Enzo Terreno

The extraordinary ligand binding properties of human serum albumin

Human serum albumin (HSA), the most prominent protein in plasma, binds different classes of ligands at multiple sites. HSA provides a depot for many compounds, affects pharmacokinetics of many drugs, holds some ligands in a strained orientation providing their metabolic modification, renders potential toxins harmless transporting them to disposal sites, accounts for most of the antioxidant capacity of human serum, and acts as a NO‐carrier. The globular domain structural organization of monomeric HSA is at the root of its allosteric properties which are reminiscent of those of multimeric proteins. Here, structural, functional, biotechnological, and biomedical aspects of ligand binding to HSA are summarized.

Challenges for Molecular Magnetic Resonance Imaging

3.3. Magnetic Particle Imaging 3029 4. Challenges for CEST Agents 3029 4.1. Technical Issues 3029 4.2. Chemical Issues 3031 4.3. Biological Issues 3032 5. Challenges for Heteronuclear MR Imaging 3033 5.1. F-Based Probes 3033 6. Challenges for Hyperpolarized Probes 3034 6.1. Brute Force 3034 6.2. Optical Pumping and Spin Exchange 3035 6.3. Dynamic Nuclear Polarization (DNP) 3035 6.4. para-Hydrogen Induced Polarization (PHIP) 3037 6.5. Use of Gd Contrast Agents with Hyperpolarized Substances 3038

Lanthanide(III) chelates for NMR biomedical applications

The peculiar magnetic properties of lanthanide(III) ions may be exploited for the development of powerful NMR probes for biomedical applications. GdIII chelates are in current clinical use as contrast agents for magnetic resonance imaging. Other paramagnetic lanthanide(III) complexes endowed with shift reagent capabilities are used for the separation of NMR resonances of species present in the inner and outer cellular compartments and for the measurement of pH and temperature.

Paramagnetic lanthanide(III) complexes as pH-sensitive chemical exchange saturation transfer (CEST) contrast agents for MRI applications

The recently introduced new class of contrast agents (CAs) based on chemical exchange saturation transfer (CEST) may have a huge potential for the development of novel applications in the field of MRI. In this work we explored the CEST properties of a series of Lanthanide(III) complexes (Ln = Eu, Dy, Ho, Er, Tm, Yb) with the macrocyclic DOTAM‐Gly ligand, which is the tetraglycineamide derivative of DOTA (1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetraacetic acid). These complexes possess two pools of exchangeable protons represented by the coordinated water and the amide protons. Yb‐DOTAM‐Gly displays the most interesting CEST properties when its amide N‐H resonance (16 ppm upfield H2O signal) is irradiated. Up to 70% suppression of the water signal is obtained at pH 8. As the exchange rate of amide protons is base‐catalyzed, Yb‐DOTAM‐Gly results to be an efficient pH‐responsive probe in the 5.5–8.1 pH range. Moreover, a ratiometric method has been set up in order to remove the dependence of the observed pH responsiveness from the absolute concentration of the paramagnetic agent. In fact, the use of a mixture of Eu‐DOTAM‐Gly and Yb‐DOTAM‐Gly, whose exchangeable proton pools are represented by the coordinated water (ca. 40 ppm downfield H2O signal at 312K) and amide protons, respectively, produces a pH‐dependent CEST effect which is the function of the concentration ratio of the two complexes. Magn Reson Med 47:639–648, 2002.

Gd(III)-BASED CONTRAST AGENTS FOR MRI

Publisher Summary Gd(III) chelates have played an important role in the development of clinical applications of Magnetic Resonance Imaging (MRI) technique by adding relevant physiological information to the superb anatomical resolution attainable with this imaging modality. The major challenges are in the emerging field of Molecular Imaging where the competition with other imaging modalities can be very tight. Targeting of thrombi and atherosclerotic plaques by peptides functionalized with Gd(III) chelates appears to be the next goal for industrial research. The search for improved Gd(III)-based agents has been highly beneficial for the growth of lanthanide coordination chemistry and has created a very fertile interdisciplinary area with contributions from Chemistry, Biology, Medicine, and Imaging technology. The requisite for more efficient Gd(III)-based probes for more advanced MRI applications will be the driving force for further goals in lanthanide coordination chemistry.

