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.     

Concentration‐independent MRI of pH with a dendrimer‐based pH‐responsive nanoprobe

The measurement of extracellular pH (pH_e) has significant clinical value for pathological diagnoses and for monitoring the effects of pH‐altering therapies. One of the major problems of measuring pH_e with a relaxation‐based MRI contrast agent is that the longitudinal relaxivity depends on both pH and the concentration of the agent, requiring the use of a second pH‐unresponsive agent to measure the concentration. Here we tested the feasibility of measuring pH with a relaxation‐based dendritic MRI contrast agent in a concentration‐independent manner at clinically relevant field strengths. The transverse and longitudinal relaxation times in solutions of the contrast agent (GdDOTA‐4AmP)₄₄‐G5, a G5–PAMAM dendrimer‐based MRI contrast agent in water, were measured at 3 T and 7 T magnetic field strengths as a function of pH. At 3 T, longitudinal relaxivity (r₁) increased from 7.91 to 9.65 mM^?1 s^?1 (on a per Gd³⁺ basis) on changing pH from 8.84 to 6.35. At 7 T, r₁ relaxivity showed pH response, albeit at lower mean values; transverse relaxivity (r₂) remained independent of pH and magnetic field strengths. The longitudinal relaxivity of (GdDOTA‐4AmP)₄₄‐G5 exhibited a strong and reversible pH dependence. The ratio of relaxation rates R₂/R₁ also showed a linear relationship in a pH‐responsive manner, and this pH response was independent of the absolute concentration of (GdDOTA‐4AmP)₄₄‐G5 agent. Importantly, the nanoprobe (GdDOTA‐4AmP)₄₄‐G5 shows pH response in the range commonly found in the microenvironment of solid tumors. 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.     

Connexin 43‐targeted T1 contrast agent for MRI diagnosis of glioma

Glioblastoma multiforme is the most aggressive form of brain tumor. Early and accurate diagnosis of glioma and its borders is an important step for its successful treatment. One of the promising targets for selective visualization of glioma and its margins is connexin 43 (Cx43), which is highly expressed in reactive astrocytes and migrating glioma cells. The purpose of this study was to synthesize a Gd‐based contrast agent conjugated with specific antibodies to Cx43 for efficient visualization of glioma C6 in vivo. We have prepared stable nontoxic conjugates of monoclonal antibody to Cx43 and polylysine–DTPA ligands complexed with Gd(III), which are characterized by higher T₁ relaxivity (6.5 mM^?1 s^?1 at 7 T) than the commercial agent Magnevist® (3.4 mM^?1 s^?1). Cellular uptake of Cx43‐specific T₁ contrast agent in glioma C6 cells was more than four times higher than the nonspecific IgG‐contrast agent, as detected by flow cytometry and confocal analysis. MRI experiments showed that the obtained agents could markedly enhance visualization of glioma C6 in vivo after their intravenous administration. Significant accumulation of Cx43‐targeted contrast agents in glioma and the peritumoral zone led not only to enhanced contrast but also to improved detection of the tumor periphery. Fluorescence imaging confirmed notable accumulation of Cx43‐specific conjugates in the peritumoral zone compared with nonspecific IgG conjugates at 24 h after intravenous injection. All these features of Cx43‐targeted contrast agents might be useful for more precise diagnosis of glioma and its borders by MRI. Copyright © 2015 John Wiley & Sons, Ltd.     

Positive MRI contrast enhancement in THP‐1 cells with Gd2O3 nanoparticles

There is a demand for more efficient and tissue‐specific MRI contrast agents and recent developments involve the design of substances useful as molecular markers and magnetic tracers. In this study, nanoparticles of gadolinium oxide (Gd₂O₃) have been investigated for cell labeling and capacity to generate a positive contrast. THP‐1, a monocytic cell line that is phagocytic, was used and results were compared with relaxivity of particles in cell culture medium (RPMI 1640). The results showed that Gd₂O₃‐labeled cells have shorter T₁ and T₂ relaxation times compared with untreated cells. A prominent difference in signal intensity was observed, indicating that Gd₂O₃ nanoparticles can be used as a positive contrast agent for cell labeling. The r₁ for cell samples was 4.1 and 3.6 s^?1 mm ^?1 for cell culture medium. The r₂ was 17.4 and 12.9 s^?1 mm ^?1, respectively. For r₁, there was no significant difference in relaxivity between particles in cells compared to particles in cell culture medium, (p_r1 = 0.36), but r₂ was significantly different for the two different series (p_r2 = 0.02). Viability results indicate that THP‐1 cells endure treatment with Gd₂O₃ nanoparticles for an extended period of time and it is therefore concluded that results in this study are based on viable cells. Copyright © 2008 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.     

