Non-ionic bulky Gd(III) DTPA-bisamide complexes as potential contrast agents for magnetic resonance imaging

Lanthanide complexes with polyaminopolycarboxylates as prospective NMR/MRI diagnostic probes: peculiarities of molecular structure, dynamics and paramagnetic properties

The paramagnetic lanthanide complexes with polyaminopolycarboxylate (PAPC) ligands attract considerable attention from the standpoint of potential applications thereof as relaxation agents used in medical magnetic resonance imaging (MRI) and in luminescent materials, as well as owing to promising use thereof as paramagnetic labels for studying the properties of biopolymers since they exhibit thermodynamic stability, good solubility in aqueous media and moderate toxicity. For the last decades, the NMR methods have been used to determine the physical and chemical properties of paramagnetic Ln compounds. The studies concerning paramagnetic NMR lanthanide-induced shifts (LISs) in dissolved Ln complexes, as well as the analysis of band shape as a function of temperature make it possible to obtain valuable information on the structure, intra- and intermolecular dynamics and paramagnetic properties thereof. This review is devoted solely to the following features: firstly, the processes of intramolecular dynamics of lanthanide complexes with polyamino-polycarboxylate ligands such as DOTA, EDTA and DTPA and their derivatives studied by NMR; secondly, the LISs of lanthanide complexes with EDTA, DOTA, DTPA and some of their derivatives depending on temperature and pH. Moreover, in this review, for the first time, the dependence of the activation energy of molecular dynamics in complexes with polydentate ligands on the atomic number of the lanthanide cation is analyzed and a monotonic change in energy is detected, which is due to the effect of lanthanide contraction. It should be noted that this phenomenon is quite general and may also appear in the future in many other series of lanthanide complexes with both other multidentate ligands and with bidentate and monodentate ligands. In the future, it is possible to predict the dependence of the properties of certain lanthanide complexes on the ionic radius of the lanthanide cation based on the approaches presented in the review. In this review, we have also presented the dynamic NMR as the main research method widely used to analyze the processes of molecular dynamics, and the structural studies based on the NMR relaxation spectroscopy and LIS analysis.

Dual modal imaging agents based on chromophore-bearing DTPA analogues

Two new DTPA analogues, centrally (L1) and terminally (L2) functionalised with a 1,8-naphthalimide chromophore, have been successfully prepared and fully characterized.

Gd(III) functionalized gold nanorods for multimodal imaging applications

We demonstrate a novel noncovalent method for producing Gd(III)-functionalized gold nanorods as multimodal contrast agents for MRI and CT imaging. The ligand is connected to the surface of the gold nanorods by a noncovalent bond making the Gd(III) ions directly accessible to water molecules, and resulting in a longitudinal relaxivity as high as 21.3 mM(-1) s(-1). In addition, compared with spherical gold nanoparticles, gold nanorods have more binding sites for Gd(III) ions due to their large surface-to-volume ratio. Benefiting from the advantages of the new type of carry material and the novel fabrication approach, the multimodal imaging probes exhibit a high longitudinal relaxivity r(1) on the order of 1.1 × 10(7) mM(-1) s(-1) on a per-particle basis, which is 24 times higher than that of Gd(III)-ion-functionalized spherical gold nanoparticles. Furthermore, CT imaging shows that such nanoprobes could induce an efficient contrast enhancement when the gold concentration is at least equal to 1.31 mg ml(-1). These results demonstrate that the as-prepared Gd functionalized gold nanorods could provide a new and versatile platform for the development of multimodal imaging probes.

A modular system for the synthesis of multiplexed magnetic resonance probes

We have developed a modular architecture for preparing high-relaxivity multiplexed probes utilizing click chemistry. Our system incorporates azide bearing Gd(III) chelates and a trialkyne scaffold with a functional group for subsequent modification. In optimizing the relaxivity of this new complex, we undertook a study of the linker length between a chelate and the scaffold to determine its effect on relaxivity. The results show a strong dependence on flexibility between the individual chelates and the scaffold with decreasing linker length leading to significant increases in relaxivity. Nuclear magnetic resonance dispersion (NMRD) spectra were obtained to confirm a 10-fold increase in the rotational correlation time from 0.049 to 0.60 ns at 310 K. We have additionally obtained a crystal structure demonstrating that modification with an azide does not impact the coordination of the lanthanide. The resulting multinuclear center has a 500% increase in per Gd (or ionic) relaxivity at 1.41 T versus small molecule contrast agents and a 170% increase in relaxivity at 9.4 T.

