Magnetic resonance imaging contrast agents: Overview and perspectives

A highly soluble gadofullerene salt and its magnetic properties

A stable complex of highly soluble Gd@C₈₂/TBPA with improved paramagnetic properties and extensive applications was investigated.

Chitosan-triphosphate nanoparticles for encapsulation of super-paramagnetic iron oxide as an MRI contrast agent

Super-paramagnetic iron oxide nanoparticles (SPIONPs) were encapsulated at various concentrations within chitosan-triphosphate (SPIONPs-CS) nanoparticles using an ionotropic gelation method. The encapsulation of SPIONPs within CS nanoparticles enhanced their dispersion ability in aqueous solution, with all particles being lower than 130 nm in size and having highly positive surface charge. The SPIONPs-CS nanoparticles exhibited crystalline structure and super-paramagnetic behavior, as seen in non-encapsulated SPIONPs. The morphology of SPIONPs-CS nanoparticles showed that they almost spherical in shape. The effect of phantom environments (culture medium and 3% agar solution) on either T1 or T2 weighted MRI was investigated using a clinical 1.5T MRI scanner. The results revealed that 3% agar solution showed relaxation values higher than the culture medium, leading to a significant decrease in the MR image intensity. Our results demonstrated that the SPIONPs-CS nanoparticles can be applied as tissue-specific MRI contrast agents.

Metal-based drugs

Metals have been considered for millennia to have medicinal values. With the advent of modern medicine, many metal-based drugs have proven to be highly effective in the clinic. Many different metal ions have shown activity against a range of diseases. The unique electronic structure of transition metals offers great versatility, not always seen in organic drugs, in terms of the ability to tune the properties of a given molecule. This review gives a brief overview of the most established therapeutic metals, and their more common applications, such as platinum-based anticancer drugs. New developments within the field of metallodrugs and novel strategies being employed to improve methods of delivery, are also discussed.

Acoustic characterization and contrast imaging of microbubbles encapsulated by polymeric shells coated or filled with magnetic nanoparticles

The combination of superparamagnetic iron oxide nanoparticles with polymeric air-filled microbubbles is used to produce two types of multimodal contrast agents to enhance medical ultrasound and magnetic resonance imaging. The nanoparticles are either covalently linked to the shell or physically entrapped into the shell. In this paper, the characterization of the acoustic properties (backscattered power, fracturing pressure, attenuation and dispersion of the ultrasonic wave) and ultrasound imaging of the two types of magnetic microbubbles are presented. In vitro B-mode images are generated using a medical ultrasound scanner by applying a nonconventional signal processing technique that is suitable to detect polymeric bubbles and based on the combination of multipulse excitation and chirp coding. Even if both types of microbubbles can be considered to be effective ultrasound contrast agents, the different structure of the shell loaded with nanoparticles has a pronounced effect on the echogenicity and the detection sensitivity of the imaging technique. The best results are obtained using microbubbles that are externally coated with nanoparticles. A backscattered power of 20 dB was achieved at lower concentration, and an increment of 8 dB in the contrast-to-tissue ratio was observed with respect to the more rigid microbubbles with particles entrapped into the shell.

Magnetic nanoparticles: surface effects and properties related to biomedicine applications

Due to finite size effects, such as the high surface-to-volume ratio and different crystal structures, magnetic nanoparticles are found to exhibit interesting and considerably different magnetic properties than those found in their corresponding bulk materials. These nanoparticles can be synthesized in several ways (e.g., chemical and physical) with controllable sizes enabling their comparison to biological organisms from cells (10–100 μm), viruses, genes, down to proteins (3–50 nm). The optimization of the nanoparticles’ size, size distribution, agglomeration, coating, and shapes along with their unique magnetic properties prompted the application of nanoparticles of this type in diverse fields. Biomedicine is one of these fields where intensive research is currently being conducted. In this review, we will discuss the magnetic properties of nanoparticles which are directly related to their applications in biomedicine. We will focus mainly on surface effects and ferrite nanoparticles, and on one diagnostic application of magnetic nanoparticles as magnetic resonance imaging contrast agents.

Magnetic polycarbonate microspheres for tumor-targeted delivery of tumor necrosis factor

OBJECTIVE

The specific expression of transferrin receptor can represent a diagnostic tool or therapeutic target in solid tumors expressing this antigen. Herein, the human transferrin receptor monoclonal antibody (T9) was investigated as a tumor-targeting group for active targeted-drug delivery systems.

