DOTA Complexes Research Articles

13- and 14-membered macrocyclic ligands containing methylcarboxylate or methylphosphonate pendant arms: Chemical and biological evaluation of their 153Sm and 166Ho complexes as potential agents for therapy or bone pain palliation

The stability constants of La 3+, Sm 3+ and Ho 3+ complexes with 13- and 14-membered macrocycles having methylcarboxylate (trita and teta) or methylphosphonate (tritp and tetp) arms were determined. All the ligands were labelled with 153Sm and 166Ho in order to evaluate the effect of the macrocyclic cavity size and type of appended arms on their in vitro and in vivo behaviour. The radiolabelling efficiency was found to be higher than 98% for all the complexes, except for those of tetp. All radiocomplexes studied are hydrophilic with an overall negative charge and low plasmatic protein binding. Good in vitro stability in physiological media and human serum was found for all complexes, except the 153Sm/ 166Ho–teta, which are unstable in phosphate buffer (pH 7.4). In vitro hydroxyapatite (HA) adsorption studies indicated that 153Sm/ 166Ho–tritp complexes bind to HA having the 166Ho complex the highest degree of adsorption (>80%, 10 mg). Biodistribution studies in mice demonstrated that 153Sm/ 166Ho–trita complexes have a fast tissue clearance with more than 95% of the injected activity excreted after 2 h, value that is comparable to the corresponding dota complexes. In contrast, the 153Sm–teta complex has a significantly lower total excretion. 153Sm/ 166Ho–tritp complexes are retained by the bone, particularly 166Ho–tritp that has 5–6% (% I.D./g) bone uptake and also a high rate of total excretion. Thus, these studies support the potential interest of 153Sm/ 166Ho–trita complexes for therapy when conjugated to a biomolecule and the potential usefulness of the 166Ho–tritp complex in bone pain palliation.

Structural and chiroptical properties of the two coordination isomers of YbDOTA-type complexes.

Studies of the structural, physical, and chemical properties of the lanthanide(III) complexes of DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) and related ligands are often complicated by the presence of two coordination isomers in solution. Since these coordination isomers are in exchange and cannot be separated, many techniques offer information only on the weighted average of the two isomers. Lanthanide ion complexes formed with the ligands S(RRRR)NO2BnDOTMA and S(SSSS)NO2BnDOTMA preferentially adopt only one of the two common coordination isomers in solution, so the ytterbium complexes of these ligands offer a unique opportunity to study the near-infrared circular dichroism (NIR-CD) characteristics of each coordination geometry in isolation. The spectra acquired support many of the conclusions and assumptions of previous studies and demonstrate that this spectroscopy is particularly sensitive to the distortion of the coordination polyhedron. This will have particularly relevant consequences when studying achiral YbDOTA-like systems forming labile adducts with (chiral) hosts and receptors.

Complexation Properties of N,N′,N″,N′′′‐[1,4,7,10‐Tetraazacyclododecane‐1,4,7,10‐tetrayltetrakis(1‐oxoethane‐2,1‐diyl)]tetrakis[glycine] (H4dotagl). Equilibrium, Kinetic, and Relaxation Behavior of the Lanthanide(III) Complexes

