Molecular imaging of angiogenesis in early-stage atherosclerosis with alpha(v)beta3-integrin-targeted nanoparticles

Nanoparticles as magnetic resonance imaging contrast agents for vascular and cardiac diseases

Advances in nanoparticle contrast agents for molecular imaging have made magnetic resonance imaging a promising modality for noninvasive visualization and assessment of vascular and cardiac disease processes. This review provides a description of the various nanoparticles exploited for imaging cardiovascular targets. Nanoparticle probes detecting inflammation, apoptosis, extracellular matrix, and angiogenesis may provide tools for assessing the risk of progressive vascular dysfunction and heart failure. The utility of nanoparticles as multimodal probes and/or theranostic agents has also been investigated. Although clinical application of these nanoparticles is largely unexplored, the potential for enhancing disease diagnosis and treatment is considerable.

Nanotechnology Applications in Vascular Disease

The confluence of nanotechnology and medicine: Nanomedicine is poised to revolutionize the delivery of health care to individuals and populations in the 21st century. Every medical specialty stands to be impacted as emerging nanotechnologies propel modalities for prevention, screening, diagnosis, and treatment. Medical interventions targeting vascular disease are becoming increasingly important, given current epidemiologic trends in morbidity and mortality due to these diseases. Vascular disease is a term that encompasses a collection of pathologies that affect the vessels (arteries, veins, and lymphatics) or vasculature in the body. The purpose of this article is to highlight current and emerging nanotechnologies aimed at vascular disease. Select, clinically relevant examples are described in the categories of drugs and drug delivery, imaging, sensors, biomaterials, diagnostics, and novel therapeutics. Several promising nanomedicine applications that target vascular disease are currently under various stages of development from bench top research to clinical trials. Given the rate of investment, promising research results and progression of many products to market nanomedicine applications in the prevention, diagnosis, and treatment of vascular disease holds great promise for improving the stand of care.

High-resolution magnetic resonance imaging enhanced with superparamagnetic nanoparticles measures macrophage burden in atherosclerosis

BACKGROUND

Macrophages contribute to the progression and acute complications of atherosclerosis. Macrophage imaging may serve as a biomarker to identify subclinical inflamed lesions, to predict future risk, and to aid in the assessment of novel therapies.

METHODS AND RESULTS

To test the hypothesis that nanoparticle-enhanced, high-resolution magnetic resonance imaging (MRI) can measure plaque macrophage accumulation, we used 3-T MRI with a macrophage-targeted superparamagnetic nanoparticle preparation (monocrystalline iron oxide nanoparticles-47 [MION-47]) in cholesterol-fed New Zealand White rabbits 6 months after balloon injury. In vivo MRI visualized thickened abdominal aortas on both T1- and T2-weighted spin-echo images (T1 spin echo, 20 axial slices per animal; T2 spin echo, 28 slices per animal). Seventy-two hours after MION-47 injection, aortas exhibited lower T2 signal intensity compared with before contrast imaging (signal intensity ratio, aortic wall/muscle: before, 1.44 ± 0.26 versus after, 0.95 ± 0.22; 164 slices; P<0.01), whereas T1 spin echo images showed no significant change. MRI on ex vivo specimens provided similar results. Histological studies colocalized iron accumulation with immunoreactive macrophages in atheromata. The magnitude of signal intensity reduction on T2 spin echo in vivo images further correlated with macrophage areas in situ (150 slices; r=0.73). Treatment with rosuvastatin for 3 months yielded diminished macrophage content (P<0.05) and reversed T2 signal intensity changes (P<0.005). Signal changes in rosuvastatin-treated rabbits correlated with reduced macrophage burden (r=0.73). In vitro validation studies showed concentration-dependent MION-47 uptake by human primary macrophages.

CONCLUSION

The magnitude of T2 signal intensity reduction in high-resolution MRI after administration of superparamagnetic phagocytosable nanoparticles can assess macrophage burden in atheromata, providing a clinically translatable tool to identify inflamed plaques and to monitor therapy-mediated changes in plaque inflammation.

High-resolution magnetic resonance imaging enhanced with superparamagnetic nanoparticles measures macrophage burden in atherosclerosis

BACKGROUND

Macrophages contribute to the progression and acute complications of atherosclerosis. Macrophage imaging may serve as a biomarker to identify subclinical inflamed lesions, to predict future risk, and to aid in the assessment of novel therapies.

METHODS AND RESULTS

To test the hypothesis that nanoparticle-enhanced, high-resolution magnetic resonance imaging (MRI) can measure plaque macrophage accumulation, we used 3-T MRI with a macrophage-targeted superparamagnetic nanoparticle preparation (monocrystalline iron oxide nanoparticles-47 [MION-47]) in cholesterol-fed New Zealand White rabbits 6 months after balloon injury. In vivo MRI visualized thickened abdominal aortas on both T1- and T2-weighted spin-echo images (T1 spin echo, 20 axial slices per animal; T2 spin echo, 28 slices per animal). Seventy-two hours after MION-47 injection, aortas exhibited lower T2 signal intensity compared with before contrast imaging (signal intensity ratio, aortic wall/muscle: before, 1.44 ± 0.26 versus after, 0.95 ± 0.22; 164 slices; P<0.01), whereas T1 spin echo images showed no significant change. MRI on ex vivo specimens provided similar results. Histological studies colocalized iron accumulation with immunoreactive macrophages in atheromata. The magnitude of signal intensity reduction on T2 spin echo in vivo images further correlated with macrophage areas in situ (150 slices; r=0.73). Treatment with rosuvastatin for 3 months yielded diminished macrophage content (P<0.05) and reversed T2 signal intensity changes (P<0.005). Signal changes in rosuvastatin-treated rabbits correlated with reduced macrophage burden (r=0.73). In vitro validation studies showed concentration-dependent MION-47 uptake by human primary macrophages.

