Strategies for increasing the sensitivity of gadolinium based MRI contrast agents

Gadolinium(III) complexes are often used in clinical MRI to increase contrast by selectively relaxing the water molecules near the complex. There is a desire to improve the sensitivity (relaxivity) of these contrast agents in order to detect molecular targets. This tutorial review describes the molecular factors that contribute to relaxivity and illustrates with recent examples how these can be optimized. It may be of interest to senior undergraduates and more advanced researchers interested in lanthanide chemistry, biophysics, and/or molecular imaging.     

High relaxivity gadolinium hydroxypyridonate-viral capsid conjugates: nanosized MRI contrast agents

High relaxivity macromolecular contrast agents based on the conjugation of gadolinium chelates to the interior and exterior surfaces of MS2 viral capsids are assessed. The proton nuclear magnetic relaxation dispersion (NMRD) profiles of the conjugates show up to a 5-fold increase in relaxivity, leading to a peak relaxivity (per Gd3+ ion) of 41.6 mM(-1) s(-1) at 30 MHz for the internally modified capsids. Modification of the exterior was achieved through conjugation to flexible lysines, while internal modification was accomplished by conjugation to relatively rigid tyrosines. Higher relaxivities were obtained for the internally modified capsids, showing that (i) there is facile diffusion of water to the interior of capsids and (ii) the rigidity of the linker attaching the complex to the macromolecule is important for obtaining high relaxivity enhancements. The viral capsid conjugated gadolinium hydroxypyridonate complexes appear to possess two inner-sphere water molecules (q = 2), and the NMRD fittings highlight the differences in the local motion for the internal (tauRl = 440 ps) and external (tauRl = 310 ps) conjugates. These results indicate that there are significant advantages of using the internal surface of the capsids for contrast agent attachment, leaving the exterior surface available for the installation of tissue targeting groups.     

Multivalent, high-relaxivity MRI contrast agents using rigid cysteine-reactive gadolinium complexes

MRI contrast agents providing very high relaxivity values can be obtained through the attachment of multiple gadolinium(III) complexes to the interior surfaces of genome-free viral capsids. In previous studies, the contrast enhancement was predicted to depend on the rigidity of the linker attaching the MRI agents to the protein surface. To test this hypothesis, a new set of Gd-hydroxypyridonate based MRI agents was prepared and attached to genetically introduced cysteine residues through flexible and rigid linkers. Greater contrast enhancements were seen for MRI agents that were attached via rigid linkers, validating the design concept and outlining a path for future improvements of nanoscale MRI contrast agents.     

The gadolinium(III)-water hydrogen distance in MRI contrast agents

The ion-nuclear distance of Gd(III) to a coordinated water proton, r(Gd)(-)(H), is central to the understanding of the efficacy of gadolinium-based MRI contrast agents. The dipolar relaxation mechanism operative for contrast agents has a 1/r(6) dependence. Estimates in the literature for this distance span 0.8 A (2.5-3.3 A). This study describes a direct determination of r(Gd)(-)(H) using the anisotropic hyperfine constant T( perpendicular ) determined from pulsed ENDOR spectra. Five Gd(III) complexes were examined: [Gd(H(2)O)(8)](3+), [Gd(DTPA)(H(2)O)](2)(-), [Gd(BOPTA)(H(2)O)](2)(-), MS-325, and [Gd(HP-DO3A)(H(2)O)]. The distance, r(Gd)(-)(H), was the same within error for all five complexes: 3.1 +/- 0.1 A. These distance estimates should aid in the design of new contrast agents, and in the interpretation of other molecular factors influencing relaxivity.     

Highly relaxing gadolinium based MRI contrast agents responsive to Mg2+ sensing

A Gd complex based on a polyphosphonated pyridyl ligand shows a very high stability in aqueous solution (log?K(EuL) = 25.7), a high relaxivity (8.5 mM(-1) s(-1) at 25 °C and 20 MHz) and a marked and selective relaxivity enhancement (37%) in the presence of Mg(2+), opening interesting perspectives for the design of cation responsive contrast agents.     

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.     

