Cyclen-based Gd3+ complexes as MRI contrast agents: Relaxivity enhancement and ligand design

Magnetic Resonance Imaging (MRI) is a noninvasive radiology technique used to examine the internal organs of human body. It is useful for the diagnosis of structural abnormalities in the body. Contrast agents are used to increase the sensitivity of this technique. 1,4,7,10-Tetraazacyclododecane (cyclen) is a macrocyclic tetraamine. Its derivatives act as useful ligands to produce stable complexes with Gd³⁺ ion. Such chelates are investigated as MRI contrast agents. Free Gd³⁺ ion is extremely toxic for in vivo use. Upon complexation with a cyclen-based ligand, it is trapped in the preformed central cavity of the ligand resulting in the formation of a highly stable Gd³⁺-chelate. Better kinetic and thermodynamic stability of cyclen-based MRI contrast agents decrease their potential toxicity for in vivo use. Consequently, such agents have proved to be safest for clinical applications. Relaxivity is the most important parameter used to measure the effectiveness of a contrast agent. A number of factors influence this parameter. This article elucidates detailed strategies to increase relaxivity of cyclen-based MRI contrast agents. 1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) and 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (DO3A) are two key ligands derived from cyclen. They also act as building blocks for the synthesis of novel ligands. A few important methodologies for the synthesis of DOTA and DO3A derivatives are described. Moreover, the coordination geometry of chelates formed by these ligands and their derivatives is discussed as well. Novel ligands can be developed by the appropriate derivatization of DOTA and DO3A. Gd³⁺-chelates of such ligands prove to be useful MRI contrast agents of enhanced relaxivity, greater stability, better clearance, lesser toxicity and higher water solubility.     

In vivo behavior of copper-64-labeled methanephosphonate tetraaza macrocyclic ligands

Copper-64 ( T(1/2)=12.7 h; beta(+): 0.653 MeV, 17.4%; beta(-): 0.578 MeV, 39%) is produced in a biomedical cyclotron and has applications in both imaging and therapy. Macrocyclic chelators are widely used as bifunctional chelators to bind copper radionuclides to antibodies and peptides owing to their relatively high kinetic stability. In this paper, we evaluated three tetraaza macrocyclic ligands with two, three, and four pendant methanephosphonate functional groups. DO2P [1,4,7,10-tetraazacyclododecane-1,7-di(methanephosphonic acid)], DO3P [1,4,7,10-tetraazacyclododecane-1,4,7-tri(methanephosphonic acid)], and DOTP [1,4,7,10-tetraazacyclododecane-1,4,7,10-tetra(methanephosphonic acid)] were all radiolabeled with (64)Cu in high radiochemical yields. Copper-64-labeled DO2P and DOTP were highly stable in rat serum out to 24 h, while (64)Cu-DO3P remained 73% intact, with the remainder possibly forming a (64)Cu(.)2DO3P dimer by 24 h. The biodistribution experiments were performed in normal Sprague-Dawley rats. Of the three complexes, (64)Cu-DO2P demonstrated the most optimal clearance through the blood and liver. Copper-64-DO3P and (64)Cu-DOTP exhibited higher liver uptake and longer retention of liver activity, possibly because of the large negative charge of the complexes under physiological conditions. All three (64)Cu-labeled complexes showed high accumulation in bone, likely due to the binding of the methanephosphonate groups to hydroxyapatite. These results suggest that this series of methanephosphonate macrocyclic ligands may be useful as potential bone-imaging agents. The thermodynamic stability constants of the Cu(II) complexes with these three ligands were determined, and were found to be significantly higher than those of their acetate analogues. The Cu(II)-DO2P complex exhibited the highest stability constant among divalent transition metal ion DO2P complexes. Metabolism studies of (64)Cu-DO2P in rat liver suggest that the DO2P ligand may be used as a bifunctional chelator for copper radionuclides in radiodiagnostic or radiotherapeutic studies.     

