Metal-based environment-sensitive MRI contrast agents

Interactions of paramagnetic metal complexes with their biological environment can modulate their magnetic resonance imaging (MRI) contrast–enhancing properties in different ways, and this has been widely exploited to create responsive probes that can provide biochemical information. We survey progress in two rapidly growing areas: the MRI detection of biologically important metal ions, such as calcium, zinc, and copper, and the use of transition metal complexes as smart MRI agents. In both fields, new imaging technologies, which take advantage of other nuclei (¹⁹F) and/or paramagnetic contact shift effects, emerge beyond classical, relaxation-based applications. Most importantly, in vivo imaging is gaining ground, and the promise of molecular MRI is becoming reality, at least for preclinical research.     

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.     

Gadonanotubes as new high-performance MRI contrast agents

Gadolinium-based carbon nanostructures are poised to make a significant impact as advanced contrast agents (CAs) for magnetic resonance imaging (MRI) in medicine. This paper reviews and forecasts gadonanotubes as synthons for the design of high-performance MRI CA probes with efficacies up to 100 times greater than current clinical CAs. This level of performance is vital for achieving the goal of cellular and molecular imaging with MRI. These new materials will be useful for in vivo MRI applications as circulating drug nanocapsules because of their low toxicities, extremely high relaxivities, and potential for cellular targeting and induced cell death by magnetic hyperthermia.     

Paramagnetic contrast agents in magnetic resonance imaging: research at Vanderbilt University

Experimental studies in animals have demonstrated the application of particulate and chelated paramagnetic oral contrast agents in magnetic resonance imaging (at 0.5 tesla). The ability of a soluble paramagnetic species, ferrous gluconate, to improve imaging studies of the pancreas presently is being evaluated in clinical trials. Two paramagnetic metal ion chelates, Cr EDTA and Gd DTPA, have been evaluated extensively as potential intravascular contrast agents. Renal function, tissue vascularity, abnormalities of the blood-brain barrier, and infarction of myocardial tissue may all be assessed with IV contrast enhanced magnetic resonance imaging. The contrast materials tested all represent first generation compounds. Improved relaxation characteristics, toxicity, distribution, and flexibility will result from development of second generation agents, primarily within the particulate and chelate classes.     

MRI contrast agents for functional molecular imaging of brain activity

Functional imaging with MRI contrast agents is an emerging experimental approach that can combine the specificity of cellular neural recording techniques with noninvasive whole-brain coverage. A variety of contrast agents sensitive to aspects of brain activity have recently been introduced. These include new probes for calcium and other metal ions that offer high sensitivity and membrane permeability, as well as imaging agents for high-resolution pH and metabolic mapping in living animals. Genetically encoded MRI contrast agents have also been described. Several of the new probes have been validated in the brain; in vivo use of other agents remains a challenge. This review outlines advantages and disadvantages of specific molecular imaging approaches and discusses current or potential applications in neurobiology.     

Paramagnetic liposomes as innovative contrast agents for magnetic resonance (MR) molecular imaging applications

This article illustrates some innovative applications of liposomes loaded with paramagnetic lanthanide-based complexes in MR molecular imaging field. When a relatively high amount of a Gd(III) chelate is encapsulated in the vesicle, the nanosystem can simultaneously affect both the longitudinal (R(1)) and the transverse (R(2)) relaxation rate of the bulk H2O H-atoms, and this finding can be exploited to design improved thermosensitive liposomes whose MRI response is not longer dependent on the concentration of the probe. The observation that the liposome compartmentalization of a paramagnetic Ln(III) complex induce a significant R(2) enhancement, primarily caused by magnetic susceptibility effects, prompted us to test the potential of such agents in cell-targeting MR experiments. The results obtained indicated that these nanoprobes may have a great potential for the MR visualization of cellular targets (like the glutamine membrane transporters) overexpressing in tumor cells. Liposomes loaded with paramagnetic complexes acting as NMR shift reagents have been recently proposed as highly sensitive CEST MRI agents. The main peculiarity of CEST probes is to allow the MR visualization of different agents present in the same region of interest, and this article provides an illustrative example of the in vivo potential of liposome-based CEST agents.     

Dendrimer-based macromolecular MRI contrast agents: characteristics and application

Numerous macromolecular MRI contrast agents prepared employing relatively simple chemistry may be readily available that can provide sufficient enhancement for multiple applications. These agents operate using a approximately 100-fold lower concentration of gadolinium ions in comparison to the necessary concentration of iodine employed in CT imaging. Herein, we describe some of the general potential directions of macromolecular MRI contrast agents using our recently reported families of dendrimer-based agents as examples. Changes in molecular size altered the route of excretion. Smaller-sized contrast agents less than 60 kDa molecular weight were excreted through the kidney resulting in these agents being potentially suitable as functional renal contrast agents. Hydrophilic and larger-sized contrast agents were found better suited for use as blood pool contrast agents. Hydrophobic variants formed with polypropylenimine diaminobutane dendrimer cores created liver contrast agents. Larger hydrophilic agents are useful for lymphatic imaging. Finally, contrast agents conjugated with either monoclonal antibodies or with avidin are able to function as tumor-specific contrast agents, which also might be employed as therapeutic drugs for either gadolinium neutron capture therapy or in conjunction with radioimmunotherapy.     