AbstractEu3+, Dy3+, and Yb3+ complexes of the dota‐derived tetramide N,N′,N″,N′′′‐[1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetrayltetrakis(1‐oxoethane‐2,1‐diyl)]tetrakis[glycine] (H4dotagl) are potential CEST contrast agents in MRI. In the [Ln(dotagl)] complexes, the Ln3+ ion is in the cage formed by the four ring N‐atoms and the amide O‐atom donor atoms, and a H2O molecule occupies the ninth coordination site. The stability constants of the [Ln(dotagl)] complexes are ca. 10 orders of magnitude lower than those of the [Ln(dota)] analogues (H4dota=1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetraacetic acid). The free carboxylate groups in [Ln(dotagl)] are protonated in the pH range 1–5, resulting in mono‐, di‐, tri‐, and tetraprotonated species. Complexes with divalent metals (Mg2+, Ca2+, and Cu2+) are also of relatively low stability. At pH>8, Cu2+ forms a hydroxo complex; however, the amide H‐atom(s) does not dissociate due to the absence of anchor N‐atom(s), which is the result of the rigid structure of the ring. The relaxivities of [Gd(dotagl)] decrease from 10 to 25°, then increase between 30–50°. This unusual trend is interpreted with the low H2O‐exchange rate. The [Ln(dotagl)] complexes form slowly, via the equilibrium formation of a monoprotonated intermediate, which deprotonates and rearranges to the product in a slow, OH−‐catalyzed reaction. The formation rates are lower than those for the corresponding Ln(dota) complexes. The dissociation rate of [Eu(dotagl)] is directly proportional to [H+] (0.1–1.0M HClO4); the proton‐assisted dissociation rate is lower for [Eu(H4dotagl)] (k1=8.1⋅10−6 M−1 s−1) than for [Eu(dota)] (k1=1.4⋅10−5 M−1 s−1).

NMR Studies Reveal Structural Differences between the Gallium and Yttrium Complexes of DOTA-d-Phe-Tyr3-octreotide

The somatostatin analogue DOTATOC, DOTA-[Tyr(3)]octreotide, is used for in vivo diagnosis and targeted therapy of somatostatin-receptor-positive tumors. DOTATOC consists of a disulfide-bridged octapeptide, d-Phe(1)-Cys(2)-Tyr(3)-d-Trp(4)-Lys(5)-Thr(6)-Cys(7)-Thr(8)-ol, connected to the metal chelator DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid). Two metal complexes, Ga(III)- and Y(III)-DOTATOC, were reported to differ significantly in somatostatin receptor affinity and tumor uptake. Our (1)H and (13)C solution NMR data and modeling studies of both compounds are in agreement with a fast conformational equilibrium of the peptide part, as previously reported for octreotide itself. However, the different coordination geometry of Ga(3+) and Y(3+) (6-fold and 8-fold, respectively, as known from model compounds) causes pronounced differences for the d-Phe(1) residue. For Y(III)-DOTATOC this leads to two conformers exchanging slowly on the NMR time scale. From various NMR measurements, they could be identified as cis-trans isomers at the amide bond between DOTA chelator and first residue (d-Phe(1)H(N)) of the peptide.

Synthesis of a bifunctional monophosphinic acid DOTA analogue ligand and its lanthanide(iii) complexes. A gadolinium(iii) complex endowed with an optimal water exchange rate for MRI applications

A new bifunctional octa-coordinating ligand containing an aminobenzyl moiety, DO3APABn (H4DO3APABn = 1,4,7,10-tetraazacyclododecane-4,7,10-triacetic-1-{methyl[(4-aminophenyl)methyl]phosphinic acid}), has been synthesized. Its lanthanide(III) complexes contain one water molecule in the first coordination sphere. The high-resolution 1H and 31P spectra of [Eu(H2O) (DO3APABn)]- show that the twisted square-antiprismatic form of the complexes is more abundant in respect to the corresponding Eu(III)-DOTA complex. The 1H NMRD and variable-temperature 17O relaxation measurements of [Gd(H2O)(DO3APABn)]- show that the water residence time is short (298tauM = 16 ns) and falls into the optimal range predicted by theory for the attainment of high relaxivities once this complex would be endowed by a slow tumbling rate. The relaxivity (298r1 = 6.7 mM(-1) s(-1) at 10 MHz) is higher than expected as a consequence of a significant contribution from the second hydration sphere. These results prompt the use of [Gd(H2O)(DO3APABn)]- as a building block for the set-up of highly efficient macromolecular MRI contrast agents.

