Developing scandium and yttrium coordination chemistry to advance theranostic radiopharmaceuticals

Actinium chelation and crystallization in a macromolecular scaffold

Targeted alpha therapy (TAT) pairs the specificity of antigen targeting with the lethality of alpha particles to eradicate cancerous cells. Actinium-225 [225Ac; t1/2 = 9.920(3) days] is an alpha-emitting radioisotope driving the next generation of TAT radiopharmaceuticals. Despite promising clinical results, a fundamental understanding of Ac coordination chemistry lags behind the rest of the Periodic Table due to its limited availability, lack of stable isotopes, and inadequate systems poised to probe the chemical behavior of this radionuclide. In this work, we demonstrate a platform that combines an 8-coordinate synthetic ligand and a mammalian protein to characterize the solution and solid-state behavior of the longest-lived Ac isotope, 227Ac [t1/2 = 21.772(3) years]. We expect these results to direct renewed efforts for 225Ac-TAT development, aid in understanding Ac coordination behavior relative to other +3 lanthanides and actinides, and more broadly inform this element's position on the Periodic Table.

Exploring Aqueous Coordination Chemistry of Highly Lewis Acidic Metals with Emerging Isotopes for Nuclear Medicine

Nuclear medicine harnesses radioisotopes for the diagnosis and treatment of disease. While the isotopes 99mTc and 111In have enabled the clinical diagnosis of millions of patients over the past 3 decades, more recent clinical translation of numerous 68Ga/177Lu-based radiopharmaceuticals for diagnostic imaging and therapy underscores the clinical utility of metal-based radiopharmaceuticals in mainstream cancer treatment. In addition to such established radionuclides, advancements in radioisotope production have enabled the production of radionuclides with a broad range of half-lives and emission properties of interest for nuclear medicine. Chemical means to form kinetically inert, in vivo-compatible species that can be modified with disease-targeting vectors is imperative. This presents a challenge for radiosiotopes of elements where the aqueous chemistry is still underdeveloped and poorly understood. Here, we discuss our efforts to date in exploring the aqueous, radioactive coordination chemistry of highly Lewis acidic metal ions and how our discoveries apply to the diagnosis and treatment of cancer in preclinical models of disease. The scope of this Account includes approaches to aqueous coordination of to-date understudied highly Lewis acidic metal ions with radioisotopes of emerging interest and the modulation of well-understood coordination environments of radio-coordination complexes to induce metal-catalyzed reactivity for separation and pro-drug applications.First, we discuss the development of seven-coordinate, small-cavity macrocyclic chelator platform mpatcn/picaga as an exemplary case study, which forms robust complexes with 44Sc/47Sc isotopes. Due to the high chemical hardness and pronounced Lewis acidity of the Sc3+ ion, the displacement of ternary ligand H2O by 18/natF- can be achieved to form an inert Sc-18/natF bond. Corresponding coordination complex natSc-18F is in vivo compatible and forms a theranostic tetrad with corresponding 44Sc/47Sc, 177Lu complexes all exhibiting homologous biodistribution profiles. Another exceptionally hard, highly Lewis acidic ion with underdeveloped aqueous chemistry and emerging interest in nuclear medicine is 45Ti4+. To develop de novo approaches to the mononuclear chelation of this ion under aqueous conditions, we employed a fragment-based bidentate ligand screening approach which identified two leads. The screen successfully predicted the formation of [45Ti][Ti(TREN-CAM)], a Ti-triscatechol complex that exhibits remarkable in vivo stability. Furthermore, the fragment-based screen also identified approaches that enabled solid-phase separation of Ti4+ and Sc3+ of interest in streamlining the isotope production of 45Ti and accessing new ways to separate 44Ti/44Sc for the development of a long-lived generator system. In addition to establishing the inert chelation of Ti4+ and Sc3+, we introduce controlled, metal-induced reactivity of corresponding coordination complexes on macroscopic and radiotracer scales. Metal-mediated autolytic amide bond cleavage (MMAAC) enables the temperature-dependent release of high-molar-activity, ready-to-inject radiopharmaceuticals; cleavage is selectively triggered by coordinated trivalent Lewis acid nat/68Ga3+ or Sc3+. Following the scope of reactivity and mechanistic studies, we validated MMAAC for the synthesis of high-molar-activity radiopharmaceuticals to image molecular targets with low expression and metal-mediated prodrug hydrolysis in vivo.This Account summarizes how developing the aqueous coordination chemistry and tuning the chemical reactivity of metal ions with high Lewis acidity at the macroscopic and tracer scales directly apply to the radiopharmaceutical synthesis with clinical potential.

