Hindered diffusion of MRI contrast agents in rat brain extracellular micro‐environment assessed by acquisition of dynamic T1 and T2 maps

The knowledge of brain tissues characteristics (such as extracellular space and tortuosity) represents valuable information for the design of optimal MR probes for specific biomarkers targeting. This work proposes a methodology based on dynamic acquisition of relaxation time maps to quantify in vivo MRI contrast agent concentration after intra‐cerebral injection in rat brain. It was applied to estimate the hindered diffusion in brain tissues of five contrast agents with different hydrodynamic diameters (Dotarem® ≈ 1 nm, P846 ≈ 4 nm, P792 ≈ 7 nm, P904 ≈ 22 nm and Gd‐based emulsion ≈ 170 nm). In vivo apparent diffusion coefficients were compared with those estimated in an obstacle‐free medium to determine brain extracellular space and tortuosity. At a 2 h imaging timescale, all contrast agents except the Gd‐based emulsion exhibited significant diffusion through brain tissues, with characteristic times compatible with MR molecular imaging (<70 min to diffuse between two capillaries). In conclusion, our experiments indicate that MRI contrast agents with sizes up to 22 nm can be used to perform molecular imaging on intra‐cerebral biomarkers. Our quantification methodology allows a precise estimation of apparent diffusion coefficients, which is helpful to calibrate optimal timing between contrast agent injection and MRI observation for molecular imaging studies. Copyright © 2012 John Wiley & Sons, Ltd.     

Effects of radiation on the NMR relaxation effects of aqueous solutions of gadolinium contrast agents

We investigate the effects of radiation on aqueous solutions of two common MRI contrast agents based on paramagnetic gadolinium chelates. Aqueous solutions of Gd–DTPA (gadolinium diethylenetriaminepentaacetic acid) and Gd–HP‐DO3A (gadoteridol), as well as Gd–DTPA in the presence of concentrations of several common metabolites (N‐acetylaspartate, Choline, Creatine and myo‐inositol) typical of conditions in vivo, were irradiated at dose levels up to 30 Gy using a 6 MV linear accelerator and imaged using a 1.5 T MRI system. In addition, the effects of radiation dose on solution relaxation rates were compared with the effects of temperature by subjecting the same samples to varying temperatures over a range of 4 K using the same experimental conditions and imaging sequences. Radiation caused small increases (1% or less at a dose of 20 Gy) in the relaxivity of solutions. The effects of radiation on relaxivities were orders of magnitude smaller than the effects of temperature over the range in this study. Copyright © 2008 John Wiley & Sons, Ltd.     

Morphology,binding behavior and MR‐properties of paramagnetic collagen‐binding liposomes

Collagen is an important component of the extracellular matrix (ECM) and plays an important role in normal tissue maturation and in pathological processes such as atherosclerosis and myocardial infarction. The diagnostics of the latter diseases using MRI could strongly benefit from the use of collagen‐specific contrast agents. The current study aimed to develop a bimodal liposomal MR contrast agent that was functionalized with CNA35, a collagen adhesion protein of the Staphylococcus aureus bacterium. The liposomes were characterized in terms of CNA35 protein conjugation and loading. The overall morphology was assessed with DLS and cryo‐TEM, while cryo‐TEM tomography was used to visualize the protein coverage of the liposomes. The binding properties of the contrast agent were investigated using a fluorescence assay based on the rhodamine content of the liposomes. The bulk relaxivity was determined using regular relaxometry while the MR‐properties of liposomes in their bound state were studied using NMR depth profiling. This CNA35 functionalized contrast agent and the set of in vitro experiments we performed indicate the potential of this technology for in vivo molecular imaging of collagen. Copyright © 2009 John Wiley & Sons, Ltd.     

How morphology influences relaxivity – comparative study of superparamagnetic iron oxide–polymer hybrid nanostructures

Superparamagnetic iron oxides (SPIOs) are widely used in MRI as T₂ contrast agents, and interest is still growing. Here, the T₂ relaxivity of three different SPIO–polymer hybrid morphologies, i.e. homogeneously distributed iron oxide within a polymer matrix, Janus‐like nanoparticles and polymer nanocapsules containing iron oxides, is studied. Making use of calculations based on theory for agglomerated systems, the obtained T₂ values could be predicted for all different morphologies, except for nanocapsules. Nanocapsules, in contrast to full spheres, allow for water exchange between encapsulated water and bulk water, and thus have two contributions to relaxivity. One originates from the capsules acting as a weakly magnetized cluster and the other stems from the individual SPIOs inside the capsule. Therefore, the relaxivities were also computed using an empirical equation found in the literature, which considers water exchange, resulting in a better T₂ forecast for the nanocapsules. The presented study is the first example of a comparison between measured and calculated relaxivities of nanocapsules. Copyright © 2015 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.     