MATERIALS AND METHODS

A tumor-targeted conjugate T9-TNF was synthesized by the attachment of both human transferrin receptor monoclonal antibody (T9) as a tumor-targeting group and human tumor necrosis factor-α (TNF) as an anti-cancer drug to two terminated hydroxyl groups of poly(ethylene glycol). Subsequently, a solvent evaporation technique was adopted to produce anti-cancer magnetic polymer microspheres T9-TNF-PC-M containing T9-TNF and Fe3O4 magnetic ultrafine powders (M) using poly(trimethylene carbonate-co-5,5-dimethyl trimethylene carbonate) (PC, P(TMC-co-DTC)) as a polymeric carrier.

RESULTS AND DISCUSSION

These magnetic polycarbonate microspheres possessed a steady TNF release rate in phosphate buffer saline solution, strong magnetic responsiveness and high T9-TNF loading capacity. In vitro cytotoxicity assays demonstrated the microspheres T9-TNF-PC-M and conjugate T9-TNF were strongly inhibitory to the human hepatic carcinoma (Bel-7204) cells. In vivo site-specific therapy in nude mice with human hepatic carcinoma indicated that the microspheres T9-TNF-PC-M and conjugate T9-TNF possessed markedly higher anti-tumor activity against Bel-7204 in mice than that of TNF.

CONCLUSIONS

These results indicated that the magnetic polycarbonate microspheres were suitable as the potential-targeted treatment for hepatic carcinoma therapeutics.

Encapsulation of particle ensembles in graphene nanosacks as a new route to multifunctional materials

Hybrid nanoparticles with multiple functions are of great interest in biomedical diagnostics, therapies, and theranostics but typically require complex multistep chemical synthesis. Here we demonstrate a general physical method to create multifunctional hybrid materials through aerosol-phase graphene encapsulation of ensembles of simple unifunctional nanoparticles. We first develop a general theory of the aerosol encapsulation process based on colloidal interactions within drying microdroplets. We demonstrate that a wide range of cargo particle types can be encapsulated, and that high pH is a favorable operating regime that promotes colloidal stability and limits nanoparticle dissolution. The cargo-filled graphene nanosacks are then shown to be open structures that rapidly release soluble salt cargoes when reintroduced into water, but can be partially sealed by addition of a polymeric filler to achieve slow release profiles of interest in controlled release or theranostic applications. Finally, we demonstrate an example of multifunctional material by fabricating graphene/Au/Fe3O4 hybrids that are magnetically responsive and show excellent contrast enhancement as multimodal bioimaging probes in both magnetic resonance imaging and X-ray computed tomography in full-scale clinical instruments.

High-relaxivity and luminescent silica nanoparticles as multimodal agents for molecular imaging

The design and synthesis of a new bimodal contrast agent for magnetic resonance imaging and optical imaging is reported. Tunable-sized silica nanoparticles were synthesized by a microemulsion-mediated pathway and used as carriers for paramagnetic and luminescent probes. The near-infrared luminescent agent was a ruthenium complex that was directly entrapped in the silica shell to provide photoluminescence enhancement and to make it highly photostable as it was protected from the surrounding environment. The paramagnetic activity came from a Gd-DTPA derivative that was grafted on the silica surface. NMRD profiles showed a strong relaxivity enhancement (increase of 432% in the r1 value at 20 MHz) when the paramagnetic complex was grafted at the nanoparticle surface, because of a reduction of its mobility. Polyethylene glycol was also grafted at the nanoparticle surface to enhance the nanoparticle residence time in the bloodstream. A thorough characterization of the material confirmed its potential as a very effective bimodal contrast agent.

Multifunctional Fe3O4-TiO2 nanocomposites for magnetic resonance imaging and potential photodynamic therapy

Multifunctional Fe(3)O(4)-TiO(2) nanocomposites with Janus structure for magnetic resonance imaging (MRI) and potential photodynamic therapy (PDT) were synthesized, in which Fe(3)O(4) was used as a MRI contrast agent and TiO(2) as an inorganic photosensitizer for PDT. Their morphology, structure, and MRI and PDT performance were characterized, respectively. Moreover, the location of Fe(3)O(4)-TiO(2) nanocomposites in MCF-7 cells was also investigated by the staining of Prussian blue and alizarin red, respectively. The results showed that the as-prepared Fe(3)O(4)-TiO(2) nanocomposites had good T(2)-weighted MRI performance, and the MCF-7 cells incubated with nanocomposites could be killed under the irradiation of UV light. Compared with traditional organic photosensitizers, TiO(2) inorganic photosensitizers could have more stable PDT performance due to their nanoscale size and anti-photodegradable stability. Therefore, the as-prepared Fe(3)O(4)-TiO(2) nanocomposites could have potential applications as a new kind of multifunctional agent for both MRI and PDT.