Sacrificial-Hydroxamate-Enabled Sizable Crystallization of Scandium Carboxylate Metal-Organic Frameworks

Starting from labile hydroxamic acid ligands that are strong chelators, here, we implemented a sacrificial modulating strategy to prepare a series of scandium carboxylate metal-organic frameworks. Overcoming conventional syntheses that use excessive carboxylate modulators, the present strategy greatly reduces the organics required and produces large single crystals of several Sc-MOFs for X-ray crystallography.

Antibody-drug conjugates in cancer therapy: innovations, challenges, and future directions

The emergence of antibody-drug conjugates (ADCs) as a potential therapeutic avenue in cancer treatment has garnered significant attention. By combining the selective specificity of monoclonal antibodies with the cytotoxicity of drug molecules, ADCs aim to increase the therapeutic index, selectively targeting cancer cells while minimizing systemic toxicity. Various ADCs have been licensed for clinical usage, with ongoing research paving the way for additional options. However, the manufacture of ADCs faces several challenges. These include identifying suitable target antigens, enhancing antibodies, linkers, and payloads, and managing resistance mechanisms and side effects. This review focuses on the strategies to overcome these hurdles, such as site-specific conjugation techniques, novel antibody formats, and combination therapy. Our focus lies on current advancements in antibody engineering, linker technology, and cytotoxic payloads while addressing the challenges associated with ADC development. Furthermore, we explore the future potential of personalized medicine, leveraging individual patients' molecular profiles, to propel ADC treatments forward. As our understanding of the molecular mechanisms driving cancer progression continues to expand, we anticipate the development of new ADCs that offer more effective and personalized therapeutic options for cancer patients.

Electron Paramagnetic Resonance, Electronic Ground State, and Electron Spin Relaxation of Seven Lanthanide Ions Bound to Lanmodulin and the Bioinspired Chelator, 3,4,3-LI(1,2-HOPO)

The electron paramagnetic resonance (EPR) spectra of lanthanide(III) ions besides Gd3+ , bound to small-molecule and protein chelators, are uncharacterized. Here, the EPR properties of 7 lanthanide(III) ions bound to the natural lanthanide-binding protein, lanmodulin (LanM), and the synthetic small-molecule chelator, 3,4,3-LI(1,2-HOPO) ("HOPO"), were systematically investigated. Echo-detected pulsed EPR spectra reveal intense signals from ions for which the normal continuous-wave first-derivative spectra are negligibly different from zero. Spectra of Kramers lanthanide ions Ce3+ , Nd3+ , Sm3+ , Er3+ , and Yb3+ , and non-Kramers Tb3+ and Tm3+ , bound to LanM are more similar to the ions in dilute aqueous:ethanol solution than to those coordinated with HOPO. Lanmodulins from two bacteria, with distinct metal-binding sites, had similar spectra for Tb3+ but different spectra for Nd3+ . Spin echo dephasing rates (1/Tm ) are faster for lanthanides than for most transition metals and limited detection of echoes to temperatures below ~6 to 12 K. Dephasing rates were environment dependent and decreased in the order water:ethanol>LanM>HOPO, which is attributed to decreasing librational motion. These results demonstrate that the EPR spectra and relaxation times of lanthanide(III) ions are sensitive to coordination environment, motivating wider application of these methods for characterization of both small-molecule and biomolecule interactions with lanthanides.

Sc-HOPO: A Potential Construct for Use in Radioscandium-Based Radiopharmaceuticals

Three isotopes of scandium─43Sc, 44Sc, and 47Sc─have attracted increasing attention as potential candidates for use in imaging and therapy, respectively, as well as for possible theranostic use as an elementally matched pair. Here, we present the octadentate chelator 3,4,3-(LI-1,2-HOPO) (or HOPO), an effective chelator for hard cations, as a potential ligand for use in radioscandium constructs with simple radiolabeling under mild conditions. HOPO forms a 1:1 Sc-HOPO complex that was fully characterized, both experimentally and theoretically. [47Sc]Sc-HOPO exhibited good stability in chemical and biological challenges over 7 days. In healthy mice, [43,47Sc]Sc-HOPO cleared the body rapidly with no signs of demetalation. HOPO is a strong candidate for use in radioscandium-based radiopharmaceuticals.

