Visualization of activated platelets by targeted magnetic resonance imaging utilizing conformation-specific antibodies against glycoprotein IIb/IIIa

Clot-targeted magnetic hyperthermia permeabilizes blood clots to make them more susceptible to thrombolysis

BACKGROUND

Thrombolysis is a frontline treatment for stroke, which involves the application of tissue plasminogen activator (tPA) to trigger endogenous clot-degradation pathways. However, it is only effective within 4.5 h of symptom onset because of clot contraction preventing tPA permeation into the clot. Magnetic hyperthermia (MH) mediated by tumor-targeted magnetic nanoparticles is used to treat cancer by using local heat generation to trigger apoptosis of cancer cells.

OBJECTIVES

To develop clot-targeting magnetic nanoparticles to deliver MH to the surface of human blood clots, and to assess whether this can improve the efficacy of thrombolysis of contracted blood clots.

METHODS

Clot-targeting magnetic nanoparticles were developed by functionalizing iron oxide nanoparticles with an antibody recognizing activated integrin αIIbβ3 (PAC-1). The magnetic properties of the PAC-1-tagged magnetic nanoparticles were characterized and optimized to deliver clot-targeted MH.

RESULTS

Clot-targeted MH increases the efficacy of tPA-mediated thrombolysis in contracted human blood clots, leading to a reduction in clot weight. MH increases the permeability of the clots to tPA, facilitating their breakdown. Scanning electron microscopy reveals that this effect is elicited through enhanced fibrin breakdown and triggering the disruption of red blood cells on the surface of the clot. Importantly, endothelial cells viability in a three-dimensional blood vessel model is unaffected by exposure to MH.

CONCLUSIONS

This study demonstrates that clot-targeted MH can enhance the thrombolysis of contracted human blood clots and can be safely applied to enhance the timeframe in which thrombolysis is effective.

Molecular magnetic resonance imaging of activated platelets allows noninvasive detection of early myocarditis in mice

MRI sensitivity for diagnosis and localization of early myocarditis is limited, although it is of central clinical interest. The aim of this project was to test a contrast agent targeting activated platelets consisting of microparticles of iron oxide (MPIO) conjugated to a single-chain antibody directed against ligand-induced binding sites (LIBS) of activated glycoprotein IIb/IIIa (= LIBS-MPIO). Myocarditis was induced by subcutaneous injection of an emulsion of porcine cardiac myosin and complete Freund’s adjuvant in mice. 3D 7 T in-vivo MRI showed focal signal effects in LIBS-MPIO injected mice 2 days after induction of myocarditis, whereas in control-MPIO injected mice no signal was detectable. Histology confirmed CD41-positive staining, indicating platelet involvement in myocarditis in mice as well as in human specimens with significantly higher LIBS-MPIO binding compared to control-MPIO in both species. Quantification of the myocardial MRI signal confirmed a signal decrease after LIBS-MPIO injection and significant less signal in comparison to control-MPIO injection. These data show, that platelets are involved in inflammation during the course of myocarditis in mice and humans. They can be imaged non-invasively with LIBS-MPIO by molecular MRI at an early time point of the inflammation in mice, which is a valuable approach for preclinical models and of interest for both diagnostic and prognostic purposes.

Controlled Synthesis and Characterization of Micrometric Single Crystalline Magnetite With Superparamagnetic Behavior and Cytocompatibility/Cytotoxicity Assessments

A new class of magnetite (Fe₃O₄) particles, coined as “Single Crystalline Micrometric Iron Oxide Particles” (SCMIOPs), were obtained by hydrothermal synthesis. Both the single Fe₃O₄ phase content and the particle sizes range, from 1 µm to 30 µm, can be controlled by synthesis. The notable finding states that these particles exhibit vanishing remanent magnetization (σr=0.28 emu/g) and coercive force (Hc=1.5 Oe), which indicate a superparamagnetic-like behavior (unexpected at micrometric particles size), and remarkably high saturation magnetization (σs=95.5 emu/g), what ensures strong magnetic response, and the lack of agglomeration after the magnetic field removal. These qualities make such particles candidates for biomedical applications, to be used instead of magnetic nanoparticles which inevitably involve some drawbacks like aglommeration and insufficient magnetic response. In this sense, cytocompatibility/cytotoxicity tests were performed on human cells, and the results have clearly indicated that SCMIOPs are cytocompatible for healthy cell lines HaCaT (human keratinocytes) and HEMa (primary epidermal melanocytes) and cytotoxic for neoplastic cell lines A375 (human melanoma) and B164A5 (murine melanoma) in a dose-dependent manner.

Targeting Human Thrombus by Liposomes Modified with Anti-Fibrin Protein Binders

Development of tools for direct thrombus imaging represents a key step for diagnosis and treatment of stroke. Nanoliposomal carriers of contrast agents and thrombolytics can be functionalized to target blood thrombi by small protein binders with selectivity for fibrin domains uniquely formed on insoluble fibrin. We employed a highly complex combinatorial library derived from scaffold of 46 amino acid albumin-binding domain (ABD) of streptococcal protein G, and ribosome display, to identify variants recognizing fibrin cloth in human thrombus. We constructed a recombinant target as a stretch of three identical fibrin fragments of 16 amino acid peptide of the Bβ chain fused to TolA protein. Ribosome display selection followed by large-scale Enzyme-Linked ImmunoSorbent Assay (ELISA) screening provided four protein variants preferentially binding to insoluble form of human fibrin. The most specific binder variant D7 was further modified by C-terminal FLAG/His-Tag or double His-tag for the attachment onto the surface of nanoliposomes via metallochelating bond. D7-His-nanoliposomes were tested using in vitro flow model of coronary artery and their binding to fibrin fibers was demonstrated by confocal and electron microscopy. Thus, we present here the concept of fibrin-targeted binders as a platform for functionalization of nanoliposomes in the development of advanced imaging tools and future theranostics.

An acoustic strategy for gold nanoparticle loading in platelets as biomimetic multifunctional carriers

In recent years, a wide variety of bioinspired colloidal particles with novel cell mimetic functions have been the subject of extensive research in materials science, chemistry, biology, physics, and engineering. However, most of the approaches are derived from natural cell membrane coatings, which are still too primitive compared with living cells. In this study, we have chosen gold nanoparticles (GNPs) to explore the bioactivity response of living platelets and nanoparticle loading efficiency under different ultrasonic intensity and frequency treatment conditions. The results show that GNPs with no surface modification could be easily loaded into intra-platelets by both incubation (30 min) and ultrasonic exposure (1 min) methods. The amount of GNP loading was (4.4 ± 0.9) × 10^-3 and (5.8 ± 2.4) × 10^-3 pg per platelet upon incubation and acoustic triggering (1 MHz, 0.25 W cm^-2), respectively. Although the other US treatment intensities (0.75, 1.50 and 2.25 W cm^-2) also promoted higher amounts of GNPs in the platelets, the higher US intensity might bring about partial damage of the platelet membrane. Compared with 1 MHz ultrasonic exposure, the change of the GNP loading amount was not significantly higher upon ultrasonic frequency treatment of 45, 80 or 100 kHz. Therefore, it has been found that an US intensity of 0.25 W cm^-2 could facilitate the intra-platelet delivery efficacy of the GNPs without damaging the biological activity. Furthermore, two possible pathways of GNPs entering into platelets upon US treatment are presented: one is the endocytosis/open canalicular system (OCS), and the other is cell membrane permeability enhancement, which is proved by the SEM and TEM results. Finally, the GNP-loaded platelets have been demonstrated as useful probes for photoacoustic imaging (PAI) and dark-field microscopy (DFM)-based imaging, which might allow a wide range of potential applications in diagnostics and therapy of platelet-related diseases.

