Study of the MR relaxation of microglia cells labeled with Gd‐DTPA‐bearing nanoparticles

Therapies involving cells as vehicles need to visualize in situ the trafficking of the cells concerned. This cellular imaging can be driven by cell contrast agent‐based nanoparticle internalization and non‐invasive MRI (magnetic resonance imaging) detection. Here, microglial cells, that would transport a suicide gene to a glioma, were incubated for different times, with various concentrations of silica nanoparticles on which numerous Gd‐DTPA were grafted. The goal of this study was to investigate the repartition of cell‐associated particles. MRI was used to quantitatively follow the particle uptake process. Fluorescence microscopy images showed that, although most of the nanoparticles were internalized, some remained adsorbed on the extracellular membrane surface. The cells were then submitted to various treatments: glycine to release bound nanoparticles and/or ultrasound to destroy the cell membranes. The R₁ relaxation rates were measured at 4.7 T. R₁ was maximal for 4 h of incubation, decreased after 8 h and remained stable for the 24 following hours. The magnetic resonance signal of ultrasonicated and glycine‐treated cells made it possible to quantify the loss of bound nanoparticles after 8 h. Nevertheless, this release did not prevent cell detection since the internalized nanoparticles are enough concentrated to visualize the labeled cells even after 4 days of cell growth. These results highlight the compartmentalization of nanoparticles in microglia and the evolution of the MR signal of the labeled cells. This study could be of importance to interpret in vivo the MR signal changes that could occur after administration of such nanoparticle‐labeled cells in therapeutic strategies. Copyright © 2009 John Wiley & Sons, Ltd.     

Evaluation of rHA labeled with Gd–DTPA for blood pool imaging and targeted contrast delivery

A new contrast agent was developed by linking Gd–DTPA chelate to recombinant human albumin in the laboratory. The molar relaxivity of the new agent was tested in aqueous solution at B₀ 1.5 T and temperature 20°C. The soluble compound had a higher molar longitudinal relaxivity and molar transverse relaxivity in water (r₁ = 7.2 s^?1 mM ^?1, r₂ = 18.4 s^?1 mM ^?1) than those measured for Gd–DTPA solution (r₁ = 3.5 s^?1 mM ^?1, r₂ = 5.5 s^?1 mM ^?1). The performance of the compound as a blood pool agent was investigated with soluble and microparticulate forms of the compound and comparisons were made with Gd–DTPA and the polymeric blood‐pool agent, Gadomer. T₁‐weighted imaging experiments show that the soluble compound acts as a highly effective blood pool agent with hyperintensity in the vasculature persisting beyond 2 h post administration, compared with free Gd–DTPA, which was cleared from the blood pool after approximately 10 min. The clearance kinetics of the new agents were examined, due to the incomplete elimination within 14 days post injection; both rHA labeled compounds are probably not suitable for development as routine blood pool contrast media. However, with free sites on the Gd‐loaded rHA molecule, there are possibilities for binding the agent to antibodies in the laboratory, which was demonstrated, and thus there exist potential applications for in vivo molecular imaging with this agent. Copyright © 2010 John Wiley & Sons, Ltd.     

Assessment of tumor angiogenesis: dynamic contrast‐enhanced MRI with paramagnetic nanoparticles compared with Gd‐DTPA in a rabbit Vx‐2 tumor model

The purpose of this study was to evaluate the suitability of a macromolecular MRI contrast agent (paramagnetic nanoparticles, PNs) for the characterization of tumor angiogenesis. Our aim was to estimate the permeability of PNs in developing tumor vasculature and compare it with that of a low molecular weight contrast agent (Gd‐DTPA) using dynamic contrast‐enhanced MRI (DCE). Male New Zealand white rabbits (n = 5) underwent DCE MRI 12–14 days after Vx‐2 tumor fragments were implanted into the left hind limb. Each contrast agent (PNs followed by Gd‐DTPA) was evaluated using a DCE protocol and transendothelial transfer coefficient (K_i) maps were calculated using a two‐compartment model. Two regions of interest (ROIs) were located within the tumor core and hindlimb muscle and five ROIs were placed within the tumor rim. Comparisons were performed using repeated measures analysis of variance (ANOVA). The K_i values estimated using PNs were significantly lower than those obtained for Gd‐DTPA (p = 0.018). When PNs and Gd‐DTPA data were analyzed separately, significant differences were identified among tumor rim ROIs for PNs (p < 0.0001), but not for Gd‐DTPA data (p = 0.34). The mean K_i for the tumor rim was significantly greater than that of either the core or the hindlimb muscle for both contrast agents (p < 0.05 for each comparison). In summary, the extravasation of Gd‐DTPA was far greater than that of PNs, suggesting that PNs can reveal regional differences in tumor vascular permeability that are not otherwise apparent with clinical contrast agents such as Gd‐DTPA. These results suggest that PNs show potential for the noninvasive delineation of tumor angiogenesis. Copyright © 2010 John Wiley & Sons, Ltd.     

