Lipid-based nanoparticles for contrast-enhanced MRI and molecular imaging

Nanoparticles for biomedical imaging

BACKGROUND

Synthetic nanoparticles are emerging as versatile tools in biomedical applications, particularly in the area of biomedical imaging. Nanoparticles 1 - 100 nm in diameter have dimensions comparable to biological functional units. Diverse surface chemistries, unique magnetic properties, tunable absorption and emission properties, and recent advances in the synthesis and engineering of various nanoparticles suggest their potential as probes for early detection of diseases such as cancer. Surface functionalization has expanded further the potential of nanoparticles as probes for molecular imaging.

OBJECTIVE

To summarize emerging research of nanoparticles for biomedical imaging with increased selectivity and reduced nonspecific uptake with increased spatial resolution containing stabilizers conjugated with targeting ligands.

METHODS

This review summarizes recent technological advances in the synthesis of various nanoparticle probes, and surveys methods to improve the targeting of nanoparticles for their application in biomedical imaging.

CONCLUSION

Structural design of nanomaterials for biomedical imaging continues to expand and diversify. Synthetic methods have aimed to control the size and surface characteristics of nanoparticles to control distribution, half-life and elimination. Although molecular imaging applications using nanoparticles are advancing into clinical applications, challenges such as storage stability and long-term toxicology should continue to be addressed.

Multimodality cardiovascular molecular imaging, part I

In Part I of this consensus article, the imaging methodology, evolving imaging technology, and development of novel targeted molecular probes relevant to the developing field of cardiovascular molecular imaging were reviewed. Novel reporter gene and reporter probe imaging approaches for tracking of cardiac transgene expression were also discussed and have important future implications for evaluation of gene- and cell-based therapies for the failing heart. The current role of metabolic and receptor imaging was also briefly reviewed, as these represent the beginning of our clinical application of molecular imaging within the cardiovascular system. Part II will summarize the available targeted imaging probes as well as specific future applications of molecular imaging for identification and evaluation of critical pathophysiological processes of the cardiovascular system.

High-density lipoprotein-based contrast agents for multimodal imaging of atherosclerosis

Lipoproteins, natural nanoparticles, have a well-recognized biological role and are highly suitable as a platform for delivering imaging agents. The ease with which both the exterior and interior of the particles can be modified permits the creation of multifunctional nanoparticles for imaging as well as the delivery of therapeutics. Importantly, their endogenous nature may make them biocompatible and biodegradable and allows them to avoid the recognition of the reticuloendothelial system. In particular, high-density lipoproteins (HDL) are of interest, because of their small size they can easily cross the endothelium and penetrate the underlying tissue. We summarize here the progress in establishing HDL as a vector for delivering a variety of diagnostically active materials to vulnerable atherosclerotic plaques in mouse models of atherosclerosis. By loading various types of image-enhancing compounds into either the core or surface of HDL, they can be visualized by different imaging modalities (MRI, CT, optical). By rerouting of HDL away from plaque macrophages, imaging of biological processes in diseases besides atherosclerosis may also be achieved.

The fabrication of uniform cylindrical nanoshells and their use as spectrally tunable MRI contrast agents

A new form of tunable magnetic resonance imaging agent based on precisely dimensioned cylindrical magnetic nanoshells is introduced. Using top-down prepatterned substrates, the nanoshells are fabricated by exploiting what is usually regarded as a detrimental processing side-effect, namely the redeposition of material back-sputtered during ion-milling. The well-resolved nuclear magnetic resonance peaks of the resulting nanostructures attest to the nanoscale fabrication control and the general feasibility of such sputter redeposition for fabrication of a variety of self-supporting, highly monodisperse nanoscale structures.

Cellular compartmentalization of internalized paramagnetic liposomes strongly influences both T1 and T2 relaxivity

In recent years, numerous Gd(3+)-based contrast agents have been developed to enable target-specific MR imaging of in vivo processes at the molecular level. The combination of powerful contrast agents and amplification strategies, aimed at increasing the contrast agent dose at the target site, is an often-used strategy to improve the sensitivity of biomarker detection. One such amplification mechanism is to target a disease-specific cell membrane receptor that can undergo multiple rounds of internalization following ligand binding and thus shuttle a sizeable amount of contrast agent into the target cell. An example of such a membrane receptor is the alpha(nu)beta(3) integrin. The goal of this study was to investigate the consequences of this amplification approach for the T(1)- and T(2)-shortening efficacy of a paramagnetic contrast agent. Cultured endothelial cells were incubated with paramagnetic liposomes that were conjugated with a cyclic RGD-peptide to enable internalization by means of the alpha(nu)beta(3) integrin receptor. Non-targeted liposomes served as a control. This study showed that alpha(nu)beta(3) targeting dramatically increased the uptake of paramagnetic liposomes. This targeting strategy, however, strongly influenced both the longitudinal and transverse relaxivity of the internalized paramagnetic liposomes.

