Contrast enhancement by differently sized paramagnetic MRI contrast agents in mice with two phenotypes of atherosclerotic plaque

Interest in the use of contrast‐enhanced MRI to enable in vivo specific characterization of atherosclerotic plaques is increasing. In this study the intrinsic ability of three differently sized gadolinium‐based contrast agents to permeate different mouse plaque phenotypes was evaluated with MRI. A tapered cast was implanted around the right carotid artery of apoE^?/? mice to induce two different plaque phenotypes: a thin cap fibroatheroma (TCFA) and a non‐TCFA lesion. Both plaques were allowed to develop over 6 and 9 weeks, leading to an intermediate and advanced lesion, respectively. Signal enhancement in the carotid artery wall, following intravenous injection of Gd‐HP‐DO3A as well as paramagnetic micelles and liposomes was evaluated. In vivo T₁‐weighted MRI plaque enhancement characteristics were complemented by fluorescence microscopy and correlated to lesion phenotype. The two smallest contrast agents, i.e. Gd‐HP‐DO3A and micelles, were found to enhance contrast in T₁‐weighted MR images of all investigated plaque phenotypes. Maximum contrast enhancement ranged between 53 and 70% at 6 min after injection of Gd‐HP‐DO3A with highest enhancement and longest retention in the non‐TCFA lesion. Twenty‐four hours after injection of micelles maximum contrast enhancement ranged between 24 and 35% in all plaque phenotypes. Administration of the larger liposomes did not cause significant contrast enhancement in the atherosclerotic plaques. Confocal fluorescence microscopy confirmed the MRI‐based differences in plaque permeation between micelles and liposomes. Plaque permeation of contrast agents was strongly dependent on size. Our results implicate that, when equipped with targeting ligands, liposomes are most suitable for the imaging of plaque‐associated endothelial markers due to low background enhancement, whereas micelles, which accumulate extravascularly on a long timescale, are suited for imaging of less abundant markers inside plaques. Low molecular weight compounds may be employed for target‐specific imaging of highly abundant extravascular plaque‐associated targets. Copyright © 2010 John Wiley & Sons, Ltd.     

Enhanced MRI relaxivity of Gd3+‐based contrast agents geometrically confined within porous nanoconstructs

Gadolinium chelates, which are currently approved for clinical MRI use, provide relaxivities well below their theoretical limit, and they also lack tissue specificity. Recently, the geometrical confinement of Gd³⁺‐based contrast agents (CAs) within porous structures has been proposed as a novel, alternative strategy to improve relaxivity without chemical modification of the CA. Here, we have characterized and optimized the performance of MRI nanoconstructs obtained by loading [Gd(DTPA)(H₂O)]^2? (Magnevist®) into the pores of injectable mesoporous silicon particles. Nanoconstructs with three different pore sizes were studied, and at 60 MHz, they exhibited longitudinal relaxivities of ~24 m m ^?1 s^?1 for 5–10 nm pores and ~10 m m ^?1 s^?1 for 30 – 40 nm pores. No enhancement in relaxivity was observed for larger pores sizes. Using an outer‐sphere compound, [GdTTHA]^3?, and mathematical modeling, it was demonstrated that the relaxivity enhancement is due to the increase in rotational correlation times (CA adsorbed on the pore walls) and diffusion correlation times (reduced mobility of the water molecules), as the pore sizes decreases. It was also observed that extensive CA adsorption on the outer surface of the silicon particles negates the advantages offered by nanoscale confinement. Upon incubation with HeLa cells, the nanoconstructs did not demonstrate significant cytotoxicity for up to 3 days post incubation, at different particle/cell ratios. In addition, the nanoconstructs showed complete degradation after 24 h of continuous agitation in phosphate‐buffered saline. These data support and confirm the hypothesis that the geometrical confinement of Gd³⁺‐chelate compounds into porous structures offers MRI nanoconstructs with enhanced relaxivity (up to 6 times for [Gd(DTPA)(H₂O)]^2?, and 4 times for [GdTTHA]^3?) and, potentially, improved stability, reduced toxicity and tissue specificity. Copyright © 2012 John Wiley & Sons, Ltd.     