High-relaxivity MRI contrast agents prepared from miniemulsion polymerization using gadolinium(III)-based metallosurfactants

Miniemulsion polymerization with amphiphilic gadolinium(III) complexes as metallosurfactants was explored as a new technique for the synthesis of high relaxivity MRI contrast agents. Well-defined metallo-colloids with up to 240% enhancement in relaxivity over their small molecular counterparts were obtained.     

Facile synthesis and relaxation properties of novel bispolyazamacrocyclic Gd3+ complexes: an attempt towards calcium-sensitive MRI contrast agents

Three novel GdDO3A-type bismacrocyclic complexes, conjugated to Ca (2+) chelating moieties like ethylenediaminetetraacetic acid and diethylenetriamine pentaacetic acid bisamides, were synthesized as potential "smart" magnetic resonance imaging contrast agents. Their sensitivity toward Ca (2+) was studied by relaxometric titrations. A maximum relaxivity increase of 15, 6, and 32% was observed upon Ca (2+) binding for Gd 2L (1), Gd 2L (2), and Gd 2L (3), respectively (L (1) = N, N-bis{1-[{[({1-[1,4,7-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane-10-yl]eth-2-yl}amino)carbonyl]methyl}-(carboxymethyl)amino]eth-2-yl}aminoacetic acid; L (2) = N, N-bis[1-({[({alpha-[1,4,7-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane-10-yl]- p-tolylamino}carbonyl)methyl]-(carboxymethyl)}amino)eth-2-yl]aminoacetic acid; L (3) = 1,2-bis[{[({1-[1,4,7-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane-10-yl]eth-2-yl}amino)carbonyl]methyl}(carboxymethyl)amino]ethane). The apparent association constants are log K A = 3.6 +/- 0.1 for Gd 2L (1) and log K A = 3.4 +/- 0.1 for Gd 2L (3). For the interaction between Mg (2+) and Gd 2L (1), log K A = 2.7 +/- 0.1 has been determined, while no relaxivity change was detected with Gd 2L (3). Luminescence lifetime measurements on the Eu (3+) complexes in the absence of Ca (2+) gave hydration numbers of q = 0.9 (Eu 2L (1)), 0.7 (Eu 2L (2)), and 1.3 (Eu 2L (3)). The parameters influencing proton relaxivity of the Gd (3+) complexes were assessed by a combined nuclear magnetic relaxation dispersion (NMRD) and (17)O NMR study. Water exchange is relatively slow on Gd 2L (1) and Gd 2L (2) ( k ex (298) = 0.5 and 0.8 x 10 (6) s (-1)), while it is faster on Gd 2L (3) (k ex (298) = 80 x 10 (6) s (-1)); in any case, it is not sensitive to the presence of Ca (2+). The rotational correlation time, tau R (298), differs for the three complexes and reflects their rigidity. Due to the benzene linker, the Gd 2L (2) complex is remarkably rigid, with a correspondingly high relaxivity despite the low hydration number ( r 1 = 10.2 mM (-1)s (-1) at 60 MHz, 298 K). On the basis of all available experimental data from luminescence, (17)O NMR, and NMRD studies on the Eu (3+) and Gd (3+) complexes of L (1) and L (3) in the absence and in the presence of Ca (2+), we conclude that the relaxivity increase observed upon Ca (2+) addition can be mainly ascribed to the increase in the hydration number, and, to a smaller extent, to the Ca (2+)-induced rigidification of the complex.     

Liver-targeting macromolecular MRI contrast agents

Macromolecular ligands with liver-targeting group (pyridoxamine, PM) PHEA-DTPA-PM and PAEA-DTPA-PM were prepared by the incorporation of different amount of diethylenetria-minepentaacetic acid monopyridoxamine group (DTPA-PM) into poly-cc, p-[N-(2-hydroxyethyl)-L-aspartamide] (PHEA) and poly-α, β-[N-(2-aminoethyl)-L-aspartamide] (PAEA). The macromolecular ligands thus obtained were further complexed with gadolinium chloride to give macromolecular MRI contrast agents with different Gd(Ⅲ) contents. These macromolecular ligands and their gadolinium complexes were characterized by 1H NMR, 1R, UV and elementary analysis. Relaxivity studies showed that these polyaspartamide gadolinium complexes possess higher relaxation effectiveness than that of the clinically used Gd-DTPA. Magnetic resonance imaging of the liver in rats and experimental data of biodistribution in mice indicate that these macromolecular MRI contrast agents containing pyridoxamine exhibit liver-targeting property.     

