Recent progress of astatine-211 in endoradiotherapy: Great advances from fundamental properties to targeted radiopharmaceuticals

Locoregional radionuclide therapy of glioblastoma with [211At]At-PDA-FAPI

Glioblastoma is the most common and aggressive tumor of the central nervous system. Locoregional administration of therapeutic radiopharmaceuticals appears to be a promising modality for recurrent glioblastomas. In this study, fibroblast activation protein alpha (FAPα) targeting molecule fibroblast activation protein inhibitor-04 (FAPI-04) was conjugated to polydopamine (PDA) nanoparticles, and then, α-emitter astatine-211 was labeled onto the nanocomposite to form [211At]At-PDA-FAPI. In vitro, [211At]At-PDA-FAPI was able to significantly reduce the cell viability, induce DSB formation, arrest cell cycle at G2/M phase and promote cell apoptosis. Furthermore, [211At]At-PDA-FAPI exhibited effective tumor inhibition ability in U87MG xenografts. Mice received 0.56 MBq [211At]At-PDA-FAPI showed a reduced tumor volume of approximately 65% on the 9th day after injection, and the median survival in this group (48 days) was obviously improved compared with that in the saline group (18 days). Meanwhile, increased apoptosis was also observed in tumor sites after [211At]At-PDA-FAPI treatment. In addition, H&E analysis of major organs confirmed the biological safety of [211At]At-PDA-FAPI. This study provides an effective and promising strategy for locoregional treatment of glioblastoma.

The halogen(I) complex of astatine: a theoretical perspective on structural and bonding properties

Halonium ions (X+) can interact with two Lewis bases to form linear 3c4e halogen-bonded halogen(I) complexes ([D⋯X⋯D]+), which have been found to be useful in organic synthesis and supramolecular chemistry. However, current research is limited to lighter halogens (F, Cl, Br, and I) and does not include the At element owing to the lack of stable isotopes for experimental studies. Herein, we explore the structural and bonding properties of an At-mediated 3c4e halogen(I) complex ([D⋯At⋯D]+) and the effects of various Lewis bases, substituents, and halogens using relativistic density functional theory (DFT) and the coupled-cluster approach with single, double and perturbative triple excitation (CCSD(T)) calculations. Theoretical calculation results show that At, similar to other halogens, can form a linear [D⋯At⋯D]+ structure with equal bond lengths from the halonium ion to two donor atoms. The physical nature of the interaction and electronic structure of the At-mediated 3c4e halogen(I) complex are the same as those of the halogen(I) complexes of light halogens. Interestingly, the positive correlation between the polarizability of the halogen and the interaction between D and [D⋯X]+ fragments (X = F to At) observed at the scalar relativistic level does not hold when considering spin-orbit coupling effects on the At atom. This work deepens the understanding of the halogen bonds of At, and the stable [D⋯At⋯D]+ structure offers new insights into At coordination chemistry and the relevant experimental study of radiolabeling of At.

Alpha Atlas: Mapping global production of α-emitting radionuclides for targeted alpha therapy

Targeted Alpha Therapy has shown great promise in cancer treatment, sparking significant interest over recent decades. However, its broad adoption has been impeded by the scarcity of alpha-emitters and the complexities related to their use. The availability of these radionuclides is often constrained by the intricate production processes and purification, as well as regulatory and logistical challenges. Moreover, the high cost and technical difficulties associated with handling and applying alpha-emitting radionuclides pose additional barriers to their clinical implementation. This Alpha Atlas provides an in-depth overview of the leading alpha-particle emitting radionuclide candidates for clinical use, focusing on their production processes and supply chains. By mapping the current facilities that produce and supply these radionuclides, this atlas aims to assist researchers, clinicians, and industries in initiating or scaling up the applications of alpha-emitters. The Alpha Atlas aspires to act as a strategic guide, facilitating collaboration and driving forward the integration of these potent therapeutic agents into cancer treatment practices.

