An Eighteen‐Membered Macrocyclic Ligand for Actinium‐225 Targeted Alpha Therapy

Oxyaapa: A Picolinate-Based Ligand with Five Oxygen Donors that Strongly Chelates Lanthanides

Coordination compounds of the lanthanide ions (Ln³⁺) have important applications in medicine due to their photophysical, magnetic, and nuclear properties. To effectively use the Ln³⁺ ions for these applications, chelators that stably bind them in vivo are required to prevent toxic side effects that arise from localization of these ions in off-target tissue. In this study, two new picolinate-containing chelators, a heptadentate ligand OxyMepa and a nonadentate ligand Oxyaapa, were prepared, and their coordination chemistries with Ln³⁺ ions were thoroughly investigated to evaluate their suitability for use in medicine. Protonation constants of these chelators and stability constants for their Ln³⁺ complexes were evaluated. Both ligands exhibit a thermodynamic preference for small Ln³⁺ ions. The log K_LuL = 12.21 and 21.49 for OxyMepa and Oxyaapa, respectively, indicating that the nonadentate Oxyaapa forms complexes of significantly higher stability than the heptadentate OxyMepa. X-ray crystal structures of the Lu³⁺ complexes were obtained, revealing that Oxyaapa saturates the coordination sphere of Lu³⁺, whereas OxyMepa leaves an additional open coordination site for a bound water ligand. Solution structural studies carried out with NMR spectroscopy revealed the presence of two possible conformations for these ligands upon Ln³⁺ binding. Density functional theory (DFT) calculations were applied to probe the geometries and energies of these conformations. Energy differences obtained by DFT are small but consistent with experimental data. The photophysical properties of the Eu³⁺ and Tb³⁺ complexes were characterized, revealing modest photoluminescent quantum yields of <2%. Luminescence lifetime measurements were carried out in H₂O and D₂O, showing that the Eu³⁺ and Tb³⁺ complexes of OxyMepa have two inner-sphere water ligands, whereas the Eu³⁺ and Tb³⁺ complexes of Oxyaapa have zero. Lastly, variable-temperature ¹⁷O NMR spectroscopy was performed for the Gd-OxyMepa complex to determine its water exchange rate constant of k_ex²⁹⁸ = (2.8 ± 0.1) × 10⁶ s^-1. Collectively, this comprehensive characterization of these Ln³⁺ chelators provides valuable insight for their potential use in medicine and garners additional understanding of ligand design strategies.

Bioconjugates of Chelators with Peptides and Proteins in Nuclear Medicine: Historical Importance, Current Innovations, and Future Challenges

Molecular radiopharmaceuticals based on bioconjugates of chelators with peptides and proteins have had significant clinical impact in the diagnosis and treatment of several types of cancers. In the 1990s, indium-111 and yttrium-90 labeled chelator-peptide/protein conjugates established the clinical utility of these radiopharmaceuticals for receptor-targeted γ-scintigraphy imaging and systemic radiotherapy. Second-generation bioconjugates based on peptides targeting the somatostatin II receptor and the prostate-specific membrane antigen are now widely used for management of neuroendocrine and prostate cancer, respectively. These bioconjugates are typically radiolabeled with gallium-68 for imaging of target receptor expression with positron emission tomography, and the β--emitter, lutetium-177, for targeted radiotherapy. Innovations in radioisotope technology and biomolecular therapies are likely to drive the future clinical development of radiopharmaceuticals based on radiometals. New chelator-peptide and chelator-protein bioconjugates will underpin nuclear medicine advances in molecular imaging and radiotherapy.

Sensing Zn2+ in Aqueous Solution with a Fluorescent Scorpiand Macrocyclic Ligand Decorated with an Anthracene Bearing Tail

Synthesis of the new scorpiand ligand L composed of a [9]aneN3 macrocyclic ring bearing a CH2CH2NHCH2-anthracene tail is reported. L forms both cation (Zn2+) and anion (phosphate, benzoate) complexes. In addition, the zinc complexes of L bind these anions. The equilibrium constants for ligand protonation and complex formation were determined in 0.1 M NaCl aqueous solution at 298.1 ± 0.1 K by means of potentiometric (pH-metric) titrations. pH Controlled coordination/detachment of the ligand tail to Zn2+ switch on and off the fluorescence emission from the anthracene fluorophore. Accordingly, L is able to sense Zn2+ in the pH range 6-10 down to nM concentrations of the metal ion. L can efficiently sense Zn2+ even in the presence of large excess of coordinating anions, such as cyanide, sulphide, phosphate and benzoate, despite their ability to bind the metal ion.

