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Objectives. The aim of this work is to evaluate energy deposition in the nucleus and cytoplasm in targeted alpha therapy of metastatic castration-resistant prostate cancer by modeling two cell lines, PC3 (osteolytic) and LNCaP C4-2 (osteoblastic), for actinium-225, astatine-211, and radium-223 and their progeny, using Monte Carlo simulations with the GATE/Geant4 code. Approach. We developed single cell and cell clusters models to Monte Carlo simulations, performed on the GATE platform version 9.3, with the GEANT4-DNA physics list emstandard_opt3_mixed_dna for At-211, Ac-225 and Ra-223 progenies. We considered three radionuclide distributions as a sources: the nucleus, the cytoplasm and the whole cell. Main results. When the nucleus was considered as a target, the S -values (N←N) calculated for At-211, Ac-225 and Ra-223 progenies were significantly higher, within 60%-90%, than S -values (N←Cy), demonstrating less influence of cytoplasm only internalization. When the cytoplasm was considering as a target, the S -values (Cy←Cy) calculated for At-211, Ac-225 and Ra-223 progeny were significantly higher, within 30%-90%, than the S -values (Cy←N). When no progeny migration occurs and for target nucleus , the cumulative S -values (N←N) calculated for At-211, Ac-225 and Ra-223 were significantly higher, within 50%-70%, than the S -values (N←N) computed for At-211, Ac-225, and Ra-223. Comparing the cumulative S -values, Ac-225 and Ra-223 therapies is more effective, in terms of deposited energy in a target, than that with At-211. Significance. The data presented in this research indicates that Ac-225 therapy may be the optimum choice due to the energy deposited in the nucleus, as long as the recoil effects and redistribution of progeny are understood. In contrast, At-211 is an alternative to avoid progeny migration. However, to completely analyze the efficacy of radionuclide therapy, other parameters must be considered, such as biological half-life, stability of the transport molecule, progeny migration, excretion pathways, and uptake in different organs., (© 2024 Institute of Physics and Engineering in Medicine. All rights, including for text and data mining, AI training, and similar technologies, are reserved.)

Astatine-211 is a nuclide with a short half-life of 7.2 h, that show promise for targeted alpha therapy of disseminated cancer. Despite nuclide production being straight-forward using a medium energy cyclotron and an uncomplicated target, not many cyclotrons are currently producing the nuclide. In this work we propose a stream-lined method to produce astatine labelled antibodies that enable production of clinical doses at other sites, remote from the nuclide producing cyclotron. Preconjugating the antibody prior to labelling, quick and efficient astatine recovery from the irradiated target in combination with optimized nuclide production logistics and an efficient synthesis for labelling are all key components to produce a clinical amount, > 300 MBq, of astatinated Trastuzumab., Competing Interests: “I have read the journal’s policy and the authors of this manuscript have the following competing interests: EA and SL are Co-founders of the company Atley Solutions AB providing the equipment Atley C100 used in this study. This does not alter our adherence to PLOS ONE policies on sharing data and materials”., (Copyright: © 2024 Aneheim et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

Background: Monte Carlo simulations have been considered for a long time the gold standard for dose calculations in conventional radiotherapy and are currently being applied for the same purpose in innovative radiotherapy techniques such as targeted radionuclide therapy (TRT)., Purpose: We present in this work a benchmarking study of the latest version of the Transport d'Ions Lourds Dans l'Aqua & Vivo (TILDA-V ) Monte Carlo track structure code, highlighting its capabilities for describing the full slowing down of α $\alpha$ -particles in water and the energy deposited in cells by α $\alpha$ -emitters in the context of TRT., Methods: We performed radiation transport simulations of α $\alpha$ -particles (10 keV u - 1 ${\rm u}^{-1}$ -100 MeV u - 1 ${\rm u}^{-1}$ ) in water with TILDA-V and the Particle and Heavy Ion Transport code System (PHITS) version 3.33. We compared the predictions of each code in terms of track parameters (stopping power, range and radial dose profiles) and cellular S-values of the promising radionuclide astatine-211 ( 211 At $^{211}{\rm At}$ ). Additional comparisons were made with available data in the literature., Results: The stopping power, range and radial dose profiles of α $\alpha$ -particles computed with TILDA-V were in excellent agreement with other calculations and available data. Overall, minor differences with PHITS were ascribed to phase effects, that is, related to the use of interaction cross sections computed for water vapor or liquid water. However, important discrepancies were observed in the radial dose profiles of monoenergetic α $\alpha$ -particles, for which PHITS results showed a large underestimation of the absorbed dose compared to other codes and experimental data. The cellular S-values of 211 At $^{211}{\rm At}$ computed with TILDA-V  agreed within 4% with the values predicted by PHITS and MIRDcell., Conclusions: The validation of the TILDA-V code presented in this work opens the possibility to use it as an accurate simulation tool for investigating the interaction of α $\alpha$ -particles in biological media down to the nanometer scale in the context of medical research. The code may help nuclear medicine physicians in their choice of α $\alpha$ -emitters for TRT. Further research will focus on the application of TILDA-V for quantifying radioinduced damage on the deoxyribonucleic acid (DNA) molecule., (© 2024 American Association of Physicists in Medicine.)

Background: Glioblastoma (GB), the most aggressive brain cancer, remains a critical clinical challenge due to its resistance to conventional treatments. Here, we introduce a locoregional targeted-α-therapy (TAT) with the rat monoclonal antibody 9E7.4 targeting murine syndecan-1 (SDC1) coupled to the α-emitter radionuclide astatine-211 ( 211 At-9E7.4)., Methods: We orthotopically transplanted 50,000 GL261 cells of murine GB into the right striatum of syngeneic female C57BL/6JRj mice using stereotaxis. After MRI validation of tumour presence at day 11, TAT was injected at the same coordinates. Biodistribution, efficacy, toxicity, local and systemic responses were assessed following application of this protocol. The 9E7.4 monoclonal antibody was labelled with iodine-125 ( 125 I) for biodistribution and with astatine-211 ( 211 At) for the other experiments., Findings: The 211 At-9E7.4 TAT demonstrated robust efficacy in reducing orthotopic tumours and achieved improved survival rates in the C57BL/6JRj model, reaching up to 70% with a minimal activity of 100 kBq. Targeting SDC1 ensured the cerebral retention of 211 At over an optimal time window, enabling low-activity administration with a minimal toxicity profile. Moreover, TAT substantially reduced the occurrence of secondary tumours and provided resistance to new tumour development after contralateral rechallenge, mediated through the activation of central and effector memory T cells., Interpretation: The locoregional 211 At-9E7.4 TAT stands as one of the most efficient TAT across all preclinical GB models. This study validates SDC1 as a pertinent therapeutic target for GB and underscores 211 At-9E7.4 TAT as a promising advancement to improve the treatment and quality of life for patients with GB., Funding: This work was funded by the French National Agency for Research (ANR) "France 2030 Investment Plan" Labex Iron [ANR-11-LABX-18-01], The SIRIC ILIAD [INCa-DGOS-INSERM-18011], the French program "Infrastructure d'Avenir en Biologie-Santé" (France Life Imaging) [ANR-11-INBS-0006], the PIA3 of the ANR, integrated to the "France 2030 Investment Plan" [ANR-21-RHUS-0012], and support from Inviscan SAS (Strasbourg, France). It was also related to: the ANR under the frame of EuroNanoMed III (project GLIOSILK) [ANR-19-ENM3-0003-01]; the "Région Pays-de-la-Loire" under the frame of the Target'In project; the "Ligue Nationale contre le Cancer" and the "Comité Départemental de Maine-et-Loire de la Ligue contre le Cancer" (CD49) under the frame of the FusTarG project and the "Tumour targeting, imaging and radio-therapies network" of the "Cancéropôle Grand-Ouest" (France). This work was also funded by the Institut National de la Santé et de la Recherche Médicale (INSERM), the University of Nantes, and the University of Angers., Competing Interests: Declaration of interests The authors declare no competing interest., (Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)

