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

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

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.     

Stem cell labeling for magnetic resonance imaging

In vivo applications of cells for the monitoring of their cell dynamics increasingly use non‐invasive magnetic resonance imaging. This imaging modality allows in particular to follow the migrational activity of stem cells intended for cell therapy strategies. All these approaches require the prior labeling of the cells under investigation for excellent contrast against the host tissue background in the imaging modality. The present review discusses the various routes of cell labeling and describes the potential to observe both cell localization and their cell‐specific function in vivo. Possibilities for labeling strategies, pros and cons of various contrast agents are pointed out while potential ambiguities or problems of labeling strategies are emphasized.     

Bioresponsive probes for molecular imaging: concepts and in vivo applications

Molecular imaging is a powerful tool to visualize and characterize biological processes at the cellular and molecular level in vivo. In most molecular imaging approaches, probes are used to bind to disease‐specific biomarkers highlighting disease target sites. In recent years, a new subset of molecular imaging probes, known as bioresponsive molecular probes, has been developed. These probes generally benefit from signal enhancement at the site of interaction with its target. There are mainly two classes of bioresponsive imaging probes. The first class consists of probes that show direct activation of the imaging label (from “off” to “on” state) and have been applied in optical imaging and magnetic resonance imaging (MRI). The other class consists of probes that show specific retention of the imaging label at the site of target interaction and these probes have found application in all different imaging modalities, including photoacoustic imaging and nuclear imaging. In this review, we present a comprehensive overview of bioresponsive imaging probes in order to discuss the various molecular imaging strategies. The focus of the present article is the rationale behind the design of bioresponsive molecular imaging probes and their potential in vivo application for the detection of endogenous molecular targets in pathologies such as cancer and cardiovascular disease. 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.     

Lanthanide ion (III) complexes of 1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetraaminophosphonate for dual biosensing of pH with chemical exchange saturation transfer (CEST) and biosensor imaging of redundant deviation in shifts (BIRDS)

Relaxivity‐based magnetic resonance of phosphonated ligands chelated with gadolinium (Gd³⁺) shows promise for pH imaging. However instead of monitoring the paramagnetic effect of lanthanide complexes on the relaxivity of water protons, biosensor (or molecular) imaging with magnetic resonance is also possible by detecting either the nonexchangeable or the exchangeable protons on the lanthanide complexes themselves. The nonexchangeable protons (e.g. –CH_x, where 3 ≥ x ≥ 1) are detected using a three‐dimensional chemical shift imaging method called biosensor imaging of redundant deviation in shifts (BIRDS), whereas the exchangeable protons (e.g. –OH or –NH_y, where 2 ≥ y ≥ 1) are measured with chemical exchange saturation transfer (CEST) contrast. Here we tested the feasibility of BIRDS and CEST for pH imaging of 1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetraaminophosphonate (DOTA‐4AmP^8?) chelated with thulium (Tm³⁺) and ytterbium (Yb³⁺). BIRDS and CEST experiments show that both complexes are responsive to pH and temperature changes. Higher pH and temperature sensitivities are obtained with BIRDS for either complex when using the chemical shift difference between two proton resonances vs using the chemical shift of a single proton resonance, thereby eliminating the need to use water resonance as reference. While CEST contrast for both agents is linearly dependent on pH within a relatively large range (i.e. 6.3–7.9), much stronger CEST contrast is obtained with YbDOTA‐4AmP^5? than with TmDOTA‐4AmP^5?. In addition, we demonstrate the prospect of using BIRDS to calibrate CEST as new platform for quantitative pH imaging. Copyright © 2014 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.     

Visualization of delayed release of compounds from pH‐sensitive capsules in vitro and in vivo in a hamster model

Delayed controlled release is an innovative strategy to locally administer therapeutic compounds (e.g. chemotherapeutics, antibodies etc.). This would improve efficiency and reduce side effects compared with systemic administration. To enable the evaluation of the efficacy of controlled release strategies both in vitro and in vivo, we investigated the release of contrast agents (¹⁹ F‐FDG and BaSO₄) to the intestinal tract from capsules coated with pH‐sensitive polymers (EUDRAGIT L‐100) by using two complementary techniques, i.e. ¹⁹ F magnetic resonance imaging (MRI) and computed tomography (CT). Using in vitro ¹⁹ F‐MRI, we were able to non‐destructively and dynamically establish a time window of 2 h during which the capsules are resistant to low pH. With ¹⁹ F‐MRI, we could establish the exact time point when the capsules became water permeable, before physical degradation of the capsule. This was complemented by CT imaging, which provided longitudinal information on physical degradation of the capsule at low pH that was only seen after 230 min. After oral administration to hamsters, ¹⁹ F‐MRI visualized the early event whereby the capsule becomes water permeable after 2 h. Additionally, using CT, the integrity and location (stomach and small intestines) of the capsule after administration could be monitored. In conclusion, we propose combined ¹⁹ F‐MRI and CT to non‐invasively visualize the different temporal and spatial events regarding the release of compounds, both in an in vitro setting and in the gastrointestinal tract of small animal models. This multimodal imaging approach will enable the in vitro and in vivo evaluation of further technical improvements to controlled release strategies. Copyright © 2015 John Wiley & Sons, Ltd.     