The gadolinium complexes with polyoxometalates as potential MRI contrast agents

The two gadolinium (Gd) polyoxometalates, K(15)[Gd(BW(11)O(39))(2)] [Gd(BW(11))(2)] and K(17)[Gd(CuW(11)O(39))(2)] [Gd(CuW(11))(2)] have been evaluated by in vivo and in vitro experiments as the candidates of potential tissue-specific magnetic resonance imaging (MRI) contrast agents. T(1) relaxivities of 17.12 mM(-1) x s(-1) for Gd(BW(11))(2) and 19.95 mM(-1) x s(-1) for Gd(CuW(11))(2) (400 MHz, 25 degrees C) were much higher than that of the commercial MRI contrast agent (GdDTPA). Their relaxivities in bovine serum albumin and human serum transferrin solutions were also reported. After administration of Gd(BW(11))(2) and Gd(CuW(11))(2) to Wistar rats, MRI showed longer and remarkable enhancement in rat liver and favorable renal excretion capability. The signal intensity increased by 37.63+/-3.45% for the liver during the whole imaging period (100 min) and by 61.47+/-10.03% for kidney within 5-40 min after injection at 40+/-1-micromol x kg(-1) dose for Gd(CuW(11))(2), and Gd(BW(11))(2) induced 50.44+/-3.51% enhancement in the liver in 5-50-min range and 61.47+/-10.03% enhancement for kidney within 5-40 min after injection at 39+/-4 micromol x kg(-1) dose. In vitro and in vivo study showed that Gd(BW(11))(2) and Gd(CuW(11))(2) are favorable candidates as tissue-specific contrast agents for MRI.

A gadolinium triacetic monoamide DOTA derivative with a methanethiosulfonate anchor group. Relaxivity properties and conjugation with albumin and thiolated particles

The gadolinium(III) complex with a new DOTA-based ligand bearing a methanethiosulfonate group (MTS) was synthesized and its relaxivity properties were investigated. MTS-ADO3A is a triacid DOTA derivative with an amide arm substituted by an ethylmethanethiosulfonate function. This ligand was obtained in two steps: tri-tert-butyl 2,2',2''-(1,4,7,10-tetraazacyclododecane-1,4,7-triyl)triacetate was reacted with S-(2-aminoethyl)methanesulfonothioate and the tert-butyl groups were removed with trifluoroacetic acid. The Gd(III) MTS-ADO3A complex readily formed disulfide bonds with albumin (BSA) in its native and reduced forms and with thiolated silica particles. Four- to five-fold relaxivity increases at 20 MHz were measured on the isolated adducts. The EuMTS-ADO3A chelate was found to be monohydrated by fluorescence and the relaxivity parameters of the Gd(III) complex were obtained by (17)O NMR and by measuring the nuclear magnetic relaxation dispersion between 0.01 and 80 MHz. The water exchange time tau(m) is increased upon forming disulfide bonds with macromolecules and particles and the relaxivity gains of all the complexes are limited by the tau(m) factor. Forming covalent or hydrophobic/electrostatic bonds with BSA seems to bring about similar relaxivity changes but the covalent BSA adducts can be isolated and their properties can be directly studied. The addition of dithiothreitol or glutathione leads to the removal of the metal chelates from the macromolecules, as indicated by the relaxation times reverting to their values before binding. It is thus expected that the chelate will stay in the body long enough for imaging but will still be excreted through the kidneys.

