靶向高分子造影剂的制备及其体外寻靶实验

目的制备一种特异性靶向肝癌细胞的高分子超声造影剂,并考察其体外寻靶能力。方法采用双乳化法制备高分子材料PLGA-COOH造影剂;并用碳二亚胺法将造影剂与抗体偶联制备出靶向高分子造影剂。检测该造影剂一般特性及体外寻靶能力,并与普通高分子造影剂做比较。结果高分子造影剂的平均粒径600.5 nm,体外寻靶试验显示,该靶向高分子造影剂较多并牢固的聚集到肝癌细胞表面;普通高分子造影剂对照组末见造影剂和细胞的结合。结论成功制备出特异性结合人肝癌抗体的靶向高分子超声造影剂。该造影剂在体外对肝癌细胞具有较强的特异性亲合力。     

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

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

Targeted ultrasound contrast agent for molecular imaging of inflammation in high‐shear flow

Targeted ultrasound contrast materials (gas‐filled microbubbles carrying ligands to endothelial selectins or integrins) have been investigated as potential molecular imaging agents. Such microbubbles normally exhibit good targeting capability at the slower flow conditions. However, in the conditions of vigorous flow, binding may be limited. Here, we describe a microbubble capable of efficient binding to targets both in slow and fast flow (exceeding 4 dyne/cm² wall shear stress) using a clustered polymeric form of the fast‐binding selectin ligand sialyl Lewis^X. Microbubbles were prepared from decafluorobutane gas and stabilized with a monolayer of phosphatidylcholine, PEG stearate and biotin‐PEG‐lipid. Biotinylated PSLe^x (sialyl Lewis^X polyacrylamide) or biotinylated anti‐P‐selectin antibody (RB40.34) was attached to microbubbles via a streptavidin bridge. In a parallel plate flow chamber targeted adhesion model, PSLe^x bubbles demonstrated specific adhesion, retention and slow rolling on P‐selectin‐coated plates. Efficiency of firm targeted adhesion to a P‐selectin surface (140 molecules/µm²) was comparable for antibody‐carrying bubbles and PSLe^x‐targeted bubbles at 0.68 dyne/cm² shear stress. At fast flow (4.45 dyne/cm²), PSLe^x‐targeted bubbles maintained their ability to bind, while antibody‐mediated targeting dropped more than 20‐fold. At lower surface density of P‐selectin (7 molecules/µm²), targeting via PSLe^x was more efficient than via antibody under all the flow conditions tested. Negative control casein‐coated plates did not retain bubbles in the range of flow conditions studied. To confirm echogenicity, targeted PSLe^x‐bubbles were visualized on P‐selectin‐coated polystyrene plates by ultrasound imaging with a clinical scanner operated in pulse inversion mode; control plates lacking targeted bubbles did not show significant acoustic backscatter. In vivo, in a murine model of inflammation in the femoral vein setting, targeting efficacy of intravenously administered PSLe^x‐microbubbles was comparable with targeting mediated by anti‐P‐selectin antibody, and significantly exceeded the accumulation of non‐targeted control bubbles. In the inflamed femoral artery setting, PSLe^x‐mediated microbubble targeting was superior to antibody‐mediated targeting. Copyright © 2006 John Wiley & Sons, Ltd.     

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

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

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.     

Adventures in multivalency,the Harry S. Fischer memorial lecture CMR 2005; Evian,France