Optical probes and the applications in multimodality imaging

Optical imaging essentially refers to in vivo fluorescence imaging and bioluminescence imaging. These types of imaging are widely used visualization methods in biomedical research and are important in molecular imaging. A new generation of imaging agents called multimodal probes have emerged in the past few years. These probes can be detected by two or more imaging modalities, which harnesses the strengths of the different modalities and enables researchers to obtain more information than can be achieved using only one modality. Owing to its low cost and the large number of probes available, the optical method plays an important role in multimodality imaging. In this mini‐review, we describe the available multimodal imaging probes for in vivo imaging that combine optical imaging with other modalities. Copyright © 2011 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.     

Nature‐inspired nanoformulations for contrast‐enhanced in vivo MR imaging of macrophages

Magnetic resonance imaging (MRI) of macrophages in atherosclerosis requires the use of contrast‐enhancing agents. Reconstituted lipoprotein particles that mimic native high‐density lipoproteins (HDL) are a versatile delivery platform for Gd‐based contrast agents (GBCA) but require targeting moieties to direct the particles to macrophages. In this study, a naturally occurring methionine oxidation in the major HDL protein, apolipoprotein (apo) A‐I, was exploited as a novel way to target HDL to macrophages. We also tested if fully functional GBCA–HDL can be generated using synthetic apo A‐I peptides. The fluorescence and MRI studies reveal that specific oxidation of apo A‐I or its peptides increases the in vitro macrophage uptake of GBCA–HDL by 2–3 times. The in vivo imaging studies using an apo E‐deficient mouse model of atherosclerosis and a 3.0 T MRI system demonstrate that this modification significantly improves atherosclerotic plaque detection using GBCA–HDL. At 24 h post‐injection of 0.05 mmol Gd kg^?1 GBCA–HDL containing oxidized apo A‐I or its peptides, the atherosclerotic wall/muscle normalized enhancement ratios were 90 and 120%, respectively, while those of GBCA–HDL containing their unmodified counterparts were 35 and 45%, respectively. Confocal fluorescence microscopy confirms the accumulation of GBCA–HDL containing oxidized apo A‐I or its peptides in intraplaque macrophages. Together, the results of this study confirm the hypothesis that specific oxidation of apo A‐I targets GBCA–HDL to macrophages in vitro and in vivo. Furthermore, our observation that synthetic peptides can functionally replace the native apo A‐I protein in HDL further encourages the development of these contrast agents for macrophage imaging. Copyright © 2014 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.     

Magnetic resonance molecular imaging of post‐stroke neuroinflammation with a P‐selectin targeted iron oxide nanoparticle

We have developed a magnetic resonance molecular imaging method using a novel iron‐oxide contrast agent targeted towards P‐selectin – MNP‐PBP (magnetic nanoparticle‐P‐selectin binding peptide) – to image endothelial activation following cerebral ischemia/reperfusion. MNP‐PBP consists of ～1000 PBP ligands (primary sequence: GSIQPRPQIHNDGDFEEIPEEYLQ GGSSLVSVLDLEPLDAAWL) conjugated to a 50 nm diameter aminated dextran iron oxide particle. In vitro P‐ and E‐selectin binding was assessed by competition ELISA. Transient focal cerebral ischemia was induced in male C57/BL 6 mice followed by contrast injection (MNP‐PBP; MNP‐NH2; Feridex; MNP‐PBP‐FITC) at 24 h after reperfusion and T₂ magnetic resonance imaging at 9.4 T was performed. Infarction and microvasculature accumulation of contrast agent was assessed in coronal brain sections. MNP‐PBP attenuated antibody binding to P‐selectin by 34.8 ± 1.7%. P‐selectin was preferentially increased in the infarct hemisphere and MNP‐PBP‐FITC accumulation in the infarct hemisphere microvasculature was observed. Compared with the nontargeted iron oxide agents MNP‐NH2 and Feridex, MNP‐PBP showed a significantly greater T₂ effect within the infarction. MR imaging of P‐selectin expression with a targeted iron oxide nanoparticle contrast agent may reveal early endothelial activation in stroke and other neuroinflammatory states. Copyright © 2009 John Wiley & Sons, Ltd.     