PEGylation of protein-based MRI contrast agents improves relaxivities and biocompatibilities

Magnetic resonance imaging (MRI) has emerged as a leading diagnostic technique in clinical and preclinical settings. However, the application of MRI to assess specific disease markers for diagnosis and monitoring drug effect has been severely hampered by the lack of desired contrast agents with high relaxivities, and optimized in vivo retention time. We have reported the development of protein-based MRI contrast agents (ProCA1) by rational design of Gd^3 + binding sites into a stable protein resulting in significantly increased longitudinal (r₁) and transverse (r₂) relaxivities compared to Gd-DTPA. Here, we report a further improvement of protein contrast agents ProCA1 for in vivo imaging by protein modification with various sizes of polyethylene glycol (PEG) chain. PEGylation results in significant increases of both r₁ and r₂ relaxivities (up to 200%), and these high relaxivities persist even at field strengths up to 9.4 T. In addition, our experimental results demonstrate that modified contrast agents have significant improvement of in vivo MR imaging and biocompatibilities including dose efficiency, protein solubility, blood retention time and decreased immunogenicity. Such improvement can be important to the animal imaging and pre-clinical research at high or ultra-high field where there is an urgent need for molecular imaging probes and optimized contrast agent.     

Development of hypoxia-sensitive Gd3+-based MRI contrast agents

Hypoxia occurs in various diseases, including cancer, ischemia, and acute and chronic vascular diseases. Here we describe the design and synthesis of the first hypoxia-sensitive MRI contrast agents, SAGds. SAGds showed a pH-dependent r(1) relaxivity change associated with intramolecular chelation of the nitrogen atom of the sulfonamide moiety to the Gd(3+) center. There was a correlation between the pK(a) of the r(1) relaxivity change and the sum of the Hammett σ constants of substituents on the aromatic ring. Among the synthesized compounds, 4NO(2)2MeOSAGd was selectively reduced to the amine by rat liver microsomes under hypoxic conditions, resulting in a 1.8-fold increment of the r(1) relaxivity owing to the change in pK(a) of the arylsulfonamide moiety. This enhancement of the r(1) relaxivity could be clearly detected in T(1)-weighted MR images. Thus, 4NO(2)2MeOSAGd is a 'smart' MRI contrast agent for the detection of hypoxia under physiological conditions.     

Comparison of divalent transition metal ion paraCEST MRI contrast agents

Transition-metal-ion-based paramagnetic chemical exchange saturation transfer (paraCEST) agents are a promising new class of compounds for magnetic resonance imaging (MRI) contrast. Members in this class of compounds include paramagnetic complexes of Fe^II, Co^II, and Ni^II. The development of the coordination chemistry for these paraCEST agents is presented with an emphasis on the choice of the azamacrocycle backbone and pendent groups with the goals of controlling the oxidation state, spin state, and stability of the complexes. Chemical exchange saturation transfer spectra and images are compared for different macrocyclic complexes containing amide or heterocyclic pendent groups. The potential of paraCEST agents that function as pH- and redox-activated MRI probes is discussed.     

Dynamic contrast enhanced MRI of mouse fibrosarcoma using small-molecular and novel macromolecular contrast agents

The aim of the study was a comparison of 2 novel macromolecular contrast agents, Gadomer-17 and Polylysine-Gd-DTPA, with commercially available Gd-DTPA in determining the quality of tumor microvasculature by dynamic contrast enhanced MRI. Three groups of 5 mice with SA-1 tumors were studied. To each group of animals one contrast agent was administered; i.e. the first group got Gd-DTPA, the second group Gadomer-17 and the third group Polylysine-Gd-DTPA. To perform dynamic contrast enhanced MRI a standard keyhole approach was used by which consecutive signal intensity change due to contrast agent accumulation in the tumor was measured. From the obtained data, tissue permeability surface area product PS and fractional blood volume BV were calculated on a pixel-by-pixel basis. PS and BV values were calculated for each contrast agent. Based on the values, contrast agents were classified according to their performance in characterizing tumor microvasculature. Results of our study suggest that Gadomer-17 and Polylysine-Gd-DTPA are significantly superior to Gd-DTPA in characterizing tumor microvasculature.     