Targets for Therapy of Bladder Cancer

In 2018 bladder cancer (urothelial carcinoma) was ranked twelfth concerning worldwide diagnosis of malignancies. At the time point of diagnosis of bladder cancer, approximately 75% of patients present with a nonmuscle-invasive disease (NMIBC), while the remaining 25% show invasion of tumor cells in the muscle layer of the bladder wall (MIBC). Among NMIBC tumors, flat, high-grade carcinoma in situ (CIS) is a therapeutic challenge. CIS shows a tendency to invade the muscle tissue of the bladder wall and thus become a MIBC. Standard therapy of NMIBC (including CIS) is done via intravesical instillation of BCG (bacillus Calmette Guerin) inducing a local immune reaction that finally promotes elimination of bladder cancer cells. However, BCG treatment of NMIBC proves to be ineffective in approximately 40% of patients. Therefore, new therapeutic approaches for the treatment of bladder cancer are urgently needed. Among promising new treatment options that are currently being investigated are the use of immune checkpoint inhibitors, and targeted approaches attacking (among others) long noncoding RNAs, micro RNAs, cancer stem cells, PARP1, and receptor signaling pathways. Moreover, the use of antibody-drug-conjugates (ADCs) is investigated also in bladder cancer therapy. Another approach that has been successfully established in preclinical studies uses the cytotoxic power of the alpha-emitter Bi-213 coupled to an antibody targeting EGFR. Overexpression of EGFR has been demonstrated in the majority of patients suffering from CIS. Feasibility, safety, toxicity and therapeutic efficacy of intravesical instillation of Bi-213-anti-EGFR have been evaluated in a pilot study. Since the results of the pilot study proved to be promising, a further optimization of alpha-emitter immunotherapy in bladder cancer seems mandatory.

A Proof-of-Concept Study on the Therapeutic Potential of Au Nanoparticles Radiolabeled with the Alpha-Emitter Actinium-225

Actinium-225 (²²⁵Ac) is receiving increased attention for its application in targeted radionuclide therapy, due to the short range of its emitted alpha particles in conjunction with their high linear energy transfer, which lead to the eradication of tumor cells while sparing neighboring healthy tissue. The objective of our study was the evaluation of a gold nanoparticle radiolabeled with ²²⁵Ac as an injectable radiopharmaceutical form of brachytherapy for local radiation treatment of cancer. Au@TADOTAGA was radiolabeled with ²²⁵Ac at pH 5.6 (30 min at 70 °C), and in vitro stability was evaluated. In vitro cytotoxicity was assessed in U-87 MG cancer cells, and in vivo biodistribution was performed by intravenous and intratumoral administration of [²²⁵Ac]²²⁵Ac-Au@TADOTAGA in U-87 MG tumor-bearing mice. A preliminary study to assess therapeutic efficacy of the intratumorally-injected radio-nanomedicine was performed over a period of 22 days, while the necrotic effect on tumors was evaluated by a histopathology study. We have shown that [²²⁵Ac]²²⁵Ac-Au@TADOTAGA resulted in the retardation of tumor growth after its intratumoral injection in U87MG tumor-bearing mice, even though very low activities were injected per mouse. This gold nanoparticle radiopharmaceutical could be applied as an unconventional brachytherapy in injectable form for local radiation treatment of cancer.

Preparation of an Actinium-228 Generator

Advances in targeted α-therapies have increased the interest in actinium (Ac), whose chemistry is poorly defined due to scarcity and radiological hazards. Challenges associated with characterizing Ac³⁺ chemistry are magnified by its 5f⁰6d⁰ electronic configuration, which precludes the use of many spectroscopic methods amenable to small amounts of material and low concentrations (like EPR, UV-vis, fluorescence). In terms of nuclear spectroscopy, many actinium isotopes (²²⁵Ac and ²²⁷Ac) are equally "unfriendly" because the actinium α-, β-, and γ-emissions are difficult to resolve from the actinium daughters. To address these issues, we developed a method for isolating an actinium isotope (²²⁸Ac) whose nuclear properties are well-suited for γ-spectroscopy. This four-step procedure isolates ²²⁸Ra from naturally occurring ²³²Th. The relatively long-lived ²²⁸Ra (t_1/2 = 5.75(3) years) radioisotope subsequently decays to ²²⁸Ac. Because the ²²⁸Ac decay rate [t_1/2 = 6.15(2) h] is fast, ²²⁸Ac rapidly regenerates after being harvested from the ²²⁸Ra parent. The resulting ²²⁸Ac generator provides frequent and long-term access (of many years) to the spectroscopically "friendly" ²²⁸Ac radionuclide. We have demonstrated that the ²²⁸Ac product can be routinely "milked" from this generator on a daily basis, in chemically pure form, with high specific activity and in excellent yield (∼95%). Hence, in the same way that developing synthesis routes to new starting materials has advanced coordination chemistry for many metals by broadening access, this ²²⁸Ac generator has the potential to broaden actinium access for the inorganic community, facilitating the characterization of actinium chemical behavior.