Fluorescent ligands derived from 2-(9-anthrylmethylamino)ethyl-appended cyclen for use in metal ion activated molecular receptors

Three new fluorescent ligands derived from 2-(9-anthrylmethylamino)ethyl-appended cyclen (cyclen = 1,4,7,10-tetraazacyclododecane) intended for future use as metal ion activated molecular receptors have been synthesised and characterised. The new ligands, 1,4,7-tris[(2″S)-acetamido-2″-(methyl-3″-phenylpropionate)]-10-(2-N-(9-anthrylmethylamino)ethyl-1,4,7,10-tetraazacyclododecane, 1,4,7-tris[(2″S)-acetamido-2″-(methyl-3″-phenylpropionate)]-10-(2-N-(9-anthrylmethylamino)ethyl-N-[(2″S)-acetamido-2″-(methyl-3″-phenylpropionate])-1,4,7,10-tetraazacyclododecane and 1,4,7-tris[2-hydroxyethyl]-10-(2-N-(9-anthrylmethylamino)ethyl)-N-(2-hydroxyethyl))-1,4,7,10-tetraazacyclododecane, provide the opportunity to investigate the consequences of alkylating the 2-(9-anthrylmethylamino)ethyl fluorophore at the anthrylamine. It was discovered that by doing this the basicity of this amine is lowered and in consequence the pH range over which the PeT induced fluorescence quenching extends is increased by about 1 pH unit. Formation constants were determined in 20% aqueous methanol for the first two ligands with Cd(II) and Cu(II). This demonstrated that alkylation of the anthrylamine significantly increases the stability of the metal complexes.     

Nuclear magnetic resonance studies of the solution chemistry of metal complexes. 26. Mixed ligand complexes of cadmium, nitrilotriacetic acid, glutathione, and related ligands

The complexation of glutathione and related ligands by the nitrilotriacetic acid complex of Cd2+ (Cd(NTA)-) has been investigated by 1H NMR as a model for the coordination chemistry of Cd2+ and GSH in biological systems. Related ligands included glycine, glutamic acid, cysteine, N-acetylcysteine, penicillamine, N-acetylpenicillamine, mercaptosuccinic acid, and the S-methyl derivative of glutathione. The nature of the complexes formed was deduced from 1H NMR spectra of Cd(NTA)- and the ligands. Mixed ligand complexes (Cd(NTA)L) and single ligand complexes (CdLx) are formed with the thiol ligands, whereas only mixed ligand complexes form with glycine, glutamic acid and S-methylglutathione. Formation constants of the mixed and the single ligand complexes were determined from NMR data. The results indicate that formation constants for binding of a thiolate donor group by Cd2+, either as the free ion or in a coordinately unsaturated complex, are in the range 10(5)-10(6).     

Mixed Chelates of Ca(II)-Pyridine-2,6-Dicarboxylate with Some Amino Acids Related to Bacterial Spores

The high resistance of bacterial spores to heat has been repeatedly postulated to be due to stabilization of spore biopolymers by metal chelate compounds. Binding of calcium dipicolinic acid (Ca(II)-DPA) with spore proteins and amino acids has been discussed in the literature, but equilibrium data are generally lacking. By means of potentiometric pH titrations at 25 degrees C and an ionic strength of 1.0 (KNO(3)), the formation of Ca(II)-DPA (1:1 and 1:2) chelates and the interactions of Ca(II)-DPA chelate with a mole of each of three typical amino acids viz., cysteine, alanine, and glycine has been investigated. Analysis of the potentiometric data indicates that calcium and DPA forms 1:1 and 1:2 chelates with log K(ML1) = 4.39 +/- 0.01 and log K(ML2) = 2.25 +/- 0.01. In the presence of an equimolar amount of each of the amino acids under consideration, the Ca(II)-DPA chelate forms mixed ligand (ternary) chelate yielding the following stepwise stability constants: log K(1) = 4.17 +/- 0.01, log K(2) = 0.78 +/- 0.01 for cysteine, log K(1) = 4.06 +/- 0.01, log K(2) = 0.65 +/- 0.01 for alanine, and log K(1) = 4.30 +/- 0.02, log K(2) = 0.11 +/- 0.01 for glycine. Methods for calculating the stability constants of the mixed ligand system have been developed. On the basis of the potentiometric equilibrium data, possible structures for the various calcium chelate species are discussed. The data suggest that the differences in heat resistance of various strains of bacterial spores may conceivably be related to the differences in composition and stability of coordination complexes in the spore.     