211At radiolabeled APBA-FAPI for enhanced targeted-alpha therapy of glioma

Fibroblast activation protein-α (FAPα) is highly expressed in tumor-associated cells and has become one of the most attractive targeting sites in cancer diagnosis and therapy. To ameliorate the rapid metabolism of FAPα inhibitor (FAPI), here, a multifunctional binding agent was introduced to simultaneously achieve 211At radiolabeling and tumor retention prolongation of corresponding radiolabeled drug. 211At-APBA-FAPI was successfully synthesized by conjugating 211At with the designed FAPI carrier in satisfactory radiochemical yield (>60 %). 211At-APBA-FAPI exhibited excellent in vitro stability, significant tumor affinity and specific killing effect on FAPα-positive U87MG cells. Molecular docking reveals that FAPI decorated with albumin binder can bind with FAPα protein via multiple intermolecular interactions with a considerable binding energy of -9.66 kcal/mol 211At-APBA-FAPI exhibits good targeting in murine xenograft models, showing obviously longer tumor retention than previously-reported radioastatinated compound. As a result, 211At-APBA-FAPI presents pronounced therapeutic effect with ignorable normal organs/tissues biotoxicity. All these indicate that introducing a multifunctional binding agent can effectively enhance the availability of FAPI for 211At conjugation and tumoricidal effect, providing vital hints for the translation of targeted-alpha therapy based on radiolabeled FAPI derivatives.

Nanoradiopharmaceuticals: An Attractive Concept in Oncotherapy

Radiopharmaceuticals are of significant importance in the fields of tumor imaging and therapy. In recent decades, the increasing role of nanotechnology has led to the attractive concept of nanoradiopharmaceuticals. Consequently, it is imperative to provide a concise summary of the necessary guidelines to facilitate the translation of nanoradiopharmaceuticals. In this work, we have presented the contents of radiolabeling strategies and some applications of nanoradiopharmaceuticals. Such a framework can assist researchers in identifying more pertinent insights or making more informed decisions in the study of nanoradiopharmaceuticals.

Strong Affinity between Astatine and Silver: An Available Approach to Anchoring 211At in Nanocarrier for Locoregional Oncotherapy

Recently, 211At-related endoradiotherapy has emerged as an important oncotherapy strategy. Conjugating 211At with a nanocarrier provides a vital candidate for radionuclide therapy of various malignant tumors. In this study, we proposed utilizing the intrinsically high affinity of heavy halogens and sulfhydryl compounds for metallic silver to achieve highly efficient conjugation between 211At and Ag-based nanoparticles in a simple way. 211At@Ag-PEG-FA was obtained via a one-pot assembly of 211At, Ag, and SH-PEG-FA in extremely high radiolabeling yield (>95%) within 15 min and maintained excellent stability in simulated physiochemical media. Additionally, the prepared 211At@Ag-PEG-FA demonstrated specific binding to the breast cancer cell line (4T1), with a high endocytosis rate and low reflux, leading to significant cell growth inhibition. 211At@Ag-PEG-FA exhibits an excellent antitumor effect that completely suppressed tumor growth during the first week, effectively prolonging the median survival of mice to 44 days, relative to 18 days in the control group. All of the mice exhibited minimal side effects from 211At@Ag-PEG-FA in the experiment, indicating its acceptable biosafety. Our work shows that the strong affinity of Ag can be utilized to produce radioactivated nanomedicines with excellent stability and high efficiency, which also provides some valuable insights for the 211At radiolabeling of general compounds.

Preparation of Medical 228Th-224Ra Radionuclide Generator Based on SiO2@TiO2 Microspheres

224Ra (T1/2 = 3.63 d), an α-emitting radionuclide, holds significant promise in cancer endoradiotherapy. Current 224Ra-related therapy is still scarce because of the lack of reliable radionuclide supply. The 228Th-224Ra radionuclide generator can undoubtedly introduce continuous and sustainable availability of 224Ra for advanced nuclear medicine. However, conventional metal oxides for such radionuclide generators manifest suboptimal adsorption capacities for the parent nuclide, primarily attributable to their limited surface area. In this work, core-shell SiO2@TiO2 microspheres were proposed to develop as column materials for the construction of a 228Th-224Ra generator. SiO2@TiO2 microspheres were well prepared and systematically characterized, which has also been demonstrated to have good adsorption capacity to 228Th and very weak binding affinity toward 224Ra via simulated chemical separation. Upon introducing 228Th-containing solution onto the SiO2@TiO2 functional column, a 228Th-224Ra generator with excellent retention of the parent radionuclide and ideal elution efficiency of daughter radionuclide was obtained. The prepared 228Th-224Ra generator can produce 224Ra with high purity and medical usability in good elution efficiency (98.72%) even over five cycles. To the best of our knowledge, this is the first time that the core-shell mesoporous materials have been applied in a radionuclide generator, which can offer valuable insights for materials chemistry, radiochemical separation, and biological medicine.