Pushing the sensitivity envelope of lanthanide-based magnetic resonance imaging (MRI) contrast agents for molecular imaging applications

Contrast in magnetic resonance imaging (MRI) arises from changes in the intensity of the proton signal of water between voxels (essentially, the 3D counterpart of pixels). Differences in intervoxel intensity can be significantly enhanced with chemicals that alter the nuclear magnetic resonance (NMR) intensity of the imaged spins; this alteration can occur by various mechanisms. Paramagnetic lanthanide(III) complexes are used in two major classes of MRI contrast agent: the well-established class of Gd-based agents and the emerging class of chemical exchange saturation transfer (CEST) agents. A Gd-based complex increases water signal by enhancing the longitudinal relaxation rate of water protons, whereas CEST agents decrease water signal as a consequence of the transfer of saturated magnetization from the exchangeable protons of the agent. In this Account, we survey recent progress in both areas, focusing on how MRI is becoming a more competitive choice among the various molecular imaging methods. Compared with other imaging modalities, MRI is set apart by its superb anatomical resolution; however, its success in molecular imaging suffers because of its intrinsic insensitivity. A relatively high concentration of molecular agents (0.01-0.1 mM) is necessary to produce a local alteration in the water signal intensity. Unfortunately, the most desirable molecules for visualization in molecular imaging are present at much lower concentrations, in the nano- or picomolar range. Therefore, augmenting the sensitivity of MRI agents is key to the development of MR-based molecular imaging applications. In principle, this task can be tackled either by increasing the sensitivity of the reporting units, through the optimization of their structural and dynamic properties, or by setting up proper amplification strategies that allow the accumulation of a huge number of imaging reporters at the site of interest. For Gd-based agents, high sensitivities can be attained by exploiting a range of nanosized carriers (micelles, liposomes, microemulsions, and the like, as well as biological structures such as apoferritin and lipoproteins) properly loaded with Gd-based chelates. Furthermore, the sensitivity of Gd-based agents can be markedly affected either by their interactions with biological structures or by their cellular localization. For CEST agents, a huge sensitivity enhancement has been obtained by using the water molecules contained in the inner cavity of liposomes as the exchangeable source of protons for magnetization transfer. Several "tricks" (for example, the use of multimeric lanthanide(III) shift reagents, changes in the shape of the liposome container, and so forth) have been devised to improve the chemical shift separation between the intraliposomal water and the "bulk" water resonances. Overall, excellent sensitivity enhancements have been obtained for both classes of agents, enabling their use in MR molecular imaging applications.

Nanoparticulate assemblies of amphiphiles and diagnostically active materials for multimodality imaging

Modern medicine has greatly benefited from recent dramatic improvements in imaging techniques. The observation of physiological events through interactions manipulated at the molecular level offers unique insight into the function (and dysfunction) of the living organism. The tremendous advances in the development of nanoparticulate molecular imaging agents over the past decade have made it possible to noninvasively image the specificity, pharmacokinetic profiles, biodistribution, and therapeutic efficacy of many novel compounds. Several types of nanoparticles have demonstrated utility for biomedical purposes, including inorganic nanocrystals, such as iron oxide, gold, and quantum dots. Moreover, natural nanoparticles, such as viruses, lipoproteins, or apoferritin, as well as hybrid nanostructures composed of inorganic and natural nanoparticles, have been applied broadly. However, among the most investigated nanoparticle platforms for biomedical purposes are lipidic aggregates, such as liposomal nanoparticles, micelles, and microemulsions. Their relative ease of preparation and functionalization, as well as the ready synthetic ability to combine multiple amphiphilic moieties, are the most important reasons for their popularity. Lipid-based nanoparticle platforms allow the inclusion of a variety of imaging agents, ranging from fluorescent molecules to chelated metals and nanocrystals. In recent years, we have created a variety of multifunctional lipid-based nanoparticles for molecular imaging; many are capable of being used with more than one imaging technique (that is, with multimodal imaging ability). These nanoparticles differ in size, morphology, and specificity for biological markers. In this Account, we discuss the development and characterization of five different particles: liposomes, micelles, nanocrystal micelles, lipid-coated silica, and nanocrystal high-density lipoprotein (HDL). We also demonstrate their application for multimodal molecular imaging, with the main focus on magnetic resonance imaging (MRI), optical techniques, and transmission electron microscopy (TEM). The functionalization of the nanoparticles and the modulation of their pharmacokinetics are discussed. Their application for molecular imaging of key processes in cancer and cardiovascular disease are shown. Finally, we discuss a recent development in which the endogenous nanoparticle HDL was modified to carry different diagnostically active nanocrystal cores to enable multimodal imaging of macrophages in experimental atherosclerosis. The multimodal characteristics of the different contrast agent platforms have proven to be extremely valuable for validation purposes and for understanding mechanisms of particle-target interaction at different levels, ranging from the entire organism down to cellular organelles.

Effect of nanoparticle and aggregate size on the relaxometric properties of MR contrast agents based on high quality magnetite nanoparticles

Colloidal dispersions of monodispersed and high-crystalline magnetite nanoparticles have been used to establish a relationship between magnetic properties and magnetic resonance (MR) relaxometric parameters in vitro. Magnetite nanoparticles with diameters between 4 and 14 nm were synthesized by thermal decomposition of Fe(acac)3 in different organic solvents and transformed to hydrophilic by changing oleic acid for dimercaptosuccinic acid (DMSA). A final treatment in alkaline water was critical to make the suspension stable at pH 7 with xi-potential values of -45 mV and hydrodynamic sizes as low as 50 nm. Samples showed superparamagnetic behavior at room temperature, which is an important parameter for biomedical applications. Susceptibility increased with both particle and aggregate size, and for particles larger than 9 nm, the aggregate size was the key factor controlling the susceptibility. Relaxivity values followed the same trend as the suspension susceptibilities, indicating that the aggregate size is an important factor above a certain particle size governing the proton relaxation times. The highest relaxivity value, r2=317 s(-1) mM(-1), much higher than those for commercial contrast agents with similar hydrodynamic size, was obtained for a suspension consisting of 9 nm particles and 70 nm of hydrodynamic size, and it was assigned to the higher particle crystallinity in comparison to particles prepared by coprecipitation. Therefore, it can be concluded that in addition to the sample crystallinity, both particle size and aggregate size should be considered in order to explain the magnetic and relaxivity values of a suspension.

