A pyrophosphate-responsive gadolinium(III) MRI contrast agent

This study shows that the relaxivity and optical properties of functionalised lanthanide‐DTPA‐bis‐amide complexes (lanthanide=Gd³⁺ and Eu³⁺, DTPA=diethylene triamine pentaacetic acid) can be successfully modulated by addition of specific anions, without direct Ln³⁺/anion coordination. Zinc(II)‐dipicolylamine moieties, which are known to bind strongly to phosphates, were introduced in the amide “arms” of these ligands, and the interaction of the resulting Gd–Zn₂ complexes with a range of anions was screened by using indicator displacement assays (IDAs). Considerable selectivity for polyphosphorylated species (such as pyrophosphate and adenosine‐5′‐triphosphate (ATP)) over a range of other anions (including monophosphorylated anions) was apparent. In addition, we show that pyrophosphate modulates the relaxivity of the gadolinium(III) complex, this modulation being sufficiently large to be observed in imaging experiments. To establish the binding mode of the pyrophosphate and gain insight into the origin of the relaxometric modulation, a series of studies including UV/Vis and emission spectroscopy, luminescence lifetime measurements in H₂O and D₂O, ¹⁷O and ³¹P NMR spectroscopy and nuclear magnetic resonance dispersion (NMRD) studies were carried out.     

Gadolinium(III)-loaded nanoparticulate zeolites as potential high-field MRI contrast agents: relationship between structure and relaxivity

The effects of dealumination, pore size, and calcination on the efficiency (as expressed in the relaxivity) of Gd3+-loaded zeolites for potential application as magnetic resonance imaging (MRI) contrast agents were studied. Partial dealumination of zeolites NaY or NaA by treatment with (NH4)2SiF6 or diluted HCl resulted in materials that, upon loading with Gd3+, had a much higher relaxivity than the corresponding non-dealuminated materials. Analysis of the 1H NMR dispersion profiles of the various zeolites showed that this can be mainly ascribed to an increase of the amount of water inside the zeolite cavities as a result of the destruction of walls between cavities. However, the average residence time of water inside the Gd3+-loaded cavities did not change significantly, which suggests that the windows of the Gd3+-loaded cavities are not affected by the dealumination. Upon calcination, the Gd3+ ions moved to the small sodalite cavities and became less accessible for water, resulting in a decrease in relaxivity. The important role of diffusion for the relaxivity was demonstrated by a comparison of the relaxivity of Gd3+-loaded zeolite NaY and NaA samples. NaA had much lower relaxivities due to the smaller pore sizes. The transversal relaxivities of the Gd3+-doped zeolites are comparable in magnitude to the longitudinal ones at low magnetic fields (<60 MHz). However at higher fields, the transversal relaxivities steeply increased, whereas the longitudinal relaxivities decreased as field strength increased. Therefore, these materials have potential as T1 MRI contrast agents at low field, and as T2 agents at higher fields.     

pH-sensitive modulation of the second hydration sphere in lanthanide(III) tetraamide-DOTA complexes: a novel approach to smart MR contrast media

The lanthanide(III) complexes of three tetraamide DOTA bearing pyridyl, phenolic and hydroxypyridyl substituents have been studied by NMR, luminescence and cyclic voltammetry. The relaxivity profiles of the gadolinium complexes of the pyridyl and phenolic ligands were flat and essentially the same between pH 2 and 8. The hydroxypyridyl ligand, however, exhibited two regions of enhanced relaxivity. The small relaxivity enhancement (25 %) at lower pH (pH 2-4) has been attributed to an increase in the prototropic exchange of the coordinated water molecule while the slightly larger enhancement (84 %) at higher pH (pH 6-9) reflects deprotonation of the ligand amide protons. Deprotonation of the amides results in the formation of an intramolecular acid-base pair interaction with the phenolic protons and this, in turn, causes a highly organized second hydration sphere to come into effect, thereby increasing the relaxivity. The water relaxivity of the Gd(3+)-hydroxypyridyl complex is further enhanced upon binding to serum albumin.     

Determination of the hydration number of gadolinium(III) complexes by high-field pulsed 17O ENDOR spectroscopy.

Pulsed 17O Mims electron-nuclear double resonance (ENDOR) spectroscopy at the W band (95 GHz) and D band (130 GHz) is used for the direct determination of the water coordination number (q) of gadolinium-based magnetic resonance imaging (MRI) contrast agents. Spectra of metal complexes in frozen aqueous solutions at approximately physiological concentrations can be obtained either in the presence or absence of protein targets. This method is an improvement over the 1H ENDOR method described previously, which involved the difference ENDOR spectrum of exchangeable protons from spectra taken in H2O and D2O. In addition to exchangeable water protons, the 1H ENDOR method is also sensitive to other exchangeable protons, and it is shown here that this method can overestimate hydration numbers for complexes with exchangeable protons at GdH distances similar to that of the coordinated water, for example, from NH groups. The 17O method does not suffer from this limitation. 17O ENDOR spectroscopy is applied to Gd(III) complexes containing zero, one, or two inner-sphere water molecules. In addition, 13C and 1H ENDOR studies were performed to assess the extent of methanol coordination, since methanol is used to produce a glass in these experiments. Under the experimental conditions used for the hydration number determination (30 mol % methanol), fewer than 15 % of the coordination sites were found to be occupied by methanol.     

