Chelating agents for radiolanthanides: Applications to imaging and therapy

161Tb-DOTATOC Production Using a Fully Automated Disposable Cassette System: A First Step Toward the Introduction of 161Tb into the Clinic

¹⁶¹Tb is an interesting radionuclide for application in the treatment of neuroendocrine neoplasms' small metastases and single cancer cells because of its conversion and Auger-electron emission. Tb has coordination chemistry similar to that of Lu; therefore, like ¹⁷⁷Lu, it can stably radiolabel DOTATOC, one of the leading peptides used for the treatment of neuroendocrine neoplasms. However, ¹⁶¹Tb is a recently developed radionuclide that has not yet been specified for clinical use. Therefore, the aim of the current work was to characterize and specify ¹⁶¹Tb and to develop a protocol for the synthesis and quality control of ¹⁶¹Tb-DOTATOC with a fully automated process conforming to good-manufacturing-practice guidelines, in view of its clinical use. Methods: ¹⁶¹Tb, produced by neutron irradiation of ¹⁶⁰Gd in high-flux reactors followed by radiochemical separation from its target material, was characterized regarding its radionuclidic purity, chemical purity, endotoxin level, and radiochemical purity (RCP) in analogy to what is described in the European Pharmacopoeia for no-carrier-added ¹⁷⁷Lu. In addition, ¹⁶¹Tb was introduced into a fully automated cassette-module synthesis to produce ¹⁶¹Tb-DOTATOC, as used for ¹⁷⁷Lu-DOTATOC. The quality and stability of the produced radiopharmaceutical in terms of identity, RCP, and ethanol and endotoxin content were assessed by means of high-performance liquid chromatography, gas chromatography, and an endotoxin test, respectively. Results: ¹⁶¹Tb produced under the described conditions showed, as the no-carrier-added ¹⁷⁷Lu, a pH of 1-2, radionuclidic purity and RCP of more than 99.9%, and an endotoxin level below the permitted range (175 IU/mL), indicating its appropriate quality for clinical use. In addition, an efficient and robust procedure for the automated production and quality control of ¹⁶¹Tb-DOTATOC with clinically applicable specifications and activity levels, that is, 1.0-7.4 GBq in 20 mL, was developed. The radiopharmaceutical's quality control was also developed using chromatographic methods, which confirmed the product's stability (RCP ≥ 95%) over 24 h. Conclusion: The current study demonstrated that ¹⁶¹Tb has appropriate features for clinical use. The developed synthesis protocol guarantees high yields and safe preparation of injectable ¹⁶¹Tb-DOTATOC. The investigated approach could be translated to other DOTA-derivatized peptides; thus, ¹⁶¹Tb could be successfully applied in clinical practice for radionuclide therapy.

Increasing Reaction Rates of Water-Soluble Porphyrins for 64Cu Radiopharmaceutical Labeling

Searching for new compounds and synthetic routes for medical applications is a great challenge for modern chemistry. Porphyrins, natural macrocycles able to tightly bind metal ions, can serve as complexing and delivering agents in nuclear medicine diagnostic imaging utilizing radioactive nuclides of copper with particular emphasis on ⁶⁴Cu. This nuclide can, due to multiple decay modes, serve also as a therapeutic agent. As the complexation reaction of porphyrins suffers from relatively poor kinetics, the aim of this study was to optimize the reaction of copper ions with various water-soluble porphyrins in terms of time and chemical conditions, that would meet pharmaceutical requirements and to develop a method that can be applied for various water-soluble porphyrins. In the first method, reactions were conducted in a presence of a reducing agent (ascorbic acid). Optimal conditions, in which the reaction time was 1 min, comprised borate buffer at pH 9 with a 10-fold excess of ascorbic acid over Cu²⁺. The second approach involved a microwave-assisted synthesis at 140 °C for 1-2 min. The proposed method with ascorbic acid was applied for radiolabeling of porphyrin with ⁶⁴Cu. The complex was then subjected to a purification procedure and the final product was identified using high-performance liquid chromatography with radiometric detection.