Coating didodecyldimethylammonium bromide onto Au nanoparticles increases the stability of its complex with DNA

The stability of the complex of cationic lipid with nucleic acid, especially when facing serum, is crucial for the efficiency of gene delivery. Here, we demonstrated that the stability of the complex of didodecyldimethylammonium bromide (DDAB, a cationic lipid) with DNA in the presence of serum dramatically increased after coating DDAB onto the surface of the gold nanoparticles. The stability of the complex was demonstrated with dye intercalation assay, and agarose gel electrophoresis. The process of the interaction was characterized with UV–vis spectra and the morphology of the complex was observed with atomic force microscope (AFM). Cell viability assays demonstrated that the cytotoxicity of DDAB was also decreased.     

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.     

Ultra‐small gadolinium oxide nanoparticles to image brain cancer cells in vivo with MRI

The majority of contrast agents used in magnetic resonance imaging (MRI) is based on the rare‐earth element gadolinium. Gadolinium‐based nanoparticles could find promising applications in pre‐clinical diagnostic procedures of certain types of cancer, such as glioblastoma multiforme. This is one of the most malignant, lethal and poorly accessible forms of cancer. Recent advances in colloidal nanocrystal synthesis have led to the development of ultra‐small crystals of gadolinium oxide (US‐Gd₂O₃, 2–3 nm diameter). As of today, this is the smallest and the densest of all Gd‐containing nanoparticles. Cancer cells labeled with a sufficient quantity of this compound appear bright in T₁‐weighted MRI images. Here we demonstrate that US‐Gd₂O₃ can be used to label GL‐261 glioblastoma multiforme cells, followed by localization and visualization in vivo using MRI. Very high amounts of Gd are efficiently internalized and retained in cells, as confirmed with TEM and ICP‐MS. Labeled cells were visualized in vivo at 1.5 T using the chicken embryo model. This is one more step toward the development of “positively contrasted” cell tracking procedures with MRI. Copyright © 2010 John Wiley & Sons, Ltd.     

Bifunctional Eu3+‐doped Gd2O3 nanoparticles as a luminescent and T1 contrast agent for stem cell labeling

Magnetic resonance tracking of stem cells has recently become an emerging application for investigating cell–tissue interactions and guiding the development of effective stem cell therapies for regeneration of damaged tissues and organs. In this work, anionic Eu³⁺‐doped Gd₂O₃ hybrid nanoparticles were applied as a contrast agent both for fluorescence microscopy and T₁‐weighted MRI. The nanoparticles were synthesized through the polyol method and further modified with citric acid to obtain anionic nanoparticles. These nanoparticles were internalized into human mesenchymal stem cells (hMSCs) as confirmed by confocal laser scanning microscopy and quantified by inductively coupled plasma–mass spectrometry. MTT assay of the labeled cells showed that the nanoparticles did not possess significant cytotoxicity. In addition, the osteogenic, adipogenic and chondrogenic differentiation of the hMSCs was not influenced by the labeling process. With MRI, the in vitro detection threshold of cells after incubation with nanoparticles at a Gd concentration of 0.5 mM for 2 h was estimated to be about 10 000 cells. The results from this study indicate that the biocompatible anionic Gd₂O₃ nanoparticles doped with Eu³⁺ show promise both as a luminescent and T₁ contrast agent for use in visualizing hMSCs. Copyright © 2010 John Wiley & Sons, Ltd.     