Cellular uptake and transport of gold nanoparticles incorporated in a liposomal carrier

Recent interest in using gold nanoparticles (Au NPs) for therapy in radiation medicine has motivated development of a liposome-based system to enhance their delivery to cells. In this study, liposomes were demonstrated to perform like a "Trojan Horse" to deliver small (1.4 nm) Au NPs into tumor cells by overcoming the energetically unfavorable endocytosis process for small NPs. The results reveal that the liposomal approach provides a thousand-fold enhancement in the cellular uptake of the small Au NPs. Real-time intracellular tracking of the Au NP-liposomes revealed an average speed of 12.48 +/- 3.12 microm/hr for their intracellular transport. Analysis of the time-dependent intracellular spatial distribution of the Au NP-liposomes demonstrated that they reside in lysosomes (final degrading organelles) within 40 minutes of incubation. Knowledge gained in these studies opens the door to pursuing liposomes as a viable strategy for delivery of Au NPs in radiation therapy applications.

FROM THE CLINICAL EDITOR

Gold nanoparticles (Au NPs) as part of an optimized liposome-based delivery system have been proposed for therapy in radiation medicine. The approach resulted in a thousand-fold enhancement in the cellular uptake of Au NPs compared to conventional delivery methods, with the nanoparticles residing in lysosomes within 40 minutes of incubation.

Magnetoliposomes: versatile innovative nanocolloids for use in biotechnology and biomedicine

The high biocompatibility and versatile nature of liposomes have made these particles keystone components in many hot-topic biomedical research areas. Liposomes can be combined with a large variety of nanomaterials, such as superparamagnetic iron oxide nanocores. Because the unique features of both the magnetizable colloid and the versatile lipid bilayer can be joined, the resulting so-called magnetoliposomes can be exploited in a great array of biotechnological and biomedical applications. In this article, we highlight the use of magnetoliposomes in immobilizing enzymes, both water-soluble and hydrophobic ones, as well as their potential in several biomedical applications, including MRI, hyperthermia cancer treatment and drug delivery. The goal of this article is not to list all known uses of magnetoliposomes but rather to present some conspicuous applications in comparison to other currently used nanoparticles.

Cell labeling and tracking for experimental models using magnetic resonance imaging

Magnetic Resonance Imaging (MRI), as one of the most powerful methods in clinical diagnosis, has emerged as an additional method in the field of molecular and cellular imaging. Compared to established molecular imaging methods, MRI provides in vivo images with high resolution. In particularly in the field of cell-based therapy, non-invasively acquired information on temporal changes of cell location linked to high-resolution anatomical information is of great interest. Relatively new approaches like responsive contrast agents or MR imaging reporter gene expression are MRI applications beyond temporal and spatial information on labeled cells towards investigations on functional changes of cells in vivo. MRI-based cell monitoring and tracking studies require prior labeling of the cells under investigation for excellent contrast against the background of host tissue. Here, an overview is provided on contrast generation strategies for MRI of cells. This includes MR contrast agents, various approaches of cell labeling and MRI as well as MR spectroscopic methods used for cell tracking in vivo. Advantages and disadvantages of the particular labeling approaches and methods are discussed. In addition to description of the methods, the emphasis is on the potential but also challenges and shortcomings of this imaging technique for applications that aim to visualize cellular processes in vivo.

Self-assembling DNA quadruplex conjugated to MRI contrast agents

We report the preparation and magnetic resonance (MR) characterization of new MRI contrast agents based on gadolinium complexes conjugated to a self-assembling DNA quadruplex scaffold. As a single gadolinium-DOTA chelated DNA strand, the r(1) molar relaxivity is 6.4 mM(-1) s(-1) per Gd and increases to 11.7 mM(-1) s(-1) per Gd upon formation of a DNA quadruplex. Similar results were obtained when a gadolinium-DOTA dendrimer was conjugated to DNA, with the r(1) molar relaxivity increasing to 12.9 mM(-1) s(-1) per Gd upon the formation of DNA quadruplex, compared to that of 6.0 mM(-1) s(-1) for a single strand of gadolinium-DOTA dendrimer chelate. This yields an r(1) molar relaxivity of 154.8 and 46.8 mM(-1) s(-1) per DNA quadruplex based on DOTA dendrimer or monomer, respectively. Importantly, the DNA quadruplex scaffold is approximately 2.5 nm(3) in size, potentially enabling this type of contrast agent to be used for targeted delivery in vivo to detect specific cells or tissues, even behind intact blood vessels.

Well-defined, multifunctional nanostructures of a paramagnetic lipid and a lipopeptide for macrophage imaging

In the field of nanomedicine there is a great demand for technologies that allow the creation of self-assembled structures of which the size and morphology can be accurately controlled. In the current study, we report a nanoparticle platform that is composed of a paramagnetic lipid and a fluorescently labeled lipopeptide. By judiciously controlling the ratio of the aforementioned amphiphilic molecules, a variety of well-defined nanosized supramolecular structures with different sizes and morphologies could be created. The hydrodynamic radii of the different structures were determined by dynamic light scattering. Cryo-TEM revealed the aggregate morphology to vary from small micellar structures to plate-like and even full grown ribbons of which the aspect ratios varied from a diameter of 5-8 nm to structures with a width of up to 25 nm and infinite length. Interestingly, nuclear magnetic resonance dispersion profiling revealed excellent properties for MRI and also showed that the relaxivity of the structures was tunable and morphology dependent. Finally, macrophage cells were treated with two selected nanoparticles and were shown to be avidly taken up. In conclusion we demonstrate a methodology to create structures that (1) are paramagnetic to enable their detection with MRI, (2) exhibit fluorescent properties, (3) can be tuned to defined sizes and shapes, and (4) are efficiently taken up by macrophage cells in vitro.

