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With great interest, we read the recent paper published in Pharmaceuticals, titled “Study of Interactions Between Gadolinium-Based Contrast Agents and Collagen by Taylor Dispersion Analysis and Frontal Analysis Continuous Capillary Electrophoresis” [...]

Objectives The purpose of this study was to compare Gd levels in rat tissues after cumulative exposure to four commercially available macrocyclic gadolinium-based contrast agents (GBCAs). Methods Sixty-five male Sprague-Dawley rats were randomized to four exposure groups ( n  = 15 per group) and one control group ( n  = 5). Animals in each exposure group received 20 GBCA administrations (four per week of ProHance®, Dotarem®, Clariscan™, or Gadovist® for 5 consecutive weeks) at a dose of 0.6 mmol/kg bodyweight. After 28-days’ recovery, animals were sacrificed and tissues harvested for Gd determination by inductively coupled plasma-mass spectroscopy (ICP-MS). Histologic assessment of the kidney tissue was performed for all animals. Results Significantly ( p  ≤ 0.005; all evaluations) lower Gd levels were noted with ProHance® than with Dotarem®, Clariscan™, or Gadovist® in all soft tissue organs: 0.144 ± 0.015 nmol/g vs. 0.342 ± 0.045, 0.377 ± 0.042, and 0.292 ± 0.047 nmol/g, respectively, for cerebrum; 0.151 ± 0.039 nmol/g vs. 0.315 ± 0.04, 0.345 ± 0.053, and 0.316 ± 0.040 nmol/g, respectively, for cerebellum; 0.361 ± 0.106 nmol/g vs. 0.685 ± 0.330, 0.823 ± 0.495, and 1.224 ± 0.664 nmol/g, respectively, for liver; 38.6 ± 25.0 nmol/g vs. 172 ± 134, 212 ± 121, and 294 ± 127 nmol/g, respectively, for kidney; and 0.400 ± 0.112 nmol/g vs. 0.660 ± 0.202, 0.688 ± 0.215, and 0.999 ± 0.442 nmol/g, respectively, for skin. No GBCA-induced macroscopic or microscopic findings were noted in the kidneys. Conclusions Less Gd is retained in the brain and body tissues of rats 28 days after the last exposure to ProHance® compared to other macrocyclic GBCAs, likely due to unique physico-chemical features that facilitate more rapid and efficient clearance.

Magnetic Resonance Imaging (MRI) plays a vital role in the accurate diagnosis of numerous human diseases and disorders, with Gd(III)‐based contrast agents (GBCAs) being used in ≈30%–40% of procedures, resulting in ≈30 million doses administered annually worldwide. The careful design of a rigid macrocyclic chelator featuring a highly hydrophilic periphery leads to the development of gadopiclenol, the first bis‐hydrated Gd(III)‐based MRI contrast agent, recently approved for clinical use by both the FDA and EMA. The stereochemistry of the coordinating arms is found to play a crucial role in the remarkable thermodynamic stability and inertness of the Gd(III)‐complex with the RRR/SSS‐stereoisomer of this heptadentate chelating agent, ensuring its safety in vivo. The exceptional stability of the most effective gadopiclenol enantiomeric pair (RRR/SSS), coupled with a relaxivity 2 to 3 times higher than that of currently used GBCAs, has enabled the use of reduced doses while ensuring non‐inferior image contrast. The stereochemistry of the coordinating side arms of the polyaminocarboxylic ligand piclenol is found to be crucial in determining the MRI contrast efficiency of the corresponding Gd(III)‐complex (“gadopiclenol”). This finding allowed to identify the most effective enantiomer of gadopiclenol, the latter being used as the first clinically approved q = 2 Gd‐based contrast agent.

Background Gd levels are higher in tissues of animals with compromised renal function, but studies to compare levels after exposure to different macrocyclic gadolinium-based contrast agents (GBCAs) are lacking. We compared Gd levels in tissues of subtotally nephrectomised (SN) rats after repeated exposure to macrocyclic GBCAs. Methods Sprague–Dawley SN male rats (19 per group) received 16 injections of gadoteridol, gadobutrol, or gadoterate meglumine at 0.6 mmol Gd/kg 4 times/weeks over 4 weeks. A control group of healthy male rats ( n  = 10) received gadoteridol at the same dosage. Plasma urea and creatinine levels were monitored. Blood, cerebrum, cerebellum, liver, femur, kidney(s), skin and peripheral nerves were harvested for Gd determination by inductively coupled plasma-mass spectrometry at 28 and 56 days after the end of treatment. Results Plasma urea and creatinine levels were roughly twofold higher in SN rats than in healthy rats at all timepoints. At day 28, Gd levels in the peripheral nerves of gadobutrol- or gadoterate-treated SN animals were 5.4 or 7.2 times higher than in gadoteridol-treated animals ( p  < 0.001). Higher Gd levels after administration of gadobutrol or gadoterate versus gadoteridol were also determined in kidneys ( p  ≤ 0.002), cerebrum ( p  ≤ 0.001), cerebellum ( p  ≤ 0.003), skin ( p  ≥ 0.244), liver ( p  ≥ 0.053), and femur ( p  ≥ 0.271). At day 56, lower Gd levels were determined both in SN and healthy rats for all GBCAs and tissues, except the femur. Conclusions Gd tissue levels were lower following gadoteridol exposure than following gadobutrol or gadoterate exposure.