Metal-Ligand Interactions in Scandium Complexes with Radiopharmaceutical Applications

The radioisotopes of scandium (43Sc, 44Sc, and 47Sc) are potential candidates for use in imaging and therapy both separately and as elementally matched pairs for radiotheranostics. In the present study the bonding interactions of Sc3+ with 18 hepta- to decadentate ligands are compared using density functional theory (DFT) calculations. The bonding analysis is based on the natural bond orbital (NBO) model. The main contributions to the bonding were assessed using natural energy decomposition analysis (NEDA). Most of the ligands have anionic character (charges from 2- to 8-); thus the electrical term determines the major differences in the interaction energies. However, interesting features were found in the covalent contributions manifested by the ligand → Sc3+ charge transfer (CT) interactions. Significant differences could be observed in the energetic contributions of the N and O donors to the total CT.

De Novo Approaches to the Solid-Phase Separation of Titanium(IV) and Scandium(III): Translating Speciation Data to Selective On-Bead Chelation toward Applications in Nuclear Medicine

The solution chemistry of the hydrolytic, early-transition-metal ions Ti4+ and Sc3+ represents a coordination chemistry challenge with important real-world implications, specifically in the context of 44Ti/44Sc and 45Ti/NatSc radiochemical separations. Unclear speciation of the solid and solution phases and tertiary mixtures of mineral acid, organic chelators, and solid supports are common confounds, necessitating tedious screening of multiple variables. Herein we describe how thermodynamic speciation data in solution informs the design of new solid-phase chelation approaches enabling separations of Ti4+ and Sc3+. The ligands catechol (benzene-1,2-diol) and deferiprone [3-hydroxy-1,2-dimethyl-4(1H)-pyridone] bind Ti4+ at significantly more acidic conditions (2-4 pH units) than Sc3+. Four chelating resins were synthesized using either catechol or deferiprone with two different solid supports. Of these, deferiprone appended to carboxylic acid polymer-functionalized silica (CA-Def) resin exhibited excellent binding affinity for Ti4+ across a wide range of HCl concentrations (1.0-0.001 M), whereas Sc3+ was only retained in dilute acidic conditions (0.01-0.001 M HCl). CA-Def resin produced separation factors of >100 (Ti/Sc) in 0.1-0.4 M HCl, and the corresponding Kd values (>1000) show strong retention of Ti4+. A model 44Ti/44Sc generator was produced, showing 65 ± 3% yield of 44Sc in 200 μL of 0.2 M HCl with a significant 44Ti breakthrough of 0.1%, precluding use in its current form. Attempts, however, removed natSc in loading fractions and a dilute (0.4 M HCl) wash and recovered 80% of the loaded 45Ti activity in 400 μL of 6 M HCl. The previously validated 45Ti chelator TREN-CAM was used for comparative proof-of-concept reactions with the CA-Def eluent (in HCl) and literature-reported hydroxamate-based resin eluents (in citric acid). CA-Def shows improved radiolabeling efficiency with an apparent molar activity (AMA) of 0.177 mCi nmol-1, exceeding the established methods (0.026 mCi nmol-1) and improving the separation and recovery of 45Ti for positron emission tomography imaging applications.

Siderocalin fusion proteins enable a new 86Y/90Y theranostic approach

The mammalian protein siderocalin binds bacterial siderophores and their iron complexes through cation-π and electrostatic interactions, but also displays high affinity for hydroxypyridinone complexes of trivalent lanthanides and actinides. In order to circumvent synthetic challenges, the use of siderocalin-antibody fusion proteins is explored herein as an alternative targeting approach for precision delivery of trivalent radiometals. We demonstrate the viability of this approach in vivo, using the theranostic pair 90Y (β-, t1/2 = 64 h)/86Y (β+, t1/2 = 14.7 h) in a SKOV-3 xenograft mouse model. Ligand radiolabeling with octadentate hydroxypyridinonate 3,4,3-LI(1,2-HOPO) and subsequent protein binding were achieved at room temperature. The results reported here suggest that the rapid non-covalent binding interaction between siderocalin fusion proteins and the negatively charged Y(iii)-3,4,3-LI(1,2-HOPO) complexes could enable purification-free, cold-kit labeling strategies for the application of therapeutically relevant radiometals in the clinic.