Carboxymethyl-substituted bifunctional chelators: preparation of aryl isothiocyanate derivatives of 3-(carboxymethyl)-3-azapentanedioic acid, 3,12-bis(carboxymethyl)-6,9-dioxa-3,12-diazatetradecanedioic++ + acid, and 1,4,7,10-tetraazacyclododecane-N,N',N",N'-tetraacetic acid for use as protein labels

3-(Carboxymethyl)-3-azapentanedioic acid (NTA), 3,12-bis(carboxymethyl)-6,9-dioxa-3,12-diazatetradecanedioic acid (EGTA), and 1,4,7,10-tetraazacyclododecane-N,N',N",N'-tetraacetic acid (DOTA) structures having a 4-nitrophenyl substituent attached via an alkyl spacer to the methylene carbon atom of one carboxymethyl arm of the chelator were obtained by alkylation of 4-nitrophenylalanine with bromoacetic acid (NTA), by reductive alkylation of 1,8-diamino-3,6-dioxaoctane with (4-nitrophenyl)-pyruvic acid followed by alkylation with bromoacetic acid (EGTA), and by alkylation of the trimethyl ester of 1,4,7,10-tetraazacyclododecane-N,N',N"-triacetic acid with the methyl ester of alpha-bromo-4-(4-nitrophenyl)pentanoic acid and subsequent saponification (DOTA). The nitrophenyl-substituted chelators were converted to the corresponding amines by hydrogenation then reacted with thiophosgene to give the protein-reactive aryl isothiocyanate derivatives.     

Backbone-substituted DTPA ligands for 90Y radioimmunotherapy

Four new bifunctional diethylenetriaminepentaacetic acid (DTPA) ligands were synthesized to provide an improved chelating agent for radioimmunotherapy with 90Y. The new DTPA ligands contained a 4-isothiocyanatobenzyl group (pSCNBz) substituted onto the carbon backbone of DTPA for use in linkage to immunoprotein. Methyl groups were strategically incorporated onto the backbone of the ligands via a peptide route to provide 2-pSCNBz-5-Me-DTPA (2) and 3-Me-6-pSCNBz-DTPA (3). Addition of these functionalities was expected to sterically hinder the release of radiometal from the chelate. A new monosubstituted ligand, 3-pSCNBz-DTPA (4), was also prepared in order to determine whether a shift in position of the linking group had an effect on the in vivo stability of the yttrium complex. Additionally, by modification of known methods, a disubstituted DTPA ligand, 2-pSCNBz-6-Me-DTPA (1), was prepared.     

Speciation studies of ternary complexes of some essential divalent metal ions with L-histidine and L-glutamic acid in DMSO-water mixtures

Abstract

Chemical speciation of ternary complexes of Ca(II), Mg(II) and Zn(II) ions with L-histidine as the primary ligand (L) and L-glutamic acid as the secondary ligand (X) has been studied pH metrically in the concentration range of 0.0-60.0% v/v DMSO-water mixtures maintaining an ionic strength of 0.16 mol L^-1 using sodium chloride at 303.0 K. Titrations were carried out in different relative concentrations (M:L:X = 1.0:2.5:2.5, 1.0:2.5:5.0, 1.0:5.0:2.5) of metal (M) to L-histidine to L-glutamic acid with sodium hydroxide. Stability constants of ternary complexes were refined with MINIQUAD75. The best-fit chemical models were selected based on statistical parameters and residual analysis. The predominant species detected for Ca(II), Mg(II) and Zn(II) are ML₂XH₂, MLXH₂ and MLX₂. Extra stability of ternary complexes compared to their binary complexes was explained to be due to electrostatic interactions of the side chains of ligands, charge neutralisation, chelate effect, stacking interactions and hydrogen bonding. The species distribution with pH at different compositions of DMSO and the plausible equilibria for the formation of species are discussed.     

