Synthesis of an Ethylenediaminetetraacetic Acid-like Ligand Based on Sucrose Scaffold and Complexation and Proton Relaxivity Studies of Its Gadolinium(III) Complex in Solution

Sucrose constitutes a non-toxic, biodegradable, low-cost and readily available natural product. To expand its utility, we developed total synthesis for a ligand based on a sucrose scaffold for potential use as a metal chelation agent. The designed target (compound 2) has a metal-chelating functionality at both the C-6 and C-6' positions, which can provide a first coordination sphere of eight valencies. The designed total synthesis was highly efficient. To demonstrate the utility of the ligand, we studied its complexation with Gd(III). Using potentiometric titration and high-resolution mass spectrometry, we confirmed the formation of a 1:1 complex with Gd(III), which has a respectable formation constant of ~1013.4. Further NMR relaxivity studies show that the Gd(III) complex has a relaxivity (r1) of 7.6958 mmol-1 s-1.

Rigidified Derivative of the Non-macrocyclic Ligand H4OCTAPA for Stable Lanthanide(III) Complexation

The stability constants of lanthanide complexes with the potentially octadentate ligand CHXOCTAPA^4–, which contains a rigid 1,2-diaminocyclohexane scaffold functionalized with two acetate and two picolinate pendant arms, reveal the formation of stable complexes [log K_LaL = 17.82(1) and log K_YbL = 19.65(1)]. Luminescence studies on the Eu³⁺ and Tb³⁺ analogues evidenced rather high emission quantum yields of 3.4 and 11%, respectively. The emission lifetimes recorded in H₂O and D₂O solutions indicate the presence of a water molecule coordinated to the metal ion. ¹H nuclear magnetic relaxation dispersion profiles and ¹⁷O NMR chemical shift and relaxation measurements point to a rather low water exchange rate of the coordinated water molecule (k_ex²⁹⁸ = 1.58 × 10⁶ s^–1) and relatively high relaxivities of 5.6 and 4.5 mM^–1 s^–1 at 20 MHz and 25 and 37 °C, respectively. Density functional theory calculations and analysis of the paramagnetic shifts induced by Yb³⁺ indicate that the complexes adopt an unprecedented cis geometry with the two picolinate groups situated on the same side of the coordination sphere. Dissociation kinetics experiments were conducted by investigating the exchange reactions of LuL occurring with Cu²⁺. The results confirmed the beneficial effect of the rigid cyclohexyl group on the inertness of the Lu³⁺ complex. Complex dissociation occurs following proton- and metal-assisted pathways. The latter is relatively efficient at neutral pH, thanks to the formation of a heterodinuclear hydroxo complex.

A non-macrocyclic ligand containing a rigid cyclohexyl spacer forms thermodynamically stable complexes with the lanthanide(III) ions in aqueous solution. The complexes also show remarkable kinetic inertness, though a structural change facilitates dissociation through the metal-assisted mechanism for the small lanthanides. The Gd(III) complex displays a relatively high relaxivity due to the presence of a water molecule coordinated to the metal ion, while the Eu(III) and Tb(III) analogues display strong metal-centered luminescence.

An Unusual Pair: Facile Formation and In Vivo Validation of Robust Sc-18 F Ternary Complexes for Molecular Imaging

Fluorine-18 remains the most widely clinically utilized radionuclide globally for positron emission tomography (PET). The emergence of therapeutic isotopes for the management of disease has produced a pronounced interest in matched, theranostic isotope pairs that can be employed in tandem for the diagnosis and stratification of patients for subsequent radiotherapy. ¹⁸ F, however, does not have a suitable therapeutic isotopologue. Here, we demonstrate that the formation of [¹⁸ F][Sc-F] ternary complexes is feasible under mild, aqueous conditions, producing chemically robust radiopharmaceuticals in high radiochemical yield and specific activity. A corresponding in vivo study with a cancer-targeting [¹⁸ F][Sc-F] tracer indicates excellent in vivo stability and produces exquisite PET image quality, rendering the ¹⁸ F/⁴⁷ Sc isotope pair an unusual, yet chemically matched theranostic pair with excellent potential for clinical translation.

