Michael Azure

Noninvasive Targeted Imaging of Matrix Metalloproteinase Activation in a Murine Model of Postinfarction Remodeling

Background— Time-dependent activation of matrix metalloproteinases (MMPs) after myocardial infarction (MI) contributes to adverse left ventricular (LV) remodeling; however, noninvasive methods to monitor this process serially are needed. Methods and Results— MMP-targeted radiotracers were developed that displayed selective binding kinetics to the active MMP catalytic domain. Initial nonimaging studies were performed with a 111In-labeled MMP-targeted radiotracer (111In-RP782) and negative control compound (111In-RP788) in control mice (Ctrl) and in mice 1 week after surgically induced MI. Localization of 111In-RP782 was demonstrated within the MI by microautoradiography. A 334±44% increase (P<0.001 versus Ctrl) in relative retention of 111In-RP782 was confirmed by gamma well counting of myocardium. Subsequent high-resolution dual-isotope planar and hybrid micro–single-photon emission computed tomography/CT imaging studies with an analogous 99mTc-labeled MMP-targeted radiotracer (99mTc-RP805) and 201Tl demonstrated favorable biodistribution and clearance kinetics of 99mTc-RP805 for in vivo cardiac imaging, with robust retention 1 to 3 weeks after MI in regions of decreased 201Tl perfusion. Gamma well counting yielded a similar ≈300% increase in relative myocardial retention of 99mTc-RP805 in MI regions (Ctrl, 102±9%; 1 week, 351±77%; 2 weeks, 291±45%; 3 weeks, 292±41%; P<0.05 versus Ctrl). Myocardial uptake in the MI region was also significantly increased ≈5-fold when expressed as percentage injected dose per gram tissue. There was also a significant 2-fold increase in myocardial activity in remote regions relative to control mice, suggesting activation of MMPs in regions remote from the MI. Conclusions— This novel noninvasive targeted MMP radiotracer imaging approach holds significant diagnostic potential for in vivo localization of MMP activation and tracking of MMP-mediated post-MI remodeling.

Molecular imaging of matrix metalloproteinase in atherosclerotic lesions : resolution with dietary modification and statin therapy

OBJECTIVES This study sought to evaluate the feasibility of noninvasive detection of matrix metalloproteinase (MMP) activity in experimental atherosclerosis using technetium-99m-labeled broad matrix metalloproteinase inhibitor (MPI) and to determine the effect of dietary modification and statin treatment on MMP activity. BACKGROUND The MMP activity in atherosclerotic lesions contributes to the vulnerability of atherosclerotic plaques to rupture. METHODS Atherosclerosis was produced in 34 New Zealand White rabbits by balloon de-endotheliazation of the abdominal aorta and a high-cholesterol diet. In addition, 12 unmanipulated rabbits were used as controls and 3 for blood clearance characteristics. In vivo micro-single-photon emission computed tomography (SPECT) imaging was performed after radiolabeled MPI administration. Subsequently, aortas were explanted to quantitatively measure percent injected dose per gram (%ID/g) MPI uptake. Histological and immunohistochemical characterization was performed and the extent of MMP activity was determined by gel zymography or enzyme-linked immunosorbent assays. RESULTS The MPI uptake in atherosclerotic lesions (n = 18) was clearly visualized by micro-SPECT imaging; MPI uptake was markedly reduced by administration of unlabeled MPI before the radiotracer (n = 4). The MPI uptake was also significantly reduced after diet withdrawal (n = 6) and fluvastatin treatment (n = 6); no uptake was observed in normal control rabbits (n = 12). The %ID/g MPI uptake (0.10 +/- 0.03%) in the atherosclerotic lesions was significantly higher than the uptake in control aorta (0.016 +/- 0.004%, p < 0.0001). Uptake in fluvastatin (0.056 +/- 0.011%, p < 0.0005) and diet withdrawal groups (0.043 +/- 0.011%, p < 0.0001) was lower than the untreated group. The MPI uptake correlated with immunohistochemically verified macrophage infiltration (r = 0.643, p < 0.0001), and MMP-2 (r = 0.542, p < 0.0001) or MMP-9 (r = 0.578, p < 0.0001) expression in plaques. CONCLUSIONS The present data show the feasibility of noninvasive detection of MMP activity in atherosclerotic plaques, and confirm that dietary modification and statin therapy reduce MMP activity.

