even if some previous literature reported on the role of RGD-based tracers in the imaging of atherosclerosis in humans, it needs to be further established. Thus far, only two recent studies have evaluated the imaging of atherosclerotic lesions with RGD-based PET agents in humans. Beer et al. documented the expression of αvβ3 integrin in macrophage infiltrates of plaque specimens obtained from a small sample of patients with high-grade carotid artery stenosis (12). Jenkins et al. have demonstrated that in vivo expression of αvβ3 integrin with 18F-fluciclatide PET/CT in human aortic atheroma is associated with plaque burden and is increased in patients with recent myocardial infarction (13). Our results are consistent with the promising results of these existing findings.
Because integrins αvβ3 are expressed in neovessels in plaques but also in CD68-positive macrophages, we are unable to ascertain whether 68Ga-NODAGA-RGD was binding preferentially to one or the other of these processes. However, non-selectivity between inflammation and angiogenesis is not necessarily a disadvantage. Both inflammation and angiogenesis are hallmark of unstable atheroma and to combine both processes could be of additional value. There was no histopathological correlation in our study to examine the relation between αvβ3 integrin expression by 68Ga-NODAGA-RGD uptake and plaque composition. One other limiting factor is the relatively lower spatial resolution of PET systems compared with other imaging techniques. Further developments in hybrid imaging promise to enhance the scope of molecular imaging. Although the use of 68Ga is widespread with ease of use and good availability of this radioisotope through a 68Ge/68Ga generator, the lower positron energy of 18F compared to 68Ga could potentially improve spatial resolution and reduce blurring effects. Given the difficulty in identifying the exact borders of the coronary arteries on the non-contrast-enhanced and non-gated CT image scans, we did not evaluate the coronary arteries. Furthermore, given the relatively limited number of patients studied using univariate analyses, with a heterogeneous sample, we cannot exclude confounding of our results by other confounding factors.