Real-time magnetic resonance imaging and quantification of lipoprotein metabolism in vivo using nanocrystals

Semiconductor quantum dots and superparamagnetic iron oxide nanocrystals have physical properties that are well suited for biomedical imaging. Previously, we have shown that iron oxide nanocrystals embedded within the lipid core of micelles show optimized characteristics for quantitative imaging. Here, we embed quantum dots and superparamagnetic iron oxide nanocrystals in the core of lipoproteins--micelles that transport lipids and other hydrophobic substances in the blood--and show that it is possible to image and quantify the kinetics of lipoprotein metabolism in vivo using fluorescence and dynamic magnetic resonance imaging. The lipoproteins were taken up by liver cells in wild-type mice and displayed defective clearance in knock-out mice lacking a lipoprotein receptor or its ligand, indicating that the nanocrystals did not influence the specificity of the metabolic process. Using this strategy it is possible to study the clearance of lipoproteins in metabolic disorders and to improve the contrast in clinical imaging.

Towards new functional nanostructures for medical imaging

Nanostructures represent a promising new type of contrast agent for clinical medical imaging modalities, including magnetic resonance imaging, x-ray computed tomography, ultrasound, and nuclear imaging. Currently, most nanostructures are simple, single-purpose imaging agents based on spherical constructs (e.g., liposomes, micelles, nanoemulsions, macromolecules, dendrimers, and solid nanoparticle structures). In the next decade, new clinical imaging nanostructures will be designed as multi-functional constructs, to both amplify imaging signals at disease sites and deliver localized therapy. Proposals for nanostructures to fulfill these new functions will be outlined. New functional nanostructures are expected to develop in five main directions: Modular nanostructures with additive functionality; cooperative nanostructures with synergistic functionality; nanostructures activated by their in vivo environment; nanostructures activated by sources outside the patient; and novel, nonspherical nanostructures and components. The development and clinical translation of next-generation nanostructures will be facilitated by a combination of improved clarity of the in vivo imaging and biological challenges and the requirements to successfully overcome them; development of standardized characterization and validation systems tailored for the preclinical assessment of nanostructure agents; and development of streamlined commercialization strategies and pipelines tailored for nanostructure-based agents for their efficient translation to the clinic.

Paramagnetic lipid-coated silica nanoparticles with a fluorescent quantum dot core: a new contrast agent platform for multimodality imaging

Silica particles as a nanoparticulate carrier material for contrast agents have received considerable attention the past few years, since the material holds great promise for biomedical applications. A key feature for successful application of this material in vivo is biocompatibility, which may be significantly improved by appropriate surface modification. In this study, we report a novel strategy to coat silica particles with a dense monolayer of paramagnetic and PEGylated lipids. The silica nanoparticles carry a quantum dot in their center and are made target-specific by the conjugation of multiple alphavbeta3-integrin-specific RGD-peptides. We demonstrate their specific uptake by endothelial cells in vitro using fluorescence microscopy, quantitative fluorescence imaging, and magnetic resonance imaging. The lipid-coated silica particles introduced here represent a new platform for nanoparticulate multimodality contrast agents.

Adapting the clinical MRI software environment for real-time navigation of an endovascular untethered ferromagnetic bead for future endovascular interventions

A dedicated software architecture for a novel interventional method allowing the navigation of ferromagnetic endovascular devices using a standard real-time clinical MRI system is shown. Through a specially developed software environment integrating a tracking method and a real-time controller algorithm, a clinical 1.5T Siemens Avanto MRI system is adapted to provide new functionality for potential automated interventional applications. The proposed software architecture was successfully validated through in vivo controlled navigation inside the carotid artery of a swine. Here we present how this MRI-upgraded software environment could also be used in more complex vasculature models through the real-time navigation of a 1.5 mm diameter chrome steel bead in two different MR-compatible phantoms with flowless and quiescent flow conditions. The developed platform and software modules needed for such navigation are also presented. Real-time tracking achieved through a dedicated positioning method based on an off-resonance excitation technique has also been successfully integrated in the software platform while maintaining adequate real-time performance. These preliminary feasibility experiments suggest that navigation of such devices can be achieved using a similar software architecture on other conventional clinical MRI systems at an operational closed-loop control frequency of 32 Hz.

Multimodality nanotracers for cardiovascular applications

Targeted imaging and therapeutics is becoming a field of prime importance in the study and treatment of cardiovascular disease; it promises to enable early diagnosis, promote improved understanding of pathology, and offer a way to improve therapeutic efficacy. Agents, particularly for cardiovascular disease, have been reported to permit the in vivo imaging, by multiple modalities, of macrophages, vascular targets such as vascular cell adhesion molecule 1, and markers for angiogenesis such as alpha(v)beta(3) integrin. In this Article, we first discuss the general concept of multimodality nanoparticles and then focus in greater depth on their clinical application for molecular imaging and therapy. Lastly, several examples of cardiovascular applications are discussed, including combined imaging and therapy approaches.

