Dual molecular imaging for targeting metalloproteinase activity and apoptosis in atherosclerosis: molecular imaging facilitates understanding of pathogenesis

Macrophage apoptosis and MMP activity contribute to vulnerability of atherosclerotic plaques to rupture. By employing molecular imaging techniques, we investigated if apoptosis and MMP release are interlinked. Atherosclerosis was produced in rabbits receiving high-cholesterol diet (HC), who underwent dual radionuclide imaging with 99mTc-labeled matrix metalloproteinase inhibitor (MPI) and 111In-labeled annexin A5 (AA5) using micro-SPECT/CT. %ID/g MPI and AA5 uptake was measured, followed by histological characterization. Unmanipulated animals were used as disease controls. Correlation between MPI and AA5 uptake was undertaken and relationship confirmed in culture study of activated THP-1 monocytes. MPI and AA5 uptake was best visualized in HC diet animals (n = 6) and reduced significantly after fluvastatin treatment (n = 4) or diet withdrawal (n = 3). %ID/g MPI (.087 ± .018%) and AA5 (.03 ± .01%) uptake was higher in HC than control (n = 6) animals (.014 ± .004%, P < .0001; .0007 ± .0002%, P < .0001), and reduced substantially after diet or statin intervention. There was a significant correlation between MPI and AA5 uptake (r = .62, P < .0001), both correlated with pathologically verified MMP-9 activity, macrophage content, and TUNEL staining. In vitro studies demonstrated MMP-9 release in culture medium from apoptotic THP-1 monocytes. The present study suggests that apoptosis and MMP are interrelated in atherosclerotic lesions and the targeting of more than one molecular candidate is feasible by molecular imaging

Small-animal SPECT and SPECT/CT: application in cardiovascular research

Preclinical cardiovascular research using noninvasive radionuclide and hybrid imaging systems has been extensively developed in recent years. Single photon emission computed tomography (SPECT) is based on the molecular tracer principle and is an established tool in noninvasive imaging. SPECT uses gamma cameras and collimators to form projection data that are used to estimate (dynamic) 3-D tracer distributions in vivo. Recent developments in multipinhole collimation and advanced image reconstruction have led to sub-millimetre and sub-half-millimetre resolution SPECT in rats and mice, respectively. In this article we review applications of microSPECT in cardiovascular research in which information about the function and pathology of the myocardium, vessels and neurons is obtained. We give examples on how diagnostic tracers, new therapeutic interventions, pre- and postcardiovascular event prognosis, and functional and pathophysiological heart conditions can be explored by microSPECT, using small-animal models of cardiovascular disease

Abstract 1143: Imaging of Vasa Vasorum in Atherosclerotic Plaque with ^99m Tc-labeled single chain-VEGF

Introduction: Adventitial vasa vasorum proliferation and neointimal neovascularization are associated with intraplaque hemorrhage, expansion of necrotic core and hence plaque vulnerability. Increased expression of VEGF and its receptors accompany neoangiogenic process. We used 99m Tc -labeled single chain VEGF (TcV) for developing potentially noninvasive imaging modality in experimentally induced aortic atherosclerotic lesion. Methods : Noninva-sive radionuclide imaging was performed with TcV (6.85 ±0. 27 mCi) in 6 NZW rabbits receiving high cholesterol diet (0.2% cholesterol, 4% fat) for one year and compared with 3 control rabbits receiving normal rabbit chow. Four hours after intravenous administration of TcV, micro SPECT/microCT imaging was performed for in vivo localization of tracer activity. Aortas were then explanted, and gamma counted for determination of % injected dose per gram (%ID/g). The aortas were then submitted for histopathologic characterization. Results : The uptake in thoracic aorta was clearly visualized non-invasively by TcV in vivo imaging in 4 of 5 rabbits in hypercholesterolemic rabbits, but not in the control animals. The %ID/g of each parts of aorta in hypercholesterolemic rabbits (Arch : 0.036 ± 0.020 %, Thoracic : 0.026 ± 0.012 %, Abd : 0.019 ± 0.009 %) was about 2.5-fold higher than that in control group (Arch : 0.014 ± 0.004 %, Thoracic : 0.009 ± 0.003 %, Abd : 0.009 ± 0.003 %) (figure a ). Ex vivo images of each group are shown as figure b . Conclusions : This preliminary study suggests a potentially novel strategy for non-invasive imaging of neoangiogenesis in atherosclerotic plaque and may allow identification of unstable plaques. </jats:p

Abstract 5790: Dual Molecular Imaging for Assessment of Matrix Metalloproteinase and Apoptosis in Atherosclerotic Lesions before and after Statin Therapy