MRI, the technology for imaging of thrombi and inflammation

Atherosclerosis and its sequelae have a major impact on morbidity and mortality. The rupture of an inflamed atherosclerotic plaque is a crucial event, since it can result in acute thrombotic closure of an arterial vessel, resulting e. g. in myocardial infarction or stroke. Not only detection of early plaque rupture with imminent closure is therefore of clinical interest, but also timely detection of vascular inflammation and atherosclerotic plaque progression. However, plaque inflammation or even plaque rupture without vessel occlusion is not reliably detectable by current imaging techniques. Coronary angiography is the gold standard for evaluation of the coronary vessels, but only allows visualization of the vessel lumen without characterizing the important pathophysiology of the vessel wall. Therefore, highly inflamed and rupture prone plaques can be missed, or appear as a minor vessel narrowing. Although currently available techniques such as intravascular ultrasound or optical coherence tomography allow a further characterization of atherosclerotic plaques, it would be desirable to detect plaque inflammation, early plaque rupture or vascular thrombosis by non-invasive techniques such as magnetic resonance imaging (MRI), since they could allow early identification of patients at risk or triage of symptomatic patients.

In this manuscript, different strategies for detection of vascular inflammation, plaque-rupture and thrombosis by MRI will be discussed, with a special focus on molecular imaging contrast agents.

Iron oxide core oil-in-water nanoemulsion as tracer for atherosclerosis MPI and MRI imaging

PURPOSE

For early atherosclerosis imaging, magnetic oil-in-water nanoemulsion (NE) decorated with atheroma specific monoclonal antibody was designed for Magnetic Particle Imaging (MPI) and Magnetic Resonance Imaging (MRI). MPI is an emerging technique based on direct mapping of superparamagnetic nanoparticles which may advantageously complement MRI.

METHODS

NE oily droplets were loaded with superparamagnetic iron oxide nanoparticles of 7, 11 and 18nm and biofunctionalized with atheroma specific scFv-Fc TEG4-2C antibody.

RESULTS

Inclusion of nanoparticles inside NE did not change the hydrodynamic diameter of the oil droplets, close to 180nm, nor the polydispersity. The droplets were negatively charged (ζ=-30mV). In vitro MPI signal was assessed by Magnetic Particle Spectroscopy (MPS). NE displayed MRI and MPS signals confirming its potential as new contrast agent. NE MPS signal increase with NPs size close to the gold standard (Resovist). In MRI, NE displayed R₂* transversal relaxivity of 45.45, 96.04 and 218.81mM^-1s^-1 for 7, 11 and 18nm respectively. NE selectively bind atheroma plaque both in vitro and ex vivo in animal models of atherosclerosis.

CONCLUSION

Magnetic NE showed reasonable MRI/MPS signals and a significant labelling of the atheroma plaque. These preliminary results support that NE platform could selectively image atherosclerosis.

Molecular Imaging of Atherothrombotic Diseases: Seeing Is Believing

Molecular imaging, with major advances in the development of both innovative targeted contrast agents/particles and radiotracers, as well as various imaging technologies, is a fascinating, rapidly growing field with many preclinical and clinical applications, particularly for personalized medicine. Thrombosis in either the venous or the arterial system, the latter typically caused by rupture of unstable atherosclerotic plaques, is a major determinant of mortality and morbidity in patients. However, imaging of the various thrombotic complications and the identification of plaques that are prone to rupture are at best indirect, mostly unreliable, or not available at all. The development of molecular imaging toward diagnosis and prevention of thrombotic disease holds promise for major advance in this clinically important field. Here, we review the medical need and clinical importance of direct molecular imaging of thrombi and unstable atherosclerotic plaques that are prone to rupture, thereby causing thrombotic complications such as myocardial infarction and ischemic stroke. We systematically compare the advantages/disadvantages of the various molecular imaging modalities, including X-ray computed tomography, magnetic resonance imaging, positron emission tomography, single-photon emission computed tomography, fluorescence imaging, and ultrasound. We further systematically discuss molecular targets specific for thrombi and those characterizing unstable, potentially thrombogenic atherosclerotic plaques. Finally, we provide examples for first theranostic approaches in thrombosis, combining diagnosis, targeted therapy, and monitoring of therapeutic success or failure. Overall, molecular imaging is a rapidly advancing field that holds promise of major benefits to many patients with atherothrombotic diseases.

A Unique Recombinant Fluoroprobe Targeting Activated Platelets Allows In Vivo Detection of Arterial Thrombosis and Pulmonary Embolism Using a Novel Three-Dimensional Fluorescence Emission Computed Tomography (FLECT) Technology

Progress in pharmaceutical development is highly-dependent on preclinical in vivo animal studies. Small animal imaging is invaluable for the identification of new disease markers and the evaluation of drug efficacy. Here, we report for the first time the use of a three-dimensional fluorescence bioimager called FLuorescence Emission Computed Tomography (FLECT) for the detection of a novel recombinant fluoroprobe that is safe, easily prepared on a large scale and stably stored prior to scan. This novel fluoroprobe (Targ-Cy7) comprises a single-chain antibody-fragment (scFv_Targ), which binds exclusively to activated-platelets, conjugated to a near-infrared (NIR) dye, Cy7, for detection. Upon mouse carotid artery injury, the injected fluoroprobe circulates and binds within the platelet-rich thrombus. This specific in vivo binding of the fluoroprobe to the thrombus, compared to its non-targeting control-fluoroprobe, is detected by the FLECT imager. The analyzed FLECT image quantifies the NIR signal and localizes it to the site of vascular injury. The detected fluorescence is further verified using a two-dimensional IVIS^® Lumina scanner, where significant NIR fluorescence is detected in vivo at the thrombotic site, and ex vivo, at the injured carotid artery. Furthermore, fluorescence levels in various organs have also been quantified for biodistribution, with the highest fluoroprobe uptake shown to be in the injured artery. Subsequently, this live animal imaging technique is successfully employed to monitor the response of the induced thrombus to treatment over time. This demonstrates the potential of using longitudinal FLECT scanning to examine the efficacy of candidate drugs in preclinical settings. Besides intravascular thrombosis, we have shown that this non-invasive FLECT-imaging can also detect in vivo pulmonary embolism. Overall, this report describes a novel fluorescence-based preclinical imaging modality that uses an easy-to-prepare and non-radioactive recombinant fluoroprobe. This represents a unique tool to study mechanisms of thromboembolic diseases and it will strongly facilitate the in vivo testing of antithrombotic drugs. Furthermore, the non-radiation nature, low-cost, high sensitivity, and the rapid advancement of optical scanning technologies make this fluorescence imaging an attractive development for future clinical applications.

