Giovanni B. Giovenzana

The synthesis and application of polyamino polycarboxylic bifunctional chelating agents

Bifunctional chelating agents (BFCAs) are molecules which contain two different moieties: a strong metal chelating unit and a reactive functional group. The latter is directed to react with amines, thiols, alcohols or other reactive molecules to form stable covalent bonds while the chelating moiety is able to strongly coordinate a metal ion. In this way, it is possible to label a molecule of interest (e.g. an antibody or a peptide) with a metal or a radioactive metal ion. Of all the ligands reported so far, those based on a polyamino polycarboxylic structure are most efficient and are widely employed for the chelation of metal ions. The resulting metal complexes have found a broad range of applications in chemistry, biology and medicine. Diagnostic imaging (MRI, SPECT, PET), molecular imaging, tumour therapy and luminescent materials are only a few examples. The present critical review gives an overview of the syntheses and most important applications of polyamino polycarboxylic BFCAs (334 references).

NOVEL PARAMAGNETIC MACROMOLECULAR COMPLEXES DERIVED FROM THE LINKAGE OF A MACROCYCLIC Gd(III) COMPLEX TO POLYAMINO ACIDS THROUGH A SQUARIC ACID MOIETY

Macromolecular Gd(III) complexes may find useful application as contrast agents for magnetic resonance angiography (MRA). Herein two novel systems are reported, namely Gd(DO3ASQ)3-lys16 and Gd(DO3ASQ)30-orn114. Their syntheses are based on the ability of the squaric acid moiety to act as a linker between the DO3A (1,4,7, 10-tetraazacyclododecane-1,4,7-triacetic acid) chelate moiety and the polyamino acidic chain. Moreover, the squaric acid participates in the coordination cage of the Gd(III) ion. The investigation of 1H and 17O NMR relaxation processes of solvent water nuclei allowed a detailed characterization of the systems under study. Gd(DO3ASQ)30-orn114 displays a remarkable ability to enhance the water proton relaxation rate of its solutions, and it may be considered as potential contrast agent for MRA applications.

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.

Non-covalent conjugates between cationic polyamino acids and GdIII chelates: a route for seeking accumulation of MRI-contrast agents at tumor targeting sites.

Three novel Gd chelates containing on their external surface pendant phosphonate and carboxylate groups, which promote the interaction with the positively charged groups of polyornithine and polyarginine, have been synthesized. Their solution structures have been assessed on the basis of 1H- and 31P-NMR spectra of the Eu and Yb analogues. A thorough investigation of the relaxometric (1H and 17O) properties of the Gd chelates has been carried out and the observed relaxivities have been accounted for the sum of three contributions arising from water molecules in the first, second, and outer coordination layers, respectively. It has been found that the occurrence of a tight second coordination coating renders the dissociation of the water molecule directly coordinated to the Gd ion more difficult. The binding interactions between the negatively charged Gd chelates and the positively charged groups of polyornithine (ca. 140 residues) and polyarginine (ca. 204 residues) have been evaluated by means of the proton relaxation enhancement (PRE) method. Although the binding interaction decreases markedly in the presence of competitive anions in the solution medium, the affinity is strong enough that in blood serum it is possible to meet the conditions where most of the chelate is bound to the polyamino acid substrate. On this basis one may envisage a novel route for a MRI location of tumors as it is known that positively charged polyamino acids selectively bind to tumors having a greater negative charge than non-tumor cells.

Equilibrium and Kinetic Properties of the Lanthanoids(III) and Various Divalent Metal Complexes of the Heptadentate Ligand AAZTA

