Highly relaxing gadolinium based MRI contrast agents responsive to Mg2+ sensing

Synthesis and Photophysical Properties of Lanthanide Pyridinylphosphonic Tacn and Pyclen Derivatives: From Mononuclear Complexes to Supramolecular Heteronuclear Assemblies

Synthetic methodologies were developed to achieve the preparation of ligands L1 and L2 consisting of tacn- and pyclen-based chelators decorated with pyridinylphosphonic pendant arms combined with ethylpicolinamide or acetate coordinating functions, respectively. Phosphonate functions have been selected for their high affinity toward Ln3+ ions compared to their carboxylated counterparts and for their steric hindrance that favors the formation of less-hydrated complexes. Thanks to regiospecific N-functionalization of the macrocyclic backbones, the two ligands were isolated with good yields and implicated in a comprehensive photophysical study for the complexation of Eu3+, Tb3+, and Yb3+. The coordination behavior of L1 and L2 with these cations has been first investigated by means of a combination of UV-vis absorption spectroscopy, steady-state and time-resolved luminescence spectroscopy, and 1H and 31P NMR titration experiments. Structural characterization in solution was assessed by NMR spectroscopy, corroborated by theoretical calculations. Spectroscopic characterization of the Ln3+ complexes of L1 and L2 was done in water and D2O and showed the effective sensitization of the lanthanide metal-centered emission spectra, each exhibiting typical lanthanide emission bands. The results obtained for the phosphonated ligands were compared with those reported previously for the corresponding carboxylated analogues.

Assembly of Poly(vinylphosphonic acid)-Based Double Hydrophilic Block Copolymers by Gadolinium Ions for the Formation of Highly Stable MRI Contrast Agents

Mixing double-hydrophilic block copolymers containing a poly(vinylphosphonic acid) block with gadolinium ions in water leads to the spontaneous formation of polymeric nanoparticles. With an average diameter near 20 nm, the nanoparticles are stable after dilution or change of pH and ionic strength. High magnetic relaxivities were measured in vitro, and in vivo magnetic resonance imaging on rats demonstrates the high potential of such polymeric assemblies.

55/52Mn2+ Complexes with a Bispidine-Phosphonate Ligand: High Kinetic Inertness for Imaging Applications

Bispidine (3,7-diazabicyclo[3.3.1]nonane) provides a rigid and preorganized scaffold that is particularly interesting for the stable and inert complexation of metal ions, especially for their application in medical imaging. In this study, we present the synthesis of two bispidine ligands with N-methanephosphonate (H₄L¹) and N-methanecarboxylate (H₃L²) substituents as well as the physico-chemical properties of the corresponding Mn²⁺ and Zn²⁺ complexes. The two complexes [Mn(L¹)]^2- and [Mn(L²)]^- have relatively moderate thermodynamic stability constants according to potentiometric titration data. However, they both display an exceptional kinetic inertness, as assessed by transmetallation experiments in the presence of 50 equiv excess of Zn²⁺, showing only ∼40 and 20% of dissociation for [Mn(L¹)]^2- and [Mn(L²)]^-, respectively, after 150 days at pH 6 and 37 °C. Proton relaxivities amount to r₁ = 4.31 mM^-1 s^-1 ([Mn(L¹)]^2-) and 3.64 mM^-1 s^-1 ([Mn(L²)]^-) at 20 MHz, 25 °C, and are remarkable for Mn²⁺ complexes with one inner-sphere water molecule (q = 1); they are comparable to that of the commercial contrast agent [Gd(DOTA)(H₂O)]^-. The presence of one inner-sphere water molecule and an associative water exchange mechanism was confirmed by temperature-dependent transverse ¹⁷O relaxation rate measurements, which yielded k_ex²⁹⁸ = 0.12 × 10⁷ and 5.5 × 10⁷ s^-1 for the water exchange rate of the phosphonate and the carboxylate complex, respectively. In addition, radiolabeling experiments with ⁵²Mn were also performed with H₂(L¹)^2- showing excellent radiolabeling properties and quantitative complexation at pH 7 in 15 min at room temperature as well as excellent stability of the complex in various biological media over 24 h.