CONCLUSION

The magnitude of T2 signal intensity reduction in high-resolution MRI after administration of superparamagnetic phagocytosable nanoparticles can assess macrophage burden in atheromata, providing a clinically translatable tool to identify inflamed plaques and to monitor therapy-mediated changes in plaque inflammation.

The time window of MRI of murine atherosclerotic plaques after administration of CB2 receptor targeted micelles: inter-scan variability and relation between plaque signal intensity increase and gadolinium content of inversion recovery prepared versus non-prepared fast spin echo

Single fast spin echo scans covering limited time frames are mostly used for contrast-enhanced MRI of atherosclerotic plaque biomarkers. Knowledge on inter-scan variability of the normalized enhancement ratio of plaque (NER(plaque)) and relation between NER(plaque) and gadolinium content for inversion-recovery fast spin echo is limited. Study aims were: evaluation of (1) timing of MRI after intravenous injection of cannabinoid-2 receptor (CB2-R) (expressed by human and mouse plaque macrophages) targeted micelles; (2) inter-scan variability of inversion-recovery fast spin echo and fast spin echo; (3) relation between NER(plaque) and gadolinium content for inversion-recovery fast spin echo and fast spin echo. Inversion-recovery fast spin echo/fast spin echo imaging was performed before and every 15 min up to 48 h after injection of CB2-R targeted or control micelles using several groups of mice measured in an interleaved fashion. NER(plaque) (determined on inversion-recovery fast spin echo images) remained high (∼2) until 48 h after injection of CB2-R targeted micelles, whereas NER(plaque) decreased after 36 h in the control group. The inter-scan variability and relation between NER(plaque) and gadolinium (assessed with inductively coupled plasma- mass spectrometry) were compared between inversion-recovery fast spin echo and fast spin echo. Inter-scan variability was higher for inversion-recovery fast spin echo than for fast spin echo. Although gadolinium and NER(plaque) correlated well for both techniques, the NER of plaque was higher for inversion-recovery fast spin echo than for fast spin echo. In mice injected with CB2-R targeted micelles, NER(plaque) can be best evaluated at 36-48 h post-injection. Because NER(plaque) was higher for inversion-recovery fast spin echo than for fast spin echo, but with high inter-scan variability, repeated inversion-recovery fast spin echo imaging and averaging of the obtained NER(plaque) values is recommended.

The role of noninvasive imaging in promoting cardiovascular health

Cardiovascular disease (CVD) is the leading cause of death worldwide, and its prevalence is likely to increase in the near future. The morbidity and mortality associated with CVD causes an enormous economic burden, which has become a major problem for many societies across the globe. The current prevention strategies are aimed at identifying and reducing established risk factors for atherosclerosis including hypertension, hypercholesterolemia, diabetes, obesity, smoking, and sedentary lifestyle. However, some of our prevention goals, such as reducing LDL cholesterol, change dramatically once a subject has been diagnosed with coronary atherosclerosis. At the present time, atherosclerosis is frequently diagnosed relatively late in the course of the disease, when a patient develops symptoms or presents with acute events such as an acute coronary syndrome or a stroke. Several studies have demonstrated that novel noninvasive imaging techniques have the potential to identify subclinical atherosclerosis and high-risk plaques. Early detection of subclinical atherosclerosis may enable clinicians to improve the control of cardiovascular risk factors in affected patients earlier, thereby helping to prevent some of the manifestations of CVD.

High-performance nanostructured MR contrast probes

Magnetic resonance imaging (MRI) has become a powerful technique in biological molecular imaging and clinical diagnosis. With the rapid progress in nanoscale science and technology, nanostructure-based MR contrast agents are undergoing rapid development. This is in part due to the tuneable magnetic and cellular uptake properties, large surface area for conjugation and favourable biodistribution. In this review, we describe our recent progress in the development of high-performance nanostructured MR contrast agents. Specifically, we report on Gd-enriched nanostructured probes that exhibit T(1) MR contrast and superparamagnetic Fe(3)O(4) and CoFe(2)O(4) nanostructures that display T(2) MR contrast enhancement. The effects of nanostructure size, shape, assembly and surface modification on relaxivity are described. The potential of these contrast agents for in vitro and in vivo MR imaging with respect to colloidal stability under physiological conditions, biocompatibility, and surface functionality are also evaluated.

Molecular imaging of inflammation and intraplaque vasa vasorum: a step forward to identification of vulnerable plaques?

Current developments in cardiovascular biology and imaging enable the noninvasive molecular evaluation of atherosclerotic vascular disease. Intraplaque neovascularization sprouting from the adventitial vasa vasorum has been identified as an independent predictor of intraplaque hemorrhage and plaque rupture. These intraplaque vasa vasorum result from angiogenesis, most likely under influence of hypoxic and inflammatory stimuli. Several molecular imaging techniques are currently available. Most experience has been obtained with molecular imaging using positron emission tomography and single photon emission computed tomography. Recently, the development of targeted contrast agents has allowed molecular imaging with magnetic resonance imaging, ultrasound and computed tomography. The present review discusses the use of these molecular imaging techniques to identify inflammation and intraplaque vasa vasorum to identify vulnerable atherosclerotic plaques at risk of rupture and thrombosis. The available literature on molecular imaging techniques and molecular targets associated with inflammation and angiogenesis is discussed, and the clinical applications of molecular cardiovascular imaging and the use of molecular techniques for local drug delivery are addressed.

