Célia S. Bonnet

A Gadolinium-Binding Cyclodecapeptide with a Large High-Field Relaxivity Involving Second-Sphere Water

A new cyclodecapeptide incorporating two prolylglycine sequences as beta-turn inducers and bearing four side chains with acidic carboxyl groups for cation complexation has been prepared. Structural analysis in water by (1)H NMR spectroscopy and CD shows that this template adopts a conformation suitable for the complexation of lanthanide ions Ln(3+), with its carboxyl groups oriented on the same face of the peptide scaffold. Luminescence titrations show that mononuclear Ln-PA complexes are formed with apparent stability constants of log beta(110) approximately 6.5 (pH 7). The high-field water relaxivity values arising from the Gd-PA complex at 200-500 MHz have been interpreted with molecular parameters determined independently. The experimentally determined water relaxivities are undoubtedly 30% higher than the expected values for this complex with two inner-sphere (IS) water molecules and a medium-range rotational correlation time (tau(R) = 386 ps (+/-10%)). This led us to propose the existence of a large second-sphere (2S) contribution to the relaxivity caused by the interaction of water molecules with the hydrophilic peptide ligand by hydrogen-bonding.

A Rigorous Framework To Interpret Water Relaxivity. The Case Study of a Gd(III) Complex with an α-Cyclodextrin Derivative

We present a general theoretical framework suitable for an economical, but rigorous, analysis of the relaxivity and EPR data of paramagnetic metal complexes. This framework is based on the so-called Grenoble method that properly accounts for the fluctuations of the static zero-field splitting Hamiltonian and avoids the misinterpretation of experimental data, which occurs with the Solomon, Bloembergen, and Morgan (SBM) formalism and may lead to erroneous conclusions. The applicability of the SBM approximation is discussed. Our approach is implemented in the case of a new Gd(3+) chelate with a cyclodextrin derivative ligand hexakis(2-O-carboxymethyl-3,6-anhydro)-alpha-cyclodextrin (ACX), designed to obtain lanthanide complexes of enhanced stability in comparison to natural cyclodextrins. The introduction of carboxymethyl units on the six residual hydroxyl groups of an alpha-per-3,6-anhydro cyclodextrin leads to mono- and binuclear Ln(3+) complexes with log beta(110) approximately = 7.5. The GdACX complex induces large water proton relaxivity in 0.1 M KCl aqueous solution. The molecular parameters governing the longitudinal (r1) and transverse (r2) relaxivities above 1 T are obtained through simple SBM-like theoretical expressions and complementary experimental techniques. The metal hydration state, the translational diffusion coefficient of the complex, and its rotational correlation time are derived from luminescence lifetime studies, pulse-field gradient NMR, and deuteron quadrupolar relaxation, respectively. The high relaxivity induced by the GdACX complex is attributed to its high hydration state in the presence of potassium ions and to a rotational correlation time lengthened by the hydrophilic character of the ACX scaffold.

Outer-Sphere Investigation of MRI Relaxation Contrast Agents. Example of a Cyclodecapeptide Gadolinium Complex with Second-Sphere Water

We show how the purely outer-sphere (OS) relaxivity of a probe solute due to a Gd(3+) complex can help characterize the outer (O), inner (I), and second (2) sphere (S) contributions to the water proton relaxivity. Because of the difficulties of accurate theoretical predictions, we propose an experimental determination of the OS dipolar time correlation function (OS-DTCF) of the relative position of Gd(3+) with respect to any of the equivalent protons of the purely OS probe p-dioxane, which moves around the complex without binding to it. The method is illustrated by the GdPA complex with PA = c(AspArgGluProGlyGluTrpAspProGly). The experimental DTCF for dioxane is obtained by a model-free analysis of the high-field relaxivity of its protons. The time-modulation of the dioxane DTCF by the Gd(3+) electronic spin relaxation yields a measurable quenching of the longitudinal relaxivity at low-to-medium field, which serves us to deduce the fluctuating zero-field splitting (ZFS) Hamiltonian causing this electronic relaxation. The DTCF for water is derived from that for dioxane by appropriate scaling of the geometry of collision and relative diffusion coefficients of these molecules with respect to GdPA. The information obtained on the OS motion for water and the ZFS Hamiltonian together with an independent characterization of the IS contribution allows us to disentangle the OS, IS, and 2S mechanisms and interpret the relaxivity profile of the water protons from 2.35 mT to 18.8 T. The presence of a large 2S contribution is confirmed.

Prototypes of Lanthanide(III) Agents Responsive to Enzymatic Activities in Three Complementary Imaging Modalities: Visible/Near-Infrared Luminescence, PARACEST-, and T1-MRI

We report first prototypes of responsive lanthanide(III) complexes that can be monitored independently in three complementary imaging modalities. Through the appropriate choice of lanthanide(III) cations, the same reactive ligand can be used to form complexes providing detection by (i) visible (Tb(3+)) and near-infrared (Yb(3+)) luminescence, (ii) PARACEST- (Tb(3+), Yb(3+)), or (iii) T1-weighted (Gd(3+)) MRI. The use of lanthanide(III) ions of different natures for these imaging modalities induces only a minor change in the structure of complexes that are therefore expected to have a single biodistribution and cytotoxicity.