The Gd3+ complex of 1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetraacetic acid mono(p‐isothiocyanatoanilide) conjugated to inulin: a potential stable macromolecular contrast agent for MRI

Reaction of DOTA–NCSA [1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetraacetic acid mono(p‐isothiocyanatoanilide)] with O‐(aminopropyl)inulin (degree of polymerization 25) provided a chelate that formed a kinetically extremely stable Gd³⁺ complex. No transmetalation was observed with Zn²⁺. The conjugate has a relaxivity of 21.7 s^?1 m m ^?1 at 20 MHz and 37 °C, and each molecule of the inulin carries on average 35 Gd³⁺ ions. The parameters governing the relaxivity of this material and of a low‐molecular‐weight model compound prepared by conjugation of DOTA–NCSA and propylamine were evaluated by investigation of their water ¹H longitudinal relaxation rate enhancements at different magnetic fields (NMRD) and by studying variable temperature ¹⁷O NMR data. The high relaxivity of the inulin conjugate can be ascribed to the efficient slowing down of the molecular tumbling by this carrier. The rotational correlation time at 37 °C of this material is 1460 ps, whereas that of the model compound is 84 ps. Furthermore, both complexes do not interact significantly with human serum albumin, as shown by their NMRD profiles, and do not undergo transmetallation by zinc ions. The inulin conjugate thus has potential for application as a contrast agent for MRI, particularly as a blood pool agent. Copyright © 2011 John Wiley & Sons, Ltd.     

Quantitative imaging of the tissue contrast agent [Gd(DTPA)]2− in articular cartilage by laser ablation inductively coupled plasma mass spectrometry

Laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) is an emerging analytical technique in the generation of quantitative images of MR contrast agent distribution in thin tissue sections of articular cartilage. An analytical protocol is described that includes sample preparation by cryo‐cutting of tissue sections, mass spectrometric measurements by LA‐ICP‐MS and quantification of gadolinium images by one‐point calibration, standard addition method (employing matrix‐matched laboratory standards) and isotope dilution analysis using highly enriched stable Gd‐155 isotope (abundance 92 vs 14.8% in the [Gd(DTPA)]^2? contrast agent). The tissue contrast agent concentrations of [Gd(DTPA)]^2? in cartilage measured in this work are in agreement with findings obtained by magnetic resonance imaging and other analytical methodologies. The LA‐ICP‐MS imaging data also confirm the observation that the spatial distribution of [Gd(DTPA)]^2? in the near‐equilibrium state is highly inhomogeneous across cartilage thickness with the highest concentration measured in superficial cartilage and a strong decrease toward the subchondral bone. In the present work, it is shown for the first time that LA‐ICP‐MS can be applied to validate the results from quantitative gadolinium‐enhanced MRI technique of articular cartilage. Copyright © 2012 John Wiley & Sons, Ltd.     

A novel one‐step synthesis of Gd3+‐incorporated mesoporous SiO2 nanoparticles for use as an efficient MRI contrast agent

Molecular imaging has generated a demand for more sensitive contrast agents for magnetic resonance (MR) imaging. We synthesized, by a novel one‐step method, Gd³⁺ incorporated mesoporous SiO₂ nanoparticles, Gd₂O₃@SiO₂, for use as an efficient contrast agent. The prepared nanoparticles were also coated with poly(lactic‐co‐glycolic acid) (PLGA). The size, morphology, composition and Brunauer–Emmett–Teller specific surface area of the nanoparticles were evaluated. The Gd₂O₃@SiO₂ nanoparticles possess intragranular network morphology with a uniform size distribution and an average size of approximately 20–40 nm. The PLGA‐coated nanoparticles were spherical or near‐spherical in shape with a diameter of approximately 120 nm, a smooth surface, and neither aggregation nor adhesion tendencies. No free Gd ions were detected to dissociate from Gd₂O₃@SiO₂ even up to the limit (<0.03 mg/l) of the ICP equipment (IRIS Advantage). Our theoretical computation based on density functional theory (using DMol3, Materials Studio) revealed that the Gd₂O₃ molecules are fully absorbed on the interface of mesoporous SiO₂ with a stable state of lower energy. Both Gd₂O₃@SiO₂ and PLGA‐coated Gd₂O₃@SiO₂ samples have a larger T₁ relaxivitiy than commercial gadolinium diethylene triaminepentaacetate (Gd‐DTPA). In vitro and in vivo MR images using the Gd₂O₃@SiO₂ nanoparticles were observed with a 1.5 T clinical MR scanner and compared with the images using Gd‐DTPA. The Gd₂O₃@SiO₂ nanoparticles display a better magnetic property than commercial Gd‐DTPA. In vivo MR imaging demonstrated that the nanoparticles were mainly distributed in the liver. Strong enhancement was also detected in nasopharyngeal carcinoma CNE‐2 xenografted tumors. The Gd₂O₃@SiO₂ nanoparticles are not only potential candidates for highly efficient contrast agents for MR imaging, but also might be developed into potent targeted probes for in vivo molecular imaging of cancer. Copyright © 2010 John Wiley & Sons, Ltd.     