Chemical and functional characterization of metal-binding pseudotripeptides with different functionalized N-alkyl residues Dedicated to Professor Dr. Ernst-Gottfried Jaeger on occasion of his 65th birthday

Pseudotripeptide ligands with 4 different N-functionalized glycine residues were qualitatively, semiquantitatively and quantitatively tested for their complexation of the bivalent transition metal ions Zn²⁺, Cu²⁺, Co²⁺, Ni²⁺ and Mn²⁺. The functional side chains have different length and different groups available for complexation. MALDI-MS and ESI-MS were used for more qualitative or semiquantitative estimation of the complex formation tendencies. The found ranking differs by these two methods only for Zn²⁺ and Ni²⁺. For one of the pseudotripeptide ligands, the ligand L1, complex formation with certain transition metal was estimated quantitatively by potentiometric titration. The Zn-complex of that ligand polarizes bound water strongly, resulting in a low pK_a-value. Complexes of pseudotripeptide ligand L1 with certain metal ions were tested for their hydrolytic activity. The pseudo first order rate constants of the hydrolysis of the substrates 4-nitrophenyl acetate and bis(4-nitrophenyl)phosphate were compared to complexes with the same metal ions formed with a very well studied ligand from the literature, the 1,4,7,10-tetraaza cyclododecane (cyclen). The hydrolysis of the phosphate ester occurs very slowly compared to the acetate ester. No correlation exists between the estimated pK_a values of complexes formed from ligand L1 with different metal ions and the phosphate ester hydrolysis. The Ni ions give totally different hydrolytic activities for pseudotripeptide ligand L1 and cyclen. With one exception, the Ni-cyclen complex, all other complexes have only a low or moderate catalytic activity.     

Effects of terbium chelate structure on dipicolinate ligation and the detection of Bacillus spores

Terbium-sensitized luminescence and its applicability towards the detection of Bacillus spores such as anthrax are of significant interest to research in biodefense and medical diagnostics. Accordingly, we have measured the effects of terbium chelation upon the parameters associated with dipicolinate ligation and spore detection. Namely, the dissociation constants, intrinsic brightness, luminescent lifetimes, and biological stabilities for several Tb(chelate)(dipicolinate)_x complexes were determined using linear, cyclic, and aromatic chelators of differing structure and coordination number. This included the chelator array of NTA, BisTris, EGTA, EDTA, BAPTA, DO2A, DTPA, DO3A, and DOTA (respectively, 2,2′,2″-nitrilotriacetic acid; 2,2-bis(hydroxymethyl)-2,2′,2″-nitrilotriethanol; ethylene glycol-bis(2-aminoethyl ether)-N,N,N′,N′-tetraacetic acid; ethylenediamine-N,N,N′,N′-tetraacetic acid; 1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid; 1,4,7,10-tetraazacyclododecane-1,7-diacetic acid; diethylenetriamine-N,N,N′,N″,N″-pentaacetic acid; 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid; and 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid). Our study has revealed that the thermodynamic and temporal emission stabilities of the Tb(chelate)(dipicolinate)_x complexes are directly related to chelate rigidity and a ligand stoichiometry of x = 1, and that chelators possessing either aromaticity or low coordination numbers are destabilizing to the complexes when in extracts of an extremotolerant Bacillus spore. Together, our results demonstrate that both Tb(EDTA) and Tb(DO2A) are chemically and biochemically stable and thus applicable as respectively low and high-cost luminescent reporters for spore detection, and thereby of significance to institutions with developing biodefense programs.     

