Biomedical subjects
Publications and source records attributed to Silvio Aime.
The Mannich reaction was successfully employed to obtain two pyridine-based ligands 1 and 2. The latter represents the first example of a Mannich reaction leading to a 12-membered pyridine-containing macrocycle. [reaction: see text]
A simple labeling procedure of stem/progenitor cells based on the use of Gd-HPDO3A and Eu-HPDO3A, respectively, is described. The Gd-chelate acts as T(1)-agent for MRI visualization, whereas the corresponding Eu-chelate acts as reporter in fluorescence microscopy. Owing to their substantial chemical equivalence, the two chelates are equally internalized in EPCs (endothelial progenitor cells), thus allowing their visualization by both techniques. The lanthanide chelates are entrapped in endosomic vesicles and the labeled cells retain biological activity with preservation of viability and pro-angiogenesis capacity. Hyperintense spots in MR have been observed for Gd-labeled EPCs injected under mice kidney capsule or grafted on a subcutaneous Matrigel plug up to 14 days after transplantation.
At present, the so-called "substantial equivalence" is the only widely accepted criterion for deciding whether or not a transgenic food is, from an alimentary point of view, to be considered totally correspondent to the "traditional" one from which it derives. Although never exactly defined, it deals with a comparison between the chemical composition of the two foods. A more in-depth analysis can be performed by one of the most suitable methods that allows for the simultaneous screening of many components without prior identification, the analysis of the proteome. As a model for testing this kind of approach, we compared protein expression of two types of tomato plants, having the same genetic background, except for a virus resistance trait introduced by genetic engineering. When proteins extracted from seedlings of the two types were analyzed by two-dimensional electrophoresis, no significant differences, either qualitative or quantitative, were detected, indicating that in this case the expression of major proteins was unmodified by the genetic manipulation. Fifteen proteins were identified by peptide mass fingerprinting.
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.
The stability and water exchange dynamics of gadolinium (GdIII) complexes are critical characteristics that determine their effectiveness as contrast agents for magnetic resonance imaging (MRI). A new heteropodal GdIII chelate, [Gd-TREN-bis(6-Me-HOPO)-(TAM-TRI)(H2O)2] (Gd-2), is presented which is based on a hydroxypyridinate (HOPO)-terephthalamide (TAM) ligand design. Thermodynamic equilibrium constants for the acid-base properties and the GdIII complexation strength of TREN-bis(6-Me-HOPO)-(TAM-TRI) (2) were measured by potentiometric and spectrophotometric titration techniques, respectively. The pGd of 2 is 20.6 (pH 7.4, 25 degrees C, I = 0.1 M), indicating that Gd-2 is of more than sufficient thermodynamic stability for in vivo MRI applications. The water exchange rate of Gd-2 (kex = 5.3(+/-0.6) x 107 s-1) was determined by variable temperature 17O NMR and is in the fast exchange regime - ideal for MRI. Variable pressure 17O NMR was used to determine the volume of activation (DeltaV) of Gd-2. DeltaV for Gd-2 is -5 cm3 mol-1, indicative of an interchange associative (Ia) water exchange mechanism. The results reported herein are important as they provide insight into the factors influencing high stability and fast water exchange in the HOPO series of complexes, potentially future clinical contrast agents.
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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.
The interaction between different HO(z.rad;) radical scavengers in a three-component antioxidant system has been investigated by means of 19F-NMR spectroscopy. This system is composed of bovine serum albumin (BSA), trolox, and N-(4-hydroxyphenyl)-trifluoroacetamide (CF(3)PAF). The antioxidant capacity of BSA and trolox has been assessed by measuring the amount of trifluoroacetamide (TFAM) arising from the radical mediated decomposition of CF(3)PAF. When assayed separately, both trolox and BSA behaved as antioxidants, as they were effective to protect CF(3)PAF from HO* radical-mediated decomposition. By contrast, trolox enhanced the production of TFAM in the presence of BSA, thus behaving as a pro-oxidant. Urate, carnosine, glucose, and propylgallate showed antioxidant properties both with or without BSA. CF(3)PAF and trolox were found to bind to BSA with association constants in the order of 5 x 10(3)M(-1) and to compete for the same binding sites. These results have been discussed in terms of BSA-catalysed cross-reactions between trolox-derived secondary radicals and CF(3)PAF.