Electron transfer pathways in photoexcited lanthanide(iii) complexes of picolinate ligands

A series of luminescent lanthanide(iii) complexes consisting of 1,4,7-triazacyclononane frameworks and three secondary amide-linked carbostyril antennae were synthesised. The metal binding sites were augmented with two pyridylcarboxylate donors yielding octadentate ligands. The antennae carried methyl, methoxymethyl or trifluoromethyl substituents in their 4-positions, allowing for a range of excited state energies and antenna electronic properties. The ¹H NMR spectra of the Eu(iii) complexes were found to be analogous to each other. Similar results were obtained in the solid-state by single-crystal X-ray crystallography, which showed the structures to have nine-coordinate metal ions with heavily distorted tricapped trigonal prismatic geometries. Steady-state and time-resolved luminescence spectroscopy showed that the antennae could sensitize both Tb(iii) and Eu(iii), however, quantum yields were lower than in other octadentate complexes lacking pyridylcarboxylate. Complexes with more electron-poor pyridines were less emissive even when equipped with the same antenna. The oxidation and reduction potentials of the antennae and the pyridinecarboxylates, respectively, were determined by cyclic voltammetry. The obtained values were consistent with electron transfer from the excited antenna to the pyridine providing a previously unexplored quenching pathway that could efficiently compete with energy transfer to the lanthanide. These results show the crucial impact that photophysically innocent ligand binding sites can have on lanthanide luminescence.

Detection of Gadolinium with an Impedimetric Platform Based on Gold Electrodes Functionalized by 2-Methylpyridine-Substituted Cyclam

Gadolinium is extensively used in pharmaceuticals and is very toxic, so its sensitive detection is mandatory. This work presents the elaboration of a gadolinium chemical sensor based on 2-methylpyridine-substituted cyclam thin films, deposited on gold electrodes, using electrochemical impedance spectroscopy (EIS). The 2-methylpyridine-substituted cyclam (bis-N-MPyC) was synthesized in three steps, including the protection of cyclam by the formation of its CH₂-bridged aminal derivative; the product was characterized by liquid ¹H and ¹³C NMR spectroscopy. Spin-coated thin films of bis-N-MPyC on gold wafers were characterized by means of infrared spectroscopy in ATR (Attenuated Total Reflectance) mode, contact angle measurements and atomic force microscopy. The impedimetric chemical sensor was studied in the presence of increasing concentrations of lanthanides (Gd³⁺, Eu³⁺, Tb³⁺, Dy³⁺). Nyquist plots were fitted with an equivalent electrical circuit including two RC circuits in series corresponding to the bis-N-MPyC film and its interface with the electrolyte. The main parameter that varies with gadolinium concentration is the resistance of the film/electrolyte interface (R_p), correlated to the rate of exchange between the proton and the lanthanide ion. Based on this parameter, the detection limit obtained is 35 pM. The bis-N-MPyC modified gold electrode was tested for the detection of gadolinium in spiked diluted negative urine control samples.

Pyclen-Based Ligands Bearing Pendant Picolinate Arms for Gadolinium Complexation

We report the synthesis of two pyclen-based regioisomer ligands (pyclen = 3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene) functionalized with picolinic acid pendant arms either at positions 3,9-pc2pa (L5) or 3,6-pc2pa (L6) of the macrocyclic fragment. The ligands were prepared by the regiospecific protection of one of the amine nitrogen atoms of the macrocycle using Boc and Alloc protecting groups, respectively. The X-ray structure of the Gd(III) complex of L5 contains trinuclear [(GdL5)₃(H₂O)₃]³⁺ entities in which the monomeric units are joined by μ₂-η¹:η¹-carboxylate groups. However, the ¹H and ⁸⁹Y NMR spectra of its Y(III) analogue support the formation of monomeric complexes in solution. The Tb(III) complexes are highly luminescent, with emission quantum yields of up to 28% for [TbL5]⁺. The luminescence lifetimes recorded in H₂O and D₂O solutions indicate the presence of a water molecule coordinated to the metal ion, as also evidenced by the ¹H relaxivities measured for the Gd(III) analogues. The Gd(III) complexes present very different exchange rates of the coordinated water molecule (k_ex²⁹⁸ = 87.1 × 10⁶ and 1.06 × 10⁶ s^-1 for [GdL5]⁺ and [GdL6]⁺, respectively). The very high water exchange rate of [GdL5]⁺ is associated with the steric hindrance originating from the coordination of the ligand around the water binding site, which favors a dissociatively activated water exchange process. The Gd(III) complexes present rather high thermodynamic stabilities (log K_GdL = 20.47 and 19.77 for [GdL5]⁺ and [GdL6]⁺, respectively). Furthermore, these complexes are remarkably inert with respect to their acid-assisted dissociation, in particular the complex of L5.