Contrast enhancement by differently sized paramagnetic MRI contrast agents in mice with two phenotypes of atherosclerotic plaque

Interest in the use of contrast-enhanced MRI to enable in vivo specific characterization of atherosclerotic plaques is increasing. In this study the intrinsic ability of three differently sized gadolinium-based contrast agents to permeate different mouse plaque phenotypes was evaluated with MRI. A tapered cast was implanted around the right carotid artery of apoE(-/-) mice to induce two different plaque phenotypes: a thin cap fibroatheroma (TCFA) and a non-TCFA lesion. Both plaques were allowed to develop over 6 and 9 weeks, leading to an intermediate and advanced lesion, respectively. Signal enhancement in the carotid artery wall, following intravenous injection of Gd-HP-DO3A as well as paramagnetic micelles and liposomes was evaluated. In vivo T(1) -weighted MRI plaque enhancement characteristics were complemented by fluorescence microscopy and correlated to lesion phenotype. The two smallest contrast agents, i.e. Gd-HP-DO3A and micelles, were found to enhance contrast in T(1) -weighted MR images of all investigated plaque phenotypes. Maximum contrast enhancement ranged between 53 and 70% at 6 min after injection of Gd-HP-DO3A with highest enhancement and longest retention in the non-TCFA lesion. Twenty-four hours after injection of micelles maximum contrast enhancement ranged between 24 and 35% in all plaque phenotypes. Administration of the larger liposomes did not cause significant contrast enhancement in the atherosclerotic plaques. Confocal fluorescence microscopy confirmed the MRI-based differences in plaque permeation between micelles and liposomes. Plaque permeation of contrast agents was strongly dependent on size. Our results implicate that, when equipped with targeting ligands, liposomes are most suitable for the imaging of plaque-associated endothelial markers due to low background enhancement, whereas micelles, which accumulate extravascularly on a long timescale, are suited for imaging of less abundant markers inside plaques. Low molecular weight compounds may be employed for target-specific imaging of highly abundant extravascular plaque-associated targets.

Quantitative magnetic resonance fluorine imaging: today and tomorrow

Fluorine (19F) is a promising moiety for quantitative magnetic resonance imaging (MRI). It possesses comparable magnetic resonance (MR) sensitivity to proton (1H) but exhibits no tissue background signal, allowing specific and selective assessment of the administrated 19F-containing compounds in vivo. Additionally, the MR spectra of 19F-containing compounds exhibited a wide range of chemical shifts (>200 ppm). Therefore, both MR parameters (e.g., spin-lattice relaxation rate R1) and the absolute quantity of molecule can be determined with 19F MRI for unbiased assessment of tissue physiology and pathology. This article reviews quantitative 19F MRI applications for mapping tumor oxygenation, assessing molecular expression in vascular diseases, and tracking labeled stem cells.

Imaging biomarkers of cardiovascular disease

Cardiovascular disease (CVD) is a leading cause of morbidity and mortality worldwide. Current clinical techniques that rely on stenosis measurement alone appear to be insufficient for risk prediction in atherosclerosis patients. Many novel imaging methods have been developed to study atherosclerosis progression and to identify new features that can predict future clinical risk. MRI of atherosclerotic vessel walls is one such method. It has the ability to noninvasively evaluate multiple biomarkers of the disease such as luminal stenosis, plaque burden, tissue composition and plaque activity. In addition, the accuracy of in vivo MRI has been validated against histology with high reproducibility, thus paving the way for application to epidemiological studies of disease pathogenesis and, by serial MRI, in monitoring the efficacy of therapeutic intervention. In this review, we describe the various MR techniques used to evaluate aspects of plaque progression, discuss imaging-based measurements (imaging biomarkers), and also detail their validation. The application of plaque MRI in clinical trials as well as emerging imaging techniques used to evaluate plaque compositional features and biological activities are also discussed.

Multifunctional agents for concurrent imaging and therapy in cardiovascular disease

The development of agents for the simultaneous detection and treatment of disease has recently gained significant attention. These multifunctional theranostic agents posses a number of advantages over their monofunctional counterparts, as they potentially allow for the concomitant determination of agent localization, release, and efficacy. Whereas the development of these agents for use in cancers has received the majority of the attention, their use in cardiovascular disease is steadily increasing. As such, this review summarized some of the most poignant recent advances in the development of theranostic agents for the treatment of this class of diseases.