Switchable MRI contrast agents based on morphological changes of pH-responsive polymers

Magnetic resonance imaging (MRI) contrast agents are effective tools in both medical diagnosis and life science research. Various smart contrast agents have been developed for the visualization of biological phenomena. These contrast agents have molecular switches that increase or reduce MRI signal intensity in response to the target biological reaction. Therefore, novel approaches to the design of molecular switches for versatile in vivo studies using MRI are eagerly anticipated. Here, we report one such approach for the development of molecular switches based on morphological changes of pH-responsive polymers. We designed and synthesized three types of contrast agents based on a linear homopolymer or spherical copolymers with two different cross-linking degrees. The relaxivity measurements showed that these agents have molecular switches that respond to pH changes, and fluorescence studies indicated that these switches are based on the alteration of the molecular tumbling caused by pH-responsive morphological changes. As a result, the spherical polymers possess promising characteristics for the development of switchable MRI contrast agents.     

Synthesis and evaluation of Gd-DTPA-labeled arabinogalactans as potential MRI contrast agents

Arabinogalactan derivatives conjugated with gadolinium-diethylenetriaminepentaacetic acid (Gd-DTPA) by ethylenediamine (Gd-DTPA-CMAG-A2) or hexylamine (Gd-DTPA-CMAG-A6) have been synthesized and characterized by means of Fourier transform infrared spectra (FTIR), 13C nuclear magnetic resonance (13C NMR), size exclusion chromatography (SEC), and inductively coupled plasma atomic emission spectrometry (ICP-AES). Relaxivity studies showed that arabinogalactan-bound complexes possessed higher relaxation effectiveness compared with the clinically used Gd-DTPA, and the influence of the spacer arm lengths on the T1 relaxivities was studied. Their stability was investigated by competition study with Ca2+, EDTA, and DTPA. MR imaging of Wistar rats showed remarkable enhancement in rat liver and kidney after i.v. injection of Gd-DTPA-CMAG-A2 (0.079+/-0.002 mmol/kg Gd3+): The mean percentage enhancement of the liver parenchyma and kidney was 38.7+/-6.4% and 69.4+/-4.4% at 10-30 min. Our preliminary in vivo and in vitro study indicates that the arabinogalactan-bound complexes are potential liver-specific contrast agents for MRI.     

Synthesis and characterization of multifunctional hyperbranched polyesters as prospective contrast agents for targeted MRI

Based on a commercially available hyperbranched aliphatic polyester, novel multifunctional gadolinium complexes were prepared bearing protective PEG chains, a folate targeting ligand and EDTA or DTPA chelate moieties. Their relatively high water relaxivity values coupled with biodegradability of the hyperbranched scaffold, folate receptor specificity render these non-toxic dendritic polymers promising candidates for MRI applications.     

Differential uptake of MRI contrast agents indicates charge-selective blood-brain interface in the crayfish

This study provides a new perspective on the long-standing problem of the nature of the decapod crustacean blood-brain interface. Previous studies of crustacean blood-brain interface permeability have relied on invasive histological, immunohistochemical and electrophysiological techniques, indicating a leaky non-selective blood-brain barrier. The present investigation involves the use of magnetic resonance imaging (MRI), a method for non-invasive longitudinal tracking of tracers in real-time. Differential uptake rates of two molecularly distinct MRI contrast agents, namely manganese (Mn(II)) and Magnevist? (Gd-DTPA), were observed and quantified in the crayfish, Cherax destructor. Contrast agents were injected into the pericardium and uptake was observed with longitudinal MRI for approximately 14.5?h. Mn(II) was taken up quickly into neural tissue (within 6.5?min), whereas Gd-DTPA was not taken up into neural tissue and was instead restricted to the intracerebral vasculature or excreted into nearby sinuses. Our results provide evidence for a charge-selective intracerebral blood-brain interface in the crustacean nervous system, a structural characteristic once considered too complex for a lower-order arthropod.     

Development of contrast agents targeted to macrophage scavenger receptors for MRI of vascular inflammation

Atherosclerosis is a leading cause of death in the U.S. Because there is a potential to prevent coronary and arterial disease through early diagnosis, there is a need for methods to image arteries in the subclinical stage as well as clinical stage using various noninvasive techniques, including magnetic resonance imaging (MRI). We describe a development of a novel MRI contrast agent targeted to plaques that will allow imaging of lesion formation. The contrast agent is directed to macrophages, one of the earliest components of developing plaques. Macrophages are labeled through the macrophage scavenger receptor A, a macrophage specific cell surface protein, using an MRI contrast agent derived from scavenger receptor ligands. We have synthesized and characterized these contrast agents with a range of relaxivities. In vitro studies show that the targeted contrast agent accumulates in macrophages, and solution studies indicate that micromolar concentrations are sufficient to produce contrast in an MR image. Cell toxicity and initial biodistribution studies indicate low toxicity, no detectable retention in normal blood vessels, and rapid clearance from blood. The promising performance of this contrast agent targeted toward vascular inflammation opens doors to tracking of other inflammatory diseases such as tumor immunotherapy and transplant acceptance using MRI.