Lanthanide(III) complexes of novel mixed carboxylic-phosphorus acid derivatives of diethylenetriamine: a step towards more efficient MRI contrast agents

Three novel phosphorus-containing analogues of H(5)DTPA (DTPA = diethylenetriaminepentaacetate) were synthesised (H6L1, H5L2, H5L3). These compounds have a -CH2-P(O)(OH)-R function (R = OH, Ph, CH2NBn2) attached to the central nitrogen atom of the diethylenetriamine backbone. An NMR study reveals that these ligands bind to lanthanide(III) ions in an octadentate fashion through the three nitrogen atoms, a P-O oxygen atom and four carboxylate oxygen atoms. The complexed ligand occurs in several enantiomeric forms due to the chirality of the central nitrogen atom and the phosphorus atom upon coordination. All lanthanide complexes studied have one coordinated water molecule. The residence times (tau(M)298) of the coordinated water molecules in the gadolinium(III) complexes of H6L1 and H5L2 are 88 and 92 ns, respectively, which are close to the optimum. This is particularly important upon covalent and noncovalent attachment of these Gd(3+) chelates to polymers. The relaxivity of the complexes studied is further enhanced by the presence of at least two water molecules in the second coordination sphere of the Gd(3+) ion, which are probably bound to the phosphonate/phosphinate moiety by hydrogen bonds. The complex [Gd(L3)(H2O)](2-) shows strong binding ability to HSA, and the adduct has a relaxivity comparable to MS-325 (40 s(-1) mM(-1) at 40 MHz, 37 degrees C) even though it has a less favourable tau(M) value (685 ns). Transmetallation experiments with Zn(2+) indicate that the complexes have a kinetic stability that is comparable to-or better than-those of [Gd(dtpa)(H2O)](2-) and [Gd(dtpa-bma)(H2O)].     

Towards the rational design of MRI contrast agents: a practical approach to the synthesis of gadolinium complexes that exhibit optimal water exchange

The gadolinium(iii) complex of S-SSSS-NO(2)BnDOTMA exhibits water exchange kinetics that are optimal for use in high relaxivity or targeted contrast agents. However, the synthesis of this ligand is hampered by the steric encumbrance imparted upon the cyclen ring by the nitrobenzyl substituent. A relatively simple modification has been used to enable the synthesis of larger quantities of a bifunctional ligand that retains similar fast water exchange properties. The gadolinium complex of S-SSS-NO(2)BnDO3MA-1A is shown to retain the rapid water exchange kinetics characteristic of a twisted square antiprismatic (TSAP) coordination geometry (tau(M)= 6 +/- 0.4 ns).     

Tuning the coordination number of hydroxypyridonate-based gadolinium complexes: implications for MRI contrast agents

Eight-coordinate hydroxypyridinone/terephthalamide GdIII complexes display high relaxivities due to their two inner sphere water molecules. This relaxivity can be further increased by functionalizing the terephthalamide moiety with an amine. A significant hydrogen bonding interaction between the amine and another water molecule close to the GdIII apparently facilitates its coordination on the open site of the metal. The resulting nine-coordinate complex has three inner sphere water molecules, while maintaining high stability and fast ligand exchange rates.     

A multinuclear NMR relaxometry study of ternary adducts formed between heptadentate Gd(III) chelates and L-lactate

Dramatic relaxation enhancements of L-lactate resonances have been observed upon formation of ternary adducts with Gd(III) complexes of heptadentate DO3A and DO3A-like ligands (DO3A = 1,4,7,10-tetraazaciclododecane-1,4,7-triacetic acid). Detailed 1H and 17O NMR relaxometry investigations allow us to obtain structural, dynamic and thermodynamic information on the ternary complexes in which L-lactate acts as a bidentate ligand replacing two water molecules in the inner coordination sphere of the Gd(III) ion. It has been found that the exchange rate of the coordinated L-lactate is modulated by the structural and electronic properties of the parent Gd-heptacoordinated macrocyclic chelate. In addition to the characterisation of the relaxation behaviour of the 1H methyl resonance of L-lactate, this study has been extended to its 13C isomer (fully enriched at the three positions) and to the trifluoro-L-lactate. The obtained results may be relevant to the development of relaxation agents able to promote the relaxation enhancement of specific substrates detectable by in vivo magnetic resonance spectroscopy.     

