Dynamics of Coordinated Phosphonate Group Directly Observed by 17O-NMR in Lanthanide(iii) Complexes of a Mono(ethyl phosphonate) DOTA Analogue.

Biological phosphates can coordinate metal ions and their complexes are common in living systems. Dynamics of mutual oxygen atom exchange in the tetrahedral group in complexes has not been investigated. Here, we present a direct experimental proof of exchange ("phosphonate rotation") in model Ln(III) complexes of monophosphonate H4dota analogue which alters phosphorus atom chirality of coordinated phosphonate monoester. Combination of macrocycle-based isomerism with P-based chirality leads to several diastereoisomers. (Non)-coordinated oxygen atoms were distinguished through 17O-labelled phosphonate group and their mutual exchange was followed by various NMR techniques and DFT calculations. The process is sterically demanding and occurs through bulky bidentate (κ2-PO2)- coordination and was observed only in twisted-square antiprism (TSA) diastereoisomer of large Ln(III) ions. Its energy demands increase for smaller Ln(III) ions (298ΔG≠(exp./DFT)=51.8/52.1 and 61.0/71.5 kJ mol-1 for La(III) and Eu(III), respectively). These results are helpful in design of such complexes as MRI CA and for protein paramagnetic NMR probes. It demonstrates usefulness of 17O NMR to study solution dynamics in complexes involving phosphorus acid derivatives and it may inspire use of this method to study dynamics of phosphoric acid derivatives (as e. g. phosphorus acid-based inhibitors of metalloenzymes) in different areas of chemistry.

Inorganic Chemistry of the Tripodal Picolinate Ligand Tpaa with Gallium(III) and Radiolabeling with Gallium-68.

We report here the improved synthesis of the tripodal picolinate chelator Tpaa, with an overall yield of 41% over five steps, in comparison to the previously reported 6% yield. Tpaa was investigated for its coordination chemistry with Ga(III) and radiolabeling properties with gallium-68 (68Ga). The obtained crystal structure for [Ga(Tpaa)] shows that the three picolinate arms coordinate to the Ga(III) ion, fully occupying the octahedral coordination geometry. This is supported by 1H NMR which shows that the three arms are symmetrical when coordinated to Ga(III). Assessment of the thermodynamic stability through potentiometry gives log KGa-Tpaa = 21.32, with a single species being produced across the range of pH 3.5-7.5. Tpaa achieved >99% radiochemical conversion with 68Ga under mild conditions ([Tpaa] = 6.6 µM, pH 7.4, 37 °C) with a molar activity of 3.1 GBq µmol-1. The resulting complex, [68Ga][Ga(Tpaa)], showed improved stability over the previously reported [68Ga][Ga(Dpaa)(H2O)] in a serum challenge, with 32% of [68Ga][Ga(Tpaa)] remaining intact after 30 min of incubation with fetal bovine serum.

Bn2DT3A, a Chelator for 68Ga Positron Emission Tomography: Hydroxide Coordination Increases Biological Stability of [68Ga][Ga(Bn2DT3A)(OH)].

The chelator Bn2DT3A was used to produce a novel 68Ga complex for positron emission tomography (PET). Unusually, this system is stabilized by a coordinated hydroxide in aqueous solutions above pH 5, which confers sufficient stability for it to be used for PET. Bn2DT3A complexes Ga3+ in a hexadentate manner, forming a mer-mer complex with log K([Ga(Bn2DT3A)]) = 18.25. Above pH 5, the hydroxide ion coordinates the Ga3+ ion following dissociation of a coordinated amine. Bn2DT3A radiolabeling displayed a pH-dependent speciation, with [68Ga][Ga(Bn2DT3A)(OH)]- being formed above pH 5 and efficiently radiolabeled at pH 7.4. Surprisingly, [68Ga][Ga(Bn2DT3A)(OH)]- was found to show an increased stability in vitro (for over 2 h in fetal bovine serum) compared to [68Ga][Ga(Bn2DT3A)]. The biodistribution of [68Ga][Ga(Bn2DT3A)(OH)]- in healthy rats showed rapid clearance and excretion via the kidneys, with no uptake seen in the lungs or bones.

Paramagnetic Cobalt(II) Complexes with Cyclam Derivatives: Toward 19F MRI Contrast Agents.