In vivo molecular imaging of vascular stress

Noninvasive in vivo imaging is an emerging specialty in experimental radiology aiming at developing hardware and appropriate contrast agents to visualize the molecular basis and pathophysiological processes of many pathological conditions, including atherosclerosis. The list of potentially useful tracers and targets for in vivo molecular imaging in the cascade of early atherosclerotic events has been narrowed down to some very promising endothelial factors, i.e., cell adhesion molecules, macrophages, apoptosis, lipoproteins, heat shock proteins, and others. In this review, we will update on the progress of recent developments in the field of noninvasive molecular imaging in experimental atherosclerosis.

Fluorescent liposomes as contrast agents for in vivo optical imaging of edemas in mice

This study assesses if specially designed fluorescent liposomes can be used as contrast agent for near-infrared fluorescence (NIRF) optical imaging of cultured macrophages in vitro and for NIRF imaging of inflammatory processes, like edema, in an in vivo mouse model. Fluorescent liposomes are prepared by the film hydration and extrusion method using cholesterol, L-phosphatidylcholine, and the NIR fluorescent dye DY-676-C(18) ester. Photon correlation spectroscopy and flow cytometry reveal that fluorescent liposomes are structurally stable for up to 133 days. Distinct uptake/labeling of cultured murine J774 macrophages is demonstrated by confocal laser scanning microscopy (CLSM), flow cytometry, and macroscopic NIRF imaging system at wavelengths >670 nm. Moreover, CLSM analysis reveals fluorescence signals within intracellular compartments. Ear edema is induced in mice (n = 16) by subcutaneous injection of zymosan A. Whole-body NIRF imaging is performed after intravenous injection (0-24 h) of fluorescent liposomes (55 nmol dye per kg body weight). Distinctly higher fluorescence intensities (1613.6 +/- 61.7 a.u.) are detected at inflamed areas of diseased mice as compared to controls (892.8 +/- 19.4 a.u.). Furthermore, cell isolated from ear lavage reveals the presence of labeled F4/80 positive tissue macrophages. Taken together, the results indicate both that mouse macrophages labeled with fluorescent liposomes can be detected in vitro with fluoro-optical methods and that in vivo optical imaging of inflammatory processes with fluorescent liposomes as contrast agent is feasible. Possibly, early stages of other inflammatory diseases could also be detected by the proposed diagnostic tool in the long term.

A stimulus-responsive magnetic nanoparticle drug carrier: magnetite encapsulated by chitosan-grafted-copolymer

We describe a magnetic nanoparticle drug carrier for controlled drug release that responds to the change in external temperature or pH, with characteristics of longer circulation time and reduced side effects. The novel nanocarrier is characterized by a functionalized magnetite (Fe(3)O(4)) core that is conjugated with drug via acid-labile hydrazone-bond and encapsulated by the thermosensitive smart polymer, chitosan-g-poly(N-isopropylacrylamide-co-N,N-dimethylacrylamide) [chitosan-g-poly(NIPAAm-co-DMAAm)]. The chitosan-g-poly(NIPAAm-co-DMAAm) smart polymer exhibits a lower critical solution temperature (LCST) of approximately 38 degrees C, signifying phase transition behavior of the smart polymer and enabling its use for triggering on-off mechanisms. The drug release response was appreciably low at a temperature less than the LCST as compared with a temperature above the LCST. In each case, there was an initial rapid drug release, followed by a controlled released in the second stage, especially in a mild acidic buffer solution of pH 5.3. We believe that the drug release occurs via a collapse of the encapsulated thermosensitive polymer and cleavage of the acid-labile hydrazone linkage.

Stem cell labeling for magnetic resonance imaging

In vivo applications of cells for the monitoring of their cell dynamics increasingly use non-invasive magnetic resonance imaging. This imaging modality allows in particular to follow the migrational activity of stem cells intended for cell therapy strategies. All these approaches require the prior labeling of the cells under investigation for excellent contrast against the host tissue background in the imaging modality. The present review discusses the various routes of cell labeling and describes the potential to observe both cell localization and their cell-specific function in vivo. Possibilities for labeling strategies, pros and cons of various contrast agents are pointed out while potential ambiguities or problems of labeling strategies are emphasized.

Synthesis of superparamagnetic nanotubes as MRI contrast agents and for cell labeling

AIMS

Magnetic nanoparticles have been studied widely as MRI contrast agents to increase the sensitivity of this technique. This work describes the synthesis and characterization of magnetic nanotubes (MNTs) as a novel MRI contrast agent.

METHODS

MNTs with high saturation magnetization were fabricated by the synthesis of superparamagnetic iron oxide nanoparticles (SPIONs) directly in the pores of silica nanotubes (SNTs). The MNTs were characterized by electron microscopy, superconducting quantum interference device and MRI. Preliminary studies on in vitro cytotoxicity and cell labeling were carried out.

RESULTS

The MNTs retained the superparamagnetic characteristics in bulk solutions with a considerably high saturation magnetization of 95 emu/gFe. The nuclear magnetic resonance (NMR) relaxivities for MNTs of 500 nm in length and of 60 nm in diameter were r(1) = 1.6 +/- 0.3 mM(-1)s(-1) and r(2) = 264 +/- 56 mM(-1)s(-1) and, for the MNTs of 2 microm in length and 70 nm in diameter, the r(1) and r(2) were 3.0 +/- 1.3 and 358 +/- 65 mM(-1)s(-1), respectively. In vitro cell labeling showed promising results with excellent labeling efficiency. No cellular toxicity was observed in vitro.

