Supramolecular adducts between macrocyclic Gd(iii) complexes and polyaromatic systems: a route to enhance the relaxivity through the formation of hydrophobic interactions

Seeking for Innovation with Magnetic Resonance Imaging Paramagnetic Contrast Agents: Relaxation Enhancement via Weak and Dynamic Electrostatic Interactions with Positively Charged Groups on Endogenous Macromolecules

Gd-L1 is a macrocyclic Gd-HPDO3A derivative functionalized with a short spacer to a trisulfonated pyrene. When compared to Gd-HPDO3A, the increased relaxivity appears to be determined by both the higher molecular weight and the occurrence of an intramolecularly catalyzed prototropic exchange of the coordinated OH moiety. In water, Gd-L1 displayed a relaxivity of 7.1 mM-1 s-1 (at 298 K and 0.5 T), slightly increasing with the concentration likely due to the onset of intermolecular aggregation. A remarkably high and concentration-dependent relaxivity was measured in human serum (up to 26.5 mM-1 s-1 at the lowest tested concentration of 0.005 mM). The acquisition of 1H-nuclear magnetic relaxation dispersion (NMRD) and 17O-R2 vs T profiles allowed to get an in-depth characterization of the system. In vitro experiments in the presence of human serum albumin, γ-globulins, and polylysine, as well as using media mimicking the extracellular matrix, provided strong support to the view that the trisulfonated pyrene fosters binding interactions with the exposed positive groups on the surface of proteins, responsible for a remarkable in vivo hyperintensity in T1w MR images. The in vivo MR images of the liver, kidneys, and spleen showed a marked contrast enhancement in the first 10 min after the i.v. injection of Gd-L1, which was 2-6-fold higher than that for Gd-HPDO3A, while maintaining a very similar excretion behavior. These findings may pave the way to an improved design of MRI GBCAs, for the first time, based on the setup of weak and dynamic interactions with abundant positive groups on serum and ECM proteins.

Kinetically Inert Macrocyclic Europium(III) Receptors for Phosphate

The significant role that phosphate plays in environmental water pollution and biomedical conditions such as hyperphosphatemia highlights the need to develop robust receptors that can sequester the anion effectively and selectively from complex aqueous media. Toward that goal, four macrocyclic tris-bidentate 1,2-hydroxypyridonate (HOPO) europium(III) complexes containing either a cyclen, cyclam, TACN, or TACD ligand cap were synthesized and evaluated as phosphate receptors. The solubility of EuIII-TACD-HOPO in water was insufficient for luminescent studies. Whereas EuIII-cyclen-HOPO is eight coordinate with two inner-sphere water molecules, both EuIII-cyclam-HOPO and EuIII-TACN-HOPO are nine coordinate with three inner-sphere water molecules, suggesting that the two coordination states are very close in energy. As observed previously with linear analogues of tripodal HOPO complexes, there is no relationship between the number of inner-sphere water molecules and the affinity of the complex for phosphate. Whereas all three complexes do bind phosphate, EuIII-cyclen-HOPO has the highest affinity for phosphate with the anion displacing both of its inner-sphere water molecules. On the other hand, only one or two of the three inner-sphere water molecules of EuIII-TACN-HOPO and EuIII-cyclam-HOPO are displaced by phosphate, respectively. All three complexes are highly selective for phosphate over other anions, including arsenate. All three complexes are highly stable. EuIII-cyclen-HOPO and, to a lesser extent, EuIII-TACN-HOPO are more kinetically inert than the linear EuIII-Ser-HOPO. EuIII-cyclam-HOPO, on the other hand, is not. This study highlights the significant effect that minor changes in the ligand cap can have on both the ligand exchange rate and affinity for phosphate of tripodal 1,2-dihydroxypyridinonate complexes.