In order to develop novel, more efficient, and/or selective contrast agents for magnetic resonance imaging (MRI), different modi operandi are explored as alternatives to water-relaxation enhancement. In this work, cobalt(II/III) complexes of bis(N-trifluoroethyl)cyclam derivatives with two acetate or two phosphonate pendant arms, H2te2f2a and H4te2f2p, were prepared and investigated. X-ray diffraction structures confirmed octahedral coordination with a very stable trans-III cyclam conformation and with fluorine atoms located about 5.3 Å from the metal center. The Co(II) complexes are kinetically inert, decomposing slowly even in 1 M aqueous HCl at 80 °C. The Co(II) complexes exhibited well-resolved paramagnetically shifted NMR spectra. These were interpreted with the help of quantum chemistry calculations. The 13C NMR shifts of the trans-[CoII(te2f2p)]2- complex were successfully assigned based on spin density delocalization within the ligand molecule. The obtained spin density also helps to describe d-metal-induced NMR relaxation properties of 19F nuclei, including the contribution of a Fermi contact relaxation mechanism. The paramagnetic complexes show convenient relaxation properties to be used as 19F MRI contrast agents.

Cross-Bridged Cyclam with Phosphonate and Phosphinate Pendant Arms: Chelators for Copper Radioisotopes with Fast Complexation.

Cross-bridged cyclam derivatives bearing two phosphonate (H4L1), bis(phosphinate) (H4L2), or phosphinate (H2L3) pendant arms were synthesized and studied with respect to their application as copper radioisotope carriers in nuclear medicine. The ligands show high macrocycle basicity (pK1 > 14) and high Cu(II) complex stability (log K = 20-24). The complexation and dissociation kinetics of the Cu(II) complexes were studied by ultraviolet-visible spectroscopy. Phosphonate Cu(II)-H4L1 and bis(phosphinate) Cu(II)-H4L2 complexes form very quickly, reaching quantitative formation within 1 s at pH ∼6 and millimolar concentrations. Conversely, the formation of the phosphinate complex Cu(II)-H2L3 is much slower (9 min at pH ∼6) due to the low stability of the out-of-cage reaction intermediate. All studied complexes are highly kinetically inert, showing half-lives of 120, 11, and 111 h for Cu(II)-H4L1, Cu(II)-H4L2, and Cu(II)-H2L3 complexes, respectively, in 1 M HClO4 at 90 °C. The high thermodynamic stability, fast formation, and extreme kinetic inertness of Cu(II) complexes indicate that phosphonate and bis(phosphinate) derivatives are promising ligands for nuclear medicine.

Lanthanide Complexes of DO3A-(Dibenzylamino)methylphosphinate: Effect of Protonation of the Dibenzylamino Group on the Water-Exchange Rate and the Binding of Human Serum Albumin.

Protonation of a distant, noncoordinated group of metal-based magnetic resonance imaging contrast agents potentially changes their relaxivity. The effect of a positive charge of the drug on the human serum albumin (HSA)-drug interaction remains poorly understood as well. Accordingly, a (dibenzylamino)methylphosphinate derivative of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) was efficiently synthesized using pyridine as the solvent for a Mannich-type reaction of tBu3DO3A, formaldehyde, and Bn2NCH2PO2H2 ethyl ester. The ligand protonation and metal ion (Gd3+, Cu2+, and Zn2+) stability constants were similar to those of the parent DOTA, whereas the basicity of the side-chain amino group of the complexes (log KA = 5.8) was 1 order of magnitude lower than that of the free ligand (log KA = 6.8). The presence of one bound water molecule in both deprotonated and protonated forms of the gadolinium(III) complex was deduced from the solid-state X-ray diffraction data [gadolinium(III) and dysprosium(III)], from the square antiprism/twisted square antiprism (SA/TSA) isomer ratio along the lanthanide series, from the fluorescence data of the europium(III) complex, and from the 17O NMR measurements of the dysprosium(III) and gadolinium(III) complexes. In the gadolinium(III) complex with the deprotonated amino group, water exchange is extremely fast (τM = 6 ns at 25 °C), most likely thanks to the high abundance of the TSA isomer and to the presence of a proximate protonable group, which assists the water-exchange process. The interaction between lanthanide(III) complexes and HSA is pH-dependent, and the deprotonated form is bound much more efficaciously (∼13% and ∼70% bound complex at pH = 4 and 7, respectively). The relaxivities of the complex and its HSA adduct are also pH-dependent, and the latter is approximately 2-3 times increased at pH = 4-7. The relaxivity for the supramolecular HSA-complex adduct ( r1b) is as high as 52 mM-1 s-1 at neutral pH (at 20 MHz and 25 °C). The findings of this study stand as a proof-of-concept, showing the ability to manipulate an albumin-drug interaction, and thus the blood pool residence time of the drug, by introducing a positive charge in a side-chain amino group.