Structural and Thermodynamics Studies on Polyaminophosphonate Ligands for Uranyl Decorporation

The development of actinide decorporation agents with high complexation affinity, high tissue specificity, and low biological toxicity is of vital importance for the sustained and healthy development of nuclear energy. After accidental actinide intake, sequestration by chelation therapy to reduce acute damage is considered as the most effective method. In this work, a series of bis- and tetra-phosphonated pyridine ligands have been designed, synthesized, and characterized for uranyl (UO₂²⁺) decorporation. Owing to the absorption of the ligand and the luminescence of the uranyl ion, UV-vis spectroscopy and time-resolved laser-induced fluorescence spectroscopy (TRLFS) were used to probe in situ complexation and structure variation of the complexes formed by the ligands with uranyl. Density functional theory (DFT) calculations and X-ray absorption fine structure (XAFS) spectroscopy on uranyl-ligand complexes revealed the coordination geometry around the uranyl center at pH 3 and 7.4. High affinity constants (log K ∼17) toward the uranyl ion were determined by displacement titration. A preliminary in vitro chelation study proves that bis-phosphonated pyridine ligands can remove uranium from calmodulin (CaM) at a low dose and in the short term, which supports further uranyl decorporation applications of these ligands.

Macrocyclic Pyclen-Based Gd3+ Complex with High Relaxivity and pH Response

We report the synthesis and characterization of the macrocyclic ligand 2,2'-((2-(3,9-bis(carboxymethyl)-3,6,9-triaza-1(2,6)-pyridinacyclodecaphane-6-yl)ethyl)azanediyl)diacetic acid (H₄L) and several of its complexes with lanthanide ions. The structure of the free ligand was determined using X-ray diffraction measurements. Two N atoms of the pyclen moiety in the trans position are protonated in the solid state, together with the exocyclic N atom and one of the carboxylate groups of the ligand. The relaxivity of the Gd³⁺ complex was found to increase from 6.7 mM^-1 s^-1 at pH 8.6 to 8.5 mM^-1 s^-1 below pH ≈ 6.0. Luminescence lifetime measurements recorded from H₂O and D₂O solutions of the Eu³⁺ complex evidence the presence of a single complex species in solution at low pH (∼5.0) that contains two inner-sphere water molecules. DFT calculations suggest that the coordination environment of the Ln³⁺ ion is fulfilled by the four N atoms of the pyclen unit, two oxygen atoms of the macrocyclic acetate groups, and an oxygen atom of an exocyclic carboxylate group. The two inner-sphere water molecules complete coordination number nine around the metal ion. At high pH (∼9.3), the lifetime of the excited ⁵D₀ level of Eu³⁺ displays a biexponential behavior that can be attributed to the presence of two species in solution with hydration numbers of q = 0 and q = 1. The ¹H NMR and DOSY spectra recorded from solutions of the Eu³⁺ and Y³⁺ complexes reveal a structural change triggered by pH and the formation of small aggregates at high pH values.

pH-Dependent Hydration Change in a Gd-Based MRI Contrast Agent with a Phosphonated Ligand

The heptadentate ligand L was shown to form an extremely stable Gd complex at neutral pH with a pGd value of 18.4 at pH 7.4. The X-ray crystal structures of the complexes formed with Gd and Tb displayed two very different coordination behaviors being, respectively, octa- and nonacoordinated. The relaxometric properties of the Gd complex were studied by field-dependent relaxivity measurements at various temperatures and by ¹⁷ O NMR spectroscopy. The pH-dependence of the longitudinal relaxivity profile indicated large changes around neutral pH leading to a very large value of 10.1 mm^-1 ⋅s^-1 (60 MHz, 298 K) at pH 4.7. The changes were attributed to an increase of the hydration number from one water molecule in basic conditions to two at acidic pH. A similar trend was observed for the luminescence of the Eu complex, confirming the change in hydration state. DOSY experiments were performed on the Lu analogue, pointing to the absence of dimers in solution in the considered pH range. A breathing mode of the complex was postulated, which was further supported by ¹ H and ³¹ P NMR spectroscopy of the Yb complex at varying pH and was finally modeled by DFT calculations.

Smart MRI Agents for Detecting Extracellular Events In Vivo: Progress and Challenges

Many elegant inorganic designs have been developed to aid medical imaging. We know better now how to improve imaging due to the enormous efforts made by scientists in probe design and other fundamental sciences, including inorganic chemistry, physiochemistry, analytical chemistry, and biomedical engineering. However, despite several years being invested in the development of diagnostic probes, only a few examples have shown applicability in MRI in vivo. In this short review, we aim to show the reader the latest advances in the application of inorganic agents in preclinical MRI.

Molecular Upconversion in Water in Heteropolynuclear Supramolecular Tb/Yb Assemblies

Piling up excited states to reach upconversion (UC) is severely restricted by vibrational quenching mechanisms, especially when one looks at discrete molecular entities in solution. By carefully controlling the supramolecular assembly processes resulting from the strong electrostatic interactions between negatively charged Yb complexes and Tb³⁺ cations in aqueous solutions, we engineered the formation of heteropolynuclear complexes of [(YbL)₂Tb _x] compositions ( x = 1 and 2). These edifices display a phenomenon of cooperative photosensitization UC with green emission of the Tb cations upon NIR excitation at 980 nm in the Yb absorption band. The photophysical properties of the complexes were carefully investigated by steady-state and time-resolved luminescence experiments in D₂O, allowing one to quantify the impact of the composition and pD of the solution on the emission intensity as well as clarifying the exact cooperative photosensitization upconversion mechanism. Using optimized conditions, the energy transfer UC process could be observed for the first time in nondeuterated water with discrete molecular compounds.

