Eight-coordinate Zn(II), Cd(II), and Pb(II) complexes based on a 1,7-diaza-12-crown-4 platform endowed with a remarkable selectivity over Ca(II)

Tuning the Kinetic Inertness of Bi3+ Complexes: The Impact of Donor Atoms on Diaza-18-Crown-6 Ligands as Chelators for 213Bi Targeted Alpha Therapy

The radionuclide ²¹³Bi can be applied for targeted α therapy (TAT): a type of nuclear medicine that harnesses α particles to eradicate cancer cells. To use this radionuclide for this application, a bifunctional chelator (BFC) is needed to attach it to a biological targeting vector that can deliver it selectively to cancer cells. Here, we investigated six macrocyclic ligands as potential BFCs, fully characterizing the Bi³⁺ complexes by NMR spectroscopy, mass spectrometry, and elemental analysis. Solid-state structures of three complexes revealed distorted coordination geometries about the Bi³⁺ center arising from the stereochemically active 6s² lone pair. The kinetic properties of the Bi³⁺ complexes were assessed by challenging them with a 1000-fold excess of the chelating agent diethylenetriaminepentaacetic acid (DTPA). The most kinetically inert complexes contained the most basic pendent donors. Density functional theory (DFT) and quantum theory of atoms in molecules (QTAIM) calculations were employed to investigate this trend, suggesting that the kinetic inertness is not correlated with the extent of the 6s² lone pair stereochemical activity, but with the extent of covalency between pendent donors. Lastly, radiolabeling studies of ²¹³Bi (30-210 kBq) with three of the most promising ligands showed rapid formation of the radiolabeled complexes at room temperature within 8 min for ligand concentrations as low as 10^-7 M, corresponding to radiochemical yields of >80%, thereby demonstrating the promise of this ligand class for use in ²¹³Bi TAT.

Expanding the Ligand Classes Used for Mn(II) Complexation: Oxa-aza Macrocycles Make the Difference

We report two macrocyclic ligands based on a 1,7-diaza-12-crown-4 platform functionalized with acetate (tO2DO2A²⁻) or piperidineacetamide (tO2DO2AM^Pip) pendant arms and a detailed characterization of the corresponding Mn(II) complexes. The X−ray structure of [Mn(tO2DO2A)(H₂O)]·2H₂O shows that the metal ion is coordinated by six donor atoms of the macrocyclic ligand and one water molecule, to result in seven-coordination. The Cu(II) analogue presents a distorted octahedral coordination environment. The protonation constants of the ligands and the stability constants of the complexes formed with Mn(II) and other biologically relevant metal ions (Mg(II), Ca(II), Cu(II) and Zn(II)) were determined using potentiometric titrations (I = 0.15 M NaCl, T = 25 °C). The conditional stabilities of Mn(II) complexes at pH 7.4 are comparable to those reported for the cyclen-based tDO2A²⁻ ligand. The dissociation of the Mn(II) chelates were investigated by evaluating the rate constants of metal exchange reactions with Cu(II) under acidic conditions (I = 0.15 M NaCl, T = 25 °C). Dissociation of the [Mn(tO2DO2A)(H₂O)] complex occurs through both proton− and metal−assisted pathways, while the [Mn(tO2DO2AM^Pip)(H₂O)] analogue dissociates through spontaneous and proton-assisted mechanisms. The Mn(II) complex of tO2DO2A²⁻ is remarkably inert with respect to its dissociation, while the amide analogue is significantly more labile. The presence of a water molecule coordinated to Mn(II) imparts relatively high relaxivities to the complexes. The parameters determining this key property were investigated using ¹⁷O NMR (Nuclear Magnetic Resonance) transverse relaxation rates and ¹H nuclear magnetic relaxation dispersion (NMRD) profiles.

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.

On the consequences of the stereochemical activity of the Bi(iii) 6s2 lone pair in cyclen-based complexes. The [Bi(DO3A)] case

The spatial arrangement of donor atoms in Bi(iii) cyclen derivatives modulates the orientation and activity of the 6s² lone pair.

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.