Low-molecular-weight paramagnetic 19F contrast agents for fluorine magnetic resonance imaging.

OBJECTIVE: 19F MRI requires biocompatible and non-toxic soluble contrast agents with high fluorine content and with suitable 19F relaxation times. Probes based on a DOTP chelate with 12 magnetically equivalent fluorine atoms (DOTP-tfe) and a lanthanide(III) ion shortening the relaxation times were prepared and tested. METHODS: Complexes of DOTP-tfe with trivalent paramagnetic Ce, Dy, Ho, Tm, and Yb ions were synthetized and characterized. 19F relaxation times were determined and compared to those of the La complex and of the empty ligand. In vitro and in vivo 19F MRI was performed at 4.7 T. RESULTS: 19F relaxation times strongly depended on the chelated lanthanide(III) ion. T1 ranged from 6.5 to 287 ms, T2 from 3.9 to 124.4 ms, and T2* from 1.1 to 3.1 ms. All complexes in combination with optimized sequences provided sufficient signal in vitro under conditions mimicking experiments in vivo (concentrations 1.25 mM, 15-min scanning time). As a proof of concept, two contrast agents were injected into the rat muscle; 19F MRI in vivo confirmed the in vivo applicability of the probe. CONCLUSION: DOTP-based 19F probes showed suitable properties for in vitro and in vivo visualization and biological applications. The lanthanide(III) ions enabled us to shorten the relaxation times and to trim the probes according to the actual needs. Similar to the clinically approved Gd3+ chelates, this customized probe design ensures consistent biochemical properties and similar safety profiles.

Coordination Behavior of 1,4-Disubstituted Cyclen Endowed with Phosphonate, Phosphonate Monoethylester, and H-Phosphinate Pendant Arms.

Three 1,4,7,10-tetraazacyclododecane-based ligands disubstituted in 1,4-positions with phosphonic acid, phosphonate monoethyl-ester, and H-phosphinic acid pendant arms, 1,4-H4do2p, 1,4-H2do2pOEt, and 1,4-H2Bn2do2pH, were synthesized and their coordination to selected metal ions, Mg(II), Ca(II), Mn(II), Zn(II), Cu(II), Eu(III), Gd(III), and Tb(III), was investigated. The solid-state structure of the phosphonate ligand, 1,4-H4do2p, was determined by single-crystal X-ray diffraction. Protonation constants of the ligands and stability constants of their complexes were obtained by potentiometry, and their values are comparable to those of previously studied analogous 1,7-disubstitued cyclen derivatives. The Gd(III) complex of 1,4-H4do2p is ~1 order of magnitude more stable than the Gd(III) complex of the 1,7-analogue, probably due to the disubstituted ethylenediamine-like structural motif in 1,4-H4do2p enabling more efficient wrapping of the metal ion. Stability of Gd(III)-1,4-H2do2pOEt and Gd(III)-H2Bn2do2pH complexes is low and the constants cannot be determined due to precipitation of the metal hydroxide. Protonations of the Cu(II), Zn(II), and Gd(III) complexes probably takes place on the coordinated phosphonate groups. Complexes of Mn(II) and alkali-earth metal ions are significantly less stable and are not formed in acidic solutions. Potential presence of water molecule(s) in the coordination spheres of the Mn(II) and Ln(III) complexes was studied by variable-temperature NMR experiments. The Mn(II) complexes of the ligands are not hydrated. The Gd(III)-1,4-H4do2p complex undergoes hydration equilibrium between mono- and bis-hydrated species. Presence of two-species equilibrium was confirmed by UV-Vis spectroscopy of the Eu(III)-1,4-H4do2p complex and hydration states were also determined by luminescence measurements of the Eu(III)/Tb(III)-1,4-H4do2p complexes.

NOTA Complexes with Copper(II) and Divalent Metal Ions: Kinetic and Thermodynamic Studies.