Versatile Macrocyclic Platform for the Complexation of [natY/90Y]Yttrium and Lanthanide Ions

We report a macrocyclic ligand (H₃L⁶) based on a 3,6,10,13-tetraaza-1,8(2,6)-dipyridinacyclotetradecaphane platform containing three acetate pendant arms and a benzyl group attached to the fourth nitrogen atom of the macrocycle. The X-ray structures of the YL⁶ and TbL⁶ complexes reveal nine coordination of the ligand to the metal ions through the six nitrogen atoms of the macrocycle and three oxygen atoms of the carboxylate pendants. A combination of NMR spectroscopic studies (¹H, ¹³C, and ⁸⁹Y) and DFT calculations indicated that the structure of the YL⁶ complex in the solid state is maintained in an aqueous solution. The detailed study of the emission spectra of the EuL⁶ and TbL⁶ complexes revealed Ln³⁺-centered emission with quantum yields of 7.0 and 60%, respectively. Emission lifetime measurements indicate that the ligand offers good protection of the metal ions from surrounding water molecules, preventing the coordination of water molecules. The YL⁶ complex is remarkably inert with respect to complex dissociation, with a lifetime of 1.7 h in 1 M HCl. On the other hand, complex formation is fast (∼1 min at pH 5.4, 2 × 10^–5 M). Studies using the ⁹⁰Y-nuclide confirmed fast radiolabeling since [⁹⁰Y]YL⁶ is nearly quantitatively formed (radiochemical yield (RCY) > 95) in a short time over a broad range of pH values from ca. 2.4 to 9.0. Challenging experiments in the presence of excess ethylenediaminetetraacetic acid (EDTA) and in human serum revealed good stability of the [⁹⁰Y]YL⁶ complex. All of these experiments combined suggest the potential application of H₃L⁶ derivatives as Y-based radiopharmaceuticals.

A new versatile asymmetric ligand based on an 18-membered macrocycle possessing three acetate pendant arms and a benzyl moiety provides fast complexation with both ^natY(III) and ⁹⁰Y(III), as well as slow dissociation kinetics. A detailed structural study in the solid state and in solution evidences nona coordination of the metal ion by the ligand, offering good protection from solvent water molecules. These favorable properties make this ligand an attractive candidate to develop yttrium-based radiopharmaceuticals.

Scandium-microorganism interactions in new biotechnologies

Scandium (Sc) plays a special role in high-tech industries because of its wide application in green, space, and defense technologies. However, Sc mining and purification are problematic due to political, technological, and environmental difficulties. The deficit of this element limits global technological development. One sustainable solution to this problem is to use microorganisms to extract Sc from ore and waste, as well as to concentrate and separate it from other elements. Sc also demonstrates attractive metabolic effects on microbes that is of great interest in white biotechnology. Sc increases the production of proteins and secondary metabolites and activates poorly expressed genes. This review offers a comprehensive analysis of current knowledge on the application of Sc-microorganism interactions in promising biotechnologies, its perspectives, and future challenges.

Hydroxypyridinone-Based Metal Chelators towards Ecotoxicity: Remediation and Biological Mechanisms

Hydroxypyridinones (HPs) are recognized as excellent chemical tools for engineering a diversity of metal chelating agents, with high affinity for hard metal ions, exhibiting a broad range of activities and applications, namely in medical, biological and environmental contexts. They are easily made and functionalizable towards the tuning of their pharmacokinetic properties or the improving of their metal complex thermodynamic stabilities. In this review, an analysis of the recently published works on hydroxypyridinone-based ligands, that have been mostly addressed for environmental applications, namely for remediation of hard metal ion ecotoxicity in living beings and other biological matrices is carried out. In particular, herein the most recent developments in the design of new chelating systems, from bidentate mono-HP to polydentate multi-HP derivatives, with a structural diversity of soluble or solid-supported backbones are outlined. Along with the ligand design, an analysis of the relationship between their structures and activities is presented and discussed, namely associated with the metal affinity and the thermodynamic stability of the corresponding metal complexes.

