A study on the synthesis and properties of substituted EHBG-Fe(III) complexes as potential MRI contrast agents

Alkyl Chain Appended Fe(III) Catecholate Complex as a Dual-Modal T1 MRI-NIR Fluorescence Imaging Agent via Second Sphere Water Interactions

The C₁₂-alkyl chain-conjugated Fe(III) catecholate complex [Fe(C₁₂CAT)₃]^3-, Fe(C₁₂CAT)₃ [C₁₂CAT = N-(3,4-dihydroxyphenethyl)dodecanamide], was synthesized and characterized, reported as a dual-modal T₁-MRI and an optical imaging probe. The DFT-optimized structure of Fe(C₁₂CAT)₃ reveals a distorted octahedral coordination geometry around the high spin Fe(III) center. The formation constant (-log K) of Fe(C₁₂CAT)₃ was calculated as 45.4. The complex exhibited r₁-relaxivity values of 2.31 ± 0.12 and 1.52 ± 0.06 mM^-1 s^-1 at 25 and 37 °C, respectively, on 1.41 T at pH 7.3 via second-sphere water interactions. The interaction of Fe(C₁₂CAT)₃ with human serum albumin showed concomitant enhancement of r₁-relaxivity to 6.44 ± 0.15 mM^-1 s^-1. The MR phantom images are significantly brighter and directly correlate to the concentration of Fe(C₁₂CAT)₃. Adding an external fluorescent marker IR780 dye to Fe(C₁₂CAT)₃ leads to the formation of self-assembly by C₁₂-alkyl chains. It resulted in the fluorescence quenching of the dye, and its critical aggregation concentration was calculated as 70 μM. The aggregated matrix of Fe(C₁₂CAT)₃ and IR780 dye is spherical, with an average hydrodynamic diameter of 189.5 nm. This self-assembled supramolecular system is found to be non-fluorescent and was "turn-on" under acidic pH via dissociation of aggregates. The r₁-relaxivity is found to be unchanged during the matrix aggregation and disaggregation. The probe showed MRI ON and fluorescent OFF under physiological conditions and MRI ON and fluorescent ON under acidic pH. The cell viability experiments showed that the cells are 80% viable at 1 mM probe concentration. Fluorescence experiments and MR phantom images showed that Fe(C₁₂CAT)₃ is a potential dual model imaging probe to visualize the acidic pH environment of the cells.

Characterization of the Fe(III)-Tiron System in Solution through an Integrated Approach Combining NMR Relaxometric, Thermodynamic, Kinetic, and Computational Data

The Fe(III)-Tiron system (Tiron = 4,5-dihydroxy-1,3-benzenedisulfonate) was investigated using a combination of ¹H and ¹⁷O NMR relaxometric studies at variable field and temperature and theoretical calculations at the DFT and NEVPT2 levels. These studies require a detailed knowledge of the speciation in aqueous solution at different pH values. This was achieved using potentiometric and spectrophotometric titrations, which afforded the thermodynamic equilibrium constants characterizing the Fe(III)-Tiron system. A careful control of the pH of the solution and the metal-to-ligand stoichiometric ratio allowed the relaxometric characterization of [Fe(Tiron)₃]^9-, [Fe(Tiron)₂(H₂O)₂]^5-, and [Fe(Tiron)(H₂O)₄]^- complexes. The ¹H nuclear magnetic relaxation dispersion (NMRD) profiles of [Fe(Tiron)₃]^9- and [Fe(Tiron)₂(H₂O)₂]^5- complexes evidence a significant second-sphere contribution to relaxivity. A complementary ¹⁷O NMR study provided access to the exchange rates of the coordinated water molecules in [Fe(Tiron)₂(H₂O)₂]^5- and [Fe(Tiron)(H₂O)₄]^- complexes. Analyses of the NMRD profiles and NEVPT2 calculations indicate that electronic relaxation is significantly affected by the geometry of the Fe³⁺ coordination environment. Dissociation kinetic studies indicated that the [Fe(Tiron)₃]^9- complex is relatively inert due to the slow release of one of the Tiron ligands, while the [Fe(Tiron)₂(H₂O)₂]^5- complex is considerably more labile.

A class of water-soluble Fe(III) coordination complexes as T1-weighted MRI contrast agents

Iron-based coordination complexes are showing increasing potential to be alternatives for T1-weighted magnetic resonance imaging (MRI) and contribute to the safety of gadolinium-based compounds. In this work, three water-soluble iron-based complexes constructed using catechol ligands exhibiting T1-weighted MRI contrast behavior are described. The longitudinal relaxivity (r1) increase from 0.88 to 1.43 mM-1 s-1 mainly depends on the sizes and the number of water molecules in the second and outer spheres around the discrete complexes.