Multimodal liposomes for SPECT/MR imaging as a tool for in situ relaxivity measurements

One of the major challenges of MR imaging is the quantification of local concentrations of contrast agents. Cellular uptake strongly influences different parameters such as the water exchange rate and the pool of water protons, and results in alteration of the contrast agent's relaxivity, therefore making it difficult to determine contrast agent concentrations based on the MR signal only. Here, we propose a multimodal radiolabeled paramagnetic liposomal contrast agent that allows simultaneous imaging with SPECT and MRI. As SPECT‐based quantification allows determination of the gadolinium concentration, the MRI signal can be deconvoluted to get an understanding of the cellular location of the contrast agent. The cell experiments indicated a reduction of the relaxivity from 2.7 ± 0.1 m m ^?1 s^?1 to a net relaxivity of 1.7 ± 0.3 m m ^?1 s^?1 upon cellular uptake for RGD targeted liposomes by means of the contrast agent concentration as determined by SPECT. This is not observed for nontargeted liposomes that serve as controls. We show that receptor targeted liposomes in comparison to nontargeted liposomes are taken up into cells faster and into subcellular structures of different sizes. We suggest that the presented multimodal contrast agent provides a functional readout of its response to the biological environment and is furthermore applicable in in vivo measurements. As this approach can be extended to several MRI‐based contrast mechanisms, we foresee a broader use of multimodal SPECT/MRI nanoparticles to serve as in vivo sensors in biological or medical research. Copyright © 2011 John Wiley & Sons, Ltd.     

Molecular imaging: challenges of bringing imaging of intracellular targets into common clinical use

Molecular imaging (MI) takes advantage of several new techniques to detect biomarkers or biochemical and cellular processes, with the goal of obtaining high sensitivity, specificity and signal‐to‐noise ratio imaging of disease. The imaging modalities bearing the most promise for MI are positron emission tomography (PET), single photon emission computer tomography (SPECT) and different optical imaging techniques with high sensitivity. Also magnetic resonance imaging (MRI) with contrast agents like ultra‐small superparamagnetic iron oxide particles (USPIO), magnetic resonance spectroscopy and ultrasound imaging with contrast agents may be useful approaches. MI techniques have been used in the clinic for many years, i.e. PET imaging using ¹⁸ F‐labeled fluorodeoxyglucose. Animal studies have during the last years revealed great potential for MI also with several other agents. The focus of the present article is the challenges of clinical imaging of intracellular targets following intravenous injection of the agents. Thus, the great challenge of getting enough contrast agent into the cytosol and at the same time obtaining a low signal from tissue just outside the diseased area is discussed. Copyright © 2012 John Wiley & Sons, Ltd.     

NMR relaxation and magnetic properties of superparamagnetic nanoworms

Maghemite particles are used as T₂ contrast agents for magnetic resonance imaging, especially for molecular and cellular imaging. Linear clusters of particles – called nanoworms – were recently developed to enhance the targeting efficiency. In this work, the magnetic and NMR relaxation properties of these nanoworms are studied at multiple magnetic fields. After the usual saturation at 0.5 T, the magnetization of the worms is still increasing, which results in an appreciable increase of the transverse relaxivity at high magnetic fields. The obtained relaxivities are typical of superparamagnetic particles of iron oxide (SPIOs). The transverse relaxation of the worms is clearly more efficient than for the isolated grains, which is confirmed by computer simulations. At high field, the longitudinal relaxation of the worms is less pronounced than for the grains, as expected for SPIOs. The nanoworms thus constitute a promising T₂ agent for cellular and molecular imaging. Copyright © 2010 John Wiley & Sons, Ltd.     