Sensing Uranyl(VI) Ions by Coordination and Energy Transfer to a Luminescent Europium(III) Complex

The release of uranyl(VI) is a hazardous environmental issue, with limited ways to monitor accumulation in situ. Here, we present a method for the detection of uranyl(VI) ions through the utilization of a unique fluorescence energy transfer process to europium(III). Our system displays the first example of a "turn-on" europium(III) emission process with a small, water-soluble lanthanide complex triggered by uranyl(VI) ions.

Fluorescent single-stranded DNA-based assay for detecting unchelated Gadolinium(III) ions in aqueous solution

The main concern pertaining to the safety of Gadolinium(III)-based contrast agents (GBCAs) is the toxicity caused by the unchelated ion, which may be inadvertently present in the solution due most commonly to excess unreacted starting material or dissociation of the complexes. Detecting the aqueous free ion during the synthesis and preparation of GBCA solutions is therefore instrumental in ensuring the safety of the agents. This paper reports the development of a sensitive fluorogenic sensor for aqueous unchelated Gadolinium(III) (Gd(III)). Our design utilizes single-stranded oligodeoxynucleotides with a specific sequence of 44 bases as the targeting moiety. The fluorescence-based assay may be run at ambient pH with very small amounts of samples in 384-well plates. The sensor is able to detect nanomolar concentration of Gd(III), and is relatively unresponsive toward a range of biologically relevant ions and the chelated Gd(III). Although some cross-reactivity with other trivalent lanthanide ions, such as Europium(III) and Terbium(III), is observed, these are not commonly found in biological systems and contrast agents. This convenient and rapid method may be useful in ascertaining a high purity of GBCA solutions. Graphical abstract Fluorescent aptamer-based assay for detecting unchelated Ln(III) ions in aqueous solution.

Lanthanide-based luminescence biolabelling

Multiplexing, time-resolution, FRET…lanthanide-based biolabels reveal exceptional spectroscopic properties for bioanalytical applications.

AMPED: a new platform for picolinate based luminescent lanthanide chelates

Anchoring three picolinate units on a AMPED scaffold affords a strongly pre-organized ligand for the formation of luminescent lanthanide complexes.

Tetraphosphonated thiophene ligand: mixing the soft and the hard

A tetraphosphonated thiophene based ligand was synthesized, which showed decreased stability but increased selectivity relative to similar (hetero)aromatic analogues.

Promising strategies for Gd-based responsive magnetic resonance imaging contrast agents

Magnetic resonance imaging is a powerful imaging modality that is often coupled with paramagnetic contrast agents based on gadolinium to enhance sensitivity and image quality. Responsive contrast agents are key to furthering the diagnostic potential of MRI, both to provide anatomical information and to discern biochemical activity. Recent design of responsive gadolinium-based T₁ agents has made interesting progress, with the development of novel complexes which sense their chemical environment through changes in the coordination of water molecules, the molecular tumbling time or the number of metal centres. Particular promising design strategies include the use of multimeric systems, and the development of dual imaging probes.

Synthesis of an activated phosphonated bifunctional chelate with potential for PET imaging and radiotherapy

The synthesis of a phosphonated acyclic bifunctional chelate L* for the labeling of biomaterial is described. L* is based on a pyridine backbone, functionalized in ortho positions by aminomethyl-bis-methylphosphonic acids, and, in the para position, by a side chain containing a reactive NHS carbamate function. The stability of L* in aqueous solutions at different pH values was studied by mass spectrometry, showing the activated function to be sensitive to hydrolysis above neutral pH. The reactivity of L* towards amine functions was tested using ethylamine under different conditions of pH and concentrations, and by the labeling of two reference peptides containing both an N-terminal amino function and a ε-amino group of a lysine residue in the backbone, and a supplementary thiol group of a cysteine residue for one of these two peptides. The results showed the coupling to be efficient at pH 8.0, with a total selectivity for the terminal amine function with respect to lysine and cysteine. The labeling was further performed on B28-13, a mouse monoclonal antibody specifically recognizing tenascin-C protein in human cancer. The labeled antibody was characterized by means of mass spectrometry and spectrofluorimetry, unraveling a labeling ratio of one chelate per antibody. Finally, the affinity of the labeled antibody towards its target was controlled by immunofluorescence staining experiments on human colon cancer biopsies, confirming the affinity of the labeled peptide for tenascin-C.