Modulating water-exchange rates of lanthanide(III)-containing polyaminopolycarboxylate-type complexes using polyethylene glycol

Water exchange in lanthanide complexes for MRI applications. Lessons learned over the last 25 years

Coordination chemistry offers convenient strategies to modulate the exchange of coordinated water molecules in lanthanide-based contrast agents.

Aqueous Lanthanide Chemistry in Asymmetric Catalysis and Magnetic Resonance Imaging

This account describes lanthanide coordination chemistry with a focus on the similarities between lanthanide complexes used in catalysis and those used as contrast agents in magnetic resonance imaging.

Thermodynamic stability, kinetic inertness and relaxometric properties of monoamide derivatives of lanthanide(III) DOTA complexes

A complete thermodynamic and kinetic solution study on lanthanide(III) complexes with monoacetamide (DOTAMA, L1) and monopropionamide (DOTAMAP, L2) derivatives of DOTA (DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) was undertaken with the aim to elucidate their stability and inertness in aqueous media. The stability constants of GdL1 and GdL2 are comparable, whereas a more marked difference is found in the kinetic inertness of the two complexes. The formation of the Eu(III) and Ce(III) complexes takes place via the formation of the protonated intermediates which can deprotonate and transform into the final complex through a OH(-) assisted pathway. GdL2 shows faster rates of acid catalysed decomplexation with respect to GdL1, which has a kinetic inertness comparable to GdDOTA. Nevertheless, GdL2 is one order of magnitude more inert than GdDO3A. A novel DOTAMAP-based bifunctional chelating ligand and its deoxycholic acid derivative (L5) were also synthesized. Since the coordinated water molecule in GdL2 is characterized by an exchange rate ca. two orders of magnitude greater than in GdL1, the relaxivity of the macromolecular derivatives of L5 should not be limited by the slow water exchange process. The relaxometric properties of the supramolecular adduct of GdL5 with human serum albumin (HSA) were investigated in aqueous solution by measuring the magnetic field dependence of the (1)H relaxivity which, at 20 MHz and 298 K, shows a 430% increase over that of the unbound GdL5 chelate. Thus, Gd(III) complexes with DOTAMAP macrocyclic ligands can represent good candidates for the development of stable and highly effective bioconjugate systems for molecular imaging applications.

Matrix-Assisted Ionization-Ion Mobility Spectrometry-Mass Spectrometry: Selective Analysis of a Europium-PEG Complex in a Crude Mixture

The analytical utility of a new and simple to use ionization method, matrix-assisted ionization (MAI), coupled with ion mobility spectrometry (IMS) and mass spectrometry (MS) is used to characterize a 2-armed europium(III)-containing poly(ethylene glycol) (Eu-PEG) complex directly from a crude sample. MAI was used with the matrix 1,2-dicyanobenzene, which affords low chemical background relative to matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI). MAI provides high ion abundance of desired products in comparison to ESI and MALDI. Inductively coupled plasma-MS measurements were used to estimate a maximum of 10% of the crude sample by mass was the 2-arm Eu-PEG complex, supporting evidence of selective ionization of Eu-PEG complexes using the new MAI matrix, 1,2-dicyanobenzene. Multiply charged ions formed in MAI enhance the IMS gas-phase separation, especially relative to the singly charged ions observed with MALDI. Individual components are cleanly separated and readily identified, allowing characterization of the 2-arm Eu-PEG conjugate from a mixture of the 1-arm Eu-PEG complex and unreacted starting materials. Size-exclusion chromatography, liquid chromatography at critical conditions, MALDI-MS, ESI-MS, and ESI-IMS-MS had difficulties with this analysis, or failed. Graphical Abstract ᅟ.

A (Fluoroalkyl)Guanidine Modulates the Relaxivity of a Phosphonate-Containing T1-Shortening Contrast Agent

Responsive magnetic resonance imaging (MRI) contrast agents, those that change their relaxivity according to environmental stimuli, have promise as next generation imaging probes in medicine. While several of these are known based on covalent modification of the contrast agents, fewer are known based on controlling non-covalent interactions. We demonstrate here accentuated relaxivity of a T1-shortening contrast agent, Gd-DOTP5- based on non-covalent, hydrogen bonding of Gd-DOTP5- with a novel fluorous amphiphile. By contrast to the phosphonate-containing Gd-DOTP5- system, the relaxivity of the analogous clinically approved contrast agent, Gd-DOTA- is unaffected by the same fluorous amphiphile under similar conditions. Mechanistic studies show that placing the fluorous amphiphile in proximity of the gadolinium center in Gd-DOTP5- caused an increase in τ m (bound-water residence lifetime or the inverse of water exchange rate, τ m = 1/kex) and an increase in τ R (rotational correlation time), with τ R being the factor driving enhanced relaxivity. Further, these effects were not observed when Gd-DOTA- was treated with the same fluorous amphiphile. Thus, Gd-DOTP5- and Gd-DOTA- respond to the fluorous amphiphile differently, presumably because the former binds to the amphiphile with higher affinity. (DOTP = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraphosphonic acid; DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid).

Strategies for optimizing water-exchange rates of lanthanide-based contrast agents for magnetic resonance imaging

This review describes recent advances in strategies for tuning the water-exchange rates of contrast agents for magnetic resonance imaging (MRI). Water-exchange rates play a critical role in determining the efficiency of contrast agents; consequently, optimization of water-exchange rates, among other parameters, is necessary to achieve high efficiencies. This need has resulted in extensive research efforts to modulate water-exchange rates by chemically altering the coordination environments of the metal complexes that function as contrast agents. The focus of this review is coordination-chemistry-based strategies used to tune the water-exchange rates of lanthanide(III)-based contrast agents for MRI. Emphasis will be given to results published in the 21st century, as well as implications of these strategies on the design of contrast agents.