New simulation approach using classical formalism to water nuclear magnetic relaxation dispersions in presence of superparamagnetic particles used as MRI contrast agents

Superparamagnetic nanoparticles are used as negative contrast agents in magnetic resonance imaging: owing to their large magnetic moment the water proton spins are dephased, which accelerates the nuclear magnetic relaxation of an aqueous sample containing these particles. Transverse and longitudinal relaxation times depend on several parameters of the nanoparticles such as radius and magnetization and on experimental parameters such as the static magnetic field or echo time. In this work, we introduce a new simulation methodology, using a classical formalism, allowing the simulation of the NMR signal during transverse and longitudinal relaxation induced by superparamagnetic particles in an aqueous solution, which, to our knowledge has never been done before. Nuclear magnetic relaxation dispersion profiles are obtained for a wide range of nanoparticle radii and magnetizations. The results can be classified in two regimes--the well-known motional averaging and static regimes. This generalizes previous studies focusing on transverse relaxation at high magnetic field (larger than 1 T). Simulation results correspond to analytical theories in their validity range and so far unknown dependences of the relaxation with magnetization and radii of the NMR dispersions profiles are observed, which could be used to characterize experimental samples containing large superparamagnetic particles.

Tobacco mosaic virus rods and spheres as supramolecular high-relaxivity MRI contrast agents

To compensate for the low sensitivity of magnetic resonance imaging (MRI), nanoparticles have been developed to deliver high payloads of contrast agents to sites of disease. Here, we report the development of supramolecular MRI contrast agents using the plant viral nanoparticle tobacco mosaic virus (TMV). Rod-shaped TMV nanoparticles measuring 300×18 nm were loaded with up to 3,500 or 2,000 chelated paramagnetic gadolinium (III) ions selectively at the interior (iGd-TMV) or exterior (eGd-TMV) surface, respectively. Spatial control is achieved through targeting either tyrosine or carboxylic acid side chains on the solvent exposed exterior or interior TMV surface. The ionic T₁ relaxivity per Gd ion (at 60 MHz) increases from 4.9 mM^-1s^-1 for free Gd(DOTA) to 18.4 mM^-1s^-1 for eGd-TMV and 10.7 mM^-1s^-1 for iGd-TMV. This equates to T₁ values of ~ 30,000 mM^-1s^-1 and ~ 35,000 mM^-1s^-1 per eGd-TMV and iGd-TMV nanoparticle. Further, we show that interior-labeled TMV rods can undergo thermal transition to form 170 nm-sized spherical nanoparticles containing ~ 25,000 Gd chelates and a per particle relaxivity of almost 400,000 mM^-1s^-1 (15.2 mM^-1s^-1 per Gd). This work lays the foundation for the use of TMV as a contrast agent for MRI.

Preclinical molecular imaging using PET and MRI

Molecular imaging fundamentally changes the way we look at cancer. Imaging paradigms are now shifting away from classical morphological measures towards the assessment of functional, metabolic, cellular, and molecular information in vivo. Interdisciplinary driven developments of imaging methodology and probe molecules utilizing animal models of human cancers have enhanced our ability to non-invasively characterize neoplastic tissue and follow anti-cancer treatments. Preclinical molecular imaging offers a whole palette of excellent methodology to choose from. We will focus on positron emission tomography (PET) and magnetic resonance imaging (MRI) techniques, since they provide excellent and complementary molecular imaging capabilities and bear high potential for clinical translation. Prerequisites and consequences of using animal models as surrogates of human cancers in preclinical molecular imaging are outlined. We present physical principles, values and limitations of PET and MRI as molecular imaging modalities and comment on their high potential to non-invasively assess information on hypoxia, angiogenesis, apoptosis, gene expression, metabolism, and cell trafficking in preclinical cancer research.

Gd(III)-DOTA-modified sonosensitive liposomes for ultrasound-triggered release and MR imaging

Ultrasound-sensitive (sonosensitive) liposomes for tumor targeting have been studied in order to increase the antitumor efficacy of drugs and decrease the associated severe side effects. Liposomal contrast agents having Gd(III) are known as a nano-contrast agent system for the efficient and selective delivery of contrast agents into pathological sites. The objective of this study was to prepare Gd(III)-DOTA-modified sonosensitive liposomes (GdSL), which could deliver a model drug, doxorubicin (DOX), to a specific site and, at the same time, be capable of magnetic resonance (MR) imaging. The GdSL was prepared using synthesized Gd(III)-DOTA-1,2-distearoyl-sn-glycero-3-phosphoethanolamine lipid. Sonosensitivity of GdSL to 20-kHz ultrasound induced 33% to 40% of DOX release. The relaxivities (r1) of GdSL were 6.6 to 7.8 mM-1 s-1, which were higher than that of MR-bester®. Intracellular uptake properties of GdSL were evaluated according to the intensity of ultrasound. Intracellular uptake of DOX for ultrasound-triggered GdSL was higher than that for non-ultrasound-triggered GdSL. The results of our study suggest that the paramagnetic and sonosensitive liposomes, GdSL, may provide a versatile platform for molecular imaging and targeted drug delivery.