Cutting edge rare earth radiometals: prospects for cancer theranostics

Background

With recent advances in novel approaches to cancer therapy and imaging, the application of theranostic techniques in personalised medicine has emerged as a very promising avenue of research inquiry in recent years. Interest has been directed towards the theranostic potential of Rare Earth radiometals due to their closely related chemical properties which allow for their facile and interchangeable incorporation into identical bifunctional chelators or targeting biomolecules for use in a diverse range of cancer imaging and therapeutic applications without additional modification, i.e. a “one-size-fits-all” approach. This review will focus on recent progress and innovations in the area of Rare Earth radionuclides for theranostic applications by providing a detailed snapshot of their current state of production by means of nuclear reactions, subsequent promising theranostic capabilities in the clinic, as well as a discussion of factors that have impacted upon their progress through the theranostic drug development pipeline.

Main body

In light of this interest, a great deal of research has also been focussed towards certain under-utilised Rare Earth radionuclides with diverse and favourable decay characteristics which span the broad spectrum of most cancer imaging and therapeutic applications, with potential nuclides suitable for α-therapy (¹⁴⁹Tb), β⁻-therapy (⁴⁷Sc, ¹⁶¹Tb, ¹⁶⁶Ho, ¹⁵³Sm, ¹⁶⁹Er, ¹⁴⁹Pm, ¹⁴³Pr, ¹⁷⁰Tm), Auger electron (AE) therapy (¹⁶¹Tb, ¹³⁵La, ¹⁶⁵Er), positron emission tomography (⁴³Sc, ⁴⁴Sc, ¹⁴⁹Tb, ¹⁵²Tb, ¹³²La, ¹³³La), and single photon emission computed tomography (⁴⁷Sc, ¹⁵⁵Tb, ¹⁵²Tb, ¹⁶¹Tb, ¹⁶⁶Ho, ¹⁵³Sm, ¹⁴⁹Pm, ¹⁷⁰Tm). For a number of the aforementioned radionuclides, their progression from ‘bench to bedside’ has been hamstrung by lack of availability due to production and purification methods requiring further optimisation.

Conclusions

In order to exploit the potential of these radionuclides, reliable and economical production and purification methods that provide the desired radionuclides in high yield and purity are required. With more reactors around the world being decommissioned in future, solutions to radionuclide production issues will likely be found in a greater focus on linear accelerator and cyclotron infrastructure and production methods, as well as mass separation methods. Recent progress towards the optimisation of these and other radionuclide production and purification methods has increased the feasibility of utilising Rare Earth radiometals in both preclinical and clinical settings, thereby placing them at the forefront of radiometals research for cancer theranostics.

A general and stereoselective approach to 14-membered cyclic bis-semicarbazones involving BF3-catalyzed amidoalkylation of 2-(trimethylsilyloxy)propene

A general and stereoselective five-step approach to 14-membered cyclic bis-semicarbazones, 5,12-diaryl-7,14-dimethyl-1,2,4,8,9,11-hexaazacyclotetradeca-7,14-diene-3,10-diones, starting from aldehyde semicarbazones has been developed. The key intermediates, 4-(3-oxobut-1-yl)semicarbazones, were prepared by BF₃-catalyzed amidoalkylation of 2-(trimethylsilyloxy)propene with 4-[(aryl)(methoxy)methyl]- or 4-[(aryl)(tosyl)methyl]semicarbazones. Treatment of these intermediates with excess of hydrazine gave hydrazones of 4-(3-oxobut-1-yl)semicarbazones or 4-(3-oxobut-1-yl)semicarbazides, which in the presence of TsOH were converted into the target macrocycles. All steps of this approach could be scaled up easily to the multi-gram level.

Porphyrins as Chelating Agents for Molecular Imaging in Nuclear Medicine

Porphyrin ligands, showing a significant affinity for cancer cells, also have the ability to chelate metallic radioisotopes to form potential diagnostic radiopharmaceuticals. They can be applied in single-photon emission computed tomography (SPECT) and positron emission tomography (PET) to evaluate metabolic changes in the human body for tumor diagnostics. The aim of this paper is to present a short overview of the main metallic radionuclides complexed by porphyrin ligands and used in these techniques. These chelation reactions are discussed in terms of the complexation conditions and kinetics and the complex stability.