Design of polyazamacrocyclic Gd3+ theranostic agents combining magnetic resonance imaging and two-photon photodynamic therapy

New “all-in-one” theranostic systems, combining a magnetic resonance imaging contrast agent with a biphotonic photodynamic therapy photosensitiser generating cytotoxic singlet oxygen, were successfully developed and characterized.

Chelation with a twist: a bifunctional chelator to enable room temperature radiolabeling and targeted PET imaging with scandium-44

Scandium-44 has emerged as an attractive, novel PET radioisotope with ideal emission properties and half-life (t 1/2 = 3.97 h, E mean β+ = 632 keV) well matched to the pharmacokinetics of small molecules, peptides and small biologics. Conjugates of the current gold-standard chelator for 44Sc, 1,4,7,10-tetraaza-cyclododecane-1,4,7,10-tetraacetic acid (DOTA), require heating to achieve radiochemical complexation, limiting application of this isotope in conjunction with temperature-sensitive biologics. To establish Sc(iii) isotopes as broadly applicable tools for nuclear medicine, development of alternative bifunctional chelators is required. To address this need, we characterized the Sc(iii)-chelation properties of the small-cavity triaza-macrocycle-based, picolinate-functionalized chelator H3mpatcn. Spectroscopic and radiochemical studies establish the [Sc(mpatcn)] complex as kinetically inert and appropriate for biological applications. A proof-of-concept bifunctional conjugate targeting the prostate-specific membrane antigen (PSMA), picaga-DUPA, chelates 44Sc to form 44Sc(picaga)-DUPA at room temperature with an apparent molar activity of 60 MBq μmol-1 and formation of inert RRR-Λ and SSS-Δ-twist isomers. Sc(picaga)-DUPA exhibits a K i of 1.6 nM for PSMA, comparable to the 18F-based imaging probe DCFPyL (K i = 1.1 nM) currently in phase 3 clinical trials for imaging prostate cancer. Finally, we successfully employed 44Sc(picaga)-DUPA to image PSMA-expressing tumors in a preclinical mouse model, establishing the picaga bifunctional chelator as an optimal choice for the 44Sc PET nuclide.

The role of ligand to metal charge-transfer states on the luminescence of Europium complexes with 18-membered macrocyclic ligands

The stabilization of a divalent Europium provides an efficient pathway for the quenching of the luminescence in ten-coordinate macrocyclic complexes.

Water exchange in lanthanide complexes for MRI applications. Lessons learned over the last 25 years

Coordination chemistry offers convenient strategies to modulate the exchange of coordinated water molecules in lanthanide-based contrast agents.

A new heptadentate picolinate-based ligand and its corresponding Gd(iii) complex: the effect of pendant picolinate versus acetate on complex properties

Replacing one picolinate pendant by acetate group in H₄bpeda ligand, the synthesised bis(aquated) Gd(iii) complex of ligand H4peada showed better stability and r₁ relaxivity for its potential use as MRI contrast agent.

The role of the capping bond effect on pyclen natY3+/90Y3+ chelates: full control of the regiospecific N-functionalization makes the difference

A dissymmetric pyclen based ligand shows astonishing ^natY³⁺ and ⁹⁰Y³⁺ complexation properties.

Sensitisation of visible and NIR lanthanide emission by InPZnS quantum dots in bi-luminescent hybrids

The synthesis of stable hybrid nanoparticles combining InPZnS@ZnSe/ZnS quantum dots (QDs) and grafted lanthanide complexes has been performed using two different approaches in organic and aqueous media. The final bi-luminescent hybrids exhibit Ln(III) (Ln = Eu and Yb) centred luminescence upon QD excitation, suggesting that an energy transfer occurs from the QD to the lanthanide.