This review discusses multivalency in the context of drug discovery, specifically the discovery of new diagnostic imaging and related agents. The aim is to draw attention to the powerful role that multivalency plays throughout research involving molecular biology, in general, and much of biochemically targeted contrast agent research, in particular. Two examples from the author's laboratory are described. We created small (～5 kDa) peptide ‘dimers’ composed of two different, chemically linked peptides. The monomer peptides both bound to the same target protein with K_d ≈ 100 s nM , while the heterodimers had sub‐nM K_d values. Biological activity was evident in the heterodimers where none or very little existed in homodimers, monomers or monomer mixtures. Two different tyrosine kinases (KDR and C‐Met) and four peptide families produced consistent results: multivalent heterodimers were uniquely different. The second example begins with making two micron ultrasound bubbles coated with the peptide, TKPPR (a Tuftsin antagonist) as a negative control for bubbles targeted with angiogenesis target‐binding peptides. Unexpected binding of a ‘negative’ control, (TKPPR)‐targeted bubble to endothelial cells expressing angiogenesis targets, led to the surprising result that TKPPR, only when multimerized, binds avidly, specifically and actively to neuropilin‐1, a VEGF co‐receptor. VEGF is the primary stimulator of angiogenesis. Tuftsin is a small peptide (TKPR) derived from IgG that binds to macrophages during inflammation, and has been studied for over 30 years. The receptor has never been cloned. The results led to new conclusions about Tuftsin, neuropilin‐1 and the purpose, up to now unknown, of exon 8 in VEGF. Multivalency can be used rationally to solve practical problems in drug discovery. When targeting larger structures, multivalency is frequently unavoidable, and can lead to unpredictable and useful biochemical information, as well as to new drug candidates. Copyright © 2006 John Wiley & Sons, Ltd.     

Contrast enhancement by lipid‐based MRI contrast agents in mouse atherosclerotic plaques; a longitudinal study

The use of contrast‐enhanced MRI to enable in vivo specific characterization of atherosclerotic plaques is increasing. In this study the intrinsic ability of two differently sized gadolinium‐based contrast agents to enhance atherosclerotic plaques in ApoE^?/? mice was evaluated with MRI. We obtained a kinetic profile for contrast enhancement, as the literature data on optimal imaging time points is scarce, and assessed the longer‐term kinetics. Signal enhancement in the wall of the aortic arch, following intravenous injection of paramagnetic micelles and liposomes, was followed for 1 week. In vivo T₁‐weighted MRI plaque enhancement characteristics were complemented by fluorescence microscopy of NIR₆₆₄ incorporated in the contrast agents and quantification of tissue and blood Gd–DTPA. Both micelles and liposomes enhanced contrast in T₁‐weighted MR images of plaques in the aortic arch. The average contrast‐to‐noise ratio increased after liposome or micelle injection to 260 or 280% respectively, at 24 h after injection, compared with a pre‐scan. A second wave of maximum contrast enhancement was observed around 60–72 h after injection, which only slowly decreased towards the 1 week end‐point. Confocal fluorescence microscopy and whole body fluorescence imaging confirmed MRI‐findings of accumulation of micelles and liposomes. Plaque permeation of contrast agents was not strongly dependent on the contrast agent size in this mouse model. Our results show that intraplaque accumulation over time of both contrast agents leads to good plaque visualization for a long period. This inherent intraplaque accumulation might make it difficult to discriminate passive from targeted accumulation. This implies that, in the development of targeted contrast agents on a lipid‐based backbone, extensive timing studies are required. Copyright © 2012 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.     

Targeting of lanthanide(III) chelates of DOTA‐type glycoconjugates to the hepatic asyaloglycoprotein receptor: cell internalization and animal imaging studies