Alkyl-Substituted Aminobis(phosphonates)-Efficient Precipitating Agents for Rare Earth Elements, Thorium, and Uranium in Aqueous Solutions

The efficient and environmentally sustainable separation process for rare earth elements (REE), especially for adjacent lanthanoids, remains a challenge due to the chemical similarity of REEs. Tetravalent actinoids, thorium, and traces of uranium are also present in concentrates of REEs, making their separation relevant. This study reports six simple water-soluble aminobis(phosphonate) ligands, RN[CH2P(O)(OH)2]2 (1 R = CH2CH3, 2 R = (CH2)2CH3, 3 R = (CH2)3CH3, 4 R = (CH2)4CH3, 5 R = (CH2)5CH3, 6 R = CH2CH(C2H5)(CH2)3CH3) as precipitating agents for REEs, Th, and U, as well as gives insight into the coordination modes of the utilized ligands with REEs at the molecular level. Aminobis(phosphonates) 4-6 with longer carbon chains were found to separate selectively thorium, uranium, and scandium from REEs with short precipitation time (15 min) and excellent separation factors that generally range from 100 to 2000 in acidic aqueous solution. Ligands 1-6 also improved separation factors for adjacent lanthanoids in comparison to traditional oxalate precipitation agents. Importantly, precipitated metals can be recovered from the ligands with 3 molar HNO3 with no observed ligand decomposition enabling the possibility of recycling the ligands in the separation process. NMR-monitored pH titrations for 1 showed deprotonation steps at pK a 1.3, 5.55, and >10.5, which indicate that the ligands remain in a deprotonated [L]-1 form in the pH range of 0-4 used in the precipitation studies. 31P NMR titration studies between 1 and M(NO3)3 (M = Y, La, Lu) gave satisfactory fits for 1:3, 1:2, and 1:1 metal-ligand stoichiometries for Y, La, and Lu, respectively, according to an F-test. Therefore, aminobis(phosphonate) precipitation agents 1-6 are likely to form metal complexes with fewer ligands than traditional separation agents like DEHPA, which coordinates to REEs in 1:6 metal-ligand ratio.

Radiolabeling of Human Serum Albumin With Terbium-161 Using Mild Conditions and Evaluation of in vivo Stability

Targeted radionuclide therapy (TRNT) is a promising approach for cancer therapy. Terbium has four medically interesting isotopes (¹⁴⁹Tb, ¹⁵²Tb, ¹⁵⁵Tb and ¹⁶¹Tb) which span the entire radiopharmaceutical space (TRNT, PET and SPECT imaging). Since the same element is used, accessing the various diagnostic or therapeutic properties without changing radiochemical procedures and pharmacokinetic properties is advantageous. The use of (heat-sensitive) biomolecules as vector molecule with high affinity and selectivity for a certain molecular target is promising. However, mild radiolabeling conditions are required to prevent thermal degradation of the biomolecule. Herein, we report the evaluation of potential bifunctional chelators for Tb-labeling of heat-sensitive biomolecules using human serum albumin (HSA) to assess the in vivo stability of the constructs. p-SCN-Bn-CHX-A”-DTPA, p-SCN-Bn-DOTA, p-NCS-Bz-DOTA-GA and p-SCN-3p-C-NETA were conjugated to HSA via a lysine coupling method. All HSA-constructs were labeled with [¹⁶¹Tb]TbCl₃ at 40°C with radiochemical yields higher than 98%. The radiolabeled constructs were stable in human serum up to 24 h at 37°C. ¹⁶¹Tb-HSA-constructs were injected in mice to evaluate their in vivo stability. Increasing bone accumulation as a function of time was observed for [¹⁶¹Tb]TbCl₃ and [¹⁶¹Tb]Tb-DTPA-CHX-A”-Bn-HSA, while negligible bone uptake was observed with the DOTA, DOTA-GA and NETA variants over a 7-day period. The results indicate that the p-SCN-Bn-DOTA, p-NCS-Bz-DOTA-GA and p-SCN-3p-C-NETA are suitable bifunctional ligands for Tb-based radiopharmaceuticals, allowing for high yield radiolabeling in mild conditions.

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.

Water-Soluble Rhenium Phosphine Complexes Incorporating the Ph2C(X) Motif (X = O-, NH-): Structural and Cytotoxicity Studies