We have explored the manifold physical scenario emerging from a model of Dynamic Nuclear Polarization (DNP) via thermal mixing under the hypothesis of highly effective electron-electron interaction. When the electron and nuclear reservoirs are also assumed to be in strong thermal contact and the microwave irradiation saturates the target electron transition, the enhancement of the nuclear polarization is expected to be considerably high even if the irradiation frequency is set far away from the centre of the ESR line (as already observed by Borghini) and the typical polarization time is reduced on moving towards the boundaries of said line. More reasonable behaviours are obtained by reducing the level of microwave saturation or the contact between electrons and nuclei in presence of nuclear leakage. In both cases the function describing the dependency of the steady state nuclear polarization on the frequency of irradiation becomes sharper at the edges and the build up rate decreases on moving off-resonance. If qualitatively similar in terms of the effects produced on nuclear polarization, the degree of microwave saturation and of electron-nucleus contact has a totally different impact on electron polarization, which is of course strongly correlated to the effectiveness of saturation and almost insensitive, at the steady state, to the magnitude of the interactions between the two spin reservoirs. The likelihood of the different scenario is discussed in the light of the experimental data currently available in literature, to point out which aspects are suitably accounted and which are not by the declinations of thermal mixing DNP considered here.

\\(^1\\)H nuclear spin-lattice relaxation and Dynamic Nuclear Polarization (DNP) have been studied in amorphous samples of trehalose sugar doped with TEMPO radicals by means of mechanical milling, in the 1.6 K \\(\\) 4.2 K temperature range. The radical concentration was varied between 0.34 and 0.81 \\(\\%\\). The highest polarization of 15 \\% at 1.6 K, observed in the sample with concentration \\(0.50 \\%\\), is of the same order of magnitude of that reported in standard frozen solutions with TEMPO. The temperature and concentration dependence of the spin-lattice relaxation rate \\(1/T_1\\), dominated by the coupling with the electron spins, were found to follow power laws with an exponent close to \\(3\\) in all samples. The observed proportionality between \\(1/T_1\\) and the polarization rate \\(1/T_pol\\), with a coefficient related to the electron polarization, is consistent with the presence of Thermal Mixing (TM) and a good contact between the nuclear and the electron spins. At high electron concentration additional relaxation channels causing a decrease in the nuclear polarization must be considered. These results provide further support for a more extensive use of amorphous DNP-ready samples, obtained by means of comilling, in dissolution DNP experiments and possibly for \\(in vivo\\) metabolic imaging.

The temperature dependence of \\(^1\\)H and \\(^13\\)C nuclear spin-lattice relaxation rate \\(1/T_1\\) has been studied in the 1.6 K - 4.2 K temperature range in pure pyruvic acid and in pyruvic acid containing trityl radicals at a concentration of 15 mM. The temperature dependence of \\(1/T_1\\) is found to follow a quadratic power law for both nuclei in the two samples. Remarkably the same temperature dependence is displayed also by the electron spin-lattice relaxation rate \\(1/T_1e\\) in the sample containing radicals. These results are explained by considering the effect of the structural dynamics on the relaxation rates in pyruvic acid. Dynamic nuclear polarization experiments show that below 4 K the \\(^13\\)C build up rate scales with \\(1/T_1e\\), in analogy to \\(^13\\)C \\(1/T_1\\) and consistently with a thermal mixing scenario where all the electrons are collectively involved in the dynamic nuclear polarization process and the nuclear spin reservoir is in good thermal contact with the electron spin system.

A novel mathematical treatment is proposed for computing the time evolution of dynamic nuclear polarization processes in the low temperature thermal mixing regime. Without assuming any a priori analytical form for the electron polarization, our approach provides a quantitative picture of the steady state that recovers the well known Borghini prediction based on thermodynamics arguments, as long as the electrons-nuclei transition rates are fast compared to the other relevant time scales. Substantially different final polarization levels are achieved instead when the latter assumption is relaxed in the presence of a nuclear leakage term, even though very weak, suggesting a possible explanation for the deviation between the measured steady state polarizations and the Borghini prediction. The proposed methodology also allows to calculate nuclear polarization and relaxation times, once specified the electrons/nuclei concentration ratio and the typical rates of the microscopic processes involving the two spin species. Numerical results are shown to account for the manifold dynamical behaviours of typical DNP samples.

The available theoretical approaches aiming at describing Dynamic Nuclear spin Polarization (DNP) in solutions containing molecules of biomedical interest and paramagnetic centers are not able to model the behaviour observed upon varying the concentration of trityl radicals or the polarization enhancement caused by moderate addition of gadolinium complexes. In this manuscript, we first show experimentally that the nuclear steady state polarization reached in solutions of pyruvic acid with 15 mM trityl radicals is substantially independent from the average internuclear distance. This evidences a leading role of electron (over nuclear) spin relaxation processes in determining the ultimate performances of DNP. Accordingly, we have devised a variant of the Thermal Mixing model for inhomogenously broadened electron resonance lines which includes a relaxation term describing the exchange of magnetic anisotropy energy of the electron spin system with the lattice. Thanks to this additional term, the dependence of the nuclear polarization on the electron concentration can be properly accounted for. Moreover, the model predicts a strong increase of the final polarization on shortening the electron spin-lattice relaxation time, providing a possible explanation for the effect of gadolinium doping.