Targeted alpha therapy (TAT) is a methodology that is being developed as a promising cancer treatment using the α-particle decay of radionuclides. This technique involves the use of heavy radioactive elements being placed near the cancer target area to cause maximum damage to the cancer cells while minimizing the damage to healthy cells. Using gold nanoparticles (AuNPs) as carriers, a more effective therapy methodology may be realized. AuNPs can be good candidates for transporting these radionuclides to the vicinity of the cancer cells since they can be labeled not just with the radionuclides, but also a host of other proteins and ligands to target these cells and serve as additional treatment options. Research has shown that astatine and iodine are capable of adsorbing onto the surface of gold, creating a covalent bond that is quite stable for use in experiments. However, there are still many challenges that lie ahead in this area, whether they be theoretical, experimental, and even in real-life applications. This review will cover some of the major developments, as well as the current state of technology, and the problems that need to be tackled as this research topic moves along to maturity. The hope is that with more workers joining the field, we can make a positive impact on society, in addition to bringing improvement and more knowledge to science.

The low range of alpha particles provides an opportunity to better target cancer cells theoretically leading to the introduction of interesting alpha emitter radiopharmaceuticals including 225 Ac, 212 Pb, etc. The combination of high energy and short range of alpha emitters differentiates targeted radiotherapy from other methods and reduces unwanted cytotoxicity of the cells around the tumoral tissue. Among interesting alpha emitters candidates for targeted therapy, 211 At, one of the radioisotopes with the best optimal decay properties, shows great promise for targeted radiotherapy in some animal prostate cancer xenograft studies and bone micro tumors with significant effects compared to other beta and alpha emitters and also demonstrates interesting properties for clinical applications. However, production and application of this alpha emitter in the development of actinium-based radiopharmaceuticals is hampered by many obstacles. This mini-review demonstrates 211 At production methods, chemical separation, radiolabeling procedures, 211 At-radiopharmaceuticals and their clinical trials, transport, logistics, and costs and future trends in the field for ultimate clinical applications. This review showed that there are limited clinical trials on 211 Ac-based radiopharmaceuticals, which is due to the low accessibility of this radioisotope and other limitations. However, the development programs of major industries indicate the development of 211 Ac-based radiopharmaceuticals in the future., (Copyright© Bentham Science Publishers; For any queries, please email at epub@benthamscience.net.)

The alpha emitter astatine-211 ( 211 At) 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 211 At. However, challenges remain regarding strategic methods for shipping 211 At in a form adaptable to advanced radiochemistry reactions and other uses of the radioisotope., Purpose: Our method allows shipment of 211 At 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 211 At from the bismuth matrix on site at the production accelerator (Texas A&M) in preparation for shipping. Aliquots of 6 M HNO 3 containing up to ≈2.22 GBq of 211 At 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 211 At. 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 211 At. The method to prepare and ship 211 At described in this manuscript has also been used to ship larger quantities of 211 At a greater distance to University of Alabama at Birmingham., Principal Conclusions: The successful proof of this method paves the way for the distribution of 211 At 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 211 At for preclinical and clinical studies., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: The authors declare the following competing financial interest(s): J.D.B., E.E.T., L.A. McIntosh, G.C.T., S.J.Y. have patent #PCT/US21/25156 Rapid At-211 purification method and E.E.T., J.D.B., L.A. McIntosh, G.C.T., S.J.Y. have patent #PCT/US21/63241 Systems and Methods for Automated Separation and Recovery of Astatine pending to Texas A&M University., (Copyright © 2023 Elsevier Inc. All rights reserved.)

Purpose: Targeted α-therapy (TAT) for prostate-specific membrane antigen (PSMA) is a promising treatment for metastatic castration-resistant prostate cancer (CRPC). Astatine is an α-emitter (half-life=7.2 h) that can be produced by a 30-MeV cyclotron. This study evaluated the treatment effect of 211 At-labeled PSMA compounds in mouse xenograft models., Methods: Tumor xenograft models were established by subcutaneous transplantation of human prostate cancer cells (LNCaP) in NOD/SCID mouse. [ 211 At]PSMA1, [ 211 At]PSMA5, or [ 211 At]PSMA6 was administered to LNCaP xenograft mice to evaluate biodistribution at 3 and 24 h. The treatment effect was evaluated by administering [ 211 At]PSMA1 (0.40 ± 0.07 MBq), [ 211 At]PSMA5 (0.39 ± 0.03 MBq), or saline. Histopathological evaluation was performed for the at-risk organs at 3 and 6 weeks after administration., Results: [ 211 At]PSMA5 resulted in higher tumor retention compared to [ 211 At]PSMA1 and [ 211 At]PSMA6 (30.6 ± 17.8, 12.4 ± 4.8, and 19.1 ± 4.5 %ID/g at 3 h versus 40.7 ± 2.6, 8.7 ± 3.5, and 18.1 ± 2.2%ID/g at 24 h, respectively), whereas kidney excretion was superior in [ 211 At]PSMA1 compared to [ 211 At]PSMA5 and [ 211 At]PSMA6. An excellent treatment effect on tumor growth was observed after [ 211 At]PSMA5 administration. [ 211 At]PSMA1 also showed a substantial treatment effect; however, the tumor size was relatively larger compared to that with [ 211 At]PSMA5. In the histopathological evaluation, regenerated tubules were detected in the kidneys at 3 and 6 weeks after the administration of [ 211 At]PSMA5., Conclusion: TAT using [ 211 At]PSMA5 resulted in excellent tumor growth suppression with minimal side effects in the normal organs. [ 211 At]PSMA5 should be considered a new possible TAT for metastatic CRPC, and translational prospective trials are warranted., (© 2022. The Author(s).)

Purpose: [131I]meta-iodobenzylguanidine ([131I]MIBG) is a targeted radiotherapeutic administered systemically to deliver beta particle radiation in neuroblastoma. However, relapses in the bone marrow are common. [211At]meta-astatobenzylguanidine ([211At] MABG) is an alpha particle emitter with higher biological effectiveness and short path length which effectively sterilizes microscopic residual disease. Here we investigated the safety and antitumor activity [211At]MABG in preclinical models of neuroblastoma., Experimental Design: We defined the maximum tolerated dose (MTD), biodistribution, and toxicity of [211At]MABG in immunodeficient mice in comparison with [131I]MIBG. We compared the antitumor efficacy of [211At]MABG with [131I]MIBG in three murine xenograft models. Finally, we explored the efficacy of [211At]MABG after tail vein xenografting designed to model disseminated neuroblastoma., Results: The MTD of [211At]MABG was 66.7 MBq/kg (1.8 mCi/kg) in CB17SC scid-/- mice and 51.8 MBq/kg (1.4 mCi/kg) in NOD.Cg-Prkdcscid Il2rgtm1Wjl/SzJ (NSG) mice. Biodistribution of [211At]MABG was similar to [131I]MIBG. Long-term toxicity studies on mice administered with doses up to 41.5 MBq/kg (1.12 mCi/kg) showed the radiotherapeutic to be well tolerated. Both 66.7 MBq/kg (1.8 mCi/kg) single dose and fractionated dosing 16.6 MBq/kg/fraction (0.45 mCi/kg) × 4 over 11 days induced marked tumor regression in two of the three models studied. Survival was significantly prolonged for mice treated with 12.9 MBq/kg/fraction (0.35 mCi/kg) × 4 doses over 11 days [211At]MABG in the disseminated disease (IMR-05NET/GFP/LUC) model (P = 0.003) suggesting eradication of microscopic disease., Conclusions: [211At]MABG has significant survival advantage in disseminated models of neuroblastoma. An alpha particle emitting radiopharmaceutical may be effective against microscopic disseminated disease, warranting clinical development., (©2022 The Authors; Published by the American Association for Cancer Research.)