Reaction of [ReOCl₃(PPh₃)₂] or [ReO₂I(PPh₃)₂] with 2,2'-diphenylglycine (dpgH₂) in refluxing ethanol afforded the air-stable complex [ReO(dpgH)(dpg)(PPh₃)] (1). Treatment of [ReO(OEt)I₂(PPh₃)₂] with 1,2,3-triaza-7-phosphaadamantane (PTA) afforded the complex [ReO(OEt)I₂(PTA)₂] (2). Reaction of [ReOI₂(PTA)₃] with dpgH₂ led to the isolation of the complex [Re(NCPh₂)I₂(PTA)₃]·0.5EtOH (3·0.5EtOH). A similar reaction but using [ReOX₂(PTA)₃] (X = Cl, Br) resulted in the analogous halide complexes [Re(NCPh₂)Cl₂(PTA)₃]·2EtOH (4·2EtOH) and [Re(NCPh₂)(PTA)₃Br₂]·1.6EtOH (5·1.6EtOH). Using benzilic acid (2,2'-diphenylglycolic acid, benzH) with 2 afforded the complex [ReO(benz)₂(PTA)][PTAH]·EtOH (6·EtOH). The potential for the formation of complexes using radioisotopes with relatively short half-lives suitable for nuclear medicine applications by developing conditions for [Re(NCPh₂)(dpg)I(PTA)₃] (7)[ReO₄]^- in a 4 h time scale was investigated. A procedure for the technetium analog of complex [Re(NCPh₂)I₂(PTA)₃] (3) from ^99mTc[TcO₄]^- was then investigated. The molecular structures of 1-7 are reported; complexes 3-7 have been studied using in vitro cell assays (HeLa, HCT116, HT-29, and HEK 293) and were found to have IC₅₀ values in the range of 29-1858 μM.

The solid-state structures and ligand cavity evaluation of lanthanide(iii) complexes of a DOTA analogue with a (dibenzylamino)methylphosphinate pendant arm

A series of lanthanide(iii) complexes of a monophosphinate analogue of H4dota, 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic-10-methyl[(N,N-dibenzylamino)methyl]phosphinic acid (H4do3apDBAm = H4L1), were prepared and their solid-state structures were studied using single-crystal X-ray diffraction. In all structures, the ligand anion was octadentately coordinated to the Ln(iii) or Sc(iii) ions similarly to other DOTA-like ligands, i.e. forming parallel N4- and O4-planes. The lighter lanthanide(iii) complexes (till dysprosium) were nonacoordinated in the twisted square-antiprismatic (TSA) configuration with the apical coordination of water molecules or oxygen atoms from the neighbouring complex unit. The heavier lanthanide(iii) complexes (from terbium) were found as the "anhydrous" octacoordinated twisted square-antiprismatic (TSA') isomer. For the terbium(iii) ion, both forms were structurally characterized. The structural data of the Ln(iii)-H4L1 complexes and complexes of several related DOTA-like ligands were analysed. It clearly showed that the structural parameters for the square-antiprismatic (SA) isomers were clustered in a small range while those for the TSA/TSA' isomers were significantly more spread. The analysis also gave useful information about the influence of various pendant arms on the structure of the complexes of the DOTA-like ligands. The twist angle (torsion) of the chelate ring containing a larger phosphorus atom was similar to those of the remaining three acetate pendants. It led to a larger separation of the N4O4 planes and to smaller trans-O-Ln-O angles than the parameters found in the complexes of H4dota and its tetraamide derivatives dotam(R). It resulted in a relatively long bond between the metal ion and the coordinated water molecule. It led, together with the negative charge of the oxygen atoms forming the O4-plane, to an extremely fast water exchange rate reported for the Gd(iii)-H4L1 complex and, generally, to a fast water exchange of Gd(iii) complexes with the monophosphorus acid analogues of H4dota, H5do3ap/H4do3apR.

Stability of europium(ii) in aqueous nitrate solutions

In the lanthanide series, Eu3+ is most easily reduced to its divalent state. Reduction of Eu3+ has been studied extensively in aqueous media that are insensitive to reducing conditions. Recently, it has been reported that reduction of Eu3+ is also feasible in aqueous nitrate solutions and that Eu2+ remained sufficiently stable in these media to conduct separation experiments. However, additional fundamental research on the reduction efficiency of Eu3+ and stability of Eu2+ in these media has not been reported yet. In this paper, cyclic voltammetry, magnetic susceptibility measurements, UV-vis absorption spectroscopy and X-ray absorption near edge structure (XANES) spectroscopy were used to gain more insights into the reduction of Eu3+ in aqueous nitrate media. Within the parameters used in this work, near-quantitative reduction of Eu3+ could be achieved within 120 min in highly concentrated nitrate salt solutions, using both chemical and electrochemical reduction techniques. Moreover, Eu2+ was remarkably stable in these solutions, showing just a small percentage of back-oxidation after 5 h in a sealed measurement cell.