Radiolabelled Peptides for Positron Emission Tomography and Endoradiotherapy in Oncology

This review deals with the development of peptide-based radiopharmaceuticals for the use with positron emission tomography and peptide receptor radiotherapy. It discusses the pros and cons of this class of radiopharmaceuticals as well as the different labelling strategies, and summarises approaches to optimise metabolic stability. Additionally, it presents different target structures and addresses corresponding tracers, which are already used in clinical routine or are being investigated in clinical trials.

Chelation with a twist: a bifunctional chelator to enable room temperature radiolabeling and targeted PET imaging with scandium-44

Scandium-44 has emerged as an attractive, novel PET radioisotope with ideal emission properties and half-life (t 1/2 = 3.97 h, E mean β+ = 632 keV) well matched to the pharmacokinetics of small molecules, peptides and small biologics. Conjugates of the current gold-standard chelator for 44Sc, 1,4,7,10-tetraaza-cyclododecane-1,4,7,10-tetraacetic acid (DOTA), require heating to achieve radiochemical complexation, limiting application of this isotope in conjunction with temperature-sensitive biologics. To establish Sc(iii) isotopes as broadly applicable tools for nuclear medicine, development of alternative bifunctional chelators is required. To address this need, we characterized the Sc(iii)-chelation properties of the small-cavity triaza-macrocycle-based, picolinate-functionalized chelator H3mpatcn. Spectroscopic and radiochemical studies establish the [Sc(mpatcn)] complex as kinetically inert and appropriate for biological applications. A proof-of-concept bifunctional conjugate targeting the prostate-specific membrane antigen (PSMA), picaga-DUPA, chelates 44Sc to form 44Sc(picaga)-DUPA at room temperature with an apparent molar activity of 60 MBq μmol-1 and formation of inert RRR-Λ and SSS-Δ-twist isomers. Sc(picaga)-DUPA exhibits a K i of 1.6 nM for PSMA, comparable to the 18F-based imaging probe DCFPyL (K i = 1.1 nM) currently in phase 3 clinical trials for imaging prostate cancer. Finally, we successfully employed 44Sc(picaga)-DUPA to image PSMA-expressing tumors in a preclinical mouse model, establishing the picaga bifunctional chelator as an optimal choice for the 44Sc PET nuclide.

Establishing Radiolanthanum Chemistry for Targeted Nuclear Medicine Applications

We report the first targeted nuclear medicine application of the lanthanum radionuclides ^132/135 La. These isotopes represent a matched pair for diagnosis via the positron emissions of ¹³² La and therapy mediated by the Auger electron emissions of ¹³⁵ La. We identify two effective chelators, known as DO3Apic and macropa, for these radionuclides. The 18-membered macrocycle, macropa, bound ^132/135 La with better molar activity than DO3Apic under similar conditions. These chelators were conjugated to the prostate-specific membrane antigen (PSMA)-targeting agent DUPA to assess the use of radiolanthanum for in vivo imaging. The ^132/135 La-labeled targeted constructs showed high uptake in tumor xenografts expressing PSMA. This study validates the use of these radioactive lanthanum isotopes for imaging applications and motivates future work to assess the therapeutic effects of the Auger electron emissions of ¹³⁵ La.

Medical isotope collection from ISAC targets

The ISAC facility (Isotope Separation and Acceleration) at TRIUMF has recently started to provide isotopes for pre-clinical nuclear medicine studies. By irradiating ISOL (Isotope Separation OnLine) targets with a 480 MeV proton beam from the TRIUMF H- cyclotron, the facility can deliver a large variety of radioactive isotope beams (RIB) for research in the fields of nuclear astrophysics, nuclear structure and material science with half-lives down to a few milliseconds via an electrostatic beamline network. For the collection of medical isotopes, typically with half-lives in the range of hours or days, we have developed a compact apparatus for the implantation of mass-separated RIB on a target disc at energies between 20-55 keV. In this paper, we also discuss two different retrieval methods of the implanted activity from the implantation target: by chemical etching of the target surface and by recoil collection of implanted alpha emitters.

