An octahedral coordination cage with six Fe(III) centers as a T1 MRI probe

The incorporation of multiple Fe(III) centers bridged by rigid ligands into a coordination cage represents a powerful approach for designing effective MRI contrast agents. In this context, an octahedral coordination cage with six high-spin Fe(III) centers is shown to be water soluble, robust towards dissociation and has effective relaxivity as a T1 MRI probe in solution and in mice.

Comparison of phosphonate, hydroxypropyl and carboxylate pendants in Fe(III) macrocyclic complexes as MRI contrast agents

Fe(III) macrocyclic complexes containing a macrocycle and three pendant groups including phosphonate (NOTP =1,4,7-triazacyclononane-1,4,7-triyl-tris(methylenephosphonic acid), carboxylate (NOTA = 1,4,7 - triazacyclononane - N,N',N″ - triacetate) or hydroxypropyl (NOHP =(2S,2'S,2"S)-1,1',1″-(1,4,7-triazonane-1,4,7-triyl)tris(propan-2-ol)) were studied in order to compare the effect of these donor groups on solution chemistry and water proton relaxivity. All three complexes, Fe(NOTP), Fe(NOHP) and Fe(NOTA), display a large degree of kinetic inertness to dissociation in the presence of phosphate and carbonate, under acidic conditions of 100 mM HCl or 1 M HCl or to trans-metalation with Zn(II). The r1 proton relaxivity of the complexes at 1.4 T, 33 °C is compared over the pH range of 1 to 10. At pH 7.4, 33 °C, 1.4 T, Fe(NOHP) has the largest relaxivity (1.5 mM-1 s-1), Fe(NOTP) is second at 1.0 mM-1 s-1, whereas Fe(NOTA) is the lowest at 0.61 mM-1 s-1. Fe(NOTP), Fe(NOHP) and Fe(NOTA) all show an increase in relaxivity at very acidic pH values (< 3) that is consistent with an acid-catalyzed process. Variable temperature 17O NMR studies at near neutral pH are consistent with the absence of an inner-sphere water molecule for Fe(NOTP) and Fe(NOHP), supporting second-sphere or outer-sphere water contributions to proton relaxation. Fe(NOTP) shows contrast enhancement in T1 weighted MRI studies in mice and clears through a renal pathway.

Underlining the Importance of Peripheral Protic Functional Groups to Enhance the Proton Exchange of Gd-Based MRI Contrast Agents

In this study, we report the synthesis and the equilibrium, kinetic, relaxation, and structural properties of two new Gd^III complexes based on modified 10-(2-hydroxypropyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (HPDO3A) designed to modulate the relaxivity at acidic and basic pH due to intra- and intermolecular proton exchange. The presence of a carboxylic or ester moieties in place of the methyl group of HPDO3A allowed differentiation of a protic and nonprotic functional group, highlighting the importance of the formation of an intramolecular hydrogen bond between the coordinated hydroxyl and the carboxylate groups for proton exchange (k_H = 1.5 × 10¹¹ M^–1 s^–1, k_OH = 1.7 × 10⁹ M^–1 s^–1). The determination of the thermodynamic stability and kinetic inertness of the Gd^III complexes confirmed that the modification of peripheral groups does not significantly affect the coordination environment and thus the stability (log K_GdL = 19.26, t_1/2 = 2.14 × 10⁷ hours, pH = 7.4, 0.15 M NaCl, 25 °C). The relaxivity (r₁) was measured as a function of pH to investigate the proton exchange kinetics, and as a function of the magnetic field strength to extrapolate the relaxometric parameters (r₁^GdL1 = 4.7 mM^–1 s^–1 and r₁^GdL2 = 5.1 mM^–1 s^–1 at 20 MHz, 25 °C, and pH 7.4). Finally, the X-ray crystal structure of the complex crystallized at basic pH showed the formation of a tetranuclear dimer with alkoxide and hydroxide groups bridging the Gd^III ions.