Increased transverse relaxivity in ultrasmall superparamagnetic iron oxide nanoparticles used as MRI contrast agent for biomedical imaging

Synthesis of a contrast agent for biomedical imaging is of great interest where magnetic nanoparticles are concerned, because of the strong influence of particle size on transverse relaxivity. In the present study, biocompatible magnetic iron oxide nanoparticles were synthesized by co‐precipitation of Fe²⁺ and Fe³⁺ salts, followed by surface adsorption with reduced dextran. The synthesized nanoparticles were spherical in shape, and 12 ± 2 nm in size as measured using transmission electron microscopy; this was corroborated with results from X‐ray diffraction and dynamic light scattering studies. The nanoparticles exhibited superparamagnetic behavior, superior T₂ relaxation rate and high relaxivities (r₁ = 18.4 ± 0.3, r₂ = 90.5 ± 0.8 s^?1 mM^?1, at 7 T). MR image analysis of animals before and after magnetic nanoparticle administration revealed that the signal intensity of tumor imaging, specific organ imaging and whole body imaging can be clearly distinguished, due to the strong relaxation properties of these nanoparticles. Very low concentrations (3.0 mg Fe/kg body weight) of iron oxides are sufficient for early detection of tumors, and also have a clear distinction in pre‐ and post‐enhancement of contrast in organs and body imaging. Many investigators have demonstrated high relaxivities of magnetic nanoparticles at superparamagnetic iron oxide level above 50 nm, but this investigation presents a satisfactory, ultrasmall, superparamagnetic and high transverse relaxivity negative contrast agent for diagnosis in pre‐clinical studies. Copyright © 2016 John Wiley & Sons, Ltd.     

Improving T1 and T2 magnetic resonance imaging contrast agents through the conjugation of an esteramide dendrimer to high‐water‐coordination Gd(III) hydroxypyridinone complexes

Commercial gadolinium magnetic resonance imaging (MRI) contrast agents are limited by low relaxivity (r₁) and coordination to only a single water molecule (q = 1). Consequently, gram quantities of these agents must be injected to obtain sufficient diagnostic contrast. In this study, MRI contrast agents for T₁ and T₂ relaxivity were synthesized using hydroxypyridinone and terephthalamide chelators with mesityl and 1,4,7‐triazacyclononane capping moieties. When covalently conjugated to a highly biocompatible esteramide dendrimer, T₂ relaxation rates up to 52 m m ^?1 s^?1 and T₁ relaxation rates up to 31 m m ^?1 s^?1 per gadolinium were observed under clinically relevant conditions. These values are believed to be brought about by using a dendritic macromolecule to decrease the molecular tumbling time of the small molecule complexes. These agents also show high aqueous solubility and low toxicity in vitro. In this study we report six new compounds: three discrete complexes and three dendrimer conjugates. Copyright © 2012 John Wiley & Sons, Ltd.     

Disruptive chemical doping in a ferritin‐based iron oxide nanoparticle to decrease r2 and enhance detection with T1‐weighted MRI

Inorganic doping was used to create flexible, paramagnetic nanoparticle contrast agents for in vivo molecular magnetic resonance imaging (MRI) with low transverse relaxivity (r₂). Most nanoparticle contrast agents formed from superparamagnetic metal oxides are developed with high r₂. While sensitive, they can have limited in vivo detection due to a number of constraints with T₂ or T₂*‐weighted imaging. T₁‐weighted imaging is often preferred for molecular MRI, but most T₁‐shortening agents are small chelates with low metal payload or are nanoparticles that also shorten T₂ and limit the range of concentrations detectable with T₁‐weighting. Here we used tungsten and iron deposition to form doped iron oxide crystals inside the apoferritin cavity to form a WFe nanoparticle with a disordered crystal and un‐coupled atomic magnetic moments. The atomic magnetic moments were thus localized, resulting in a principally paramagnetic nanoparticle. The WFe nanoparticles had no coercivity or saturation magnetization at 5 K and sweeping up to ±20 000 Oe, while native ferritin had a coercivity of 3000 Oe and saturation at ±20 000 Oe. This tungsten–iron crystal paramagnetism resulted in an increased WFe particle longitudinal relaxivity (r₁) of 4870 mm ^?1 s^?1 and a reduced transverse relaxivity (r₂) of 9076 mm ^?1 s^?1 compared with native ferritin. The accumulation of the particles was detected with T₁‐weighted MRI in concentrations from 20 to 400 nm in vivo, both injected in the rat brain and targeted to the rat kidney glomerulus. The WFe apoferritin nanoparticles were not cytotoxic up to 700 nm particle concentrations, making them potentially important for targeted molecular MRI. Copyright © 2014 John Wiley & Sons, Ltd.     