Fe3O4-based PLGA nanoparticles as MR contrast agents for the detection of thrombosis

Thrombotic disease is a great threat to human health, and early detection is particularly important. Magnetic resonance (MR) molecular imaging provides noninvasive imaging with the potential for early disease diagnosis. In this study, we developed Fe₃O₄-based poly(lactic-co-glycolic acid) (PLGA) nanoparticles (NPs) surface-modified with a cyclic Arg-Gly-Asp (cRGD) peptide as an MR contrast agent for the detection of thrombosis. The physical and chemical characteristics, biological toxicity, ability to target thrombi, and biodistribution of the NPs were studied. The Fe₃O₄-PLGA-cRGD NPs were constructed successfully, and hematologic and pathologic assays indicated no in vivo toxicity of the NPs. In a rat model of FeCl₃-induced abdominal aorta thrombosis, the NPs readily and selectively accumulated on the surface of the thrombosis and under vascular endothelial cells ex vivo and in vivo. In the in vivo experiment, the biodistribution of the NPs suggested that the NPs might be internalized by the macrophages of the reticuloendothelial system in the liver and the spleen. The T2 signal decreased at the mural thrombus 10 min after injection and then gradually increased until 50 min. These results suggest that the NPs are suitable for in vivo molecular imaging of thrombosis under high shear stress conditions and represent a very promising MR contrast agent for sensitive and specific detection of thrombosis.

Molecular imaging of atherosclerosis with nanoparticle-based fluorinated MRI contrast agents

As atherosclerosis remains one of the most prevalent causes of patient mortality, the ability to diagnose early signs of plaque rupture and thrombosis represents a significant clinical need. With recent advances in nanotechnology, it is now possible to image specific molecular processes noninvasively with MRI, using various types of nanoparticles as contrast agents. In the context of cardiovascular disease, it is possible to specifically deliver contrast agents to an epitope of interest for detecting vascular inflammatory processes, which serve as predecessors to atherosclerotic plaque development. Herein, we review various applications of nanotechnology in detecting atherosclerosis using MRI, with an emphasis on perfluorocarbon nanoparticles and fluorine imaging, along with theranostic prospects of nanotechnology in cardiovascular disease.

Noninvasive Molecular Imaging of Mouse Atherosclerosis

Molecular imaging offers great potential for noninvasive visualization and quantitation of the cellular and molecular components involved in atherosclerotic plaque stability. In this chapter, we review emerging molecular imaging modalities and approaches for quantitative, noninvasive detection of early biological processes in atherogenesis, including vascular endothelial permeability, endothelial adhesion molecule up-regulation, and macrophage accumulation, with special emphasis on mouse models. We also highlight a number of targeted imaging nanomaterials for assessment of advanced atherosclerotic plaques, including extracellular matrix degradation, proteolytic enzyme activity, and activated platelets using mouse models of atherosclerosis. The potential for clinical translation of molecular imaging nanomaterials for assessment of atherosclerotic plaque biology, together with multimodal approaches is also discussed.

Thrombus-Targeted Theranostic Microbubbles: A New Technology towards Concurrent Rapid Ultrasound Diagnosis and Bleeding-free Fibrinolytic Treatment of Thrombosis

Rationale: Myocardial infarction and stroke are leading causes of morbidity/mortality. The typical underlying pathology is the formation of thrombi/emboli and subsequent vessel occlusion. Systemically administered fibrinolytic drugs are the most effective pharmacological therapy. However, bleeding complications are relatively common and this risk as such limits their broader use. Furthermore, a rapid non-invasive imaging technology is not available. Thereby, many thrombotic events are missed or only diagnosed when ischemic damage has already occurred.

Objective: Design and preclinical testing of a novel 'theranostic' technology for the rapid non-invasive diagnosis and effective, bleeding-free treatment of thrombosis.

Methods and Results: A newly created, innovative theranostic microbubble combines a recombinant fibrinolytic drug, an echo-enhancing microbubble and a recombinant thrombus-targeting device in form of an activated-platelet-specific single-chain antibody. After initial in vitro proof of functionality, we tested this theranostic microbubble both in ultrasound imaging and thrombolytic therapy using a mouse model of ferric-chloride-induced thrombosis in the carotid artery. We demonstrate the reliable highly sensitive detection of in vivo thrombi and the ability to monitor their size changes in real time. Furthermore, these theranostic microbubbles proofed to be as effective in thrombolysis as commercial urokinase but without the prolongation of bleeding time as seen with urokinase.

Conclusions: We describe a novel theranostic technology enabling simultaneous diagnosis and treatment of thrombosis, as well as monitoring of success or failure of thrombolysis. This technology holds promise for major progress in rapid diagnosis and bleeding-free thrombolysis thereby potentially preventing the often devastating consequences of thrombotic disease in many patients.

Dual-contrast molecular imaging allows noninvasive characterization of myocardial ischemia/reperfusion injury after coronary vessel occlusion in mice by magnetic resonance imaging

BACKGROUND

Inflammation and myocardial necrosis play important roles in ischemia/reperfusion injury after coronary artery occlusion and recanalization. The detection of inflammatory activity and the extent of myocardial necrosis itself are of great clinical and prognostic interest. We developed a dual, noninvasive imaging approach using molecular magnetic resonance imaging in an in vivo mouse model of myocardial ischemia and reperfusion.

METHODS AND RESULTS

Ischemia/reperfusion injury was induced in 10-week-old C57BL/6N mice by temporary ligation of the left anterior descending coronary artery. Activated platelets were targeted with a contrast agent consisting of microparticles of iron oxide (MPIOs) conjugated to a single-chain antibody directed against a ligand-induced binding site (LIBS) on activated glycoprotein IIb/IIIa (LIBS-MPIOs). After injection and imaging of LIBS-MPIOs, late gadolinium enhancement was used to depict myocardial necrosis; these imaging experiments were also performed in P2Y12 (-/-) mice. All imaging results were correlated to immunohistochemistry findings. Activated platelets were detectable by magnetic resonance imaging via a significant signal effect caused by LIBS-MPIOs in the area of left anterior descending coronary artery occlusion 2 hours after reperfusion. In parallel, late gadolinium enhancement identified the extent of myocardial necrosis. Immunohistochemistry confirmed that LIBS-MPIOs bound significantly to microthrombi in reperfused myocardium. Only background binding was found in P2Y12 (-/-) mice.

