Modified Calix[4]crowns as Molecular Receptors for Barium

Theranostic Imaging Surrogates for Targeted Alpha Therapy: Progress in Production, Purification, and Applications

This article highlights recent developments of SPECT and PET diagnostic imaging surrogates for targeted alpha particle therapy (TAT) radiopharmaceuticals. It outlines the rationale for using imaging surrogates to improve diagnostic-scan accuracy and facilitate research, and the properties an imaging-surrogate candidate should possess. It evaluates the strengths and limitations of each potential imaging surrogate. Thirteen surrogates for TAT are explored: 133La, 132La, 134Ce/134La, and 226Ac for 225Ac TAT; 203Pb for 212Pb TAT; 131Ba for 223Ra and 224Ra TAT; 123I, 124I, 131I and 209At for 211At TAT; 134Ce/134La for 227Th TAT; and 155Tb and 152Tb for 149Tb TAT.

The Curies' element: state of the art and perspectives on the use of radium in nuclear medicine

BACKGROUND

The alpha-emitter radium-223 (223Ra) is presently used in nuclear medicine for the palliative treatment of bone metastases from castration-resistant prostate cancer. This application arises from its advantageous decay properties and its intrinsic ability to accumulate in regions of high bone turnover when injected as a simple chloride salt. The commercial availability of [223Ra]RaCl2 as a registered drug (Xofigo®) is a further additional asset.

MAIN BODY

The prospect of extending the utility of 223Ra to targeted α-therapy of non-osseous cancers has garnered significant interest. Different methods, such as the use of bifunctional chelators and nanoparticles, have been explored to incorporate 223Ra in proper carriers designed to precisely target tumor sites. Nevertheless, the search for a suitable scaffold remains an ongoing challenge, impeding the diffusion of 223Ra-based radiopharmaceuticals.

CONCLUSION

This review offers a comprehensive overview of the current role of radium radioisotopes in nuclear medicine, with a specific focus on 223Ra. It also critically examines the endeavors conducted so far to develop constructs capable of incorporating 223Ra into cancer-targeting drugs. Particular emphasis is given to the chemical aspects aimed at providing molecular scaffolds for the bifunctional chelator approach.

Synthesis of Ionizable Calix[4]arenes for Chelation of Selected Divalent Cations

Two sets of functionalised calix[4]arenes, either with a 1,3-crown ether bridge or with an open-chain oligo ether moiety in 1,3-position were prepared and further equipped with additional deprotonisable sulfonamide groups to establish chelating systems for selected cations Sr2+, Ba2+, and Pb2+ ions. To improve the complexation behaviour towards these cations, calix[4]arenes with oligo ether groups and modified crowns of different sizes were synthesized. Association constants were determined by UV/Vis titration in acetonitrile using the respective perchlorate salts and logK values between 3.2 and 8.0 were obtained. These findings were supported by the calculation of the binding energies exemplarily for selected complexes with Ba2+.

The impact of barium isotopes in radiopharmacy and nuclear medicine - From past to presence

With the exception of beryllium, divalent cations of every alkaline earth metal are characterized by their calcimimetic behavior. Thus, in vivo biodistribution of these cations mostly occurs in form of a massive accumulation in bone tissues, consisting of hydroxyapatite to a major extent. Apart from the lightest elements beryllium and magnesium, animal studies and human studies regarding the overall in vivo behavior were carried out by using radioisotopes of the elements calcium, strontium, barium and radium. To date, only strontium with its radioisotopes and radium gained importance for applications in nuclear medicine, mainly for pain-reducing and palliative treatment of bone metastases. In contrast, barium radioisotopes can be ascertained as useful imaging agents and possible diagnostic analogues for theranostic approaches. This review focuses on the characteristic and chemical behavior of barium compounds, possible radioactive barium isotopes for future applications in nuclear medicine and radiopharmacy as well as recent results regarding barium-131 as diagnostic match for radium isotopes used in targeted alpha therapy.