Molecular Imaging of Activated Matrix Metalloproteinases in Vascular Remodeling

Background— Matrix metalloproteinase (MMP) activation plays a key role in vascular remodeling. RP782 is a novel indium 111In–labeled tracer with specificity for activated MMPs. We hypothesized that RP782 can detect injury-induced vascular remodeling in vivo. Methods and Results— Left common carotid artery injury was induced with a guidewire in apolipoprotein E−/− mice. Sham surgery was performed on the contralateral artery, which served as control for imaging experiments. Carotid wire injury led to significant hyperplasia and expansive remodeling over a period of 4 weeks. MMP activity, detected by in situ zymography, increased in response to injury and was maximal by 3 to 4 weeks after injury. RP782 (11.1 MBq) was injected intravenously into apolipoprotein E−/− mice at 1, 2, 3, and 4 weeks after left carotid injury. MicroSPECT imaging was performed at 2 hours and was followed by CT angiography to localize the carotid arteries. In vivo images revealed focal uptake of RP782 in the injured carotid artery at 2, 3, and 4 weeks. Increased tracer uptake in the injured artery was confirmed by quantitative autoradiography. Pretreatment with 50-fold excess nonlabeled tracer significantly reduced RP782 uptake in injured carotids, thus demonstrating uptake specificity. Weekly changes in the vessel-wall area closely paralleled and correlated with RP782 uptake (Spearman r=0.95, P=0.001). Conclusions— Injury-induced MMP activation in the vessel wall can be detected by RP782 microSPECT/CT imaging in vivo. RP782 uptake tracks the hyperplastic process in vascular remodeling and provides an opportunity to track the remodeling process in vivo.

Molecular Imaging of Matrix Metalloproteinase Expression in Atherosclerotic Plaques of Mice Deficient in Apolipoprotein E or Low-Density-Lipoprotein Receptor

Matrix metalloproteinases (MMPs) are expressed in atherosclerotic plaques and play an important role in plaque instability. Methods: Using 99mTc-labeled broad-spectrum MMP inhibitor (MPI), we performed noninvasive imaging of MMP expression with micro-SPECT/micro-CT in mice deficient in apolipoprotein E (ApoE−/−, n = 14), mice deficient in low-density-lipoprotein receptor (LDLR−/−, n = 14), and C57/BL6 mice as controls (n = 7). Seven ApoE−/− and 7 LDLR−/− received a high-cholesterol diet. After in vivo imaging, aortas were explanted, ex vivo images acquired, and the percent injected dose of MPI per gram (%ID/g) determined, followed by histologic characterization of atherosclerotic lesions. Results: MPI uptake was noninvasively visualized in atherosclerotic lesions by micro-SPECT, with confirmation by micro-CT of anatomic location and aortic calcification. %ID/g in each part of the aorta was highest in ApoE−/− that were fed a high-cholesterol diet, followed by LDLR−/− that were fed a high-cholesterol diet, ApoE−/− that were fed normal chow, and LDLR−/− that were fed normal chow. The control mice had minimal MPI uptake. A significant correlation was noted between %ID/g and % area positive for macrophages (r = 0.81, P = 0.009), MMP-2 (r = 0.65, P = 0.013), and MMP-9 (r = 0.62, P = 0.008). Conclusion: This study demonstrates the usefulness of molecular imaging for noninvasive assessment of the extent of MMP expression in various transgenic mouse models of atherosclerosis receiving a normal or hyperlipidemic diet. It is conceivable that such a strategy may be translationally developed for identification of unstable atherosclerotic plaques.

Evaluation of LMI1195, a Novel 18F-Labeled Cardiac Neuronal PET Imaging Agent, in Cells and Animal Models