MR molecular imaging of HER‐2 in a murine tumor xenograft by SPIO labeling of anti‐HER‐2 affibody

In vivo molecular imaging is a rapidly growing research area both for basic and clinical science. Non‐invasive imaging of in vivo conditions at the molecular level increases understanding of the biological characteristics of normal and diseased tissues without the need for invasive surgical procedures. Among the various imaging modalities, magnetic resonance imaging (MRI) has garnered interest as a molecular imaging modality due to its high spatial resolution. Here, we have demonstrated that the combined use of HER‐2 targeting affibody, a small 7 kDa molecule that behaves similarly to antibodies, and superparamagnetic iron oxide (SPIO) can non‐invasively image HER‐2 expressing cells or tissues both in vitro and in vivo by MRI. This preliminary study demonstrates that affibody‐SPIO is a feasible, target‐specific contrast agent for in vivo MR molecular imaging. Copyright © 2010 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.     

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.     

Anthranilic acid analogs as diamagnetic CEST MRI contrast agents that feature an intramolecular‐bond shifted hydrogen

Diamagnetic chemical exchange saturation transfer (diaCEST) agents are a new class of imaging agents, which have unique magnetic resonance (MR) properties similar to agents used for optical imaging. Here we present a series of anthranilic acid analogs as examples of diaCEST agents that feature an exchangeable proton shifted downfield, namely, an intramolecular‐bond shifted hydrogen (IM‐SHY), which produces significant and tunable contrast at frequencies of 4.8–9.3 ppm from water. Five analogs of N‐sulfonyl anthranilic acids are all highly soluble and produced similar CEST contrast at ~6–8 ppm. We also discovered that flufenamic acid, a commercial nonsteroidal anti‐inflammatory drug, displayed CEST contrast at 4.8 ppm. For these N–H IM‐SHY agents, the contrast produced was insensitive to pH, making them complementary to existing diaCEST probes. This initial IM‐SHY library includes the largest reported shifts for N–H protons on small organic diaCEST agents, and should find use as multifrequency MR agents for in vivo applications. Copyright © 2014 John Wiley & Sons, Ltd.     

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

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

MR imaging for the longevity of mesenchymal stem cells labeled with poly‐L‐lysine–Resovist complexes

Superparamagnetic iron oxide (SPIO) nanoparticles are emerging as ideal probes for noninvasive cell tracking. In this study, poly‐L ‐lysine (PLL) was mixed with Resovist to form the PLL–Resovist complexes and the control of the complexes formed by PLL and Resovist and their subsequent properties was easily achievable. MSCs could be safely and efficiently labeled for MR imaging using PLL–Resovist complexes (w/w 0.01:1) and the labeled MSCs could be detected to have definite decreased signal intensity on T₂‐weight imaging until 20 days with standard 1.5 T MR equipment. This study describes a simple protocol to label MSCs using PLL–Resovist complexes and the results presented in our study can provide a basis for the application of PLL–Resovist complexes cell labeling. Copyright © 2010 John Wiley & Sons, Ltd.     

Thermodynamic stability and kinetic inertness of a Gd–DTPA bisamide complex grafted onto gold nanoparticles

Gold nanoparticles coated by gadolinium (III) chelates (Au@DTDTPA) where DTDTPA is a dithiolated bisamide derivative of diethylenetriamine‐N,N,N′,N′′,N′′‐pentaacetic acid (DTPA), constituted contrast agents for both X‐ray computed tomography and magnetic resonance imaging. In an MRI context, highly stable Gd³⁺ complexes are needed for in vivo applications. Thus, knowledge of the thermodynamic stability and kinetic inertness of these chelates, when grafted onto gold nanoparticles, is crucial since bisamide DTPA chelates are usually less suited for Gd³⁺ coordination than DTPA. Therefore, these parameters were evaluated by means of potentiometric titrations and relaxivity measurements. The results showed that, when the chelates were grafted onto the nanoparticle, not only their thermodynamic stability but also their kinetic inertness were improved. These positive effects were correlated to the chelate packing at the nanoparticle surface that stabilized the corresponding Gd³⁺ complexes and greatly enhanced their kinetic inertness. Copyright © 2014 John Wiley & Sons, Ltd.