CONCLUSIONS

The integration of SPIONs with SNTs imparts the superparamagnetic characteristics of SPIONs onto the SNTs, creating unique magnetic nanoparticles with multifunctionality. The MNTs showed promising results as a MRI contrast agent with high NMR relaxivities, little cytotoxicity and high cell-labeling efficiency.

Gd(III)-EPTPAC16, a new self-assembling potential liver MRI contrast agent: in vitro characterization and in vivo animal imaging studies

The recently reported amphiphilic chelate, [Gd(EPTPAC16)(H2O)]2-, forms supramolecular aggregates in aqueous solution by self-assembly of the monomers with a relaxometrically determined critical micellar concentration (CMC) of 0.34 mM. The effect of sonication on the aggregate size was characterized by dynamic light scattering and relaxometry, indicating the presence of premicellar aggregates and an overall decrease in aggregate size and polydispersity upon sonication, slightly below the CMC. [[153Sm](EPTPAC16)(H2O)]2- radiotracer was evaluated in vivo from gamma scintigraphy and biodistribution in Wistar rats. It was found to depend strongly on the sample concentration, below or above the CMC, and its sonication, in a way that correlates with the effect of the same factors on the size of the aggregates formed in solution. Below CMC, the very large aggregates of the [153Sm]3+ -labeled chelate were persistently and mainly taken up by the lungs, and also by the macrophage-rich liver and spleen. Sonication of this solution led to loss of the lung uptake. Above CMC, the metal chelate was mainly taken up by the liver, with very little uptake by the spleen and lungs. In vivo, dynamic contrast-enhanced (DCE)-MRI evaluation of the micellar [Gd(EPTPAC16)(H2O)]2- compound in Wistar rats showed a persistent hepatic positive-contrast effect in T1-weighted images, qualitatively similar to the clinically established Gd(III)-based hepatobiliary-selective agents. No enhancement effect was observed in the lungs because of the scarcity of mobile protons in this organ, despite the scintigraphic evidence of significant lung retention of the [153Sm]3+ -labeled chelate at concentrations below the CMC.

Nanoengineering artificial lipid envelopes around adenovirus by self-assembly

We have developed a novel, reproducible, and facile methodology for the construction of artificial lipid envelopes for adenoviruses (Ad) by self-assembly of lipid molecules around the viral capsid. No alteration of the viral genome or conjugation surface chemistry at the virus capsid was necessary, therefore difficulties in production and purification associated with generating most surface-modified viruses can be eliminated. Different lipid bilayer compositions produced artificially enveloped Ad with physicochemical and biological characteristics determined by the type of lipid used. Physicochemical characteristics such as vector size, degree of aggregation, stability, and surface charge of the artificially enveloped Ad were correlated to their biological (gene transfer) function. In monolayer cell cultures, binding to the coxsackie and adenovirus receptor (CAR) was blocked using a zwitterionic envelope, whereas enhanced binding to the cell membrane was achieved using a cationic envelope. Envelopment of Ad by both zwitterionic and cationic lipid bilayers led to almost complete ablation of gene expression in cell monolayers, due to blockage of virion endosomal escape. Alternatively, artificial Ad envelopes built from lipid bilayers at the fluid phase in physiological conditions led to enhanced penetration of the vectors inside a three-dimensional tumor spheroid cell culture model and delayed gene expression in the tumor spheroid compared to nonenveloped adenovirus. These results indicate that construction of artificial envelopes for nonenveloped viruses by lipid bilayer wrapping of the viral capsids constitutes a general strategy to rationally engineer viruses at the nanoscale with control over their biological properties.

Coating thickness of magnetic iron oxide nanoparticles affects R2 relaxivity

PURPOSE

To evaluate the effect of coating thickness on the relaxivity of iron oxide nanoparticles.

MATERIALS AND METHODS

Monocrystalline superparamagnetic iron oxide nanoparticles (MIONs), coated with a polyethylene glycol (PEG)-modified, phospholipid micelle coating, with different PEG molecular weights, were prepared. The particle diameters were measured with dynamic light scattering (DLS) and electron microscopy (EM). The R1 and R2 of MIONs were measured using a bench-top nuclear magnetic resonance (NMR) relaxometer. pH was varied for some measurements. Monte Carlo simulations of proton movement in a field with nanometer-sized magnetic inhomogeneities were performed.

RESULTS

Increasing the molecular weight of the PEG portion of the micelle coating increased overall particle diameter. As coating thickness increases, the R2 decreases and the R1 increases. Changing pH has no effect on relaxivity. The Monte Carlo simulations suggest that the effect of coating size on R2 relaxivity is determined by two competing factors: the physical exclusion of protons from the magnetic field and the residence time for protons within the coating zone.

CONCLUSION

Coating thickness can significantly impact the R2, and the R2/R1 ratio, of a MION contrast agent. An understanding of the relationship between coating properties and changes in relaxivity is critical for designing magnetic nanoparticle probes for molecular imaging applications using MRI.