Equilibrium and NMR studies on GdIII, YIII, CuIIand ZnIIcomplexes of various DTPA–N,N″-bis(amide) ligands. Kinetic stabilities of the gadolinium(iii) complexes

Three DTPA-derivative ligands, the non-substituted DTPA-bis(amide) (L(0)), the mono-substituted DTPA-bis(n-butylamide) (L(1)) and the di-substituted DTPA-bis[bis(n-butylamide)] (L(2)) were synthesized. The stability constants of their Gd3+ complexes (GdL) have been determined by pH-potentiometry with the use of EDTA or DTPA as competing ligands. The endogenous Cu2+ and Zn2+ ions form ML, MHL and M(2)L species. For the complexes CuL(0) and CuL(1) the dissociation of the amide hydrogens (CuLH(-1)) has also been detected. The stability constants of complexes formed with Gd3+, Cu2+ and Zn2+ increase with an increase in the number of butyl substituents in the order ML(0) < ML(1) < ML(2). NMR studies of the diamagnetic YL(0) show the presence of four diastereomers formed by changing the chirality of the terminal nitrogens of their enantiomers. At 323 K, the enantiomerization process, involving the racemization of central nitrogen, falls into the fast exchange range. By the assignment and interpretation of 1H and 13C NMR spectra, the fractions of the diastereomers were found to be equal at pH = 5.8 for YL(0). The kinetic stabilities of GdL(0), GdL(1) and GdL(2) have been characterized by the rates of the exchange reactions occurring between the complexes and Eu3+, Cu2+ or Zn2+. The rates of reaction with Eu3+ are independent of the [Eu3+] and increase with increasing [H+], indicating the rate determining role of the proton assisted dissociation of complexes. The rates of reaction with Cu2+ and Zn2+ increase with rising metal ion concentration, which shows that the exchange can take place with direct attack of Cu2+ or Zn2+ on the complex, via the formation of a dinuclear intermediate. The rates of the proton, Cu2+ and Zn2+ assisted dissociation of Gd3+ complexes decrease with increasing number of the n-butyl substituents, which is presumably the result of steric hindrance hampering the formation or dissociation of the intermediates. The kinetic stabilities of GdL(0) and GdL(1) at pH = 7.4, [Cu2+] = 1 x 10(-6) M and [Zn(2+)] = 1 x 10(-5) M are similar to that of Gd(DTPA)2-, while the complex GdL2 possesses a much higher kinetic stability.

Investigation of sandwiched gadolinium (III) complexes with tungstosilicates as potential MRI contrast agents

Two gadolinium-sandwiched complexes with tungstosilicates, K(13)[Gd(SiW(11)O(39))(2)] (Gd(SiW(11))(2)) and K(11)H(6)[Gd(3)O(3)(SiW(9)O(34))(2)] (Gd(3)(SiW(9))(2)), have been investigated by in vitro and in vivo experiments as potential contrast agents for magnetic resonance imaging (MRI). T(1)-relaxivity of Gd(SiW(11))(2)was 6.59 mM(-1).s(-1) in aqueous solution and 6.85 mM(-1).s(-1) in 0.725 mmol.L(-1) bovine serum albumin solution at 25 degrees C and 9.39 T, respectively. The corresponding T(1)-relaxivity of Gd(3)(SiW(9))(2) was 12.6 and 19.3 mM(-1).s(-1) per Gd, respectively. MRI for Sprague-Dawley rats showed longer and more remarkable enhancement in rat liver after i.v. injection of these two complexes: 39.4 +/- 3.9% and 57.4 +/- 11.6% within the first 30 min after injection, 31.2 +/- 2.6% and 39.9 +/- 7.6% in the next 60 min for Gd(SiW(11))(2) and Gd(3)(SiW(9))(2) at doses of 0.081 and 0.084 mmol Gd/kg, respectively. Our preliminary in vitro and in vivo study indicates that Gd(SiW(11))(2) and Gd(3)(SiW(9))(2) are favorable candidates for hepatic contrast agents for MRI. However, the two complexes exhibit higher acute toxicity and need to be modified and studied further before clinical use.