Multifunctional, histidine-tagged polymers: antibody conjugation and signal amplification

A polymer scaffold, with multiple reactive centres, was synthesised by RAFT polymerisation and conjugated to the antibody herceptin. A hexahistidine RAFT agent enabled simple purification of polymer–protein conjugates.

Advancing Chelation Chemistry for Actinium and Other +3 f-Elements, Am, Cm, and La

A major chemical challenge facing implementation of ²²⁵Ac in targeted alpha therapy-an emerging technology that has potential for treatment of disease-is identifying an ²²⁵Ac chelator that is compatible with in vivo applications. It is unclear how to tailor a chelator for Ac binding because Ac coordination chemistry is poorly defined. Most Ac chemistry is inferred from radiochemical experiments carried out on microscopic scales. Of the few Ac compounds that have been characterized spectroscopically, success has only been reported for simple inorganic ligands. Toward advancing understanding in Ac chelation chemistry, we have developed a method for characterizing Ac complexes that contain highly complex chelating agents using small quantities (μg) of ²²⁷Ac. We successfully characterized the chelation of Ac³⁺ by DOTP^8- using EXAFS, NMR, and DFT techniques. To develop confidence and credibility in the Ac results, comparisons with +3 cations (Am, Cm, and La) that could be handled on the mg scale were carried out. We discovered that all M³⁺ cations (M = Ac, Am, Cm, La) were completely encapsulated within the binding pocket of the DOTP^8- macrocycle. The computational results highlighted the stability of the M(DOTP)^5- complexes.

Development of Targeted Alpha Particle Therapy for Solid Tumors

Targeted alpha-particle therapy (TAT) aims to selectively deliver radionuclides emitting α-particles (cytotoxic payload) to tumors by chelation to monoclonal antibodies, peptides or small molecules that recognize tumor-associated antigens or cell-surface receptors. Because of the high linear energy transfer (LET) and short range of alpha (α) particles in tissue, cancer cells can be significantly damaged while causing minimal toxicity to surrounding healthy cells. Recent clinical studies have demonstrated the remarkable efficacy of TAT in the treatment of metastatic, castration-resistant prostate cancer. In this comprehensive review, we discuss the current consensus regarding the properties of the α-particle-emitting radionuclides that are potentially relevant for use in the clinic; the TAT-mediated mechanisms responsible for cell death; the different classes of targeting moieties and radiometal chelators available for TAT development; current approaches to calculating radiation dosimetry for TATs; and lead optimization via medicinal chemistry to improve the TAT radiopharmaceutical properties. We have also summarized the use of TATs in pre-clinical and clinical studies to date.

Cyclobutane-Containing Scaffolds as Useful Intermediates in the Stereoselective Synthesis of Suitable Candidates for Biomedical Purposes: Surfactants, Gelators and Metal Cation Ligands

Efficient and versatile synthetic methodologies are reported for the preparation of products that are suitable candidates to be used as surfactants, gelators for hydroxylic solvents or metal cation ligands, with potential use in several fields including biomedical applications. The common structural feature of all the synthesized products is the presence of a cis or trans-1,2- or cis-1,3-difunctionalized cyclobutane ring. In the two first cases, the key intermediates including enantiomerically pure 1,3-diamines and 1,3-amino alcohols have been prepared from β-amino acid derivatives obtained, in turn, from a chiral half-ester. This compound is also precursor of γ-amino esters. Furthermore, two kind of polydentate ligands have also been synthesized from a symmetric 1,5-diamine obtained from norpinic acid, which was easily prepared from commercial verbenone.

Cationic radionuclides and ligands for targeted therapeutic radiopharmaceuticals

This review considers the already used and potential α- and β-emitting cationic radionuclides for targeted radionuclide therapy. Recent results of laboratory, preclinical and clinical applications of these radionuclides are discussed. As opposed to β-emitters, which are already used in nuclear medicine, α-emitters involved in targeted radiopharmaceuticals were subjected to clinical trials only recently and were found to be therapeutically effective. The review summarizes recent trends in the development of ligands as components of radiopharmaceuticals addressing specific features of short-lived cationic radionuclides applied in medicine. Despite a steadily growing number of chelating ligands, 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) and diethylenetriaminepentaacetic acid (DTPA) remain the most widely used agents in nuclear medicine. The drawbacks of these compounds restrict the application of radionuclides in medicine. Variations in the macrocycle size, the introduction and modification of substituents can significantly improve the chelating ability of ligands, enhance stability of radionuclide complexes with these ligands and eliminate the influence of ligands on the affinity of biological targeting vectors.

