Optical visualization of cathepsin K activity in atherosclerosis with a novel, protease-activatable fluorescence sensor

Imaging of the unstable plaque: how far have we got?

Rupture of unstable plaques may lead to myocardial infarction or stroke and is the leading cause of morbidity and mortality in western countries. Thus, there is a clear need for identifying these vulnerable plaques before the rupture occurs. Atherosclerotic plaques are a challenging imaging target as they are small and move rapidly, especially in the coronary tree. Many of the currently available imaging tools for clinical use still provide minimal information about the biological characteristics of plaques, because they are limited with respect to spatial and temporal resolution. Moreover, many of these imaging tools are invasive. The new generation of imaging modalities such as magnetic resonance imaging, nuclear imaging such as positron emission tomography and single photon emission computed tomography, computed tomography, fluorescence imaging, intravascular ultrasound, and optical coherence tomography offer opportunities to overcome some of these limitations. This review discusses the potential of these techniques for imaging the unstable plaque.

Molecular Imaging of Cathepsin B Proteolytic Enzyme Activity Reflects the Inflammatory Component of Atherosclerotic Pathology and Can Quantitatively Demonstrate the Antiatherosclerotic Therapeutic Effects of Atorvastatin and Glucosamine

Inflammation in atherosclerotic plaques causes plaque vulnerability and rupture, leading to thromboembolic complications. Cathepsin B (CatB) proteases secreted by macrophages play a major role in plaque inflammation. We used a CatB-activatable near-infrared fluorescence (NIRF) imaging agent to demonstrate the inflammatory component in mice atheromata and the atherosclerosis-modulating effects of atorvastatin or glucosamine treatments. Apolipoprotein E knockout mice ( n = 35) were fed normal chow, a Western diet, a Western diet + atorvastatin, a Western diet + glucosamine, or a Western diet + atorvastatin + glucosamine for 14 weeks. Twenty-four hours after the intravenous injection of a CatB-activatable probe, ex vivo NIRF imaging of the aortas and brains was performed, followed by histology. The CatB-related signal, observed in the aortas but not in the cerebral arteries, correlated very well with protease activity and the presence of macrophages on histology. Animals on Western diets could be distinguished from animals on a normal diet. The antiatherosclerotic effects of atorvastatin and glucosamine could be demonstrated, with reduced CatB-related signal compared with untreated animals. Plaque populations were heterogeneous within individuals, with some plaques showing a high and others a lower CatB-related signal. These differences in signal intensity could not be predicted by visual inspection of the plaques but did correlate with histologic evidence of inflammation in every case. This suggests that vulnerable inflamed plaques can be identified by optical molecular imaging.

Molecular Imaging of Proteases in Cancer

Proteases play important roles during tumor angiogenesis, invasion, and metastasis. Various molecular imaging techniques have been employed for protease imaging: optical (both fluorescence and bioluminescence), magnetic resonance imaging (MRI), single-photon emission computed tomography (SPECT), and positron emission tomography (PET). In this review, we will summarize the current status of imaging proteases in cancer with these techniques. Optical imaging of proteases, in particular with fluorescence, is the most intensively validated and many of the imaging probes are already commercially available. It is generally agreed that the use of activatable probes is the most accurate and appropriate means for measuring protease activity. Molecular imaging of proteases with other techniques (i.e. MRI, SPECT, and PET) has not been well-documented in the literature which certainly deserves much future effort. Optical imaging and molecular MRI of protease activity has very limited potential for clinical investigation. PET/SPECT imaging is suitable for clinical investigation; however the optimal probes for PET/SPECT imaging of proteases in cancer have yet to be developed. Successful development of protease imaging probes with optimal in vivo stability, tumor targeting efficacy, and desirable pharmacokinetics for clinical translation will eventually improve cancer patient management. Not limited to cancer, these protease-targeted imaging probes will also have broad applications in other diseases such as arthritis, atherosclerosis, and myocardial infarction.

Atherosclerosis and matrix metalloproteinases: experimental molecular MR imaging in vivo

PURPOSE

To evaluate the capability of P947, a magnetic resonance (MR) imaging contrast agent that molecularly targets matrix metalloproteinases (MMPs), to aid detection and imaging of MMPs in atherosclerotic lesions in vivo; its specificity compared with that of P1135; expression and distribution of MMPs in atherosclerotic vessels; and in vivo distribution and molecular localization of fluorescent europium (Eu) P947.