Astatine ( 211 At) is an alpha-emitter with a better treatment efficacy against differentiated thyroid cancer compared with iodine ( 131 I), a conventional beta-emitter. However, its therapeutic comparison has not been fully evaluated. In this study, we compared the therapeutic effect between [ 211 At]NaAt and [ 131 I]NaI. In vitro analysis of a double-stranded DNA break (DSB) and colony formation assay were performed using K1-NIS cells. The therapeutic effect was compared using K1-NIS xenograft mice administered with [ 211 At]NaAt (0.4 MBq (n = 7), 0.8 MBq (n = 9), and 1.2 MBq (n = 4)), and [ 131 I]NaI (1 MBq (n = 4), 3 MBq (n = 4), and 8 MBq (n = 4)). The [ 211 At]NaAt induced higher numbers of DSBs and had a more reduced colony formation than [ 131 I]NaI. In K1-NIS mice, dose-dependent therapeutic effects were observed in both [ 211 At]NaAt and [ 131 I]NaI. In [ 211 At]NaAt, a stronger tumour-growth suppression was observed, while tumour regrowth was not observed until 18, 25, and 46 days after injection of 0.4, 0.8, and 1.2 MBq of [ 211 At]NaAt, respectively. While in [ 131 I]NaI, this was observed within 12 days after injection (1, 3, and 8 MBq). The superior therapeutic effect of [ 211 At]NaAt suggests the promising clinical applicability of targeted alpha therapy using [ 211 At]NaAt in patients with differentiated thyroid cancer refractory to standard [ 131 I]NaI treatment.

Radioactive iodine has long been used clinically for patients with differentiated thyroid cancer. Radioiodine(131I) is used for the ablation of thyroid remnants or treatment of metastatic thyroid cancer. However, some patients with multiple metastases are refractory to repetitive 131I treatment, despite the targeted regions showing sufficient iodine uptake. In such patients, β- particle therapy using 131I is inadequate and another strategy is needed using more effective radionuclide targeting the sodium/iodide symporter(NIS). Astatine(211At)is receiving increasing attention as an α-emitter for targeted radionuclide therapy. 211At is a halogen element with similar chemical properties to iodine. α particles emitted from 211At has higher linear energy transfer as compared to β particles from 131I and exert a better therapeutic effect by inducing DNA double strand breaks and free radical formation. We showed that increase of the radiochemical purity of astatide of 211At solution by addition of ascorbic acid was associated with significantly enhanced uptake of 211At by both normal thyroid tissue and differentiated thyroid cancer cells. The treatment effect of 211At solution in the K1-NIS xenograft model was dose-dependent and was associated with prolonged survival, suggesting the potential applicability of targeted α therapy for the treatment of advanced differentiated thyroid cancer. Thus, targeted α therapy using 211At is highly promising for the treatment of advanced differentiated thyroid cancer. We have already started the clinical trial of 211At-NaAt in Osaka University Hospital since November 2021 after getting the approval by IRB and PMDA investigation. We would like to get the proof of concept that astatine can be used safely and effectively in patients, aiming at the drug approval as a targeted α therapeutic from Japan.

Radioimmunotherapy (RIT) has long been pursued to improve outcomes in acute leukemia and higher-risk myelodysplastic syndrome (MDS). Of increasing interest are alpha-particle-emitting radionuclides such as astatine-211 ( 211 At) as they deliver large amounts of radiation over just a few cell diameters, enabling efficient and selective target cell kill. Here, we developed 211 At-based RIT targeting CD123, an antigen widely displayed on acute leukemia and MDS cells including underlying neoplastic stem cells. We generated and characterized new murine monoclonal antibodies (mAbs) specific for human CD123 and selected four, all of which were internalized by CD123+ target cells, for further characterization. All mAbs could be conjugated to a boron cage, isothiocyanatophenethyl-ureido-closo-decaborate(2-) (B10), and labeled with 211 At. CD123+ cell targeting studies in immunodeficient mice demonstrated specific uptake of 211 At-labeled anti-CD123 mAbs in human CD123+ MOLM-13 cell tumors in the flank. In mice injected intravenously with MOLM-13 cells or a CD123 NULL MOLM-13 subline, a single dose of up to 40 µCi of 211 At delivered via anti-CD123 mAb decreased tumor burdens and substantially prolonged survival dose dependently in mice bearing CD123+ but not CD123- leukemia xenografts, demonstrating potent and target-specific in vivo anti-leukemia efficacy. These data support the further development of 211 At-CD123 RIT toward clinical application., (© 2022. The Author(s), under exclusive licence to Springer Nature Limited.)

Background: 211 At is a high-energy α-ray emitter with a relatively short half-life and a high cytotoxicity for cancer cells. Its dispersion can be imaged using clinical scanners, and it can be produced in cyclotrons without the use of nuclear fuel material. This study investigated the biodistribution and the antitumor effect of 211 At-labeled gold nanoparticles ( 211 At-AuNP) administered intratumorally., Results: AuNP with a diameter of 5, 13, 30, or 120 nm that had been modified with poly (ethylene glycol) methyl ether (mPEG) thiol and labeled with 211 At ( 211 At-AuNP-S-mPEG) were incubated with tumor cells, or intratumorally administered to C6 glioma or PANC-1 pancreatic cancers subcutaneously transplanted into rodent models. Systemic and intratumoral distributions of the particles in the rodents were then evaluated using scintigraphy and autoradiography, and the changes in tumor volumes were followed for about 40 days. 211 At-AuNP-S-mPEG was cytotoxic when it was internalized by the tumor cells. After intratumoral administration, 211 At-AuNP-S-mPEG became localized in the tumor and did not spread to systemic organs during a time period equivalent to 6 half-lives of 211 At. Tumor growth was strongly suppressed for both C6 and PANC-1 by 211 At-AuNP-S-mPEG. In the C6 glioma model, the strongest antitumor effect was observed in the group treated with 211 At-AuNP-S-mPEG with a diameter of 5 nm., Conclusions: The intratumoral single administration of a simple nanoparticle, 211 At-AuNP-S-mPEG, was shown to suppress the growth of tumor tissue strongly in a particle size-dependent manner without radiation exposure to other organs caused by systemic spread of the radionuclide., (© 2021. The Author(s).)

