Synthesis and characterization of ethosomal contrast agents containing iodine for computed tomography (CT) imaging applications

As a first step in the development of novel liver-specific contrast agents using ethosomes for computed tomography (CT) imaging applications, we entrapped iodine within ethosomes, which are phospholipid vesicular carriers containing relatively high alcohol concentrations, synthesized using several types of alcohol, such as methanol, ethanol, and propanol. The iodine containing ethosomes that were prepared using methanol showed the smallest vesicle size (392?nm) and the highest CT density (1107 HU). The incorporation of cholesterol into the ethosomal contrast agents improved the stability of the ethosomes but made the vesicle size large. The ethosomal contrast agents were taken up well by macrophage cells and showed no cellular toxicity. The results demonstrated that ethosomes containing iodine, as prepared in this study, have potential as contrast agents for applications in CT imaging.     

Molecular imaging: The latest generation of contrast agents and tissue characterization techniques

Molecular Imaging technologies will have a profound impact on both basic research and clinical imaging in the near future. As the field covers many different specialties and scientific disciplines it is not possible to review all in a single article. In the current article we will turn our attention to those modalities that are either currently in use or in development for the medical imaging clinic.     

Inorganic nanoparticle-based contrast agents for molecular imaging

Inorganic nanoparticles (NPs) including semiconductor quantum dots (QDs), iron oxide NPs and gold NPs have been developed as contrast agents for diagnostics by molecular imaging. Compared with traditional contrast agents, NPs offer several advantages: their optical and magnetic properties can be tailored by engineering the composition, structure, size and shape; their surfaces can be modified with ligands to target specific biomarkers of disease; the contrast enhancement provided can be equivalent to millions of molecular counterparts; and they can be integrated with a combination of different functions for multimodal imaging. Here, we review recent advances in the development of contrast agents based on inorganic NPs for molecular imaging, and also touch on contrast enhancement, surface modification, tissue targeting, clearance and toxicity. As research efforts intensify, contrast agents based on inorganic NPs that are highly sensitive, target-specific and safe to use are expected to enter clinical applications in the near future.     

Molecular magnetic resonance imaging with targeted contrast agents

Magnetic resonance imaging (MRI) produces high-resolution three-dimensional maps delineating morphological features of the specimen. Differential contrast in soft tissues depends on endogenous differences in water content, relaxation times, and/or diffusion characteristics of the tissue of interest. The specificity of MRI can be further increased by exogenous contrast agents (CA) such as gadolinium chelates, which have been successfully used for imaging of hemodynamic parameters including blood perfusion and vascular permeability. Development of targeted MR CA directed to specific molecular entities could dramatically expand the range of MR applications by combining the noninvasiveness and high spatial resolution of MRI with specific localization of molecular targets. However, due to the intrinsically low sensitivity of MRI (in comparison with nuclear imaging), high local concentrations of the CA at the target site are required to generate detectable MR contrast. To meet these requirements, the MR targeted CA should recognize targeted cells with high affinity and specificity. They should also be characterized by high relaxivity, which for a wide variety of CA depends on the number of contrast-generating groups per single molecule of the agent. We will review different designs and applications of targeted MR CA and will discuss feasibility of these approaches for in vivo MRI.     

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.     

Water-soluble polysaccharides as carriers of paramagnetic contrast agents for magnetic resonance imaging: synthesis and relaxation properties.

Water-soluble, carbohydrate-based, paramagnetic metal chelate derivatives have been investigated as potential organ-selective contrast media for magnetic resonance imaging (m.r.i.). The in vitro proton spin-lattice relaxation properties of compounds with different paramagnetic metals, chelating agents, and carbohydrate matrixes have been studied. Typically, these complexes were 60-260% more efficient proton-relaxation agents than the corresponding low-molecular-weight metal chelates at 10 MHz, but less efficient than the corresponding protein derivatives. As expected, carbohydrates that contained manganese or gadolinium were more effective relaxation agents than iron, copper, erbium, or nickel derivatives.     

Peptidyl molecular imaging contrast agents using a new solid-phase peptide synthesis approach

A versatile method is disclosed for solid-phase peptide synthesis (SPPS) of molecular imaging contrast agents. A DO3A moiety was derivatized to introduce a CBZ-protected amino group and then coupled to a polymeric support. CBZ cleavage with Et2AlCl/thioanisole was optimized for SPPS. Amino acids were then coupled to the aminoDOTA-loaded resin using conventional stepwise Fmoc SPPS to create a product with DOTA coupled to the C-terminus of the peptide. In a second study, the DO3A moiety was coupled to a glycine-loaded polymeric support, and amino acids were then coupled to the amino-DOTA-peptide-loaded resin using SPPS to incorporate DOTA within the peptide sequence. The peptide-(Tm3+-DOTA) amide showed a paramagnetic chemical exchange saturation transfer (PARACEST) effect, which demonstrated the utility of this contrast agent for molecular imaging. These results demonstrate the advantages of exploiting SPPS methodologies through development of unique DOTA derivatives to create peptide-based molecular imaging contrast agents.     

Polymeric gadolinium chelate magnetic resonance imaging contrast agents: design, synthesis, and properties.