Background—Heart failure has been associated with impaired cardiac sympathetic neuronal function. Cardiac imaging with radiolabeled agents that are substrates for the neuronal norepinephrine transporter (NET) has demonstrated the potential to identify individuals at risk of cardiac events. N-[3-Bromo-4-(3-[18F]fluoro-propoxy)-benzyl]-guanidine (LMI1195) is a newly developed 18F-labeled NET substrate designed to allow cardiac neuronal imaging with the high sensitivity, resolution, and quantification afforded by positron emission tomography (PET). Methods and Results—LMI1195 was evaluated in comparison with norepinephrine (NE) in vitro and 123I-meta-iodobenzylguanidine (MIBG) in vivo. The affinity (Ki) of LMI1195 for NET was 5.16±2.83 &mgr;mol/L, similar to that of NE (3.36±2.77 &mgr;mol/L) in a cell membrane–binding assay. Similarly, LMI1195 uptake kinetics examined in a human neuroblastoma cell line had Km and Vmax values of 1.44±0.76 &mgr;mol/L and 6.05±3.09 pmol/million cells per minute, comparable to NE (2.01±0.85 &mgr;mol/L and 6.23±1.52 pmol/million cells per minute). In rats, LMI1195 heart uptake at 15 and 60 minutes after intravenous administration was 2.36±0.38% and 2.16±0.38% injected dose per gram of tissue (%ID/g), similar to 123I-MIBG (2.14±0.30 and 2.19±0.27%ID/g). However, the heart to liver and lung uptake ratios were significantly higher for LMI1195 than for 123I-MIBG. In rabbits, desipramine (1 mg/kg), a selective NET inhibitor, blocked LMI1195 heart uptake by 82%, which was more effective than 123I-MIBG (53%), at 1 hour after dosing. Sympathetic denervation with 6-hydroxydopamine, a neurotoxin, resulted in a marked (79%) decrease in LMI1195 heart uptake. Cardiac PET imaging with LMI1195 in rats, rabbits, and nonhuman primates revealed clear myocardium with low radioactivity levels in the blood, lung, and liver. Imaging in rabbits pretreated with desipramine showed reduced heart radioactivity levels in a dose-dependent manner. Additionally, imaging in sympathetically denervated rabbits resulted in low cardiac image intensity with LMI1195 but normal perfusion images with flurpiridaz F 18, a PET myocardial perfusion imaging agent. In nonhuman primates pretreated with desipramine (0.5 mg/kg), imaging with LMI1195 showed a 66% decrease in myocardial uptake. In a rat model of heart failure, the LMI1195 cardiac uptake decreased as heart failure progressed. Conclusions—LMI1195 is a novel 18F imaging agent retained in the heart through the NET and allowing evaluation of the cardiac sympathetic neuronal function by PET imaging.

Atherosclerosis Plaque Heterogeneity and Response to Therapy Detected by In Vivo Molecular Imaging of Matrix Metalloproteinase Activation

Matrix metalloproteinases (MMPs) play a key role in the development of atherosclerosis and its complications. In vivo detection and quantification of MMP activation can help track the propensity to complications and response to therapy. We sought to establish an in vivo imaging approach for monitoring MMP activation in atherosclerotic mouse aorta and use it to assess the response to dietary modification. Method: Apolipoprotein-deficient mice were fed normal chow or a high-fat diet (HFD) for up to 3 mo or a HFD for 2 mo, followed by 1 mo on normal chow. Then they underwent micro-SPECT/CT, along with autoradiography and oil red O staining of tissues. Results: After 3 mo of HFD, there was considerable atherosclerosis in the aorta. In vivo micro-SPECT/CT using RP782 (an 111In-labeled tracer targeting activated MMPs) showed a heterogeneous pattern of tracer uptake along the aorta. Heterogeneity of RP782 uptake was confirmed by autoradiography, and specificity was demonstrated using excess unlabeled precursor. Tracer uptake quantified by micro-SPECT significantly correlated with uptake quantified by autoradiography. Comparison of oil red O staining with autoradiography demonstrated areas of discordance between plaque presence and tracer uptake. HFD withdrawal led to significant reduction in RP782 uptake beyond the effect on plaque area. MMP expression and macrophage infiltration were similarly heterogeneous along the aorta and significantly reduced after withdrawal from the HFD. Finally, RP782 uptake significantly correlated with aortic macrophage content. Conclusion: Molecular imaging of MMP activation reveals the heterogeneity of atherosclerotic plaques and is a useful tool for tracking plaque biology and response to therapy.

Molecular Imaging of Matrix Metalloproteinase Activation to Predict Murine Aneurysm Expansion In Vivo

Rupture and dissection are major causes of morbidity and mortality in arterial aneurysm and occur more frequently in rapidly expanding aneurysms. Current imaging modalities provide little information on aneurysm beyond size. Matrix metalloproteinase (MMP) activation plays a key role in the pathogenesis of aneurysm. We investigated whether imaging MMP activation in aneurysm helps predict its propensity to expansion. Methods: We used a model of carotid aneurysm in apolipoprotein E–deficient (apoE−/−) mice. Radiotracers with specificity for activated MMPs were used to detect and quantify MMP activation by micro-SPECT/CT in vivo. Tracer uptake was confirmed by autoradiography and γ-well counting, and specificity was demonstrated using an excess of unlabeled precursor and a specific MMP inhibitor. Results: We demonstrated that several MMPs are expressed with distinct temporal patterns in aneurysm. Significant focal uptake was observed in aneurysmal carotid arteries, peaking at 4 wk after aneurysm induction. In a group of animals imaged serially at 2 and 4 wk after aneurysm induction, MMP tracer uptake at 2 wk correlated well with the vessel area assessed by histology at 4 wk. Conclusion: Molecular imaging of MMP activation is a useful experimental, and potentially clinical, tool to noninvasively predict the propensity of an aneurysm to expansion in vivo.