Characterization of MRI contrast agent‐loaded polymeric nanocapsules as versatile vehicle for targeted imaging

Various contrast agents (Magnevist®, Gadovist® and Multihance®) loaded into polymeric nanocapsules were synthesized by the inverse miniemulsion technique. The relaxivity of the resultant contrast agents was assessed at 1.5 T magnetic field strength. The ionic relaxivity of the contrast agents could at least be maintained after their encapsulation in different polymer capsules. The chemical composition of the nanocapsules was characterized by Fourier transform infrared spectroscopy. The distribution of the contrast agent in the nanocapsules could be identified by energy filtered transmission electron microscopy and energy dispersive X‐ray spectroscopy. The results indicate entrapment of the gadolinium complex into the inner shell of the polymeric nanocapsules. The payload of contrast agent per nanocapsule resulted in some 2.5 × 10⁶ Gd³⁺ complexes yielding a particle‐based relaxivity of 10.75 × 10⁶ mM ^?1 s^?1. Maintained or even slightly increased ionic relaxivity of the different contrast agents after encapsulation in combination with high payloads and the possibility of functionalization of the capsules' surface facilitate the application of the nanocapsules as promising targeted contrast agents for MRI. Copyright © 2010 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.     

Impact of rigidification on relaxometric properties of a tricyclic tetraazatriacetic gadolinium chelate

A constrained derivative of Gd–PCTA12, Gd–cyclo‐PCTA12, in which one ethylene bridge connecting two nitrogen atoms of the triamine block is replaced by a cyclohexylene bridge, was synthesized and the impact of rigidification was studied by comparing the physicochemical and relaxometric properties of both gadolinium MRI contrast agents, Gd–PCTA12 and Gd–cyclo‐PCTA12. The new complex has higher proton relaxivity than the parent compound (r₁ = 6.1 s^?1 mM ^?1 at 20 MHz and 310 K). The rigidification of the PCTA12 scaffold proved to have no impact on the inertness towards transmetallation by endogenous ions such as Zn²⁺. Moreover, for both contrast agents, the relaxivity was not quenched by endogenous anions. The oxygen‐17 NMR study and the NMRD profile demonstrated that the rigidification of the PCTA scaffold had no impact on the electronic relaxation of Gd–cyclo‐PCTA12. However, the rigidity of this complex induced an acceleration of the exchange rate of the inner‐sphere water molecules as a result of steric crowding around the gadolinium ion. The value of τ_M³¹⁰ thus approached the optimal value required to attain high relaxivity once the chelate is immobilized by covalent or non‐covalent binding to macromolecules. Copyright © 2006 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.     

Mn‐citrate and Mn‐HIDA: intermediate‐affinity chelates for manganese‐enhanced MRI

In this study we investigated two manganese chelates in order to improve the image enhancement of manganese‐enhanced MRI and decrease the toxicity of free manganese ions. Since both MnCl₂ and a low‐affinity chelate were associated with a slow continuous decrease of cardiac functions, we investigated intermediate‐affinity chelates: manganese N‐(2‐hydroxyethyl)iminodiacetic acid (Mn‐HIDA) and Mn‐citrate. The T₁ relaxivity values for Mn‐citrate (4.4 m m ^?1 s^?1) and Mn‐HIDA (3.3 m m ^?1 s^?1) in artificial cerebrospinal fluid (CSF) were almost constant in a concentration range from 0.5 to 5 m m at 37 °C and 4.7 T. In human plasma, the relaxivity values increased when the concentrations of these Mn chelates were decreased, suggesting the presence of free Mn²⁺ bound with serum albumin. Mn‐HIDA and Mn‐citrate demonstrated a tendency for better contractility when employed with an isolated perfused frog heart, compared with MnCl₂. Only minimal changes were demonstrated after a venous infusion of 100 m m Mn‐citrate or Mn‐HIDA (8.3 µmol kg^?1 min^?1) in rats and a constant heart rate, arterial pressure and sympathetic nerve activity were maintained, even after breaking the blood–brain barrier (BBB). Mn‐citrate and Mn‐HIDA could not cross the intact BBB and appeared in the CSF, and then diffused into the brain parenchyma through the ependymal layer. The responses in the supraoptic nucleus induced by the hypertonic stimulation were detectable. Therefore, Mn‐citrate and Mn‐HIDA appear to be better choices for maintaining the vital conditions of experimental animals, and they may improve the reproducibility of manganese‐enhanced MRI of the small nuclei in the hypothalamus and thalamus. Copyright © 2012 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.     