Physicochemical properties of mixed micellar aggregates containing CCK peptides and Gd complexes designed as tumor specific contrast agents in MRI

New amphiphilic molecules containing a bioactive peptide or a claw moiety have been prepared in order to obtain mixed micelles as target-specific contrast agents in magnetic resonance imaging. The first molecule, C(18)H(37)CONH(AdOO)(2)-G-CCK8 (C18CCK8), contains a C18 hydrophobic moiety bound to the C-terminal cholecystokinin octapeptide amide (CCK 26-33 or CCK8). The second amphiphilic compound, C(18)H(37)CONHLys(DTPAGlu)CONH(2) (C18DTPAGlu) or its gadolinium complex, (C18DTPAGlu(Gd)), contains the same C18 hydrophobic moiety bound, through a lysine residue, to the DTPAGlu chelating agent. The mixed aggregates as well as the pure C18DTPAGlu aggregate, in the presence and absence of Gd, have been fully characterized by surface tension measurements, FT-PGSE-NMR, fluorescence quenching, and small-angle neutron scattering measurements. The structural characterization of the mixed aggregates C18DTPAGlu(Gd)-C18CCK8 indicates a spherical arrangement of the micelles with an external shell of approximately 21 A and an inner core of approximately 20 A. Both the DTPAGlu(Gd) complexes and the CCK8 peptides point toward the external surface. The measured values for relaxivity in saline medium at 20 MHz proton Larmor frequency and 25 degrees C are 18.7 mM(-)(1) s(-)(1). These values show a large enhancement in comparison with the isolated DTPAGlu(Gd) complex.     

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.     

Supramolecular aggregates of amphiphilic gadolinium complexes as blood pool MRI/MRA contrast agents: physicochemical characterization

In this paper, we present the development of a new potential blood pool contrast agent for magnetic resonance imaging applications (MRA/MRI) based on gadolinium complexes containing amphiphilic supramolecular aggregates. A novel amphiphilic unimer, containing the DTPAGlu chelating agent covalently bound to two C18 alkylic chains, has been synthesized. DTPAGlu is a well-known chelating agent for a wide number of ions such as the paramagnetic metal ion Gd3+ used as contrast agent in MRA/MRI. The wide aggregation behavior of this surfactant, as free base or as gadolinium complex, has been studied and compared by means of dynamic light scattering, small-angle neutron scattering and cryogenic transmission electron microscopy techniques. Near neutral pH in both cases, the dominant aggregates are micelles.The high negative actual charge of the surfactant headgroup causes a strong headgroups repulsion, promoting the formation of large and high curvature aggregates. By decreasing pH and less markedly increasing the ionic strength, we observe a micelle-to-vesicle transition driven by a decreased electrostatic repulsion. A straightforward switch between different aggregation states can be particularly useful in the development of pH-responsive MRA/MRI contrast agents.     

Towards the rational design of MRI contrast agents: electron spin relaxation is largely unaffected by the coordination geometry of gadolinium(III)-DOTA-type complexes

Electron-spin relaxation is one of the determining factors in the efficacy of MRI contrast agents. Of all the parameters involved in determining relaxivity it remains the least well understood, particularly as it relates to the structure of the complex. One of the reasons for the poor understanding of electron-spin relaxation is that it is closely related to the ligand-field parameters of the Gd(3+) ion that forms the basis of MRI contrast agents and these complexes generally exhibit a structural isomerism that inherently complicates the study of electron spin relaxation. We have recently shown that two DOTA-type ligands could be synthesised that, when coordinated to Gd(3+), would adopt well defined coordination geometries and are not subject to the problems of intramolecular motion of other complexes. The EPR properties of these two chelates were studied and the results examined with theory to probe their electron-spin relaxation properties.     