Towards MRI contrast agents responsive to Ca(II) and Mg(II) ions: metal‐induced oligomerization of dota–bisphosphonate conjugates

In magnetic resonance imaging (MRI), paramagnetic complexes are utilized as contrast agents. Much attention has been paid to the development of new contrast agents responsive to pH, temperature or concentration of various components of body liquids. We report a new type of MRI probe sensing the concentrations of calcium and magnesium in biological media. The ligand do3ap^BP combines a dota‐like chelator with a bisphosphonate group. In the complex, the Gd(III ) ion is entrapped in the macrocyclic cavity whereas the bisphosphonate group is not coordinated and therefore is available for coordination with endogenous metal ions. In the presence of metal ions, Gd–do3ap^BP appears to show formation of coordination oligomers leading to an unprecedented increase in r₁ up to 200–500%. The extremely high relaxivity response makes this type of compound interesting for further studies as MRI ion‐responsive probes for biomedical research. Copyright © 2010 John Wiley & Sons, Ltd.     

Antibody‐functionalized nanoparticles for imaging cancer: influence of conjugation to gold nanoparticles on the biodistribution of 89Zr‐labeled cetuximab in mice

Antibody‐labeled gold nanoparticles represent a promising novel tool regarding cancer imaging and therapy. Nevertheless, the characterization of biodistribution of such immunonanocarriers has been poorly documented. In this study, the biodistribution of ⁸⁹Zr‐labeled cetuximab before and after the coupling reaction to gold nanoparticles (AuNPs) was compared and the quantitative imaging performance of ⁸⁹Zr immuno‐PET was evaluated. Cetuximab was functionalized with the desferal moiety and labeled with ⁸⁹Zr (⁸⁹Zr–Df–Bz–NCS–cetuximab). AuNPs with a mean diameter of 5 nm were synthesized according a new method developed in the laboratory, and conjugated to ⁸⁹Zr–Df–Bz–NCS–cetuximab using carbodiimide chemistry (AuNPs–PPAA–cetuximab–⁸⁹Zr). The two tracers were injected in A431 xenograft‐bearing mice. Tumor and liver uptakes were assessed at different times after injection using quantitative PET imaging. The in vivo specificity of the binding was investigated using a saturating dose of unlabeled cetuximab. Radiolabeled cetuximab was conjugated to AuNPs with a coupling reaction yield >75%. All conjugates were stable in vitro and to a lesser extent in plasma. In vivo distribution studies revealed no significant difference in tumor uptake for cetuximab conjugated to nanoparticles up to 72 h after injection, compared with unconjugated cetuximab. Immuno‐PET studies showed that AuNPs–PPAA–cetuximab–⁸⁹Zr provided high tumor‐to‐background ratio. The liver uptake of AuNPs–PPAA–cetuximab–⁸⁹Zr was higher, compared with ⁸⁹Zr–Df–Bz–NCS–cetuximab. In vivo blocking experiments demonstrated selective tumor targeting after coupling reaction. This study showed that the conjugation of AuNPs to cetuximab did not affect its tumor accumulation and that the efficacy of EGFR‐targeted nanoparticles was unaltered. The ⁸⁹Zr‐labeled cetuximab‐targeted gold nanoparticles could be a valuable tool for theranostic purposes. Copyright © 2013 John Wiley & Sons, Ltd.     