Comparison between Gd-DTPA and several bisamide derivatives as potential MRI contrast agents

Four neutral gadolinium complexes of diethylenetriaminepentaacetic acid (DTPA)-bisamide derivatives have been synthesized and characterized. Their potential application as tissue-specific and low-osmolarity MRI contrast agents has been evaluated by in vitro and in vivo experiments. Their measured relaxivities in D(2)O, bovine serum albumin and human serum transferrin solutions showed favorable relaxation ability. In vivo studies have proven that Gd(DTPA-BDMA), Gd(DTPA-BIN), and Gd(cyclic-DTPA-1,2-pn) could be promising liver-specific MRI contrast agents and Gd(DTPA-BDMA), and Gd(cyclic-DTPA-1,2-pn) have favorable renal excretion capability. Among them, Gd(cyclic-DTPA-1,2-pn) is a more powerful hepatic contrast agent and Gd(DTPA-BIN) provides the stable imaging contrast for several hours. They also show a lower toxicity.

Quantitative assessment of Gd-DTPA contrast agent from signal enhancement: an in-vitro study

Quantitative determination of in-vivo gadolinium diethylenetriamine-pentaacid (Gd-DTPA) concentration is attractive in various studies involving perfusion, tracer kinetics and permeability constants. Using a 1.5 T clinical system and a 7 T small-bore system, we evaluated a method for absolute determination of Gd-DTPA concentrations in plasma solutions. Different solutions of Gd-DTPA and (99m)Tc-DTPA were mixed in human plasma and concentrations in the range of 0-5.0 mmol/l (1.5 T system) or 0-3.0 mmol/l (7 T system) of Gd-DTPA were divided into thirteen tubes. All MRI measurements were carried out using conventional sequences (SE, FLASH and GRASS). The MR measured intensity was converted to Gd-DTPA concentration by mathematical interpretation of the sequences. All MRI sequences showed, that the measured concentrations of Gd-DTPA revealed a slight non-linear difference compared with the calculated Gd-DTPA concentrations determined by the plasma (99m)Tc-DTPA using gamma counting. This non-linearity was most pronounced at high Gd-DTPA concentrations, suggesting that the discrepancy could be a result of an increased plasma relaxivity at higher concentrations. Adjustment of measured Gd-DTPA concentration was therefore performed using a selected power function, A[Gd-DTPA](a), which yielded the best linear relationship. Regression analysis showed that the scaling constant (A) varied from 0.11 to 97.45 and the power constant (a) varied from 0.83 to 1.6. Based on these constants, the MRI measured concentrations of Gd-DTPA did not differ from the calculated concentrations of Gd-DTPA obtained from reference measurements of (99m)Tc-DTPA. In the 1.5 T system, a linear relationship (r(2) > or = 0.95) was demonstrated in the range of 0-5.0 mmol/l Gd-DTPA, and in the 7 T system, a linear relationship (r(2) > or = 0.92) was demonstrated in the range of 0-3.0 mmol/l Gd-DTPA. Additionally, the effect of signal-to-noise on measured concentrations of Gd-DTPA was simulated using MR data of the mixed solutions of Gd-DTPA in plasma and the analytical expression of the pulse sequences. The simulations showed that the concentrations were most sensitive to noise in the GRASS sequence. In conclusion, this study demonstrates a novel approach to quantify accurately the Gd-DTPA concentration directly from MRI signal data using different routine sequences.

Application of NMRD to hydration of rubredoxin and a variant containing a (Cys-S)3FeIII(OH) site

Hydration of oxidized rubredoxin (Fe(III)(S-Cys)(4) center) was investigated by (1)H and (17)O relaxation measurements of bulk water as a function of the applied magnetic field (nuclear magnetic relaxation dispersion). Oxidized rubredoxin showed an increased water (1)H relaxation profile with respect to the diamagnetic gallium derivative or reduced species. Analysis of the data shows evidence of exchangeable proton(s) approximately 4.0-4.5 A from the metal ion, the exchange time being longer than 10(-10) s and shorter than 10(-5) s. The correlation time for the proton-electrons interaction is 7 x 10(-11) s and is attributed to the effective electron relaxation time. Its magnitude is consistent with the large signal linewidths of the protein donor nuclei, observed in high resolution NMR spectra. For reduced rubredoxin, such correlation time is proposed to be smaller than 10(-11) s. (17)O relaxation measurements suggest the presence of at least one long-lived protein-bound water molecule. Analogous relaxation measurements were performed on the C6S rubredoxin variant, whose iron(III) center has been previously shown to be coordinated to three cysteine residues and a hydroxide ion above pH 6. (1)H nuclear magnetic relaxation dispersion profiles indicate increased hydration with respect to the wild-type.