Nano drug delivery systems: a new paradigm for treating metal toxicity

INTRODUCTION

The standard medical treatment for metal toxicity is chelation therapy. Chelating agents work by forming less toxic complexes with the toxic metal ions which are readily excreted from the body. These compounds, based on their hydrophilic/lipophilic property, can either remove toxic metal ions from extracellular sites or can penetrate the intracellular compartments to facilitate the removal of toxic metal ions. However, there are various disadvantages associated with this kind of therapy, notably, selectivity. Other problems and challenges are that the therapy regime is expensive, time consuming and has poor patient compliance. Two chelating agents, dimercaptosuccinic acid (DMSA) and dimercaptopropionicsulfonate (DMPS) have gained increased acceptance among clinicians, undoubtedly improving the management of metal intoxications.

AREAS COVERED

The present review provides an insight into the conventional chelating agents, new chelators under development, and the new opportunities presented by the use of nanotherapy for the treatment of metal poisoning cases.

EXPERT OPINION

Today's research should not only focus towards development of alternate chelators but also targeted therapy such as the nanotherapy.

Investigating the Complexation of the Pb(2+)/Bi(3+) Pair with Dipicolinate Cyclen Ligands

The complexation properties toward Pb(2+) and Bi(3+) of the macrocyclic ligands 6,6'-((1,4,7,10-tetraazacyclododecane-1,7-diyl)bis(methylene))dipicolinic acid (H2do2pa) and 6,6'-((4,10-dimethyl-1,4,7,10-tetraazacyclododecane-1,7-diyl)bis(methylene))dipicolinic acid (H2Me-do2pa) have been investigated. A new three-step synthesis of H2do2pa following the bisaminal methodology has also been developed. The X-ray structures of [Pb(Me-do2pa)]·6H2O and [Bi(Me-do2pa)](NO3)·H2O show that the two metal ions are eight-coordinated by the ligand. The two complexes exist as the racemic Δ(δδδδ)/Λ(λλλλ) mixture both in the solid state and in solution, as indicated by NMR and DFT studies. The stability constants of the lead(II) and bismuth(III) complexes of the two ligands were determined in 0.5 M KCl using potentiometric and spectrophotometric techniques. The stability constants determined for the complexes of Pb(2+) are relatively high (log KML = 16.44 and 18.44 for H2do2pa and H2Me-do2pa, respectively) and exceptionally high for the complexes of Bi(3+) (log KML = 32.0 and 34.2 for H2do2pa and H2Me-do2pa, respectively). The [Pb(Me-do2pa)] complex presents rather fast formation and very good kinetic inertness toward transchelation. Additionally, the [Bi(Me-do2pa)](+) complex was found to present a remarkably fast complexation rate (full complexation in ∼2 min at pH 5.0, acetate buffer) and a very good kinetic inertness with respect to metal ion dissociation (half-life of 23.9 min in 1 M HCl), showing promise for potential applications in α-radioimmunotherapy.

Matching chelators to radiometals for radiopharmaceuticals

Radiometals comprise many useful radioactive isotopes of various metallic elements. When properly harnessed, these have valuable emission properties that can be used for diagnostic imaging techniques, such as single photon emission computed tomography (SPECT, e.g.(67)Ga, (99m)Tc, (111)In, (177)Lu) and positron emission tomography (PET, e.g.(68)Ga, (64)Cu, (44)Sc, (86)Y, (89)Zr), as well as therapeutic applications (e.g.(47)Sc, (114m)In, (177)Lu, (90)Y, (212/213)Bi, (212)Pb, (225)Ac, (186/188)Re). A fundamental critical component of a radiometal-based radiopharmaceutical is the chelator, the ligand system that binds the radiometal ion in a tight stable coordination complex so that it can be properly directed to a desirable molecular target in vivo. This article is a guide for selecting the optimal match between chelator and radiometal for use in these systems. The article briefly introduces a selection of relevant and high impact radiometals, and their potential utility to the fields of radiochemistry, nuclear medicine, and molecular imaging. A description of radiometal-based radiopharmaceuticals is provided, and several key design considerations are discussed. The experimental methods by which chelators are assessed for their suitability with a variety of radiometal ions is explained, and a large selection of the most common and most promising chelators are evaluated and discussed for their potential use with a variety of radiometals. Comprehensive tables have been assembled to provide a convenient and accessible overview of the field of radiometal chelating agents.