In situ beam reduction of Pu(IV) and Bk(IV) as a route to trivalent transuranic coordination complexes with hydroxypyridinone chelators

The solution-state interactions of plutonium and berkelium with the octadentate chelator 3,4,3-LI(1,2-HOPO) (343-HOPO) were investigated and characterized by X-ray absorption spectroscopy, which revealed in situ reductive decomposition of the tetravalent species of both actinide metals to yield Pu(III) and Bk(III) coordination complexes. X-ray absorption near-edge structure (XANES) measurements were the first indication of in situ synchrotron redox chemistry as the Pu threshold and white-line position energies for Pu-343-HOPO were in good agreement with known diagnostic Pu(III) species, whereas Bk-343-HOPO results were found to mirror the XANES behavior of Bk(III)-DTPA. Extended X-ray absorption fine structure results revealed An-O_HOPO bond distances of 2.498 (5) and 2.415 (2) Å for Pu and Bk, respectively, which match well with bond distances obtained for trivalent actinides and 343-HOPO via density functional theory calculations. Pu(III)- and Bk(III)-343-HOPO data also provide initial insight into actinide periodicity as they can be compared with previous results with Am(III)-, Cm(III)-, Cf(III)-, and Es(III)-343-HOPO, which indicate there is likely an increase in 5f covalency and heterogeneity across the actinide series.

Hydroxypyridinones as a Very Promising Platform for Targeted Diagnostic and Therapeutic Radiopharmaceuticals

Hydroxypyridinones (HOPOs) have been used in the chelation therapy of iron and actinide metals. Their application in metal-based radiopharmaceuticals has also been increasing in recent years. This review article focuses on how multidentate HOPOs can be used in targeted radiometal-based diagnostic and therapeutic radiopharmaceuticals. The general structure of radiometal-based targeted radiopharmaceuticals, a brief description of siderophores, the basic structure and properties of bidentate HOPO, some representative HOPO multidentate chelating agents, radiopharmaceuticals based on HOPO multidentate bifunctional chelators for gallium-68, thorium-227 and zirconium-89, as well as the future prospects of HOPO multidentate bifunctional chelators in other metal-based radiopharmaceuticals are described and discussed in turn. The HOPO metal-based radiopharmaceuticals that have shown good prospects in clinical and preclinical studies are gallium-68, thorium-227 and zirconium-89 radiopharmaceuticals. We expect HOPO multidentate bifunctional chelators to be a very promising platform for building novel targeted radiometal-based diagnostic and therapeutic radiopharmaceuticals.

Hydroxypyridinones as a Very Promising Platform for Targeted Diagnostic and Therapeutic Radiopharmaceuticals

Hydroxypyridinones (HOPOs) have been used in the chelation therapy of iron and actinide metals. Their application in metal-based radiopharmaceuticals has also been increasing in recent years. This review article focuses on how multidentate HOPOs can be used in targeted radiometal-based diagnostic and therapeutic radiopharmaceuticals. The general structure of radiometal-based targeted radiopharmaceuticals, a brief description of siderophores, the basic structure and properties of bidentate HOPO, some representative HOPO multidentate chelating agents, radiopharmaceuticals based on HOPO multidentate bifunctional chelators for gallium-68, thorium-227 and zirconium-89, as well as the future prospects of HOPO multidentate bifunctional chelators in other metal-based radiopharmaceuticals are described and discussed in turn. The HOPO metal-based radiopharmaceuticals that have shown good prospects in clinical and preclinical studies are gallium-68, thorium-227 and zirconium-89 radiopharmaceuticals. We expect HOPO multidentate bifunctional chelators to be a very promising platform for building novel targeted radiometal-based diagnostic and therapeutic radiopharmaceuticals.

Hydroxypyridinones as a Very Promising Platform for Targeted Diagnostic and Therapeutic Radiopharmaceuticals

Hydroxypyridinones (HOPOs) have been used in the chelation therapy of iron and actinide metals. Their application in metal-based radiopharmaceuticals has also been increasing in recent years. This review article focuses on how multidentate HOPOs can be used in targeted radiometal-based diagnostic and therapeutic radiopharmaceuticals. The general structure of radiometal-based targeted radiopharmaceuticals, a brief description of siderophores, the basic structure and properties of bidentate HOPO, some representative HOPO multidentate chelating agents, radiopharmaceuticals based on HOPO multidentate bifunctional chelators for gallium-68, thorium-227 and zirconium-89, as well as the future prospects of HOPO multidentate bifunctional chelators in other metal-based radiopharmaceuticals are described and discussed in turn. The HOPO metal-based radiopharmaceuticals that have shown good prospects in clinical and preclinical studies are gallium-68, thorium-227 and zirconium-89 radiopharmaceuticals. We expect HOPO multidentate bifunctional chelators to be a very promising platform for building novel targeted radiometal-based diagnostic and therapeutic radiopharmaceuticals.

