Optical and multimodality molecular imaging: insights into atherosclerosis

Molecular imaging of macrophage protease activity in cardiovascular inflammation in vivo

Macrophages contribute pivotally to cardiovascular diseases (CVD), notably to atherosclerosis. Imaging of macrophages in vivo could furnish new tools to advance evaluation of disease and therapies. Proteolytic enzymes serve as key effectors of many macrophage contributions to CVD. Therefore, intravital imaging of protease activity could aid evaluation of the progress and outcome of atherosclerosis, aortic aneurysm formation, or rejection of cardiac allografts. Among the large families of proteases, matrix metalloproteinases (MMPs) and cysteinyl cathepsins have garnered the most interest because of their participation in extracellular matrix remodelling. These considerations have spurred the development of dedicated imaging agents for protease activity detection. Activatable fluorescent probes, radiolabelled inhibitors, and nanoparticles are currently under exploration for this purpose. While some agents and technologies may soon see clinical use, others will require further refinement. Imaging of macrophages and protease activity should provide an important adjunct to understanding pathophysiology in vivo, evaluating the effects of interventions, and ultimately aiding clinical care.

Optical techniques for the molecular imaging of angiogenesis

The process of angiogenesis, an essential hallmark for tumour development as well as for several inflammatory diseases and physiological phenomena, is of growing interest for diagnosis and therapy in oncology. In the context of biochemical characterisation of key molecules involved in angiogenesis, several targets for imaging and therapy could be identified in the last decade. Optical imaging (OI) relies on the visualisation of near infrared (NIR) light, either its absorption and scattering in tissue (non-enhanced OI) or using fluorescent contrast agents. OI offers excellent signal to noise ratios due to virtually absent background fluorescence in the NIR range and is thus a versatile tool to image specific molecular target structures in vivo. This work intends to provide a survey of the different approaches to imaging of angiogenesis using OI methods in preclinical research as well as first clinical trials. Different imaging modalities as well as various optical contrast agents are briefly discussed.

In vivo imaging in cancer

Imaging has become an indispensable tool in the study of cancer biology and in clinical prognosis and treatment. The rapid advances in high resolution fluorescent imaging at single cell level and MR/PET/CT image registration, combined with new molecular probes of cell types and metabolic states, will allow the physical scales imaged by each to be bridged. This holds the promise of translation of basic science insights at the single cell level to clinical application. In this article, we describe the recent advances in imaging at the macro- and micro-scale and how these advances are synergistic with new imaging agents, reporters, and labeling schemes. Examples of new insights derived from the different scales of imaging and relevant probes are discussed in the context of cancer progression and metastasis.

Ultrasmall superparamagnetic iron oxide-enhanced magnetic resonance imaging of abdominal aortic aneurysms--a feasibility study

OBJECTIVES

Abdominal aortic aneurysms (AAAs), being predominantly atherosclerotic in nature, have underlying inflammatory activity. As it is well established that ultrasmall superparamagnetic iron oxide (USPIO) particles accumulate in the macrophages within atheromatous lesions, USPIO-enhanced magnetic resonance (MR) imaging can be potentially effective in the quantification of the associated inflammatory processes.

METHODS

A total of 14 patients underwent USPIO-enhanced MR imaging using a 1.5T-MR system. Quantitative T(2)* and T(2) relaxation time data were acquired before and 36 h after UPSIO infusion at identical AAA locations. The pre- and post-USPIO-infusion relaxation times (T(2)(∗) and T(2)) were quantified and the correlation between pre- and post-USPIO infusion T(2)* and T(2) values was investigated.

RESULTS

There was a significant difference between pre- and post-infusion T(2)* and T(2) values (both respective p-values = 0.005). A significant correlation between T(2)* and T(2) values post-USPIO infusion was observed (r = 0.90, p < 0.001), which indicates USPIO uptake by the aortic wall.

CONCLUSIONS

Aortic wall inflammation using USPIO-enhanced MR imaging is feasible. Use of quantitative T(2) and T(2)* pulse sequences provides a quantitative method for assessing USPIO uptake by the aortic wall.

Pioglitazone suppresses inflammation in vivo in murine carotid atherosclerosis: novel detection by dual-target fluorescence molecular imaging

OBJECTIVE

To investigate the effects of pioglitazone (PIO), a peroxisome proliferator-activated receptor γ agonist, on plaque matrix metalloproteinase (MMP) and macrophage (Mac) responses in vivo in a molecular imaging study.

METHODS AND RESULTS

In vitro, PIO suppressed MMP-9 protein expression in murine peritoneal Macs (P<0.05). To assess PIO's effects on plaque inflammation, nondiabetic apolipoprotein E(-/-) mice receiving a high-cholesterol diet (HCD) were administered an MMP-activatable fluorescence imaging agent and a spectrally distinct Mac-avid fluorescent nanoparticle. After 24 hours, mice underwent survival dual-target intravital fluorescence microscopy of carotid arterial plaques. These mice were then randomized to HCD or HCD plus 0.012% PIO for 8 weeks, followed by a second intravital fluorescence microscopy study of the same carotid plaque. In the HCD group, in vivo MMP and Mac target-to-background ratios increased similarly (P<0.01 versus baseline). In contrast, PIO reduced MMP and Mac target-to-background ratios (P<0.01) versus HCD. Changes in MMP and Mac signals correlated strongly (r ≥0.75). Microscopy demonstrated MMP and Mac reductions in PIO-treated mice and a PIO-modulated increase in plaque collagen.