Cobalt nanoparticles as a novel magnetic resonance contrast agent—relaxivities at 1.5 and 3 Tesla

Two samples of polymer‐coated cobalt nanoparticles were synthesized and dispersed in agarose gel and water. The relaxivities r₁ and r₂ of the two samples were obtained at different temperatures (25, 37 and 40°C) and magnetic field strengths (1.5 and 3 T). The average cobalt core diameters of the two samples were 3.3 and 3.9 nm (measured by transmission electron microscopy); the corresponding average total diameters (cobalt core + polymer coating) were 13 and 28 nm (measured by dynamic light scattering). The larger particles had the higher r₁ relaxivity, whilst r₂ was similar for the two samples. There was no significant change in r₁ or r₂ relaxivities with temperature but r₁ at 1.5 T was approximately double the value at 3 T. The highest relaxivities were obtained at 1.5 T with values for r₁ and r₂ of 7.4 and 88 mM ^?1 s^?1, respectively. These values are similar to those reported for iron oxide with larger core size, suggesting the potential of the cobalt nanoparticles for development and future use as a negative contrast agent. Copyright © 2008 John Wiley & Sons, Ltd.     

The effects of intramolecular H‐bond formation on the stability constant and water exchange rate of the Gd(III)‐diethylenetriamine‐N′‐(3‐amino‐1,1‐propylenephosphonic)‐N,N,N″,N″‐tetraacetate complex

The binding interaction of metal chelates to biological macromolecules, though driven by properly devoted recognition synthons, may cause dramatic changes in some property associated with the coordination cage such as the thermodynamic stability or the exchange rate of the metal coordinated water. Such changes are due to electrostatic and H‐bonding interactions involving atoms of the coordination cage and atoms of the biological molecule at the binding site. To mimic this type of H‐bonding interactions, lanthanide(III) complexes with a DTPA–monophosphonate ligand bearing a propylamino moiety (H₆NP–DTPA) were synthesized. Their thermodynamic stabilities and the exchange lifetime of the coordinated water molecule (for the Gd‐complex) were compared with those of the analog complexes with DTPA and the parent DTPA–monophosphonate derivative (H₆P–DTPA). It was found that the intramolecular H‐bond between the ε‐amino group and the phosphonate moiety in NP–DTPA complexes causes displacements of electric charges in their coordination cage that are markedly pH dependent. In turn, this affects the characteristic properties of the coordination cage. In particular it results in a marked elongation of the exchange lifetime of the coordinated water molecule. Copyright © 2007 John Wiley & Sons, Ltd.     

Utilization of nanoparticles as X‐ray contrast agents for diagnostic imaging applications

Among all the diagnostic imaging modalities, X‐ray imaging techniques are the most commonly used owing to their high resolution and low cost. The improvement of these techniques relies heavily on the development of novel X‐ray contrast agents, which are molecules that enhance the visibility of internal structures within the body in X‐ray imaging. To date, clinically used X‐ray contrast agents consist mainly of small iodinated molecules that might cause severe adverse effects (e.g. allergies, cardiovascular diseases and nephrotoxicity) in some patients owing to the large and repeated doses that are required to achieve good contrast. For this reason, there is an increasing interest in the development of alternative X‐ray contrast agents utilizing elements with high atomic numbers (e.g. gold, bismuth, ytterbium and tantalum), which are well known for exhibiting high absorption of X‐rays. Nanoparticles (NPs) made from these elements have been reported to have better imaging properties, longer blood circulation times and lower toxicity than conventional iodinated X‐ray contrast agents. Additionally, the combination of two or more of these elements into a single carrier allows for the development of multimodal and hybrid contrast agents. Herein, the limitations of iodinated X‐ray contrast agents are discussed and the parameters that influence the efficacy of X‐ray contrast agents are summarized. Several examples of the design and production of both iodinated and iodine‐free NP‐based X‐ray contrast agents are then provided, emphasizing the studies performed to evaluate their X‐ray attenuation capabilities and their toxicity in vitro and in vivo. Copyright © 2014 John Wiley & Sons, Ltd.     

Synthesis and evaluation of a polydisulfide with Gd–DOTA monoamide side chains as a biodegradable macromolecular contrast agent for MR blood pool imaging