The peripheral carboxylic moiety of a 2-hydroxypropanoic pendant arm of a GdHPDO3A-like complex forms an intramolecular hydrogen bond with the −OH group that allows both acid- and base-catalyzed proton exchange and thus a relaxivity enhancement. Conversely, the nonprotic ester group in the same position permits only the base-catalyzed mechanism.

Dinuclear Fe(III) Hydroxypropyl-Appended Macrocyclic Complexes as MRI Probes

Four high-spin Fe(III) macrocyclic complexes, including three dinuclear and one mononuclear complex, were prepared toward the development of more effective iron-based magnetic resonance imaging (MRI) contrast agents. All four complexes contain a 1,4,7-triazacyclononane macrocyclic backbone with two hydroxypropyl pendant groups, an ancillary aryl or biphenyl group, and a coordination site for a water ligand. The pH potentiometric titrations support one or two deprotonations of the complexes, most likely deprotonation of hydroxypropyl groups at near-neutral pH. Variable-temperature ¹⁷O NMR studies suggest that the inner-sphere water ligand is slow to exchange with bulk water on the NMR time scale. Water proton T₁ relaxation times measured for solutions of the Fe(III) complexes at pH 7.2 showed that the dinuclear complexes have a 2- to 3-fold increase in r₁ relaxivity in comparison to the mononuclear complex per molecule at field strengths ranging from 1.4 T to 9.4 T. The most effective agent, a dinuclear complex with macrocycles linked through para-substitution of an aryl group (Fe₂(PARA)), has an r₁ of 6.7 mM^-1 s^-1 at 37 °C and 4.7 T or 3.3 mM^-1 s^-1 per iron center in the presence of serum albumin and shows enhanced blood pool and kidney contrast in mice MRI studies.

H-Bonding and intramolecular catalysis of proton exchange affect the CEST properties of EuIII complexes with HP-DO3A-like ligands

Eu(HP-DO3A) is present in solution as a mixture of two diastereoisomers whose alcoholic groups are the source of the mobile protons for the CEST effect. The exchange is base catalyzed. Two novel Eu^III complexes of HP-DO3A-like ligands containing an amino or a carboxylate functionality in the proximity of the -OH groups showed the occurrence of intramolecular catalysis of the prototropic exchange. New insights into the role of the intramolecular proton exchange on the CEST properties have been gained.

Enhanced relaxivity of GdIII-complexes with HP-DO3A-like ligands upon the activation of the intramolecular catalysis of the prototropic exchange

The simple modification of the hydroxypropyl arm in Gd(HP-DO3A) complex allows to achieve an increased relaxivity by the activation of the intramolecular catalysis of the proton exchange process.

Scrutinising the role of intramolecular hydrogen bonding in water exchange dynamics of Gd(iii) complexes

The water exchange rate in Gd^III-complexes bearing substituted acetophenone moieties is modulated by the ability of peripherical substituents to establish hydrogen bonds with the coordinated and/or second sphere water molecules.

Acid-catalyzed proton exchange as a novel approach for relaxivity enhancement in Gd-HPDO3A-like complexes

A current challenge in medical diagnostics is how to obtain high MRI relaxation enhancement using Gd^III-based contrast agents (CAs) containing the minimum concentration of Gd^III ions. We report that in GdHPDO3A-like complexes a primary amide group located in close proximity to the coordinated hydroxyl group can provide a strong relaxivity enhancement at slightly acidic pH. A maximum relaxivity of r₁ = 9.8 mM⁻¹ s⁻¹ (20 MHz, 298 K) at acidic pH was achieved, which is more than double that of clinically approved MRI contrast agents under identical conditions. This effect was found to strongly depend on the number of amide protons, i.e. it decreases with a secondary amide group and almost completely vanishes with a tertiary amide. This relaxivity enhancement is attributed to an acid-catalyzed proton exchange process between the metal-coordinated OH group, the amide protons and second sphere water molecules. The mechanism and kinetics of the corresponding H⁺ assisted exchange process are discussed in detail and a novel simultaneous double-site proton exchange mechanism is proposed. Furthermore, ¹H and ¹⁷O NMR relaxometry, Chemical Exchange Saturation Transfer (CEST) on the corresponding Eu^III complexes, and thermodynamic and kinetic studies are reported. These highlight the optimal physico-chemical properties required to achieve high relaxivity with this series of Gd^III-complexes. Thus, proton exchange provides an important opportunity to enhance the relaxivity of contrast agents, providing that labile protons close to the paramagnetic center can contribute.