Novel procedures for preparing 99mTc(III) complexes with tetradentate/monodentate coordination of varying lipophilicity and adaptation to 188Re analogues

Improved methods are presented for the preparation of 99mTc and 188Re mixed-ligand complexes with tetradentate and monodentate ligands of the general formula [MIII(Lm)(Ln)] (M = Tc, Re; Lm = NS3 or NS3COOH; Ln = isocyanide or phosphine). To avoid the undesired formation of reduced-hydrolyzed species of both metals, the preparation of complexes is performed in a two-step procedure. At first the Tc(III)- or Re(III)-EDTA complex is formed which reacts in a second step with the tripodal ligand 2,2',2' '-nitrilotris(ethanethiol) (NS3) or its carboxyl derivative NS3COOH (a) and the monodentate phosphine ligands (triphenylphosphine L1, dimethylphenylphosphine L2) or isocyanides (tert-butyl isonitrile L3, methoxyisobutyl isonitrile L4, 4-isocyanomethylbenzoic acid-L-arginine L5, 4-isocyanomethylbenzoic acid-L-arginyl-L-arginine L6, 4-isocyanomethylbenzoic acid-neurotensin(8-13) L7) to the so-called '4+1' complex. Copper(I) isocyanide complexes are used for preparing the '4+1' complexes. That facilitates storage stability and allows kit formulations, and, moreover, enables the formation of 188Re complexes in acidic solution. Only micromolar amounts of the monodentate ligand are needed, and that results in high specific activity labeling of interesting molecules. The lipophilicity of complexes can be controlled by introducing a carboxyl group into the tetradentate ligand and/or derivatization of the monodentate ligands. Furthermore, the carboxyl group enables the conjugation of biomolecules. As an example, the neurotensin derivative CN-NT(8-13) was prepared and labeled with 99mTc according to the '4+1' approach, and its behavior in vivo was studied.     

How can aluminium(III) generate fluorescence?

In a previous paper [F. Launay, V. Alain, E. Destandau, N. Ramos, E. Bardez, P. Baret, J. L. Pierre, New J. Chem. 25 (2001) 1269-1280] [New J. Chem. 25 (2001) 1269], we showed that the hexadentate tripodal ligand O-TRENSOX (O-TR), incorporating three 8-hydroxy-5-sulfoquinoline subunits, was an efficient chelator of Al(III), quantitatively giving the 1:1 chelate in stoichiometric conditions even at the 10(-5) mol L(-1) concentration scale. However, the 1:1 Al:O-TR chelate turned out to be not significantly more fluorescent than the free ligand, whereas fluorescence enhancement by factors of at least 100 occurred either with the 3:1 Al:O-TR chelate, or with the 1:1 complex obtained with n-BUSOX, a ligand similar to one arm of O-TRENSOX. The present paper addresses the unresolved question of the magnitude of the fluorescence enhancement. Time-resolved fluorescence measurements, and additional complexation experiments carried out with the tripod TRENSOXCAMS2 (one 8-HQS and two 5-sulfocatechol subunits) and with n-BUCAMS analogous to one catechol arm of TRENSOXCAMS2, show that stoichiometry between Al(III) and the bound bidentate subunits is the key factor of fluorescence enhancement. The charge density on Al(III), tuned by the number of chelating groups and by their formal charges, influences the photoinduced charge transfer which tends to quench the fluorescence emission of the 8-hydroxyquinoline ligand. Transposition can be done to other bifunctional amphoterous ligands such as morin.     

Mn2+, Co2+, Cu2+ and Zn2+ complexes with two macrocyclic ligands bearing L-lactate-like functions: potentiometric studies and evaluation of superoxide-scavenging properties of the Mn2+ complex

