Multimodal molecular imaging of phagocytic and proteolytic activity in abdominal aortic aneurysm

Objectives: Abdominal aortic aneurysm (AAA) rupture occurs in a subset of small aneurysms and is often lethal. Inflammation plays a major role in AAA progression and rupture, and imaging of vessel wall inflammation can lead to better AAA rupture risk assessment. Macrophages are professional phagocytes and a major source of matrix metalloproteinases (MMPs), key mediators of tissue remodeling in AAA. In preclinical models of AAA, MMP activity may be detected by single-photon emission computed tomography (SPECT) imaging using a Tc-99m labeled pan-MMP inhibitor, ^99mTc-RYM1. The ability of macrophages to internalize nanoparticles may be leveraged for X-ray computed tomography (CT) imaging of vessel wall inflammation. The objective of this study was to evaluate nanoparticulate CT contrast agent-based imaging of phagocytic activity in comparison with ^99mTc-RYM1 SPECT imaging of vessel wall MMP activity in AAA.

Methods: Two murine models of AAA were used. Male apolipoprotein E-deficient mice were administrated angiotensin II for up to 4 weeks to induce aneurysm in the supra-renal abdominal aorta (Ang II model). C57Bl/6J mice treated with β-aminopropionitrile (BAPN) underwent topical elastase application to induce infra-renal AAA after 5 (El+BAPN model). In a cross-sectional study, the animals were imaged by CT at 5 min and at 24 h after intravenous injection of a nanoparticulate CT contrast agent (Np) for CT angiography, and imaging of phagocytic activity, respectively. The animals (Ang II model, n = 13 and El+BAPN model, n = 6) were euthanized following CT imaging, and the aorta was collected for tissue analysis. In a longitudinal study, another group of animals from the Ang II model (n = 18) where imaged by Np CT at 4-7 days of Ang II infusion. A subset of these animals (n = 5) underwent ^99mTc-RYM1 SPECT/CT and repeat Np CT imaging at 2-3 weeks of Ang II infusion. After 4 weeks, the animals were euthanized, and the abdominal aorta was harvested for tissue analysis. Np uptake in AAA was evaluated on images acquired at 24 h post-injection using a thresholding method. Aortic tissue was processed for morphometry, electron microscope and gene expression analysis. The biological correlates of the Np CT signal in AAA, and the predictive value of Np CT imaging at 1 week for AAA size at 4 weeks were determined. Np CT images of phagocytic activity were compared with ^99mTc-RYM1 SPECT images of MMP activity in the same animal.

Results: Transmission electron microscopy showed the Np are concentrated in adventitial macrophages in AAA. Threshold-based quantification of the Np signal on CT images acquired at 24 h post-injection showed different levels of Np uptake in both models of AAA. In the Ang II model, gene expression analysis showed a significant correlation (r² = 0.67, P < 0.001) between Np uptake and CD68 (macrophage marker), but not with Myh11 (smooth muscle cell marker) nor VE-cadherin (endothelial cell marker) gene expression. In the longitudinal study, the Np CT signal on early images correlated with the maximal aortic diameter at 4 weeks (r² = 0.34, P < 0.05). Qualitative analysis of Np CT and ^99mTc-RYM1 SPECT images in animals that underwent multimodality imaging revealed partially overlapping, yet discordant phagocytic and proteolytic activity signals.

Conclusions: This study demonstrated the feasibility and predictive value of CT imaging phagocytic activity in two preclinical models of AAA. The distinct patterns of the phagocytic and MMP activity signals highlights the value of multimodality molecular imaging in detecting complementary aspects of the inflammatory process. Clinical translation of these findings may lead to better understanding of AAA pathophysiology and better tools for AAA risk stratification.