CONCLUSIONS

Dual molecular imaging of myocardial ischemia/reperfusion injury allows characterization of platelet-driven inflammation by LIBS-MPIOs and myocardial necrosis by late gadolinium enhancement. This noninvasive imaging strategy is of clinical interest for both diagnostic and prognostic purposes and highlights the potential of molecular magnetic resonance imaging for characterizing ischemia/reperfusion injury.

Immunobiology of Plasmodium in liver and brain

Malaria remains one of the most serious health problems globally, but our understanding of the biology of the parasite and the pathogenesis of severe disease is still limited. Multiple cellular effector mechanisms that mediate parasite elimination from the liver have been described, but how effector cells use classical granule-mediated cytotoxicity to attack infected hepatocytes and how cytokines and chemokines spread via the unique fluid pathways of the liver to reach the parasites over considerable distances remains unknown. Similarly, a wealth of information on cerebral malaria (CM), one of the most severe manifestations of the disease, was gained from post-mortem analyses of human brain and murine disease models, but the cellular processes that ultimately cause disease are not fully understood. Here, we discuss how imaging of the local dynamics of parasite infection and host response as well as consideration of anatomical and physiological features of liver and brain can provide a better understanding of the initial asymptomatic hepatic phase of the infection and the cascade of events leading to CM. Given the increasing drug resistance of both parasite and vector and the unavailability of a protective vaccine, the urgency to reduce the tremendous morbidity and mortality associated with severe malaria is obvious.

Detection of activated platelets in a mouse model of carotid artery thrombosis with 18 F-labeled single-chain antibodies

INTRODUCTION

Activated platelets are key players in thrombosis and inflammation. We previously generated single-chain antibodies (scFv) against ligand-induced binding sites (LIBS) on the highly abundant platelet glycoprotein integrin receptor IIb/IIIa. The aim of this study was the construction and characterisation of a novel (18)F PET radiotracer based on this antibody.

METHODS

ScFv(anti-LIBS) and control antibody mut-scFv were reacted with N-succinimidyl-4-[(18)F]fluorobenzoate (S[(18)F]FB). Radiolabeled scFv was incubated with in vitro formed platelet clots and injected into mice with FeCl(3) induced thrombus in the left carotid artery. Clots were imaged in the PET scanner and amount of radioactivity measured using an ionization chamber and image analysis. Assessment of vessel injury as well as the biodistribution of the radiolabeled scFv was studied.

RESULTS

After incubation with increasing concentrations of (18)F-scFv(anti-LIBS) clots had retained significantly higher amounts of radioactivity compared to clots incubated with radiolabeled (18)F-mut-scFv (13.3 ± 3.8 vs. 3.6 ± 1 KBq, p < 0.05, n = 9, decay corrected). In the in vivo experiments we found an high uptake of the tracer in the injured vessel compared with the non-injured vessel, with 12.6 ± 4.7% injected dose per gram (ID/g) uptake in the injured vessel and 3.7 ± 0.9% ID/g in the non-injured vessel 5 minutes after injection (p < 0.05, n = 6).

CONCLUSIONS

Our results show that the novel antibody radiotracer (18)F-scFv(anti-LIBS) is useful for the sensitive detection of activated platelets and thrombosis.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

We describe the first (18)F variant of a scFv(anti-LIBS) against activated platelets. This diagnostic agent could provide a powerful tool for the assessment of acute thrombosis and inflammation in patients in the future.

Molecular imaging of inflammation in atherosclerosis

Acute rupture of vulnerable plaques frequently leads to myocardial infarction and stroke. Within the last decades, several cellular and molecular players have been identified that promote atherosclerotic lesion formation, maturation and plaque rupture. It is now widely recognized that inflammation of the vessel wall and distinct leukocyte subsets are involved throughout all phases of atherosclerotic lesion development. The mechanisms that render a stable plaque unstable and prone to rupture, however, remain unknown and the identification of the vulnerable plaque remains a major challenge in cardiovascular medicine. Imaging technologies used in the clinic offer minimal information about the underlying biology and potential risk for rupture. New imaging technologies are therefore being developed, and in the preclinical setting have enabled new and dynamic insights into the vessel wall for a better understanding of this complex disease. Molecular imaging has the potential to track biological processes, such as the activity of cellular and molecular biomarkers in vivo and over time. Similarly, novel imaging technologies specifically detect effects of therapies that aim to stabilize vulnerable plaques and silence vascular inflammation. Here we will review the potential of established and new molecular imaging technologies in the setting of atherosclerosis, and discuss the cumbersome steps required for translating molecular imaging approaches into the clinic.

Future non-invasive imaging to detect vascular plaque instability and subclinical non-obstructive atherosclerosis

Atherosclerosis underlies the major causes of death in the Western World. Our main goal is to detect early changes of atherosclerosis and to identify subjects at highest cardiovascular risk that may aid in the development of prevention approaches and better management that will decrease cardiovascular morbidity and mortality. The new methods that are of interest include the advanced vascular ultrasound methods, the infra red and near infra red imaging techniques, the EndoPat device that reflects peripheral arterial tone, the electron beam computed tomography, the magnetic resonance imaging, and the molecular imaging techniques. In this review we will focus on the future of advanced imaging techniques that are being developed to detect early (pre-clinical) development of atherosclerosis.

Cardiac magnetic resonance imaging of rapid VCAM-1 up-regulation in myocardial ischemia-reperfusion injury

Inflammatory response plays an important role in myocardial ischaemia-reperfusion (IR) injury. Up-regulation of vascular cell adhesion molecule-1 (VCAM) contributes to this. We examined the feasibility of using intravenously administered VCAM-MPIO (microparticle iron oxide) to characterize VCAM expression patterns in myocardial IR injury. Myocardial ischemia was simulated by 30 min of transient ligation of the left coronary vessel in rats. Purified, monoclonal, rat-specific, mouse VCAM antibody coupled to MPIO was administered through the tail vein at 3 h post reperfusion and the rats were sacrificed 1 h later. High resolution 3D ex vivo MRI images were acquired at 9.4 Tesla. Extensive foci of signal voids were observed on T2*-weighted gradient-echo sequences, which corresponded to focal deposits of MPIOs observed in histological sections. The spatial density of the signal voids (expressed as a percentage of pixels below a threshold value) was increased in the peri-infarct zone compared with non-infarct zone (32.5 ± 4% vs. 13.9 ± 5%; n = 6; p < 0.05) and was substantially greater than the signal loss due to non-specific binding seen in rats administered IgG control MPIO (2.0 ± 1%; n = 6; p < 0.05). The VCAM-specific MPIO signal was also seen in myocardium and pericardium in segments remote from the IR injury, but not in rats undergoing a sham operation. In conclusion, molecular imaging in a model of myocardial IR injury is possible using high field MRI and VCAM-MPIOs and may provide novel insights beyond those achieved by standard histological and molecular analysis.