A novel GdHPDO3A-like complex featuring primary amide side chain induces extraordinary high relaxivity by virtue of a simultaneous double-site proton exchange mechanism under slight acidic conditions.

Exploring Inner-Sphere Water Interactions of Fe(II) and Co(II) Complexes of 12-Membered Macrocycles To Develop CEST MRI Probes

Several paramagnetic Co(II) and Fe(II) macrocyclic complexes were prepared with the goal of introducing a bound water ligand to produce paramagnetically shifted water ¹H resonances and for paramagnetic chemical exchange saturation transfer (paraCEST) applications. Three 12-membered macrocycles with amide pendent groups including 1,7-bis(carbamoylmethyl)-1,4,7,10-tetraazacyclodocane (DCMC), 4,7,10-tris(carbamoylmethyl)-,4,7,10-triaza-12-crown-ether (N3OA), and 4,10-bis(carbamoylmethyl)-4,10-diaza-12-crown-ether (NODA) were prepared and their Co(II) complexes were characterized in the solid state and in solution. The crystal structure of [Co(DCMC)]Br₂ featured a six-coordinated Co(II) center with distorted octahedral geometry, while [Co(NODA)(OH₂)]Cl₂ and [Co(N3OA)](NO₃)₂ were seven-coordinated. The analogous Fe(II) complexes of NODA and NO3A were successfully prepared, but the complex of DCMC oxidized rapidly to the Fe(III) form. Similarly, [Fe(NODA)]²⁺ oxidized over several days, forming crystals of the Fe(III) complex isolated as the μ-O bridged dimer. Magnetic susceptibility values and paramagnetic NMR spectra of the Fe(II) complexes of NODA and N3OA, as well as Co(II) complexes of DCMC, NODA, and N3OA, were consistent with high spin complexes. CEST peaks ranging from 60 ppm to 70 ppm, attributed to NH groups of the amide pendents, were identified. Variable-temperature ¹⁷O NMR spectra of Co(II) and Fe(II) NODA complexes were consistent with rapid exchange of the water ligand with bulk water. Notably, the Co(II) and Fe(II) complexes presented here produced substantial paramagnetic shifts of bulk water ¹H resonances, independent of having an inner-sphere water.

Metal-Free Air Oxidation in a Convenient Cascade Approach for the Access to Isoquinoline-1,3,4(2H)-triones

Herein we describe a very useful application of the readily available trifunctional aromatic ketone methyl-2-(2-bromoacetyl)benzoate in reactions with primary amines. An unexpected in situ air oxidation that follows a cascade process allowed the access to a series of isoquinoline-1,3,4(2H)-triones, a class of heterocyclic compounds of great interest containing an oxygen-rich heterocyclic scaffold. A modification of the original protocol, utilizing a Staudinger reaction in the presence of trimethylphosphine, was necessary for the synthesis of Caspase inhibitor trione with free NH group.

Accelerating water exchange in GdIII–DO3A-derivatives by favouring the dissociative mechanism through hydrogen bonding

The water exchange rate in Gd^III-complexes increases by one order of magnitude due to H-bonding between the phenol(ate) group and the water molecules involved in the dissociative exchange mechanism.