Tissue factor (TF), the trigger protein of the extrinsic blood coagulation cascade, is abundantly expressed in various cancers including gastric cancer. Anti-TF monoclonal antibodies (mAbs) capable of targeting cancers have been successfully applied to armed antibodies such as antibody-drug conjugates (ADCs) and molecular imaging probes. We prepared an anti-TF mAb, clone 1084, labeled with astatine-211 ( 211 At), as a promising alpha emitter for cancer treatment. Alpha particles are characterized by high linear energy transfer and a range of 50-100 µm in tissue. Therefore, selective and efficient tumor accumulation of alpha emitters results in potent antitumor activities against cancer cells with minor effects on normal cells adjacent to the tumor. Although the 211 At-conjugated clone 1084 ( 211 At-anti-TF mAb) was disrupted by an 211 At-induced radiochemical reaction, we demonstrated that astatinated anti-TF mAbs eluted in 0.6% or 1.2% sodium ascorbate (SA) solution were protected from antibody denaturation, which contributed to the maintenance of cellular binding activities and cytocidal effects of this immunoconjugate. Although body weight loss was observed in mice administered a 1.2% SA solution, the loss was transient and the radioprotectant seemed to be tolerable in vivo. In a high TF-expressing gastric cancer xenograft model, 211 At-anti-TF mAb in 1.2% SA exerted a significantly greater antitumor effect than nonprotected 211 At-anti-TF mAb. Moreover, the antitumor activities of the protected immunoconjugate in gastric cancer xenograft models were dependent on the level of TF in cancer cells. These findings suggest the clinical availability of the radioprotectant and applicability of clone 1084 to 211 At-radioimmunotherapy., (© 2021 The Authors. Cancer Science published by John Wiley & Sons Australia, Ltd on behalf of Japanese Cancer Association.)

Introduction: The high potency and short tissue range of α-particles are attractive features for targeted radionuclide therapy, particularly for cancers with micro-metastases. In the current study, we describe the synthesis of a series of 211 At-labeled prostate-specific membrane antigen (PSMA) inhibitors and their preliminary evaluation as potential agents for metastatic prostate cancer treatment., Methods: Four novel Glu-urea based PSMA ligands containing a trialkyl stannyl group were synthesized and labeled with 211 At, and for comparative purposes, 131 I, via halodestannylation reactions with N-chlorosuccinimide as the oxidant. A PSMA inhibitory assay was performed to evaluate PSMA binding of the unlabeled, iodinated compounds. A series of paired-label biodistribution experiments were performed to compare each 211 At-labeled PSMA ligand to its 131 I-labeled counterpart in mice bearing subcutaneous PC3 PSMA+ PIP xenografts., Results: Radiochemical yields ranged from 32% to 65% for the 211 At-labeled PSMA inhibitors and were consistently lower than those obtained with the corresponding 131 I-labeled analogue. Good localization in PC3 PSMA+ PIP but not control xenografts was observed for all labeled molecules studied, which exhibited a variable degree of in vivo dehalogenation as reflected by thyroid and stomach activity levels. Normal tissue uptake and in vivo stability for several of the compounds was markedly improved compared with the previously evaluated compounds, [ 211 At]DCABzL and [*I]DCIBzL., Conclusions and Implications for Patient Care: Compared with the first generation compound [ 211 At]DCABzL, several of the novel 211 At-labeled PSMA ligands exhibited markedly improved stability in vivo and higher tumor-to-normal tissue ratios. [ 211 At]GV-620 has the most promising characteristics and warrants further evaluation as a targeted radiotherapeutic for prostate cancer., Competing Interests: Declaration of competing interest G.V., R.M., I.L., M.P. and M.R.Z. are co-inventors on filed patent applications that cover (211)At- and (131)I-labeled PSMA inhibitors including the molecules comprising this study. We declare no other competing interests related to this work., (Copyright © 2021 Elsevier Inc. All rights reserved.)

Background: In a previous clinical study, the authors evaluated the potential of antitenascin C monoclonal antibody (mAb) 81C6 labeled with 211 At via the prosthetic agent N -succinimidyl 3-[ 211 At]astatobenzoate (SAB) for the treatment of primary brain tumors. Although encouraging results were obtained, labeling chemistry failed while attempting to escalate the dose to 370 MBq. The goal of the current study was to develop a revised procedure less susceptible to radiolysis-mediated effects on 211 At labeling that would be suitable for use at higher activity levels of this α-emitter. Materials and Methods: Addition of N -chlorosuccinimide to the methanol used to remove the 211 At from the cryotrap after bismuth target distillation was done to thwart radiolytic decomposition of reactive 211 At and the tin precursor. A series of 11 reactions were performed to produce SAB at initial 211 At activity levels of 0.31-2.74 GBq from 50 μg of N -succinimidyl 3-trimethylstannylbenzoate (Me-STB), which was then reacted with murine 81C6 mAb without purification of the SAB intermediate. Radiochemical purity, immunoreactive fraction, sterility, and apyrogenicity of the 211 At-labeled 81C6 preparations were evaluated. Results: Murine 81C6 mAb was successfully labeled with 211 At using these revised procedures with improved radiochemical yields and decreased overall synthesis time compared with the original clinical labeling procedure. Conclusions: With 2.74 GBq of 211 At, it was possible to produce 1.0 GBq of 211 At-labeled 81C6 with an immunoreactive fraction of 92%. These revised procedures permit production of 211 At-labeled mAbs suitable for use at clinically relevant activity levels.

Despite the consensus around the clinical potential of the α-emitting radionuclide astatine-211 ( 211 At), there are only a limited number of research facilities that work with this nuclide. There are three main reasons for this: (1) Scarce availability of the nuclide. Despite a relatively large number of globally existing cyclotrons capable of producing 211 At, few cyclotron facilities produce the nuclide on a regular basis. (2) Lack of a chemical infrastructure, that is, isolation of 211 At from irradiated targets and the subsequent synthesis of an astatinated product. At present, the research groups that work with 211 At depend on custom systems for recovering 211 At from the irradiated targets. Setting up and implementing such custom units require long lead times to provide a proper working system. (3) The chemistry of 211 At. Compared with radiometals there are no well-established and generally accepted synthesis methods for forming sufficiently stable bonds between 211 At and the tumor-specific vector to allow for systemic applications. Herein we present an overview of the infrastructure of producing 211 At radiopharmaceuticals, from target to radiolabeled product including chemical strategies to overcome hurdles for advancement into clinical trials with 211 At.

To explore stem-cell-targeted radioimmunotherapy with α-particles in acute myelogenous leukemia (AML), pharmacokinetics and dosimetry of the 211 At-labeled anti-C-X-C chemokine receptor type 4 monoclonal antibody ( 211 At-CXCR4 mAb) were conducted using tumor xenografted mice. The biological half-life of 211 At-CXCR4 mAb in blood was 15.0 h. The highest tumor uptake of 5.05%ID/g with the highest tumor-to-muscle ratio of 8.51 ± 6.14 was obtained at 6 h. Radiation dosimetry estimated with a human phantom showed absorbed doses of 0.512 mGy/MBq in the bone marrow, 0.287 mGy/MBq in the kidney, and <1 mGy/MBq in other major organs except bone. Sphere model analysis revealed 22.8 mGy/MBq in a tumor of 10 g; in this case, the tumor-to-bone marrow and tumor-to-kidney ratios were 44.5 and 79.4, respectively. The stem-cell-targeted α-particle therapy using 211 At-CXCR4 mAb for AML appears possible and requires further therapeutic studies.

