Ernő Brücher

Dissociation Kinetics of Open‐Chain and Macrocyclic Gadolinium(III)‐Aminopolycarboxylate Complexes Related to Magnetic Resonance Imaging: Catalytic Effect of Endogenous Ligands

The kinetics of the metal exchange reactions between open-chain Gd(DTPA)(2-) and Gd(DTPA-BMA), macrocyclic Gd(DOTA)(-) and Gd(HP-DO3A) complexes, and Cu(2+)  ions were investigated in the presence of endogenous citrate, phosphate, carbonate and histidinate ligands in the pH range 6-8 in NaCl (0.15 M) at 25 °C. The rates of the exchange reactions of Gd(DTPA)(2-) and Gd(DTPA-BMA) are independent of the Cu(2+) concentration in the presence of citrate and the reactions occur via the dissociation of Gd(3+)  complexes catalyzed by the citrate ions. The HCO(3)(-)/CO(3)(2-) and H(2)PO(4)(-) ions also catalyze the dissociation of complexes. The rates of the dissociation of Gd(DTPA-BMA), catalyzed by the endogenous ligands, are about two orders of magnitude higher than those of the Gd(DTPA)(2-). In fact near to physiological conditions the bicarbonate and carbonate ions show the largest catalytic effect, that significantly increase the dissociation rate of Gd(DTPA-BMA) and make the higher pH values (when the carbonate ion concentration is higher) a risk-factor for the dissociation of complexes in body fluids. The exchange reactions of Gd(DOTA)(-) and Gd(HP-DO3A) with Cu(2+) occur through the proton assisted dissociation of complexes in the pH range 3.5-5 and the endogenous ligands do not affect the dissociation rates of complexes. More insights into the interaction scheme between Gd(DTPA-BMA) and Gd(DTPA)(2-) and endogenous ligands have been obtained by acquiring the (13)C NMR spectra of the corresponding diamagnetic Y(III)-complexes, indicating the increase of the rates of the intramolecular rearrangements in the presence of carbonate and citrate ions. The herein reported results may have implications in the understanding of the etiology of nephrogenic systemic fibrosis, a rare disease that has been associated to the administration of Gd-containing agents to patients with impaired renal function.

Formation and dissociation kinetics of the complexes Gd(DOTP)5- and Gd(DOTPMB)-

The monobutyl ester of H8DOTP, the ligand H4DOTPMB, was synthesized, and the protonation constants (KHi) and the stability constant of Gd(DOTPMB) were determined by pH-potentiometry (25 °C, 0.1 M Me4NCl): logKHi, (i = 1, 2, and 3) = 10.34(0.02), 7.72(0.025), and 2.42(0.030), respectively, and logKGdL = 12.19(0.05). The rates of formation of Gd(DOTPMB) and Gd(DOTP) were studied by 1H relaxometry in the pH range 5.4−7, and also by spectrophotometry in the case of Gd(DOTP) (7 < pH < 8). For both reactions, first-order rate constants were obtained at different concentration ratios of the reactants, which indicated the rapid formation of a reaction intermediate. The compositions of the intermediates are Gd(HiDOTPMB) (i = 1, 2) and HnGd(H2DOTP) (n = 0−4), respectively, where one or two protons are attached to the nitrogen atoms of the ligand. The rate of rearrangement (kr) of the intermediate Gd(HiDOTPMB) to the product Gd(DOTPMB) increases with increasing [OH−]: kr = kOH[OH−] + k2OH[OH−]2, where kOH = (1.3±0.25) ×103M−1s−1 and k2OH = (7.8±0.2) ×1011M−2s−1. For the formation reaction of Gd(DOTP), only the first term exists and kOH = (7.2±0.1) ×103M−1s−1. For the complexation reactions, similar mechanisms were proposed in which deprotonation of the species Gd(HDOTPMB) and HnGd(HDOTP) plays the rate-determining role. For the deprotonation, general base catalysis was found to be satisfactory. The rate of dissociation of Gd(DOTPMB) in 0.025−1.0 M HCl solution ([HCl] + [Me4NCl] = 1.0 M) was lower than that of Gd(DOTP) and the first-order rate constants exhibited saturation curves with increasing [H+]. Based on the assumption that the protonated species HGd(DOTPMB) and H5Gd(DOTP) dissociate, the rate constants (and protonation constants) were found to be (5.4±0.2) ×10−4M−1s−1 (KHGdL = 1.7±0.1) and (2.1±0.1) ×10−4M−1s−1 (KHH4GdL = 1.9±0.1), respectively.