Albumin-based nanoparticles as magnetic resonance contrast agents: II. Physicochemical characterisation of purified and standardised nanoparticles

We are developing a nanoparticulate histochemical reagent designed for histochemistry in living animals (molecular imaging), which should finally be useful in clinical imaging applications. The iterative development procedure employed involves conceptual design of the reagent, synthesis and testing of the reagent, then redesign based on data from the testing; each cycle of testing and development generates a new generation of nanoparticles, and this report describes the synthesis and testing of the third generation. The nanoparticles are based on human serum albumin and the imaging modality selected is magnetic resonance imaging (MRI). Testing the second particle generation with newly introduced techniques revealed the presence of impurities in the final product, therefore we replaced dialysis with diafiltration. We introduced further testing methods including thin layer chromatography, arsenazo III as chromogenic assay for gadolinium, and several versions of polyacrylamide gel electrophoresis, for physicochemical characterisation of the nanoparticles and intermediate synthesis compounds. The high grade of chemical purity achieved by combined application of these methodologies allowed standardised particle sizes to be achieved (low dispersities), and accurate measurement of critical physicochemical parameters influencing particle size and imaging properties. Regression plots confirmed the high purity and standardisation. The good degree of quantitative physicochemical characterisation aided our understanding of the nanoparticles and allowed a conceptual model of them to be prepared. Toxicological screening demonstrated the extremely low toxicity of the particles. The high magnetic resonance relaxivities and enhanced mechanical stability of the particles make them an excellent platform for the further development of MRI molecular imaging.

In vivo evaluation of (64)Cu-labeled magnetic nanoparticles as a dual-modality PET/MR imaging agent

A novel nanoparticle-based dual-modality positron emission tomograph/magnetic resonance imaging (PET/MRI) contrast agent was developed. The probe consisted of a superparamagnetic iron oxide (SPIO) core coated with PEGylated phospholipids. The chelator 1,4,7,10-tetraazacyclo-dodecane-1,4,7,10-tetraacetic acid (DOTA) was conjugated to PEG termini to allow labeling with positron-emitting (64)Cu. Radiolabeling with (64)Cu at high yield and high purity was readily achieved. The (64)Cu-SPIO probes produced strong MR and PET signals and were stable in mouse serum for 24 h at 37 degrees C. Biodistribution and in vivo PET/CT imaging studies of the probes showed a circulation half-life of 143 min and high initial blood retention with moderate liver uptake, making them an attractive contrast agent for disease studies.

Biomimetic MRI contrast agent for imaging of inflammation in atherosclerotic plaque of ApoE-/- mice: a pilot study

OBJECTIVE

Atherosclerosis involves an inflammatory process characterized by cellular and molecular responses. A slow-clearance blood-pool paramagnetic agent (CMD-A2-Gd-DOTA: P717) chemically modified to create a functionalized product (F-P717) for targeting inflammation in vessel walls was evaluated in vivo in mice.

METHODS AND RESULTS

Carboxylate and sulfate groups were grafted onto the macromolecular paramagnetic Gd-DOTA-dextran backbone. Products were also fluorescently labeled with rhodamine isothiocyanate. Pre- and postcontrast MRI was performed on a 2-Tesla magnet in ApoE-/- and control C57BL/6 mice after P717 or F-P717 injection at a dose of 60 micromol Gd/kg. Axial T1-weighted images of the abdominal aorta were obtained using a 2D multislice spin-echo sequence. F-P717 significantly enhanced the magnetic resonance imaging (MRI) signal in the abdominal aortic wall of ApoE-/- mice (>50% signal-to-noise ratio increase between 10 and 30 minutes), but not of control mice. P717 produced only moderate (<20%) MRI signal enhancement within the same time frame. The MRI data were correlated to histopathology. Immunofluorescence in ApoE-/- mice colocalized F-P717 but not P717 with the inflammatory area revealed by P-selectin labeling.

CONCLUSION

This study demonstrates the efficacy of F-P717 as a new molecular imaging agent for noninvasive in vivo MRI location of inflammatory vascular tree lesions in ApoE-/- mice.

CBP knockdown inhibits angiotensin II-induced vascular smooth muscle cells proliferation through downregulating NF-kB transcriptional activity

CREB binding protein (CBP), a powerful transcriptional co-activator for various transcriptional factors, regulates cell behavior in many cell types. Angiotensin II (Ang II) contributes to vascular lesion by promoting vascular smooth muscle cells (VSMCs) proliferation and migration. Therefore, we examined whether CBP knockdown could suppress Ang II-induced VSMCs proliferation, and elucidated its underlying molecular mechanism. We constructed lentiviral vector expressing CBP-specific short hairpin RNAs (shRNAs) that efficiently silenced CBP. VSMCs proliferation was evaluated by bromodeoxyuridine (BrdU) incorporation assay. Protein and mRNA expression of CBP and relevant cytokines were examined by Western blot, ELISA, and real-time PCR, respectively. We also used luciferase reporter gene and electrophoretic mobility shift assay (EMSA) to detect Nuclear factor kappaB (NF-kB) transcriptional activity and DNA binding. Meanwhile, NF-kB p65 subunit nuclear translocation was confirmed by immunoblotting. Lentiviral-mediated CBP-shRNAs at different multiplicities of infection (MOI = 100, 150) both significantly suppressed Ang II-induced CBP expression. Knockdown of CBP markedly inhibited Ang II-stimulated VSMCs proliferation and cytokines (TNF-alpha and IL-6) production. However, this inhibitory effect was not enhanced at MOI of 150 compared with MOI of 100 (P > 0.05). CBP siRNA showed the potent inhibition on Ang II-induced NF-kB transcriptional activity. Similarly, no significant difference was found between CBP siRNA lentivirus treatment groups. Furthermore, CBP gene silencing had no effect on NF-kB nuclear translocation and DNA binding. These findings suggest that CBP knockdown inhibits Ang II-induced VSMCs proliferation and the mechanism is involved with downregulation of NF-kB transcriptional activity, not through reduction in NF-kB nuclear translocation or DNA binding. Maintaining proper CBP level may be a potential therapeutic target for Ang II-induced cardiovascular disorders.