Coordination Behavior of 1,4-Disubstituted Cyclen Endowed with Phosphonate, Phosphonate Monoethylester, and H-Phosphinate Pendant Arms

Three 1,4,7,10-tetraazacyclododecane-based ligands disubstituted in 1,4-positions with phosphonic acid, phosphonate monoethyl-ester, and H-phosphinic acid pendant arms, 1,4-H4do2p, 1,4-H2do2pOEt, and 1,4-H2Bn2do2pH, were synthesized and their coordination to selected metal ions, Mg(II), Ca(II), Mn(II), Zn(II), Cu(II), Eu(III), Gd(III), and Tb(III), was investigated. The solid-state structure of the phosphonate ligand, 1,4-H4do2p, was determined by single-crystal X-ray diffraction. Protonation constants of the ligands and stability constants of their complexes were obtained by potentiometry, and their values are comparable to those of previously studied analogous 1,7-disubstitued cyclen derivatives. The Gd(III) complex of 1,4-H4do2p is ~1 order of magnitude more stable than the Gd(III) complex of the 1,7-analogue, probably due to the disubstituted ethylenediamine-like structural motif in 1,4-H4do2p enabling more efficient wrapping of the metal ion. Stability of Gd(III)-1,4-H2do2pOEt and Gd(III)-H2Bn2do2pH complexes is low and the constants cannot be determined due to precipitation of the metal hydroxide. Protonations of the Cu(II), Zn(II), and Gd(III) complexes probably takes place on the coordinated phosphonate groups. Complexes of Mn(II) and alkali-earth metal ions are significantly less stable and are not formed in acidic solutions. Potential presence of water molecule(s) in the coordination spheres of the Mn(II) and Ln(III) complexes was studied by variable-temperature NMR experiments. The Mn(II) complexes of the ligands are not hydrated. The Gd(III)-1,4-H4do2p complex undergoes hydration equilibrium between mono- and bis-hydrated species. Presence of two-species equilibrium was confirmed by UV-Vis spectroscopy of the Eu(III)-1,4-H4do2p complex and hydration states were also determined by luminescence measurements of the Eu(III)/Tb(III)-1,4-H4do2p complexes.

Multifunctionality in Two Families of Dinuclear Lanthanide(III) Complexes with a Tridentate Schiff-Base Ligand

The employment of N-(2-carboxyphenyl)salicylideneimine in 4f metal chemistry has led to two families of dinuclear complexes depending on the lanthanide(III) used. Representative members exhibit interesting magnetic, optical, and catalytic properties.

Combinatorial design of multimeric chelating peptoids for selective metal coordination

The combinatorial synthesis of a new library of tetrameric peptoid ligands is introduced, enabling coordination and characterization of f-block metals.

Current methods for metal chelation are generally based on multidentate organic ligands, which are generated through cumbersome multistep synthetic processes that lack flexibility for systematically varying metal-binding motifs. Octadentate ligands incorporating hydroxypyridinone or catecholamide binding moieties onto a spermine scaffold are known to display some of the highest affinities towards f-elements. Enhancing binding affinity for specific lanthanide or actinide ions however, necessitates ligand architectures that allow for modular and high throughput synthesis. Here we introduce a high-throughput combinatorial library of 16 tetrameric N-substituted glycine oligomers (peptoids) containing hydroxypyridinone or catecholamide chelating units linked via an ethylenediamine bridge and, for comparison, we also synthesized the corresponding mixed ligands derived from the spermine scaffold: 3,4,3-LI(1,2-HOPO)₂(CAM)₂ and 3,4,3-LI(CAM)₂(1,2-HOPO)₂. Coordination-based luminescence studies were carried out with Eu³⁺ and Tb³⁺ to begin probing the properties of the new ligand architecture and revealed higher sensitization efficiency with the spermine scaffold as well as different spectroscopic features among the structural peptoid isomers. Solution thermodynamic properties of selected ligands revealed different coordination properties between the spermine and peptoid analogues with Eu³⁺ stability constants log β₁₁₀ ranging from 28.88 ± 3.45 to 43.97 ± 0.49. The general synthetic strategy presented here paves the way for precision design of new specific and versatile ligands, with a variety of applications tailored towards the use of f-elements, including separations, optical device optimization, and pharmaceutical development.