H3nota derivatives are among the most studied macrocyclic ligands and are widely used for metal ion binding in biology and medicine. Despite more than 40 years of chemical research on H3nota, the comprehensive study of its solution chemistry has been overlooked. Thus, the coordination behavior of H3nota with several divalent metal ions was studied in detail with respect to its application as a chelator for copper radioisotopes in medical imaging and therapy. In the solid-state structure of the free ligand in zwitterionic form, one proton is bound in the macrocyclic cavity through a strong intramolecular hydrogen-bond system supporting the high basicity of the ring amine groups (log Ka = 13.17). The high stability of the [Cu(nota)]- complex (log KML = 23.33) results in quantitative complex formation, even at pH <1.5. The ligand is moderately selective for Cu(II) over other metal ions (e.g., log KML(Zn) = 22.32 and log KML(Ni) = 19.24). This ligand forms a more stable complex with Mg(II) than with Ca(II) and forms surprisingly stable complexes with alkali-metal ions (stability order Li(I) > Na(I) > K(I)). Thus, H3nota shows high selectivity for small metal ions. The [Cu(nota)]- complex is hexacoordinated at neutral pH, and the equatorial N2O2 interaction is strengthened by complex protonation. Detailed kinetic studies showed that the Cu(II) complex is formed quickly (millisecond time scale at cCu ≈ 0.1 mM) through an out-of-cage intermediate. Conversely, conductivity measurements revealed that the Zn(II) complex is formed much more slowly than the Cu(II) complex. The Cu(II) complex has medium kinetic inertness (τ1/2 46 s; pH 0, 25 °C) and is less resistant to acid-assisted decomplexation than Cu(II) complexes with H4dota and H4teta. Surprisingly, [Cu(nota)]- decomplexation is decelerated in the presence of Zn(II) ions due to the formation of a stable dinuclear complex. In conclusion, H3nota is a good carrier of copper radionuclides because the [Cu(nota)]- complex is predominantly formed over complexes with common impurities in radiochemical formulations, Zn(II) and Ni(II), for thermodynamic and, primarily, for kinetic reasons. Furthermore, the in vivo stability of the [Cu(nota)]- complex may be increased due to the formation of dinuclear complexes when it interacts with biometals.

Eu(III) Complex with DO3A-amino-phosphonate Ligand as a Concentration-Independent pH-Responsive Contrast Agent for Magnetic Resonance Spectroscopy (MRS).

A new DOTA-like ligand H5do3aNP with a 2-[amino(methylphosphonic acid)]ethyl-coordinating pendant arm was prepared, and its coordinating properties were studied by NMR spectroscopy and potentiometry. The study revealed a rare slow exchange (on the 1H and 31P NMR time scale) between protonated and unprotonated complex species with a corresponding acidity constant pKA ∼ 8.0. This unusually slow time scale associated with protonation is caused by a significant geometric change from square-antiprismatic (SA) arrangement observed for protonated complex SA-[Eu(Hdo3aNP)]- to twisted-square-antiprismatic (TSA) arrangement found for deprotonated complex TSA-[Eu(do3aNP)]2-. This behavior results in simultaneous occurrence of the signals of both species in the 31P NMR spectra at approximately -118 and +70 ppm, respectively. Such an unprecedented difference in the chemical shifts between species differing by a proton is caused by a significant movement of the principal magnetic axis and by a change of phosphorus atom position in the coordination sphere of the central Eu(III) ion (i.e., by relative movement of the phosphorus atom with respect to the principal magnetic axis). It changes the sign of the paramagnetic contribution to the 31P NMR chemical shift. The properties discovered can be employed in the measurement of pH by MRS techniques as presented by proof-of-principle experiments on phantoms.

Paramagnetic 19F Relaxation Enhancement in Nickel(II) Complexes of N-Trifluoroethyl Cyclam Derivatives and Cell Labeling for 19F MRI.

1,8-Bis(2,2,2-trifluoroethyl)cyclam (te2f) derivatives with two coordinating pendant arms involving methylenecarboxylic acid (H2te2f2a), methylenephosphonic acid (H4te2f2p), (2-pyridyl)methyl (te2f2py), and 2-aminoethyl arms (te2f2ae) in 4,11-positions were prepared, and their nickel(II) complexes were investigated as possible 19F MR tracers. The solid-state structures of several synthetic intermediates, ligands, and all complexes were confirmed by X-ray diffraction analysis. The average Ni···F distances were determined to be about 5.2 Å. All complexes exhibit a trans-III cyclam conformation with pendant arms bound in the apical positions. Kinetic inertness of the complexes is increased in the ligand order te2f2ae ≪ te2f < te2f2py ≈ H4te2f2p ≪ H2te2f2a. The [Ni(te2f2a)] complex is the most kinetically inert Ni(II) complex reported so far. Paramagnetic divalent nickel caused a shortening of 19F NMR relaxation time down to the millisecond range. Solubility, stability, and cell toxicity were only satisfactory for the [Ni(te2f2p)]2- complex. This complex was visualized by 19F MRI utilizing an ultrashort echo time (UTE) imaging pulse sequence, which led to an increase in sensitivity gain. Mesenchymal stem cells were successfully loaded with the complex (up to 0.925/5.55 pg Ni/F per cell).19F MRI using a UTE pulse sequence provided images with a good signal-to-noise ratio within the measurement time, as short as tens of minutes. The data thus proved a major sensitivity gain in 19F MRI achieved by utilization of the paramagnetic (transition) metal complex as 19F MR tracers coupled with the optimal fast imaging protocol.