Lanthanide(III) complexes of DOTA-glycoconjugates: a potential new class of lectin-mediated medical imaging agents.

The synthesis and characterization of a new class of DOTA (1,4,7,10-tetrakis(carboxymethyl)-1,4,7,10-tetraazacyclododecane) monoamide-linked glycoconjugates (glucose, lactose and galactose) of different valencies (mono, di and tetra) and their Sm(III), Eu(III) and Gd(III) complexes are reported. The 1H NMR spectrum of Eu(III)-DOTALac2 shows the predominance of a single structural isomer of square antiprismatic geometry of the DOTA chelating moiety and fast rotation about the amide bond connected to the targeting glycodendrimer. The in vitro relaxivity of the Gd(III)-glycoconjugates was studied by 1H nuclear magnetic relaxation dispersion (NMRD), yielding parameters close to those reported for other DOTA monoamides. The known recognition of sugars by lectins makes these glycoconjugates good candidates for medical imaging agents (MRI and gamma scintigraphy).

Dynamic NMR properties of DOTA ligand: variable pH and temperature 1H NMR study on [K(HxDOTA)](3-x)- species.

The (1)H NMR spectra of [H(x)DOTA]((4-x)-) species are reported as a function of pH and temperature in aqueous solution. The spectra show line broadening both in ligand proton signals and also in the water proton signal by titration with KOH solution. The formation of different [K(H(x)DOTA)]((3-x)-) complexes is found to be responsible for this behaviour. At high pH the usual fluxional motions, i.e. the ring inversion and the change in the acetate arms' helicity, which are also characteristic for other but inert metal-DOTA complexes, have been detected. However, because of the kinetic lability of K(+)-O and K(+)-N coordinative bonds a new type of rearrangement appears. This new motion requires breaking of coordinative bonds in the complex and can be described as a certain type of "ring slewing" around the ring C-C bonds. At low temperature (about 270 K) the ring slewing slows down and becomes negligible compared with the ring inversion and the change in the arms' helicity. These two latter processes have the same rate. When the temperature is higher (about 320 K) the ring slewing accelerates and its rate exceeds the rate of ring inversion. At this temperature the change in the acetate arms' helicity has the same rate as the ring slewing. Additionally, in the pH range 4-5 a slow intermolecular proton exchange process has been observed between the water and the dissociable protons of [K(H(x)DOTA)]((3-x)-). A water-assisted proton exchange mechanism is proposed on the basis of the activation parameters. This finding supports the previously suggested slow proton motion hypothesis for the formation of DOTA complexes.

Structural analysis of nine-coordinate lanthanide complexes: steric control of the metal-water distance across the series.

The structures of 10 isomorphous lanthanide (Ln) complexes of a chiral DOTA tetra-amide ligand (L(1)), [LnL(1)(H(2)O)](CF(3)SO(3))(3).3H(2)O, crystallizing in space group P2(1), have been studied by single-crystal X-ray diffraction. The Ln coordination is a O(4)N(4) square antiprism, the O(4) base of which is capped by an aqua ligand. The sterically demanding position of the latter results in the lengthening of the Ln-OH(2) distance along the Pr to Lu series by 0.06 A (after allowing for the lanthanide contraction). In parallel, the distance between the bound water oxygen and the second-sphere water oxygen is reduced from 3.17 A (Pr) to 3.04 A (Lu), consistent with the enhanced hydrogen bond acceptor ability of the coordinated water oxygen across the series. A Cambridge Structural Database survey of [Ln(H(2)O)(9)](CF(3)SO(3))(3) salts (space group P6(3)/m) and of six reported isostructural complexes of DOTA [L(2)] revealed a similar trend. The implications of the resultant destabilization of the ground state structure for the water interchange process are discussed.

Crystal structures of two complexes of the rare-earth-DOTA-binding antibody 2D12.5: ligand generality from a chiral system.

