Stable Chelation of the Uranyl Ion by Acyclic Hexadentate Ligands: Potential Applications for 230U Targeted α-Therapy

Macrocyclic 1,2-Hydroxypyridinone-Based Chelators as Potential Ligands for Thorium-227 and Zirconium-89 Radiopharmaceuticals

Thorium-227 (227Th) is an α-emitting radionuclide that has shown preclinical and clinical promise for use in targeted α-therapy (TAT), a type of molecular radiopharmaceutical treatment that harnesses high energy α particles to eradicate cancerous lesions. Despite these initial successes, there still exists a need for bifunctional chelators that can stably bind thorium in vivo. Toward this goal, we have prepared two macrocyclic chelators bearing 1,2-hydroxypyridinone groups. Both chelators can be synthesized in less than six steps from readily available starting materials, which is an advantage over currently available platforms. The complex formation constants (log βmlh) of these ligands with Zr4+ and Th4+, measured by spectrophotometric titrations, are greater than 34 for both chelators, indicating the formation of exceedingly stable complexes. Radiolabeling studies were performed to show that these ligands can bind [227Th]Th4+ at concentrations as low as 10-6 M, and serum stability experiments demonstrate the high kinetic stability of the formed complexes under biological conditions. Identical experiments with zirconium-89 (89Zr), a positron-emitting radioisotope used for positron emission tomography (PET) imaging, demonstrate that these chelators can also effectively bind Zr4+ with high thermodynamic and kinetic stability. Collectively, the data reported herein highlight the suitability of these ligands for use in 89Zr/227Th paired radioimmunotheranostics.

Rapid Elution of 226Th from a Two-Column 230U/226Th Generator with Diluted and Buffer Solutions

A radionuclide generator of the short-lived alpha emitter ²²⁶Th was proposed. An original scheme consisting of two in-series chromatographic columns was developed for rapidly producing a neutral citric-buffered eluate of high purity ²²⁶Th. The first column filled with TEVA resin retained the parent ²³⁰U, while ²²⁶Th was eluted with 7 M HCl solution to be immediately adsorbed on the second column containing DGA resin or UTEVA resin. Having substituted the strongly acidic medium of second column with neutral salt solution, ²²⁶Th was desorbed with diluted citric buffer solution. One cycle of generator milking took 5-7 min and produced >90% of ²²⁶Th in 1.5 mL of eluate (pH 4.5-5.0) appropriate for direct use in radiopharmaceutical synthesis. The ²³⁰U impurity in ²²⁶Th eluate was less than 0.01%. The proposed two-column ²³⁰U/²²⁶Th generator was tested over 2 months including a second loading of ²³⁰U additionally accumulated from ²³⁰Pa.

Hybrids of 1,4-Quinone with Quinoline Derivatives: Synthesis, Biological Activity, and Molecular Docking with DT-Diaphorase (NQO1)

Hybrids 1,4-quinone with quinoline were obtained by connecting two active structures through an oxygen atom. This strategy allows to obtain new compounds with a high biological activity and suitable bioavailability. Newly synthesized compounds were characterized by various spectroscopic methods. The enzymatic assay used showed that these compounds were a suitable DT-diaphorase (NQO1) substrates as evidenced by increasing enzymatic conversion rates relative to that of streptonigrin. Hybrids were tested in vitro against a panel of human cell lines including melanoma, breast, and lung cancers. They showed also a high cytotoxic activity depending on the type of 1,4-quinone moiety and the applied tumor cell lines. It was found that cytotoxic activity of the studied hybrids was increasing against the cell lines with higher NQO1 protein level, such as breast (MCF-7 and T47D) and lung (A549) cancers. Selected hybrids were tested for the transcriptional activity of the gene encoding a proliferation marker (H3 histone), cell cycle regulators (p53 and p21) and the apoptosis pathway (BCL-2 and BAX). The molecular docking was used to examine the probable interaction between the hybrids and NQO1 protein.

Medicinal inorganic chemistry - challenges, opportunities and guidelines to develop the next generation of radioactive, photoactivated and active site inhibiting metal-based medicines

Medicinal inorganic chemistry is a burgeoning subfield of medicinal chemistry that focuses on the development of metal-based diagnostic and therapeutic agents. This tutorial review aims to provide an introductory primer, present a timely overview of recent discoveries and identify current challenges and opportunities of the field. Three specific areas of discovery are highlighted herein. The first part focuses on metal-based radiopharmaceuticals for diagnostic and therapeutic purposes and specific design criteria for the development of radiopharmaceuticals that combine fundamental aqueous coordination chemistry with elucidation of pharmacokinetics. The second part describes approaches to photodynamic therapy with metal complexes. Here, photophysical characterization, combined with the challenge of careful control of the chemical behavior and selective biological deposition of transition metals with significant off-target toxicity, is discussed. In the third part, we summarize emerging strategies to modulate enzyme inhibition with coordination chemistry, while also highlighting the utility of the unique properties of metal ions for the characterization of mechanisms of action of these emerging diagnostic and therapeutic agents.

Circularly Polarized Luminescence from Uranyl Improves Resolution of Electronic Transitions

The first reported example of circularly polarized luminescence from a chiral, molecular uranyl (UO₂²⁺) complex in solution is presented. This uranyl chiroptical activity is enabled by complexation with ibuprofen, an enantiopure chiral carboxylate ligand. Salt metathesis between [UO₂Cl₂(thf)₂]₂ and the sodium ibuprofenate salts results in the formation of the anionic tris complexes; these complexes are found to be luminescent in solution, both under visible excitation, directly targeting the metal, and through sensitization by UV absorption and energy transfer from the ligand. Each enantiomer displays both circular dichroism and circularly polarized luminescence (CPL) with |g_abs| ≤ 8.1 × 10^-2 and |g_lum| ≤ 8.0 × 10^-3 under UV excitation, comparable to chiral transition metal complexes or purely organic emitters. The strength of the CPL emission is found to be comparable following excitation of either the ligand or metal directly. Further, use of CPL allows for resolution of subcomponents of the emission spectrum not previously possible at room temperature using standard fluorescence techniques. Observation of CPL following direct uranyl excitation presents a new tool for probing speciation of uranyl complexes when chiral ligands are used, without the need for synthetic modification to incorporate a suitable chromophore, and could enable the design of improved ligands for uranyl extraction from wastewater.

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.