Dramatic Increase of Selectivity for Heavy Lanthanide(III) Cations by Tuning the Flexibility of Polydentate Chelators

Two novel octadentate ligands have been synthesized by attaching two terminal iminodiacetic groups to either 1,4-diazepane (BCAED) or piperazine (BCAEP) as central scaffold. The introduction of the seven- or six-membered ring into the ligand backbone is expected to modify their overall flexibility and then to affect the stability of the corresponding lanthanide(III) complexes. In this work, thermodynamic stability data are determined for the formation of the complexes of BCAED and BCAEP with La(3+), Nd(3+), Eu(3+), Gd(3+), Ho(3+), and Lu(3+). The ligand BCAED shows a strong binding affinity for Lu(3+) (logK = 20.99), moderate for Gd(3+) (logK = 17.15) and rather weak for La(3+) (logK = 12.77). Thus, the variation of logK across the Ln series assumes the remarkable value of 8.22, the largest so far reported. This points to a predominant role of a suitable size match between the metal ion and the ligand cavity, determined by its structure and flexibility. The ligand BCAEP forms less stable complexes with lanthanide(III) cations although it retains a good selectivity (DeltalogK(La-Lu) = 5.66). The Gd(III) complexes have been investigated in aqueous solution by measuring their relaxivity as a function of pH, at 20 MHz and 25 degrees C. The results can be interpreted very well in terms of the species distribution curves calculated from the thermodynamic data and indicate that in these complexes Gd(3+) is octacoordinated, without any bound water molecule. This coordination geometry is maintained in the solid state as shown by the X-ray crystal structure of [Na(H(2)O)(2)][Gd(BCAED)] where the metal ion is at the center of a bicapped-trigonal prism. Finally, the (13)C NMR spectra (9.4 T, 25 degrees C) of the diamagnetic La(3+), Y(3+), and Lu(3+) complexes show that a pronounced stereochemical rigidity is associated with the thermodynamically more stable complexes.

The Role of Equilibrium and Kinetic Properties in the Dissociation of Gd[DTPA‐bis(methylamide)] (Omniscan) at near to Physiological Conditions

[Gd(DTPA-BMA)] is the principal constituent of Omniscan, a magnetic resonance imaging (MRI) contrast agent. In body fluids, endogenous ions (Zn(2+), Cu(2+), and Ca(2+)) may displace the Gd(3+). To assess the extent of displacement at equilibrium, the stability constants of DTPA-BMA(3-) complexes of Gd(3+), Ca(2+), Zn(2+), and Cu(2+) have been determined at 37 °C in 0.15 M NaCl. The order of these stability constants is as follows: GdL≈CuL>ZnL≫CaL. Applying a simplified blood plasma model, the extent of dissociation of Omniscan (0.35 mM [Gd(DTPA-BMA)]) was found to be 17% by the formation of Gd(PO4), [Zn(DTPA-BMA)](-) (2.4%), [Cu(DTPA-BMA)](-) (0.2%), and [Ca(DTPA-BMA)](-) (17.7%). By capillary electrophoresis, the formation of [Ca(DTPA-BMA)](-) has been detected in human serum spiked with [Gd(DTPA-BMA)] (2.0 mM) at pH 7.4. Transmetallation reactions between [Gd(DTPA-BMA)] and Cu(2+) at 37 °C in the presence of citrate, phosphate, and bicarbonate ions occur by dissociation of the complex assisted by the endogenous ligands. At physiological concentrations of citrate, phosphate, and bicarbonate ions, the half-life of dissociation of [Gd(DTPA-BMA)] was calculated to be 9.3 h at pH 7.4. Considering the rates of distribution and dissociation of [Gd(DTPA-BMA)] in the extracellular space of the body, an open two-compartment model has been developed, which allows prediction of the extent of dissociation of the Gd(III) complex in body fluids depending on the rate of elimination of the contrast agent.