A combined NMR and DFT study of conformational dynamics in lanthanide complexes of macrocyclic DOTA-like ligands.

The solution dynamics of the Eu(iii) complexes of H4dota (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetracarboxylic acid) and H5do3ap (1,4,7,10-tetraazacyclododecane-4,7,10-tris(carboxymethyl)-1-methylphosphonic acid, bound in both monoprotonated and fully deprotonated forms) were investigated by using a combination of NMR measurements and DFT calculations. In solution, an equilibrium between the square antiprismatic (SAP) and twisted-square antiprismatic isomers (TSAP) of these complexes is present. These two isomers interconvert by rotation of the pendant arms or inversion of the cyclen chelate rings. 1D EXSY NMR spectra were used to determine these exchange rates with unprecedented accuracy. It was found that the two processes occur at different rates. Additional variable-temperature measurements allowed determination of the corresponding activation parameters for the two processes. DFT calculations were then used to obtain mechanistic information at the molecular level. The results show that the cyclen inversion pathway involves stepwise inversion of the four chelate rings formed upon metal ion coordination. However, the arm rotation process may operate through a synchronous rotation of the pendant arms or a stepwise mechanism depending on the system. A mixed cluster-continuum approach was required to improve the agreement between experimental and calculated activation parameters for the arm rotation process. The obtained results will aid the design of MRI contrast agents. Furthermore, the methodology developed in this work can be further applied for the investigation of other dynamic paramagnetic systems, e.g. peptides with Ln(iii) probes or natively paramagnetic metalloproteins.

Bifunctional cyclam-based ligands with phosphorus acid pendant moieties for radiocopper separation: thermodynamic and kinetic studies.

Two macrocyclic ligands based on cyclam with trans-disposed N-methyl and N-(4-aminobenzyl) substituents as well as two methylphosphinic (H2L1) or methylphosphonic (H4L2) acid pendant arms were synthesised and investigated in solution. The ligands form stable complexes with transition metal ions. Both ligands show high thermodynamic selectivity for divalent copper over nickel(II) and zinc(II)-K(CuL) is larger than K(Ni/ZnL) by about seven orders of magnitude. Complexation is significantly faster for the phosphonate ligand H4L2, probably due to the stronger coordination ability of the more basic phosphonate groups, which efficiently bind the metal ion in an "out-of-cage" complex and thus accelerate its "in-cage" binding. The rate of Cu(II) complexation by the phosphinate ligand H2L1 is comparable to that of cyclam itself and its derivatives with non-coordinating substituents. Acid-assisted decomplexation of the copper(II) complexes is relatively fast (τ1/2 = 44 and 42 s in 1 M aq. HClO4 at 25 °C for H2L1 and H4L2, respectively). This combination of properties is convenient for selective copper removal/purification. Thus, the title ligands were employed in the preparation of ion-selective resins for radiocopper(II) separation. Glycidyl methacrylate copolymer beads were modified with the ligands through a diazotisation reaction. The separation ability of the modified polymers was tested with cold copper(II) and non-carrier-added (64)Cu in the presence of a large excess of both nickel(II) and zinc(II). The experiments exhibited high overall separation efficiency leading to 60-70% recovery of radiocopper with high selectivity over the other metal ions, which were originally present in 900-fold molar excess. The results showed that chelating resins with properly tuned selectivity of their complexing moieties can be employed for radiocopper separation.

Cyclam Derivatives with a Bis(phosphinate) or a Phosphinato-Phosphonate Pendant Arm: Ligands for Fast and Efficient Copper(II) Complexation for Nuclear Medical Applications.