The characterization of a new class of hydrophilic liver‐targeted agents for γ‐scintigraphy and MRI, consisting, respectively, of [¹⁵³Sm]³⁺ or Gd³⁺ complexes of DOTA monoamide or bisamide linked glycoconjugates (DOTA = 1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetraacetic acid), is reported. In vitro studies show high uptake of radiolabeled [¹⁵³Sm]‐DOTAGal₂ by the human hepatocyte carcinoma cell line Hep G2 containing the asialoglycoprotein receptor (ASGP‐R), which is decreased to less than 50% by the presence of its high‐affinity ligand asialofetuin (ASF). In vivo biodistribution, γ‐imaging and pharmacokinetic studies on Wistar rats using the [¹⁵³Sm]³⁺‐labeled glycoconjugates show a high uptake in the receptor‐rich organ liver of the radiolabeled compounds containing terminal galactosyl groups, but very little uptake for those compounds with terminal glycosyl groups. Blocking the receptor in vivo reduced liver uptake by 90%, strongly suggesting that the liver uptake of these compounds is mediated by their binding to the asyaloglycoprotein receptor (ASGP‐R). This study also demonstrated that the valency increase improves the targeting capability of the glycoconjugates, which is also affected by their topology. However despite the specific liver uptake of the radiolabeled galactose‐bearing multivalent compounds, the animal MRI assessment of the corresponding Gd³⁺ chelates shows liver‐to‐kidney contrast effects which are not significantly better than those shown by GdDTPA. This probably results from the quick wash‐out from the liver of these highly hydrophilic complexes, before they can be sufficiently concentrated within the hepatocytes via receptor‐mediated endocytosis. Copyright © 2006 John Wiley & Sons, Ltd.     

In vivo quantification of ultrasound targeted microbubbles to enhance cancer assessment

Contrast‐enhanced ultrasound with targeted microbubble contrast agents is an emerging technique for imaging biological processes at the molecular level. The accumulation of targeted microbubbles at tissue sites overexpressing specific molecular markers increases the backscattered signal for noninvasive evaluations of diseases. The aim of this preliminary study was to combine molecular imaging with an in vivo contrast agent quantification to support the early diagnosis of the pathology and to enhance the assessment of neoplastic tissues. Tumor growth was induced by subcutaneous injection of prostate cancer cells in four rats. Microbubbles targeted to tissue factor (TF) were administered. A vascularized region located in proximity to the tumor and centered around the focus depth was analyzed in each animal. The backscattered signals (i.e. the radio‐frequency data) were acquired during two different perfusion conditions to evaluate the contribution of attached microbubbles. After image generation by means of a multi‐pulse contrast‐enhanced technique, a nonlinear regression method based on the support vector machine was employed to estimate the contrast agent concentrations in cubic voxels (1‐mm side length). The number of attached microbubbles per mm³ was estimated based on a multi‐dimensional vector of features extracted from the processed radio‐frequency signals. A significant correlation (p < 0.05) between the size of the tumors and the estimated microbubble concentration was found, thus opening the possibility for combining molecular imaging and contrast agent concentration mapping to refine pathology evaluation. Copyright © 2016 John Wiley & Sons, Ltd.     

Fluorometry,a fast screening technique for non‐covalent binding of contrast agents to human serum albumin?

The non‐covalent interactions of gadolinium‐based MRI contrast agents with macromolecules, such as human serum albumin (HSA), increase their efficacy. The identification of contrast agents that interact with HSA is a crucial first step in the complex, lengthy and expensive developmental process of a new potential HSA‐targeting contrast agent. Fluorometry has been used as a possibly simpler and more effective tool of screening. In this study, the affinity of four compounds (Gd–DTPA, Gd–BOPTA, Gd–EOB–DTPA and MS‐325) for HSA was investigated. The results show that the fluorescence method is a convenient tool that can easily detect this kind of non‐covalent interaction owing to the small amount of required compound, the simplicity of the procedure and the popularity of the instrument, compared with the other approaches reported in the literature. However, fluorescence screening tests should be interpreted with caution since false‐negative results will occur when the binding site of a gadolinium‐based agent is far away from the location of the sole Trp residue of HSA or when an unsuitable site‐marker is selected. Copyright © 2013 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.     