Equilibrium studies on the AlIII-, GaIII-, InIII- and TlIII-ethylenediaminetetraacetate-halide and -sulphide systems

Abstract The formation of mixed ligand complexes was studied in the Al(edta)−, Ga(edta)−, In(edta)− and Tl(edta)−-halide and -sulphide ion systems by potentiometry, using glass and anion-selective electrodes. The stability constants (log KMLF) of the mixed ligand complexes M(edta)F2− were found to be 4.8±0.2, 1.9±0.1 and 0.9±0.1 for Al3+, Ga3+ and In3+, respectively. Values of log KMLX = 2.3±0.2, 3.5±0.15 and 5.9±0.2 were measured for Tl(edta)X2−, X = Cl−, Br− and I−, respectively. Complexes M(edta)S3− are described. The values of log KMLS were calculated to be 10.3, 10.6 and 9.4 for Al3+, Ga3+ and In3+, respectively. Ionic-covalent interactions in the mixed complexes are discussed. The stability constant of Tl(edta)− was reinvestigated via spectrophotometric study of the competition reaction in the Tl3+-H+-(edta)4−-X− system.

Kinetics of the Exchange Reactions between Gd(DTPA)2−, Gd(BOPTA)2−, and Gd(DTPA-BMA) Complexes, Used As MRI Contrast Agents, and the Triethylenetetraamine-Hexaacetate Ligand

The kinetics of ligand exchange reactions occurring between the Gd(DTPA), Gd(BOPTA), and Gd(DTPA-BMA) complexes, used as contrast agents in MRI, and the ligand TTHA, have been studied in the pH range 6.5-11.0 by measuring the water proton relaxation rates at 25 °C in 0.15 M NaCl. The rates of the reactions are directly proportional to the concentration of TTHA, indicating that the reactions take place with the direct attack of the H(i)TTHA((6-i)-) (i = 0, 1, 2 and 3) species on the Gd(3+) complexes, through the formation of ternary intermediates. The rates of the exchange reactions of the neutral Gd(DTPA-BMA) increase when the pH is increased from 6.5 to 9, because the less protonated H(i)TTHA((6-i)-) species can more efficiently attack the Gd(3+) complex. The rates of the exchange reactions of [Gd(DTPA)](2-) and [Gd(BOPTA)](2-) also increase from pH 8.5 to 11, but from 6.5 to 8.5 an unexpected decrease was observed in the reaction rates. The decrease has been interpreted by assuming the validity of general acid catalysis. The protons from the H(i)TTHA((6-i)-) species (i = 2 and 3) can be transferred to the coordinated DTPA or BOPTA in the ternary intermediates when the dissociation of the Gd(3+) complexes occurs faster. The kinetic inertness of Gd(DTPA), Gd(BOPTA), and Gd(DTPA-BMA) differs very considerably; the rates of the ligand exchange reactions of Gd(DTPA-BMA), thus the rates of its dissociation, are 2 to 3 orders of magnitude higher than those of Gd(DTPA) and Gd(BOPTA). The rates of the ligand exchange reactions increase with increasing concentration of the endogenous citrate, phosphate, or carbonate ions at a pH of 7.4, but the effect of citrate and phosphate is negligible at their physiological concentrations. The increase in the reaction rates at the physiological concentration of the carbonate ion is significant (20-60%), and the effect is the largest for the Gd(DTPA-BMA) complex.

Determination of gadolinium-based magnetic resonance imaging contrast agents by micellar electrokinetic capillary chromatography

MEKC with DAD was applied to detect six Gd‐based contrasting agents (CAs) (Gd‐DTPA‐BMA (Omniscan), Gd‐HPDO3A (ProHance), Gd‐DOTA (Dotarem), Gd‐AAZTA, Gd‐BOPTA (Multihance) and Gd‐DTPA (Magnevist)) commonly used in MRI diagnostics. The achieved LODs ranged between 0.40 and 20 μM and the optimized method gave excellent precision, especially when two internal standards were applied (less than 0.34 RSD% for migration time). The MEKC technique made it possible to determine the CAs in urine and serum samples of patients having a therapeutic dose. Due to the SDS content of the running buffer, the serum samples can be directly injected to analyze Gd‐based CAs without interference of high protein content.