We report the crystal structures of antibody 2D12.5 Fab bound to an yttrium-DOTA analogue and separately to a gadolinium-DOTA analogue. The rare earth elements have many useful properties as probes, and 2D12.5 binds the DOTA (1,4,7,10-tetraazacyclododecane-N,N',N' ',N' "-tetraacetic acid) complexes of all of them (Corneillie et al. J. Am. Chem. Soc. 2003, 125, 3436-3437). The structures show that there are no direct protein-metal interactions: a bridging water acts as a link between the protein and metal, with the chelate present as the M isomer in each case. DOTA forms an amphipathic cylinder with the charged carboxylate groups toward the face of the chelate near the metal ion, while nonpolar methylene groups from the macrocycle and the carboxymethyl groups occupy the rear and sides of the molecule. The orientation of the metal-DOTA in the 2D12.5 complex places most of the methylene carbon atoms of DOTA in hydrophobic contact with aromatic protein side chains. Other binding interactions between the metal complex and the antibody include a bidentate salt bridge, four direct H-bonds, and four to five water-mediated H-bonds. We find that 2D12.5 exhibits enantiomeric binding generality, binding yttrium chelates in both Lambda(deltadeltadeltadelta) and Delta(lambdalambdalambdalambda) configurations with modestly different affinities. This develops from the symmetrical nature of the DOTA chelate, which places heteroatoms and hydrophobic atoms in approximately the same relative positions regardless of the helicity of DOTA.

Studies of inclusion complexes between cyclodextrins and polyazamacrocyclic chelates of lanthanide (III) ions

The complexes between γ-cyclodextrin and lanthanide (III) chelates of the polyazamacrocycles DOTA (DOTA ≡ 1,4,7,10-tertraazacyclododecane-1,4,7,10-tetraacetate) and DOTP (DOTP ≡ 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetramethylenephosphonate) have been thought out to enhance the potential of such chelates as contrast agents for MRI. Given the actual demand for the design of new contrast agents, we thought it worthwhile to confirm previous results for the equilibrium constant K obtained by one of us by NMR on the DOTP complex, as well as to determine K for a new one with DOTA. Further, we wanted to study and quantify the interactions present in these complexes, with a view to improve them in newly designed complexes. The interactions between γ-cyclodextrin and the lanthanide (III)-polyazamacrocyclic chelates, [Tm(DOTP)] 5−, and [Gd(DOTA)] − were then studied by isothermal calorimetry (ITC) and molecular dynamics. The calorimetric experiments can be interpreted by considering that in both cases there is a weak association, characterized by low values for the equilibrium constant as well as for the molar enthalpy change for complex formation, at T=298.15 K. The K value for the complex with DOTP obtained now by ITC is of the same order of magnitude of the one determined previously by NMR. Further, the complex formation seems rather insensitive to the macrocycle, as the values now obtained by ITC for the DOTA complex are very similar to the ones obtained for the DOTP complex. We have also carried out molecular dynamics simulations on these very same inclusion complexes, which provided quantitative data on the interactions present, as well as a plausible explanation for the data obtained, leading to the proposal of possible solutions to improve the modelling of new contrast agents on a host–guest basis.

Application of 1H and 23Na magic angle spinning NMR spectroscopy to define the HRBC up-taking of MRI contrast agents