MATERIALS AND METHODS

The Animal Care and Use Committee approved all experiments. P947 was synthesized by attaching a gadolinium chelate (1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid) to a peptide that specifically binds MMPs. Scrambled form of P947 (P1135) was synthesized by replacing the targeting moiety of P947 with a scrambled peptide lacking the ability to bind MMPs. P947, P1135, and gadoterate meglumine were injected into atherosclerotic apolipoprotein E-deficient and wild-type mice. The aortic MR imaging enhancement produced by the contrast agents was measured at different times and was compared by using one-way analysis of variance. MMP expression was investigated in the aortas by using MMP immunostaining and in situ MMP zymography. A fluorescent form of P947 (Eu-P947) was synthesized to compare the in vivo distribution of the contrast agent (Eu-P947) with specific MMP immunofluorescent staining.

RESULTS

MMP-targeted P947 facilitated a 93% increase (P < .001) in MR image signal intensity (contrast-to-noise ratio [CNR], 17.7 compared with 7.7; P < .001) of atherosclerotic lesions in vivo. Nontargeted P1135 (scrambled P947) provided 33% MR image enhancement (CNR, 10.8), whereas gadoterate meglumine provided 5% (CNR, 6.9). Confocal laser scanning microscopy demonstrated colocalization between fluorescent Eu-P947 and MMPs in atherosclerotic plaques. Eu-P947 was particularly present in the fibrous cap region of plaques.

CONCLUSION

P947 improved MR imaging for atherosclerosis through MMP-specific targeting. The results were validated and provide support for further assessment of P947 as a potential tool for the identification of unstable atherosclerosis.

Collagens in the progression and complications of atherosclerosis

Collagens constitute a major portion of the extracellular matrix in the atherosclerotic plaque, where they contribute to the strength and integrity of the fibrous cap, and also modulate cellular responses via specific receptors and signaling pathways. This review focuses on the diverse roles that collagens play in atherosclerosis; regulating the infiltration and differentiation of smooth muscle cells and macrophages; controlling matrix remodeling through feedback signaling to proteinases; and influencing the development of atherosclerotic complications such as plaque rupture, aneurysm formation and calcification. Expanding our understanding of the pathways involved in cell-matrix interactions will provide new therapeutic targets and strategies for the diagnosis and treatment of atherosclerosis.

Heterogeneous in vivo behavior of monocyte subsets in atherosclerosis

Monocytes and macrophages play active roles in atherosclerosis, a chronic inflammatory disease that is a leading cause of death in the developed world. The prevailing paradigm states that, during human atherogenesis, monocytes accumulate in the arterial intima and differentiate into macrophages, which then ingest oxidized lipoproteins, secrete a diverse array of proinflammatory mediators, and eventually become foam cells, the key constituents of a vulnerable plaque. Yet monocytes are heterogeneous. In the mouse, one subset (Ly-6C(hi)) promotes inflammation, expands in hypercholesterolemic conditions, and selectively gives rise to macrophages in atheromata. A different subset (Ly-6C(lo)) attenuates inflammation and promotes angiogenesis and granulation tissue formation in models of tissue injury, but its role in atherosclerosis is largely unknown. In the human, monocyte heterogeneity is preserved but it is still unresolved how subsets correspond functionally. The contradistinctive properties of these cells suggest commitment for specific function before infiltrating tissue. Such commitment argues for discriminate targeting of deleterious subsets while sparing host defense and repair mechanisms. In addition to advancing our understanding of atherosclerosis, the ability to target and image monocyte subsets would allow us to evaluate drugs designed to selectively inhibit monocyte subset recruitment or function, and to stratify patients at risk for developing complications such as myocardial infarction or stroke. In this review we summarize recent advances of our understanding of the behavioral heterogeneity of monocytes during disease progression and outline emerging molecular imaging approaches to address key questions in the field.