The bibliography includes 189 references.

3D-printed automation for optimized PET radiochemistry

Reproducible batch synthesis of radioligands for imaging by positron emission tomography (PET) in a manner that maximizes ligand yield, purity, and molar activity, and minimizes cost and exposure to radiation, remains a challenge, as new and synthetically complex radioligands become available. Commercially available automated synthesis units (ASUs) solve many of these challenges but are costly to install and cannot always accommodate diverse chemistries. Through a reiterative design process, we exploit the proliferation of three-dimensional (3D) printing technologies to translate optimized reaction conditions into ASUs composed of 3D-printed, electronic, and robotic parts. Our units are portable and robust and reduce radiation exposure, shorten synthesis time, and improve the yield of the final radiopharmaceutical for a fraction of the cost of a commercial ASU. These 3D-printed ASUs highlight the gains that can be made by designing a fit-for-purpose ASU to accommodate a synthesis over accommodating a synthesis to an unfit ASU.

Evaluation of polydentate picolinic acid chelating ligands and an α-melanocyte-stimulating hormone derivative for targeted alpha therapy using ISOL-produced 225Ac

BACKGROUND

Actinium-225 (²²⁵Ac, t_1/2 = 9.9 d) is a promising candidate radionuclide for use in targeted alpha therapy (TAT), though the currently limited global supply has hindered the development of a suitable Ac-chelating ligand and ²²⁵Ac-radiopharmaceuticals towards the clinic. We at TRIUMF have leveraged our Isotope Separation On-Line (ISOL) facility to produce ²²⁵Ac and use the resulting radioactivity to screen a number of potential ²²⁵Ac-radiopharmaceutical compounds.

RESULTS

MBq quantities of ²²⁵Ac and parent radium-225 (²²⁵Ra, t_1/2 = 14.8 d) were produced and separated using solid phase extraction DGA resin, resulting in a radiochemically pure ²²⁵Ac product in > 98% yield and in an amenable form for radiolabeling of ligands and bioconjugates. Of the many polydentate picolinic acid ("pa") containing ligands evaluated (H₄octapa [N₄O₄], H₄CHXoctapa [N₄O₄], p-NO₂-Bn-H₄neunpa [N₅O₄], and H₆phospa [N₄O₄]), all out-performed the current gold standard, DOTA for ²²⁵Ac radiolabeling ability at ambient temperature. Moreover, a melanocortin 1 receptor-targeting peptide conjugate, DOTA-modified cyclized α-melanocyte-stimulating hormone (DOTA-CycMSH), was radiolabeled with ²²⁵Ac and proof-of-principle biodistribution studies using B16F10 tumour-bearing mice were conducted. At 2 h post-injection, tumour-to-blood ratios of 20.4 ± 3.4 and 4.8 ± 2.4 were obtained for the non-blocking (molar activity [M.A.] > 200 kBq/nmol) and blocking (M.A. = 1.6 kBq/nmol) experiment, respectively.

CONCLUSION

TRIUMF's ISOL facility is able to provide ²²⁵Ac suitable for preclinical screening of radiopharmaceutical compounds; [²²⁵Ac(octapa)]^-, [²²⁵Ac(CHXoctapa)]^-, and [²²⁵Ac(DOTA-CycMSH)] may be good candidates for further targeted alpha therapy studies.

Production and in vivo PET/CT imaging of the theranostic pair 132/135La

The present study describes a novel method for the low energy cyclotron production and radiochemical isolation of no-carrier-added ^132/135La³⁺ from bulk ^natBa. This separation strategy combines precipitation and single-column extraction chromatography to afford an overall radiochemical yield (92 ± 2%) and apparent molar activity (22 ± 4 Mbq/nmol) suitable for the radiolabeling of DOTA-conjugated vectors. The produced ^132/135La³⁺ has a radiochemical and radionuclidic purity amenable for ¹³²La/¹³⁵La-based cancer theranostic applications. Longitudinal PET/CT images acquired using the positron-emitting ¹³²La and ex vivo biodistribution data separately corroborated the accumulation of unchelated ^132/135La³⁺ ions in bone and the liver.