Polyaspartamide spin probes containing isoindoline nitroxide and porphyrin groups

Water-soluble polyaspartamide isoindoline nitroxides 5-(4′-aminophenyl)-10,15,20-tris (4′-sulfonatophenyl) porphyrin, trisodium salt–poly[α,β- N-(2-hydroxyethyl)-l-aspartamide]–5-carboxy-1,1,3,3-tetramethylisoindolin-2-yloxyl were synthesized by the incorporation of 5-(4′-aminophenyl)-10,15,20-tris(4′-sulfonatophenyl) porphyrin, trisodium salt, as a tumor-targeting group, and 5-carboxy-1,1,3,3-tetramethylisoindolin-2-yloxyl into poly[α,β- N-(2-hydroxyethyl)-l-aspartamide]. These compounds were characterized, and the in vitro properties were evaluated. The polyaspartamide isoindoline nitroxides had higher relaxation effectiveness and had greater toxicity to HeLa cells than that of 5-carboxy-1,1,3,3-tetramethylisoindolin-2-yloxyl. The polyaspartamide isoindoline nitroxides retained similar electrochemical properties and redox reaction mechanisms as the parent nitroxides. The electron paramagnetic resonance spectra of polyaspartamide isoindoline nitroxides exhibited characteristic hyperfine electron paramagnetic resonance spectra of tetramethyl isoindoline nitroxides, with typical nitroxide g-values and nitrogen isotropic hyperfine coupling constants. Therefore, the water-soluble 5-(4′-aminophenyl)-10,15,20-tris(4′-sulfonatophenyl) porphyrin, trisodium salt–poly[α,β- N-(2-hydroxyethyl)-l-aspartamide]–5-carboxy-1,1,3,3-tetramethylisoindolin-2-yloxyl is considered to be a novel potential spin probe for electron paramagnetic resonance.

Magnetite nanoparticles can be coupled to microbubbles to support multimodal imaging

Microbubbles (MBs) are commonly used as injectable ultrasound contrast agent (UCA) in modern ultrasonography. Polymer-shelled UCAs present additional potentialities with respect to marketed lipid-shelled UCAs. They are more robust; that is, they have longer shelf and circulation life, and surface modifications are quite easily accomplished to obtain enhanced targeting and local drug delivery. The next generation of UCAs will be required to support not only ultrasound-based imaging methods but also other complementary diagnostic approaches such as magnetic resonance imaging or computer tomography. This work addresses the features of MBs that could function as contrast agents for both ultrasound and magnetic resonance imaging. The results indicate that the introduction of iron oxide nanoparticles (SPIONs) in the poly(vinyl alcohol) shell or on the external surface of the MBs does not greatly decrease the echogenicity of the host MBs compared with the unmodified one. The presence of SPIONs provides enough magnetic susceptibility to the MBs to accomplish good detectability both in vitro and in vivo. The distribution of SPIONs on the shell and their aggregation state seem to be key factors for the optimization of the transverse relaxation rate.

Nanotechnology in medicine: from inception to market domination

Born from the marriage of nanotechnology and medicine, nanomedicine is set to bring advantages in the fight against unmet diseases. The field is recognized as a global challenge, and countless worldwide research and business initiatives are in place to obtain a significant market position. However, nanomedicine belongs to those emerging sectors in which business development methods have not been established yet. Open issues include which type of business model best fits these companies and which strategies would lead them to sustained growth. This paper describes the financial and strategic decisions by nanomedicine start-ups to reach the market successfully, obtain a satisfactory market share, and build and maintain a competitive defendable advantage. Walking nanomedicine-product from the hands of the inventor to those of the doctor, we explored the technological transfer process, which connects laboratories or research institutions to the marketplace. The process involves detailed analysis to evaluate the potentials of end-products, and researches to identify market segment, size, structure, and competitors, to ponder a possible market entry and the market share that managers can realistically achieve at different time horizons. Attracting funds is crucial but challenging. However, investors are starting to visualize the potentials of this field, magnetized by the business of “nano.”

