Surface modification effect on contrast agent efficiency for X-ray based spectral photon-counting scanner/luminescence imaging: from fundamental study to in vivo proof of concept.

X-Ray imaging techniques are among the most widely used modalities in medical imaging and their constant evolution has led to the emergence of new technologies. The new generation of computed tomography (CT) systems - spectral photonic counting CT (SPCCT) and X-ray luminescence optical imaging - are examples of such powerful techniques. With these new technologies the rising demand for new contrast agents has led to extensive research in the field of nanoparticles and the possibility to merge the modalities appears to be highly attractive. In this work, we propose the design of lanthanide-based nanocrystals as a multimodal contrast agent with the two aforementioned technologies, allowing SPCCT and optical imaging at the same time. We present a systematic study on the effect of the Tb3+ doping level and surface modification on the generation of contrast with SPCCT and the luminescence properties of GdF3:Tb3+ nanocrystals (NCs), comparing different surface grafting with organic ligands and coatings with silica to make these NCs bio-compatible. A comparison of the luminescence properties of these NCs with UV revealed that the best results were obtained for the Gd0.9Tb0.1F3 composition. This property was confirmed under X-ray excitation in microCT and with SPCCT. Moreover, we could demonstrate that the intensity of the luminescence and the excited state lifetime are strongly affected by the surface modification. Furthermore, whatever the chemical nature of the ligand, the contrast with SPCCT did not change. Finally, the successful proof of concept of multimodal imaging was performed in vivo with nude mice in the SPCCT taking advantage of the so-called color K-edge imaging method.

Multimodal Imaging with NanoGd Reveals Spatiotemporal Features of Neuroinflammation after Experimental Stroke.

The purpose of this study is to propose and validate a preclinical in vivo magnetic resonance imaging (MRI) tool to monitor neuroinflammation following ischemic stroke, based on injection of a novel multimodal nanoprobe, NanoGd, specifically designed for internalization by phagocytic cells. First, it is verified that NanoGd is efficiently internalized by microglia in vitro. In vivo MRI coupled with intravenous injection of NanoGd in a permanent middle cerebral artery occlusion mouse model results in hypointense signals in the ischemic lesion. In these mice, longitudinal two-photon intravital microscopy shows NanoGd internalization by activated CX3CR1-GFP/+ cells. Ex vivo analysis, including phase contrast imaging with synchrotron X-ray, histochemistry, and transmission electron microscopy corroborate NanoGd accumulation within the ischemic lesion and uptake by immune phagocytic cells. Taken together, these results confirm the potential of NanoGd-enhanced MRI as an imaging biomarker of neuroinflammation at the subacute stage of ischemic stroke. As far as it is known, this work is the first to decipher the working mechanism of MR signals induced by a nanoparticle passively targeted at phagocytic cells by performing intravital microscopy back-to-back with MRI. Furthermore, using a gadolinium-based rather than an iron-based contrast agent raises future perspectives for the development of molecular imaging with emerging computed tomography technologies.

Hybrid multimodal contrast agent for multiscale in vivo investigation of neuroinflammation.

Neuroinflammation is a process common to several brain pathologies. Despites its medical relevance, it still remains poorly understood; there is therefore a need to develop new in vivo preclinical imaging strategies to monitor inflammatory processes longitudinally. We here present the development of a hybrid imaging nanoprobe named NP3, that was specifically designed to get internalized by phagocytic cells and imaged in vivo with MRI and bi-photon microscopy. NP3 is composed of a 16 nm core of gadolinium fluoride (GdF3), coated with bisphosphonate polyethylene glycol (PEG) and functionalized with a Lemke-type fluorophore. It has a hydrodynamic diameter of 28 ± 8 nm and a zeta potential of -42 ± 6 mV. The MR relaxivity ratio at 7 T is r1/r2 = 20; therefore, NP3 is well suited as a T2/T2* contrast agent. In vitro cytotoxicity assessments performed on four human cell lines revealed no toxic effects of NP3. In addition, NP3 is internalized by macrophages in vitro without inducing inflammation or cytotoxicity. In vivo, uptake of NP3 has been observed in the spleen and the liver. NP3 has a prolonged vascular remanence, which is an advantage for macrophage uptake in vivo. The proof-of-concept that NP3 may be used as a contrast agent targeting phagocytic cells is provided in an animal model of ischemic stroke in transgenic CX3CR1-GFP/+ mice using three complementary imaging modalities: MRI, intravital two-photon microscopy and phase contrast imaging with synchrotron X-rays. In summary, NP3 is a promising preclinical tool for the multiscale and multimodal investigation of neuroinflammation.

Hybrid Nano-GdF3 contrast media allows pre-clinical in vivo element-specific K-edge imaging and quantification.

Computed tomography (CT) is a widely used imaging modality. Among the recent technical improvements to increase the range of detection for optimized diagnostic, new devices such as dual energy CT allow elemental discrimination but still remain limited to two energies. Spectral photon-counting CT (SPCCT) is an emerging X-ray imaging technology with a completely new multiple energy detection and high spatial resolution (200 µm). This unique technique allows detection and quantification of a given element thanks to an element-specific increase in X-ray absorption for an energy (K-band) depending on its atomic number. The main contrast media used hitherto are iodine-based compounds but the K-edge of iodine (33.2 keV) is out of the range of detection. Therefore, it is crucial to develop contrast media suitable for this advanced technology. Gadolinium, well known and used element for MRI, possess a K-edge (50.2 keV) well suited for the SPCCT modality. The use of nano-objects instead of molecular entities is pushed by the necessity of high local concentration. In this work, nano-GdF3 is validated on a clinical based prototype, to be used as efficient in vivo contrast media. Beside an extremely high stability, it presents long lasting time in the blood pool allowing perfusion imaging of small animals, without apparent toxicity.