Anion Control of Lanthanoenediyne Cyclization

A suite of lanthanoenediyne complexes of the form Ln(macrocycle)X₃ (Ln = La³⁺, Ce³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Lu³⁺; X = NO₃^-, Cl^-, OTf^-) was prepared by utilizing an enediyne-containing [2 + 2] hexaaza-macrocycle (2). The solid-state Bergman cyclization temperatures, measured via DSC, decrease with the denticity of X (bidentate NO₃^-, T = 267-292 °C; monodentate Cl^-, T = 238-262 °C; noncoordinating OTf^-, T = 170-183 °C). ¹³C NMR characterization shows that the chemical shifts of the acetylenic carbon atoms also rely on the anion identity. The alkyne carbon closest to the metal binding site, C_A, exhibits a Δδ > 3 ppm downfield shift, while the more distal alkyne carbon, C_B, displays a concomitant Δδ ≤ 2.5 ppm upfield shift, reflecting a depolarization of the alkyne on metal inclusion. For all metals studied, the degree of perturbation follows the trend 2 < NO₃^- < Cl^- < OTf^-. This belies a greater degree of electronic rearrangement in the coordinated macrocycle as the denticity of X and its accompanying shielding of the metal's Lewis acidity decrease. Computationally modeled structures of LnX₃ show a systematic increase in the lanthanide-2 coordination number (CN_La-mc = 2 (NO₃^-), 4 (Cl^-), 5 (H₂O, model for OTf^-)) and a decrease in the mean Ln-N bond length (La-N_average = 2.91 Å (NO₃^-), 2.78 Å (Cl^-), 2.68 Å (H₂O)), further suggesting that a decrease in the anion coordination number correlates with an increase in the metal-macrocycle interaction. Taken together, these data illustrate a Bergman cyclization landscape that is influenced by the bonding of metal to an enediyne ligand but whose reaction barrier is ultimately dominated by the coordinating ability of the accompanying anion.

Radiochemical aspects of alpha emitting radionuclides for medical application

The use of α-emitting radionuclides in targeted alpha therapy (TAT) holds great potential for treatment of human diseases, such as cancer, due to the short pathlength and high potency of the α particle, which can localize damage to targeted cells while minimizing effects to healthy surrounding tissues. In this review several potential α-emitting radionuclides having emission properties applicable to TAT are discussed from a radiochemical point of view. Overviews of production, radiochemical separation and chelation aspects relative to developing TAT radiopharmaceuticals are provided for the α-emitting radionuclides (and their generator systems) 211At, 224Ra/212Pb/212Bi, 225Ac/213Bi, 227Th/223Ra, 230U/226Th, 149Tb and 255Fm.

Optimization of Cation Exchange for the Separation of Actinium-225 from Radioactive Thorium, Radium-223 and Other Metals

Actinium-225 (²²⁵Ac) can be produced with a linear accelerator by proton irradiation of a thorium (Th) target, but the Th also underdoes fission and produces 400 other radioisotopes. No research exists on optimization of the cation step for the purification. The research herein examines the optimization of the cation exchange step for the purification of ²²⁵Ac. The following variables were tested: pH of load solution (1.5–4.6); rinse steps with various concentrations of HCl, HNO₃, H₂SO₄, and combinations of HCl and HNO₃; various thorium chelators to block retention; MP50 and AG50 resins; and retention of 20–45 elements with different rinse sequences. The research indicated that HCl removes more isotopes earlier than HNO₃, but that some elements, such as barium and radium, could be eluted with ≥2.5 M HNO₃. The optimal pH of the load solution was 1.5–2.0, and the optimized rinse sequence was five bed volumes (BV) of 1 M citric acid pH 2.0, 3 BV of water, 3 BV of 2 M HNO₃, 6 BV of 2.5 M HNO₃ and 20 BV of 6 M HNO₃. The sequence recovered >90% of ²²⁵Ac with minimal ²²³Ra and thorium present.

Mini-review: Targeted radiopharmaceuticals incorporating reversible, low molecular weight albumin binders

The combination of low molecular weight, reversible human serum albumin (HSA) binders with targeted radiopharmaceuticals in dual-targeted radioconjugates holds great promise, in particular for endoradiotherapy. Attachment of HSA-binders to radiopharmaceuticals extends their blood circulation time and results in an enhanced tumour uptake as well as often in an improved pharmacokinetic profile. In this mini-review, an overview of currently pursued approaches of this novel strategy is provided.