Kinetic analysis of hyaluronidase activity using a bioactive MRI contrast agent

One of the attractions of molecular imaging using ‘smart’ bioactive contrast agents is the ability to provide non‐invasive data on the spatial and temporal changes in the distribution and expression patterns of specific enzymes. The tools developed for that aim could potentially also be developed for functional imaging of enzyme activity itself, through quantitative analysis of the rapid dynamics of enzymatic conversion of these contrast agents. High molecular weight hyaluronan, the natural substrate of hyaluronidase, is a major antiangiogenic constituent of the extracellular matrix. Degradation by hyaluronidase yields low molecular weight fragments, which are proangiogenic. A novel contrast material, HA‐GdDTPA‐beads, was designed to provide a substrate analog of hyaluronidase in which relaxivity changes are induced by enzymatic degradation. We show here a first‐order kinetic analysis of the time‐dependent increase in R₂ as a result of hyaluronidase activity. The changes in R₂ and the measured relaxivity of intact HA‐GdDTPA‐beads (r_2B) and HA‐GdDTPA fragments (r_2D) were utilized for derivation of the temporal drop in concentration of GdDTPA in HA‐GdDTPA‐beads as the consequence of the release of HA‐GdDTPA fragments. The rate of dissociation of HA‐GdDTPA from the beads showed typical bell‐shaped temperature dependence between 7 and 36 °C with peak activity at 25 °C. The tools developed here for quantitative dynamic analysis of hyaluronidase activity by MRI would allow the use of activation of HA‐GdDTPA‐beads for the determination of the role of hyaluronidase in altering the angiogenic microenvironment of tumor micro metastases. Copyright © 2006 John Wiley & Sons, Ltd.     

Influence of molecular parameters and increasing magnetic field strength on relaxivity of gadolinium‐ and manganese‐based T1 contrast agents

Simulations were performed to understand the relative contributions of molecular parameters to longitudinal (r₁) and transverse (r₂) relaxivity as a function of applied field, and to obtain theoretical relaxivity maxima over a range of fields to appreciate what relaxivities can be achieved experimentally. The field‐dependent relaxivities of a panel of gadolinium and manganese complexes with different molecular parameters, water exchange rates, rotational correlation times, hydration state, etc. were measured to confirm that measured relaxivities were consistent with theory. The design tenets previously stressed for optimizing r₁ at low fields (very slow rotational motion; chelate immobilized by protein binding; optimized water exchange rate) do not apply at higher fields. At 1.5T and higher fields, an intermediate rotational correlation time is desired (0.5–4 ns), while water exchange rate is not as critical to achieving a high r₁. For targeted applications it is recommended to tether a multimer of metal chelates to a protein‐targeting group via a long flexible linker to decouple the slow motion of the protein from the water(s) bound to the metal ions. Per ion relaxivities of 80, 45, and 18 mM ^?1 s^?1 at 1.5, 3 and 9.4 T, respectively, are feasible for Gd³⁺ and Mn²⁺ complexes. Copyright © 2009 John Wiley & Sons, Ltd.     

Quantitative 1H MRI, 19F MRI,and 19F MRS of cell‐internalized perfluorocarbon paramagnetic nanoparticles

In vivo molecular imaging with targeted MRI contrast agents will require sensitive methods to quantify local concentrations of contrast agent, enabling not only imaging‐based recognition of pathological biomarkers but also detection of changes in expression levels as a consequence of disease development, therapeutic interventions or recurrence of disease. In recent years, targeted paramagnetic perfluorocarbon emulsions have been frequently applied in this context, permitting high–resolution ¹H MRI combined with quantitative ¹⁹F MR imaging or spectroscopy, under the assumption that the fluorine signal is not altered by the local tissue and cellular environment. In this in vitro study we have investigated the ¹⁹F MR–based quantification potential of a paramagnetic perfluorocarbon emulsion conjugated with RGD–peptide to target the cell–internalizing α_νβ₃–integrin expressed on endothelial cells, using a combination of ¹H MRI, ¹⁹F MRI and ¹⁹F MRS. The cells took up the targeted emulsion to a greater extent than nontargeted emulsion. The targeted emulsion was internalized into large 1–7 µm diameter vesicles in the perinuclear region, whereas nontargeted emulsion ended up in 1–4 µm diameter vesicles, which were more evenly distributed in the cytoplasm. Association of the targeted emulsion with the cells resulted in different proton longitudinal relaxivity values, r₁, for targeted and control nanoparticles, prohibiting unambiguous quantification of local contrast agent concentration. Upon cellular association, the fluorine R₁ was constant with concentration, while the fluorine R₂ increased nonlinearly with concentration. Even though the fluorine relaxation rate was not constant, the ¹⁹F MRI and ¹⁹F MRS signals for both targeted nanoparticles and controls were linear and quantifiable as function of nanoparticle concentration. Copyright © 2010 John Wiley & Sons, Ltd.     

