L. Vander Elst

Iron Oxide Based MR Contrast Agents: from Chemistry to Cell Labeling

Superparamagnetic iron oxide nanoparticles can be used for numerous applications such as MRI contrast enhancement, hyperthermia, detoxification of biological fluids, drug delivery, or cell separation. In this work, we will summarize the chemical routes for synthesis of iron oxide nanoparticles, the fluid stabilization, and the surface modification of superparamagnetic iron oxide nanoparticles. Some examples of the numerous applications of these particles in the biomedical field mainly as MRI negative contrast agents for tissue-specific imaging, cellular labeling, and molecular imaging will be given. Larger particles or particles displaying a non-neutral surface (thanks to their coating or to a cell transfection agent with which they are mixed) are very useful tools, although the cells to be labeled have no professional phagocytic function. Labeled cells can then be transplanted and monitored by MRI in a broad spectrum of applications. Direct in vivo magnetic labeling of cells is mainly performed by intravenous injection of long-circulating iron oxide-based MRI contrast agents, which can extravasate and/or undergo a cellular uptake in an amount sufficient to allow an MRI visualization of areas of interest such as inflamed regions or tumors. Particles with long circulation times, or able to induce a strong negative effect individually have been also modified by conjugation to a ligand, so that their cellular uptake, or at least their binding to the cell surface, could occur through a specific ligand-receptor interaction, in vivo as well as in vitro. Thus, experimentally as well as in a few trials on humans, iron oxide particles currently find promising applications.

How to measure the transmetallation of a gadolinium complex

The suitability of paramagnetic complexes as contrast agents depends not only on their relaxivity but also on their stability and inertness towards transmetallation processes by endogenous ions. In this work, we describe a convenient method to study the stability of paramagnetic Gd complexes through the evolution of the paramagnetic longitudinal relaxation rate of water protons at 37 °C.

RELAXATION BY METAL-CONTAINING NANOSYSTEMS

Publisher Summary Super-paramagnetic (spm) crystals are used as contrast agents for Magnetic Resonance Imaging (MRI). Improving their efficiency requires a better knowledge of their physical and morphological properties that can be deduced from experimental and theoretical studies of proton relaxation in ferrofluids. Colloidal suspensions of super-paramagnetic nanocrystals are candidates for the development of new intelligent contrast agents, opening an early detection of several pathologies. This chapter reviews the parameters influencing the proton relaxation of a nanomagnet suspension. The chapter presents an analysis of NMRD profiles, which provide the relaxivity dependence with the external field, expressed in proton Larmor frequency units. Ferrofluid NMR studies can also be used in order to determine geometrical and physical properties of the super-paramagnetic crystals, like their specific magnetization or radius. They also give valuable information on the aggregation level and on anisotropy.

Optimising the design of paramagnetic MRI contrast agents: influence of backbone substitution on the water exchange rate of Gd-DTPA derivatives

Among other factors influencing the residence time of the coordinated water (τM) of paramagnetic contrast agents, the steric hindrance around the gadolinium ion seems to play a beneficial role. Such a crowding can be achieved by substituting the Gd-DTPA backbone on the C4 position. Several Gd-DTPA complexes carrying diverse groups at this position have thus been synthesised and characterised: Gd(S)-C4-Me-DTPA, Gd(S)-C4-n-Bu-DTPA, Gd(S)-C4-iBu-DTPA, Gd(S)-C4-iPr-DTPA, and Gd-C4-diMe-DTPA. τM has been measured through the evolution of the water oxygen-17 transverse relaxation rate as a function of the temperature. The data show a reduction of τM of Gd(S)-C4-Me-DTPA, Gd(S)-C4-n-Bu-DTPA, Gd(S)-C4-iBu-DTPA, Gd(S)-C4-iPr-DTPA, and Gd-C4-diMe-DTPA (τM310=91,82, 108,98, and 57 ns respectively, as compared to Gd-DTPA (τM310=143 ns)). At 310 K, the nuclear magnetic dispersion relaxation profiles of water protons are very similar for the five complexes which present longitudinal relaxivities slightly higher than those of Gd-DTPA. Regarding zinc transmetallation, C4-monosubstituted derivatives are more stable than Gd-DTPA. These results confirm that a judicious substitution of the DTPA skeleton allows for an acceleration of the coordinated water exchange rate. This observation can be useful for the design of vectorised contrast agents for molecular imaging.

NMR relaxivity of Ln3+-based zeolite-type materials

A series of zeolite-type silicates containing framework lanthanides and termed Ln-AV-9 have been synthesized. Fundamentally different from conventional zeolites, these materials have the composition (Na4K2)(Ln2Si16O38)·10H2O (Ln = Nd, Sm, Eu, Tb, Gd, Dy). 29Si and 23Na MAS NMR spectra of Sm-AV-9 indicate the presence of six and two distinct sites, respectively, in agreement with the crystal structure. Gd-AV9 exhibits a simple Curie paramagnetic behaviour with an effective magnetic moment of 7.97 µB, while Eu-AV-9 has a much lower magnetic susceptibility, with a very shallow temperature dependence. The 1H longitudinal relaxivities of water (r1), obtained for aqueous suspensions of several Ln-AV-9 materials with different Ln3+ ions, are very small (0.04–0.09 s−1 mM−1) and almost independent of the Ln3+ ion, while their transverse relaxivities (r2) are much larger (16–60 s−1 mM−1) and proportional to μ4eff, where μeff is the magnetic moment of the lanthanide ion present. While r1 of Gd-AV-9 suspensions is very small, the r2 is substantial and proportional to B20. The dependence of r2 on μ4eff and B20 has been fitted using the model of outer-sphere diffusion of water around the particles in the long-echo limit. The results indicate that aqueous suspensions of Ln-AV-9, although inefficient in longitudinal relaxation, are very effective in enhancing transverse relaxation, particularly at high magnetic fields. They are, therefore, attractive as T2 MRI contrast agents.