Stable Mn2+, Cu2+ and Ln3+ complexes with cyclen-based ligands functionalized with picolinate pendant arms

Cyclen-based ligands containing two picolinate pendant arms form Gd³⁺ complexes remarkably stable and inert with respect to metal ion dissociation.

A pair of highly biotolerated diamagnetic and paramagnetic iron(ii) complexes displaying electroneutrality

A pair of structurally analogous macrocyclic iron(ii) complexes with a magnetic off-on relationship is reported that exhibit electroneutrality at neutral pH and high stability in physiological media.

Design of ProCAs (protein-based Gd(3+) MRI contrast agents) with high dose efficiency and capability for molecular imaging of cancer biomarkers

Magnetic resonance imaging (MRI) is the leading imaging technique for disease diagnostics, providing high resolution, three-dimensional images noninvasively. MRI contrast agents are designed to improve the contrast and sensitivity of MRI. However, current clinically used MRI contrast agents have relaxivities far below the theoretical upper limit, which largely prevent advancing molecular imaging of biomarkers with desired sensitivity and specificity. This review describes current progress in the development of a new class of protein-based MRI contrast agents (ProCAs) with high relaxivity using protein design to optimize the parameters that govern relaxivity. Further, engineering with targeting moiety allows these contrast agents to be applicable for molecular imaging of prostate cancer biomarkers by MRI. The developed protein-based contrast agents also exhibit additional in vitro and in vivo advantages for molecular imaging of disease biomarkers, such as high metal-binding stability and selectivity, reduced toxicity, proper blood circulation time, and higher permeability in tumor tissue in addition to improved relaxivities.

Modulating water-exchange rates of lanthanide(III)-containing polyaminopolycarboxylate-type complexes using polyethylene glycol

We have synthesized a series of Ln(III)-containing polyethylene glycol conjugates and studied the structural and electronic properties of these complexes. These studies demonstrate that polyethylene glycol can be used to fine-tune water-exchange rates of Ln(III)-containing polyaminopolycarboxylate-type complexes; this control is desirable in developing Ln(III)-containing contrast agents for magnetic resonance imaging.

Strategies for optimizing water-exchange rates of lanthanide-based contrast agents for magnetic resonance imaging

This review describes recent advances in strategies for tuning the water-exchange rates of contrast agents for magnetic resonance imaging (MRI). Water-exchange rates play a critical role in determining the efficiency of contrast agents; consequently, optimization of water-exchange rates, among other parameters, is necessary to achieve high efficiencies. This need has resulted in extensive research efforts to modulate water-exchange rates by chemically altering the coordination environments of the metal complexes that function as contrast agents. The focus of this review is coordination-chemistry-based strategies used to tune the water-exchange rates of lanthanide(III)-based contrast agents for MRI. Emphasis will be given to results published in the 21st century, as well as implications of these strategies on the design of contrast agents.

Is macrocycle a synonym for kinetic inertness in Gd(III) Complexes? Effect of coordinating and noncoordinating substituents on inertness and relaxivity of Gd(III) chelates with DO3A-like ligands

Gadolinium chelates with octadentate ligands are widely used as contrast agents for magnetic resonance imaging (MRI), with macrocyclic ligands based on DO3A being preferred for the high kinetic inertness of their Gd chelates. A major challenge in the design of new bifunctional MRI probes is the need to control the rotational motion of the chelate, which greatly affects its relaxivity. In this work we explored facile alkylation of a secondary amine in macrocyclic DO3A-like ligands to create a short, achiral linkage to limit the undesired internal motion of chelates within larger molecular constructs. The acetate moiety on the trans nitrogen was also replaced with either a bidentate (ethoxyacetate, L1 or methyl picolinate, L2) or bulky monodentate (methyl phosphonate, L3) donor arm to give octa- or heptadentate ligands, respectively. The resultant Gd(III) complexes were all monohydrated (q = 1) and exhibited water residency times that spanned 2 orders of magnitude (τM = 2190 ± 170, 3500 ± 90, and 12.7 ± 3.8 ns at 37 °C for GdL1, GdL2, and GdL3, respectively). Alkylation of the secondary amine with a noncoordinating biphenyl moiety resulted in coordinatively saturated q = 0 complexes of octadentate ligands L1 and L2. Relaxivities were limited by slow water exchange and/or lack of water coligand. All complexes showed decreased inertness compared to [Gd(DO3A)] despite higher ligand denticity, and inertness was further decreased upon N-alkylation. These results demonstrate that high kinetic inertness and in vivo safety of Gd chelates with macrocyclic ligands should not be generalized.