The up-take of Gd(III) complexes of BOPTA, DTPA, DOTA, EDTP, HPDO3A, and DOTP in HRBC has been evaluated by measuring the lanthanide induced shift (LIS) produced by the corresponding dysprosium complexes (DC) on the MAS-NMR resonances of water protons and free sodium ions. These complexes are important in their use as MRI contrast agents (MRI-CA) in diagnostics. 1H and 23Na MAS-NMR spectra of HRBC suspension, collected at 9.395 T, show only one signal due to extra- and intra-cellular water (or sodium). In MAS spectra, the presence of DC in a cellular compartment produces the LIS of only the nuclei (water proton or sodium) in that cellular compartment and this LIS can be related to the DC concentrations (by the experimental curves of LIS vs. DC concentrations) collected in the physiological solution. To obtain correct results about LIS, the use of MAS technique is mandatory, because it guarantees the only the nuclei staying in the same cellular compartment where the LC is present show the LIS. In all the cases considered, the addition of the DC to HRBC (100% hematocrit) produced a shift of only the extra-cellular water (or sodium) signal and the gradient of concentration ( G C) between extra- and intra-cellular compartments resulted greater than 100:1, when calculated by means of sodium signals. These high values of G C are direct proofs that none of the tested dysprosium complexes crosses the HRBC membrane. Since the DC are iso-structural to the gadolinium complexes the corresponding gadolinium ones (MRI-CA) do not cross the HRBC membrane and, consequently, they are not up-taken in HRBC. The G C values calculated by means of water proton signals resulted much lower than those obtained by sodium signals. This proves that the choice of the isotope is a crucial step in order to use this method in the best way. In fact, G C value depends on the lowest detectable LIS which, in turn, depends on the nature of the LC (lanthanide complex) and the observed isotopes.

Novel macrocyclic EuII complexes: fast water exchange related to an extreme M-O water distance.

Eu(II) complexes are potential candidates for pO(2)-responsive contrast agents in magnetic resonance imaging. In this regard, we have characterized two novel macrocyclic Eu(II) chelates, [Eu(II)(DOTA)(H(2)O)](2-) and [Eu(II)(TETA)](2-) (H(4)DOTA=1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid, H(4)TETA=1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetic acid) in terms of redox and thermodynamic complex stability, proton relaxivity, water exchange, rotation and electron spin relaxation. Additionally, solid-state structures were determined for the Sr(II) analogues. They revealed no inner-sphere water in the TETA and one inner-sphere water molecule in the DOTA complex. This hydration pattern is retained in solution, as the (17)O chemical shifts and (1)H relaxation rates proved for the corresponding Eu(II) compounds. The thermodynamic complex stability, determined from the formal redox potential and by pH potentiometry, of [Eu(II)(DOTA)(H(2)O)](2-) (lg K(Eu(II))=16.75) is the highest among all known Eu(II) complexes, whereas the redox stabilities of both [Eu(II)(DOTA)(H(2)O)](2-) and [Eu(II)(TETA)](2-) are inferior to that of 18-membered macrocyclic Eu(II) chelates. Variable-temperature (17)O NMR, NMRD and EPR studies yielded the rates of water exchange, rotation and electron spin relaxation. Water exchange on [Eu(II)(DOTA)(H(2)O)](2-) is remarkably fast (k298(ex)=2.5 x 10(9) s(-1)). The near zero activation volume (DeltaV++ =+0.1+/-1.0 cm(3) mol(-1)), determined by variable-pressure (17)O NMR spectroscopy, points to an interchange mechanism. The fast water exchange can be related to the low charge density on Eu(II), to an unexpectedly long M-O(water) distance (2.85 A) and to the consequent interchange mechanism. Electron spin relaxation is considerably slower on [Eu(II)(DOTA)(H(2)O)](2-) than on the linear [Eu(II)(DTPA)(H(2)O)](3-) (H(5)DTPA=diethylenetriaminepentaacetic acid), and this difference is responsible for its 25 percent higher proton relaxivity (r(1)=4.32 mM(-1) s(-1) for [Eu(II)(DOTA)(H(2)O)](2-) versus 3.49 mM(-1) s(-1) for [Eu(II)(DTPA)(H(2)O)](3-); 20 MHz, 298 K).