Bench to Bedside: Albumin Binders for Improved Cancer Radioligand Therapies

Radioligand therapy (RLT) relies on the use of pharmacophores to selectively deliver ionization energy to cancers to exert its tumoricidal effects. Cancer cells that are not directly targeted by a radioconjugate remain susceptible to RLT because of the crossfire effect. This is significant given the inter- and intra-heterogeneity of tumors. In recent years, reversible albumin binders have been used as simple "add-ons" for radiopharmaceuticals to modify pharmacokinetics and to enhance therapeutic efficacy. In this Review, we discuss recent advances in albumin binder platforms used in RLT, with an emphasis on 4-( p-iodophenyl)butyric acid and Evans blue derivatives. We focus on four biological systems pertinent to oncology that utilize this class of compounds: folate receptor, integrin αvβ3, somatostatin receptor, and prostate-specific membrane antigen. Finally, we offer our perspectives on albumin binders for RLT, highlighting future areas of research that will help propel the technology further for clinical use.

An Overview of Targeted Alpha Therapy with 225Actinium and 213Bismuth

Background:

Recent reports of the remarkable therapeutic efficacy of 225Ac-labeled PSMA-617 for therapy of metastatic castration-resistant prostate cancer have under-lined the clinical potential of targeted alpha therapy.

Objective and Conclusion:

This review describes methods for the production of 225Ac and its daughter nuclide 213Bi and summarizes the current clinical experience with both alpha emitters with particular focus on recent studies of targeted alpha therapy of bladder cancer, brain tu-mors, neuroendocrine tumors and prostate cancer.

Synthesis, structure and metal ion coordination of novel benzodiazamacrocyclic ligands bearing pyridyl and picolinate pendant side-arms

Complex formation of benzodiazacrown ethers with heavy and transition metal ions was studied using NMR spectroscopy, potentiometry and X-ray crystallography.

211At-labeled immunoconjugate via a one-pot three-component double click strategy: practical access to α-emission cancer radiotherapeutics

α-Emission radiotherapeutics has potential to be one of most effective cancer therapeutics. Herein, we report a facile synthesis of an ²¹¹At-labeled immunoconjugate for use as an α-emission molecular targeting therapy. We synthesized a tetrazine probe modified with closo-decaborate(2-), a prosthetic group that forms a bioavailable stable complex with ²¹¹At. Our one-pot three-component double-click labeling method was used to attach decaborate to trastuzumab (anti-HER2 antibody) using decaborate-tetrazine and TCO-aldehyde probes without reducing the antibody binding affinity. Labeling the decaborate-attached trastuzumab with ²¹¹At produced in the cyclotron at the RIKEN Nishina Center, at which highly radioactive ²¹¹At can be produced, readily furnished the ²¹¹At-labeled trastuzumab with a maximum specific activity of 15 MBq μg^-1 and retention of the native binding affinity. Intratumor injection of the ²¹¹At-labeled trastuzumab in BALB/c nude mice implanted with HER2-expressing epidermoid cancer cells yielded efficient accumulation at the targeted tumor site as well as effective suppression of tumor growth.

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.

A Single Dose of 225Ac-RPS-074 Induces a Complete Tumor Response in an LNCaP Xenograft Model

Promising biochemical responses to ²²⁵Ac-prostate-specific membrane antigen (PSMA) 617, even in patients who are refractory to β-particle radiation, illustrate the potential of targeted α-therapy for the treatment of metastatic castration-resistant prostate cancer. However, side effects such as xerostomia are severe and irreversible. To fully harness the potential of targeted α-therapy, it is necessary to increase the therapeutic index of the targeted radioligands. One emerging strategy is to increase clearance half-life through enhanced binding to serum albumin. We have evaluated the albumin-binding PSMA-targeting ligand RPS-074 in a LNCaP xenograft model to determine its potential value to the treatment of prostate cancer. Methods: ²²⁵Ac-RPS-074 was evaluated in male BALB/c mice bearing LNCaP xenograft tumors. A biodistribution study was performed over 21 d to determine the dosimetry in tumors and normal tissue. The dose response was measured in groups of 7 mice using 37, 74, and 148 kBq of ²²⁵Ac-RPS-074 and compared with positive and negative control groups. Mice were sacrificed when tumor volume exceeded 1,500 mm³ Results: ²²⁵Ac-RPS-074 was labeled in greater than 98% radiochemical yield and showed high (>10% injected dose/g) and sustained accumulation in LNCaP tumors from 24 h to beyond 14 d. Signal in blood and highly vascularized tissues was evident over the first 24 h after injection and cleared by 7 d. The tumor-to-kidney ratio was 4.3 ± 0.7 at 24 h and 62.2 ± 9.5 at 14 d. A single injection of 148 kBq induced a complete response in 6 of 7 tumors, with no apparent toxic effects. Treatment with 74 kBq induced a partial response in 7 of 7 tumors, but from 42 d, 6 of 7 experienced significant regrowth. The 37-kBq group experienced a survival benefit relative to the negative control but not compared with the positive control group. Conclusion: A single dose of 148 kBq of ²²⁵Ac-RPS-074 induced a complete response in 86% of tumors, with tumor-to-normal-tissue ratios that predict an improved therapeutic index. The use of the macropa chelator enabled quantitative radiolabeling and may facilitate the clinical translation of this promising targeted α-therapeutic.