A review of responsive MRI contrast agents: 2005–2014

This review focuses on MRI contrast agents that are responsive to a change in a physiological biomarker. The response mechanisms are dependent on six physicochemical characteristics, including the accessibility of water to the agent, tumbling time, proton exchange rate, electron spin state, MR frequency or superparamagnetism of the agent. These characteristics can be affected by changes in concentrations or activities of enzymes, proteins, nucleic acids, metabolites, or metal ions, or changes in redox state, pH, temperature, or light. A total of 117 examples are presented, including ones that employ nuclei other than ¹H, which attests to the creativity of multidisciplinary research efforts to develop responsive MRI contrast agents. Copyright © 2014 John Wiley & Sons, Ltd.     

First ex‐vivo MRI co‐localization of two LIPOCEST agents

One of the major advantages of the CEST methodology deals with the possibility of visualizing more probes in the same MR image voxels. This is a unique property within the contrast media that act on the ¹H‐NMR signal of water protons, and it might considerably improve the potential of the technique. In addition to displaying sufficiently different resonance frequencies of their mobile protons, it is also important that the CEST agents designed for this application are highly sensitive. LIPOCEST agents represent the most sensitive class of CEST systems developed so far. On this basis, two LIPOCEST samples, a spherical one and an osmotically shrunken nonspherical one, endowed with markedly different resonance frequencies of their intraliposomal water protons, 3 ppm and 15 ppm, respectively, were prepared and tested both in vitro and in ex‐vivo on a bovine muscle used as tissue‐surrogate. The response of the two agents did not interfere each other, thus allowing the multiple visualization of the two agents present at nanomolar concentrations in the same image voxels. Copyright © 2008 John Wiley & Sons, Ltd.     

Main applications of hybrid PET‐MRI contrast agents: a review

In medical imaging, the continuous quest to improve diagnostic performance and optimize treatment strategies has led to the use of combined imaging modalities. Positron emission tomography (PET) and computed tomography (CT) is a hybrid imaging existing already for many years. The high spatial and contrast resolution of magnetic resonance imaging (MRI) and the high sensitivity and molecular information from PET imaging are leading to the development of this new hybrid imaging along with hybrid contrast agents. To create a hybrid contrast agent for PET‐MRI device, a PET radiotracer needs to be combined with an MRI contrast agent. The most common approach is to add a radioactive isotope to the surface of a small superparamagnetic iron oxide (SPIO) particle. The resulting agents offer a wide range of applications, such as pH variation monitoring, non‐invasive angiography and early imaging diagnosis of atherosclerosis. Oncology is the most promising field with the detection of sentinel lymph nodes and the targeting of tumor neoangiogenesis. Oncology and cardiovascular imaging are thus major areas of development for hybrid PET‐MRI imaging systems and hybrid contrast agents. The aim is to combine high spatial resolution, high sensitivity, morphological and functional information. Future prospects include the use of specific antibodies and hybrid multimodal PET‐MRI‐ultrasound‐fluorescence imaging with the potential to provide overall pre‐, intra‐ and postoperative patient care. Copyright © 2015 John Wiley & Sons, Ltd.     

Synthesis and characterization of new low‐molecular‐weight lysine‐conjugated Gd‐DTPA contrast agents

Various blood pool contrast agents (CAs), characterized by intravascular distribution, have been developed to assist contrast enhanced magnetic resonance angiography (MRA). Among these CAs, the DTPA derivatives conjugated to synthetic polypeptides, such as polylysine, represent attractive candidates for blood pool imaging. However, due to the presence of charged residues located on their backbone, these agents are retained in the kidneys and this compromises their long blood half‐life. In order to overcome this major drawback of the polylysine compounds, two new low‐molecular‐weight CAs were synthesized in the present work by conjugating four or six 1‐p‐isothiocyanatobenzyl‐DTPA moieties to tri‐ or penta‐Lys peptides [(Gd‐DTPA)₄Lys₃ and (Gd‐DTPA)₆Lys₅], respectively. All the –NH₂ groups of Lys were thus blocked by covalent conjugation to DTPA. The stability and relaxometric properties of these compounds, as well as their pharmacokinetic and biodistribution characteristics, were then evaluated. The half‐life in blood of these new polylysine derivatives, as determined in rats, is twofold longer than that of Gd‐DTPA. The compounds could thus be optimal blood pool markers for MRA, which typically uses fast acquisition times. The absence of positive molecular charge did not limit their retention in kidneys 2 h after administration. On the other hand, (Gd‐DTPA)₄Lys₃ is retained in kidneys to a lesser extent than (Gd‐DTPA)₆Lys₅. Their moderate retention in blood and their higher stability and relaxivity in comparison with Gd‐DTPA highlight these polylysine derivatives as optimal compared with previously developed polylysine compounds. Copyright © 2010 John Wiley & Sons, Ltd.     