Quantification of the extent of glycation of human serum albumin (HSA) and of haemoglobin provides a record of average mid- and long-term blood-sugar concentrations, respectively; this is very useful for the management of diabetes. The reaction of D-glucose with propylamine affords the corresponding Schiff base, N-propylamino-D-glucoside, in the cyclic form. This compound is not stable: upon standing or treatment with acid it is converted, by an Amadori rearrangement, into N-propylfructosamine. Both amino sugars occur predominantly in the beta-pyranose form. Phenylboronate forms highly stable boronate esters through binding of the cis 1,2-diol moiety in the furanose form of N-propylfructosamine. Between pH 5 and 10, an electrostatic interaction between the protonated amino group and the negatively charged boronate moiety affords an additional stabilisation of the ester. The Schiff base, however, has no observable interaction with phenylboronate. In aqueous solution the Schiff base is in equilibrium with propylamine and glucose. Upon addition of phenylboronate, this equilibrium shifts to the side of glucose due to the formation of highly stable phenylboronate esters of the beta-furanose form of this compound. After Amadori rearrangement, the sugar moieties in glycated human serum albumin have a similar structure, they occur as an equilibrium of the beta-pyranose (59%), alpha-furanose (19%) and beta-furanose (24%) anomers. The open form was not observed. The beta-furanose anomer is selectively recognised by phenylboronate.
The (1)H and (17)O NMR relaxometric properties of two cationic complexes formed by Gd(III) with a macrocyclic heptadentate triamide ligand, L(1), and its Nmethylated analogue, L(2), have been investigated in aqueous media as a function of pH, temperature and magnetic field strength. The complexes possess two water molecules in their inner coordination sphere for which the rate of exchange has been found to be sensibly faster for the Nmethylated derivative and explained in terms of electronic effects (decrease of the charge density at the metal center) and perturbation of the network of hydrogen-bonded water molecules in the outer hydration sphere. The proton relaxivity shows a marked dependence from pH and decreases of about six units in the pH range 6.5 to 9.0. This has been accounted for by the displacement of the two water molecules by dissolved carbonate which acts as a chelating anion. The formation of ternary complexes with lactate, malonate, citrate, acetate, fluoride and hydrogenphosphate has been monitored by (1)H NMR relaxometric titrations at 20 MHz and pH 6.3 and the value of the affinity constant, K, and of the relaxivity of the adducts could be obtained. Lactate, malonate and citrate interact strongly with the complexes (log K > or =3.7) and coordinate in a bidendate mode by displacing both water molecules. Larger affinity constants have been measured for GdL(2). Acetate, fluoride and hydrogenphosphate form monoaqua ternary complexes which were investigated in detail with regard to their relaxometric properties. The NMR dispersion (NMRD) profiles indicate a large contribution to the relaxivity of the adducts from water molecules belonging to the second hydration shell of the complexes and hydrogen-bonded to the anion. A VT (17)O NMR study has shown a marked increase of the rate of water exchange upon binding which is explained by coordination of the anion in an equatorial site, thus leaving the water molecule in an apical position, more accessible for interactions with the solvent molecules of the second hydration shell which facilitate the exchange process.
A new method for the accurate determination of ferric heme-human serum albumin (heme-HSA) at concentrations down to the physiological level, i.e., in the micromolar concentration range, is proposed. This method is based on the (1)H NMR relaxometric properties of heme-HSA. Actually, the binding of the paramagnetic ferric heme to the primary binding site of HSA determines a strong paramagnetic enhancement of the water (1)H NMR relaxation rate. Although a linear relationship may be seen by operating at 20 MHz on conventional electromagnets, the method here reported is improved by working at 0.02 MHz on a field-cycling instrument. This (1)H NMR relaxometric method does not suffer from the presence in serum of heme catabolites (e.g., bilirubin) that affect significantly the optical determination of ferric heme-HSA in the micromolar concentration range. Paramagnetic ferric hemoglobin contribution may be selectively quenched by cyanide binding.
The variation of the size of the capping scaffold which connects the hydroxypyridonate (HOPO) binding units in a series of tripodal chelators for gadolinium (Gd) complexes has been investigated. A new analogue of TREN-1-Me-3,2-HOPO (1) (TREN = tri(ethylamine)amine) was synthesized: TREN-Gly-1-Me-3,2-HOPO (2) features a glycine spacer between the TREN cap and HOPO binding unit. TRPN-1-Me-3,2-HOPO (3) has a propylene-bridged cap, as compared to the ethylene bridges within the TREN cap of the parent complex. Thermodynamic equilibrium constants for the acid-base properties of 2 and the Gd(3+) complexation strength of 2 and 3 were measured and are compared with that of the parent ligand. The most basic ligand is 2 while 3 is the most acidic. Both 2 and 3 form Gd(3+) complexes of similar stability (pGd = 16.7 and 15.6, respectively) and are less stable than the parent complex Gd-1 (pGd = 19.2). Two of the three complexes are more stable than the bis(methylamide)diethylenetriamine pentaacetate complex Gd(DTPA-BMA) (pGd = 15.7) while the other is of comparable stability. Enlargement of the ligand scaffold decreases the stability of the Gd(3+) complexes and indicates that the TREN scaffold is superior to the TRPN and TREN-Gly scaffolds. The proton relaxivity of Gd-2 is 6.6 mM(-)(1) s(-)(1) (20 MHz, 25 degrees C, pH 7.3), somewhat lower than the parent Gd-1 but higher than that of the MRI contrast agents in clinical practice. The pH-independent relaxivity of Gd-2 is uncharacteristic of this family of complexes and is discussed.