Gadolinium(III) complexes as MRI contrast agents: ligand design and properties of the complexes

Magnetic resonance imaging is a commonly used diagnostic method in medicinal practice as well as in biological and preclinical research. Contrast agents (CAs), which are often applied are mostly based on Gd(III) complexes. In this paper, the ligand types and structures of their complexes on one side and a set of the physico-chemical parameters governing properties of the CAs on the other side are discussed. The solid-state structures of lanthanide(III) complexes of open-chain and macrocyclic ligands and their structural features are compared. Examples of tuning of ligand structures to alter the relaxometric properties of gadolinium(III) complexes as a number of coordinated water molecules, their residence time (exchange rate) or reorientation time of the complexes are given. Influence of the structural changes of the ligands on thermodynamic stability and kinetic inertness/lability of their lanthanide(III) complexes is discussed.     

Structure, stability, dynamics, high-field relaxivity and ternary-complex formation of a new tris(aquo) gadolinium complex

The tripodal hexadentate picolinate ligand dpaa3- (H3dpaa=N,N'-bis[(6-carboxypyridin-2-yl)methyl]glycine) has been synthesised. It can form 1:1 and 1:2 lanthanide/ligand complexes. The crystal structure of the bis(aquo) lutetium complex [Lu(dpaa)(H2O)2] has been determined by X-ray diffraction studies. The number of water molecules was determined by luminescence lifetime studies of the terbium and europium complexes. The tris(aquo) terbium complex shows a fairly high luminescence quantum yield (22 %). The [Gd(dpaa)(H2O)3] complex displays a high water solubility and an increased stability (pGd=12.3) with respect to the analogous bis(aquo) complex [Gd(tpaa)(H2O)2] (pGd=11.2). Potentiometric and relaxometric studies show the formation of a soluble GdIII hydroxo complex at high pH values. A unique aquohydroxo gadolinium complex has been isolated and its crystal structure determined. This complex crystallises as a 1D polymeric chain consisting of square-shaped tetrameric units. In heavy water, the [Gd(dpaa)-(D2O)3] complex shows a quite high HOD proton relaxivity at high field (11.93 s(-1) mM(-1) at 200 MHz and 298 K) because of the three inner-sphere water molecules. The formation of ternary complexes with physiological anions has been monitored by relaxometric studies, which indicate that even under conditions favourable to the formation of adducts with oxyanions, the mean relaxivity remains higher than those of most of the currently used commercial contrast agents except for the citrate. However, the measured relaxivity (r1=7.9 s(-1) mM(-1)) in a solution containing equimolar concentrations of [Gd(dpaa)(D2O)3] and citrate is still high. The interaction with albumin has been investigated by relaxometric and luminescence studies. Finally, a new versatile method to unravel the geometric and dynamic molecular factors that explain the high-field relaxivities has been developed. This approach uses a small, uncharged non-coordinating probe solute, the outer-sphere relaxivity of which mimics that of the water proton. Only a routine NMR spectrometer and simple mathematical analysis are required.     

Approaching the Kinetic Inertness of Macrocyclic Gadolinium(III)‐Based MRI Contrast Agents with Highly Rigid Open‐Chain Derivatives

A highly rigid open‐chain octadentate ligand (H₄cddadpa) containing a diaminocylohexane unit to replace the ethylenediamine bridge of 6,6′‐[(ethane‐1,2 diylbis{(carboxymethyl)azanediyl})bis(methylene)]dipicolinic acid (H₄octapa) was synthesized. This structural modification improves the thermodynamic stability of the Gd³⁺ complex slightly (log K_GdL=20.68 vs. 20.23 for [Gd(octapa)]^?) while other MRI‐relevant parameters remain unaffected (one coordinated water molecule; relaxivity r₁=5.73 mm ^?1 s^?1 at 20 MHz and 295 K). Kinetic inertness is improved by the rigidifying effect of the diaminocylohexane unit in the ligand skeleton (half‐life of dissociation for physiological conditions is 6 orders of magnitude higher for [Gd(cddadpa)]^? (t_1/2=1.49×10⁵ h) than for [Gd(octapa)]^?. The kinetic inertness of this novel chelate is superior by 2–3 orders of magnitude compared to non‐macrocyclic MRI contrast agents approved for clinical use.     