Molecular imaging of atherosclerosis for improving diagnostic and therapeutic development

Despite recent progress, cardiovascular and allied metabolic disorders remain a worldwide health challenge. We must identify new targets for therapy, develop new agents for clinical use, and deploy them in a clinically effective and cost-effective manner. Molecular imaging of atherosclerotic lesions has become a major experimental tool in the last decade, notably by providing a direct gateway to the processes involved in atherogenesis and its complications. This review summarizes the current status of molecular imaging approaches that target the key processes implicated in plaque formation, development, and disruption and highlights how the refinement and application of such tools might aid the development and evaluation of novel therapeutics.

In vivo detection of activated platelets allows characterizing rupture of atherosclerotic plaques with molecular magnetic resonance imaging in mice

BACKGROUND

Early and non-invasive detection of platelets on micro atherothrombosis provides a means to identify unstable plaque and thereby allowing prophylactic treatment towards prevention of stroke or myocardial infarction. Molecular magnetic resonance imaging (mMRI) of activated platelets as early markers of plaque rupture using targeted contrast agents is a promising strategy. In this study, we aim to specifically image activated platelets in murine atherothrombosis by in vivo mMRI, using a dedicated animal model of plaque rupture.

METHODS

An antibody targeting ligand-induced binding sites (LIBS) on the glycoprotein IIb/IIIa-receptor of activated platelets was conjugated to microparticles of iron oxide (MPIO) to form the LIBS-MPIO contrast agent causing a signal-extinction in T2*-weighted MRI. ApoE(-/-) mice (60 weeks-old) were fed a high fat diet for 5 weeks. Using a small needle, the surface of their carotid plaques was scratched under blood flow to induce atherothrombosis. In vivo 9.4 Tesla MRI was performed before and repetitively after intravenous injection of either LIBS-MPIO versus non-targeted-MPIO.

RESULTS

LIBS-MPIO injected animals showed a significant signal extinction (p<0.05) in MRI, corresponding to the site of plaque rupture and atherothrombosis in histology. The signal attenuation was effective for atherothrombosis occupying ≥ 2% of the vascular lumen. Histology further confirmed significant binding of LIBS-MPIO compared to control-MPIO on the thrombus developing on the surface of ruptured plaques (p<0.01).

CONCLUSION

in vivo mMRI detected activated platelets on mechanically ruptured atherosclerotic plaques in ApoE(-/-) mice with a high sensititvity. This imaging technology represents a unique opportunity for noninvasive detection of atherothrombosis and the identification of unstable atherosclerotic plaques with the ultimate promise to prevent strokes and myocardial infarctions.

Imaging inflammation: molecular strategies to visualize key components of the inflammatory cascade, from initiation to resolution

Dysregulation of inflammation is central to the pathogenesis of innumerable human diseases. Understanding and tracking the critical events in inflammation are crucial for disease monitoring and pharmacological drug discovery and development. Recent progress in molecular imaging has provided novel insights into spatial associations, molecular events and temporal sequelae in the inflammatory process. While remaining a burgeoning field in pre-clinical research, increasing application in man affords researchers the opportunity to study disease pathogenesis in humans in situ thereby revolutionizing conventional understanding of pathophysiology and potential therapeutic targets. This review provides a description of commonly used molecular imaging modalities, including optical, radionuclide and magnetic resonance imaging, and details key advances and translational opportunities in imaging inflammation from initiation to resolution.

Emerging engineered magnetic nanoparticulate probes for molecular MRI of atherosclerosis: how far have we come?

Atherosclerosis is a chronic, progressive, immunoinflammatory disease of the large and medium-sized arteries, and a major cause of cardiovascular diseases. Atherosclerosis often progresses silently for decades until the occurrence of a major catastrophic clinical event such as myocardial infarction, cardiac arrest and stroke. The main challenge in the diagnosis and management of atherosclerosis is to develop a safe, noninvasive technique that is accurate and reproducible, which can detect the biologically active high-risk vulnerable plaques (with ongoing active inflammation, angiogenesis and apoptosis) before the occurrence of an acute clinical event. This article reviews the events involved in the pathogenesis of atherosclerosis in light of recently advanced understanding of the molecular pathogenesis of the disease. Next, we elaborate on the interesting developments in molecular MRI, by describing the recently engineered magnetic nanoparticulate probes targeting clinically promising molecular and cellular players/processes, involved in early atherosclerotic lesion formation to plaque rupture and erosion.

Novel single-chain antibody-targeted microbubbles for molecular ultrasound imaging of thrombosis: validation of a unique noninvasive method for rapid and sensitive detection of thrombi and monitoring of success or failure of thrombolysis in mice

BACKGROUND

Molecular imaging is a fast emerging technology allowing noninvasive detection of vascular pathologies. However, imaging modalities offering high resolution currently do not allow real-time imaging. We hypothesized that contrast-enhanced ultrasound with microbubbles (MBs) selectively targeted to activated platelets would offer high-resolution, real-time molecular imaging of evolving and dissolving arterial thrombi.

METHODS AND RESULTS

Lipid-shell based gas-filled MBs were conjugated to either a single-chain antibody specific for activated glycoprotein IIb/IIIa via binding to a Ligand-Induced Binding Site (LIBS-MBs) or a nonspecific single-chain antibody (control MBs). Successful conjugation was assessed in flow cytometry and immunofluorescence double staining. LIBS-MBs but not control MBs strongly adhered to both immobilized activated platelets and microthrombi under flow. Thrombi induced in carotid arteries of C57Bl6 mice in vivo by ferric chloride injury were then assessed with ultrasound before and 20 minutes after MB injection through the use of gray-scale area intensity measurement. Gray-scale units converted to decibels demonstrated a significant increase after LIBS-MB but not after control MB injection (9.55±1.7 versus 1.46±1.3 dB; P<0.01). Furthermore, after thrombolysis with urokinase, LIBS-MB ultrasound imaging allows monitoring of the reduction of thrombus size (P<0.001).

CONCLUSION

We demonstrate that glycoprotein IIb/IIIa-targeted MBs specifically bind to activated platelets in vitro and allow real-time molecular imaging of acute arterial thrombosis and monitoring of the success or failure of pharmacological thrombolysis in vivo.

A leukocyte-mimetic magnetic resonance imaging contrast agent homes rapidly to activated endothelium and tracks with atherosclerotic lesion macrophage content

OBJECTIVE

Endothelial cell activation is an important mediator of monocyte recruitment to sites of vascular inflammation. We hypothesized that high-affinity dual-ligand microparticles of iron oxide (MPIO), targeted to P-selectin and vascular cell adhesion molecule-1 (PV-MPIO), would identify activated endothelial cells during atherosclerosis progression.