Evaluation of EuII‐based positive contrast enhancement after intravenous,intraperitoneal, and subcutaneous injections

Eu^II‐based contrast agents offer physiologically relevant, metal‐based redox sensing that is unachievable with Gd^III‐based contrast agents. To evaluate the in vivo contrast enhancement of Eu^II as a function of injection type, we performed intravenous, intraperitoneal, and subcutaneous injections in mice. Our data reveal a correlation between reported oxygen content and expected rates of diffusion with the persistence of Eu^II‐based contrast enhancement. Biodistribution studies revealed europium clearance through the liver and kidneys for intravenous and intraperitoneal injections, but no contrast enhancement was observed in organs associated with clearance. These data represent a step toward understanding the behavior of Eu^II‐based complexes in vivo. Copyright © 2016 John Wiley & Sons, Ltd.     

Biocompatible blood pool MRI contrast agents based on hyaluronan

Biocompatible gadolinium blood pool contrast agents based on a biopolymer, hyaluronan, were investigated for magnetic resonance angiography application. Hyaluronan, a non‐sulfated linear glucosaminoglycan composed of 2000–25,000 repeating disaccharide subunits of D ‐glucuronic acid and N‐acetylglucosamine with molecular weight up to 20 MDa, is a major component of the extracellular matrix. Two gadolinium contrast agents based on 16 and 74 kDa hyaluronan were synthesized, both with R₁ relaxivity around 5 mM ^?1 s^?1 per gadolinium at 9.4 T at 25°C. These two hyaluronan based agents show significant enhancement of the vasculature for an extended period of time. Initial excretion was primarily through the renal system. Later uptake was observed in the stomach and lower gastrointestinal tract. Macromolecular hyaluronan‐based gadolinium agents have a high clinical translation potential as hyaluronan is already approved by FDA for a variety of medical applications. Copyright © 2010 John Wiley & Sons, Ltd.     

Functionalized single‐walled carbon nanotubes containing traces of iron as new negative MRI contrast agents for in vivo imaging

Single‐walled carbon nanotubes (SWCNTs) containing traces of iron oxide were functionalized by noncovalent lipid‐PEG or covalent carboxylic acid function to supply new efficient MRI contrast agents for in vitro and in vivo applications. Longitudinal (r₁) and transversal (r₂) water proton relaxivities were measured at 300 MHz, showing a stronger T₂ feature as an MRI contrast agent (r₂/r₁ = 190 for CO₂H functionalisation). The r₂ relaxivity was demonstrated to be correlated to the presence of iron oxide in the SWNT‐carboxylic function COOH, in comparison to iron‐free ones. Biodistribution studies on mice after a systemic injection showed a negative MRI contrast in liver, suggesting the presence of the nanotubes in this organ until 48 h after i.v. injection. The presence of carbon nanotubes in liver was confirmed after ex vivo carbon extraction. Finally, cytotoxicity studies showed no apparent effect owing to the presence of the carbon nanotubes. The functionalized carbon nanotubes were well tolerated by the animals at the dose of 10 µg g^?1 body weight. Copyright © 2012 John Wiley & Sons, Ltd.     

Modified lipoproteins as contrast agents for imaging of atherosclerosis

The ability to detect and characterize atherosclerosis with targeted contrast agents may enable initiation of therapy for atherosclerotic lesions prior to becoming symptomatic. Since lipoproteins such as high‐density lipoprotein (HDL) and low‐density lipoprotein (LDL) play a critical role in the regulation of plaque biology through the transport of lipids into and out of atherosclerotic lesions, modifying HDL and LDL with radioisotopes for nuclear imaging, chelates for magnetic resonance imaging (MRI) or other possible contrast agents for computed tomography imaging techniques may aid in the detection and characterization of atherosclerosis. This review focuses on the literature employing lipoproteins as contrast agents for imaging atherosclerosis and the feasibility of this approach. Copyright © 2007 John Wiley & Sons, Ltd.     