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.

Chemical aspects of metal ion chelation in the synthesis and application antibody-based radiotracers

Radiometals are becoming increasingly accessible and are utilized frequently in the design of radiotracers for imaging and therapy. Nuclear properties ranging from the emission of γ-rays and β+ -particles (imaging) to Auger electron and β- and α-particles (therapy) in combination with long half-lives are ideally matched with the relatively long biological half-life of monoclonal antibodies in vivo. Radiometal labeling of antibodies requires the incorporation of a metal chelate onto the monoclonal antibody. This chelate must coordinate the metal under mild conditions required for the handling of antibodies, as well as provide high kinetic, thermodynamic, and metabolic stability once the metal ion is coordinated to prevent release of the radionuclide before the target site is reached in vivo. Herein, we review the role of different radiometals that have found applications of the design of radiolabeled antibodies for imaging and radioimmunotherapy. Each radionuclide is described regarding its nuclear synthesis, coordinative preference, and radiolabeling properties with commonly used and novel chelates, as well as examples of their preclinical and clinical applications. An overview of recent trends in antibody-based radiopharmaceuticals is provided to spur continued development of the chemistry and application of radiometals for imaging and therapy.

Actinium-225 for Targeted α Therapy: Coordination Chemistry and Current Chelation Approaches

The α-emitting radionuclide actinium-225 possesses nuclear properties that are highly promising for use in targeted α therapy (TAT), a therapeutic strategy that employs α particle emissions to destroy tumors. A key factor, however, that may hinder the clinical use of actinium-225 is the poor understanding of its coordination chemistry, which creates challenges for the development of suitable chelation strategies for this ion. In this article, we provide an overview of the known chemistry of actinium and a summary of the chelating agents that have been explored for use in actinium-225-based TAT. This overview provides a starting point for researchers in the field of TAT to gain an understanding of this valuable therapeutic radionuclide.

Bispidines for radiopharmaceuticals

Radiometal based radiopharmaceuticals for imaging and therapy require selective ligands (bifunctional chelators, BFCs) that form metal complexes, which are inert against trans-chelation under physiological conditions, linked to a biological vector, directing them to the targeted tissue. Bispidine ligands with a very rigid backbone and widely variable donor sets are reviewed as an ideal class of BFCs, and recent applications are discussed.

Re-thinking the role of radiometal isotopes: Towards a future concept for theranostic radiopharmaceuticals

The potential and future role of certain metal radionuclides, for example, ⁴⁴ Sc, ⁸⁹ Zr, ⁸⁶ Y, ⁶⁴ Cu, ⁶⁸ Ga, ¹⁷⁷ Lu, ²²⁵ Ac, and ²¹³ Bi, and several terbium isotopes has been controversially discussed in the past decades. Furthermore, the possible benefits of "matched pairs" of isotopes for tandem applications of diagnostics and therapeutics (theranostics) have been emphasized, while such approaches still have not made their way into routine clinical practice. Analysis of bibliographical data illustrates how popularity of certain nuclides has been promoted by cycles of availability and applications. We furthermore discuss the different practical requirements for diagnostic and therapeutic radiopharmaceuticals and the resulting consequences for efficient development of clinically useful pairs of radionuclide theranostics, with particular emphasis on the underlying economical factors. Based on an exemplary assessment of overall production costs for ⁶⁸ Ga and ¹⁸ F radiopharmaceuticals, we venture a look into the future of theranostics and predict that high-throughput PET applications, that is, diagnosis of frequent conditions, will ultimately rely on ¹⁸ F tracers. PET radiometals will occupy a niche in the clinical low-throughput sector (diagnosis of rare diseases), but above all, dominate preclinical research and clinical translation. Matched isotope pairs will be of lesser relevance for theranostics but may become important for future PET-based therapeutic dosimetry.