Cellular delivery of MRI contrast agents

Magnetic resonance imaging (MRI) is a powerful tool for acquiring images of opaque living animals with the benefit of tracking events over extended periods of time on the same specimen. Contrast agents are used to enhance regions, tissues, and cells that are magnetically similar but histologically distinct. A principal barrier to the development of MRI contrast agents for investigating biological questions is the delivery of agents across cellular membranes. Here, we describe the synthesis and in vitro testing of Gd(III)-based MRI contrast agents containing varying length polyarginine oligomers capable of permeating cell membranes. We examine the effect of the length of oligomer on T(1) enhancement and cellular uptake. Furthermore, the effect of incubation time, concentration, and cell type on uptake is explored. Toxicity and washout studies are performed in addition to MRI phantom studies.     

New designs for MRI contrast agents

New designs for Magnetic Resonance Imaging contrast agents are presented. Essentially, they all are host–guest inclusion complexes between -cyclodextrins and polyazamacrocycles of gadolinium (III) ion. Substitutions have been made to the host to optimise the host–guest association. Molecular mechanics calculations have been performed, using the UFF force field for metals, to decide on the suitability of the substitutions, and to evaluate the host–guest energies of association. Interesting general conclusions have been obtained, concerning the improvement of Magnetic Resonance Imaging contrast agents; namely, a set of rational methodologies have been deduced to improve the association between the gadolinium (III) chelates and the cyclodextrins, and their efficiency is demonstrated with a large set of substituted complexes, opening new doors to increase the diagnostic capabilities of Magnetic Resonance Imaging.     

Rational design of protein-based MRI contrast agents

We describe the rational design of a novel class of magnetic resonance imaging (MRI) contrast agents with engineered proteins (CAi.CD2, i = 1, 2,..., 9) chelated with gadolinium. The design of protein-based contrast agents involves creating high-coordination Gd(3+) binding sites in a stable host protein using amino acid residues and water molecules as metal coordinating ligands. Designed proteins show strong selectivity for Gd(3+) over physiological metal ions such as Ca(2+), Zn(2+), and Mg(2+). These agents exhibit a 20-fold increase in longitudinal and transverse relaxation rate values over the conventional small-molecule contrast agents, e.g., Gd-DTPA (diethylene triamine pentaacetic acid), used clinically. Furthermore, they exhibit much stronger contrast enhancement and much longer blood retention time than Gd-DTPA in mice. With good biocompatibility and potential functionalities, these protein contrast agents may be used as molecular imaging probes to target disease markers, thereby extending applications of MRI.     

Superparamagnetic gadonanotubes are high-performance MRI contrast agents

We report the nanoscale loading and confinement of aquated Gd3+n-ion clusters within ultra-short single-walled carbon nanotubes (US-tubes); these Gd3+n@US-tube species are linear superparamagnetic molecular magnets with Magnetic Resonance Imaging (MRI) efficacies 40 to 90 times larger than any Gd3+-based contrast agent (CA) in current clinical use.     

Lanthanoid endohedral metallofullerenols for MRI contrast agents

Water-soluble multi-hydroxyl lanthanoid (La, Ce, Gd, Dy, and Er) endohedral metallofullerenes (metallofullerenols, M@C(82)(OH)(n)()) have been synthesized and characterized for the use of magnetic resonance imaging (MRI) contrast agents. The observed longitudinal and transverse relaxivities for water protons, r(1) and r(2), of the metallofullerenols are in the range 0.8-73 and 1.2-80 (sec(-1)mM(-1)), respectively, which are significantly higher than those of the corresponding lanthanoid-DTPA chelate complexes. Among these Gd-metallofullerenols, Gd@C(82)(OH)(n)() has exhibited the highest r(1) and r(2) values in consistent with our previous results. The observed large r(1) of the current metallofullerenols can mainly be ascribed to the dipole-dipole relaxation together with a substantial decrease of the overall molecular rotational motion. The large r(2), except for the Gd-metallofullerenols, have been attributed to the so-called Curie spin relaxation. The MRI phantom studies are also performed and are consistent with these results. The metallofullerenols will be an ideal model for future MRI contrast agents with higher proton relaxivities.     

Zinc ferrite nanoparticles as MRI contrast agents

Mixed spinel hydrophobic ZnxFe1-xO x Fe2O3 (up to x = 0.34) nanoparticles encapsulated in polymeric micelles exhibited increased T2 relaxivity and sensitivity of detection over clinically used Feridex.