A Rare Earth-DOTA-Binding Antibody: Probe Properties and Binding Affinity across the Lanthanide Series

An antibody that binds rare earth complexes selectively could be used as a docking station for a set of probe molecules, of particular interest for medical imaging and therapy. The rare earths are rich in probe properties, such as the paramagnetism of Gd, the luminescence of Tb and Eu, and the nuclear properties of Lu and Y. We find that antibody 2D12.5, initially developed to bind analogues of Y-DOTA (1,4,7,10-tetraazacyclododecane-N,N',N' ',N' ''-tetraacetic acid) for radiotherapy, binds not only Y-DOTA analogues but also analogous DOTA complexes of all of the lanthanides. Surprisingly, chelates of some metals such as Gd3+ bind more tightly than the original Y3+ complex. When the shape of the complex is perturbed by either increasing or decreasing the radius of the lanthanide ion, the thermodynamic stability of the protein-ligand complex changes in a regular fashion. The behavior of DeltaDeltaG as a function of ionic radius fits a parabola, as might be expected for a system that behaves in a thermodynamically elastic way. The broad specificity and high affinity of this antibody for all rare earth-DOTA complexes make it particularly interesting for applications that take advantage of the unique characteristics of lanthanides. For example, UV excitation of the Tb-DOTA-2D12.5 complex leads to energy transfer from aromatic side chains of the antibody to bound Tb-DOTA, enhancing green terbium luminescence >104 relative to unbound Tb-DOTA.

Study of holmium (III) and yttrium(III) with DOTA complexes as candidates for radiopharmaceutical use

Reaction conditions for complexation of radionuclides with DOTA were studied using thinlayer chromatography (TLC), paper chromatography (PC) and potentiometry. It was found that all of the studied complexes can reach very high radiochemical yield about 95%. Optimal conditions for obtaining such high radiochemical yields are as follows: pH higher than 4 and the excess of chelating agent must be minimally 3∶1. Potentiometric study showed that the formation of complexes is characterised by very slow kinetics.

Structural variations across the lanthanide series of macrocyclic DOTA complexes: insights into the design of contrast agents for magnetic resonance imaging.

It was early shown that the macrocyclic Ln(DOTA) complexes (DOTA = 1,4,7,10-tetra-azacyclododecane-N,N',N' ',N' "-tetraacetic acid) exists in solution as a mixture of two enantiomeric pairs of diastereoisomers differing in the ligand conformation, namely, square antiprismatic (SA) and twisted square antiprismatic (TSA) geometries, respectively. Later, extensive (1)H NMR investigations suggested that a coordination change may be superimposed on this conformational equilibrium involving two additional structures in which the metal ion possesses a coordination number of eight (CN 8). It was predicted that these two species, lacking the apical coordinated water molecule, would maintain the SA and TSA coordination geometries, and therefore, they have been labeled as SA' and TSA', respectively. In this work we report the X-ray solid-state crystal structure determination of six Ln(DOTA) complexes representative of all four coordination geometry typologies deduced from NMR solution studies. A distinctive structural feature that discriminates SA (and SA') and TSA (and TSA') structures is represented by the twist angle between the two square planes of the antiprism, the basal four nitrogen, and the apical four oxygen planes. [Ce(DOTA)(H(2)O)](-) displays a TSA structural typology with a twist angle of 25 degrees and a Ce-O(water) distance of 2.59 A. The SA-type structure has been found in the case of complexes with Pr(III), Nd(III), and Dy(III), where the twist angle is 39, 39, and 38 degrees, respectively, and the metal-water oxygen distance varies significantly (Pr-O(w) 2.529 A; Nd-O(w) 2.508 A; and Dy-O(w) 2.474 A). [Tm(DOTA)](-) displays a TSA'-type structure with a twist angle of 24 degrees. As compared with the TSA structure of the corresponding Ce(III) complex, the Tm(III) complex shows an overall marked shrinkage of all metal-nitrogen and metal-oxygen distances (ca. 0.2 A), which reflects the contraction of the metal ionic radius across the series but also the effect associated with the decrease of the CN from 9 to 8. In [Sc(DOTA)](-), the even smaller ionic radius of Sc(III) shifts the geometry of the coordination cage to the more compact SA' typology with a twist angle of 41 degrees, a value very similar to that found in the SA structures of lanthanide(III) ions with CN 9. Finally, an investigation was made into the hydration spheres of the complexes with SA and TSA geometries to account for the experimental evidence of a markedly different rate of water exchange for the two isomeric structures. This is of fundamental importance to the understanding of the corresponding Gd(III) complexes as MRI contrast agents.