CONCLUSIONS

Serial optical molecular imaging demonstrates that plaque MMP and Mac activity in vivo intensify with hypercholesterolemia and are reduced by PIO therapy.

Imaging and quantifying the morphology of an organic-inorganic nanoparticle at the sub-nanometre level

The development of hybrid organic-inorganic nanoparticles is of interest for applications such as drug delivery, DNA and protein recognition, and medical diagnostics. However, the characterization of such nanoparticles remains a significant challenge due to the heterogeneous nature of these particles. Here, we report the direct visualization and quantification of the organic and inorganic components of a lipid-coated silica particle that contains a smaller semiconductor quantum dot. High-angle annular dark-field scanning transmission electron microscopy combined with electron energy loss spectroscopy was used to determine the thickness and chemical signature of molecular coating layers, the element atomic ratios, and the exact positions of different elements in single nanoparticles. Moreover, the lipid ratio and lipid phase segregation were also quantified.

A novel molecule integrating therapeutic and diagnostic activities reveals multiple aspects of stem cell-based therapy

Stem cells are promising therapeutic delivery vehicles; however pre-clinical and clinical applications of stem cell-based therapy would benefit significantly from the ability to simultaneously determine therapeutic efficacy and pharmacokinetics of therapies delivered by engineered stem cells. In this study, we engineered and screened numerous fusion variants that contained therapeutic (TRAIL) and diagnostic (luciferase) domains designed to allow simultaneous investigation of multiple events in stem cell-based therapy in vivo. When various stem cell lines were engineered with the optimized molecule, SRL(O)L(2)TR, diagnostic imaging showed marked differences in the levels and duration of secretion between stem cell lines, while the therapeutic activity of the molecule showed the different secretion levels translated to significant variability in tumor cell killing. In vivo, simultaneous diagnostic and therapeutic monitoring revealed that stem cell-based delivery significantly improved pharmacokinetics and anti-tumor effectiveness of the therapy compared to intravenous or intratumoral delivery. As treatment for highly malignant brain tumor xenografts, tracking SRL(O)L(2)TR showed stable stem cell-mediated delivery significantly regressed peripheral and intracranial tumors. Together, the integrated diagnostic and therapeutic properties of SRL(O)L(2)TR answer critical questions necessary for successful utilization of stem cells as novel therapeutic vehicles.

Near-infrared fluorescence catheter system for two-dimensional intravascular imaging in vivo

Detection of high-risk coronary arterial plaques prior to rupture remains an unmet clinical challenge, in part due to the stringent resolution and sensitivity requirements for in vivo human coronary arterial imaging. To address this need, we have developed a near-infrared (NIR) fluorescence imaging catheter system for intra-vascular molecular imaging of atherosclerosis in coronary artery-sized vessels, capable of resolving two-dimensional fluorescence activity in hollow organs, such as blood vessels. Based on a rotational fiber design, the catheter system illuminates and detects perpendicular to the rotational axis, while an automated pullback mechanism enables visualization along blood vessels with a scan speed of up to 1.5 mm/sec. We demonstrate the previously undocumented capacity to produce intravascular NIR fluorescence images of hollow organs in vivo and showcase the performance metrics of the system developed using blood vessel mimicking phantoms. This imaging approach is geared toward in vivo molecular imaging of atherosclerotic biomarkers and is engineered to allow seamless integration into the cardiac catheterization laboratory.

Emerging applications of nanomedicine for the diagnosis and treatment of cardiovascular diseases

Nanomedicine is an emerging field that utilizes nanotechnology concepts for advanced therapy and diagnostics. This convergent discipline merges research areas such as chemistry, biology, physics, mathematics and engineering. It therefore bridges the gap between molecular and cellular interactions, and has the potential to revolutionize medicine. This review presents recent developments in nanomedicine research poised to have an important impact on the treatment of cardiovascular disease. This will occur through improvement of the diagnosis and therapy of cardiovascular disorders as atherosclerosis, restenosis and myocardial infarction. Specifically, we discuss the use of nanoparticles for molecular imaging and advanced therapeutics, specially designed drug eluting stents and in vivo/ex vivo early detection techniques.

The vascular biology of atherosclerosis and imaging targets

The growing worldwide health challenge of atherosclerosis, together with advances in imaging technologies, have stimulated considerable interest in novel approaches to gauging this disease. The last several decades have witnessed a burgeoning in understanding of the molecular pathways involved in atherogenesis, lesion progression, and the mechanisms underlying the complications of human atherosclerotic plaques. The imaging of atherosclerosis is reaching beyond anatomy to encompass assessment of aspects of plaque biology related to the pathogenesis and complication of the disease. The harnessing of these biologic insights promises to provide a plethora of new targets for molecular imaging of atherosclerosis. The goals for the years to come must include translation of the experimental work to visualization of these appealing biologic targets in humans.