Targeted radionuclide therapy (TRT) targeting oncoproteins facilitates the delivery of therapeutic radionuclides to tumor tissues with high precision. Herein, we developed 2 new radiopharmaceuticals, 4- 131 I-iodo- and 4- 211 At-astato- N -[4-(6-(isopropylamino)pyridine-4-yl)-1,3-thiazol-2-yl]- N -methylbenzamide ( 131 I-IITM and 211 At-AITM), targeting the ectopic metabotropic glutamate receptor 1 (mGluR1) in melanomas for TRT studies. Methods: 131 I-IITM and 211 At-AITM were synthesized by reacting a stannyl precursor with 131 I-NaI and 211 At in the presence of an oxidizing agent. The therapeutic efficacy and safety of the 2 radiopharmaceuticals were investigated using mGluR1-expressing B16F10 melanoma cells and melanoma-bearing mice. Results: 131 I-IITM and 211 At-AITM were obtained with a radiochemical purity of greater than 99% and radiochemical yields of 42.7% ± 10.4% and 45.7% ± 6.5%, respectively, based on the total radioactivity of used radionuclides. 131 I-IITM and 211 At-AITM exhibited a maximum uptake of 4.66% ± 0.70 and 7.68% ± 0.71 percentage injected dose per gram (%ID/g) in the targeted melanomas, respectively, and were rapidly cleared from nontarget organs after intravenous injection. Both agents markedly inhibited melanoma growth compared with the controls (61.00% and 95.68%, respectively). In the melanoma model, considerably greater therapeutic efficacy with negligible toxicity was observed using 211 At-AITM. Conclusion: The nontoxic radiopharmaceuticals 131 I-IITM and 211 At-AITM are useful high-precision TRT agents that can be used to target the oncoprotein mGluR1 for melanoma therapy., (© 2020 by the Society of Nuclear Medicine and Molecular Imaging.)

Minimal residual disease (MRD) has become an increasingly prevalent and important entity in multiple myeloma (MM). Despite deepening responses to frontline therapy, roughly 75% of MM patients never become MRD-negative to ≤10-5, which is concerning because MRD-negative status predicts significantly longer survival. MM is highly heterogeneous, and MRD persistence may reflect survival of isolated single cells and small clusters of treatment-resistant subclones. Virtually all MM clones are exquisitely sensitive to radiation, and the α-emitter astatine-211 (211At) deposits prodigious energy within 3 cell diameters, which is ideal for eliminating MRD if effectively targeted. CD38 is a proven MM target, and we conjugated 211At to an anti-CD38 monoclonal antibody to create an 211At-CD38 therapy. When examined in a bulky xenograft model of MM, single-dose 211At-CD38 at 15 to 45 µCi at least doubled median survival of mice relative to untreated controls (P < .003), but no mice achieved complete remission and all died within 75 days. In contrast, in a disseminated disease model designed to reflect low-burden MRD, 3 studies demonstrated that single-dose 211At-CD38 at 24 to 45 µCi produced sustained remission and long-term survival (>150 days) for 50% to 80% of mice, where all untreated mice died in 20 to 55 days (P < .0001). Treatment toxicities were transient and minimal. These data suggest that 211At-CD38 offers the potential to eliminate residual MM cell clones in low-disease-burden settings, including MRD. We are optimistic that, in a planned clinical trial, addition of 211At-CD38 to an autologous stem cell transplant (ASCT) conditioning regimen may improve ASCT outcomes for MM patients., (© 2019 by The American Society of Hematology.)

Astatine-211 is a promising radionuclide for targeted radiotherapy. It is required to image the distribution of targeted radiotherapeutic agents in a patient's body for optimization of treatment strategies. We proposed to image 211 At with high-energy photons to overcome some problems in conventional planar or single-photon emission computed tomography imaging. We performed an imaging experiment of a point-like 211 At source using a Compton camera, and demonstrated the capability of imaging 211 At with the high-energy photons for the first time., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Synovial sarcoma (SS) is a rare yet refractory soft-tissue sarcoma that predominantly affects young adults. We show in a mouse model that radioimmunotherapy (RIT) with an α-particle emitting anti-Frizzled homolog 10 (FZD10) antibody, synthesized using the α-emitter radionuclide astatine-211 ( 211 At-OTSA101), suppresses the growth of SS xenografts more efficiently than the corresponding β-particle emitting anti-FZD10 antibody conjugated with the β-emitter yettrium-90 ( 90 Y-OTSA101). In biodistribution analysis, 211 At was increased in the SS xenografts but decreased in other tissues up to 1 day after injection as time proceeded, albeit with a relatively higher uptake in the stomach. Single 211 At-OTSA101 doses of 25 and 50 μCi significantly suppressed SS tumor growth in vivo, whereas a 50-μCi dose of 90 Y-OTSA101 was needed to achieve this. Importantly, 50 μCi of 211 At-OTSA101 suppressed tumor growth immediately after injection, whereas this effect required several days in the case of 90 Y-OTSA101. Both radiolabeled antibodies at the 50-μCi dosage level significantly prolonged survival. Histopathologically, severe cellular damage accompanied by massive cell death was evident in the SS xenografts at even 1 day after the 211 At-OTSA101 injection, but these effects were relatively milder with 90 Y-OTSA101 at the same timepoint, even though the absorbed doses were comparable (3.3 and 3.0 Gy, respectively). We conclude that α-particle RIT with 211 At-OTSA101 is a potential new therapeutic option for SS., (© 2018 The Authors. Cancer Science published by John Wiley & Sons Australia, Ltd on behalf of Japanese Cancer Association.)

Intraperitoneally administered radiolabeled monoclonal antibodies (mAbs) have been tested in several clinical trials, often with promising results, but have never proven curative. Methods: We have previously presented simulations of clinically relevant amounts of intraperitoneal 90 Y-mAbs for treatment of minimal disease and shown that such treatments are unlikely to eradicate microtumors. Our previous model simulated the kinetics of intraperitoneally infused radiolabeled mAbs in humans and showed the benefit of instead using α-emitters such as 211 At. In the current work, we introduce penetration of mAbs into microtumors with radii of up to 400 μm. Calculations were performed using dynamic simulation software. To determine the radiation dose distribution in nonvascularized microtumors of various sizes after intraperitoneal 211 At-radioimmunotherapy, we used an in-house-developed Monte Carlo program for microdosimetry. Our aim was to find methods that optimize the therapy for as wide a tumor size range as possible. Results: Our results show that high-specific-activity radiolabeled mAbs that are bound to a tumor surface will penetrate slowly compared with the half-lives of 211 At and shorter-lived radionuclides. The inner-core cells of tumors with radii exceeding 100 μm may therefore not be sufficiently irradiated. For lower specific activities, the penetration rate and dose distribution will be more favorable for such tumors, but the dose to smaller microtumors and single cells will be low. Conclusion: Our calculations show that the addition of a boost with unlabeled mAb 1-5 h after therapy results in sufficient absorbed doses both to single cells and throughout microtumors up to approximately 300 μm in radius. This finding should also hold for other high-affinity mAbs and short-lived α-emitters., (© 2018 by the Society of Nuclear Medicine and Molecular Imaging.)

This review summarizes recent progress and developments as well as the most important pitfalls in targeted alpha-particle therapy, covering single alpha-particle emitters as well as in vivo alpha-particle generators. It discusses the production of radionuclides like 211 At, 223 Ra, 225 Ac/ 213 Bi, labelling and delivery employing various targeting vectors (small molecules, chelators for alpha-emitting nuclides and their biomolecular targets as well as nanocarriers), general radiopharmaceutical issues, preclinical studies, and clinical trials including the possibilities of therapy prognosis and follow-up imaging. Special attention is given to the nuclear recoil effect and its impacts on the possible use of alpha emitters for cancer treatment, proper dose estimation, and labelling chemistry. The most recent and important achievements in the development of alpha emitters carrying vectors for preclinical and clinical use are highlighted along with an outlook for future developments., Competing Interests: The authors declare no conflict of interest. The funding sponsors had no role in the interpretation of data, in the writing of the manuscript, and in the decision to publish the results.