Towards the rational design of platinum(II) and gold(III) complexes as antitumour agents

Metal complexes afford an opportunity for the discovery of new antitumour drugs with truly novel mechanisms of action. Various tactics and some new concepts have been employed to improve the physico-chemical and biological properties of metal complexes. Recent advances in this area demonstrate a bright prospect for the utilization of metal complexes in cancer chemotherapy. The theme of this article focuses on the approaches towards the rational design of platinum(II) and gold(III) complexes with antitumour properties based on the updated understanding of the mechanism of action of these compounds. The complexes summarized in this work include monofunctional platinum(II) complexes, multinuclear platinum(II) complexes, hybrid and targeted platinum(II) complexes, and gold(III) complexes. Most of them violate the established structure-activity relationships and demonstrate different reactivities from cisplatin and thereby show some potential for the prevention of detoxification.     

Hydrogels Incorporating GdDOTA: Towards Highly Efficient Dual T(1) /T(2) MRI Contrast Agents

Do not tumble dry: Gadolinium-DOTA encapsulated into polysaccharide nanoparticles (GdDOTA?NPs) exhibited high relaxivity (r(1) =101.7?s(-1) mM(-1) per Gd(3+) ion at 37?°C and 20?MHz). This high relaxation rate is due to efficient Gd loading, reduced tumbling of the Gd complex, and the hydrogel nature of the nanoparticles. The efficacy of the nanoparticles as a T(1) /T(2) dual-mode contrast agent was studied in C6 cells.     

High-relaxivity magnetic resonance imaging (MRI) contrast agent based on supramolecular assembly between a gadolinium chelate, a modified dextran, and poly-beta-cyclodextrin

Nanosized contrast agents have great potential in magnetic resonance molecular imaging applications for clinical diagnosis. This study proposes new nanoparticles spontaneously formed under mild conditions and composed of a noncovalent adduct between a gadolinium complex, a polymer of beta-cyclodextrin (pbetaCD: MW 1.5 x 10(6) g mol(-1)) and a dextran grafted with alkyl chains (MD). The formation of this supramolecular nanoassembly is based upon a "lock-and-key" recognition process in which the hydrophobic alkyl chains of MD and the adamantyl moieties of macrocyclic Gd(III) chelates are included in the cavities of pbetaCD. The large number of betaCDs contained in the pbetaCD resulted in the formation of 200 nm diameter nanoparticles, each entrapping 1.8 x 10(5) molecules of a low-molecular-weight Gd complex. This system, which exhibits a great relaxivity enhancement (48.4 mM(-1) s(-1), at 20 MHz and 37 degrees C) compared to the Gd(III) chelate itself (5.2 mM(-1) s(-1)), appears to be a promising strategy for the in vivo targeted delivery of Gd(III) complexes. The mechanisms of particle formation, conjugation strategies, and relaxometric characterizations in the field of contrast-enhanced magnetic resonance imaging are discussed.     

Gadolinium(III) 1,2-hydroxypyridonate-based complexes: toward MRI contrast agents of high relaxivity

Prospective gadolinium(III) MRI contrast agent precursors [Gd-TREN-1,2-HOPO] (1) [TREN-1,2-HOPO = tris[(1-hydroxy-2-oxo-1,2-dihydropyridine-6-carboxamido)ethyl]amine] and [Gd-TREN-bis(Me-3,2-HOPO)-1,2-HOPO] (2) have been synthesized and characterized by relaxometric measurements. The water proton relaxivity values of 1 and 2 (20 MHz and 25 degrees C) are 9.5 and 9.3 mM(-)(1)s(-)(1), respectively, suggesting the presence of two coordinated water molecules. The molecular structure of [1.DMF](2) was obtained and reveals a similar eight-coordinate geometry to [Gd-TREN-Me-3,2-HOPO.2H(2)O] ([3.2H(2)O]). A shape analysis of the coordination polyhedron of 1 reveals that this geometry is best described as a bicapped trigonal prism, poised to accommodate an additional donor atom to give a tricapped trigonal prismatic intermediate. This geometry supports the model that formation of a tris-aquo intermediate for 1 enables fast and associative water exchange.     

Spatially Resolved Quantification of Gadolinium(III)‐Based Magnetic Resonance Agents in Tissue by MALDI Imaging Mass Spectrometry after In Vivo MRI

Gadolinium(III)‐based contrast agents improve the sensitivity and specificity of magnetic resonance imaging (MRI), especially when targeted contrast agents are applied. Because of nonlinear correlation between the contrast agent concentration in tissue and the MRI signal obtained in vivo, quantification of certain biological or pathophysiological processes by MRI remains a challenge. Up to now, no technology has been able to provide a spatially resolved quantification of MRI agents directly within the tissue, which would allow a more precise verification of in vivo imaging results. MALDI imaging mass spectrometry for spatially resolved in situ quantification of gadolinium(III) agents, in correlation to in vivo MRI, were evaluated. Enhanced kinetics of Gadofluorine M were determined dynamically over time in a mouse model of myocardial infarction. MALDI imaging was able to corroborate the in vivo imaging MRI signals and enabled in situ quantification of the gadolinium probe with high spatial resolution.