Targeted contrast agents for magnetic resonance imaging and ultrasound.

The development of contrast agents that can be localized to a particular tissue or cellular epitope will potentially allow the noninvasive visualization and characterization of a variety of disease states. Recent advances have been made in the field of molecular imaging with magnetic resonance imaging and ultrasound and varied approaches have been devised to overcome the high background tissue signal. The types of agents and applications developed include gadolinium-conjugated targeting molecules for imaging of fibrin, superparamagnetic iron oxide particles for stem-cell tracking, multimodal perfluorocarbon nanoparticles for visualization of angiogenesis, liposomes for targeting atheroma components, and microbubbles for imaging transplant rejection.

Magnetic resonance molecular imaging with nanoparticles.

Molecular imaging agents are extending the potential of noninvasive medical diagnosis from basic gross anatomic descriptions to complicated phenotypic characterizations based on the recognition of unique cell surface biochemical signatures. Although originally the purview of nuclear medicine, molecular imaging is now a prominent feature of most clinically relevant imaging modalities, in particular magnetic resonance (MR) imaging. MR nanoparticulate agents afford the opportunity not only for targeted diagnostic studies but also for image-monitored site-specific therapeutic delivery, much like the "magic bullet" envisioned by Paul Erhlich 100 years ago. Combining high-resolution MR molecular imaging with drug delivery will facilitate verification and quantification of treatment (ie, rational targeted therapy) and will offer new clinical approaches to many diseases.

Quantitative "magnetic resonance immunohistochemistry" with ligand-targeted (19)F nanoparticles.

Unstable atherosclerotic plaques exhibit microdeposits of fibrin that may indicate the potential for a future rupture. However, current methods for evaluating the stage of an atherosclerotic lesion only involve characterizing the level of vessel stenosis, without delineating which lesions are beginning to rupture. Previous work has shown that fibrin-targeted, liquid perfluorocarbon nanoparticles, which carry a high payload of gadolinium, have a high sensitivity and specificity for detecting fibrin with clinical (1)H MRI. In this work, the perfluorocarbon content of the targeted nanoparticles is exploited for the purposes of (19)F imaging and spectroscopy to demonstrate a method for quantifiable molecular imaging of fibrin in vitro at 4.7 T. Additionally, the quantity of bound nanoparticles formulated with different perfluorocarbon species was calculated using spectroscopy. Results indicate that the high degree of nanoparticle binding to fibrin clots and the lack of background (19)F signal allow accurate quantification using spectroscopy at 4.7 T, as corroborated with proton relaxation rate measurements at 1.5 T and trace element (gadolinium) analysis. Finally, the extension of these techniques to a clinically relevant application, the evaluation of the fibrin burden within an ex vivo human carotid endarterectomy sample, demonstrates the potential use of these particles for uniquely identifying unstable atherosclerotic lesions in vivo.

Targeted nanoparticles for quantitative imaging of sparse molecular epitopes with MRI.

Before molecular imaging with MRI can be applied clinically, certain problems, such as the potential sparseness of molecular epitopes on targeted cell surfaces, and the relative weakness of conventional targeted MR contrast agents, must be overcome. Accordingly, the conditions for diagnostic conspicuity that apply to any paramagnetic MRI contrast agent with known intrinsic relaxivity were examined in this study. A highly potent paramagnetic liquid perfluorocarbon nanoparticle contrast agent ( approximately 250 nm diameter, >90,000 Gd3+/particle) was imaged at 1.5 T and used to successfully predict a range of sparse concentrations in experimental phantoms with the use of standard MR signal models. Additionally, we cultured and targeted the smooth muscle cell (SMC) monolayers that express "tissue factor," a glycoprotein of crucial significance to hemostasis and response to vascular injury, by conjugating an anti-tissue factor antibody fragment to the nanoparticles to effect specific binding. Quantification of the signal from cell monolayers imaged at 1.5 T demonstrated, as predicted via modeling, that only picomolar concentrations of paramagnetic perfluorocarbon nanoparticles were required for the detection and quantification of tissue factor at clinical field strengths. Thus, for targeted paramagnetic agents carrying high payloads of gadolinium, it is possible to quantify molecular epitopes present in picomolar concentrations in single cells with routine MRI.

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

BACKGROUND: Angiogenesis is a critical feature of plaque development in atherosclerosis and might play a key role in both the initiation and later rupture of plaques that lead to myocardial infarction and stroke. The precursory molecular or cellular events that initiate plaque growth and that ultimately contribute to plaque instability, however, cannot be detected directly with any current diagnostic modality. METHODS AND RESULTS: Atherosclerosis was induced in New Zealand White rabbits fed 1% cholesterol for approximately 80 days. alpha(v)beta3-Integrin-targeted, paramagnetic nanoparticles were injected intravenously and provided specific detection of the neovasculature within 2 hours by routine magnetic resonance imaging (MRI) at a clinically relevant field strength (1.5 T). Increased angiogenesis was detected as a 47+/-5% enhancement in MRI signal averaged throughout the abdominal aortic wall among rabbits that received alpha(v)beta3-targeted, paramagnetic nanoparticles. Pretreatment of atherosclerotic rabbits with alpha(v)beta3-targeted, nonparamagnetic nanoparticles competitively blocked specific contrast enhancement of the alpha(v)beta3-targeted paramagnetic agent. MRI revealed a pattern of increased alpha(v)beta3-integrin distribution within the atherosclerotic wall that was spatially heterogeneous along both transverse and longitudinal planes of the abdominal aorta. Histology and immunohistochemistry confirmed marked proliferation of angiogenic vessels within the aortic adventitia, coincident with prominent, neointimal proliferation among cholesterol-fed, atherosclerotic rabbits in comparison with sparse incidence of neovasculature in the control animals. CONCLUSIONS: This molecular imaging approach might provide a method for defining the burden and evolution of atherosclerosis in susceptible individuals as well as responsiveness of individual patients to antiatherosclerotic therapies.