Multifunctional Magnetic-fluorescent Nanocomposites for Biomedical Applications

Nanotechnology is a fast-growing area, involving the fabrication and use of nano-sized materials and devices. Various nanocomposite materials play a number of important roles in modern science and technology. Magnetic and fluorescent inorganic nanoparticles are of particular importance due to their broad range of potential applications. It is expected that the combination of magnetic and fluorescent properties in one nanocomposite would enable the engineering of unique multifunctional nanoscale devices, which could be manipulated using external magnetic fields. The aim of this review is to present an overview of bimodal “two-in-one” magnetic-fluorescent nanocomposite materials which combine both magnetic and fluorescent properties in one entity, in particular those with potential applications in biotechnology and nanomedicine. There is a great necessity for the development of these multifunctional nanocomposites, but there are some difficulties and challenges to overcome in their fabrication such as quenching of the fluorescent entity by the magnetic core. Fluorescent-magnetic nanocomposites include a variety of materials including silica-based, dye-functionalised magnetic nanoparticles and quantum dots-magnetic nanoparticle composites. The classification and main synthesis strategies, along with approaches for the fabrication of fluorescent-magnetic nanocomposites, are considered. The current and potential biomedical uses, including biological imaging, cell tracking, magnetic bioseparation, nanomedicine and bio- and chemo-sensoring, of magnetic-fluorescent nanocomposites are also discussed.

Magnetic resonance imaging detection of tumor cells by targeting low-density lipoprotein receptors with Gd-loaded low-density lipoprotein particles

Gd-DO3A-diph and Gd-AAZTAC17 are lipophilic magnetic resonance imaging (MRI) agents that display high affinity for low-density lipoprotein (LDL) particles. However, on binding to LDL, Gd-DO3A-diph shows a decreased hydration that results in a lower enhancement of water proton relaxation rate. Conversely, Gd-AAZTAC17 displays a strong relaxation enhancement at the imaging fields. Each LDL particle can load up to 100 and 400 UNITS of Gd-DO3A-diph and Gd-AAZTAC17, respectively. Their LDL adducts are taken up by human hepatoblastoma G2 (HepG2) and melanoma B16 tumor cells when added to the incubation medium. T(1) measurements of the labeled cells indicate that Gd-AAZTAC17 is significantly more efficient than Gd-DO3A-diph. Furthermore, it has been found that HepG2 hepatoma cells can internalize higher amounts of Gd-AAZTAC17 than B16 cells and the involvement of LDL receptors (LDLRs) has been demonstrated in competition assays with free LDL. Gd-AAZTAC17/LDL adduct proved to be an efficient probe in the magnetic resonance (MR) visualization of subcutaneous tumors in animal models obtained by injecting B16 melanoma cells into the right flank of mice. Finally, confocal microscopy validation of the distribution of LDL-based probes in the tumor has been obtained by doping the Gd-AAZTAC17/LDL adduct with a fluorescent phospholipid moiety.

Magnetic resonance molecular imaging contrast agents and their application in atherosclerosis

Heart disease is the most prevalent cause of mortality in the Western world and is most frequently caused by rupture of lesions in the arteries, which are formed by atherosclerosis. Atherosclerosis is a progressive disease, and therefore, there is a strong motivation to be able to image the stages of this disease in vivo. The pathogenesis of this disease is now well established, and a number of markers such as macrophages, vascular adhesion molecules, fibrin, and the alphanubeta3-integrin have been identified that are of particular interest for imaging. Furthermore, the differentiation between the stable and unstable plaque with imaging is a central goal of the field. Contrast can be generated in magnetic resonance imaging through the application of several types of agents such as T1, T2, chemical exchange saturation transfer or 19F-based imaging agents. Subsequent to the discussion of the above topics, we will describe some examples of molecular imaging agents that successfully detect specific markers in atherosclerotic plaques that are of interest in several stages of this disease.

Molecular imaging of macrophages in atherosclerotic plaques using bimodal PEG-micelles

Pegylated, fluorescent, and paramagnetic micelles were developed. The micelles were conjugated with macrophage scavenger receptor (MSR)-specific antibodies. The abdominal aortas of atherosclerotic apoE-KO mice were imaged with T(1)-weighted high-resolution MRI before and 24 h after intravenous administration of the contrast agent (CA). Pronounced signal enhancement (SE) (up to 200%) was observed for apolipoprotein E knockout (apoE-KO) mice that were injected with MSR-targeted micelles, while the aortic vessel wall of mice injected with nontargeted micelles showed little SE. To allow fluorescence microscopy and optical imaging of the excised aorta, the micelles were made fluorescent by incorporating either a quantum dot (QD) in the micelle corona or rhodamine lipids in the micelle. Ultraviolet (UV) illumination of the aorta allowed the identification of regions with high macrophage content, while MSR-targeted rhodamine micelles could be detected with fluorescence microscopy and were found to be associated with macrophages. In conclusion, this study demonstrates that macrophages in apoE-KO mice can be effectively and specifically detected by molecular MRI and optical methods upon administration of a pegylated micellar CA.

Synthesis of iron oxide nanoparticles in a microfluidic device: preliminary results in a coaxial flow millichannel

A millimetric coaxial flow device operating under laminar flow has been designed to study the synthesis of iron oxide nanoparticles in a millichannel where the flow rate of the different reagents has been adjusted all over the experiments so that the magnetic and stable colloidal iron oxide particles with a size less than 7 nm have been prepared continuously.

Very-low-frequency electron paramagnetic resonance (EPR) imaging of nitroxide-loaded cells

Recent advances in electron paramagnetic resonance (EPR) imaging have made it possible to image, in real time in vivo, cells that have been labeled with nitroxide spin probes. We previously reported that cells can be loaded to high (millimolar) intracellular concentrations with (2,2,5,5-tetramethylpyrrolidin-1-oxyl-3-ylmethyl)amine-N,N-diacetic acid by incubation with the corresponding acetoxymethyl (AM) ester. Furthermore, the intracellular lifetime (t(1/e)) of this nitroxide is 114 min-sufficiently long to permit in vivo imaging studies. In the present study, at a gradient of approximately 50 mT/m, we acquire and compare EPR images of a three-tube phantom, filled with either a 200-microM solution of the nitroxide, or a suspension of cells preincubated with the nitroxide AM ester. In both cases, 3-mm resolution images can be acquired with excellent signal-to-noise ratios (SNRs). These findings indicate that cells well-loaded with nitroxide are readily imageable by EPR imaging, and that in vivo tracking studies utilizing such cells should be feasible.