Iron‐loaded PLLA nanoparticles as highly efficient intracellular markers for visualization of mesenchymal stromal cells by MRI

Monitoring of the fate of cells after injection appears paramount for the further development of cell therapies. In this context magnetic resonance imaging (MRI) is increasing in relevance owing to its unique tissue visualization properties. For assessment of cell trafficking and homing, the cells have to be labeled to become MR visible. The rather low sensitivity of MRI demands dedicated intracellular markers with high payloads of MR contrast agents for ensuring sensitive detection of local cell aggregations. In the presented work the application of custom‐designed nanometer‐sized iron oxide loaded poly‐(l ‐lactide) (iPLLA) nanoparticles was investigated. The particles were synthesized by the mini‐emulsion process and evaluated for labeling of mesenchymal stromal cells (MSCs). The efficient cellular uptake and long intracellular retention times of the particles as well as their nontoxicity are demonstrated. The average cellular iron content was 55 pg iron per cell. Further incorporation of, for example, fluorescent dye enables the generation of multireporter particles, providing the great potential for multimodal imaging. The efficiency of these nanoparticles as MRI contrast agent was evaluated in vitro using relaxation rate mapping, yielding relaxivities r₂ = 273.3, r₂^* = 545.1 mm ^?1 s^?1 at 3 T and r₂ = 415.7, r₂^* = 872.3 mm ^?1 s^?1 at 11.7 T. The high r₂^* relaxivity of the iPLLA nanoparticles enabled visualization of a single labeled cell in vitro at 50‐µm spatial resolution. In vivo evaluation in a rat injury model revealed the potential of the iPLLA particles to efficiently label MSCs for MRI monitoring of ~20 000–40 000 injected cells at 11.7 T. In conclusion the presented work demonstrates the applicability of iPLLA particles as efficient intracellular marker for MSC labeling for monitoring the fate of the cells by MRI. Copyright © 2014 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.     

Relaxivity enhancement of aquated Tris(β‐diketonate)gadolinium(III) chelates by confinement within ultrashort single‐walled carbon nanotubes

Ultrashort single‐walled carbon nanotubes loaded with gadolinium ions (gadonanotubes) have been previously shown to exhibit extremely high T₁‐weighted relaxivities (>100 mm ^?1 s^?1). To further examine the effect of nanoconfinement on the relaxivity of gadolinium‐based contrast agents for magnetic resonance imaging, a series of ultrashort single‐walled carbon nanotube (US‐tube) materials internally loaded with gadolinium chelates have been prepared and studied. US‐tubes were loaded with Gd(acac)₃ · 2H₂O, Gd(hfac)₃ · 2H₂O, and Gd(thd)₃ (acac = acetylacetone, hfac = hexafluoroacetylacetone, thd = tetramethylheptanedione). The longitudinal relaxivities of the prepared materials determined at 25°C in a 1.5 T field were 103 mm ^?1 s^?1 for Gd(acac)₃ · 2H₂O@US‐tubes, 105 mm ^?1 s^?1 for Gd(hfac)₃ · 2H₂O@US‐tubes and 26 mm ^?1 s^?1 for Gd(thd)₃@US‐tubes. Compared with the relaxivities obtained for the unloaded chelates (<10 mm ^?1 s^?1) as well as accounting for the T₁ reduction observed for the empty US‐tubes, the boost in relaxivity for chelate‐loaded US‐tubes is attributed to confinement within the nanotube and depends on the number of coordinated water molecules. Copyright © 2014 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.     

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.     

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.