Optical and multimodality molecular imaging: insights into atherosclerosis

Imaging approaches that visualize molecular targets rather than anatomic structures aim to illuminate vital molecular and cellular aspects of atherosclerosis biology in vivo. Several such molecular imaging strategies stand ready for rapid clinical application. This review describes the growing role of in vivo optical molecular imaging in atherosclerosis and highlights its ability to visualize atheroma inflammation, calcification, and angiogenesis. In addition, we discuss advances in multimodality probes, both in the context of multimodal imaging as well as multifunctional, or "theranostic," nanoparticles. This review highlights particular molecular imaging strategies that possess strong potential for clinical translation.

Pathophysiology of coronary artery disease: the case for multiparametric imaging

Interventions to treat coronary artery disease are available but they must be targeted at the correct individuals (and indeed lesions), in order to gain maximal benefit with the minimal adverse effects. Coronary contrast angiography is not able to provide all the information required for the assessment of the effects of artery disease. Other imaging modalities are of growing importance as they can reduce radiation exposure and invasiveness of screening, as well as providing important extra information. The ideal 'multiparametric' imaging technique would assess anatomy, viability and lesion activity in a single quick scan. Currently, MRI is the technology closest to achieving this ideal, although the existing technology still has some limitations. This review discusses the currently available techniques for the imaging of coronary anatomy and of myocardial viability, and considers their benefits and limitations. We also discuss the developing field of imaging molecularly targeted to active coronary lesions. Finally we provide a 5-year view of the current and likely future optimal imaging strategies.

Imaging of inflamed and vulnerable plaque in coronary arteries with 18F-FDG PET/CT in patients with suppression of myocardial uptake using a low-carbohydrate, high-fat preparation

PET/CT imaging with (18)F-FDG has been used to detect inflammation in carotid and aortic plaque; its use in detecting coronary plaque has been limited by avid (18)F-FDG uptake by the myocardium. We investigated whether (18)F-FDG PET/CT could be used to image inflammation in coronary arteries as a potential noninvasive method to detect vulnerable plaque.

METHODS

We retrospectively studied 32 patients treated for malignancy who underwent (18)F-FDG PET/CT and concomitant cardiac catheterization. As part of the recently described protocol, all patients were instructed to eat a low-carbohydrate, high-fat meal the night before and drink a vegetable oil drink the morning of the study. We reviewed the patients' baseline characteristics and their (18)F-FDG PET/CT scans for adequacy of myocardial uptake suppression and correlated the presence of angiographically apparent plaque with (18)F-FDG uptake in the major coronary arteries. Two independent observers assessed the angiographic images and (18)F-FDG PET scans.

RESULTS

A total of 95% of patients had 2 or more coronary disease risk factors, and 25% had unstable symptoms; 30% of index catheterizations resulted in intervention. In 20 of 32 patients (63%), myocardial suppression was good (12) or adequate (8). Inadequate suppression was due to self-reported dietary nonadherence. Patients with good, adequate, and poor suppression had maximal myocardial standardized uptake values of 2.8 +/- 0.7, 5.0 +/- 1.3, and 17.0 +/- 9.7, respectively. We identified (18)F-FDG uptake in 15 patients in 1 or more coronary segments. A trend to significance in correlation between presence of angiographic disease and signal in the vessel was observed (P = 0.07; 80 vessels examined). A total of 7 patients with significant coronary artery disease had aortic (18)F-FDG uptake.

CONCLUSION

In this retrospective study, we demonstrated the potential use of (18)F-FDG PET in imaging of inflammation in coronary arteries. The potential of (18)F-FDG PET is also being investigated in a prospective study.

Advances in molecular imaging of atherosclerotic vascular disease

PURPOSE OF REVIEW

Molecular imaging aims to illuminate vital molecular and cellular aspects of disease in vivo, and is rapidly translating into the clinical arena. Advantages of this field include enabling serial biological studies in living subjects, assessment of pharmaceutical efficacy, and in-vivo characterization of clinical diseases. Here we present recent exciting advances in molecular imaging of atherosclerotic vascular disease.

RECENT FINDINGS

Atherosclerosis molecular imaging approaches are now available for magnetic resonance, nuclear, computed tomography, ultrasound, and near-infrared fluorescence imaging. Advances in agent synthesis and detection technology are now enabling in-vivo imaging of endothelial cell activation, macrophages, cellular metabolism, protease activity, apoptosis, and osteogenic activity. Several agents show clinical utility for the detection of high-risk plaques.

