Determination of water permeability of paramagnetic liposomes of interest in MRI field.

The water permeability of various liposome membranes has been determined at 298K by measuring the NMR longitudinal water proton relaxation rate of vesicles encapsulating the clinically approved Gd-HPDO3A complex (HPDO3A=10-(2-hydroxypropyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid). Two basic formulations based on DPPC (dipalmitoylphosphatidylcholine) and POPC (palmitoyl-oleylphosphatidylcholine) phospholipids were selected and investigated. Furthermore, the permeability changes caused by the membrane incorporation of amphiphiles like cholesterol and/or metal complexes of interest for designing improved liposome-based MRI contrast agents, were also investigated. The incorporation of cholesterol and metal complexes bearing C18 saturated chains in POPC-based liposomes reduces the water diffusivity across the membrane bilayer. On the contrary, the incorporation of a macrocyclic metal complex bearing four C12 alkylic chains, one for each coordination arm of the ligand, considerably enhances the water permeability in DPPC-based liposomes. Finally, it is reported that the permeability of POPC-based bilayer is increased when the liposomes are subjected to an osmotic stress.

1H and 17O NMR relaxometric study in aqueous solution of Gd(III) complexes of EGTA-like derivatives bearing methylenephosphonic groups.

The Gd(III) complexes of three new octadentate chelators, prepared by substitution of four, two, and one carboxylate groups of EGTA with phosphonate groups, have been investigated by 1H and 17O NMR relaxometric techniques in aqueous solutions. The analysis of the solvent proton relaxivity data as a function of pH, temperature, and magnetic field strength (nuclear magnetic relaxation dispersion (NMRD) profiles) in combination with the 17O transverse relaxation rate data at variable temperature allowed assessing the hydration state of the complexes, the occurrence of pH-dependent oligomerization processes for the tetraphosphonate derivative, the presence of a well-defined second sphere of hydration that markedly contributes to the relaxivity, and the values of the structural and dynamic relaxation parameters. In addition, in the case of the monophosphonate derivative the presence of a coordinated water molecule has allowed evaluation of the kinetic parameters of the exchange process, highly relevant for the possible use of this Gd(III) complex as an MRI probe. The rate of exchange of the water molecule, (298)k(ex) = 4.2 x 10(8)s(-1), is one of the highest measured so far for a nonacoordinate Gd(III) chelate and optimal for developing contrast-enhancing probes of high efficacy at high magnetic fields.

Ln(III)-DOTAMGly complexes: a versatile series to assess the determinants of the efficacy of paramagnetic chemical exchange saturation transfer agents for magnetic resonance imaging applications.

RATIONALE AND OBJECTIVES: Paramagnetic Ln-DOTAMGly complexes (Ln not equal La, Lu, and Gd) are the prototypes of a novel class of contrast agents for magnetic resonance imaging based on chemical exchange saturation transfer (CEST). Their ability to reduce the water signal intensity depends on the interplay of several physico-chemical properties of the agent and instrumental parameters. This study aims to identify possible routes for their optimization METHODS: Saturation transfer (ST) has been measured in vitro at 7.05 T as a function of pH, temperature, and concentration of the agent. RESULTS: Large saturation transfer effects have been observed upon irradiating the coordinated water protons (for Ln = Pr, Nd, Eu, and Tb). The comparison of the results obtained by irradiating water versus amide protons allows the set-up of ratiometric methods through which the ST response can be made independent on the concentration of the agent. CONCLUSIONS: The modulation of the magnetic properties along the lanthanide series allows an in-depth understanding of the determinants of ST effect and provides useful insights for the design of more efficient agents.

Methods for an improved detection of the MRI-CEST effect.

CEST imaging is a recently introduced MRI contrast modality based on the use of endogenous or exogenous molecules whose exchangeable proton pools transfer saturated magnetization to bulk water, thus creating negative contrast. One of the critical issues for further development of these agents is represented by their limited sensitivity in vivo. The aim of this work is to improve the detection of CEST agents by exploring new approaches through which the saturation transfer (ST) effect can be enhanced. The performance of the proposed methods has been tested in vitro and in vivo using highly sensitive and highly shifted lipoCEST agents, and the results were compared with the standard ST evaluation mode. The acquired Z-spectra were interpolated locally and voxel-by-voxel by smoothing splines. Besides expressing the ST in the standard mode, we explore two methods, enhanced and integral ST, which better exploit all the information contained in the Z-spectrum. By combining different modes for ST assessment a significant improvement in the detection of the lipoCEST agents, both in vitro and in vivo, has been found. The results obtained from the application of the proposed methods outline the importance of post-processing analysis for highlighting the CEST-MRI contrast.

First ex-vivo MRI co-localization of two LIPOCEST agents.