METHODS AND RESULTS

In vivo magnetic resonance imaging in apolipoprotein E-deficient mice showed rapid binding of PV-MPIO to the aortic root, which was maximal 30 minutes post-MPIO injection and maintained at 60 minutes. Minimal binding was observed for control IgG-MPIO. Intensely low magnetic resonance signal areas, corresponding to PV-MPIO binding, were detected early (14 weeks), during foam cell formation. Contrast effects increased at 20 weeks during fibrofatty lesion development (P<0.05), but reduced by 30 weeks (P<0.01). Across all lesion severities, magnetic resonance imaging contrast effects correlated with lesion macrophage area quantified by immunohistochemistry (R=0.53; P<0.01). Near-infrared fluorescently labeled PV-MPIO were shown, by flow cytometry, to bind only activated endothelial cells and not to macrophages. Using en face immunofluorescence, we further demonstrate selective PV-MPIO accumulation at atherosclerosis-susceptible sites, with minimal binding to atherosclerosis-spared regions.

CONCLUSIONS

This high-affinity leukocyte-mimetic magnetic resonance imaging agent reveals endothelial activation. PV-MPIO demonstrate exceptionally rapid in vivo steady state accumulation, providing conspicuous magnetic resonance contrast effects that can be objectively quantified. In atherosclerosis progression, PV-MPIO tracked closely with the burden and distribution of plaque macrophages, not merely plaque size. On a biocompatible platform, this approach has potential for quantitative magnetic resonance imaging of inflammatory disease activity.

Development and application of endothelium-targeted microparticles for molecular magnetic resonance imaging

Molecular imaging of disease states can enhance diagnosis allowing for accurate and more effective treatment. By specifically targeting molecules differentially expressed in disease states, researchers and clinicians have a means of disease characterization at a cellular or tissue level. Targeted micron-sized particles of iron oxide (MPIO) have been used as molecule-specific contrast agents for use with magnetic resonance imaging (MRI), and early evidence suggests they may be suitable for use with other imaging modalities. Targeting of MPIO to markers of disease is commonly achieved through the covalent attachment of antibodies to the surface of the particles, providing an imaging agent that is both highly specific and which binds with high affinity. When comparing micron-sized particles with nanometre-sized particles, the former provide substantial signal dropout in MRI and confer the sensitivity to detect low levels of target. Furthermore, larger particles appear to bind to targets more potently than smaller particles. Animal models have also demonstrated favorable blood clearance characteristics of MPIO, which are important in achieving favorable signal over background and to attain clearance and disposal. Although the current generation of commercially available MPIO are not suitable for administration into humans, future work may focus on the development of biodegradable and nonimmunogenic MPIO that may allow the use of these imaging agents in a clinical setting.

Evaluation and clinical application of platelet function testing in small animal practice

Tests that evaluate many aspects of platelet function have been applied in both human and veterinary medicine for the monitoring of treatment with platelet function inhibitors and for detection of platelet function abnormalities (inherited or acquired). Interspecies variation in the response to various platelet agonists is an important consideration when methods that have been developed for people are applied in other species. At the present time, many of these assays are not readily available in standard veterinary practice. Advanced platelet function testing for veterinary patients is offered at select academic institutions. Discussion with a specialist is recommended when considering the use of these tests, and the relative strengths and limitations of each assay should be considered in the interpretation of test results.

Enzymatic single-chain antibody tagging: a universal approach to targeted molecular imaging and cell homing in cardiovascular disease

RATIONALE

Antibody-targeted delivery of imaging agents can enhance the sensitivity and accuracy of current imaging techniques. Similarly, homing of effector cells to disease sites increases the efficacy of regenerative cell therapy while reducing the number of cells required. Currently, targeting can be achieved via chemical conjugation to specific antibodies, which typically results in the loss of antibody functionality and in severe cell damage. An ideal conjugation technique should ensure retention of antigen-binding activity and functionality of the targeted biological component.

OBJECTIVE

To develop a biochemically robust, highly reproducible, and site-specific coupling method using the Staphylococcus aureus sortase A enzyme for the conjugation of a single-chain antibody (scFv) to nanoparticles and cells for molecular imaging and cell homing in cardiovascular diseases. This scFv specifically binds to activated platelets, which play a pivotal role in thrombosis, atherosclerosis, and inflammation.

METHODS AND RESULTS

The conjugation procedure involves chemical and enzyme-mediated coupling steps. The scFv was successfully conjugated to iron oxide particles (contrast agents for magnetic resonance imaging) and to model cells. Conjugation efficiency ranged between 50% and 70%, and bioactivity of the scFv after coupling was preserved. The targeting of scFv-coupled cells and nanoparticles to activated platelets was strong and specific as demonstrated in in vitro static adhesion assays, in a flow chamber system, in mouse intravital microscopy, and in in vivo magnetic resonance imaging of mouse carotid arteries.

CONCLUSIONS

This unique biotechnological approach provides a versatile and broadly applicable tool for procuring targeted regenerative cell therapy and targeted molecular imaging in cardiovascular and inflammatory diseases and beyond.

MRI of inducible P-selectin expression in human activated platelets involved in the early stages of atherosclerosis

The noninvasive imaging of atherosclerotic plaques at an early stage of atherogenesis remains a major challenge for the evaluation of the pathologic state of patients at high risk of acute coronary syndromes. Recent studies have emphasized the importance of platelet-endothelial cell interactions in atherosclerosis-prone arteries at early stages, and the prominent role of P-selectin in the initial loose contact between platelets and diseased vessel walls. A specific MR contrast agent was developed here for the targeting, with high affinity, of P-selectin expressed in large amounts on activated platelets and endothelial cells. For this purpose, PEGylated dextran/iron oxide nanoparticles [PEG, poly(ethylene glycol)], named versatile ultrasmall superparamagnetic iron oxide (VUSPIO) particles, labeled with rhodamine were coupled to an anti-human P-selectin antibody (VH10). Flow cytometry and microscopy experiments on human activated platelets were highly correlated with MRI (performed at 4.7 and 0.2 T), with a 50% signal decrease in T(2) and T(1) values corresponding to the strong labeling of activated vs resting platelets. The number of 1000 VH10-VUSPIO nanoparticles attained per activated platelet appeared to be optimal for the detection of hypo- and hyper-signals in the platelet pellet on T(2) - and T(1) -weighted MRI. Furthermore, in vivo imaging of atherosclerotic plaques in ApoE mice at 4.7 T showed a spatial resolution adapted to the imaging of intimal thickening and a hypo-signal at 4.7 T, as a result of the accumulation of VH10-VUSPIO nanoparticles in the plaque. Our work provides support for the further assessment of the use of VH10-VUSPIO nanoparticles as a promising imaging modality able to identify the early stages of atherosclerosis with regard to the pertinence of both the target and the antibody-conjugated contrast agent used.

Activated platelets in carotid artery thrombosis in mice can be selectively targeted with a radiolabeled single-chain antibody

Background

Activated platelets can be found on the surface of inflamed, rupture-prone and ruptured plaques as well as in intravascular thrombosis. They are key players in thrombosis and atherosclerosis. In this study we describe the construction of a radiolabeled single-chain antibody targeting the LIBS-epitope of activated platelets to selectively depict platelet activation and wall-adherent non-occlusive thrombosis in a mouse model with nuclear imaging using in vitro and ex vivo autoradiography as well as small animal SPECT-CT for in vivo analysis.