Macromolecular Gd(III)‐based contrast agents are effective for contrast‐enhanced blood pool and cancer MRI in preclinical studies. However, their clinical applications are impeded by potential safety concerns associated with slow excretion and prolonged retention of these agents in the body. To minimize the safety concerns of macromolecular Gd contrast agents, we have developed biodegradable macromolecular Gd contrast agents based on polydisulfide Gd(III) complexes. In this study, we designed and synthesized a new generation of the polydisulfide Gd(III) complexes containing a macrocyclic Gd(III) chelate, Gd–DOTA monoamide, to improve the in vivo kinetic inertness of the Gd(III) chelates. (N₆‐Lysyl)lysine‐(Gd–DOTA) monoamide and 3‐(2‐carboxyethyldisulfanyl)propanoic acid copolymers (GODC) were synthesized by copolymerization of (N₆‐lysyl)lysine DOTA monoamide and dithiobis(succinimidylpropionate), followed by complexation with Gd(OAc)₃. The GODC had an apparent molecular weight of 26.4 kDa and T₁ relaxivity of 8.25 m m ^?1 s^?1 per Gd at 1.5 T. The polymer chains of GODC were readily cleaved by l ‐cysteine and the chelates had high kinetic stability against transmetallation in the presence of an endogenous metal ion Zn²⁺. In vivo MRI study showed that GODC produced strong and prolonged contrast enhancement in the vasculature and tumor periphery of mice with breast tumor xenografts. GODC is a promising biodegradable macromolecular MRI contrast agent with high kinetic stability for MR blood pool imaging. Copyright © 2013 John Wiley & Sons, Ltd.     

Pharmacokinetic and in vivo evaluation of a self‐assembled gadolinium(III)‐iron(II) contrast agent with high relaxivity

A high‐molecular weight tetrametallic supramolecular complex [(Ln‐DTPA‐phen)₃Fe]^? (Ln = Gd, Eu, La) has been obtained upon self‐assembly around one iron(II) ion of three 1,10‐phenantroline‐based molecules substituted in 5′‐position with the polyaminocarboxylate diethylenetriamine‐N,N,N′,N′,N′‐pentaacetate, DTPA‐phen^4?. The ICP‐MS measurements indicated that the lanthanide:iron ratio is 3:1. Photoluminescence spectra of [Eu‐DTPA‐phen]^? and of [(Eu‐DTPA‐phen)₃Fe]^? are nearly identical, implying that the first coordination sphere of the lanthanide(III) ion has not been changed upon coordination of phenantroline unit to iron(II) ion. NMRD measurements revealed that at 20 MHz and 310 K the relaxivity of the [(Gd‐DTPA‐phen)₃Fe]^? is equal to 9.5 ± 0.3 s^?1 mM^?1 of Gd (28.5 s^?1 per millimole per liter of complex) which is significantly higher than that for Gd‐DTPA (3.9 s^?1 mM^?1). The pharmacokinetic parameters of [(Gd‐DTPA‐phen)₃Fe]^? in rats indicate that the elimination of [(Gd‐DTPA‐phen)₃Fe]^? is significantly slower than that of Gd‐DTPA and is correlated with a reduced volume of distribution. The low volume of distribution and the longer elimination time (T_e1/2) suggest that the agent is confined to the blood compartment, so it could have an important potential as a blood pool contrast agent. The biodistribution profile of [(Gd‐DTPA‐phen)₃Fe]^? 2 h after injection indicates significantly higher concentrations of [(Gd‐DTPA‐phen)₃Fe]^? as compared with Gd‐DTPA in kidney, liver, lungs, heart and spleen. The images obtained on rats by MR angiography show the enhancement of the abdominal blood vessels. The signal intensity reaches a maximum of 55% at 7 min post‐contrast and remains around 25% after 90 min. MRI‐histomorphological correlation studies of [Gd‐DTPA‐phen]^? and [(Gd‐DTPA‐phen)₃Fe]^? showed that both agents displayed potent contrast enhancement in organs including the liver. The necrosis avidity tests indicated that, in contrast to the [Gd‐DTPA‐phen]^? precursor complex, the supramolecular complex [(Gd‐DTPA‐phen)₃Fe]^? exhibits necrosis avidity. Copyright © 2006 John Wiley & Sons, Ltd.     

Polyglycerol‐grafted superparamagnetic iron oxide nanoparticles: highly efficient MRI contrast agent for liver and kidney imaging and potential scaffold for cellular and molecular imaging

Polyglycerol as a water‐soluble and biocompatible hyperbranched polymer was covalently grafted on the surface of superparamagnetic iron oxide nanoparticles. With this aim, superparamagnetic magnetite nanoparticles were prepared by coprecipitation in aqueous media, then the surface of nanoparticles was modified to introduce the reactive groups on the surface of nanoparticles. After that, polyglycerol was grafted on the surface of nanoparticles by ring‐opening anionic polymerization of glycidol using n‐bulyllithium as initiator. The magnetometry, relaxometry and phantom MRI experiments of this highly stable ferrofluid showed its high potential as a negative MRI contrast agent. Calculated r₁ and r₂ relaxivities at different magnetic fields were higher than the values reported for commercially available iron oxide contrast agents. The in vivo MRI studies showed that, after intravenous injection into mice, the particles produced a strong negative contrast in liver and kidneys, which persisted for 80 min (in liver) to 110 min (in kidneys). The negative contrast of the liver and kidneys weakened over the time, suggesting that polyglycerol coating renders the nanoparticles stealth and possibly optimal for renal excretion. Copyright © 2012 John Wiley & Sons, Ltd.     