Strategies to reverse atherosclerosis: an imaging perspective

Several treatment strategies, including lowering low-density lipoprotein cholesterol with intensive statin therapy, reducing triglycerides with fibrates, and raising high-density lipoprotein cholesterol with nicotinic acid, have the potential to induce atherosclerosis regression. Atherosclerosis imaging techniques including intravascular ultrasound, carotid ultrasound to measure carotid intima-media thickness, and cardiovascular MRI are established modalities for describing longitudinal changes in the quantity and quality of atherosclerotic plaque. An increasing number of clinical trials are using radiologic measures of subclinical atherosclerosis as surrogate end points in lieu of the traditional "hard" end points of myocardial infarction and death. This approach has great appeal, as improvements in atherosclerosis imaging now enable the characterization of early atheromas and positive remodeling within the vessel wall before the plaque becomes obstructive. Additional prospective data correlating these surrogate end points with hard outcomes are needed to determine whether atherosclerosis regression will be the major determinant of future treatment strategies.

Microfluidic platforms for studies of angiogenesis, cell migration, and cell-cell interactions. Sixth International Bio-Fluid Mechanics Symposium and Workshop March 28-30, 2008 Pasadena, California

Recent advances in microfluidic technologies have opened the door for creating more realistic in vitro cell culture methods that replicate many aspects of the true in vivo microenvironment. These new designs (i) provide enormous flexibility in controlling the critical biochemical and biomechanical factors that influence cell behavior, (ii) allow for the introduction of multiple cell types in a single system, (iii) provide for the establishment of biochemical gradients in two- or three-dimensional geometries, and (iv) allow for high quality, time-lapse imaging. Here, some of the recent developments are reviewed, with a focus on studies from our own laboratory in three separate areas: angiogenesis, cell migration in the context of tumor cell-endothelial interactions, and liver tissue engineering.

MR angiogenesis imaging with Robo4- vs. alphaVbeta3-targeted nanoparticles in a B16/F10 mouse melanoma model

The primary objective of this study was to utilize MR molecular imaging to compare the 3-dimensional spatial distribution of Robo4 and α(V)β(3)-integrin as biosignatures of angiogenesis, in a rapidly growing, syngeneic tumor. B16-F10 melanoma-bearing mice were imaged with magnetic resonance (MR; 3.0 T) 11 d postimplantation before and after intravenous administration of either Robo4- or α(V)β(3)-targeted paramagnetic nanoparticles. The percentage of MR signal-enhanced voxels throughout the tumor volume was low and increased in animals receiving α(V)β(3)- and Robo4-targeted nanoparticles. Neovascular signal enhancement was predominantly associated with the tumor periphery (i.e., outer 50% of volume). Microscopic examination of tumors coexposed to the Robo4- and α(V)β(3)-targeted nanoparticles corroborated the MR angiogenesis mapping results and further revealed that Robo4 expression generally colocalized with α(V)β(3)-integrin. Robo4- and α(V)β(3)-targeted nanoparticles were compared to irrelevant or nontargeted control groups in all modalities. These results suggest that α(V)β(3)-integrin and Robo4 are useful biomarkers for noninvasive MR molecular imaging in syngeneic mouse tumors, but α(V)β(3)-integrin expression was more detectable by MR at 3.0 T than Robo4. Noninvasive, neovascular assessments of the MR signal of Robo4, particularly combined with α(V)β(3)-integrin expression, may help define tumor character prior to and following cancer therapy.

Relation of microchannel structure identified by optical coherence tomography to plaque vulnerability in patients with coronary artery disease

Increased neovascularization in atherosclerotic plaques is associated with plaque vulnerability. The high resolution of optical coherence tomography (OCT) might provide a chance to directly visualize plaque neovascularization in vivo. The aim of the present study was to investigate the relation between microchannels in culprit plaques identified by OCT and plaque vulnerability in patients with coronary artery disease. A total of 63 consecutive patients with coronary artery disease who had undergone both OCT and intravascular ultrasound before any interventions to examine culprit lesion morphologies were enrolled. Microchannel was defined as a no-signal tubuloluminal structure on the cross-sectional optical coherence tomographic image. Microchannels were found in 24 (38%) of the 63 patients. The patients were divided into 2 groups according to the presence or absence of microchannels. The frequency of plaque rupture tended to be greater in the microchannel group (50% vs 28%, p = 0.11). The thickness of the fibrous cap (median 60 vs 100 microm, p = 0.001) was significantly less in the patients with microchannels, and significant differences were found in the frequency of thin-cap fibroatheroma (54% vs 21%, p = 0.012) and positive remodeling (67% vs 36%, p = 0.02) between the 2 groups. The high-sensitivity C-reactive protein levels in the microchannel group was significantly greater than those in the no-microchannel group (median 0.27 vs 0.13 mg/dl, p = 0.015). Moreover, increased microchannel counts were associated with greater high-sensitivity C-reactive protein levels (p = 0.01). In conclusion, a significant relation was found between the presence of microchannels in plaques identified by OCT and plaque vulnerability in patients with coronary artery disease.