Introduction: The purposes of the present work were to label substance P (5-11) with 211 At using a rhodium(III) complex with a bifunctional ligand-2-(1,5,9,13-tetrathiacyclohexadecan-3-yloxy)acetic acid ([16aneS 4 ]-COOH) and to assess the in vitro stability and toxicity of the obtained radiobioconjugate., Methods: Two approaches were evaluated to obtain 131 I/ 211 At-Rh[16aneS 4 ]-SP 5-11 radiobioconjugates, based on 2-step and 1-step syntheses. In the first method 131 I/ 211 At-Rh[16aneS 4 ]-COOH complexes were obtained that required further coupling to a biomolecule. In the second approach, the bioconjugate [16aneS 4 ]-SP 5-11 was synthesized and further labeled with 131 I and 211 At through the utilization of a Rh(III) metal cation bridge. The synthesized compounds were analyzed by HPLC, TLC and paper electrophoresis., Results: The 131 I/ 211 At-Rh[16aneS 4 ]-COOH complexes were obtained in high yield and possessed good stability in PBS and CSF. Preliminary studies on coupling of 131 I-Rh[16aneS 4 ]-COOH to substance P (5-11) in 2-step synthesis showed that this procedure was too long with respect to 211 At half-life, prompting us to improve it by finally using a 1-step synthesis. This strategy not only shortened the labeling time, but also increased final yield of 131 I/ 211 At-Rh[16aneS 4 ]-SP 5-11 radiobioconjugates. The stability of both compounds in PBS and CSF was high. Toxicity studies with the 211 At-Rh[16aneS 4 ]-SP 5-11 demonstrated that radiobioconjugate significantly reduced T98G cell viability in a dose dependent manner reaching 20% of survival at the highest radioactivity 1200kBq/mL., Conclusions: The radiobioconjugate 211 At-Rh[16aneS 4 ]-SP 5-11 revealed its potential in killing glioma T98G cells during in vitro studies; therefore further animal studies to are required to determine its in vivo stability and treatment potential in normal and xenografted mice., (Copyright © 2017 Elsevier Inc. All rights reserved.)

Prostate-specific membrane antigen (PSMA)-targeted radiotherapy of prostate cancer (PCa) has emerged recently as a promising approach to the treatment of disseminated disease. A small number of ligands have been evaluated in patients, and although early tumor response is encouraging, relapse rate is high and these compounds localize to the parotid, salivary, and lacrimal glands as well as to the kidney, leading to dose-limiting toxicities and adverse events affecting quality of life. We envision that dual-target binding ligands displaying high affinity for PSMA and appropriate affinity for human serum albumin (HSA) may demonstrate a higher therapeutic index and be suitable for treatment of PCa by targeted α-therapy. Methods: Six novel urea-based ligands with varying affinities for PSMA and HSA were synthesized, labeled with 131 I, and evaluated by in vitro binding and uptake assays in LNCaP cells. Four compounds were advanced for further evaluation in a preclinical model of PCa. The compounds were compared with MIP-1095, a PSMA ligand currently in clinical evaluation. Results: The compounds demonstrated affinity for PSMA on the order of 4-40 nM and affinity for HSA in the range of 1-53 μM. Compounds with relatively high affinity for HSA (≤2 μM) showed high and sustained blood-pool activity and reduced uptake in the kidneys. 131 I-RPS-027, with a 50% inhibitory concentration (PSMA) of 15 nM and a dissociation constant (HSA) of 11.2 μM, cleared from the blood over the course of 48 h and showed good tumor uptake (10 percentage injected dose per gram) and retention and a greater than 5-fold decrease in kidney uptake relative to MIP-1095. The tumor-to-kidney ratio of 131 I-RPS-027 was greater than 3:1 at 24 h after injection, increasing to 7:1 by 72 h. Conclusion: RPS-027 shows dual targeting to PSMA and albumin, resulting in a high tumor uptake, highly favorable tissue distribution, and promising therapeutic profile in a preclinical model of prostate cancer. In comparison to existing ligands proposed for targeted therapy of prostate cancer, RPS-027 has tumor-to-tissue ratios that predict a significant reduction in side effects during therapy. Using iodine/radioiodine as a surrogate for the radiohalogen 211 At, we therefore propose dual-target binding ligands such as RPS-027 as next-generation radiopharmaceuticals for targeted α-therapy using 211 At., (© 2017 by the Society of Nuclear Medicine and Molecular Imaging.)

211 At is a most promising radionuclide for targeted alpha therapy. However, its limited availability and poorly known basic chemistry hamper its use. Based on the analogy with iodine, labelling is performed via astatobenzoate conjugates, but in vivo deastatination occurs, particularly when the conjugates are internalized in cells. Actually, the chemical or biological mechanism responsible for deastatination is unknown. In this work, we show that the C-At "organometalloid" bond can be cleaved by oxidative dehalogenation induced by oxidants such as permanganates, peroxides or hydroxyl radicals. Quantum mechanical calculations demonstrate that astatobenzoates are more sensitive to oxidation than iodobenzoates, and the oxidative deastatination rate is estimated to be about 6 × 10 6 faster at 37 °C than the oxidative deiodination one. Therefore, we attribute the "internal" deastatination mechanism to oxidative dehalogenation in biological compartments, in particular lysosomes.

Introduction: The availability of 211 At for targeted alpha therapy research can be increased by the 211 Rn/ 211 At generator system, whereby 211 At is produced by 211 Rn electron capture decay. This study demonstrated the feasibility of using generator-produced 211 At to label monoclonal antibody (BC8, anti-human CD45) for preclinical use, following isolation from the 207 Po contamination also produced by these generators (by 211 Rn α-decay)., Methods: 211 Rn was produced by 211 Fr electron capture decay following mass separated ion beam implantation and chemically isolated in liquid alkane hydrocarbon (dodecane). 211 At produced by the resulting 211 Rn source was extracted in strong base (2N NaOH) and purified by granular Te columns. BC8-B10 (antibody conjugated with closo-decaborate(2-)) was labeled with generator-produced 211 At and purified by PD-10 columns., Results: Aqueous solutions extracted from the generator were found to contain 211 At and 207 Po, isolated from 211 Rn. High radionuclidic purity was obtained for 211 At eluted from Te columns, from which BC8-B10 monoclonal antibody was successfully labeled. If not removed, 207 Po was found to significantly contaminate the final 211 At-BC8-B10 product. High yield efficiencies (decay-corrected, n=3) were achieved for 211 At extraction from the generator (86%±7%), Te column purification (70%±10%), and antibody labeling (76%±2%)., Conclusions: The experimental 211 Rn/ 211 At generator was shown to be well-suited for preclinical 211 At-based research., Advances in Knowledge: We believe that these experiments have furthered the knowledge-base for expanding accessibility to 211 At using the 211 Rn/ 211 At generator system., Implications for Patient Care: As established by this work, the 211 Rn/ 211 At generator has the capability of facilitating preclinical evaluations of 211 At-based therapies., (Copyright © 2017 Elsevier Inc. All rights reserved.)