Some aerobic organisms devoid of SOD use Mn2+ chelates to scavenge the O2- radical. Since the Mn2+-bis(lactato)diaquo complex is known as having a high SOD-like activity, we prepared manganese(II) complexes with triazamacrocyclic ligands bearing L-lactate-like functions in order to obtain model compounds able to disproportionate the superoxide radical. Thus, two macrocyclic ligands, N,N',N"-tris[2(S)-hydroxybutyric acid]-1,4,7-triazacyclononane, L1, and N,N',N"-tris[2(S)-hydroxybutyric acid]-1,5,9-triazacyclododecane, L2, were prepared and their capacity to retain the Mn2+ ion in aqueous solution was determined from potentiometric experiments. The chelating properties in aqueous solution of each ligand towards Co2+, Cu2+ and Zn2+ ions were also determined. L1 forms complexes with Mn2+, Co2+, Cu2+ and Zn2+ ions with stability constants of 8.33(5), 15.78(5), 17.65(3) and 14.32(1), respectively. L2 forms complexes with Cu2+ and Zn2+ ions with stability constants of 10.67(1) and 6.98(3), respectively. But the constants related to the Mn2+ and Co2+ complexes were too low to be determined by the method used. The stability constants values calculated for L2 complexes are significantly lower than those for the corresponding complexes of L1. Additional spectroscopic measurements were carried out on the Mn2+-L1 system. The electronic spectrum of this system showed a pH-dependence that may be consistent with the formation of hydroxo-species as the ESR spectra recorded at 120 K did not show oxidation of the Mn2+ ion in the pH range studied. The superoxide-scavenging activity of the manganese(II)-L1 complex was investigated using the cytochrome c assay. The Mn2+-L1 system showed an IC50 value of 1.7 microM which indicates that it appears as a potent SOD mimic.     

Uranyl(VI) complexes of ethylene-1,2-dioxydiacetic acid and ethylene-1,2-diaminodiacetic acid in aqueous solution: a potentiometric and calorimetric study

The stability constants and the changes in enthalpy and entropy for the formation of uranyl(VI) complexes with dicarboxylate ligands of the type (-CH₂RCH₂COO)₂²⁻, where RO or NH, have been determined by potentiometric and calorimetric titrations at 25.0 °C in 1.0 mol dm⁻³ aqueous solution of sodium perchlorate.The ethylene-l,2-dioxydiacetate ligand forms either 1:1 and 1:2 chelated complexes or unchelated protonated complexes, owing to the low stability of the chelate ring. Because of precipitation of solid compounds only one complex of 1:1 stoichiometry was observed in the uranyl(VI)—ethylene-l,2-diaminodiacetate system.Factors influencing the stability constants and the enthalpy and entropy changes in the uranyl(VI) complexes with these potentially tetradentate ligands are discussed in comparison with analogous complexes involving bi- and tridentate dicarboxylate ligands.     

Vanadium(IV/V) speciation of pyridine-2,6-dicarboxylic acid and 4-hydroxy-pyridine-2,6-dicarboxylic acid complexes: potentiometry,EPR spectroscopy and comparison across oxidation states

Evaluation of stability of vanadium(IV) and (V) complexes under similar conditions is critical for the interpretation and assessment of bioactivity of various vanadium species. Detailed understanding of the chemical properties of these complexes is necessary to explain differences observed their activity in biological systems. These studies are carried out to link the chemistry of both vanadium(IV) and (V) complexes of two ligands, 2,6-pyridinedicarboxylic acid (dipicolinic acid, H(2)dipic) and 4-hydroxy-2,6-pyridinedicarboxylic acid (H(2)dipic-OH). Solution speciation of the two 2,6-pyridinedicarboxylic acids with vanadium(IV) and vanadium(V) ions was determined by pH-potentiometry at I=0.2 M (KCl) ionic strength and at T=298 K. The stability and the metal affinities of the ligands were compared. Vanadium(V) complexes were found to form only tridentate coordinated 1:1 complexes, while vanadium(IV) formed complexes with both 1:1 and 1:2 stoichiometries. The formation constant reflects hindered coordination of a second ligand molecule, presumably because of the relatively small size of the metal ion. The most probable binding mode of the complexes was further explored using ambient and low temperature EPR spectroscopy for vanadium(IV) and 51V NMR spectroscopy for vanadium(V) systems. Upon complex formation the pyridinol-OH in position 4 deprotonates with pK approximately 3.7-4.1, which is approximately 6 orders of magnitude lower than that of the free ligand. The deprotonation enhances the ligand metal ion affinity compared to the parent ligand dipicolinic acid. In the light of the speciation and stability data of the metal complexes, the efficiency of the two ligands in transporting the metal ion in the two different oxidation states are assessed and discussed.     