Gd(III) based MRI contrast agents: improved physical meaning in a combined analysis of EPR and NMR data?

A new global analysis of EPR, 17O NMR relaxation and chemical shift and 1H NMRD profiles with physically meaningful parameters for [Gd(DOTA)(H2O)]− and for [Gd(DTPA)(H2O)]2− in aqueous solution is presented (DOTA=1,4,7,10-tetraaza-1,4,7,10-tetrakis(carboxymethyl)-cyclododecane; DTPA=diethylenetriamine-N,N,N′,N″,N″-pentaacetic acid). The recent developments of an improved EPR relaxation theory, the inclusion of the internal motion of the bound water molecule are the principal modifications. Furthermore the better knowledge of the quadrupolar coupling constant of the bound water molecule, the neglect of the outer-sphere contribution to the chemical shift and the consideration of different isomers for the DOTA complex allowed for an improved analysis. The water exchange and parameters of rotational motion are only slightly changed. Comparison of the contributions of static zero-field-splitting shows that a more symmetric environment of the Gd(III) ion should lead to slower electron spin relaxation, a feature which can become important if all other parameters (rotational correlation time and water exchange rate) are optimised. In the actual stage the improved combined analysis of Gd(III) poly(amino carboxylate) is limited by the approximations of Redfield’s relaxation theory, i.e., very low frequency NMRD-data and slowly tumbling complexes cannot be analysed with the method presented.

Synthesis and biological evaluation of novel macrocyclic ligands with pendent donor groups as potential yttrium chelators for radioimmunotherapy with improved complex formation kinetics.

Novel 1,4,7-triazacyclononane-N,N',N' '-triacetic acid (NOTA) based octadentate ligands [2-(4,7-biscarboxymethyl[1,4,7]triazacyclononan-1-ylethyl)carbonylmethylamino]acetic acid tetrahydrochloride (1) and [3-(4,7-biscarboxymethyl[1,4,7]triazacyclononan-1-yl-propyl)carbonylmethylamino]acetic acid tetrahydrochloride (2) with pendent donor groups as potential yttrium chelators for radioimmunotherapy (RIT) have been prepared via a convenient and high-yield cyclization route. The complexation kinetics of the novel chelates with Y(III) was investigated and compared to that of 1,4,7,10-tetraazacyclododecane-N,N',N' ',N" '-tetraacetic acid (DOTA), a macrocyclic chelating agent well recognized as forming very stable complexes with yttrium but also limited in usage because of slow Y(III) complex formation rates. The in vitro stability of the corresponding (88)Y-labeled complexes in human serum was assessed by measuring the release of (88)Y from the complexes over 14 days. The in vivo biodistribution of (86)Y-labeled 1 in mice was evaluated and compared to that of the (86)Y-DOTA complex. Formation of the Y complex of 1 was significantly more rapid than that of either 2 or DOTA. Serum stability of the (88)Y complex formed with 1 was equivalent to the DOTA complex, while the complex formed with 2 proved to be significantly unstable. The results obtained from a biodistribution study indicate that the (86)Y-1 complex possesses in vivo stability comparable to the analogous DOTA complex.

Conformational characterization of lanthanide(III)-DOTA complexes by ab initio investigation in vacuo and in aqueous solution.