Mass-separated francium beams ( 211 Fr or 213 Fr) were implanted into solid targets for producing 211 Rn (14.6h half-life) or 209 At (5.41h), in situ. 211 Rn was transferred to dodecane and isolated from contaminants, providing sources for 211 At (7.21h) production by 211 Rn decay (73%). 209 At was recovered with high radionuclidic purity in aqueous solutions, directly. These experiments demonstrated Fr beam implantations as a novel method for producing preclinical quantities of 211 Rn/ 211 At (for therapy) and 209 At (for imaging)., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

The goal of this study was to investigate whether targeted α-therapy can be used to successfully treat macrotumors, in addition to its established role for treating micrometastatic and minimal disease. We used an intravenous fractionated regimen of α-radioimmunotherapy in a subcutaneous tumor model in mice. We aimed to evaluate the absorbed dose levels required for tumor eradication and growth monitoring, as well as to evaluate long-term survival after treatment. Methods: Mice bearing subcutaneous tumors (50 mm 3 , NIH:OVCAR-3) were injected repeatedly (1-3 intravenous injections 7-10 d apart, allowing bone marrow recovery) with 211 At-MX35-F(ab') 2 at different activities (close to acute myelotoxicity). Mean absorbed doses to tumors and organs were estimated from biodistribution data and summed for the fractions. Tumor growth was monitored for 100 d and survival for 1 y after treatment. Toxicity analysis included body weight, white blood cell count, and hematocrit. Results: Effects on tumor growth after fractionated α-radioimmunotherapy with 211 At-MX35-F(ab') 2 was strong and dose-dependent. Complete remission (tumor-free fraction, 100%) was found for tumor doses of 12.4 and 16.4 Gy. The administered activities were high, and long-term toxicity effects (≤60 wk) were clear. Above 1 MBq, the median survival decreased linearly with injected activity, from 44 to 11 wk. Toxicity was also seen by reduced body weight. White blood cell count analysis after α-radioimmunotherapy indicated bone marrow recovery for the low-activity groups, whereas for high-activity groups the reduction was close to acute myelotoxicity. A decrease in hematocrit was seen at a late interval (34-59 wk after therapy). The main external indication of poor health was dehydration. Conclusion: Having observed complete eradication of solid tumor xenografts, we conclude that targeted α-therapy regimens may stretch beyond the realm of micrometastatic disease and be eradicative also for macrotumors. Our observations indicate that at least 10 Gy are required. This agrees well with the calculated tumor control probability. Considering a relative biological effectiveness of 5, this dose level seems reasonable. However, complete remission was achieved first at activity levels close to lethal and was accompanied by biologic effects that reduced long-term survival., (© 2017 by the Society of Nuclear Medicine and Molecular Imaging.)

Introduction: Alpha particles are radiation of high energy and short range, properties that can lead to radiolysis-mediated complications in labeling chemistry at the high radioactivity levels required for clinical application. In previous papers in this series, we have shown that radiation dose has a profound effect on the astatine species that are present in the labeling reaction and their suitability for the synthesis of N-succinimidyl 3-[ 211 At]astatobenzoate. The purpose of this study was to evaluate the effects of adding N-chlorosuccinimide (NCS) to the methanol solution used for initial isolation of 211 At after distillation, a process referred to as 211 At stabilization, on 211 At chemistry after exposure to high radiation doses., Methods: High performance liquid chromatography was used to evaluate the distribution of 211 At species present in methanol in the 500-65,000Gy radiation dose range and the synthesis of SAB from N-succinimidyl 3-(tri-n-butylstannyl)benzoate in the 500-120,000Gy radiation dose range using different 211 At timeactivity combinations under conditions with/without 211 At stabilization., Results: In the absence of NCS stabilization, a reduced form of astatine, At(2), increased with increasing radiation dose, accounting for about half the total activity by about 15,000Gy, while with stabilization, At(2) accounted for <10% of 211 At activity even at doses >60,000Gy. SAB yields without stabilization rapidly declined with increasing dose, falling to ~20% at about 5000Gy while with stabilization, yields >80% were obtained with 211 At solutions stored for more than 23h and receiving radiation doses >100,000Gy., Conclusions: Adding NCS to the methanol solution used for initial isolation of 211 At is a promising strategy for countering the deleterious effects of radiolysis on 211 At chemistry., Advances in Knowledge and Implications for Patient Care: This strategy could facilitate the ability to perform 211 At labeling at sites remote from its production and at the high activity levels required for clinical applications., (Copyright © 2016 Elsevier Inc. All rights reserved.)

The application of (211)At to targeted cancer therapy is currently hindered by the rapid deastatination that occurs in vivo. As the deastatination mechanism is unknown, we tackled this issue from the viewpoint of the intrinsic properties of At-involving chemical bonds. An apparent correlation has been evidenced between in vivo stability of (211)At-labeled compounds and the At-R (R = C, B) bond enthalpies obtained from relativistic quantum mechanical calculations. Furthermore, we highlight important differences in the nature of the At-C and At-B bonds of interest, e.g. the opposite signs of the effective astatine charges, which implies different stabilities with respect to the biological medium. Beyond their practical use for rationalizing the labeling protocols used for (211)At, the proposed computational approach can readily be used to investigate bioactive molecules labeled with other heavy radionuclides., (Copyright © 2016 Elsevier Masson SAS. All rights reserved.)

Rationale: The sigma-2 receptor is a protein with a Heme binding region and is capable of receptor-mediated endocytosis. It is overexpressed in many cancers making it a potential vector for therapeutic drug delivery. Our objective was to introduce an alpha-emitting radionuclide, astatine-211, into a selective sigma-2 ligand moiety to provide cytotoxic capabilities without adversely altering the pharmacological characteristics. In this study we investigated the in vitro/in vivo tumor targeting and estimated dosimetry of alpha-emitting sigma-2 ligand, 5-(astato-(211)At)-N-(4-(6,7-dimethoxy-3,4-dihydroisoquinolin-2(1H)-yl)butyl)-2,3-dimethoxybenzamide ((211)At-MM3), in a pre-clinical human breast cancer model., Methods: Astatine-211 was produced in a cyclotron and isolated by dry distillation. Radiosynthesis of (211)At-MM3 was performed using a tin precursor through radioastatodestannylation. In vitro sigma-2 binding experiments using (211)At-MM3 were carried out in live EMT6 and MDA-MB-231 breast cancer cells and liver homogenate tissue. In vivo biodistribution experiments were performed using EMT6 mouse breast cancer cells in BALB/c female mice. Approximately 370 kBq of (211)At-MM3 was administered intravenously and at time points of 5 min, 1, 2, 4, 8, and 24 h organs/tissue were harvested. Estimated human dosimetry was extrapolated from biodistribution data using OLINDA/EXM (VU e-Innovations)., Results: Astatine-211 was successfully produced and isolated in quantities suitable for in vitro and small animal in vivo experiments. Radiosynthesis of (211)At-MM3 was reproducible with high radiochemical purity. Astatine-211-MM3 exhibited picomolar affinity to the sigma-2 receptor in contrast to the iodinated analog that had nanomolar affinity. Prolonged tumor targeting was measured through biodistribution studies with a maximal tumor to muscle ratio of 9.02 at 4h. Estimated human dosimetry revealed doses of up to 370 MBq in an adult female patient were below organ radiation limits with the potential to provide a high therapeutic dose to tumors., Conclusion: The sigma-2 receptor could serve as a suitable targeting platform for designing radiotherapeutics. (211)At-MM3 showed tumor targeting properties in vitro/in vivo and favorable estimated human dosimetry establishing the proof of concept for future development as a radiotherapeutic for the treatment of breast cancer., (Copyright © 2015 Elsevier Inc. All rights reserved.)