Towards Targeted Alpha Therapy with Actinium-225: Chelators for Mild Condition Radiolabeling and Targeting PSMA-A Proof of Concept Study

Currently, targeted alpha therapy is one of the most investigated topics in radiopharmaceutical cancer management. Especially, the alpha emitter 225Ac has excellent nuclear properties and is gaining increasing popularity for the treatment of various tumor entities. We herein report on the synthesis of two universal 225Ac-chelators for mild condition radiolabeling and binding to conjugate molecules of pharmacological interest via the copper-mediated click chemistry. A convenient radiolabeling procedure was investigated as well as the complex stability proved for both chelators and two PSMA (prostate-specific membrane antigen)-targeting model radioconjugates. Studies regarding affinity and cell survival were performed on LNCaP cells followed by biodistribution studies, which were performed using LNCaP tumor-bearing mice. High efficiency radiolabeling for all conjugates was demonstrated. Cell binding studies revealed a fourfold lower cell affinity for the PSMA radioconjugate with one targeting motif compared to the radioconjugate owing two targeting motifs. Additionally, these differences were verified by in vitro cell survival evaluation and biodistribution studies, both showing a higher cell killing efficiency for the same dose, a higher tumor uptake (15%ID/g) and a rapid whole body clearance after 24 h. The synthesized chelators will overcome obstacles of lacking stability and worse labeling needs regarding 225Ac complexation using the DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrayl)tetraacetic acid) chelator. Moreover, the universal functionalization expands the coverage of these chelators in combination with any sensitive bio(macro)molecule, thus improving treatment of any addressable tumor target.

Recent Insights in Barium-131 as a Diagnostic Match for Radium-223: Cyclotron Production, Separation, Radiolabeling, and Imaging

Barium-131 is a single photon emission computed tomography (SPECT)-compatible radionuclide for nuclear medicine and a promising diagnostic match for radium-223/-224. Herein, we report on the sufficient production route 133Cs(p,3n)131Ba by using 27.5 MeV proton beams. An average of 190 MBq barium-131 per irradiation was obtained. The SR Resin-based purification process led to barium-131 in high radiochemical purity. An isotopic impurity of 0.01% barium-133 was detectable. For the first time, radiolabeling of the ligand macropa with barium-131 was performed. Radiolabeling methods under mild conditions and reaction controls based on TLC systems were successfully applied. Small animal SPECT/ computed tomography (CT) measurements and biodistribution studies were performed using [131Ba]Ba(NO3)2 as reference and 131Ba-labeled macropa in healthy mice for the first time. Biodistribution studies revealed the expected rapid bone uptake of [131Ba]Ba2+, whereas 131Ba-labeled macropa showed a fast clearance from the blood, thereby showing a significantly (p < 0.001) lower accumulation in the bone. We conclude that barium-131 is a promising SPECT radionuclide and delivers appropriate imaging qualities in small animals. Furthermore, the relative stability of the 131Ba-labeled macropa complex in vivo forms the basis for the development of sufficient new chelators, especially for radium isotopes. Thereby, barium-131 will attain its goal as a diagnostic match to the alpha emitters radium-223 and radium-224.

Sub-10 nm Radiolabeled Barium Sulfate Nanoparticles as Carriers for Theranostic Applications and Targeted Alpha Therapy

The treatment of cancer patients with α-particle-emitting therapeutics continues to gain in importance and relevance. The range of radiopharmaceutically relevant α-emitters is limited to a few radionuclides, as stable chelators or carrier systems for safe transport of the radioactive cargo are often lacking. Encapsulation of α-emitters into solid inorganic systems can help to diversify the portfolio of candidate radionuclides, provided, that these nanomaterials effectively retain both the parent and the recoil daughters. We therefore focus on designing stable and defined nanocarrier-based systems for various clinically relevant radionuclides, including the promising α-emitting radionuclide 224Ra. Hence, sub-10 nm barium sulfate nanocontainers were prepared and different radiometals like 89Zr, 111In, 131Ba, 177Lu or 224Ra were incorporated. Our system shows stabilities of >90 % regarding the radiometal release from the BaSO4 matrix. Furthermore, we confirm the presence of surface-exposed amine functionalities as well as the formation of a biomolecular corona.