The conformational behavior of four [Ln(DOTA)(H(2)O)](-) systems (Ln = La, Gd, Ho, and Lu) has been characterized by means of ab initio calculations performed in vacuo and in aqueous solution, the latter by using the polarizable continuum model (PCM). Calculated molecular geometries and conformational energies of the [Ln(DOTA)(H(2)O)](-) systems, and interconversion mechanisms, barriers, and (13)C NMR spectra of the [Lu(DOTA)](-) complex are compared with experimental values. For each system, geometry optimizations, performed in vacuo and in solution at the HF/3-21G level and using a 46+4f(n) core electron effective core potential (ECP) for lanthanides, provide two minima corresponding to a square antiprismatic (A) and an inverted antiprismatic (IA) coordination geometry. All the systems are nonacoordinated, with the exception of the IA isomer of the Lu complex that, from in solution calculations, is octacoordinated, in agreement with experimental data. On comparing the in vacuo relative free energies calculated at different theory levels it can be seen that the nonacoordinated species dominates at the beginning of the lanthanide series while the octacoordinated one does so at the end. Furthermore, on passing along the series the IA isomer becomes less and less favored with respect to A and for the Lu complex a stabilization of the IAisomer is observed in solution (but not in vacuo), in agreement with the experimental data. Investigation of the A<-->IA isomerization process in the [Lu(DOTA)](-) system provides two different interconversion mechanisms: a single-step process, involving the simultaneous rotation of the acetate arms, and a multistep path, involving the inversion of the cyclen cycle configuration. While in vacuo the energy barrier for the acetate arm rotation is higher than that involved in the ring inversion (23.1 and 13.1 kcal mol(-)(1) at the B3LYP/6-311G level, respectively), in solution the two mechanisms present comparable barriers (14.7 and 13.5 kcal mol(-)(1)), in fairly good agreement with the experimental values. The NMR shielding constants for the two isomers of the [Lu(DOTA)](-) complex have been calculated by means of the ab initio GIAO and CSGT methods, and using a 46-core-electron ECP for Lu. The calculated (13)C NMR chemical shifts are in close agreement with the experimental values (rms 3.3 ppm, at the HF/6-311G level) and confirm the structural assignment of the two isomers based on experimental NMR spectra in solution. The results demonstrate that our computational approach is able to predict several physicochemical properties of lanthanide complexes, allowing a better characterization of this class of compounds for their application as contrast agents in medical magnetic resonance imaging (MRI).

Theoretical investigation on the geometries of DOTA and DOTA-like complexes and on the transition states of their conformational equilibriaElectronic supplementary information (ESI) available: Tables S1–S3, extended versions of Tables 1–3, and Table S4, comparison of structural features. See http://www.rsc.org/suppdata/nj/b1/b106168m/

Computations, mostly at the RHF level, have been performed on La3+ and Y3+ model complexes of the widely used DOTA ligand [DOTA = 1,4,7,10-tetraaza-1,4,7,10-tetrakis(carboxymethyl)cyclododecane] for the purpose of identifying properties of the transition states of geometrical isomer interconversions, which have been the object of numerous NMR studies. An analogous study has been carried out on model complexes of the same trications formed by the DOTA-like ligand 1,7-bis(1-methylimidazol-2-ylmethyl)-4,10-bis(carboxymethyl)-1,4,7,10-tetraazacyclododecane, LH2. Features of the transition states for the two types of systems are compared.

Direct Measurement of Fast Electron Spin−Lattice Relaxation: Method and Application to Nitroxide Radical Solutions and Gd3+ Contrast Agents

A modulation technique for direct measurement of extremely fast electron spin−lattice relaxation is described. The method is based on longitudinal magnetization detection. It is applied here to study the spin−lattice relaxation times T1e in aqueous solutions of perdeuterated nitroxide radical (T1e ≈ 200 ns) with various concentrations as well as to two Gd DOTA complexes (T1e ≈ 1.8−3.2 ns), which can be employed as paramagnetic contrast agents in magnetic resonance imaging.