The Different Strategies for the Radiolabeling of [211At]-Astatinated Radiopharmaceuticals

Astatine-211 (211At) has emerged as a promising radionuclide for targeted alpha therapy of cancer by virtue of its favorable nuclear properties. However, the limited in vivo stability of 211At-labeled radiopharmaceuticals remains a major challenge. This review provides a comprehensive overview of the current strategies for 211At radiolabeling, including nucleophilic and electrophilic substitution reactions, as well as the recent advances in the development of novel bifunctional coupling agents and labeling approaches to enhance the stability of 211At-labeled compounds. The preclinical and clinical applications of 211At-labeled radiopharmaceuticals, including small molecules, peptides, and antibodies, are also discussed. Looking forward, the identification of new molecular targets, the optimization of 211At production and quality control methods, and the continued evaluation of 211At-labeled radiopharmaceuticals in preclinical and clinical settings will be the key to realizing the full potential of 211At-based targeted alpha therapy. With the growing interest and investment in this field, 211At-labeled radiopharmaceuticals are poised to play an increasingly important role in future cancer treatment.

A Proof-of-Concept Study on the Theranostic Potential of 177 Lu-labeled Biocompatible Covalent Polymer Nanoparticles for Cancer Targeted Radionuclide Therapy

Theranostic nanomedicine combined bioimaging and therapy probably rises more helpful and interesting opportunities for personalized medicine. In this work, 177 Lu radiolabeling and surface PEGylation of biocompatible covalent polymer nanoparticles (CPNs) have generated a new theranostic nanoformulation (177 Lu-DOTA-PEG-CPNs) for targeted diagnosis and treatment of breast cancer. The in vitro anticancer investigations demonstrate that 177 Lu-DOTA-PEG-CPNs possess excellent bonding capacity with breast cancer cells (4T1), inhibiting the cell viability, leading to cell apoptosis, arresting the cell cycle, and upregulating the reactive oxygen species (ROS), which can be attributed to the good targeting ability of the nanocarrier and the strong relative biological effect of the radionuclide labelled compound. Single photon emission computed tomography/ computed tomography (SPECT/CT) imaging and in vivo biodistribution based on 177 Lu-DOTA-PEG-CPNs reveal that notable radioactivity accumulation at tumor site in murine 4T1 models with both intravenous and intratumoral administration of the prepared radiotracer. Significant tumor inhibition has been observed in mice treated with 177 Lu-DOTA-PEG-CPNs, of which the median survival was highly extended. More strikingly, 50 % of mice intratumorally injected with 177 Lu-DOTA-PEG-CPNs was cured and showed no tumor recurrence within 90 days. The outcome of this work can provide new hints for traditional nanomedicines and promote clinical translation of 177 Lu radiolabeled compounds efficiently.

Biocompatible conjugated polymer nanoparticles labeled with 225Ac for tumor endoradiotherapy

Recently, endoradiotherapy based on actinium-225 (225Ac) has attracted increasing attention, which is due to its α particles can generate maximal damage to cancer cells while minimizing unnecessary radiation effects on healthy tissues. Herein, 111In/225Ac-radiolabeled conjugated polymer nanoparticles (CPNs) coated with amphiphilic polymer DSPE-PEG-DOTA have been developed as a new injectable nano-radiopharmaceuticals for cancer endoradiotherapy under the guidance of nuclear imaging. Single photon emission computed tomography/computed tomography (SPECT/CT) using 111In-DOTA-PEG-CPNs as nano probe indicates a prolonged retention of radiolabeled nanocarriers, which was consistent with the in vivo biodistribution examined by direct radiometry analysis. Significant inhibition of tumor growth has been observed in murine 4T1 models treated with 225Ac-DOTA-PEG-CPNs when compared to mice treated with PBS or DOTA-PEG-CPNs. The 225Ac-DOTA-PEG-CPNs group experienced no single death within 24 days with the median survival considerably extended to 35 days, while all the mice treated with PBS or DOTA-PEG-CPNs died at 20 days post injection. Additionally, the histopathology studies demonstrated no obvious side effects on healthy tissues after treatment with 225Ac-DOTA-PEG-CPNs. All these results reveal that the new 225Ac-labeled DOTA-PEG-CPNs is promising as paradigm for endoradiotherapy.