Methodology/Principal Findings

LIBS as well as an unspecific control single-chain antibody were labeled with ¹¹¹Indium (¹¹¹In) via bifunctional DTPA ( = ¹¹¹In-LIBS/¹¹¹In-control). Autoradiography after incubation with ¹¹¹In-LIBS on activated platelets in vitro (mean 3866±28 DLU/mm², 4010±630 DLU/mm² and 4520±293 DLU/mm²) produced a significantly higher ligand uptake compared to ¹¹¹In-control (2101±76 DLU/mm², 1181±96 DLU/mm² and 1866±246 DLU/mm²) indicating a specific binding to activated platelets; P<0.05. Applying these findings to an ex vivo mouse model of carotid artery thrombosis revealed a significant increase in ligand uptake after injection of ¹¹¹In-LIBS in the presence of small thrombi compared to the non-injured side, as confirmed by histology (49630±10650 DLU/mm² vs. 17390±7470 DLU/mm²; P<0.05). These findings could also be reproduced in vivo. SPECT-CT analysis of the injured carotid artery with ¹¹¹In-LIBS resulted in a significant increase of the target-to-background ratio compared to ¹¹¹In-control (1.99±0.36 vs. 1.1±0.24; P<0.01).

Conclusions/Significance

Nuclear imaging with ¹¹¹In-LIBS allows the detection of platelet activation in vitro and ex vivo with high sensitivity. Using SPECT-CT, wall-adherent activated platelets in carotid arteries could be depicted in vivo. These results encourage further studies elucidating the role of activated platelets in plaque pathology and atherosclerosis and might be of interest for further developments towards clinical application.

Bioinformatics-led design of single-chain antibody molecules targeting DNA sequences for retinoblastoma

AIM

To analyze the relationship between the structure and function of single-chain Fv antibody (scFv) with bioinformatics methods, so as to provide theoretical basis for retinoblastoma targeted therapy.

METHODS

Single-chain antibodies are reconstructed for cancer-targeted therapy to provide good penetration into tumor tissue and to improve their pharmacokinetics in vivo, offering a clinically valuable application. The relationship needs to be analyzed that there may be some variations between the structure and function of the fusion proteins, and the relationship between the structure and function of protein molecules was obtained through analyzing relevant literature at home and abroad as well as modeling analysis.

RESULTS

Through our analysis of the interaction region between the antibody and the antigen, and of the binding sites for molecular conformation, it is clear that existing antibodies need to be modified at the DNA sequence level, enhancing the biological activity of the antibodies. Based on the view that bio-molecular computer models are closely integrated with biological experiments, a bio-molecular structure-activity relationship model can be established in terms of molecular conformation, physical and chemical properties and the biological activity of single-chain antibodies. Two enlightenments are obtained from our analysis. On the one hand, the structure-activity relationship is clear for new immune molecules at the gene expression level. On the other hand, a single-chain antibody molecule can be designed and optimized for the cancer-oriented treatment.

CONCLUSION

In this article, we provide the theoretical and experimental basis for the development of single-chain antibodies appropriate for retinoblastoma therapy.

Emerging diagnostic and therapeutic molecular imaging applications in vascular disease

Assessment of vascular disease has evolved from mere indirect and direct measurements of luminal stenosis to sophisticated imaging methods to depict millimeter structural changes of the vasculature. In the near future, the emergence of multimodal molecular imaging strategies may enable robust therapeutic and diagnostic ('theragnostic') approaches to vascular diseases that comprehensively consider structural, functional, biological and genomic characteristics of the disease in individualized risk assessment, early diagnosis and delivery of targeted interventions.This review presents a summary of recent preclinical and clinical developments in molecular imaging and theragnostic applications covering diverse atherosclerosis events such as endothelial activation, macrophage inflammatory activity, plaque neovascularization and arterial thrombosis. The main focus is on molecular targets designed for imaging platforms commonly used in clinical medicine including magnetic resonance, computed tomography and positron emission tomography. A special emphasis is given to vascular ultrasound applications, considering the important role this imaging platform plays in the clinical and research practice of the vascular medicine specialty.

Molecular MRI approaches to the detection of CNS inflammation

Inflammation is a key component of many neurological diseases, yet our understanding of the contribution of these processes to tissue damage remains poor. For many such diseases, magnetic resonance imaging (MRI) has become the method of choice for clinical diagnosis. However, many of the MRI parameters that enable disease detection, such as passive contrast enhancement across a compromised blood-brain barrier, are weighted towards late-stage disease. Moreover, whilst these methods may report on disease severity, they are not able to provide information on either disease activity or the underlying molecular processes. There is a need, therefore, to develop methods that enable earlier disease detection, potentially long before clinical symptoms become apparent, together with identification of specific molecular processes that may guide specific therapy. This chapter describes the methodology for the synthesis and validation of two novel, functional MRI-detectable probes, based on microparticles of iron oxide (MPIO), which target endothelial adhesion molecules. These contrast agents enable the detection of acute brain inflammation in vivo, at a time when pathology is undetectable by conventional MRI. Such molecular MRI methods are opening new vistas for the acute diagnosis of CNS disease, together with the possibility for individually tailored therapy and earlier, more sensitive assessment of the efficacy of novel therapies.

Detection of brain pathology by magnetic resonance imaging of iron oxide micro-particles

Contrast agents are widely used with magnetic resonance imaging (MRI) to increase the contrast between regions of interest and the background signal, thus providing better quality information. Such agents can work in one of two ways, either to specifically enhance the signal that is produced or to localize in a specific cell type of tissue. Commonly used image contrast agents are typically based on gadolinium complexes or super-paramagnetic iron oxide, the latter of which is used for imaging lymph nodes. When blood-brain barrier (BBB) breakdown is a feature of central nervous system (CNS) pathology, intravenously administered contrast agent enters into the CNS and alters contrast on MR scans. However, BBB breakdown reflects downstream or end-stage pathology. The initial recruitment of leukocytes to sites of disease such as multiple sclerosis (MS), ischemic lesions, or tumours takes place across an intact, but activated, brain endothelium. Molecular imaging affords the ability to obtain a "non-invasive biopsy" to reveal the presence of brain pathology in the absence of significant structural changes. We have developed smart contrast agents that target and reversibly adhere to sites of disease and have been used to reveal activated brain endothelium when images obtained by conventional MRI look normal. Indeed, our selectively targeted micro-particles of iron oxide have revealed the early presence of cerebral malaria pathology and ongoing MS-like plaques in clinically relevant models of disease.