Comparison between GdDTPA and two gadolinium polyoxometalates as potential MRI contrast agents

Two gadolinium polyoxometalates, Gd(2)P(2)W(18)O(62) and K(15)[(GdO)(3)(PW(9)O(34))(2)], have been evaluated by in vivo as well as in vitro experiments as the candidates of tissue-specific magnetic resonance imaging (MRI) contrast agents. T(1)-relaxivities of 28.4 mM(-1).s(-1) for Gd(2)P(2)W(18)O(62) and 11.2 mM(-1).s(-1) for K(15)[(GdO)(3)(PW(9)O(34))(2)] (400 MHz, 25 degrees C) were higher than that of the commercial MRI contrast agent (GdDTPA). Their relaxivities in bovine serum albumin and human serum transferrin were also reported. The favorable liver-specific contrast enhancement and renal excretion capability in in vivo MRI with Sprague-Dawley rats after i.v. administration of K(15)[(GdO)(3)(PW(9)O(34))(2)] was demonstrated. In vivo and in vitro assay showed that K(15)[(GdO)(3)(PW(9)O(34))(2)] is a promising liver-specific MRI contrast agent. However, Gd(2)P(2)W(18)O(62) did not show the favorable quality in vivo as expected from its high relaxivity in vitro, which was attributed to low bioavailability, indicating that it is of limited value as tissue-specific MRI contrast agent.

Design and synthesis of a novel magnetic resonance imaging contrast agent for selective sensing of zinc ion

A series of new diethylenetriaminepentaacetic acid (DTPA)-bisamide chelators has been prepared and characterized for application as zinc sensors. We have designed and synthesized (GdL(a))(2-), which contains a DTPA-bisamide moiety. The R(1) relaxivity of (GdL(a))(2-) solution decreased monotonically on the addition of Zn(2+). Moreover, (GdL(a))(2-) showed high selectivity for Zn(2+) against Ca(2+) and Mg(2+). We also measured the UV-visible spectra and the coldspray ionization (CSI) MS spectra and concluded that the 1-to-1 Zn(2+) complex of (GdL(a))(2-) is stable at higher concentrations of Zn(2+). These complexes should provide the basis for creating a superior Zn(2+)-sensitive MRI contrast agent and are excellent candidates for incorporation into sensors designed for selective detection of Zn(2+) in biological applications.

Mechanistic studies of a calcium-dependent MRI contrast agent

Intracellular Ca(2+) plays an important role in signal transduction, and we are developing new MRI techniques to study its regulation in living animals. We have reported on an MRI contrast agent (DOPTA-Gd) where the relaxivity of the complex is controlled by the presence or absence of the divalent ion Ca(2+). By structurally modulating inner-sphere access of water to a chelated Gd(3+) ion, we observe a substantial and reversible change in T(1) upon the addition of Ca(2+) and not other divalent ions. Luminescence lifetime and NMRD measurements of the complex have been acquired, and several parameters contribute to the Ca(2+) dependent relaxivity change of DOPTA-Gd. The number of inner-sphere water molecules is more than doubled after the Ca(2+) concentration is increased. This finding strongly supports the proposed conformational change of DOPTA-Gd when Ca(2+) is bound. Relaxometric measurements confirm these results and provide an indication that second-sphere water molecules are probably responsible for paramagnetic relaxation enhancement in the absence of Ca(2+). After Ca(2+) is bound to DOPTA-Gd, the molecule undergoes a substantial conformational change that opens up the hydrophilic face of the tetraazacyclododecane macrocycle. This change dramatically increases the accessibility of chelated Gd(3+) ion to bulk solvent. The design of this class of calcium-activated MR contrast agent was based primarily on the assumption that the number of coordinated inner-sphere water molecules would be the dominating factor in observed relaxivity measurements. This result has been confirmed; however, careful mechanistic studies reveal that additional factors are involved in this process.