We evaluated the correlation between matrix metalloproteinase (MMP) and apoptotic activities in atherosclerosis lesions, and the impact of lipid lowering by dietary modification and statin therapy employing molecular imaging with Tc-99m broad MMP inhibitor (MPI) and indium-111 annexin A5 (AA5). Atherosclerosis was produced in 13 New Zealand White rabbits by balloon deendothelialization and hypercholesterolemic diet for 4 months; 6 unmanipulated rabbits on normal chow were used as controls. In the last month, 3 of the 13 atherosclerotic rabbits were changed to normal chow, and 4 received fluvastatin (1mg/kg) once a day. MPI and AA5 imaging were performed using micro-SPECT/micro-CT. After in vivo imaging, aortas were explanted to acquire ex vivo images and calculate percent injected dose per gram (%ID/g) uptake. Histological and immunohistochemical characterization and extent of MMP and apoptosis were evaluated in representative aortic samples. Both MPI and Annexin uptake were clearly visualized in the atherosclerotic lesions by noninvasive imaging; No uptake were observed after diet withdrawal, fluvastatin, and in normal control groups. %ID/g MPI and AA5 uptakes in the atherosclerotic lesions (0.09 ± 0.02%, 0.03 ± 0.01%) were significantly higher than the uptake in control abdominal aorta (0.014 ± 0.004%, 0.0007 ± 0.0002%; p&lt;0.0001). The quantitative uptake in fluvastatin (0.053 ± 0.013%, 0.02 ± 0.01%; p&lt;0.05) and diet withdrawal (0.047 ± 0.005%, 0.02 ± 0.004%; p&lt;0.05) groups were statistically significantly lower compared to diet uninterrupted group. There were significant direct relationship betweem MPI and AA5 uptake in all experimental group (r=0.62, p&lt;0.0001). The decrease in the tracer uptake maintained the relationship in the fluvastatin group (r=0.62, p&lt;0.0001) but became less apparent in the diet withdrawal (r=0.24, p&lt;0.05). Histopathological and immunohisto-chemical studies revealed a significant correlation of both MPI and Annexin uptake with macrophage infiltration, MMP-2, 9 expression and the extent of apoptosis. Dual imaging with MPI and AA5 showed good correlation between apoptosis and MMP expression except diet withdrawal group.</jats:p

Use of an oxygen-carrying blood substitute to improve intravascular optical coherence tomography imaging.

Optical coherence tomography (OCT) is a catheter-based imaging technology with powerful resolution capable of identifying vulnerable plaques and guiding coronary intervention. However, a significant limitation of intravascular OCT imaging is its attenuation by blood. We propose that the use of an oxygen-carrying blood substitute could potentially optimize OCT image quality. Surgical isolation of the descending thoracic aorta of six rabbits is performed, followed by intravascular OCT imaging of the abdominal aorta. Perfluorodecalin (PFD) is oxygenated using a bubble-through technique with 100% oxygen. OCT imaging is performed and compared using three different flushing modalities: PFD; saline; and blood. OCT imaging of the rabbit abdominal aorta is successful in all of the subjects. In each of the six studied subjects, flushing with PFD consistently provides dramatically better imaging of the vessel wall tissue structures. OCT image quality is highly dependent on the ability of the flushing modality to remove blood from the imaging field. From this proof-of-concept study, we demonstrate that endovascular flushing with an oxygen-carrying blood substitute (PFD) is optically superior to saline flushing for intravascular imaging

Use of an oxygen-carrying blood substitute to improve intravascular optical coherence tomography imaging.

Optical coherence tomography (OCT) is a catheter-based imaging technology with powerful resolution capable of identifying vulnerable plaques and guiding coronary intervention. However, a significant limitation of intravascular OCT imaging is its attenuation by blood. We propose that the use of an oxygen-carrying blood substitute could potentially optimize OCT image quality. Surgical isolation of the descending thoracic aorta of six rabbits is performed, followed by intravascular OCT imaging of the abdominal aorta. Perfluorodecalin (PFD) is oxygenated using a bubble-through technique with 100% oxygen. OCT imaging is performed and compared using three different flushing modalities: PFD; saline; and blood. OCT imaging of the rabbit abdominal aorta is successful in all of the subjects. In each of the six studied subjects, flushing with PFD consistently provides dramatically better imaging of the vessel wall tissue structures. OCT image quality is highly dependent on the ability of the flushing modality to remove blood from the imaging field. From this proof-of-concept study, we demonstrate that endovascular flushing with an oxygen-carrying blood substitute (PFD) is optically superior to saline flushing for intravascular imaging