In vitro setup to study permeability characteristics of contrast agents by MRI

An experimental setup consisting of a hollow fiber module (HFM) was developed for the in vitro study of contrast agent (CA) permeability. Controllable flow and known fiber characteristics allowed permeability studies under well‐defined conditions with CAs of different molecular weight (MW). In the MRI experiments performed at 4.7 T, the system was perfused at a constant flow rate (5 ml/min) with water and four CA of different MW: Gd‐DOTA (MW = 0.6 kDa), P846 (3.5 kDa), P792 (6.5 kDa) and P717 (50.5 kDa). R₁ time courses were measured with a saturation‐recovery multi‐gradient‐echo snapshot sequence in the fiber‐free HFM input and the fiber‐filled center. Concentration time courses were calculated, and CA extravasation was analyzed with a pharmacokinetic model yielding exchange rate constant k_ie. Only Gd‐DOTA (k_ie = 2.37 ± 0.16 min^?1) and P846 (k_ie = 0.58 ± 0.17 min^?1) showed quantifiable extravasation. P717 perfusion yielded an intra‐capillary volume fraction of 15.6 ± 2.7% compared with 12% estimated from the HFM manufacturer's specifications. In conclusion, the experimental setup allowed classification of in vitro permeability characteristics for CAs with different MW and therefore holds potential for systematic comparison of CAs currently under development. Copyright © 2009 John Wiley & Sons, Ltd.     

A neutral polydisulfide containing Gd(III) DOTA monoamide as a redox‐sensitive biodegradable macromolecular MRI contrast agent

This work aims to develop safe and effective gadolinium (III)‐based biodegradable macromolecular MRI contrast agents for blood pool and cancer imaging. A neutral polydisulfide containing macrocyclic Gd‐DOTA monoamide (GOLS) was synthesized and characterized. In addition to studying the in vitro degradation of GOLS, its kinetic stability was also investigated in an in vivo model. The efficacy of GOLS for contrast‐enhanced MRI was examined with female BALB/c mice bearing 4T1 breast cancer xenografts. The pharmacokinetics, biodistribution, and metabolism of GOLS were also determined in mice. GOLS has an apparent molecular weight of 23.0 kDa with T₁ relaxivities of 7.20 mM^?1 s^?1 per Gd at 1.5 T, and 6.62 mM^?1 s^?1 at 7.0 T. GOLS had high kinetic inertness against transmetallation with Zn²⁺ ions, and its polymer backbone was readily cleaved by L‐cysteine. The agent showed improved efficacy for blood pool and tumor MR imaging. The structural effect on biodistribution and in vivo chelation stability was assessed by comparing GOLS with Gd(HP‐DO3A), a negatively charged polydisulfide containing Gd‐DOTA monoamide GODC, and a polydisulfide containing Gd‐DTPA‐bisamide (GDCC). GOLS showed high in vivo chelation stability and minimal tissue deposition of gadolinium. The biodegradable macromolecular contrast agent GOLS is a promising polymeric contrast agent for clinical MR cardiovascular imaging and cancer imaging. Copyright © 2015 John Wiley & Sons, Ltd.     

Dual nano‐sized contrast agents in PET/MRI: a systematic review

Nowadays molecular imaging plays a vital role in achieving a successful targeted and personalized treatment. Hence, the approach of combining two or more medical imaging modalities was developed. The objective of this review is to systematically compare recent dual contrast agents in Positron Emission Tomography (PET)/Magnetic Resonance Imaging (MRI) and in some cases Single photon emission computed tomography (SPECT)/MRI in terms of some their characteristics, such as tumor uptake, and reticuloendothelial system uptake (especially liver) and their relaxivity rates for early detection of primary cancer tumor. To the best of our knowledge, this is the first systematic and integrated overview of this field. Two reviewers individually directed the systematic review search using PubMed, MEDLINE and Google Scholar. Two other reviewers directed quality assessment, using the criteria checklist from the CAMARADES (Collaborative Approach to Meta‐Analysis and Review of Animal Data from Experimental Studies) tool, and differences were resolved by consensus. After reviewing all 49 studies, we concluded that a size range of 20–200 nm can be used for molecular imaging, although it is better to try to achieve as small a size as it is possible. Also, small nanoparticles with a hydrophilic coating and positive charge are suitable as a T₂ contrast agent. According to our selected data, the most successful dual probes in terms of high targeting were with an average size of 40 nm, PEGylated using peptides as a biomarker and radiolabeled with copper 64 and gallium 68. Copyright © 2017 John Wiley & Sons, Ltd.     