Surface functionalization of magnetic iron oxide nanoparticles for MRI applications – effect of anchoring group and ligand exchange protocol

Hydrophobic magnetite nanoparticles synthesized from thermal decomposition of iron salts must be rendered hydrophilic for their application as MRI contrast agents. This process requires refunctionalizing the surface of the nanoparticles with a hydrophilic organic coating such as polyethylene glycol. Two parameters were found to influence the magnetic behavior and relaxivity of the resulting hydrophilic iron oxide nanoparticles: the functionality of the anchoring group and the protocol followed for the functionalization. Nanoparticles coated with PEGs via a catecholate‐type anchoring moiety maintain the saturation magnetization and relaxivity of the hydrophobic magnetite precursor. Other anchoring functionalities, such as phosphonate, carboxylate and dopamine decrease the magnetization and relaxivity of the contrast agent. The protocol for functionalizing the nanoparticles also influences the magnetic behavior of the material. Nanoparticles refunctionalized according to a direct biphasic protocol exhibit higher relaxivity than those refunctionalized according to a two‐step procedure which first involves stripping the nanoparticles. This research presents the first systematic study of both the binding moiety and the functionalization protocol on the relaxivity and magnetization of water‐soluble coated iron oxide nanoparticles used as MRI contrast agents. Copyright © 2010 John Wiley & Sons, Ltd.     

First in vivo MRI assessment of a self‐assembled metallostar compound endowed with a remarkable high field relaxivity

{Fe[Gd₂bpy(DTTA)₂(H₂O)₄]₃}^4? is a self‐assembled, metallostar‐structured potential MRI contrast agent, with six efficiently relaxing Gd³⁺ centres confined into a small molecular space. Its proton relaxivity is particularly remarkable at very high magnetic fields (r₁ = 15.8 mM ^?1 s^?1 at 200 MHz, 37°C, in H₂O). Here we report the first in vivo MRI feasibility study, complemented with dynamic γ scintigraphic imaging and biodistribution experiments using the ¹⁵³Sm‐enriched compound. Comparative MRI studies have been performed at 4.7 T in mice with the metallostar and the small molecular weight contrast agent gadolinium(III)‐1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetraacetate ([Gd(DOTA)(H₂O)]^? = GdDOTA). The metallostar was well tolerated by the animals at the concentrations of 0.0500 (high dose) and 0.0125 (low dose) mmol Gd kg^?1 body weight; (BW). The signal enhancement in the inversion recovery fast low angle shot (IR FLASH) images after the high‐dose metallostar injection was considerably higher than after GdDOTA injection (0.1 mmol Gd kg^?1 BW), despite the higher dose of the latter. The high‐dose metallostar injection resulted in a greater drop in the spin‐lattice relaxation time (T₁), as calculated from the inversion recovery true fast imaging with steady‐state precession (IR TrueFISP) data for various tissues, than the GdDOTA or the low dose metallostar injection. In summary, these studies have confirmed that the approximately four times higher relaxivity measured in vitro for the metallostar is retained under in vivo conditions. The pharmacokinetics of the metallostar was found to be similar to that of GdDOTA, involving fast renal clearance, a leakage to the extracellular space in the muscle tissue and no leakage to the brain. As expected on the basis of its moderate molecular weight, the metallostar does not function as a blood pool agent. The dynamic γ scintigraphic studies performed in Wistar rats with the metallostar compound having ¹⁵³Sm enrichment also proved the renal elimination pathway. The biodistribution experiments are in full accordance with the MR and scintigraphic imaging. At 15 min post‐injection the activity is primarily localized in the urine, while at 24 h post‐injection almost all radioactivity is cleared from tissues and organs. Copyright © 2006 John Wiley & Sons, Ltd.