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

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

Imaging atherosclerosis and vulnerable plaque

Identifying patients at high risk for an acute cardiovascular event such as myocardial infarction or stroke and assessing the total atherosclerotic burden are clinically important. Currently available imaging modalities can delineate vascular wall anatomy and, with novel probes, target biologic processes important in plaque evolution and plaque stability. Expansion of the vessel wall involving remodeling of the extracellular matrix can be imaged, as can angiogenesis of the vasa vasorum, plaque inflammation, and fibrin deposits on early nonocclusive vascular thrombosis. Several imaging platforms are available for targeted vascular imaging to acquire information on both anatomy and pathobiology in the same imaging session using either hybrid technology (nuclear combined with CT) or MRI combined with novel probes targeting processes identified by molecular biology to be of importance. This article will discuss the current state of the art of these modalities and challenges to clinical translation.

Morphing low-affinity ligands into high-avidity nanoparticles by thermally triggered self-assembly of a genetically encoded polymer

Multivalency is the increase in avidity resulting from the simultaneous interaction of multiple ligands with multiple receptors. This phenomenon, seen in antibody-antigen and virus-cell membrane interactions, is useful in designing bioinspired materials for targeted delivery of drugs or imaging agents. While increased avidity offered by multivalent targeting is attractive, it can also promote nonspecific receptor interaction in nontarget tissues, reducing the effectiveness of multivalent targeting. Here, we present a thermal targeting strategy--dynamic affinity modulation (DAM)--using elastin-like polypeptide diblock copolymers (ELP(BC)s) that self-assemble from a low-affinity to high-avidity state by a tunable thermal "switch", thereby restricting activity to the desired site of action. We used an in vitro cell binding assay to investigate the effect of the thermally triggered self-assembly of these ELP(BC)s on their receptor-mediated binding and cellular uptake. The data presented herein show that (1) ligand presentation does not disrupt ELP(BC) self-assembly; (2) both multivalent ligand presentation and upregulated receptor expression are needed for receptor-mediated interaction; (3) increased size of the hydrophobic segment of the block copolymer promotes multivalent interaction with membrane receptors, potentially due to changes in the nanoscale architecture of the micelle; and (4) nanoscale presentation of the ligand is important, as presentation of the ligand by micrometer-sized aggregates of an ELP showed a low level of binding/uptake by receptor-positive cells compared to its presentation on the corona of a micelle. These data validate the concept of thermally triggered DAM and provide rational design parameters for future applications of this technology for targeted drug delivery.

Theragnostics for tumor and plaque angiogenesis with perfluorocarbon nanoemulsions

Molecular imaging agents are extending the potential of noninvasive medical diagnosis from basic gross anatomical descriptions to complicated phenotypic characterizations based upon the recognition of unique cell-surface biochemical signatures. Although originally the purview of nuclear medicine, "molecular imaging" is now studied in conjunction with all clinically relevant imaging modalities. Of the myriad of particles that have emerged as prospective candidates for clinical translation, perfluorocarbon nanoparticles offer great potential for combining targeted imaging with drug delivery, much like the "magic bullet" envisioned by Paul Ehrlich 100 years ago. Perfluorocarbon nanoparticles, once studied in Phase III clinical trials as blood substitutes, have found new life for molecular imaging and drug delivery. The particles have been adapted for use with all clinically relevant modalities and for targeted drug delivery. In particular, their intravascular constraint due to particle size provides a distinct advantage for angiogenesis imaging and antiangiogenesis therapy. As perfluorocarbon nanoparticles have recently entered Phase I clinical study, this review provides a timely focus on the development of this platform technology and its application for angiogenesis-related pathologies.

Approaches to Multimodality Imaging of Angiogenesis

Angiogenesis, defined as the formation of new capillaries by cellular outgrowth from existing microvessels, can be assessed by the evaluation of perfusion, function, and metabolism. However, more recently, novel, noninvasive imaging strategies for the evaluation of molecular events associated with the angiogenic process have been developed.

Albumin-based nanoparticles as magnetic resonance contrast agents: I. Concept, first syntheses and characterisation

To develop a platform for molecular magnetic resonance imaging, we prepared gadolinium-bearing albumin-polylactic acid nanoparticles in the size range 20-40 nm diameter. Iterative cycles of design and testing upscaled the synthesis procedures to gram amounts for physicochemical characterisation and for pharmacokinetic testing. Morphological analyses showed that the nanoparticles were spheroidal with rough surfaces. Particle sizes were measured by direct transmission electron microscopical measurements from negatively contrasted preparations, and by use of photon correlation spectroscopy; the two methods each documented nanoparticle sizes less than 100 nm and generally 10-40 nm diameter, though with significant intrabatch and interbatch variability. The particles' charge sufficed to hold them in suspension. HSA retained its tertiary structure in the particles. The nanoparticles were stable against turbulent flow conditions and against heat, though not against detergents. MRI imaging of liquid columns was possible at nanoparticle concentrations below 10 mg/ml. The particles were non-cytotoxic, non-thrombogenic and non-immunogenic in a range of assay systems developed for toxicity testing of nanoparticles. They were micellar prior to lyophilisation, but loosely structured aggregated masses after lyophilisation and subsequent resuspension. These nanoparticles provide a platform for further development, based on non-toxic materials of low immunogenicity already in clinical use, not expensive, and synthesized using methods which can be upscaled for industrial production.