Nano assembly and encapsulation; a versatile platform for slowing the rotation of polyanionic Gd3+‐based MRI contrast agents

Encapsulating discrete Gd³⁺ chelates in nano‐assembled capsules (NACs) is a simple and effective method of preparing an MRI contrast agent capable of delivering a large payload of high relaxivity imaging agent. The preparation of contrast agent containing NACs had previously focussed on preparations incorporating GdDOTP^5‐ into the internal aggregate. In this report we demonstrate that other Gd³⁺ chelates bearing overall charges as low as 2‐ can also be used to prepare NACs. This discovery opens up the possibility of using Gd³⁺ chelates that have inner‐sphere water molecules that could further increase the relaxivity enhancement associated with the long τ_R that arises from encapsulation. However, encapsulation of the q = 1 chelate GdDTPA^2‐ did not give rise to a significant increase in relaxivity relative to encapsulation of the outer‐sphere chelate GdTTHA^3‐. This leads us to the conclusion that in the NAC interior proton transport is not mediated by movement of whole water molecules and the enhanced relaxivity of Gd³⁺ chelate encapsulated within NACs arises primarily from second sphere effects. Copyright © 2015 John Wiley & Sons, Ltd.     

The reliability of parameters obtained by fitting of 1H NMRD profiles and 17O NMR data of potential Gd3+‐based MRI contrast agents

Synthetic variable temperature ¹H NMRD profiles and ¹⁷O NMR relaxation and shift data were generated with a model based on the Solomon–Bloembergen–Morgan and Freed theories and then fitted simultaneously or individually. The effects of the fitting procedure and of experimental uncertainties on the resulting best‐fit parameters were investigated. The most reliable best‐fit parameters were obtained when all data were included in a simultaneous fitting procedure. Fitting of only NMRD and/or ¹⁷O NMR data provided considerably less accurate best‐fit parameters. Very large deviations from the values of the parameters used for the construction of the datasets were obtained due to the combined effects of uncertainties resulting from the fitting and from the data. For these fittings, the accuracy of the best‐fit parameters appeared to be strongly dependent on the magnitude of synthetic parameters applied. For example, the accuracy of τ_M^C was low around τ_M^S = 10^?8 s. The parameters τ_V and Δ² are strongly correlated in fittings of only ¹⁷O NMR data. Consequently, only the ratio of these parameters can be evaluated in this way. The observations underline the need to reduce the number of adjustable parameters by constraining as many as possible of them at values obtained by independent techniques. The inaccuracies observed in these simulations come in addition to those caused by the inadequacy of the Solomon–Bloembergen–Morgan theory, particularly at low magnetic field strengths. Copyright © 2015 John Wiley & Sons, Ltd.     

Real‐time 3D MRI of contrast agents in whole living mice

A specific mouse whole body coil and a dedicated gradient system at 4.7 T were coupled with an ultra‐fast 3D gradient echo MRI and keyhole reconstruction technique to obtain 3D whole‐body dynamic T₁‐weighted or T₂*‐weighted imaging. The technique was used to visualize the real‐time distribution of non‐targeting T₁ and T₂* contrast agent (CA) in a glioma‐bearing mouse model. T₁ dynamic contrast‐enhancement imaging was performed with a fast imaging with steady‐state precession sequence [echo time/repetition time (TE/TR), 1.32/3.7 ms] before and after CA injection (Gd–DOTA and BSA–Gd–DOTA) for 21 min. The temporal resolution was 1 image/6.5 s. T₂* imaging (TE/TR, 4/8 ms) was performed before and after iron‐based (small and ultra‐small particles of iron oxide) CA injection for 45 min. The temporal resolution was 1 image/14 s. Signal‐to‐noise ratio curves were determined in various mouse organs. The whole‐body coil and gradient systems made it possible to acquire data with sufficient and homogeneous signal‐to‐noise ratio on the whole animal. The spatial resolution allowed adequate depiction of the major organs, blood vessels and brain glioma. The distribution and the time‐course of T₁ and T₂* contrasts upon contrast agent injection were also assessed. 3D whole‐body mouse MRI is feasible at high spatial resolution in movie mode and can be applied successfully to visualize real‐time contrast agent distribution. This method should be effective in future preclinical molecular imaging studies. Copyright © 2011 John Wiley & Sons, Ltd.     