The heptadentate ligand 1,4-bis(hydroxycarbonylmethyl)-6-[bis(hydroxycarbonylmethyl)]amino-6-methylperhydro-1,4-diazepine (AAZTA) and its derivatives were recently reported to give stable complexes with Gd(3+) with superior efficiency as MRI contrast agents. Nevertheless, only preliminary data are available on the coordination behavior of this interesting ligand. In this work, thermodynamic and kinetic stability data are determined for the formation of complexes with AAZTA and the lanthanoid metal ions, and other divalent metal ions of interest for this application. The AAZTA ligand binds the lanthanoid ions with log K(ML) values of 17.53-21.85 with its affinity steadily increasing from La(3+) to Lu(3+), suggesting that the seven-membered skeleton is better suited to accommodate smaller metal ions. Even though the denticity is lower, the stability of the heavier lanthanoid complexes is comparable to those of the classical ligand diethylenetriaminepentaacetic acid (DTPA). The transmetalation reactions of [Gd(AAZTA)](-) with Cu(2+) and Eu(3+) predominantly occur through proton-assisted dissociation of the complex. The role of the direct attack of Cu(2+) or Eu(3+) in the exchange reactions is limited, although the formation of dinuclear complexes decreases the proton-assisted dissociation. Near physiological conditions, [Gd(AAZTA)](-) is significantly more inert than [Gd(DTPA)](2-), allowing its potentially safe use as contrast agent in magnetic resonance imaging.

High-Relaxivity Gadolinium-Modified High-Density Lipoproteins as Magnetic Resonance Imaging Contrast Agents

There is an ongoing desire to produce high-relaxivity, Gd-based magnetic resonance imaging (MRI) contrast agents. These may allow for lower doses to be used, which is especially important in view of the current safety concerns surrounding Gd in patients. Here we report the synthesis of a high-relaxivity MRI contrast agent, by incorporating Gd-chelating lipids that coordinate two water molecules into high-density lipoprotein (q = 2 HDL). We compared the properties of q = 2 HDL with those of an analogous HDL particle labeled with Gd-chelating lipids that coordinate only one water molecule (q = 1 HDL). We found that the q = 2 HDL possessed an elevated r(1) of 41 mM(-1) s(-1) compared to 9 mM(-1) s(-1) for q = 1 HDL at 20 MHz, but the q = 2 HDL exhibited high R(2)* values at high fields, precluding imaging above 128 MHz. While carrying out this investigation we observed that enlarged, disrupted particles were formed when the synthesis was carried out above the lipid critical micelle concentration (cmc), indicating the importance of synthesis below the cmc when modifying lipoproteins in this manner. The high relaxivity of q = 2 HDL means it will be an efficacious contrast agent for future MR imaging studies.

A straightforward entry into enantiomerically enriched β-amino-α-hydroxyphosphonic acid derivatives

Abstract The asymmetric aminohydroxylation (AA) reaction of β-substituted vinylphosphonates under Sharpless protocol followed by hydrolysis afforded β-amino-α-hydroxyphosphonic acids in moderate to good ee.

Fast and easy access to efficient bifunctional chelators for MRI applications.

Novel bifunctional agents based on the AAZTA (6-amino-6-methylperhydro-1,4-diazepinetetraacetic acid) structure with hydroxyl, carboxyl and chloro functional groups were prepared. The Gd(III) complexes show optimal magnetic properties making them very good candidates for conjugation to biomolecules for molecular imaging applications. An example of conjugation to a bile acid derivative is also reported.

A macromolecular Gd(III) complex as pH-responsive relaxometric probe for MRI applications

The ability of the paramagnetic macromolecular complex (GdDO3ASQ)30-Orn114 to enhance the water proton relaxation rate has been found to be a function of pH; this behaviour is related to the structural changes occurring in the polypeptide upon protonation of the side-chain NH2 groups.

15N Magnetic Resonance Hyperpolarization via the Reaction of Parahydrogen with 15N-Propargylcholine

(15)N-Propargylcholine has been synthesized and hydrogenated with para-H(2). Through the application of a field cycling procedure, parahydrogen spin order is transferred to the (15)N resonance. Among the different isomers formed upon hydrogenation of (15)N-propargylcholine, only the nontransposed derivative contributes to the observed N-15 enhanced emission signal. The parahydrogen-induced polarization factor is about 3000. The precise identification of the isomer responsible for the observed (15)N enhancement has been attained through a retro-INEPT ((15)N-(1)H) experiment. T(1) of the hyperpolarized (15)N resonance has been estimated to be ca. 150 s, i.e., similar to that reported for the parent propargylcholine (144 s). Experimental results are accompanied by theoretical calculations that stress the role of scalar coupling constants (J(HN) and J(HH)) and of the field dependence in the formation of the observed (15)N polarized signal. Insights into the good cellular uptake of the compound have been gained.