The thulium complex of 1,4,7,10-tetrakis{[N-(1H-imidazol-2-yl)carbamoyl]methyl}-1,4,7,10-tetraazacyclododecane (dotami) as a ParaCEST contrast agent

1,4,7,10-Tetrakis{[N-(1H-imidazol-2-yl)carbamoyl]methyl}-1,4,7,10-tetraazacyclododecane (dotami), a tetra(1H-imidazol-2-yl) derivative of the well-studied octadentate 1,4,7,10-tetrakis[(carbamoyl)methyl]-1,4,7,10-tetraazacyclododecane (dotam) ligand, was synthesized by reaction of 1,4,7,10-tetraazacyclododecane with N-(1H-imidazol-2-yl)chloroacetamide in high yield. Its tricationic thulium complex was isolated as a water-soluble chloride salt. The detection of the mildly acidic amide and amine protons by direct proton NMR in aqueous solution was unsuccessful, but such exchangeable protons could be detected via their chemical exchange-dependent saturation transfer (CEST) effect. The observed CEST effect was distinctly different from that found for respective dotam complexes and is, therefore, ascribed to exchangeable protons associated with the imidazole substituent.     

Chemical and functional characterization of metal-binding pseudotripeptides with different functionalized N-alkyl residues Dedicated to Professor Dr. Ernst-Gottfried Jaeger on occasion of his 65th birthday

Summary Pseudotripeptide ligands with 4 different N-functionalized glycine residues were qualitatively, semiquantitatively and quantitatively tested for their complexation of the bivalent transition metal ions Zn²⁺, Cu²⁺, Co²⁺, Ni²⁺ and Mn²⁺. The functional side chains have different length and different groups available for complexation. MALDI-MS and ESI-MS were used for more qualitative or semiquantitative estimation of the complex formation tendencies. The found ranking differs by these two methods only for Zn²⁺ and Ni²⁺. For one of the pseudotripeptide ligands, the ligand L1, complex formation with certain transition metal was estimated quantitatively by potentiometric titration. The Zn-complex of that ligand polarizes bound water strongly, resulting in a low pK _a -value. Complexes of pseudotripeptide ligand L1 with certain metal ions were tested for their hydrolytic activity. The pseudo first order rate constants of the hydrolysis of the substrates 4-nitrophenyl acetate and bis(4-nitrophenyl)phosphate were compared to complexes with the same metal ions formed with a very well studied ligand from the literature, the 1,4,7,10-tetraaza cyclododecane (cyclen). The hydrolysis of the phosphate ester occurs very slowly compared to the acetate ester. No correlation exists between the estimated pK _a values of complexes formed from ligand L1 with different metal ions and the phosphate ester hydrolysis. The Ni ions give totally different hydrolytic activities for pseudotripeptide ligand L1 and cyclen. With one exception, the Ni-cyclen complex, all other complexes have only a low or moderate catalytic activity. Dedicated to Professor Dr. Ernst-Gottfried Jaeger on occasion of his 65th birthday.     

Large-scale synthesis of the bifunctional chelating agent 2-(p-nitrobenzyl)-1,4,7,10-tetraazacyclododecane-N,N',N',N'-tetr aacetic acid, and the determination of its enantiomeric purity by chiral chromatography.

The attachment of radiometals to monoclonal antibodies for medical applications requires extreme stability under physiological conditions, with no significant release of metal. Chelators that can hold radiometals like 111In, 67Ga, and 90Y with high stability under these conditions are essential for radiotherapy or immunoscintigraphy. 2-(p-Nitrobenzyl)-1,4,7,10-tetraazacyclododecane- N,N',N',N'-tetraacetic acid (nitrobenzyl-DOTA) is one of the most promising bifunctional chelating agents. A large-scale synthesis of nitrobenzyl-DOTA is described. The overall yield for the nine-step synthesis sequence starting from nitrophenylalanine is 5.6%. Synthesis of nitrobenzyl-DOTA according to the new procedure yields up to approximately 10 g without special apparatus. Both enantiomers of the chiral chelate nitrobenzyl-DOTA have been prepared, and their enantiomeric purity has been checked by chiral chromatography.     