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

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

Modified lipoproteins as contrast agents for imaging of atherosclerosis

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

Sub‐second proton imaging of 13C hyperpolarized contrast agents in water

Indirect proton detection of ¹³C hyperpolarized contrast agents potentially enables greater sensitivity. Presented here is a study of sub‐second projection imaging of hyperpolarized ¹³C contrast agent addressing the obstacle posed by water suppression for indirect detection in vivo. Sodium acetate phantoms were used to develop and test water suppression and sub‐second imaging with frequency‐selective RF pulses using spectroscopic and imaging indirect proton detection. A 9.8 mm aqueous solution of ¹³C PHIP hyperpolarized 2‐hydroxyethyl‐¹³C‐propionate‐d_2,3,3 (HEP), <P > ~25% was used for demonstration of indirect proton sub‐second imaging detection. Balanced 2D FSSFP (fast steady‐state free precession) allowed the recording of proton images with a field of view of 64 × 64 mm² and spatial resolution 2 × 2 mm² with total acquisition time of less than 0.2 s. In thermally polarized sodium 1‐¹³C‐acetate, ¹³C to ¹H polarization transfer efficiency of 45.1% of the theoretically predicted values was observed in imaging detection corresponding to an 11‐fold overall sensitivity improvement compared with direct ¹³C FSSFP imaging. ¹³C to ¹H polarization transfer efficiency of 27% was observed in imaging detection, corresponding to a 3.25‐fold sensitivity improvement compared with direct ¹³C FSSFP imaging with hyperpolarized HEP. The range of potential applications and limitations of this sub‐second and ultra‐sensitive imaging approach are discussed. 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.     

A physiologically based pharmacokinetic model of vascular–extravascular exchanges during liver carcinogenesis: application to MRI contrast agents

The extraction of physiological parameters by non‐invasive imaging techniques such as dynamic magnetic resonance imaging (MRI) or positron emission tomography requires a knowledge of molecular distribution and exchange between microvascularization and extravascular tissues. These phenomena not only depend on the physicochemical characteristics of the injected molecules but also the pathophysiological state of the targeted organ. We developed a five‐compartment physiologically based pharmacokinetic model focused on hepatic carcinogenesis and MRI contrast agents. This model includes physical characteristics of the contrast agent, dual specific liver supply, microvessel wall properties and transport parameters that are compatible with hepatocarcinoma development. The evolution of concentrations in the five compartments showed significant differences in the distribution of three molecules (differentiated by their diameters and diffusion coefficients ranging, respectively, from 0.9 to 62 nm and from 68.10^?9 to 47.10^?7 cm² s^?1) in simulated regeneration nodules and dysplastic nodules, as well as in medium‐ and poorly differentiated hepatocarcinoma. These results are in agreement with known vascular modifications such as arterialization that occur during hepatocarcinogenesis. This model can be used to study the pharmacokinetics of contrast agents and consequently to extract parameters that are characteristic of the tumor development (like permeability), after fitting simulated to in vivo data. Copyright © 2007 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.     

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.     

Clinical and biological consequences of transmetallation induced by contrast agents for magnetic resonance imaging: a review

Gadolinium-based contrast agents (CAs) are widely used to enhance the contrast of images in magnetic resonance imaging procedures. Two categories of gadolinium chelates exist: the macrocyclic molecules where Gd3+ is caged in the pre-organized cavity of the ligand and the linear molecules. Gadolinium chelates differ in their thermodynamic stability constants and in their kinetic stability. In general, macrocyclic chelates such as Gd-DOTA or Gd-HP-DO3A are more stable than linear molecules. Even among linear agents, differences can be found. There is increasing evidence that transmetallation can be found in vivo, in the case of certain CAs (especially linear chelates), with body cations such as zinc, calcium or iron. Furthermore, analytical interference with colorimetric determination of calcium has been clinically evidenced with two linear chelates, Gd-DTPA-BMA and Gd-DTPA-BMEA. Clinical cases of spurious hypocalcaemia have been reported with these molecules. Such interference with some colorimetric assays for calcium is clinically relevant in that it can lead to unnecessary and potentially harmful treatment for hypocalcaemia.