A comparative evaluation of calix[4]arene-1,3-crown-6 as a ligand for selected divalent cations of radiopharmaceutical interest

Metals, which form divalent cations, including the alkaline earth metals offer radionuclides like lead-203, lead-212, barium-131, and strontium-89, which are promising candidates for radiopharmaceutical applications. Besides, the heavy homologous nuclides radium-223 and radium-224 - with similar properties to barium - are suitable alpha-emitters for targeted alpha-particle therapy. However, there is a lack of suitable chelation agents, especially for heavy group 2 metals. The macrocycle calix[4]arene-1,3-crown-6 seems to interact with these metals strongly. Therefore, this ligand and its coordination to the divalent cations of barium, strontium, and lead have been investigated. The complex formation was analyzed by NMR and UV/Vis titration experiments in acetonitrile, and stability constants were determined to be >4 with both methods. It was found that the stability of these complexes increase in the order strontium, barium, and lead. Additional to these investigations, X-ray crystallography, solvent-dependent 1H NMR, and 207Pb NMR measurements were performed to deliver deeper insight into the coordination chemistry of this ligand.

Polymeric Composite Materials for the Detection of Barium Ions in Aqueous Solutions

The design of polymeric composite materials (PCM) for the optical control of chemical substances is currently one of the actively developing fields of science “at the junction” of polymer, organic, and analytical chemistry. The purpose of this work is the preparation of PCM containing derivatives of crown ethers for the optical determination of barium ions. The polymeric composite materials containing a novel optical molecular sensor have been obtained and investigated on the basis of a number of film-forming polymers. The best results have been obtained for PCM based on polyvinyl butyral films (since the fluorescence and absorption maxima shifted by 9 and 16 nm, respectively) in the fluorescence and absorption spectra of this PCM in the presence of barium ions. This makes the proposed PCM highly promising as sensor elements for the detection of the barium ions in the aqueous solutions.

Facile preparation of radium-doped, functionalized nanoparticles as carriers for targeted alpha therapy

A facile preparation of nanoparticles with enhanced properties obtained by co-precipitation containing radium-224 and functional groups to connect target (bio)molecules for therapeutic applications in oncology is described.

Rapid Dissolution of BaSO4 by Macropa, an 18-Membered Macrocycle with High Affinity for Ba2

Insoluble BaSO4 scale is a costly and time-consuming problem in the petroleum industry. Clearance of BaSO4-impeded pipelines requires chelating agents that can efficiently bind Ba2+, the largest nonradioactive +2 metal ion. Due to the poor affinity of currently available chelating agents for Ba2+, however, the dissolution of BaSO4 remains inefficient, requiring very basic solutions of ligands. In this study, we investigated three diaza-18-crown-6 macrocycles bearing different pendent arms for the chelation of Ba2+ and assessed their potential for dissolving BaSO4 scale. Remarkably, the bis-picolinate ligand macropa exhibits the highest affinity reported to date for Ba2+ at pH 7.4 (log K' = 10.74), forming a complex of significant kinetic stability with this large metal ion. Furthermore, the BaSO4 dissolution properties of macropa dramatically surpass those of the state-of-the-art ligands DTPA and DOTA. Using macropa, complete dissolution of a molar equivalent of BaSO4 is reached within 30 min at room temperature in pH 8 buffer, conditions under which DTPA and DOTA only achieve 40% dissolution of BaSO4. When further applied for the dissolution of natural barite, macropa also outperforms DTPA, showing that this ligand is potentially valuable for industrial processes. Collectively, this work demonstrates that macropa is a highly effective chelator for Ba2+ that can be applied for the remediation of BaSO4 scale.