SUMMARY

Molecular imaging is actively unraveling the biological basis of atherosclerosis in living subjects. In the near-term, molecular imaging will play an important role in assessing novel atherosclerosis pharmacotherapies in clinical trials. Longer term, molecular imaging should enable accurate identification of high-risk plaques responsible for myocardial infarction, stroke, and ischemic limbs.

Nanoparticles for optical molecular imaging of atherosclerosis

Molecular imaging contributes to future personalized medicine dedicated to the treatment of cardiovascular disease, the leading cause of mortality in industrialized countries. Endoscope-compatible optical imaging techniques would offer a stand-alone alternative and high spatial resolution validation technique to clinically accepted imaging techniques in the (intravascular) assessment of vulnerable atherosclerotic lesions, which are predisposed to initiate acute clinical events. Efficient optical visualization of molecular epitopes specific for vulnerable atherosclerotic lesions requires targeting of high-quality optical-contrast-enhancing particles. In this review, we provide an overview of both current optical nanoparticles and targeting ligands for optical molecular imaging of atherosclerotic lesions and speculate on their applicability in the clinical setting.

Molecular imaging in atherosclerosis, thrombosis, and vascular inflammation

Appreciation of the molecular and cellular processes of atherosclerosis, thrombosis, and vascular inflammation has identified new targets for imaging. The common goals of molecular imaging approaches are to accelerate and refine diagnosis, provide insights that reveal disease diversity, guide specific therapies, and monitor the effects of those therapies. Here we undertake a comparative analysis of imaging modalities that have been used in this disease area. We consider the elements of contrast agents, emphasizing how an understanding of the biology of atherosclerosis and its complications can inform optimal design. We address the potential and limitations of current contrast approaches in respect of translation to clinically usable agents and speculate on future applications.

Non-invasive optical detection of cathepsin K-mediated fluorescence reveals osteoclast activity in vitro and in vivo

Osteoclasts degrade bone matrix by demineralization followed by degradation of type I collagen through secretion of the cysteine protease, cathepsin K. Current imaging modalities are insufficient for sensitive observation of osteoclast activity, and in vivo live imaging of osteoclast resorption of bone has yet to be demonstrated. Here, we describe a near-infrared fluorescence reporter probe whose activation by cathepsin K is shown in live osteoclast cells and in mouse models of development and osteoclast upregulation. Cathepsin K probe activity was monitored in live osteoclast cultures and correlates with cathepsin K gene expression. In ovariectomized mice, cathepsin K probe upregulation precedes detection of bone loss by micro-computed tomography. These results are the first to demonstrate non-invasive visualization of bone degrading enzymes in models of accelerated bone loss, and may provide a means for early diagnosis of upregulated resorption and rapid feedback on efficacy of treatment protocols prior to significant loss of bone in the patient.

Contrast agents for MRI

Molecular imaging is a rapidly growing field with the potential to revolutionize cardiovascular medicine by shifting diagnostic focus from functional abnormalities which occur late in a disease process to the biochemical events which precipitate the earliest stages of disease. MRI is a modality well suited to this task as it allows a variety of contrast mechanisms for detection of epitopes of interest, as well as high-resolution anatomical localization and functional information. In this review, we discuss the widerange of available molecular MRI contrast agents and their application to diseases such as atherosclerosis, thrombus imaging, and stem cell tracking, along with opportunities for molecularly targeted drug delivery.

MR-optical imaging of cardiovascular molecular targets

Our understanding of the intricate inflammation biology underlying atherosclerosis is rapidly progressing. Molecular imaging strategies, harnessing this body of knowledge, have been developed to visualize some key cellular and molecular events in plaque evolution and vulnerability. Here, we discuss recent advances in magnetic resonance and fluorescence imaging of key biomarkers including adhesion molecules, inflammatory cells, and enzyme activity. We discuss strengths and limitations of respective imaging technologies, and comment on the potential of multi-modality imaging approaches.

Near-infrared fluorescent probes for imaging vascular pathophysiology

Light in the near-infrared (NIR) region between 700-900 nm can penetrate deep into living tissue, thereby offering a unique opportunity to use near-infrared fluorescence (NIRF) imaging techniques to detect and visualize fluorescent probes in-vivo. In the past few years, many novel NIR fluorescent probes have been designed, synthesized and studied in a variety of disease conditions. Recent research has focused primarily on the class of cyanines dyes as non-specific agents and as part of specific NIR fluorescent probes. The publications reviewed herein discuss the characteristics of cyanine dyes and their conjugates and present examples for the application of these probes for imaging vascular pathophysiology.