Preparation of amyloid-like fibrils containing magnetic iron oxide nanoparticles: effect of protein aggregation on proton relaxivity

A method to prepare amyloid-like fibrils functionalized with magnetic nanoparticles has been developed. The amyloid-like fibrils are prepared in a two step procedure, where insulin and magnetic nanoparticles are mixed simply by grinding in the solid state, resulting in a water soluble hybrid material. When the hybrid material is heated in aqueous acid, the insulin/nanoparticle hybrid material self assembles to form amyloid-like fibrils incorporating the magnetic nanoparticles. This results in magnetically labeled amyloid-like fibrils which has been characterized by Transmission Electron Microscopy (TEM) and electron tomography. The influence of the aggregation process on proton relaxivity is investigated. The prepared materials have potential uses in a range of bio-imaging applications.

Magnetoliposomes as multimodal contrast agents for molecular imaging and cancer nanotheragnostics

In the emerging field of molecular and cellular imaging, flexible strategies to synthesize multimodal contrast agents with targeting ligands are required. Liposomes have the ability to combine with a large variety of nanomaterials, including superparamagnetic iron oxide nanoparticles, to form magnetoliposomes (MLs). MLs can be used as highly efficient MRI contrast agents. Owing to their high flexibility, MLs can be associated with other imaging modality probes to be used as multimodal contrast agents. By using a thermosensitive lipid bilayer in the ML structure, these biocompatible systems offer many possibilities for targeting and delivering therapeutic agents for ‘theragnostics’, a coincident therapy and diagnosis strategy. This article deals with the fast-growing field of MLs as biomedical diagnostic tools. Different kinds of MLs, their preparation methods, as well as their surface modification with different imaging probes, are discussed. ML applications as multimodal contrast agents and in theragnostics are reviewed. Some important issues for the biomedical uses of magnetic liposomes, such as toxicity, are summarized.

A biotechnological perspective on the application of iron oxide magnetic colloids modified with polysaccharides

Iron oxide magnetic nanoparticles (MNPs) alone are suitable for a broad spectrum of applications, but the low stability and heterogeneous size distribution in aqueous medium represent major setbacks. These setbacks can however be reduced or diminished through the coating of MNPs with various polymers, especially biopolymers such as polysaccharides. Polysaccharides are biocompatible, non-toxic and renewable; in addition, they possess chemical groups that permit further functionalization of the MNPs. Multifunctional entities can be created through decoration with specific molecules e.g. proteins, peptides, drugs, antibodies, biomimetic ligands, transfection agents, cells, and other ligands. This development opens a whole range of applications for iron oxide nanoparticles. In this review the properties of magnetic structures composed of MNPs and several polysaccharides (Agarose, Alginate, Carrageenan, Chitosan, Dextran, Heparin, Gum Arabic, Pullulan and Starch) will be discussed, in view of their recent and future biomedical and biotechnological applications.

Anticancer drug-loaded nanospheres based on biodegradable amphiphilic ε-caprolactone and carbonate copolymers

PURPOSE

The aim was to investigate anticancer drug-loaded poly(carbonate-ester) nanospheres as potential drug delivery systems for cancer therapy.

METHODS

Functional poly(carbonate-ester) copolymers (HPCP-SD) were synthesized by the incorporation of sulfadiazine as the tumor-targeting groups to hydroxyl groups of poly(carbonate-ester) copolymers. Two types of anticancer drug-loaded poly(carbonate-ester) nanospheres I and II were further prepared by dialysis method and high-voltage electrostatic field-assisted atomization, respectively, using HPCP-SD as polymeric carriers. These carriers and anticancer drug-loaded nanospheres were characterized, and their properties in vitro and in vivo were evaluated.

RESULTS

These anticancer drug-loaded poly(carbonate-ester) nanospheres had steady drug release rates and good controlled release properties. Moreover, anticancer drug-loaded poly(carbonate-ester) nanospheres II had faster drug release rates than those of anticancer drug-loaded nanospheres I. These anticancer drug-loaded nanospheres possessed lower cytotoxicity to HEK 293 cells and exhibited obviously higher anticancer efficiencies to the HeLa tumor cells than that of 5-fluorouracil. Anticancer drug-loaded nanospheres I possessed lower cytotoxicity to HEK 293 cells and higher anticancer activity to HeLa cells than those of anticancer drug-loaded nanospheres II.

CONCLUSIONS

These anticancer drug-loaded poly(carbonate-ester) nanospheres showed the potential as drug delivery systems for cancer therapy.