Effects of Cations on HPTS Fluorescence and Quantification of Free Gadolinium Ions in Solution; Assessment of Intracellular Release of Gd3+ from Gd-Based MRI Contrast Agents

8-Hydroxypyrene-1,3,6-trisulfonate (HPTS) is a small, hydrophilic fluorescent molecule. Since the pKa of the hydroxyl group is close to neutrality and quickly responds to pH changes, it is widely used as a pH-reporter in cell biology for measurements of intracellular pH. HPTS fluorescence (both excitation and emission spectra) at variable pH was measured in pure water in the presence of NaCl solution or in the presence of different buffers (PBS or hepes in the presence or not of NaCl) and in a solution containing BSA. pKa values have been obtained from the sigmoidal curves. Herein, we investigated the effect of mono-, di-, and trivalent cations (Na+, Ca2+, La3+, Gd3+) on fluorescence changes and proposed its use for the quantification of trivalent cations (e.g., gadolinium ions) present in solution as acqua-ions. Starting from the linear regression, the LoD value of 6.32 µM for the Gd3+ detection was calculated. The effects on the emission were also analyzed in the presence of a combination of Gd3+ at two different concentrations and the previously indicated mono and di-valent ions. The study demonstrated the feasibility of a qualitative method to investigate the intracellular Gd3+ release upon the administration of Gd-based contrast agents in murine macrophages.

The interaction between iodinated X‐ray contrast agents and macrocyclic GBCAs provides a signal enhancement in T 1 ‐weighted MR images: Insights into the renal excretion pathways of Gd‐HPDO3A and iodixanol in healthy mice

Purpose

This work aims to investigate the supramolecular binding interactions that occur between iodinated X‐ray contrast agents (CAs) and macrocyclic gadolinium (Gd)–based MRI contrast agents (GBCAs). This study provides some new insights in the renal excretion pathways of the two types of imaging probes.

Methods

The water‐proton relaxivities (r₁) of clinically approved macrocyclic and linear GBCAs have been measured in the presence of different iodinated X‐ray contrast agents at different magnetic field strengths in buffer and in serum. The in vivo MRI and X‐ray CT of mice injected with either Gd‐HPDO3A or a Gd‐HPDO3A + iodixanol mixture were then acquired to assess the biodistribution of the two probes.

Results

A significant increase in r₁ (up to approximately 200%) was observed for macrocyclic GBCAs when measured in the presence of an excess of iodinated X‐ray CAs (1:100 mol:mol) in serum. The co‐administration of Gd‐HPDO3A and iodixanol in vivo resulted in a marked increase in the signal intensity of the kidney regions in T₁‐weighted MR images. Moreover, the co‐presence of the two agents resulted in the extended persistence of the MRI signal enhancement, suggesting that the Gd‐HPDO3A/iodixanol adduct was eliminated more slowly than the typical washing out of Gd‐HPDO3A.

Conclusions

The reported results show that it is possible to detect the co‐presence of iodinated agents and macrocyclic GBCAs in contrast‐enhanced MR images. The new information may be useful in the design of novel experiments toward improved diagnostic outcomes.

Studies of the hydrophobic interaction between a pyrene - containing dye and a tetra-aza macrocyclic gadolinium complex

An in vivo and in vitro investigation of the hydrophobic interaction between HPTS and gadolinium(III)-complex of tetra-aza macrocyclic ligand HP-DO3A‡ (Gd(HP-DO3A)) is reported. UV-spectra at variable pH showed that the...

Defining the conditions for the development of the emerging class of FeIII-based MRI contrast agents

Fe(iii) complexes are attracting growing interest in chemists developing diagnostic probes for Magnetic Resonance Imaging because they leverage on an endogenous metal and show superior stability. However, in this case a detailed understanding of the relationship between the chemical structure of the complexes, their magnetic, thermodynamic, kinetic and redox properties and the molecular parameters governing the efficacy (relaxivity) is still far from being available. We have carried out an integrated ¹H and ¹⁷O NMR relaxometric study as a function of temperature and magnetic field, on the aqua ion and three complexes chosen as reference models, together with theoretical calculations, to obtain accurate values of the parameters that control their relaxivity. Moreover, thermodynamic stability and dissociation kinetics of the Fe(iii) chelates, measured in association with the ascorbate reduction behaviour, highlight their role and mutual influence in achieving the stability required for use in vivo.

An integrated ¹H and ¹⁷O NMR relaxometric study on model systems allowed to highlight that the Fe(III) complexes might represent the best alternative to Gd-based MRI contrast agents at the magnetic fields of current and future clinical scanners.