Cyclam derivatives bearing one geminal bis(phosphinic acid), -CH2PO2HCH2PO2H2 (H2L(1)), or phosphinic-phosphonic acid, -CH2PO2HCH2PO3H2 (H3L(2)), pendant arm were synthesized and studied as potential copper(II) chelators for nuclear medical applications. The ligands showed good selectivity for copper(II) over zinc(II) and nickel(II) ions (log KCuL = 25.8 and 27.7 for H2L(1) and H3L(2), respectively). Kinetic study revealed an unusual three-step complex formation mechanism. The initial equilibrium step leads to out-of-cage complexes with Cu(2+) bound by the phosphorus-containing pendant arm. These species quickly rearrange to an in-cage complex with cyclam conformation II, which isomerizes to another in-cage complex with cyclam conformation I. The first in-cage complex is quantitatively formed in seconds (pH ≈5, 25 °C, Cu:L = 1:1, cM ≈ 1 mM). At pH >12, I isomers undergo nitrogen atom inversion, leading to III isomers; the structure of the III-[Cu(HL(2))] complex in the solid state was confirmed by X-ray diffraction analysis. In an alkaline solution, interconversion of the I and III isomers is mutual, leading to the same equilibrium isomeric mixture; such behavior has been observed here for the first time for copper(II) complexes of cyclam derivatives. Quantum-chemical calculations showed small energetic differences between the isomeric complexes of H3L(2) compared with analogous data for isomeric complexes of cyclam derivatives with one or two methylphosphonic acid pendant arm(s). Acid-assisted dissociation proved the kinetic inertness of the complexes. Preliminary radiolabeling of H2L(1) and H3L(2) with (64)Cu was fast and efficient, even at room temperature, giving specific activities of around 70 GBq of (64)Cu per 1 µmol of the ligand (pH 6.2, 10 min, ca. 90 equiv of the ligand). These specific activities were much higher than those of H3nota and H4dota complexes prepared under identical conditions. The rare combination of simple ligand synthesis, very fast copper(II) complex formation, high thermodynamic stability, kinetic inertness, efficient radiolabeling, and expected low bone tissue affinity makes such ligands suitably predisposed to serve as chelators of copper radioisotopes in nuclear medicine.

The influence of the combination of carboxylate and phosphinate pendant arms in 1,4,7-triazacyclononane-based chelators on their 68Ga labelling properties.

In order to compare the coordination properties of 1,4,7-triazacyclononane (tacn) derivatives bearing varying numbers of phosphinic/carboxylic acid pendant groups towards 68Ga, 1,4,7-triazacyclononane-7-acetic-1,4-bis(methylenephosphinic) acid (NOPA) and 1,4,7- triazacyclononane-4,7-diacetic-1-[methylene(2-carboxyethyl)phosphinic] acid (NO2AP) were synthesized using Mannich reactions with trivalent or pentavalent forms of H-phosphinic acids as phosphorus components. Stepwise protonation constants logK1-3 12.06, 3.90 and 1.95, and stability constants with GaIII and CuII, logKGaL 24.01 and logKCuL 16.66, were potentiometrically determined for NOPA. Both ligands were labelled with 68Ga and compared with NOTA (tacn-N,N',N″-triacetic acid) and NOPO, a TRAP-type [tacn-N,N',N″- tris(methylenephosphinic acid)] chelator. At pH 3, NOPO and NOPA showed higher labelling efficiency (binding with lower ligand excess) at both room temperature and 95 °C, compared to NO2AP and NOTA. Labelling efficiency at pH = 0-3 correlated with a number of phosphinic acid pendants: NOPO >> NOPA > NO2AP >> NOTA; however, it was more apparent at 95 °C than at room temperature. By contrast, NOTA was found to be labelled more efficiently at pH > 4 compared to the ligands with phosphinic acids. Overall, replacement of a single phosphinate donor with a carboxylate does not challenge 68Ga labelling of TRAP-type chelators. However, the presence of carboxylates facilitates labelling at neutral or weakly acidic pH.

Thermodynamic and kinetic study of scandium(III) complexes of DTPA and DOTA: a step toward scandium radiopharmaceuticals.