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

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

High-affinity alphavbeta3 integrin targeted optical probe as a new imaging biomarker for early atherosclerosis: initial studies in Watanabe rabbits

PURPOSE

A newly developed synthetic alpha v beta 3 integrin targeted optical probe (ITOP) has been demonstrated to target cancer cells, in vivo. Compared to the commercially available cyclic peptide c[RGDfv], this optical probe has at least 20 times better binding affinity for the alpha v beta 3 receptor. The present in vitro study was designed to investigate the possibility of detecting early atherosclerotic plaque by using this ITOP.

PROCEDURES

Experiments were performed on five Watanabe heritable hyperlipidemic rabbits and two New Zealand White rabbits for control. Our ITOP was used for detecting the presence of alpha v beta 3 receptors in vitro.

RESULTS

Segments of plaque accumulation from two distinct regions of ascending and descending aortas were labeled in Watanabe rabbits. The signal was found principally in the adventitia and proximal intima of the aortic vessel, corresponding directly to the expression of integrin alpha v beta 3 as determined by antibody assay. Moreover, there was a close association between the level of labeling with the alpha v beta 3 targeted probe and the thickness of the adventitia.

CONCLUSIONS

This high-affinity ITOP identifies the site and extent of alpha v beta 3 expression and correlates with adventitial thickness. Recent evidence associates alpha v beta 3 expression with the inflammatory process in early vulnerable plaque, making this compound a promising potential biomarker for early atherosclerotic disease.

Anti-angiogenic perfluorocarbon nanoparticles for diagnosis and treatment of atherosclerosis

Complementary developments in nanotechnology, genomics, proteomics, molecular biology and imaging offer the potential for early, accurate diagnosis. Molecularly-targeted diagnostic imaging agents will allow noninvasive phenotypic characterization of pathologies and, therefore, tailored treatment close to the onset. For atherosclerosis, this includes anti-angiogenic therapy with specifically-targeted drug delivery systems to arrest the development of plaques before they impinge upon the lumen. Additionally, monitoring the application and effects of this targeted therapy in a serial fashion will be important. This review covers the specific application of alpha(nu)beta(3)-targeted anti-angiogenic perfluorocarbon nanoparticles in (1) the detection of molecular markers for atherosclerosis, (2) the immediate verification of drug delivery with image-based prediction of therapy outcomes, and (3) the serial, noninvasive observation of therapeutic efficacy.

Vulnerable plaque: definition, diagnosis, and treatment

This article provides a systematic approach to vulnerable plaques. It is divided into 4 sections. The first section is devoted to definition, incidence, anatomic distribution, and clinical presentation. The second section is devoted to plaque composition, setting up the foundations to understand plaque vulnerability. The third section relates to invasive plaque imaging. The fourth section is devoted to therapy, from conservative pharmacologic options to aggressive percutaneous coronary intervention alternatives.

Nanomedicine strategies for molecular targets with MRI and optical imaging

The science of 'theranostics' plays a crucial role in personalized medicine, which represents the future of patient management. Over the last decade an increasing research effort has focused on the development of nanoparticle-based molecular-imaging and drug-delivery approaches, emerging as a multidisciplinary field that shows promise in understanding the components, processes, dynamics and therapies of a disease at a molecular level. The potential of nanometer-sized agents for early detection, diagnosis and personalized treatment of diseases is extraordinary. They have found applications in almost all clinically relevant biomedical imaging modality. In this review, a number of these approaches will be presented with a particular emphasis on MRI and optical imaging-based techniques. We have discussed both established molecular-imaging approaches and recently developed innovative strategies, highlighting the seminal studies and a number of successful examples of theranostic nanomedicine, especially in the areas of cardiovascular and cancer therapy.

Enabling individualized therapy through nanotechnology

Individualized medicine is the healthcare strategy that rebukes the idiomatic dogma of 'losing sight of the forest for the trees'. We are entering a new era of healthcare where it is no longer acceptable to develop and market a drug that is effective for only 80% of the patient population. The emergence of "-omic" technologies (e.g. genomics, transcriptomics, proteomics, metabolomics) and advances in systems biology are magnifying the deficiencies of standardized therapy, which often provide little treatment latitude for accommodating patient physiologic idiosyncrasies. A personalized approach to medicine is not a novel concept. Ever since the scientific community began unraveling the mysteries of the genome, the promise of discarding generic treatment regimens in favor of patient-specific therapies became more feasible and realistic. One of the major scientific impediments of this movement towards personalized medicine has been the need for technological enablement. Nanotechnology is projected to play a critical role in patient-specific therapy; however, this transition will depend heavily upon the evolutionary development of a systems biology approach to clinical medicine based upon "-omic" technology analysis and integration. This manuscript provides a forward looking assessment of the promise of nanomedicine as it pertains to individualized medicine and establishes a technology "snapshot" of the current state of nano-based products over a vast array of clinical indications and range of patient specificity. Other issues such as market driven hurdles and regulatory compliance reform are anticipated to "self-correct" in accordance to scientific advancement and healthcare demand. These peripheral, non-scientific concerns are not addressed at length in this manuscript; however they do exist, and their impact to the paradigm shifting healthcare transformation towards individualized medicine will be critical for its success.