Synthesis and Biological Evaluation of Pyridazinone Analogues as Potential Cardiac Positron Emission Tomography Tracers

A series of fluorinated pyridazinone derivatives with IC50 values ranging from 8 to 4000 nM for the mitochondrial complex 1 (MC1) have been prepared. Structure-activity relationship (SAR) assessment indicated preference of the fluorine label to be incorporated on an alkyl side chain rather than directly on the pyridazinone moiety. Tissue distribution studies of a series of analogues ([18F] 22-28) in Sprague-Dawley (SD) rats identified [18F]27 as the most promising radiotracer with high uptake in cardiac tissue (3.41%ID/g; 30 min post injection) in addition to favorable heart to nontarget organ distribution ratios. MicroPET images of SD rats and nonhuman primates after [18F]27 administration allowed easy assessment of the myocardium through 60 min with minimal lung or liver interference.

αvβ3-Targeted detection of arteriopathy in transplanted human coronary arteries: an autoradiographic study

Graft arteriopathy (GA), characterized by diffuse concentric narrowing of coronary arteries, is the major cause of late graft failure in cardiac transplantation. αvβ3 Integrin is up‐regulated in proliferating vascular cells and may constitute an appropriate target for imaging GA. We used a human/mouse chimeric model of GA, in which segments of human coronary artery were transplanted to severe combined immunodeficiency mice, followed by reconstitution with allogeneic human peripheral blood mononuclear cells (PBMC). This led to vascular remodeling characterized by neointima formation over a period of 4 wk. αvβ3 expression in the graft was minimal in animals without PBMC, considerably increased by 2 wk, and decreased toward baseline by 4 wk after PBMC reconstitution. Cell proliferation was maximal at 2 wk, correlating with peak αvβ3 expression. RP748, an 111In‐labeled αvβ3 (active conformation)‐targeted radiotracer was injected into groups of 5 recipients at 0, 2, and 4 wk after PBMC reconstitution. Relative uptakes, defined as autoradiographic intensity in the graft/native aortas closely tracked the proliferative process. Specificity of uptake was demonstrated using excess nonlabeled tracer. In conclusion, αvβ3 integrin is transiently up‐regulated (and activated) in GA and may be targeted by RP748 for detection of the proliferative process in early GA.

Cardiac retention of PET neuronal imaging agent LMI1195 in different species: Impact of norepinephrine uptake-1 and -2 transporters

INTRODUCTION Released sympathetic neurotransmitter norepinephrine (NE) in the heart is cleared by neuronal uptake-1 and extraneuronal uptake-2 transporters. Cardiac uptake-1 and -2 expression varies among species, but the uptake-1 is the primary transporter in humans. LMI1195 is an NE analog labeled with (18)F for PET evaluation of cardiac neuronal function. This study investigated the impact of cardiac neuronal uptake-1 associated with different species on LMI1195 heart uptake. METHODS Cardiac uptake-1 was blocked by desipramine, a selective uptake-1 inhibitor, and sympathetic neuronal denervation was induced by 6-hydroxydopamine, a neurotoxin, in rats, rabbits and nonhuman primates (NHP). Tissue biodistribution and cardiac imaging of LMI1195 and (123)I-metaiodobenzylguanidine (MIBG) were performed. RESULTS In rats, uptake-1 blockade did not alter LMI1195 heart uptake compared to the control at 60-min post injection [1.41 ± 0.07 vs. 1.47 ± 0.23 % injected dose per gram tissue (%ID/g)]. In contrast, LMI1195 heart uptake was reduced by 80% in uptake-1 blocked rabbits. In sympathetically denervated rats, LMI1195 heart uptake was similar to the control (2.18 ± 0.40 vs. 2.58 ± 0.76 %ID/g). However, the uptake decreased by 79% in denervated rabbits. Similar results were found in MIBG heart uptake in rats and rabbits with uptake-1 blockade. Consistently, LMI1195 cardiac imaging showed comparable myocardial activity in uptake-1 blocked or sympathetically denervated rats to the control, but marked activity reduction in uptake-1 blocked or denervated rabbits and NHPs. CONCLUSIONS LMI1195 is retained in the heart of rabbits and NHPs primarily via the neuronal uptake-1 with high selectivity and can be used for evaluation of cardiac sympathetic denervation. Similar to the human, the neuronal uptake-1 is the dominant transporter for cardiac retention of NE analogs in rabbits and NHPs, but not in rats.