Atropisomeric amides: Achiral ligands with chiral conformations

A new class of achiral ligands with atropisomeric conformations has been coordinated to titanium(IV). The ligands are ortho-hydroxy benzamide derivatives which are deprotonated on reaction with titanium tetraisopropoxide to furnish Ti(L)(2)(O-iPr)(2) complexes (L=ortho-phenoxy benzamide). In these octahedral titanium compounds, the ortho-phenoxy benzamide ligands chelate to titanium, bonding through the phenoxide oxygen and the amide carbonyl oxygen. The benzamide ligands adopt atropisomeric conformations with an angle between the aryl and amide groups of approximately 35 degrees. The ligand precursor, ligand, and titanium complexes have been characterized by X-ray crystallography. Only one diastereomer of each titanium complex was observed in the solid state structures.     

Influence of the denticity of ligand systems on the in vitro and in vivo behavior of (99m)Tc(I)-tricarbonyl complexes: a hint for the future functionalization of biomolecules

Functionalization of biologically relevant molecules for the labeling with the novel fac-[(99m)Tc(OH(2))(3)(CO)(3)](+) precursor has gained considerable attention recently. Therefore, we tested seven different tridentate (histidine L(1)(), iminodiacetic acid L(2)(), N-2-picolylamineacetic acid L(3)(), N, N-2-picolylaminediacetic acid L(4)()) and bidentate (histamine L(5)(), 2-picolinic acid L(6)(), 2,4-dipicolinic acid L(7)()) ligand systems, with the potential to be bifunctionalized and attached to a biomolecule. The ligands allowed mild radiolabeling conditions with fac-[(99m)Tc(OH(2))(3)(CO)(3)](+) (30 min, 75 degrees C). The ligand concentrations necessary to obtain yields of >95% of the corresponding organometallic complexes 1-7 ranged from 10(-)(6) to 10(-)(4) M. Complexes of the general formula "fac-[(99m)TcL(CO)(3)]" (L = tridentate ligand) and "fac-[(99m)Tc(OH(2))L'(CO)(3)]" (L' = bidentate ligand), respectively, were produced. Challenge studies with cysteine and histidine revealed significant displacement of the ligands in complexes 5-7 but only little exchange with complexes 1-4 after 24 h at 37 degrees C in PBS buffer. However, no decomposition to (99m)TcO(4)(-) was observed under these conditions. All complexes showed a hydrophilic character (log P(o/w) values ranging from -2.12 to 0.32). Time-dependent FPLC analyses of compounds 1-7 incubated in human plasma at 37 degrees C showed again no decomposition to (99m)TcO(4)(-) after 24 h at 37 degrees C. However, the complexes with bidentate ligands (5-7) became almost completely protein bound after 60 min, whereas the complexes with tridentate coordinated ligands (1-4) showed no reaction with serum proteins. The compounds were tested for their in vivo stability and the biodistribution characteristics in BALB/c mice. The complexes with tridentate coordinated ligand systems (1-4) revealed generally a good and fast clearance from all organs and tissues. On the other hand, the complexes with only bidentate coordinated ligands (5-7) showed a significantly higher retention of activity in the liver, the kidneys, and the blood pool. Detailed radiometric analyses of murine plasma samples, 30 min p.i. of complex fac-[(99m)TcL(1)(CO)(3)], 1, revealed almost no reaction of the radioactive complex with the plasma proteins. By contrast, in plasma samples of mice, which were injected with complex fac-[(99m)Tc(OH(2))L(5)(CO)(3)](+), 5, the entire radioactivity coeluded with the proteins. On the basis of these in vitro and in vivo experiments, it appears that functionalization of biomolecules with tridentate-chelating ligand systems is preferable for the labeling with fac-[(99m)Tc(OH(2))(3)(CO)(3)](+), since this will presumably result in radioactive bioconjugates with better pharmacokinetic profiles.