We have synthesized and evaluated five series of polymeric gadolinium chelates which are of interest as potential MRI blood pool contrast agents. The polymers were designed so that important physical properties including molecular weight, relaxivity, metal content, viscosity, and chelate stability could be varied. We have shown that, by selecting polymers of the appropriate MW, extended blood pool retention can be achieved. In addition, relaxivity can be manipulated by changing the polymer rigidity, metal content affected by monomer selection, viscosity by polymer shape, and chelate stability by chelator selection.     

Dendritic MRI contrast agents: an efficient prelabeling approach based on CuAAC

The Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) allows the efficient and complete functionalization of dendrimers with preformed Gd chelates (prelabeling) to give monodisperse macromolecular contrast agents (CAs) for magnetic resonance imaging (MRI). This monodispersity contrasts with the typical distribution of materials obtained by classical routes and facilitates the characterization and quality control demanded for clinical applications. The potential of a new family of PEG-dendritic CA based on a gallic acid-triethylene glycol (GATG) core functionalized with up to 27 Gd complexes has been explored in vitro and in vivo, showing contrast enhancements similar to those of Gadomer-17, which reveals them to be a promising platform for the development of CA for MRI.     

Paramagnetic oligonucleotides: contrast agents for magnetic resonance imaging with proton relaxation enhancement effects

An antisense paramagnetic oligonucleotide analogue targeted to a model macromolecular receptor (5S rRNA) was prepared. The paramagnetic agent's relaxivity (dependence of the relaxation rate on paramagnetic agent concentration) in the presence and absence of the macromolecular receptor was measured at 1.5 and 6.3 T. The relaxivity of the targeted agent increased specifically in the presence of the macromolecular receptor (16% at 6.3 T and 15% at 1.5 T). This effect was specific for a paramagnetic oligonucleotide targeted to the receptor and was larger than the relaxivity enhancement due simply to receptor-induced viscosity differences. Maximizing this relaxivity enhancement of tumor targeted paramagnetic oligonucleotides will aid in contrast agent development for magnetic resonance imaging.     

Fluorescence-enhanced,near infrared diagnostic imaging with contrast agents

The deep tissue propagation of near-infrared (NIR) light between 700-900 nm offers new opportunities for diagnostic imaging when employing sensitive detection techniques and NIR excitable fluorescent agents that target and report disease and metabolism. Herein, we highlight approaches for illuminating tissues and monitoring the re-emitted fluorescence for tomographic reconstruction, strategies for developing fluorescent dye constructs, and clinical opportunities for fluorescence-enhanced NIR optical imaging.     

Conformation and structure of polymeric contrast agents for medical imaging

The structure of Gd-DTPA-polylysine, Gd-DOTA-polylysine, Gd-SCN-Bz-DOTA-polylysine, and Gd-DTPA-poly(glu:lys) was investigated with circular dichroism, gel permeation chromatography, low angle light scattering, and proton longitudinal relaxivity. Molecular modeling calculations were performed and predicted helical secondary structure for charged Gd-chelator residues, i.e., Gd-DTPA, when the DTPA conjugation levels reached 90% and higher. This helical secondary structure was observed with circular dichroism. The conformational transition from coiled to extended linear was observed also by gel permeation chromatography and by proton relaxivity measurements. The helical secondary structure was not observed when the chelator was changed to DOTA. The residue charge interactions were eliminated in this case since the Gd-DOTA complex had no net charge. For this construct, the gel permeation and relaxivity measurements indicated a coiled conformation. An extended linear conformation was regained when the chelator complex was changed to Gd-SCN-Bz-DOTA, which had a net negative charge. The functional aspects of these structures were investigated by MR imaging of an animal tumor model. The linear extended polymer constructs gave 10-fold higher tumor signals then the coiled-collapsed constructs, indicating a much higher degree of trans-endothelial transport in the tumors.     

Promising strategies for Gd-based responsive magnetic resonance imaging contrast agents

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Highlights► Responsive MRI contrast agents enable the study of biochemical events. ► Recent Gd-based contrast agents are reviewed here. ► Responsive agents act by modulating hydration state, molecular tumbling or number of metal centres. ► Promising strategies for future probe design are identified.     

Semiconductor quantum dots as contrast agents for whole animal imaging

Recent developments in quantum dot (QD) technology have paved the way for using QDs as optical contrast agents for in vivo imaging. Pioneering papers showed the use of QDs as luminescent contrast agents for imaging cancer and guiding cancer surgery. The possible future use of QDs for clinical applications is expected to have a significant impact, however many challenges in this field have yet to be overcome.     

Aptamer-targeted gold nanoparticles as molecular-specific contrast agents for reflectance imaging

Targeted metallic nanoparticles have shown potential as a platform for development of molecular-specific contrast agents. Aptamers have recently been demonstrated as ideal candidates for molecular targeting applications. In this study, we investigated the development of aptamer-based gold nanoparticles as contrast agents, using aptamers as targeting agents and gold nanoparticles as imaging agents. We devised a novel conjugation approach using an extended aptamer design where the extension is complementary to an oligonucleotide sequence attached to the surface of the gold nanoparticles. The chemical and optical properties of the aptamer-gold conjugates were characterized using size measurements and oligonucleotide quantitation assays. We demonstrate this conjugation approach to create a contrast agent designed for detection of prostate-specific membrane antigen (PSMA), obtaining reflectance images of PSMA(+) and PSMA(-) cell lines treated with the anti-PSMA aptamer-gold conjugates. This design strategy can easily be modified to incorporate multifunctional agents as part of a multimodal platform for reflectance imaging applications.