An evaluation of gadolinium polyoxometalates as possible MRI contrast agent

Two gadolinium polyoxometalates, K(9)GdW(10)O(36) and K(11)[Gd(PW(11)O(39))(2)], have been evaluated both in vivo and in vitro as candidates for tissue-specific MRI contrast agents. T(1)-relaxivities of 6.89 mM(-1). s(-1) for K(9)GdW(10)O(36) and 5.27 mM(-1). s(-1) for K(11)[Gd(PW(11)O(39))(2)] are slightly higher than that of the commercial MRI contrast agent (Gd-DTPA). Both compounds bind with bovine serum albumin and human serum transferrin and favorable liver-specific contrast enhancement in in vivo MRI with Sprague-Dawley rats after i.v. administration has been demonstrated. Imaging studies demonstrate that the two agents have a long residence time, showing MR signal enhancement in the liver for more than 40 min, longer than commercially available contrast agents. In vivo and in vitro assays showed that GdW(10) and Gd(PW(11))(2) are promising liver-specific MRI contrast agents and GdW(10) may be used in the diagnosis of the pathological state. However, with the higher acute toxicity, the two gadolinium polyoxometalates need to be modified and studied further before clinical use.

Stability of MRI paramagnetic contrast media: a proton relaxometric protocol for transmetallation assessment

RATIONALE AND OBJECTIVES

The suitability of paramagnetic complexes as magnetic resonance contrast agents depends on various factors such as their relaxivity, stability, selectivity, and the inertness toward transmetallation by endogenous ions. The transmetallation of a series of 18 gadolinium complexes by the Zn2+ ion was studied in vitro by proton relaxometry.

METHODS

Transmetallation was analyzed through the evolution of the paramagnetic longitudinal relaxation rate of water protons at 37 degrees C in pH = 7 phosphate buffer solutions containing 2.5 mmol/L of the gadolinium complexes and 2.5 mmol/L zinc chloride. The measurements were performed at 0.47 T over a period of at least 3 days.

RESULTS

The results confirm the high stability of macrocyclic systems and a high sensitivity of Gd-diethylenetriamine-pentaacetic acid (DTPA) derivatives to transmetallation by Zn2+ ions. The decreasing order of stability with respect to metal exchange is as follows: Gd-macrocyclics > Gd-C-functionalized DTPA > Gd-DTPA > primary and secondary Gd-DTPA bisamides. The ternary bisamide analyzed in this study [Gd-DTPA-1,11-bisbismethylamino-1,11-dioxo-3,6,9-triaza-3,6,9-tris(carboxymethyl)undecane] is more stable than the parent compound Gd-DTPA.

CONCLUSIONS

A simple relaxometric protocol has been successfully developed to study the in vitro transmetallation process of gadolinium complexes. The importance of the functionalization and substitution of the DTPA-like complexes is clearly shown.

Characterization of (111)In(3+) complexes of DTPA amide derivatives: biodistribution and clearance studied by gamma imaging

A large series of structurally related diethylenetriaminepentaacetic acid amide derivatives with different structures and lipophilic properties were synthesized and radiolabeled with (111)In(3+). Two of the more hydrophobic compounds studied ([(111)In]L(9) and [(111)In]L(10)) showed high affinity for human serum albumin (HSA). The biodistribution and clearance properties shown by all complexes upon injection in Wistar rats were followed by gamma imaging. The blood retention time of the chelates correlates better with their binding to HSA than with their hydrophilic/lipophilic ratio. Hydrophilic and negatively charged complexes undergo renal retention, while the majority of the lipophilic complexes are retained in the blood for a longer period of time and are cleared through the liver.