The peculiar spectral properties of amino-substituted uracils: a combined theoretical and experimental study.

A detailed experimental and computational study of the absorption and fluorescence spectra of 5-aminouracil (5 AU) and 6-aminouracil (6 AU) in aqueous solution is reported. The lowest energy band of the steady-state absorption spectra of 5 AU is considerably red-shifted, noticeably less intense, and broader than its counterpart in uracil (U). On the contrary, the 6 AU lowest energy absorption peak is close in energy to that of U, but it is much narrower and the transition is much more intense. The emission properties of 5 AU, 6 AU, and U are also very different. Both amino-substituted compounds exhibit indeed a much larger Stokes shift as compared to U, and the emission band of 5 AU is much narrower than that of 6 AU. Those features are fully rationalized with the help of PCM/TD-PBE0 calculations in aqueous solution and MS-CASPT2/CASSCF calculations in the gas phase. A stable minimum on the potential energy surface of the lowest energy bright state is found for 5 AU, both in the gas phase and in aqueous solution. For 6 AU a barrierless path leads to the conical intersection with the ground electronic state, but a nonplanar plateau region is predicted in aqueous solution, which is responsible for the very large Stokes shift. Some general considerations on the excited-state dynamics of uracil derivatives are also reported.

UVA-induced cyclobutane pyrimidine dimers in DNA: a direct photochemical mechanism?

The carcinogenic action of UVA radiation is commonly attributed to DNA oxidation mediated by endogenous photosensitisers. Yet, it was recently shown that cyclobutane pyrimidine dimers (CPD), well known for their involvement in UVB genotoxicity, are produced in larger yield than oxidative lesions in UVA-irradiated cells and skin. In the present work, we gathered mechanistic information on this photoreaction by comparing formation of all possible bipyrimidine photoproducts upon UVA irradiation of cells, purified genomic DNA and dA(20):dT(20) oligonucleotide duplex. We observed that the distribution of photoproducts, characterized by the sole formation of CPD and the absence of (6-4) photoproducts was similar in the three types of samples. The CPD involving two thymines represented 90% of the amount of photoproducts. Moreover, the yields of formation of the DNA lesions were similar in cells and isolated DNA. In addition, the effect of the wavelength of the incident photons was found to be the same in isolated DNA and cells. This set of data shows that UVA-induced cyclobutane pyrimidine dimers are formed via a direct photochemical mechanism, without mediation of a cellular photosensitiser. This is possible because the double-stranded structure increases the capacity of DNA bases to absorb UVA photons, as evidenced in the case of the oligomer dA(20):dT(20). These results emphasize the need to consider UVA in the carcinogenic effects of sunlight. An efficient photoprotection is needed that can only be complete by completely blocking incident photons, rather than by systemic approaches such as antioxidant supplementation.

Influence of intermolecular interactions on the Mössbauer quadrupole splitting of organotin(IV) compounds as studied by DFT calculations.

The influence of intermolecular interactions on the Mössbauer quadrupole splitting (Delta) of 119Sn was investigated in detail by density functional theory (DFT) calculations. Six organotin(IV) complexes [Me2Sn(acac)2 (1), Ph3SnCl (2), Me3Sn-succinimide (3), Me3Sn-phthalimide (4), Me3SnCl (5), and cHex3SnCl (6)] of known solid-state structures and quadrupole splittings were selected. Theoretical Delta values were calculated for both fully optimized geometries and experimental solid-state structures of different size, and the results were compared to the experimental Delta values. Compared to a synthetic procedure described in the literature for compound 4, a more convenient synthesis is reported here. The experimental Delta of this compound has also been redetermined at 80 K. For compounds with negligible intermolecular interactions in the solid state, calculated Delta values obtained did not vary significantly. In contrast, the calculated Delta values turned out to be very sensitive to the size of the supramolecular moiety considered in the crystal lattice. The crystal structure of compound 2 shows no significant intermolecular interactions; however, the calculated and the experimental Delta values remained very different, even when the supramolecular moiety considered was extended. Distortion of the coordination sphere of tin in the molecule of 2 toward a trigonal bipyramidal geometry was considered, and a possible weak intermolecular Sn...Cl interaction was included in the model. Steps of the distortion followed the new structure correlation function, which was found for the R3SnCl (R=alkyl, aryl) compounds. The experimental Delta value could be approached by this method. These results suggest that compound 2 is involved in some unexpected intermolecular interaction at 80 K.

Probing the 3d spin momentum with X-ray emission spectroscopy: the case of molecular-spin transitions.

We report X-ray emission spectra of Fe(III), Fe(II), and Co(II) spin-crossover compounds in their high-spin and low-spin forms. It is shown that all X-ray emission features are sensitive to the spin state. Variations of the Kbeta and the Kalpha emission line shapes, which are in agreement with theory, can be used as quantitative probes of the spin state; it is suggested that with appropriate reference experiments one can extract the spin momentum for a general case. Resonant X-ray emission spectra unveil details of the redistribution of electrons on the 3d levels associated with the spin-state change by revealing features at the X-ray absorption preedge not accessible through standard absorption measurements.