Next generation fibroblast activation protein (FAP) targeting PET tracers - The tetrazine ligation allows an easy and convenient way to 18F-labeled (4-quinolinoyl)glycyl-2-cyanopyrrolidines

Small-molecular fibroblast activation protein inhibitor (FAPI)-based tracer have been shown to be promising Positron Emission Tomography (PET) 68Ga-labeled radiopharmaceuticals to image a variety of tumors including pancreatic, breast, and colorectal cancers, among others. In this study, we developed a novel 18F-labeled FAPI derivative. [18F]6 was labeled using a synthon approach based on the tetrazine ligation. It showed subnanomolar affinity for the FAP protein and a good selectivity profile against known off-target proteases. Small animal PET studies revealed high tumor uptake and good target-to-background ratios. [18F]6 was excreted via the liver. Overall, [18F]6 showed promising characteristics to be used as a PET tracer and could serve as a lead for further development of halogen-based theranostic FAPI radiopharmaceuticals.

Novel Computational Chemistry Infrastructure for Simulating Astatide in Water: From Basis Sets to Force Fields Using Particle Swarm Optimization

Using the example of astatine, the heaviest naturally occurring halogen whose isotope At-211 has promising medical applications, we propose a new infrastructure for large-scale computational models of heavy elements with strong relativistic effects. In particular, we focus on developing an accurate force field for At- in water based on reliable relativistic density functional theory (DFT) calculations. To ensure the reliability of such calculations, we design novel basis sets for relativistic DFT, via the particle swarm optimization algorithm to optimize the coefficients of the new basis sets and the polarization-consistent basis set idea's extension to heavy elements to eliminate the basis set error from DFT calculations. The resulting basis sets enable the well-grounded evaluation of relativistic DFT against "gold-standard" CCSD(T) results. Accounting for strong relativistic effects, including spin-orbit interaction, via our redesigned infrastructure, we elucidate a noticeable dissimilarity between At- and I- in halide-water force field parameters, radial distribution functions, diffusion coefficients, and hydration energies. This work establishes the framework for the systematic development of polarization-consistent basis sets for relativistic DFT and accurate force fields for molecular dynamics simulations to be used in large-scale models of complex molecular systems with elements from the bottom of the periodic table, including actinides and even superheavy elements.

Production, isolation, and shipment of clinically relevant quantities of astatine-211: A simple and efficient approach to increasing supply

The alpha emitter astatine-211 (211At) is a promising candidate for cancer treatment based on Targeted Alpha (α) Therapy (TAT). A small number of facilities, distributed across the United States, are capable of accelerating α-particle beams to produce 211At. However, challenges remain regarding strategic methods for shipping 211At in a form adaptable to advanced radiochemistry reactions and other uses of the radioisotope.

PURPOSE

Our method allows shipment of 211At in various quantities in a form convenient for further radiochemistry.

PROCEDURES

For this study, a 3-octanone impregnated Amberchrom CG300M resin bed in a column cartridge was used to separate 211At from the bismuth matrix on site at the production accelerator (Texas A&M) in preparation for shipping. Aliquots of 6 M HNO3 containing up to ≈2.22 GBq of 211At from the dissolved target were successfully loaded and retained on columns. Exempt packages (<370 MBq) were shipped to a destination radiochemistry facility, University of Texas MD Anderson Cancer Center, in the form of a convenient air-dried column. Type A packages have been shipped overnight to University of Alabama at Birmingham.

MAIN FINDINGS

Air-dried column hold times of various lengths did not inhibit simple and efficient recovery of 211At. Solution eluted from the column was sufficiently high in specific activity to successfully radiolabel a model compound, 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)aniline (1), with 211At. The method to prepare and ship 211At described in this manuscript has also been used to ship larger quantities of 211At a greater distance to University of Alabama at Birmingham.

PRINCIPAL CONCLUSIONS

The successful proof of this method paves the way for the distribution of 211At from Texas A&M University to research institutions and clinical oncology centers in Texas and elsewhere. Use of this simple method at other facilities has the potential increase the overall availability of 211At for preclinical and clinical studies.