Exploiting the Proton Exchange as an Additional Route to Enhance the Relaxivity of Paramagnetic MRI Contrast Agents

The relaxivity of Gd(HP-DO3A) was studied as a function of pH and buffer composition in order to identify the main factors of the observed relaxation enhancement due to the exchange of the coordinated hydroxyl proton. It was established that the paramagnetic relaxation time, T1M, of the coordinated hydroxyl proton is about 50% shorter than that of the protons in the coordinated water molecule. The control of the p K of the coordinated alcoholic -OH moiety in the ligand is fundamental to utilize the proton exchange enhanced relaxivity under physio/pathologic conditions. A new derivative of Gd(HP-DO3A) was synthesized by replacing the -CH3 group with a -CF3 moiety. In this complex, the -OH group becomes more acidic. Consequently, the maximum contribution of the proton exchange to the relaxivity is shifted to a lower pH region with the fluorinated ligand.

Synthesis, Conformation and Equilibrium Study of New Piperazine and Azamacrocyclic Ligands with N-(Tetrahydro-2-oxofuran-3-yl) and N-[(Carboxy)(2-hydroxyethyl)methyl] Pendant Arms

New piperazine (1), homopiperazine (2), 1-tosyl-1,4,7-triazacyclononane (3), 1,4,7-triazacyclononane (4), and 1,4,10,13-tetraoxa-7,16-diazacyclooctadecane (5) derivatives 1a−5a with 2-oxotetrahydrofuran-3-yl pendant arms have been synthesized from the parent cyclic polyamines 1−5 and 2-bromobutyrolactone in acetonitrile. Their conformational properties have been studied by molecular mechanics and NMR spectroscopy, and new sets of Karplus parameters for calculation of the vicinal coupling constants of the butyrolactone moieties have been determined. Compounds 1a−5a were hydrolysed to (carboxy)(2-hydroxyethyl)methyl derivatives 1b−5b by treatment with aqueous sodium hydroxide. The protonation and complexation properties of 4b (HOET-NOTA) were studied by pH potentiometry, photometry, and 1H NMR titrations, and the results were compared with the corresponding values for NOTA. It was found that, although complexation of 4b with smaller metal ions was approximately two orders of magnitude weaker, its stability constants with the lanthanides remained unchanged.

Taking the next step toward inert Mn2+ complexes of open-chain ligands: the case of the rigid PhDTA ligand

In line with our research to find inert Mn(II) complexes as contrast agents for magnetic resonance imaging, we have studied the aromatic-ring rigidified EDTA-analogue o-phenylenediamine-N,N,N′,N′-tetraacetic acid (PhDTA). The protonation constants (KHi) of PhDTA and stability constants of complexes formed between this open-chain ligand and several different biogenic metal ions (Ca2+, Mg2+, Zn2+, Cu2+, Mn2+) have been determined in 0.15 M NaCl at 25 °C and compared with the values reported in the literature previously. The protonation constants are lower than those of the corresponding cis- and trans-CDTA complexes, which might be attributed to the electron withdrawing effect of the phenylene group. The lower total basicity of the ligand leads to lower stability constants for all the examined metal complexes. On the contrary, we have found that the conditional stability constants of [Mn(PhDTA)]2− and [Mn(trans-CDTA)]2− are approximately the same, as both complexes are completely formed at pH 5 and their pM values are also comparable. The relaxivity of [Mn(PhDTA)]2− is nearly identical (r1 = 3.72 mM−1 s−1) to that determined previously for the [Mn(trans-CDTA)]2− complex (r1 = 3.62 mM−1 s−1), and its pH-dependence confirms the equilibrium model used for the fitting of the titration data. The results of the kinetic studies of the metal exchange reactions reveal that the [Mn(PhDTA)]2− complex possesses a slightly better dissociation kinetics profile than that of [Mn(trans-CDTA)]2−, which has been tested in vivo recently (including human injections). The half-life of the dissociation of the complex near physiological pH at 25 °C is 19 hours. By using the rate constant calculated for the dissociation (pH = 7.4, cCu2+ = 10 μM) and the half-life of excretion (1.6 hour), the ratio of the dissociated complex is estimated to represent 8% of the injected dose. DFT studies reveal that the metal coordination environment of [Mn(PhDTA)]2− is very similar to that of [Mn(EDTA)]2−, both containing an inner-sphere water molecule. Cyclic voltammetry studies indicate that [Mn(PhDTA)]2− is slightly more resistant towards oxidation to the Mn3+ complex than the EDTA analogue.