Structural and relaxometric characterization of peptide aggregates containing gadolinium complexes as potential selective contrast agents in MRI

The structural and relaxometric characterization of a novel class of supramolecular aggregates, as potential tumor-specific contrast agents in magnetic resonance imaging (MRI), is reported. The aggregates are based on a new monomer with an upsilon shape (MonY) that contains, in the same molecule, all three fundamental tasks that are required: 1) a hydrophobic moiety that allows the formation of supramolecular aggregates; 2) the bioactive CCK8 peptide for target recognition; and 3) a chelating agent able to give stable gadolinium complexes. As indicated by dynamic light scattering and small-angle neutron scattering (SANS) measurements, MonY and its gadolinium complex MonY(Gd) aggregate in aqueous solution to give ellipsoidal micelles with a ratio between the micellar axes of approximately 1.7 and an aggregation number N(agg) of approximately 30. There are no differences in the aggregation behavior of MonY and MonY(Gd), which indicates that the presence of metal ions, and therefore the reduction of the net charge, does not influence the aggregation behavior. When MonY or MonY(Gd) are blended with dioleoyl phosphatidylcholine (DOPC), the aggregation behavior is dictated by the tendency of DOPC to give liposomes. Only when the amount of MonY or MonY(Gd) is higher than 20 % is the coexistence of liposomes and micelles observed. The thickness d of the bilayer is estimated by SANS to be approximately 35-40 A, whereas cryogenic transmission electron microscopy images show that the diameter of the liposomes ranges from approximately 50 to 150 nm. Self-assembling micelles of MonY(Gd) present high relaxivity values (r(1p)=15.03 mM(-1) s(-1)) for each gadolinium complex in the aggregate. Liposomes containing MonY(Gd) inserted in the DOPC bilayer at a molar ratio of 20:80 present slightly lower relaxivity values (r(1p)=12.7 mM(-1) s(-1)), independently of their internal or external position in the liposome.

Hydrophilic Monodisperse Magnetic Nanoparticles Protected by an Amphiphilic Alternating Copolymer

Iron oxide nanoparticles (NPs) with diameters of 16.1, 20.5, and 20.8 nm prepared from iron oleate precursors were coated with poly(maleic acid-alt-1-octadecene) (PMAcOD). The coating procedure exploited hydrophobic interactions of octadecene and oleic acid tails while hydrolysis of maleic anhydride moieties allowed the NP hydrophilicity. The PMAcOD nanostructure in water and the PMAcOD-coated NPs were studied using transmission electron microscopy, zeta-potential measurements, small-angle X-ray scattering, and fluorescence measurements. The combination of several techniques suggests that independently of the iron oxide core and oleic acid shell structures, PMAcOD encapsulates NPs, forming stable hydrophilic shells which withstand absorption of hydrophobic molecules, such as pyrene, without shell disintegration. Moreover, the PMAcOD molecules are predominantly attached to a single NP instead of self-assembling into the PMAcOD disklike nanostructures or attachment to several NPs. This leads to highly monodisperse aqueous samples with only a small fraction of NPs forming large aggregates due to cross-linking by the copolymer macromolecules.

Magnetic nanoparticles for gene and drug delivery

Investigations of magnetic micro- and nanoparticles for targeted drug delivery began over 30 years ago. Since that time, major progress has been made in particle design and synthesis techniques, however, very few clinical trials have taken place. Here we review advances in magnetic nanoparticle design, in vitro and animal experiments with magnetic nanoparticle-based drug and gene delivery, and clinical trials of drug targeting.

Micelles based on biodegradable poly(L-glutamic acid)-b-polylactide with paramagnetic Gd ions chelated to the shell layer as a potential nanoscale MRI-visible delivery system

There is much interest in the development of a nanoscale drug delivery system with MRI visibility to optimize the delivery efficiency and therapeutic efficacy under image guidance. Here we report on the successful fabrication of nanoscale micelles based on biodegradable poly( L-glutamic acid)- b-polylactide (PG- b-PLA) block copolymer with paramagnetic Gd3+ ions chelated to their shell. PG- b-PLA was synthesized by sequential polymerization reactions: anionic polymerization of L-lactide followed by ring-opening polymerization of benzyl glutamate N-carboxylic anhydride. The metal chelator p-aminobenzyldiethylenetriaminepenta(acetic acid) (DTPA) was readily conjugated to the side chain carboxylic acids of poly( L-glutamic acid). The resulting copolymer formed spherical micelles in aqueous solution with an average diameter of 230 nm at pH 7.4. The size of PG(DTPA)- b-PLA micelles decreased with increasing pH value. DTPA-Gd chelated to the shell layer of the micelles exhibited significantly higher spin-lattice relaxivity (r1) than a small-molecular-weight MRI contrast agent, indicating that water molecules could readily access the Gd ions in the micelles. Because of the presence of multiple carboxylic acid functional groups in the shell layer, polymeric micelles based on biodegradable PG(DTPA-Gd)- b-PLA may be a suitable platform for the development of MRI-visible, targeted nanoscale drug delivery systems.