Four Gadolinium(III) Complexes Appended to a Porphyrin: A Water-Soluble Molecular Theranostic Agent with Remarkable Relaxivity Suited for MRI Tracking of the Photosensitizer

A molecular theranostic agent for magnetic resonance imaging (MRI) and photodynamic therapy (PDT) consisting of four [GdDTTA](-) complexes (DTTA(4-) = diethylenetriamine-N,N,N″,N″-tetraacetate) linked to a meso-tetraphenylporphyrin core, as well as its yttrium(III) analogue, was synthesized. A variety of physicochemical methods were used to characterize the gadolinium(III) conjugate 1 both as an MRI contrast agent and as a photosensitizer. The proton relaxivity measured in H2O at 20 MHz and 25 °C, r1 = 43.7 mmol(-1) s(-1) per gadolinium center, is the highest reported for a bishydrated gadolinium(III)-based contrast agent of medium size and can be related to the rigidity of the molecule. The complex displays also a remarkable singlet oxygen quantum yield of ϕΔ = 0.45 in H2O, similar to that of a meso-tetrasulfonated porphyrin. We also evidenced the ability of the gadolinium(III) conjugate to penetrate in cancer cells with low cytotoxicity. Its phototoxicity on Hela cells was evaluated following incubation at low micromolar concentration and moderate light irradiation (21 J cm(-2)) induced 50% of cell death. Altogether, these results demonstrate the high potential of this conjugate as a theranostic agent for MRI and PDT.

Smart Contrast Agents for Magnetic Resonance Imaging.

By visualizing bioactive molecules or biological parameters in vivo, molecular imaging is searching for information at the molecular level in living organisms. In addition to contributing to earlier and more personalized diagnosis in medicine, it also helps understand and rationalize the molecular factors underlying physiological and pathological processes. In magnetic resonance imaging (MRI), complexes of paramagnetic metal ions, mostly lanthanides, are commonly used to enhance the intrinsic image contrast. They rely either on the relaxation effect of these metal chelates (T(1) agents), or on the phenomenon of paramagnetic chemical exchange saturation transfer (PARACEST agents). In both cases, responsive molecular magnetic resonance imaging probes can be designed to report on various biomarkers of biological interest. In this context, we review recent work in the literature and from our group on responsive T(1) and PARACEST MRI agents for the detection of biogenic metal ions (such as calcium or zinc), enzymatic activities, or neurotransmitter release. These examples illustrate the general strategies that can be applied to create molecular imaging agents with an MRI detectable response to biologically relevant parameters.

Proton Exchange in a Paramagnetic Chemical Exchange Saturation Transfer Agent from Experimental Studies and ab Initio Metadynamics Simulation

The proton-exchange process between water and a carbamate has been studied experimentally and theoretically in a lanthanide-based paramagnetic chemical exchange saturation transfer agent endowed with potential multimodality detection capabilities (optical imaging, or T1 MRI for the Gd(III) analogue). In addition to an in-depth structural analysis by a combined approach (using X-ray crystallography, NMR, and molecular dynamics), our ab initio simulation in aqueous solution sheds light on the reaction mechanism for this proton exchange, which involves structural Grotthuss diffusion.

Paramagnetic relaxation enhancements in acetate and its fluorine derivatives interacting with Gd3+: complex formation, structure, and transmetallation.

The relative spatial distribution and motion with respect to Gd(3+) of the (1)H and (19)F nuclei in the acetate ion and its fluorine derivatives are studied in D(2)O solutions through the paramagnetic relaxation rate enhancements (PREs) of these nuclei. We derive general theoretical expressions of the longitudinal PRE in terms of the analytical concentrations of metal and ligands, formation constants of the complexes, metal-nucleus distances, and coordination lifetimes of the ligands. The observed formation constants of the 1 metal: 1 ligand complexes markedly decrease with increasing number of fluorine atoms, the electronegativity of which reduces the negative partial charge of the coordinating COO(-) group. The coordination lifetimes are very short at the scale of the relaxation times of the protons of metal bound acetate, that is, shorter than about 10 μs. The average distance of the acetate protons from Gd(3+) is in fair agreement with independent crystallographic determination. The release of free Gd(3+) from the very stable Gddtpa (dtpa=diethylene-triaminepentaacetate) complex caused by the competition of Zn(2+) for dtpa, is evidenced by an increase of the PREs with Zn(2+) concentration. The observed PRE increase is consistent with the known equilibrium constants governing the speciation involving Gd(3+), Zn(2+), and dtpa. The present case study illustrates a method which easily yields experimental tunable properties suitable to test the ongoing theories of lanthanide Ln(3+) complexation in solution.