Diethylenetriamine-N,N,N',N'',N''-pentaacetic acid (DTPA) and 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) scandium(III) complexes were investigated in the solution and solid state. Three (45)Sc NMR spectroscopic references suitable for aqueous solutions were suggested: 0.1 M Sc(ClO4)3 in 1 M aq. HClO4 (δSc =0.0 ppm), 0.1 M ScCl3 in 1 M aq. HCl (δSc =1.75 ppm) and 0.01 M [Sc(ox)4](5-) (ox(2-) = oxalato) in 1 M aq. K2C2O4 (δSc =8.31 ppm). In solution, [Sc(dtpa)](2-) complex (δSc = 83 ppm, Δν = 770 Hz) has a rather symmetric ligand field unlike highly unsymmetrical donor atom arrangement in [Sc(dota)](-) anion (δSc = 100 ppm, Δν = 4300 Hz). The solid-state structure of K8[Sc2(ox)7]⋅13 H2O contains two [Sc(ox)3](3-) units bridged by twice "side-on" coordinated oxalate anion with Sc(3+) ion in a dodecahedral O8 arrangement. Structures of [Sc(dtpa)](2-) and [Sc(dota)](-) in [(Hguanidine)]2[Sc(dtpa)]⋅3 H2O and K[Sc(dota)][H6 dota]Cl2⋅4 H2O, respectively, are analogous to those of trivalent lanthanide complexes with the same ligands. The [Sc(dota)](-) unit exhibits twisted square-antiprismatic arrangement without an axial ligand (TSA' isomer) and [Sc(dota)](-) and (H6 dota)(2+) units are bridged by a K(+) cation. A surprisingly high value of the last DOTA dissociation constant (pKa =12.9) was determined by potentiometry and confirmed by using NMR spectroscopy. Stability constants of scandium(III) complexes (log KScL 27.43 and 30.79 for DTPA and DOTA, respectively) were determined from potentiometric and (45)Sc NMR spectroscopic data. Both complexes are fully formed even below pH 2. Complexation of DOTA with the Sc(3+) ion is much faster than with trivalent lanthanides. Proton-assisted decomplexation of the [Sc(dota)](-) complex (τ1/2 =45 h; 1 M aq. HCl, 25 °C) is much slower than that for [Ln(dota)](-) complexes. Therefore, DOTA and its derivatives seem to be very suitable ligands for scandium radioisotopes.

A bis(pyridine N-oxide) analogue of DOTA: relaxometric properties of the Gd(III) complex and efficient sensitization of visible and NIR-emitting lanthanide(III) cations including Pr(III) and Ho(III).

We report the synthesis of a cyclen-based ligand (4,10-bis[(1-oxidopyridin-2-yl)methyl]-1,4,7,10-tetraazacyclododecane-1,7-diacetic acid=L1) containing two acetate and two 2-methylpyridine N-oxide arms anchored on the nitrogen atoms of the cyclen platform, which has been designed for stable complexation of lanthanide(III) ions in aqueous solution. Relaxometric studies suggest that the thermodynamic stability and kinetic inertness of the Gd(III) complex may be sufficient for biological applications. A detailed structural study of the complexes by (1) H NMR spectroscopy and DFT calculations indicates that they adopt an anti-Δ(λλλλ) conformation in aqueous solution, that is, an anti-square antiprismatic (anti-SAP) isomeric form, as demonstrated by analysis of the (1) H NMR paramagnetic shifts induced by Yb(III) . The water-exchange rate of the Gd(III) complex is ${k{{298\hfill \atop {\rm ex}\hfill}}}$=6.7×10(6)  s(-1) , about a quarter of that for the mono-oxidopyridine analogue, but still about 50 % higher than the ${k{{298\hfill \atop {\rm ex}\hfill}}}$ of GdDOTA (DOTA=1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid). The 2-methylpyridine N-oxide chromophores can be used to sensitize a wide range of Ln(III) ions emitting in both the visible (Eu(III) and Tb(III) ) and NIR (Pr(III) , Nd(III) , Ho(III) , Yb(III) ) spectral regions. The emission quantum yield determined for the Yb(III) complex (${Q{{{\rm L}\hfill \atop {\rm Yb}\hfill}}}$=7.3(1)×10(-3) ) is among the highest ever reported for complexes of this metal ion in aqueous solution. The sensitization ability of the ligand, together with the spectroscopic and relaxometric properties of its complexes, constitute a useful step forward on the way to efficient dual probes for optical imaging (OI) and MRI.

Tailored Gallium(III) chelator NOPO: synthesis, characterization, bioconjugation, and application in preclinical Ga-68-PET imaging.