Fe-HBED Analogs: A Promising Class of Iron-Chelate Contrast Agents for Magnetic Resonance Imaging

Contrast-enhanced magnetic resonance imaging is an essential tool for disease diagnosis and management; all marketed clinical magnetic resonance imaging (MRI) contrast agents (CAs) are gadolinium (Gd) chelates and most are extracellular fluid (ECF) agents. After intravenous injection, these agents rapidly distribute to the extracellular space and are also characterized by low serum protein binding and predominant renal clearance. Gd is an abiotic element with no biological recycling processes; low levels of Gd have been detected in the central nervous system and bone long after administration. These observations have prompted interest in the development of new MRI contrast agents based on biotic elements such as iron (Fe); Fe-HBED (HBED = N,N′-bis(2-hydroxyphenyl)ethylenediamine-N,N′-diacetic acid), a coordinatively saturated iron chelate, is an attractive MRI CA platform suitable for modification to adjust relaxivity and biodistribution. Compared to the parent Fe-HBED, the Fe-HBED analogs reported here have lower serum protein binding and higher relaxivity as well as lower relative liver enhancement in mice, comparable to that of a representative gadolinium-based contrast agent (GBCA). Fe-HBED analogs are therefore a promising class of non-Gd ECF MRI CA.

Studies on the redox activity of iron N,O-complexes: Potential T1-contrast agents

OBJECTIVES

The goal of this study was to determine the redox activity of iron (ethylenebis[2-(o-hydroxyphenyl)glycine]) (EHPG) and (ethylenebis[2-(o-hydroxybenzyl)glycine]) (EHBG) (N,N'-bis(2-hydroxybenzyl)ethylenediamine-N,N'-diacetic acid) derivative complexes and of some N,O-salan complexes of iron. The hexadentate chelate (EHPG and EHBG) ligands varied in their substituents (polar OMe, NHAc, or lipophilic Ph), while the latter had different charge and lipophilicity. The low redox activity of these complexes is important in their potential applications as magnetic resonance imaging contrast agents.

METHODS

Redox activity was assessed in the entire Haber-Weiss cycle and separately in the Fenton reaction. The spin-trapping method with 5,5-dimethyl-1-pyrroline-N-oxide monitored in electron paramagnetic resonance was used. The standard Mn marker was applied as a reference for quantitative analysis. Additionally, ascorbate oxidation was analyzed with UV-Vis spectrophotometry.

RESULTS

Both the Haber-Weiss cycle and in particular the Fenton reaction showed low redox activity of the studied complexes, which did not exceed 30% of [Fe(EDTA)]^- or FeCl₃ activity. The N,O-salan complexes expressed even lower activity, i.e. 10-20% activity of [Fe(EDTA)]^-.

DISCUSSION

For the EHPG and EHBG complexes, it is likely that hydrophobicity and the possibility of H-bond formation play a major role in the resulting redox effects. For this reason, chelates equipped with phenyl groups in the majority belong to less redox-active complexes. For N,O-salan complexes, activity is not correlated with the charge of the coordination sphere, but again, the highly hydrophobic character of the groups and the non-pendant substituents capable of H-bonding that are present in these ligands limit the affinity of hydrophilic species.

Mono-, bi-, and trinuclear bis-hydrated Mn(2+) complexes as potential MRI contrast agents

We report a series of ligands containing pentadentate 6,6′-((methylazanediyl)bis(methylene))dipicolinic acid binding units that form mono- (H2dpama), di- (mX(H2dpama)2), and trinuclear (mX(H2dpama)3) complexes with Mn2+ containing two coordinated water molecules per metal ion, which results in pentagonal bipyramidal coordination around the metal ions. In contrast, the hexadentate ligand 6,6′-((ethane-1,2-diylbis(azanediyl))bis(methylene))dipicolinic acid (H2bcpe) forms a complex with distorted octahedral coordination around Mn2+ that lacks coordinated water molecules. The protonation constants of the ligands and the stability constants of the Mn2+, Cu2+, and Zn2+ complexes were determined using potentiometric and spectrophotometric titrations in 0.15 M NaCl. The pentadentate dpama2– ligand and the di- and trinucleating mX(dpama)24– and mX(dpama)36– ligands provide metal complexes with stabilities that are very similar to that of the complex with the hexadentate ligand bcpe2–, with log β101 values in the range 10.1–11.6. Cyclic voltammetry experiments on aqueous solutions of the [Mn(bcpe)] complex reveal a quasireversible system with a half-wave potential of +595 mV versus Ag/AgCl. However, [Mn(dpama)] did not suffer oxidation in the range 0.0–1.0 V, revealing a higher resistance toward oxidation. A detailed 1H NMRD and 17O NMR study provided insight into the parameters that govern the relaxivity for these systems. The exchange rate of the coordinated water molecules in [Mn(dpama)] is relatively fast, kex298 = (3.06 ± 0.16) × 108 s–1. The trinuclear [mX(Mn(dpama)(H2O)2)3] complex was found to bind human serum albumin with an association constant of 1286 ± 55 M–1 and a relaxivity of the adduct of 45.2 ± 0.6 mM–1 s–1 at 310 K and 20 MHz.