One of the major advantages of the CEST methodology deals with the possibility of visualizing more probes in the same MR image voxels. This is a unique property within the contrast media that act on the (1)H-NMR signal of water protons, and it might considerably improve the potential of the technique. In addition to displaying sufficiently different resonance frequencies of their mobile protons, it is also important that the CEST agents designed for this application are highly sensitive. LIPOCEST agents represent the most sensitive class of CEST systems developed so far. On this basis, two LIPOCEST samples, a spherical one and an osmotically shrunken nonspherical one, endowed with markedly different resonance frequencies of their intraliposomal water protons, 3 ppm and 15 ppm, respectively, were prepared and tested both in vitro and in ex-vivo on a bovine muscle used as tissue-surrogate. The response of the two agents did not interfere each other, thus allowing the multiple visualization of the two agents present at nanomolar concentrations in the same image voxels.

A multinuclear NMR relaxometry study of ternary adducts formed between heptadentate Gd(III) chelates and L-lactate.

Dramatic relaxation enhancements of L-lactate resonances have been observed upon formation of ternary adducts with Gd(III) complexes of heptadentate DO3A and DO3A-like ligands (DO3A = 1,4,7,10-tetraazaciclododecane-1,4,7-triacetic acid). Detailed 1H and 17O NMR relaxometry investigations allow us to obtain structural, dynamic and thermodynamic information on the ternary complexes in which L-lactate acts as a bidentate ligand replacing two water molecules in the inner coordination sphere of the Gd(III) ion. It has been found that the exchange rate of the coordinated L-lactate is modulated by the structural and electronic properties of the parent Gd-heptacoordinated macrocyclic chelate. In addition to the characterisation of the relaxation behaviour of the 1H methyl resonance of L-lactate, this study has been extended to its 13C isomer (fully enriched at the three positions) and to the trifluoro-L-lactate. The obtained results may be relevant to the development of relaxation agents able to promote the relaxation enhancement of specific substrates detectable by in vivo magnetic resonance spectroscopy.

Enantioselective recognition between chiral alpha-hydroxy-carboxylates and macrocyclic heptadentate lanthanide(III) chelates.

Three novel heptacoordinated Ln(III) complexes (Ln = Gd and Yb) have been synthesized and investigated by (1)H NMR spectroscopy. These complexes contain two stereogenic centers, one associated with a deltadeltadeltadelta or lambdalambdalambdalambda conformation of the ethylenediamine moieties in the tetraazamacrocycle and the latter arises from the orientation (Delta or Lambda) of the coordinating arms. Evidence has been gained for the occurrence of a fast exchange between all the possible conformers. Upon addition of several (S)-alpha-hydroxy-carboxylate substrates, the formation of stable ternary adducts has been obtained. Their (1)H NMR spectra are consistent with the presence of two diastereoisomers differing in the conformation adopted by the macrocyclic ligand wrapping the lanthanide(III) ion. The interaction leading to the formation of the ternary complexes is enantioselective depending on the hydrophilicity of the alpha-hydroxy-carboxylate.

Spectroscopic and computational investigations of stable radical anions of triosmium benzoheterocycle clusters.

The variable temperature (1)H and (13)C NMR and EPR spectra of the stable radical anions [Os(3)(CO)(9)(micro(3)-eta(2)-L)(micro-H)] (LH=phenanthridine, 1; 5,6-benzoquinoline, 2), and [Os(3)(CO)(10)(micro(3)-eta(2)-L)(micro-H)] (LH=quinoxaline, 3) are reported. The radical anions 1(-), 2(-), and 3(-) can be prepared by both exhaustive electrolysis and partially by chemical reduction with cobaltocene and with sodium dispersion (only with sodium dispersion in the case of 3(-)). DFT calculations on 1-3 reveal that the LUMO for the electron-deficient compounds 1 and 2 involves significant contributions from both the heterocyclic ligand and the two metal atoms bridged by the ligand and the micro-hydride. The character of this orbital rationalizes the previously observed regioselective reactions of these complexes with nucleophiles. In contrast, the LUMO for the electron precise 3 involves only ligand-based orbitals. Partial chemical reduction of 1 and 2 requires an excess of either cobaltocene or sodium, and their (1)H and (13)C NMR spectra reveal selective line broadening of those proton resonances that are predicted by DFT calculations to bear the greatest amount of free spin density. The variable temperature behavior of the partially chemically reduced species of 1 and 2 indicates that electron transfer between the reduced/unreduced cluster pair and between the cobaltocene/cobaltocenium pair occurs on the NMR timescale. The radical anions of 1 and 2 prepared by exhaustive electrolysis show an EPR signal at room temperature, while the NMR signals are uniformly broadened. Compound 3 appears to be partially reduced by sodium at room temperature and shows uniformly broadened (1)H NMR resonances at room temperature that sharpen significantly at -80 degrees C. The temperature dependence of the spectra are discussed in terms of the effects of relative electron nuclear relaxation processes, chemical exchange, and the results of the DFT calculations.