The bifunctional chelator NOPO (1,4,7-triazacyclononane-1,4-bis[methylene(hydroxymethyl)phosphinic acid]-7-[methylene(2-carboxyethyl)phosphinic acid]) shows remarkably high Ga(III) complexation efficiency and comprises one carboxylic acid moiety which is not involved into metal ion coordination. An improved synthetic protocol affords NOPO with 45% overall yield. Stepwise protonation constants (log Ka), determined by potentiometry, are 11.96, 5.22, 3.77, and 1.54; the stability constant of the Ga(III) complex is log KGaL = 25.0. Within 5 min, (68)Ga(III) incorporation by NOPO is virtually quantitative at room temperature between pH 3 and 4, and at 95 °C at pH ranging from 0.5 to 7, at NOPO concentrations of 30 µM and 10 µM, respectively. During amide bond formation at the distant carboxylate using the HATU coupling reagent, an intramolecular phosphinic acid ester (phosphilactone) is formed, which is cleaved during (68)Ga complexation or in acidic media, such as trifluoroacetic acid (TFA). Phosphilactone formation can also be suppressed by complexation of Zn(2+) prior to conjugation, the resulting zinc-containing conjugates nevertheless being suitable for direct (68)Ga-labeling. In AR42J (rat pancreatic carcinoma) xenografted CD-1 nude mice, (68)Ga-labeled NOPO-NaI(3)-octreotide conjugate ((68)Ga-NOPO-NOC) showed high and fully blockable tumor uptake (13.9 ± 5% ID/g, 120 min p.i., compared to 0.9 ± 0.4% ID/g with 5 mg/kg of nonlabeled peptide). Uptake in other tissues was generally below 3% ID/g, except appearance of excretion-related activity accumulation in kidneys. NOPO-functionalized compounds tend to be more hydrophilic than the corresponding DOTA- and NODAGA-conjugates, thus promoting fast and extensive renal excretion of (68)Ga-NOPO-radiopharmaceuticals. NOPO-functionalized peptides provide suitable pharmacokinetics in vivo and meet all requirements for efficient (68)Ga-labeling even at room temperature in a kit-like manner.

A UV-Vis method for investigation of gallium(III) complexation kinetics with NOTA and TRAP chelators: advantages, limitations and comparison with radiolabelling.

An easy and cheap method for measurement of GaIII complexation kinetics was developed. The method is based on UV-Vis quantification of non-complexed chelators after the addition of CuII ions at individual time points. The method was evaluated using established ligands, H3nota and H6notPPr, and was utilized to study the kinetics of GaIII complexation with four new symmetric derivatives of 1,4,7-triazacyclononane bearing methylphosphonate/phosphinate pendant arms - TRAP ligands. Chelators bearing ethoxy groups (H3L1) or 2,2,2-trifluoroethyl groups (H3L2) on the phosphorus atoms showed fast formation (t99% = 21 and 10 min, respectively, at pH 2.0) and efficient radiolabelling which were comparable to the previously reported chelators bearing the 2-carboxyethyl group (H6notPPr). Chelators bearing (N,N-dibenzyl-amino)methyl (H3L3) and aminomethyl (H3L4) substituents showed a significantly slower complexation (t99% = 4.4 and 3.6 h, respectively, at pH 2.0) and inefficient radiolabelling, mainly at room temperature or low pH. This was caused by protonation of the amino groups of the pendant arms leading to coulombic repulsion between the GaIII ion and the positively charged protonated amines. The trends in complexation rates determined by the UV-Vis method correlated well with the results of the 68Ga radiolabelling study.

Transition metal complexes of the (2,2,2-trifluoroethyl)phosphinate NOTA analogue as potential contrast agents for 19F magnetic resonance imaging.

A new hexadentate 1,4,7-triazacyclononane-based ligand bearing three coordinating methylene-(2,2,2-trifluoroethyl)phosphinate pendant arms was synthesized and its coordination behaviour towards selected divalent (Mg2+, Ca2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+) and trivalent (Cr3+, Fe3+, Co3+) transition metal ions was studied. The ligand forms stable complexes with late divalent transition metal ions (from Co2+ to Zn2+) and the complexes of these metal ions are formed above pH ∼3. A number of complexes with divalent metal ions were structurally characterized by means of single-crystal X-ray diffraction. The complex of the larger Mn2+ ion adopts a twisted trigonally antiprismatic geometry with a larger coordination cavity and smaller torsion of the pendant arms, whereas the smaller ions Ni2+, Cu2+ and Zn2+ form octahedral species with a smaller cavity and larger pendant arm torsion. In the case of the Co2+ complexes, both coordination arrangements were observed. The complexes with paramagnetic metal ions were studied from the point of view of potential utilization in 19F magnetic resonance imaging. A significant shortening of the 19F NMR longitudinal relaxation times was observed: a sub-millisecond range for complexes of Cr3+, Mn2+ and Fe3+ with symmetric electronic states (t2g3 and HS-d5), the millisecond range for the Ni2+ and Cu2+ complexes and tens of milliseconds for the Co2+ complex. Such short relaxation times are consistent with a short distance between the paramagnetic metal ion and the fluorine atoms (∼5.5-6.5 Å). Among the redox-active complexes (Mn3+/Mn2+, Fe3+/Fe2+, Co3+/Co2+, Cu2+/Cu+), the cobalt complexes show sufficient stability and a paramagnetic-diamagnetic changeover with the redox potential lying in a physiologically relevant range. Thus, the Co3+/Co2+ complex pair can be potentially used as a smart redox-responsive contrast agent for 19F MRI.