Emerging chelators for nuclear imaging

Chelators are necessary in nuclear medicine imaging to direct an inorganic radionuclide, a radiometal, to a desired target; unfortunately, there is no 'one-size-fits-all' chelator. As the toolbox of radiometals is expanding, new chelators are required to prevent off-target side effects. 1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) is the current gold standard chelator for several radiometals, but typically, chelation requires harsh conditions, making it unsuitable to label biological vectors. The ideal chelator would allow labelling under mild conditions (near-neutral pH and low temperatures [∼37 °C]) and be both thermodynamically and kinetically stable. Over the past 2-3 years, several exciting chelators have been developed that have superior properties to make them worth investigating for future clinical applications.

Production of scandium radionuclides for theranostic applications: towards standardization of quality requirements

In the frame of "precision medicine", the scandium radionuclides have recently received considerable interest, providing personalised adjustment of radiation characteristics to optimize the efficiency of medical care or therapeutic benefit for particular groups of patients. Radionuclides of scandium, namely scandium-43 and scandium-44 (43/44Sc) as positron emitters and scandium-47 (47Sc), beta-radiation emitter, seem to fit ideally into the concept of theranostic pair. This paper aims to review the work on scandium isotopes production, coordination chemistry, radiolabeling, preclinical studies and the very first clinical studies. Finally, standardized procedures for scandium-based radiopharmaceuticals have been proposed as a basis to pave the way for elaboration of the Ph.Eur. monographs for perspective scandium radionuclides.

Combining the Best of Two Chelating Titans: A Hydroxypyridinone-Decorated Macrocyclic Ligand for Efficient and Concomitant Complexation and Sensitized Luminescence of f-Elements

An ideal chelator for f-elements features rapid kinetics of complexation, high thermodynamic stability, and slow kinetics of dissociation. Here we present the facile synthesis of a macrocyclic ligand bearing four 1-hydroxy-2-pyridinone units linked to a cyclen scaffold that rapidly forms thermodynamically stable complexes with lanthanides (Sm³⁺ , Eu³⁺ , Tb³⁺ , Dy³⁺ ) and a representative late actinide (Cm³⁺ ) in aqueous media and concurrently sensitizes them. Extended X-ray absorption fine structure (EXAFS) spectroscopy revealed an increase in the Ln/An-O bond lengths following the trend Cm>Eu>Tb and EXAFS data were compatible with time-resolved luminescence studies, which indicated one to two water molecules in the inner metal coordination sphere of Eu(III) and two water molecules for the Cm(III) complex. Spectrofluorimetric ligand competition titrations against DTPA confirmed the high thermodynamic stability of DOTHOPO complexes, with pM values between 19.9(1) and 21.9(2).

Macromolecular crystallography for f-element complex characterization

Single crystal X-ray diffraction is a technique that measures interatomic distances with atomic resolution. Utilizing this technique for metal complexes featuring lanthanide and actinide elements is complicated by the scarcity and radioactivity of many of the metals of the f-block, as sub-milligram samples are difficult to crystallize for small molecule X-ray diffraction experiments. In this chapter, we present a protocol developed in our group that circumvents these challenges by exploiting macromolecular crystallography, wherein a protein with a large and well-characterized binding calyx is used as a scaffold to crystallize small-molecule metal complexes. Highlighting several examples, we identify the structural and chemical information that can be acquired by this method, and delineate the benefits of directing crystal growth with proteins, such as decreasing the amount of metal used to the sub-microgram scale. Moreover, since protein recognition depends on the nature of the metal-chelator bonds, subtle effects in the lanthanide and actinide coordination chemistry, such as metal-ligand covalency, can be qualitatively assessed.

Aqueous chemistry of the smallest rare earth: Comprehensive characterization of radioactive and non-radioactive scandium complexes for biological applications

The aqueous chemistry of scandium(III) is of emerging interest for biological applications, specifically in nuclear medicine, as radioactive isotopes of scandium are becoming more readily accessible. In contrast to other rare earths, Sc³⁺ has no d or f electrons, limiting characterization of corresponding coordination complexes to spectroscopic techniques that do not rely on the characteristic electronic transitions of f-elements or transition metal ions. Herein, we provide a comprehensive overview on characterization techniques suitable to elucidate the solution behavior of small and macromolecular complexes of the smallest rare earth.