Lanthanide dota-like complexes containing a picolinate pendant: structural entry for the design of Ln(III)-based luminescent probes

In this contribution we present two ligands based on a do3a platform containing a picolinate group attached to the fourth nitrogen atom of the cyclen unit, which are designed for stable lanthanide complexation in aqueous solutions. Potentiometric measurements reveal that the thermodynamic stability of the complexes is very high (log K = 21.2-23.5), being comparable to that of the dota analogues. Luminescence lifetime measurements performed on solutions of the Eu(III) and Tb(III) complexes indicate that the complexes are nine coordinate with no inner-sphere water molecules. A combination of density functional theory (DFT) calculations and NMR measurements shows that for the complexes of the heaviest lanthanides there is a major isomer in solution consisting of the enantiomeric pair Λ(δδδδ) and Δ(λλλλ), which provides square antiprismatic coordination (SAP) around the metal ion. Analysis of the Yb(III)-induced paramagnetic shifts unambiguously confirms that these complexes have SAP coordination in aqueous solution. For the light lanthanide ions however both the SAP and twisted-square antiprismatic (TSAP) isomers are present in solution. Inversion of the cyclen ring appears to be the rate-determining step for the Λ(δδδδ) ↔ Δ(λλλλ) enantiomerization process observed in the Lu(III) complexes. The energy barriers obtained from NMR measurements for this dynamic process are in excellent agreement with those predicted by DFT calculations. The energy barriers calculated for the arm-rotation process are considerably lower than those obtained for the ring-inversion path. Kinetic studies show that replacement of an acetate arm of dota by a picolinate pendant results in a 3-fold increase in the formation rate of the corresponding Eu(III) complexes and a significant increase of the rates of acid-catalyzed dissociation of the complexes. However, these rates are 1-2 orders of magnitude lower than those of do3a analogues, which shows that the complexes reported herein are remarkably inert with respect to metal ion dissociation.

The effect of ring size variation on the structure and stability of lanthanide(III) complexes with crown ethers containing picolinate pendants

The coordination properties of the macrocyclic receptor N,N'-bis[(6-carboxy-2-pyridyl)methylene]-1,10-diaza-15-crown-5 (H(2)bp15c5) towards the lanthanide ions are reported. Thermodynamic stability constants were determined by pH-potentiometric titration at 25 °C in 0.1 M KCl. A smooth decrease in complex stability is observed upon decreasing the ionic radius of the Ln(III) ion from La [log K(LaL) = 12.52(2)] to Lu [log K(LuL) = 10.03(6)]. Luminescence lifetime measurements recorded on solutions of the Eu(III) and Tb(III) complexes confirm the absence of inner-sphere water molecules in these complexes. (1)H and (13)C NMR spectra of the complexes formed with the diamagnetic La(III) metal ion were obtained in D(2)O solution and assigned with the aid of HSQC and HMBC 2D heteronuclear experiments, as well as standard 2D homonuclear COSY and NOESY spectra. The (1)H NMR spectra of the paramagnetic Ce(III), Eu(III) and Yb(III) complex suggest nonadentate binding of the ligand to the metal ion. The syn conformation of the ligand in [Ln(bp15c5)](+) complexes implies the occurrence of two helicities, one associated with the layout of the picolinate pendant arms (absolute configuration Δ or Λ), and the other to the five five-membered chelate rings formed by the binding of the crown moiety (absolute configuration δ or λ). A detailed conformational analysis performed with the aid of DFT calculations (B3LYP model) indicates that the complexes adopt a Λ(λδ)(δδλ) [or Δ(δλ)(λλδ)] conformation in aqueous solution. Our calculations show that the interaction between the Ln(III) ion and several donor atoms of the crown moiety is weakened as the ionic radius of the metal ion decreases, in line with the decrease of complex stability observed on proceeding to the right across the lanthanide series.