Myelin‐targeted,texaphyrin‐based multimodal imaging agent for magnetic resonance and optical imaging

Reliable methods of imaging myelin are essential to investigate the causes of demyelination and to study drugs that promote remyelination. Myelin‐specific compounds can be developed into imaging probes to detect myelin with various imaging techniques. The development of multimodal myelin‐specific imaging probes enables the use of orthogonal imaging techniques to accurately visualize myelin content and validate experimental results. Here, we describe the synthesis and application of multimodal myelin‐specific imaging agents for light microscopy and magnetic resonance imaging. The imaging agents were synthesized by incorporating the structural features of luxol fast blue MBS, a myelin‐specific histological stain, into texaphyrins coordinated to Gd^III. These new complexes demonstrated absorption of visible light, emission of near‐IR light, and relaxivity values greater than clinically approved contrast agents for magnetic resonance imaging. These properties enable the use of optical imaging and magnetic resonance imaging for visualization of myelin. We performed section‐ and en block‐staining of ex vivo mouse brains to investigate the specificity for myelin of the new compounds. Images obtained from light microscopy and magnetic resonance imaging demonstrate that our complexes are retained in white matter structures and enable detection of myelin. Copyright © 2016 John Wiley & Sons, Ltd.     

Effect of a mesitylene‐based ligand cap on the relaxometric properties of Gd(III) hydroxypyridonate MRI contrast agents

A series of new Gd(III) hydroxypyridonate complexes featuring a mesitylene (ME)‐derived ligand cap has been prepared. Relaxometric characterization reveals that the complexes tend to form large aggregates in solution with slow tumbling rates, as estimated from NMRD analysis, and unique pH‐dependent relaxivities. The solution behavior and relaxometric properties are compared with those observed for analogous TREN‐capped compounds, and the potential for use of these new ME‐capped complexes as pH‐responsive MRI contrast agents is explored. Copyright © 2009 John Wiley & Sons, Ltd.     

A new temperature‐sensitive contrast mechanism for MRI: Curie temperature transition‐based imaging

A temperature‐sensitive MRI contrast mechanism is proposed based on the physical property, the Curie temperature (T_c), at which a ferromagnetic material transitions to paramagnetic state and vice versa. To evaluate the feasibility of this new contrast mechanism, experiments were performed with solid gadolinium metal, which has a T_c of 20°C. In phantom and ex vivo experiments, the magnetic susceptibility artifact area decreased with increasing temperature transitioning across T_c (p < 0.05). Similar results would be expected for a variety of ferromagnetic substances with substance‐specific T_c values. Temperature‐sensitive MRI contrast agents harnessing this mechanism may be used to (1) indicate regional attainment of specific temperatures in thermotherapy, (2) render an accumulated contrast agent more or less visible by the external application of appropriate heating or cooling, or (3) quantify tissue temperature based on MR image characteristics and magnetic susceptibility artifact caused by a ferromagnetic–paramagnetic transitioning substance. Copyright © 2007 John Wiley & Sons, Ltd.     

可透过血脑屏障的MRI造影剂的实验研究

目的探索开发一种可透过血脑屏障的MRI造影剂,为进一步开发靶向脑内的特异性分子探针打下基础。方法用聚山梨酯80包被Fe3O4纳米颗粒制成注射液,注入20只新西兰兔耳背静脉内。结果注入造影剂后第5、10、30分钟行MR扫描,20只实验兔T2WI脑实质信号随着时间延长而逐渐降低,12h后行MR扫描,信号显著降低。结论聚山梨酯80包被的Fe3O4纳米颗粒,是一种理想的穿透血脑屏障的MRI造影剂,可用于靶向脑内的分子探针的开发。