Cardiovascular magnetic resonance in carotid atherosclerotic disease

Atherosclerosis is a chronic, progressive, inflammatory disease affecting many vascular beds. Disease progression leads to acute cardiovascular events such as myocardial infarction, stroke and death. The diseased carotid alone is responsible for one third of the 700,000 new or recurrent strokes occurring yearly in the United States. Imaging plays an important role in the management of atherosclerosis, and cardiovascular magnetic resonance (CMR) of the carotid vessel wall is one promising modality in the evaluation of patients with carotid atherosclerotic disease. Advances in carotid vessel wall CMR allow comprehensive assessment of morphology inside the wall, contributing substantial disease-specific information beyond luminal stenosis. Although carotid vessel wall CMR has not been widely used to screen for carotid atherosclerotic disease, many trials support its potential for this indication. This review summarizes the current state of knowledge regarding carotid vessel wall CMR and its potential clinical application for management of carotid atherosclerotic disease.

Will the real plaque vasculature please stand up? Why we need to distinguish the vasa plaquorum from the vasa vasorum

Many studies of experimental atherosclerosis and pathologic observations of human specimens have provided evidence supporting a correlation between vascularization of the atherosclerotic plaque and its natural growth and progression toward acute failure, associated with clinical events. The growing interest in the topic is illustrated by several excellent recent reviews discussing the molecular mechanisms that might play a role in the formation of plaque vasculature and that could explain some of the observed associations with pathologic features of experimental and human atherosclerotic lesions. At the same time, these reviews also emphasize that the field is still largely in uncharted territory. Hoping to spark some new investigations, we are taking this opportunity to question some of the common assumptions and to highlight less explored mechanisms. Finally, we are proposing to adopt the term vasa plaquorum to refer to the neovasculature located within the atherosclerotic plaque to distinguish it clearly from vasa vasorum, the native, supporting vasculature of the artery. We suggest that this new nomenclature offers a potential solution to eliminate ambiguity regarding implicit, but frequently neglected, differences between these structures. We think these points are relevant for future efforts to tailor diagnostic tools and therapeutic interventions targeting plaque neovascularization for the clinical management of atherosclerosis.

Increased neovascularization in advanced lipid-rich atherosclerotic lesions detected by gadofluorine-M-enhanced MRI: implications for plaque vulnerability

BACKGROUND

Inflammation and neovascularization may play a significant role in atherosclerotic plaque progression and rupture. We evaluated gadofluorine-M-enhanced MRI for detection of plaque inflammation and neovascularization in an animal model of atherosclerosis.

METHODS AND RESULTS

Sixteen rabbits with aortic plaque and 6 normal control rabbits underwent gadofluorine-M-enhanced MRI. Eight rabbits had advanced atherosclerotic lesions, whereas the remaining 8 had early lesions. Magnetic resonance atherosclerotic plaque enhancement was meticulously compared with plaque inflammation and neovessel density as assessed by histopathology. Advanced plaques and early atheroma were enhanced after gadofluorine-M injection. Control animals displayed no enhancement. After accounting for the within-animal correlation of observations, mean contrast-to-noise ratio was significantly higher in advanced plaques than compared with early atheroma (4.29+/-0.21 versus 3.00+/-0.32; P=0.004). Macrophage density was higher in advanced plaques in comparison to early atheroma (geometric mean=0.50 [95% CI, 0.19 to 1.03] versus 0.25 [0.07 to 0.42]; P=0.05). Furthermore, higher neovessel density was observed in advanced plaques (1.83 [95% CI, 1.51 to 2.21] versus 1.29 [0.99 to 1.69]; P=0.05). The plaque accumulation of gadofluorine-M correlated with increased neovessel density as shown by linear regression analysis (r=0.67; P<0.001). Confocal and fluorescence microscopy revealed colocalization of gadofluorine-M with plaque areas containing a high density of neovessels.

CONCLUSIONS

Gadofluorine-M-enhanced MRI is effective for in vivo detection of atherosclerotic plaque inflammation and neovascularization in an animal model of atherosclerosis. These findings suggest that gadofluorine-M enhancement reflects the presence of high-risk plaque features believed to be associated with plaque rupture. Gadofluorine-M plaque enhancement may therefore provide functional assessment of atherosclerotic plaque in vivo.

Molecular imaging: a primer for interventionalists and imagers

The characterization of human diseases by their underlying molecular and genomic aberrations has been the hallmark of molecular medicine. From this, molecular imaging has emerged as a potentially revolutionary discipline that aims to visually characterize normal and pathologic processes at the cellular and molecular levels within the milieu of living organisms. Molecular imaging holds promise to provide earlier and more precise disease diagnosis, improved disease characterization, and timely assessment of therapeutic response. This primer is intended to provide a broad overview of molecular imaging with specific focus on future clinical applications relevant to interventional radiology.

Coronary computed tomography and magnetic resonance imaging

Cardiac computed tomography and magnetic resonance are relatively new imaging modalities that can exceed the ability of established imaging modalities to detect present pathology or predict patient outcomes. Coronary calcium scoring may be useful in asymptomatic patients at intermediate risk. Computed tomographic coronary angiography is a first-line indication to evaluate congenitally abnormal coronary arteries and, along with stress magnetic resonance myocardial perfusion imaging, is useful in symptomatic patients with nondiagnostic conventional stress tests. Cardiac magnetic resonance is indicated for visualizing cardiac structure and function, and delayed enhancement magnetic resonance is a first-line indication for assessing myocardial viability. Imaging plaque and molecular mechanisms related to plaque rupture holds great promise for the presymptomatic detection of patients at risk for coronary events but is not yet suitable for routine clinical use.