A dual CT‐MR dendrimer contrast agent as a surrogate marker for convection‐enhanced delivery of intracerebral macromolecular therapeutic agents

The feasibility of using Gd dendrimer‐based macromolecules (Gd‐G8 dendrimer) as a dual CT and MR contrast agent for monitoring convection‐enhanced delivery of therapy in the brain is evaluated both in vitro and in vivo with optimal dosing established. In vitro CT attenuation values of the Gd‐based agents (～6.0 HU mM ^?1) were ～1.6 times greater than iodine‐based agents and the attenuation of the Gd‐DTPA was comparable to Gd‐G8 dendrimer. Visible enhancement was observed on both CT and MR using Gd‐G8 dendrimer over a range of 23–78 mM ; however, a concentration of at least 47 mM in Gd was required for adequate delineation of the injection site on both CT and MR. MR offers greater sensitivity than CT in estimating the volume of distribution (V_d) and effectively quantified the agent's concentration and diffusion using T₁ mapping at much lower concentrations of Gd (<10 mM in [Gd]). Copyright © 2008 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.     

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.     

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.     

T2 relaxation time of hyaline cartilage in presence of different gadolinium‐based contrast agents

The transverse relaxation time, T₂, of native cartilage is used to quantify cartilage degradation. T₂ is frequently measured after contrast administration, assuming that the impact of gadolinium‐based contrast agents on cartilage T₂ is negligible. To verify this assumption the depth‐dependent variation of T₂ in the presence of gadopentetate dimeglumine, gadobenate dimeglumine and gadoteridol was investigated. Furthermore, the r₂/r₁ relaxivity ratios were quantified in different cartilage layers to demonstrate differences between T₂ and T₁ relaxation effects. Transverse high‐spatial‐resolution T₁‐ and T₂‐maps were simultaneously acquired on a 1.5 T MR scanner before and after contrast administration in nine bovine patellae using a turbo‐mixed sequence. The r₂/r₁ ratios were calculated for each contrast agent in cartilage. Profiles of T₁, T₂ and r₂/r₁ across cartilage thickness were generated in the absence and presence of contrast agent. The mean values in different cartilage layers were compared for global variance using the Kruskal–Wallis test and pairwise using the Mann–Whitney U‐test. T₂ of unenhanced cartilage was 98 ± 5 ms at 1 mm and 65 ± 4 ms at 3 mm depth. Eleven hours after contrast administration significant differences (p < 0.001) were measurable for all three contrast agents. T₂ values were 58 ± 2 and 62 ± 3 ms for gadopentetate dimeglumine, 46 ± 2 and 57 ± 2 ms for gadobenate dimeglumine, and 38 ± 2 and 42 ± 2 ms for gadoteridol at 1 and 3 mm depths, respectively. The r₂/r₁ relaxivity ratios across cartilage thickness were close to 1.0 (range 0.9–1.3). At 1.5 T, T₂ decreased significantly in the presence of contrast agents, more pronounced in superficial than in deep cartilage. The change in T₂ relaxation rate was similar to the change in T₁. Cartilage T₂ measurements after contrast administration will lead to systematic errors in the quantification of cartilage degradation. 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.     

Targeted RGD nanoparticles for highly sensitive in vivo integrin receptor imaging

A new magnetic resonance imaging (MRI) contrast bearing RGD peptide is reported. In this study, ultrasmall superparamagnetic iron oxide (USPIO) nanoparticles with various sizes were prepared. Particles sizes between 6 and 13 nm were tuned by varying the stirring rate. Remarkable negative contrast was observed because USPIO nanoparticles (13.1 ± 2.1 nm) generated high transversal relaxivity r₂ (188 ± 3 m m ^?1 s^?1) and saturation magnetization (94 emu g^?1 Fe). The USPIO nanoparticles were coated with PDA [2‐(pyridyldithio)‐ethylamine; PDA nanoparticles] containing functional polymer, which can be readily synthesized by Michael addition. The PDA nanoparticles were conjugated with RGD peptide (RGD nanoparticles) for targeting the specific site. The target specificity and high relaxivity allowed RGD nanoparticles to differentiate the expression level of integrin receptor on several cell lines and tumors (MCF‐7, A‐549, HT‐29 and HT‐1080) by in vitro and in vivo MR imaging. Importantly, a remarkable negative contrast (?51.3 ± 6.7%) was observed for in vivo MR imaging of MCF‐7 tumor. This result implies that the RGD nanoparticles that greatly enhance the MR imaging are highly sensitive for early stage tumor detection. Copyright © 2012 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.