Optical imaging of vascular pathophysiology

Pathophysiological processes in the vascular system are the major cause of mortality and disease. Atherosclerosis, an inflammatory process in arterial walls, can lead to formation of plaques, whose rupture can lead to thrombus formation, obstruction of vessels (thrombosis), reduction of the blood flow (ischemia), cell death in the tissue fed by the occluded vessel, and depending on the affected vessel, to myocardial infarction or stroke. Imaging techniques enabling visualization of the biological processes involved in this scenario are therefore highly desirable. In recent years, a number of reporter agents and reporter systems have been developed to visualize these processes using different imaging modalities including nuclear imaging techniques, such as positron emission tomography or single photon emission computed tomography, magnetic resonance imaging, and ultrasound. This article comprises a brief overview of optical imaging techniques, such as fluorescence imaging and bioluminescence imaging for the visualization of vascular pathophysiology.

Molecular pathology in vulnerable carotid plaques: correlation with [18]-fluorodeoxyglucose positron emission tomography (FDG-PET)

OBJECTIVES

Atherosclerosis is recognised as an inflammatory disease, and new diagnostic tools are warranted to evaluate plaque inflammatory activity and risk of cardiovascular events. We investigated [18]-fluorodeoxyglucose (FDG) uptake in vulnerable carotid plaques visualised by positron emission tomography (PET). Uptake was correlated to quantitative gene expression of known markers of inflammation and plaque vulnerability.

METHODS

Ten patients with recent transient ischaemic attack and carotid artery stenosis (>50%) underwent combined FDG-PET and computed tomography angiography (CTA) the day before carotid endarterectomy. Plaque mRNA expression of the inflammatory cytokine interleukin 18 (IL-18), the macrophage-specific marker CD68 and the two proteinases, Cathepsin K and matrix metalloproteinase 9 (MMP-9), were quantified using real-time quantitative polymerase chain reaction.

RESULTS

Consistent up-regulation of CD68 (3.8-fold+/-0.9; mean+/-standard error), Cathepsin K (2.1-fold+/-0.5), MMP-9 (122-fold+/-65) and IL-18 (3.4-fold+/-0.7) were found in the plaques, compared to reference-artery specimens. The FDG uptake by plaques was strongly correlated with CD68 gene expression (r=0.71, P=0.02). Any correlations with Cathepsin K, MMP-9 or IL-18 gene expression were weaker.

CONCLUSIONS

FDG-PET uptake in carotid plaques is correlated to gene expression of CD68 and other molecular markers of inflammation and vulnerability.

Leucocyte cathepsin K affects atherosclerotic lesion composition and bone mineral density in low-density lipoprotein receptor deficient mice

AIMS

Cathepsin K (CatK), an established drug target for osteoporosis, has been reported to be upregulated in atherosclerotic lesions. Due to its proteolytic activity, CatK may influence the atherosclerotic lesion composition and stability. In this study, we investigated the potential role of leucocyte CatK in atherosclerotic plaque remodelling.

METHODS AND RESULTS

To assess the biological role of leucocyte CatK, we used the technique of bone marrow transplantation to selectively disrupt CatK in the haematopoietic system. Total bone marrow progenitor cells from CatK(+/+), CatK(+/-), and CatK(-/-) mice were transplanted into X-ray irradiated low-density lipoprotein receptor knockout (LDLr(-/-)) mice. The selective silencing of leucocyte CatK resulted in phenotypic changes in bone formation with an increased total bone mineral density in the CatK(-/-) chimeras and an effect of gene dosage. The absence of leucocyte CatK resulted in dramatically decreased collagen and increased macrophage content of the atherosclerotic lesions while lesion size was not affected. The atherosclerotic lesions also demonstrated less elastic lamina fragmentation and a significant increase in the apoptotic and necrotic area in plaques of mice transplanted with CatK(-/-) bone marrow.