Magnetic resonance imaging of brain inflammation using microparticles of iron oxide

For molecular magnetic resonance imaging (mMRI), microparticles of iron oxide (MPIO) create potent hypointense contrast effects that extend a distance far exceeding their physical size. The potency of the contrast effects derive from their high iron content and are significantly greater than that of ultra-small particles of iron oxide (USPIO), commonly used for MRI. Due to their size and incompressible nature, MPIO are less susceptible to nonspecific vascular egress or uptake by endothelial cells. Therefore, MPIO may be useful contrast agents for detection of endovascular molecular targets by MRI. This Chapter describes the methodology of a novel, functional MPIO probe targeting vascular cell adhesion molecule-1 (VCAM-1), for detection of acute brain inflammation in vivo, at a time when pathology is undetectable by conventional MRI. Protocols are included for conjugation of MPIO to mouse monoclonal antibodies against VCAM-1 (VCAM-MPIO), the validation of VCAM-MPIO binding specificity to activated endothelial cells in vitro, and the application of VCAM-MPIO for in vivo targeted MRI of acute brain inflammation in mice. This functional molecular imaging tool may potentially accelerate accurate diagnosis of early cerebral vascular inflammation by MRI, and guide specific therapy.

Thrombocytopenia in early malaria is associated with GP1b shedding in absence of systemic platelet activation and consumptive coagulopathy

Thrombocytopenia develops early in malaria, but the underlying mechanisms remain incompletely understood. We studied the aetiology of malaria-associated thrombocytopenia in volunteers experimentally infected with Plasmodium falciparum malaria, in Indonesian malaria patients and in ex vivo studies. In experimental human malaria, the decrease in platelet counts was associated with a concurrent rise in young platelets (immature platelet fraction) and thrombopoietin. D-dimer concentrations were moderately elevated without a prolongation in the activated partial thromboplastin time or decrease in fibrinogen. There was no increase in expression of the platelet surface markers CD62P, PAC-1 and CD63 and in plasma concentrations of the platelet factors P-selectin, CXCR4, CXCL7, RANTES and CD40L. In contrast, concentrations of soluble glycoprotein-1b (sGP1b), the external domain of the platelet receptor for von Willebrand factor (VWF), increased early. Indonesian malaria patients also had elevated concentrations of sGP1b, which correlated with VWF concentrations. Finally, incubation of platelets with parasitized erythrocytes in vitro failed to induce platelet aggregation or activation. We concluded that neither compromised platelet production nor platelet activation or consumptive coagulopathy were responsible for the early thrombocytopenia in malaria. We hypothesize that the increase in sGP1b concentrations results from VWF-mediated GP1b shedding; a process that may prevent excessive adhesion of platelets and parasitized erythrocytes.

In vivo quantification of VCAM-1 expression in renal ischemia reperfusion injury using non-invasive magnetic resonance molecular imaging

Rationale and Objective

Vascular cell adhesion molecule-1 (VCAM-1) is upregulated in ischemia reperfusion injury (IRI), persisting after restoration of blood flow. We hypothesized that microparticles of iron oxide targeting VCAM-1 (VCAM-MPIO) would depict “ischemic memory” and enable in vivo assessment of VCAM-1 expression.

Methodology and Findings

Mice subject to unilateral, transient (30 minutes) renal ischemia and subsequent reperfusion received intravenous VCAM-MPIO (4.5 mg iron/kg body weight). Contrast agent bound rapidly (<30 minutes) in IRI-kidneys and appeared as intensely low signal areas by MRI in vivo. Automated segmentation and quantification yielded MPIO contrast volumes of 5991±354×10⁶ µm³ in IRI vs. 87±7×10⁶ µm³ in kidneys with no surgical intervention (P<0.001); 90±8×10⁶ µm³ in IRI kidneys exposed to control (IgG-MPIO) and 625±80×10⁶ µm³, in IRI kidneys pre-treated with a blocking dose of VCAM-1 antibody (P<0.001). In keeping with quantitative MRI data, VCAM-1 mRNA expression in IRI was 65-fold higher than in kidneys without surgical intervention (3.06±0.63 vs. 0.05±0.02, P<0.001). Indeed VCAM-1 mRNA expression and VCAM-MPIO contrast volume were highly correlated (R² = 0.901, P<0.01), indicating that quantification of contrast volume reflected renal VCAM-1 transcription. Serial imaging showed VCAM-MPIO accumulation at target within 30 minutes, persisting for ≥90 minutes, while unbound VCAM-MPIO was cleared rapidly from blood, with sequestration by mac-3 positive Kupffer cells in the liver and monocyte/macrophages in the spleen.

Conclusions

(1) VCAM-MPIO detected VCAM-1 expression and defined its 3-dimensional distribution, revealing “ischemic memory” in renal IRI; (2) automated volumetric quantification of VCAM-MPIO accurately reflected tissue levels of VCAM-1 mRNA; and (3) VCAM-MPIO bound rapidly to target with active sequestration of unbound MPIO in the liver and spleen.

Target: ligand interactions of the vascular endothelium. Implications for molecular imaging in inflammation

Molecular imaging refers to the non-invasive visualisation of biological processes at the molecular and cellular levels within a living organism, and offers a wide range of potential benefits to both clinical medicine and research into novel therapeutic agents. Inflammation plays an important role in a wide variety of pathological processes and imaging the molecular and cellular machinery that underlies chronic inflammation is attractive and feasible. In this review, we present an overview of molecular imaging of inflammation. We start by characterising molecular and cellular events in early inflammation, identifying current and potential future imaging targets. We focus on the imaging of endothelial cells, which mediate the important first steps in inflammation in any tissue, are readily accessible to imaging probes and which present an approach that can be applied across multiple modalities. We then review the generic requirements for imaging contrast agents and focus on the important considerations in respect of ligands, ligand-target interactions and contrast vehicles. We aim to provide an integrated view of current progress with a focus on promising recent developments in experimental and translational molecular imaging.

Chapter 4 - Applications of nanotechnology in molecular imaging of the brain

Rapid advances in the field of nanotechnology promise revolutionary improvements in the diagnosis and therapy of neuroinflammatory disorders. An array of iron oxide nano- and microparticle agents have been developed for in vivo molecular magnetic resonance imaging (mMRI) of cerebrovascular endothelial targets, such as vascular cell adhesion molecule-1 (VCAM-1), E-selectin, and the glycoprotein receptor GP IIb/IIIa expressed on activated platelets. Molecular markers of glioma cells, such as matrix metalloproteinase-2 (MMP-2), and markers for brain tumor angiogenesis, such as alpha (v) beta (3) integrin (alpha(v)beta(3)), have also been successfully targeted using nanoparticle imaging probes. This chapter provides an overview of targeted, iron oxide nano- and microparticles that have been applied for in vivo mMRI of the brain in experimental models of multiple sclerosis (MS), brain ischemia, cerebral malaria (CM), brain cancer, and Alzheimer's disease. The potential of targeted nanoparticle agents for application in clinical imaging is also discussed, including multimodal and therapeutic approaches.