Validation of a dendron concept to tune colloidal stability, MRI relaxivity and bioelimination of functional nanoparticles

The functionalization of spherical superparamagnetic iron oxide nanoparticles (SPION) of 10 nm with a linear monophosphonate (L1) and also PEGylated mono-phosphonated dendrons of growing generation (D2-G1, -G2 and -G3) yielded dendritic nano-objects of 15 to 30 nm in size, stable in physiological media and showing both renal and hepatobiliary elimination. The grafting of the different molecules has been confirmed by IR spectroscopy and elemental analysis. The colloidal stability of functionalized NS10 has been evaluated in water and in different physiological media. All functionalized NS10 were stable over a long period of time and displayed a mean hydrodynamic diameter smaller than 50 nm whatever the molecule architecture or dendron generation. Only the NS10@L1 showed less stability in biological media at high ionic concentration. NMRD profiles and relaxivity measurements highlighted the influence of the molecule architecture on the water diffusion close to the magnetic core thus influencing the relaxation properties at low magnetic field. Coupling of a fluorescent dye on the functionalized NS10 allowed investigating their biodistribution and highlighting urinary and hepato-biliary eliminations.

Multimodal assessment of early tumor response to chemotherapy: comparison between diffusion-weighted MRI, 1H-MR spectroscopy of choline and USPIO particles targeted at cell death

The aim of this study was to determine the value of different magnetic resonance (MR) protocols to assess early tumor response to chemotherapy. We used a murine tumor model (TLT) presenting different degrees of response to three different cytotoxic agents. As shown in survival curves, cyclophosphamide (CP) was the most efficient drug followed by 5‐fluorouracil (5‐FU), whereas the etoposide treatment had little impact on TLT tumors. Three different MR protocols were used at 9.4 Tesla 24 h post‐treatment: diffusion‐weighted (DW)‐MRI, choline measurement by 1H MRS, and contrast‐enhanced MRI using ultrasmall iron oxide nanoparticles (USPIO) targeted at phosphatidylserine. Accumulation of contrast agent in apoptotic tumors was monitored by T2‐weighted images and quantified by EPR spectroscopy. Necrosis and apoptosis were assessed by histology. Large variations were observed in the measurement of choline peak areas and could not be directly correlated to tumor response. Although the targeted USPIO particles were able to significantly differentiate between the efficiency of each cytotoxic agent and best correlated with survival endpoint, they present the main disadvantage of non‐specific tumor accumulation, which could be problematic when transferring the method to the clinic. DW‐MRI presents a better compromise by combining longitudinal studies with a high dynamic range; however, DW‐MRI was unable to show any significant effect for 5‐FU. This study illustrates the need for multimodal imaging in assessing tumor response to treatment to compensate for individual limitations. Copyright

How to perform accurate and reliable measurements of longitudinal and transverse relaxation times of MRI contrast media in aqueous solutions.

Knowledge of the longitudinal and transverse relaxation times (T(1) and T(2)) of water protons in an aqueous solution of an MRI contrast agent is essential for its characterization. These parameters can be measured at low field on low resolution spectrometers or at high field on high resolution spectrometers. The reliability and the accuracy of T(1) and T(2) measurements rely on several experimental settings and on the equation used to fit the data. Examples of the importance of careful adjustment of the most important parameters are illustrated through several measurements performed on a low-resolution, low-magnetic field instrument. In addition, some specificities of T(1) and T(2) measurements on high-resolution, high-magnetic field spectrometers are pointed out.

Study of non-covalent interactions between MRI contrast agents and human serum albumin by NMR diffusometry

The NMR diffusometry technique, based on the measurement of the diffusion coefficient of a ligand in the absence and in the presence of its macromolecular partner, was used to study the affinity for human serum albumin (HSA) of four gadolinium complexes, potential or already used magnetic resonance imaging contrast agents. Diamagnetic lanthanum(III) ion or europium(III) ion, which has the advantage of shifting the NMR signals far away from those of the macromolecule, was used to avoid the excessive broadening of the NMR signals induced by the gadolinium(III) ion. Titration experiments, in which the HSA concentration was kept constant and the concentration of the europium or lanthanum chelate was varied, were performed to evaluate the association constant and the number of binding sites. Some additional information about the kinetics of the exchange between the free and the bound chelate was also obtained. Competition experiments with ibuprofen and salicylate, which are ligands with a known affinity for the macromolecule and for which the binding site is known, were also performed to get information about the binding site of the contrast agents.

Dy-complexes as high field T2 contrast agents: influence of water exchange rates.

Dy complexes can act as suitable negative (T2) contrast agents for Magnetic Resonance Imaging (MRI). As clinical MRI moves toward higher fields, tuning of the exchange rate of coordinated water molecules will become necessary to optimize the r2 relaxivity. For Dy complexes, this will require lengthening of the water residence time, a strategy opposite that required to optimize the r1 relaxivity of Gd complexes. However, very slow water exchange can be deleterious. This is illustrated here by a Dy complex that is characterized by a very slow water exchange. This complex, Dy-DOTA-4AmCE, is compared with several Dy-DTPA derivatives known for their efficacy as T2 contrast agents at high magnetic fields.