CONCLUSION

Leucocyte CatK is an important determinant of atherosclerotic plaque composition, vulnerability, and bone remodelling, rendering CatK an attractive target for pharmaceutical modulation in atherosclerosis and osteoporosis.

Real-time catheter molecular sensing of inflammation in proteolytically active atherosclerosis

BACKGROUND

To enable intravascular detection of inflammation in atherosclerosis, we developed a near-infrared fluorescence (NIRF) catheter-based strategy to sense cysteine protease activity during vascular catheterization.

METHODS AND RESULTS

The NIRF catheter design was based on a clinical coronary artery guidewire. In phantom studies of NIRF plaques, blood produced only a mild (<30%) attenuation of the fluorescence signal compared with saline, affirming the favorable optical properties of the NIR window. Catheter evaluation in vivo used atherosclerotic rabbits (n=11). Rabbits received an injection of a cysteine protease-activatable NIRF imaging agent (Prosense750; excitation/emission, 750/770 nm) or saline. Catheter pullbacks through the blood-filled iliac artery detected NIRF signals 24 hours after injection of the probe. In the protease agent group, the in vivo peak plaque target-to-

BACKGROUND

<0.05). Ex vivo fluorescence reflectance imaging corroborated these results (target-to-

BACKGROUND

<0.01). In the protease group only, saline flush-modulated NIRF signal profiles further distinguished atheromata from normal segments in vivo (P<0.01). Good correlation between the in vivo and ex vivo plaque target-to-

BACKGROUND

=0.82, P<0.01). Histopathological analyses demonstrated strong NIRF signal in plaques only from the protease agent group. NIRF signals colocalized with immunoreactive macrophages and the cysteine protease cathepsin B.

CONCLUSIONS

An intravascular fluorescence catheter can detect cysteine protease activity in vessels the size of human coronary arteries in real time with an activatable NIRF agent. This strategy could aid in the detection of inflammation and high-risk plaques in small arteries.

Applications of gold nanoparticles in cancer nanotechnology

It has been almost 4 decades since the "war on cancer" was declared. It is now generally believed that personalized medicine is the future for cancer patient management. Possessing unprecedented potential for early detection, accurate diagnosis, and personalized treatment of cancer, nanoparticles have been extensively studied over the last decade. In this review, we will summarize the current state-of-the-art of gold nanoparticles in biomedical applications targeting cancer. Gold nanospheres, nanorods, nanoshells, nanocages, and surface enhanced Raman scattering nanoparticles will be discussed in detail regarding their uses in in vitro assays, ex vivo and in vivo imaging, cancer therapy, and drug delivery. Multifunctionality is the key feature of nanoparticle-based agents. Targeting ligands, imaging labels, therapeutic drugs, and other functionalities can all be integrated to allow for targeted molecular imaging and molecular therapy of cancer. Big strides have been made and many proof-of-principle studies have been successfully performed. The future looks brighter than ever yet many hurdles remain to be conquered. A multifunctional platform based on gold nanoparticles, with multiple receptor targeting, multimodality imaging, and multiple therapeutic entities, holds the promise for a "magic gold bullet" against cancer.

Assessing the sensitivity of commercially available fluorophores to the intracellular environment

The use of fluorescence has become commonplace in the biological sciences, with many studies utilizing probes based on commercially available fluorophores to provide insight into cell function and behavior. As these imaging applications become more advanced, it becomes increasingly important to acquire accurate quantitative measurements of the fluorescence signal. Absolute quantification of fluorescence, however, requires the fluorophores themselves to be insensitive to environmental factors such as nonspecific protein interactions and pH. Here, we present a method for characterizing the sensitivity of fluorophores to the cytosolic environment by comparing their fluorescent intensity to an environment-insensitive reference signal before and after intracellular delivery. Results indicated that although the fluorescent intensity of a few fluorophores, e.g., fluorescein, were highly susceptible to the intracellular environment, other fluorophores, e.g., Dylight 649, Alexa647, and Alexa750, were insensitive to the intracellular environment. It was also observed that the sensitivity of the fluorophore could be dependent on the biomolecule to which it was attached. In addition to assessing the environmental sensitivity of fluorophores, a method for quantifying the amount of fluorophores within living cells is also introduced. Overall, the present study provides a means to select fluorophores for studies that require an absolute quantification of fluorescence in the intracellular environment.