A Smart Benzothiazole-Based Conjugated Polymer Nanoplatform with Multistimuli Response for Enhanced Synergistic Chemo-Photothermal Cancer Therapy

The combination of chemotherapy and phototherapy has received tremendous attention in multimodal cancer therapy. However, satisfactory therapeutic outcomes of chemo-photothermal therapy (chemo-PTT) still remain challenging. Herein, a biocompatible smart nanoplatform based on benzothiazole-linked conjugated polymer nanoparticles (CPNs) is rationally designed, for effectively loading doxorubicin (DOX) and Mo-based polyoxometalate (POM) through both dynamic chemical bond and intermolecular interactions, with an expectation to obtain new anticancer drugs with multiple stimulated responses to the tumor microenvironment (TME) and external laser irradiation. Controlled drug release of DOX from the obtained nanoformulation (CPNs-DOX-PEG-cRGD-BSA@POM) triggered by both endogenous stimulations (GSH and low pH) and exogenous laser irradiation has been well demonstrated by pharmacodynamics investigations. More intriguingly, incorporating POM into the nanoplatform not only enables the nanomedicine to achieve mild hyperthermia but also makes it exhibit self-assembly behavior in acidic TME, producing enhanced tumor retention. Benefiting from the versatile functions, the prepared CPNs-DOX-PEG-cRGD-BSA@POM exhibited excellent tumor targeting and therapeutic effects in murine xenografted models, showing great potential in practical cancer therapy.

Investigating the Heaviest Halogen: Lessons Learned from Modeling the Electronic Structure of Astatine's Small Molecules

We present a systematic study of electron-correlation and relativistic effects in diatomic molecular species of the heaviest halogen astatine (At) within relativistic single- and multireference coupled-cluster approaches and relativistic density functional theory. We establish revised reference ab initio data for the ground states of At₂, HAt, AtAu, and AtO⁺ using a highly accurate relativistic effective core potential model and in-house basis sets developed for accurate modeling of molecules with large spin-orbit effects. Spin-dependent relativistic effects on chemical bonding in the ground state are comparable to the binding energy or even exceed it in At₂. Electron-correlation effects near the equilibrium internuclear separation are mostly dynamical and can be adequately captured using single-reference CCSD(T). However, bond elongation in At₂ and, especially, AtO⁺ results in rapid manifestation of its multireference character. While useful for evaluating the spin-orbit effects on the ground-state bonding and properties, the two-component density functional theory lacks predictive power, especially in combination with popular empirically adjusted exchange-correlation functionals. This drawback supports the necessity to develop new functionals for reliable quantum-chemical models of heavy-element compounds with strong relativistic effects.

Targeted Alpha Therapy of Glioma Using 211At-Labeled Heterodimeric Peptide Targeting Both VEGFR and Integrins

Targeted radionuclide therapy based on α-emitters plays an increasingly important role in cancer treatment. In this study, we proposed to apply a heterodimeric peptide (iRGD-C6-lys-C6-^DA7R) targeting both VEGFR and integrins as a new vector for ²¹¹At radiolabeling to obtain high-performance radiopharmaceuticals with potential in targeted alpha therapy (TAT). An astatinated peptide, iRGD-C6-lys(²¹¹At-ATE)-C6-^DA7R, was prepared with a radiochemical yield of ∼45% and high radiochemical purity of >95% via an electrophilic radioastatodestannylation reaction. iRGD-C6-lys(²¹¹At-ATE)-C6-^DA7R showed good stability in vitro and high binding ability to U87MG (glioma) cells. Systematic in vitro antitumor investigations involving cytotoxicity, apoptosis, distribution of the cell cycle, and reactive oxygen species (ROS) clearly demonstrated that ²¹¹At-labeled heterodimeric peptides could significantly inhibit cell viability, induce cell apoptosis, arrest the cell cycle in G2/M phase, and increase intracellular ROS levels in a dose-dependent manner. Biodistribution revealed that iRGD-C6-lys(²¹¹At-ATE)-C6-^DA7R had rapid tumor accumulation and fast normal tissue/organ clearance, which was mainly excreted through the kidneys. Moreover, in vivo therapeutic evaluation indicated that iRGD-C6-lys(²¹¹At-ATE)-C6-^DA7R was able to obviously inhibit tumor growth and prolong the survival of mice bearing glioma xenografts without notable toxicity to normal organs. All these results suggest that TAT mediated by iRGD-C6-lys(²¹¹At-ATE)-C6-^DA7R can provide an effective and promising strategy for the treatment of glioma and some other tumors.