Radioactive Main Group and Rare Earth Metals for Imaging and Therapy

Radiometals possess an exceptional breadth of decay properties and have been applied to medicine with great success for several decades. The majority of current clinical use involves diagnostic procedures, which use either positron-emission tomography (PET) or single-photon imaging to detect anatomic abnormalities that are difficult to visualize using conventional imaging techniques (e.g., MRI and X-ray). The potential of therapeutic radiometals has more recently been realized and relies on ionizing radiation to induce irreversible DNA damage, resulting in cell death. In both cases, radiopharmaceutical development has been largely geared toward the field of oncology; thus, selective tumor targeting is often essential for efficacious drug use. To this end, the rational design of four-component radiopharmaceuticals has become popularized. This Review introduces fundamental concepts of drug design and applications, with particular emphasis on bifunctional chelators (BFCs), which ensure secure consolidation of the radiometal and targeting vector and are integral for optimal drug performance. Also presented are detailed accounts of production, chelation chemistry, and biological use of selected main group and rare earth radiometals.

Probing Optical Anisotropy and Polymorph-Dependent Photoluminescence in [Ln2 ] Complexes by Hyperspectral Imaging on Single Crystals

Two homodinuclear and one heterodinuclear lanthanide (Ln)-based complexes of the general formula [Ln₂ (bpm)(tfaa)₆ ] (Ln=Eu (1), Tb (2), Eu-Tb (3), bpm=2,2'-bipyrimidine, tfaa^- =1,1,1-trifluoroacetylacetonate) were synthesized and characterized by single-crystal photoluminescence spectroscopy and hyperspectral imaging. Complexes 1 and 2 crystallize in two polymorphic structures, while three polymorphs were isolated for 3, namely having needle-, plate-, and block-like morphologies. Single-crystal photoluminescence spectroscopy and imaging on Eu³⁺ -containing 1 and 3 revealed polymorph-dependent J-splitting of the hypersensitive ⁵ D₀ →⁷ F₂ Eu³⁺ transition as well as electric-to-magnetic dipole emission intensity ratios. According to these observations, the lowest symmetry chemical environment was attributed to the Eu³⁺ ions present in the needle-like polymorph, also in agreement with single-crystal X-ray diffraction analysis. More importantly, hyperspectral imaging on all three single-crystal polymorphs of 3 exhibits optical anisotropy with photoluminescence enhancement at specific crystallographic faces. This behavior was ascribed to the distinct molecular packing of the Ln-Ln dimers in each polymorphic crystal as well as to face-specific local symmetry of the Eu³⁺ centers. Overall, opto-structural relationships of three Ln-Ln dimers and their single-crystal polymorphs were established as a particularly promising avenue for control of photoluminescence by chemical crystal engineering.

Separation of samarium and europium by solvent extraction with an undiluted quaternary ammonium ionic liquid: towards high-purity medical samarium-153

Long-lived europium-154 impurities are formed during the production of medical samarium-153 in a high-flux nuclear reactor. A method to separate these europium impurities from samarium was investigated using the hydrophobic quaternary ammonium ionic liquid Aliquat 336 nitrate. The separation method consists of the selective reduction of Eu³⁺ by zinc metal in an aqueous feed solution containing a high nitrate salt concentration. Subsequent extraction using undiluted Aliquat 336 nitrate leads to an efficient separation of both lanthanides in a relatively short time frame. Sm³⁺ was extracted to the neat ionic liquid phase much more efficiently than Eu²⁺. An initial approach using the addition of dicyclohexano-18-crown-6 to capture Eu²⁺ in the ionic liquid phase was less efficient.

After selective reduction of europium(iii), samarium is separated from europium by Aliquat 336 nitrate with high separation factors.

A cocktail of 165Er(iii) and Gd(iii) complexes for quantitative detection of zinc using SPECT and MRI

Quantitative zinc determination by nuclear and MR imaging using two Ln³⁺ complexes, including purified ¹⁶⁵Er³⁺, indispensable for metal ion quantification.

Stoichiometry of lanthanide(iii) complexes with tripodal aminophosphonic ligands – a new solution to an old problem

The [Eu(NP₂py)₂]⁵⁻ complex crystallized as a [C(NH₂)₃]₅[Eu(NP₂py)₂]·12 compound. However, the formation of the [Ln(NP₂py)₂]^5– species in aqueous solution starts at pH as high as 8.

H4octapa: synthesis, solution equilibria and complexes with useful radiopharmaceutical metal ions

H₄octapa is synthesized and complexed to nine metals of medicinal interest. Crystal structures of the ligand and its La complex were obtained. Solution equilibria for the ligand and several lanthanide complexes were investigated.