Radionuclide imaging: a molecular key to the atherosclerotic plaque

Despite primary and secondary prevention, serious cardiovascular events such as unstable angina or myocardial infarction still account for one-third of all deaths worldwide. Therefore, identifying individual patients with vulnerable plaques at high risk for plaque rupture is a central challenge in cardiovascular medicine. Several noninvasive techniques, such as magnetic resonance imaging, multislice computed tomography, and electron beam tomography are currently being tested for their ability to identify such patients by morphological criteria. In contrast, molecular imaging techniques use radiolabeled molecules to detect functional aspects in atherosclerotic plaques by visualizing their biological activity. Based upon the knowledge about the pathophysiology of atherosclerosis, various studies in vitro and in vivo and the first clinical trials have used different tracers for plaque imaging studies, including radioactive-labeled lipoproteins, components of the coagulation system, cytokines, mediators of the metalloproteinase system, cell adhesion receptors, and even whole cells. This review gives an update on the relevant noninvasive plaque imaging approaches using nuclear imaging techniques to detect atherosclerotic vascular lesions.

Imaging the boundaries-innovative tools for microscopy of living cells and real-time imaging

Recently, light microscopy moved back into the spotlight, which is mainly due to the development of revolutionary technologies for imaging real-time events in living cells. It is truly fascinating to see enzymes “at work” and optically acquired images certainly help us to understand biological processes better than any abstract measurements. This review aims to point out elegant examples of recent cell-biological imaging applications that have been developed with a chemical approach. The discussed technologies include nanoscale fluorescence microscopy, imaging of model membranes, automated high-throughput microscopy control and analysis, and fluorescent probes with a special focus on visualizing enzyme activity, free radicals, and protein–protein interaction designed for use in living cells.

The molecular mechanisms of the thrombotic complications of atherosclerosis

Our evolving knowledge of the cellular and molecular mechanisms underlying atherosclerosis has helped uncover the underlying causes behind thrombotic complications of this disease. Most fatal coronary thrombosis result from fibrous cap rupture or superficial erosion. Recent research has established a role for matrix metalloproteinases in the regulation of aspects of plaque structure related to propensity to disrupt and provoke thrombosis. Inflammatory pathways impinge on proteinase activity and aspects of oxidative stress that may favour plaque disruption. Novel molecular imaging strategies may permit visualization of proteinase activity in vivo, providing a new functional window on pathophysiology.

Imaging of atherosclerotic cardiovascular disease

Atherosclerosis is characterized by thickening of the walls of the arteries, a process that occurs slowly and 'silently' over decades. This prolonged course of disease provides a window of opportunity for diagnosis before symptoms occur. But, until recently, only advanced atherosclerotic disease could be observed. Now, developments in imaging technology offer many enticing prospects, including detecting atherosclerosis early, grouping individuals by the probability that they will develop symptoms of atherosclerosis, assessing the results of treatment and improving the current understanding of the biology of atherosclerosis.

A new atherosclerotic lesion probe based on hydrophobically modified chitosan nanoparticles functionalized by the atherosclerotic plaque targeted peptides

We developed a new imaging probe for atherosclerotic lesion imaging by chemically conjugating an atherosclerotic plaque-homing peptide (termed the AP peptide) to hydrophobically modified glycol chitosan (HGC) nanoparticles. The AP peptide was previously discovered by using an in vivo phage display screening method. HGC nanoparticles were labeled with the near-infrared (NIR) fluorophore Cy5.5, yielding nanoparticles 314 nm in diameter. The binding characteristics of nanoparticles to cytokine (TNF-alpha)-activated bovine aortic endothelial cells (BAECs) were studied in vitro under static conditions and in a dynamic flow environment. AP-tagged HGC-Cy5.5 nanoparticles (100 microg/ml, 2 h incubation) bound more avidly to TNF-alpha-activated BAECs than to unactivated BAECs. Nanoparticles were mostly located in the membranes of BAECs, although some were taken up by the cells and were visible in the cytoplasm, suggesting that the AP peptides in HGC nanoparticles retained target selectivity for activated BAECs. Binding selectivity of AP-tagged HGC-Cy5.5 nanoparticles was also studied in vivo. NIR fluorescence imaging demonstrated that AP-tagged HGC-Cy5.5 nanoparticles bound better to atherosclerotic lesions in a low-density lipoprotein receptor-deficient (Ldlr(-/-)) atherosclerotic mouse than to such lesions in a normal mouse. These results suggest that the newly designed AP-tagged HGC-Cy5.5 nanoparticles may be useful for atherosclerotic lesion imaging, and may also be employed to elucidate pathophysiological changes, at the molecular level, on atherosclerotic endothelium.