Effect of the intracellular localization of a Gd-based imaging probe on the relaxation enhancement of water protons

Gd‐HPDO3A has been internalized into rat hepatocarcinoma cells in the cytoplasm (by electroporation) or in intracellular vesicles (by pinocytosis), respectively. In the former case, the observed relaxation rates are likely dependent upon the amount of internalized paramagnetic complex, whereas in the latter case the relaxation enhancement is “quenched” to a plateau value (about 3 s−1) when the entrapped amount of Gd‐chelate is higher than 1 × 1010 Gd/cell. The observed behavior has been accounted in terms of a theoretical treatment based on equations formally derived by Labadie et al. (J Magn Reson B 1994;105:99–102). On this basis, entrapment into intracellular vesicles has been treated as a three‐site water exchange (extracellular/cytoplasm/vesicle compartments), whereas the cell pellets containing the paramagnetic agent spread out in the cytoplasm can be analyzed by a two‐site exchange system. Magn Reson Med, 2006.

Gd(III) complexes as contrast agents for magnetic resonance imaging: a proton relaxation enhancement study of the interaction with human serum albumin

Abstract The non-covalent interaction between human serum albumin (HSA) and DOTA-like Gd(III) complexes containing hydrophobic benzyloxymethyl (BOM) substituents has been thoroughly investigated by measuring the solvent proton relaxation rates of their aqueous solutions. The binding association constants (KA) to HSA are directly related to the number of hydrophobic substituents present on the surface of the complexes. Furthermore, an estimation of ΔH° and ΔS° has been obtained by the temperature dependence of KA. Assays performed with the competitor probes warfarin and ibuprofen established that the complexes interact with HSA through two nearly equivalent binding sites located in the subdomains IIA and IIIA of the protein. Strong relaxation enhancements, promoted by the formation of slowly tumbling paramagnetic adducts, have been measured at 20 MHz for complexes containing two and three hydrophobic substituents. The macromolecular adduct with the latter species has a relaxivity of 53.2±0.7 mM–1 s–1, which represents the highest value so far reported for a Gd(III) complex. The temperature dependence of the relaxivity for the paramagnetic adducts with HSA indicates long exchange lifetimes for the water molecules dipolarly interacting with the paramagnetic centre. This is likely to be related to the formation, upon hydrophobic interaction of the complexes with HSA, of a clathrate-like, second-coordination-sphere arrangement of water molecules. Besides affecting the dissociative pathway of the coordinated water molecule, this water arrangement may itself significantly contribute to enhancement of the bulk solvent relaxation rate.

Iopamidol as a responsive MRI-chemical exchange saturation transfer contrast agent for pH mapping of kidneys: In vivo studies in mice at 7 T.

Iopamidol (Isovue®—Bracco Diagnostic Inc.) is a clinically approved X‐Ray contrast agent used in the last 30 years for a wide variety of diagnostic applications with a very good clinical acceptance. Iopamidol contains two types of amide functionalities that can be exploited for the generation of chemical exchange saturation transfer effect. The exchange rate of the two amide proton pools is markedly pH‐dependent. Thus, a ratiometric method for pH assessment has been set‐up based on the comparison of the saturation transfer effects induced by selective irradiation of the two resonances. This ratiometric approach allows to rule out the concentration effect of the contrast agent and provides accurate pH measurements in the 5.5–7.4 range. Upon injection of Iopamidol into healthy mice, it has been possible to acquire pH maps of kidney regions. Furthermore, it has been also shown that the proposed method is able to report about pH‐changes induced in control mice fed with acidified or basified water for a period of a week before image acquisition. Magn Reson Med, 2010.

Large relaxivity enhancement of paramagnetic lipid nanoparticles by restricting the local motions of the Gd(III) chelates.

A Gd(III)-DOTA-like complex bearing two aliphatic chains on adjacent acetic arms was designed, synthesized, and compared with its analogous monofunctionalized complex. A 1/T(1) NMR relaxometric study of the two amphiphilic complexes incorporated into micelles and liposomes showed an unprecedented relaxivity enhancement for the complex with two lipophilic side arms. This remarkable result, which is attributed to a favorable water exchange rate and increased rigidity of the system resulting from limiting of the local motion of the gadolinium center, represents an important advance in the development of highly efficient nanosystems for MRI applications.

Encoding the frequency dependence in MRI contrast media: the emerging class of CEST agents.

CEST agents represent a very promising class of MRI contrast media as they encode a frequency dependence that is not like the classical relaxation-based agents. This peculiar property enables novel applications such as the detection of more than one agent in the same MR image as well as the set-up of ratiometric methods for the quantitative assessment of physico-chemical and biological parameters that characterize the micro-environment in which they are distributed. This survey is aimed at providing the reader with the basic properties and the potential of these compounds. Fundamental aspects, such as the theoretical basis of the saturation transfer via chemical exchange, the generation of the CEST contrast, the classification and sensitivity of CEST agents, and some representative examples displaying their potential in the field of MR-molecular imaging, are presented and discussed in detail.