Two novel mono-PEGylated derivatives of hGRF(1-29)-NH(2) [human growth hormone-releasing factor, fragment 1-29] have been synthesized by regio-specific conjugation of Lys(12) or Lys(21) to a monomethoxy-PEG(5000) chain (compounds Lys(12)PEG-GRF and Lys(21)PEG-GRF). The PEG moiety has been covalently linked at the amino group of a norleucine residue via a carbamate bond. The Lys(12)PEG-GRF regioisomer was found to be slightly less active in vitro than both the unmodified peptide and Lys(21)PEG-GRF. To assess whether the differences in the biological activity of the PEGylated analogues could be related to conformational rearrangements induced by the PEG moiety, the structure of these PEGylated derivatives has been worked out (TFE solution) by means of NMR spectroscopy and molecular dynamics. Secondary structure shifts, hydrogen/deuterium exchange kinetics, temperature coefficients of amide protons, and NOE-based molecular models point out that hGRF(1-29)-NH(2), Lys(21)PEG-GRF and Lys(12)PEG-GRF share a remarkably similar pattern of secondary structure. All three compounds adopt an alpha-helix conformation which spans the whole length of the molecule, and which becomes increasingly rigid on going from the N-terminus to the C-terminus. Residues Lys(12) and Lys(21) are enclosed in all the compounds considered into well-defined alpha-helical domains, indicating that PEGylation either at Lys(12) or Lys(21) does not alter the tendency of the peptide to adopt a stable alpha-helix conformation, nor does it induce appreciable conformational mobility in the proximity of the PEGylation sites. No significant variation of the amphiphilic organization of the alpha-helix is observed among the three peptides. Therefore, the different biological activities observed for the PEGylated analogues are not due to conformational effects, but are rather due to sterical hindrance effects. The relationship between the biological activitiy of the mono-PEGylated derivatives and sterical hindrance is discussed in terms of the topology of interaction between hGRF(1-29)-NH(2) and its receptor.
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Solid Lipid Nanoparticles (SLN) are already under investigation as a pharmaceutical tool able to change the pharmacokinetic and biodistribution of carried molecules. SLN are able to target drugs to lymph after duodenal administration and to overcome the Blood Brain Barrier (BBB). In this study, superparamagnetic SLN have been prepared, have colloidal size, in vitro analysis showed relaxometric properties similar to Endorem. In vivo Magnetic Resonance Imaging (MRI) of the central nervous system (CNS) with both SLN and Endorem showed that superparamagnetic SLN have slower blood clearance than Endorem. MRI data are consistent with CNS uptake of SLN lasting up to the end of the experiment (135 min). These findings confirm the ability of SLN to overcome the BBB; SLN might be used as a CNS MRI contrast agent.
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The nature of the ternary complexes formed in aqueous media at ambient pH on reversible binding of acetate, lactate, citrate, and selected amino acids and peptides to chiral diaqua europium, gadolinium, or ytterbium cationic complexes has been examined. Crystal structures of the chelated ytterbium acetate and lactate complexes have been defined in which the carboxylate oxygen occupies an "equatorial" site in the nine-coordinate adduct. The zwitterionic adduct of the citrate anion with [EuL1] was similar to the chelated lactate structure, with a 5-ring chelate involving the apical 3-hydroxy group and the alpha-carboxylate. Analysis of Eu and Yb emission CD spectra and lifetimes (H2O and D2O) for each ternary complex, in conjunction with 1H NMR analyses of Eu/Yb systems and 17O NMR and relaxometric studies of the Gd analogues, suggests that carbonate, oxalate, and malonate each form a chelated (q = 0) square-antiprismatic complex in which the dipolar NMR paramagnetic shift (Yb, Eu) and the emission circular polarization (gem for Eu) are primarily determined by the polarizability of the axial ligand. The ternary complexes with hydrogen phosphate, with fluoride, and with Phe, His, and Ser at pH 6 are suggested to be monoaqua systems with Eu/Gd with an apical bound water molecule. However, for the ternary complexes of simple amino acids with [YbL1]3+, the enhanced charge demand favors a chelate structure with the amine N in an apical position. Crystal structures of the Gly and Ser adducts confirm this. In peptides and proteins (e.g. albumin) containing Glu or Asp residues, the more basic side chain carboxylate may chelate to the Ln ion, displacing both waters.