Atherosclerosis and thrombosis: identification of targets for magnetic resonance imaging

Imaging techniques are needed that will allow earlier and more refined diagnosis, guide targeted treatment in individual patients and monitor response to that treatment. Magnetic resonance imaging is well-suited to these tasks as it can provide anatomical, structural, and functional data on the arterial wall. Its capabilities are further enhanced by the use of a range of increasingly sophisticated contrast agents that target specific molecules, cells, and biological processes. This article will consider the pathogenesis of atherosclerosis and systematically identify biologically relevant targets for imaging at different stages of disease process.

Veni, vidi, vici: in vivo molecular imaging of immune response

"I came, I saw, I conquered," Julius Caesar proclaimed, highlighting the importance of direct visualization as a winning strategy. Continuing the "From the Field" series (see Editorial [2007] 26, 131), Gross et al. summarize how modern molecular imaging techniques can successfully dissect the complexities of immune response in vivo.

Osteogenesis associates with inflammation in early-stage atherosclerosis evaluated by molecular imaging in vivo

BACKGROUND

Arterial calcification is associated with cardiovascular events; however, mechanisms of calcification in atherosclerosis remain obscure.

METHODS AND RESULTS

We tested the hypothesis that inflammation promotes osteogenesis in atherosclerotic plaques using in vivo molecular imaging in apolipoprotein E-/- mice (20 to 30 weeks old, n=35). A bisphosphonate-derivatized near-infrared fluorescent imaging agent (excitation 750 nm) visualized osteogenic activity that was otherwise undetectable by x-ray computed tomography. Flow cytometry validated the target specifically in osteoblast-like cells. A spectrally distinct near-infrared fluorescent nanoparticle (excitation 680 nm) was coinjected to simultaneously image macrophages. Fluorescence reflectance mapping demonstrated an association between osteogenic activity and macrophages in aortas of apolipoprotein E-/- mice (R2=0.93). Intravital dual-channel fluorescence microscopy was used to further monitor osteogenic changes in inflamed carotid arteries at 20 and 30 weeks of age and revealed that macrophage burden and osteogenesis concomitantly increased during plaque progression (P<0.01 and P<0.001, respectively) and decreased after statin treatment (P<0.0001 and P<0.05, respectively). Fluorescence microscopy on cryosections colocalized near-infrared fluorescent osteogenic signals with alkaline phosphatase activity, bone-regulating protein expression, and hydroxyapatite nanocrystals as detected by electron microscopy, whereas von Kossa and alizarin red stains showed no evidence of calcification. Real-time reverse-transcription polymerase chain reaction revealed that macrophage-conditioned media increased alkaline phosphatase mRNA expression in vascular smooth muscle cells.

CONCLUSIONS

This serial in vivo study demonstrates the real-time association of macrophage burden with osteogenic activity in early-stage atherosclerosis and offers a cellular-resolution tool to identify preclinical microcalcifications.

Biochemical properties and regulation of cathepsin K activity

Cysteine cathepsins (11 in humans) are mostly located in the acidic compartments of cells. They have been known for decades to be involved in intracellular protein degradation as housekeeping proteases. However, the discovery of new cathepsins, including cathepsins K, V and F, has provided strong evidence that they also participate in specific biological events. This review focuses on the current knowledge of cathepsin K, the major bone cysteine protease, which is a drug target of clinical interest. Nevertheless, we will not discuss recent developments in cathepsin K inhibitor design since they have been extensively detailed elsewhere. We will cover features of cathepsin K structure, cellular and tissue distribution, substrate specificity, and regulation (pH, propeptide, glycosaminoglycans, oxidants), and its putative roles in physiological or pathophysiological processes. Finally, we will review the kinetic data of its inhibition by natural endogenous inhibitors (stefin B, cystatin C, H- and L-kininogens).