Will molecular optical imaging have clinically important roles in stroke management, and how?

Molecular imaging is a novel technology to visualize biological processes at the cellular and molecular levels, which is reshaping both biomedical research and clinical practice. By providing molecular information to supplement and augment conventional anatomy-based imaging, molecular imaging is expected to allow 1) the earlier detection of diseases, 2) precise evaluation of disease stages, and 3) both diagnostic and therapeutic monitoring of disease progression in a quantitative manner. In this brief review, we present our view on the prospects of molecular optical imaging in the field of stroke practice, focusing on the imaging vulnerability of atherosclerotic plaques, thrombolytic resistance, real-time cerebral perfusion, and penumbra.

Nanomedicine and Cardiovascular Disease

Nanomedicine has become an important tool in the imaging and therapy of numerous diseases. This is due, in large part, to the ability to generate multifunctional nanoagents bearing combinations of targeting, diagnostic, and therapeutic moieties, allowing for the tailoring of the properties of the synthesized nanomaterials. With respect to cardiovascular disease and its sequelae, nanomedicine has the potential to detect and treat some of the leading causes of death and disability in the developed world, including atherosclerosis, thrombosis, and myocardial infarction. As such, this review focuses on some of the most poignant examples of the utility of nanomedicine in the detection and treatment of cardiovascular disease that have been recently reported.

Protease imaging of human atheromata captures molecular information of atherosclerosis, complementing anatomic imaging

OBJECTIVE

There is hope that molecular imaging can identify vulnerable atherosclerotic plaques. However, there is a paucity of clinical translational data to guide the future development of this field. Here, we cross-correlate cathepsin-B or matrix metalloproteinase-2/-9 molecular optical imaging data of human atheromata or emboli with conventional imaging data, clinical data, and histopathologic data.

METHODS AND RESULTS

Fifty-two patients undergoing carotid endarterectomy (41 atheromata) or carotid stenting (15 captured emboli) were studied with protease-activatable imaging probes. We show that protease-related fluorescent signal in carotid atheromata or in emboli closely reflects the pathophysiologic alterations of plaque inflammation and statin-mediated therapeutic effects on plaque inflammation. Inflammation-related fluorescent signal was observed in the carotid bifurcation area and around ulcero-hemorrhagic lesions. Pathologically proven unstable plaques had high cathepsin-B-related fluorescent signal. The distribution patterns of the mean cathepsin-B imaging signals showed a difference between the symptomatic vs asymptomatic plaque groups. However, the degree of carotid stenosis or ultrasonographic echodensity was weakly correlated with the inflammatory proteolytic enzyme-related signal, suggesting that molecular imaging yields complimentary new information not available to conventional imaging.

CONCLUSIONS

These results could justify and facilitate clinical trials to evaluate the use of protease-sensing molecular optical imaging in human atherosclerosis patients.

Optical molecular imaging in atherosclerosis

Current imaging techniques focus on evaluating the anatomical structure of blood vessel wall and atherosclerotic plaque. These techniques fail to evaluate the biological processes which take place in the vessel wall and inside the plaque. Novel imaging techniques like optical imaging can evaluate the biological and cellular processes inside the plaque and provide information which can be vital for better patient risk stratification. This review highlights the various optical imaging techniques and their application in assessing biological processes in atherosclerosis.

Comparative immunohistochemical staining of atherosclerotic plaques using F16, F8 and L19: Three clinical-grade fully human antibodies

OBJECTIVE

F16, F8 and L19 are three fully human monoclonal antibodies, specific to splice isoforms of tenascin-C and fibronectin, which stain sites of active tissue remodeling and which are currently in Phase I and II clinical trials as radio-immunoconjugates and immunocytokines in patients with cancer and arthritis. The characterization of atherosclerosis using these antibodies may open novel pharmacodelivery options for the imaging and treatment of cardiovascular conditions. It may also allow a better assessment of the corresponding immunoconjugates in polymorbid patients with atherosclerotic plaques.

METHODS

We performed a comparative immunohistochemical analysis with the F16, F8 and L19 antibodies in 28 freshly frozen human carotid plaques and in 11 normal arteries. Furthermore, we assessed the localization of the antibodies in relation to the infiltrating macrophages, vasa vasorum and Ki67-positive proliferating cells of the plaque.

RESULTS

The F16 antibody, specific to the extra-domain A1 of tenascin-C, stained plaques with a selective and intense pattern, while F8 and L19, specific to the EDA and EDB domains of fibronectin, respectively, exhibited a less selective and intense staining. In immunofluorescence, F16 was found to bind regions rich in macrophages, vasa vasorum and proliferating cells, while showing no detectable vs. weak staining of normal arteries and of quiescent plaque structures.

CONCLUSION

The human monoclonal antibody F16 stains areas of active tissue remodeling in atherosclerotic plaques and may thus deserve to be investigated as a suitable building block for the development of radiopharmaceuticals for plaque imaging or for the antibody-based targeted delivery of therapeutic agents to atherosclerotic lesions.