Introduction: Sigma receptors are overexpressed in a variety of human tumors, making them potential targets for radionuclide receptor therapy. We have previously synthesized and evaluated (131)I-labeled (+)-2-[4-(4-iodophenyl)piperidino]cyclohexanol [(+)-[(131)I]pIV], which has a high affinity for sigma receptors. Therefore, (+)-[(131)I]pIV significantly inhibited tumor cell proliferation in tumor-bearing mice. In the present study, we report the synthesis and the in vitro and in vivo characterization of (+)-[(211)At]pAtV, an (211)At-labeled sigma receptor ligand, that has potential use in alpha-radionuclide receptor therapy., Methods: The radiolabeled sigma receptor ligand (+)-[(211)At]pAtV was prepared using a standard halogenation reaction generating a 91% radiochemical yield with 98% purity after HPLC purification. The partition coefficient of (+)-[(211)At]pAtV was measured. Cellular uptake experiments and in vivo biodistribution experiments were performed using a mixed solution of (+)-[(211)At]pAtV and (+)-[(125)I]pIV; the human prostate cancer cell line DU-145, which expresses high levels of the sigma receptors, and DU-145 tumor-bearing mice., Results: The lipophilicity of (+)-[(211)At]pAtV was similar to that of (+)-[(125)I]pIV. DU-145 cellular uptake and the biodistribution patterns in DU-145 tumor-bearing mice at 1h post-injection were also similar between (+)-[(211)At]pAtV and (+)-[(125)I]pIV. Namely, (+)-[(211)At]pAtV demonstrated high uptake and retention in tumor via binding to sigma receptors., Conclusion: These results indicate that (+)-[(211)At]pAtV could function as an new agent for alpha-radionuclide receptor therapy., (Copyright © 2015 Elsevier Inc. All rights reserved.)

α-Emitting radionuclides deposit a large amount of energy within a few cell diameters and may be particularly effective for radioimmunotherapy targeting minimal residual disease (MRD). To evaluate this hypothesis, (211)At-labeled 1F5 monoclonal antibody (mAb) (anti-CD20) was studied in both bulky lymphoma tumor xenograft and MRD animal models. Superior treatment responses to (211)At-labeled 1F5 mAb were evident in the MRD setting. Lymphoma xenograft tumor-bearing animals treated with doses of up to 48 µCi of (211)At-labeled anti-CD20 mAb ([(211)At]1F5-B10) experienced modest responses (0% cures but two- to threefold prolongation of survival compared with negative controls). In contrast, 70% of animals in the MRD lymphoma model demonstrated complete eradication of disease when treated with (211)At-B10-1F5 at a radiation dose that was less than one-third (15 µCi) of the highest dose given to xenograft animals. Tumor progression among untreated control animals in both models was uniformly lethal. After 130 days, no significant renal or hepatic toxicity was observed in the cured animals receiving 15 µCi of [(211)At]1F5-B10. These findings suggest that α-emitters are highly efficacious in MRD settings, where isolated cells and small tumor clusters prevail.

In this work a new coupling reagent, N-[2-(maleimido)ethyl]-3-(trimethylstannyl)benzamide, for radiohalogenation has been synthesized and characterized. The reagent is intended to either be attached to reduced disulfide bridges of proteins (making the halogenation site-specific) or to free terminal cysteine groups on peptides. The new reagent was also shown to be easily halogenated with inactive bromine and iodine as well as (125)I and (211)At, indicating potential use within targeted radiotherapy., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

131I is widely used for therapy in the clinic and 125I and 131I, and increasingly 211At, are often used in experimental studies. It is important to know the biodistribution and dosimetry for these radionuclides to determine potential risk organs when using radiopharmaceuticals containing these radionuclides. The purpose of this study was to investigate the biodistribution of 125I-, 131I-, and free 211At in rats and to determine absorbed doses to various organs and tissues. Male Sprague Dawley rats were injected simultaneously with 0.1-0.3 MBq 125I- and 0.1-0.3 MBq 131I-, or 0.05-0.2 MBq 211At and sacrificed 1 hour to 7 days after injection. The activities and activity concentrations in organs and tissues were determined and mean absorbed doses were calculated. The biodistribution of 125I- was similar to that of 131I- but the biodistribution of free 211At was different compared to 125I- and 131I-. The activity concentration of radioiodine was higher compared with 211At in the thyroid and lower in all extrathyroidal tissues. The mean absorbed dose per unit injected activity was highest to the thyroid. 131I gave the highest absorbed dose to the thyroid, and 211At gave the highest absorbed dose to all other tissues studied.

Unlabelled: (211)At is a promising therapeutic radionuclide because of the nearly optimal biological effectiveness of emitted α-particles. Unbound (211)At accumulates in the thyroid gland and in other vital normal tissues. However, few studies have been performed that assess the (211)At-induced normal-tissue damage in vivo. Knowledge about the extent and quality of resulting responses in various organs offers a new venue for reducing risks and side effects and increasing the overall well-being of the patient during and after therapy., Methods: Female BALB/c nude mice were injected intravenously with 0.064-42 kBq of (211)At or mock-treated, and the kidneys, liver, lungs, and spleen were excised 24 h after injection. A transcriptional gene expression analysis was performed in triplicate using RNA microarray technology. Biological processes associated with regulated transcripts were grouped into 8 main categories with 31 subcategories according to gene ontology terms for comparison of regulatory profiles., Results: A substantial decrease in the total number of regulated transcripts was observed between 0.64 and 1.8 kBq of (211)At for all investigated tissues. Few genes were differentially regulated in each tissue at all absorbed doses. In all tissues, most of these genes showed a nonmonotonous dependence on absorbed dose. However, the direction of regulation generally remained uniform for a given gene. Few known radiation-associated genes were regulated on the transcriptional level, and their expression profile generally appeared to be dose-independent and tissue-specific. The regulatory profiles of categorized biological processes were tissue-specific and reflected the shift in regulatory intensity between 0.64 and 1.8 kBq of (211)At. The profiles revealed strongly regulated and nonregulated subcategories., Conclusion: The strong regulatory change observed between 0.64 and 1.8 kBq is hypothesized to result not only from low-dose effects in each tissue but also from physiologic responses to ionizing radiation-induced damage to, for example, the (211)At-accumulating thyroid gland. The presented results demonstrate the complexity of responses to radionuclides in vivo and highlight the need for further research to also consider physiology in ionizing radiation-induced responses.

Despite aggressive chemotherapy combined with hematopoietic stem cell transplantation (HSCT), many patients with acute myeloid leukemia (AML) relapse. Radioimmunotherapy (RIT) using monoclonal antibodies labeled with β-emitting radionuclides has been explored to reduce relapse. β emitters are limited by lower energies and nonspecific cytotoxicity from longer path lengths compared with α emitters such as (211)At, which has a higher energy profile and shorter path length. We evaluated the efficacy and toxicity of anti-CD45 RIT using (211)At in a disseminated murine AML model. Biodistribution studies in leukemic SJL/J mice showed excellent localization of (211)At-anti-murine CD45 mAb (30F11) to marrow and spleen within 24 hours (18% and 79% injected dose per gram of tissue [ID/g], respectively), with lower kidney and lung uptake (8.4% and 14% ID/g, respectively). In syngeneic HSCT studies, (211)At-B10-30F11 RIT improved the median survival of leukemic mice in a dose-dependent fashion (123, 101, 61, and 37 days given 24, 20, 12, and 0 µCi, respectively). This approach had minimal toxicity with nadir white blood cell counts >2.7 K/µL 2 weeks after HSCT and recovery by 4 weeks. These data suggest that (211)At-anti-CD45 RIT in conjunction with HSCT may be a promising therapeutic option for AML.

(211)At is a promising radionuclide for α-particle therapy of cancers. Its physical characteristics make this radionuclide particularly interesting to consider when bound to cancer-targeting biomolecules for the treatment of microscopic tumors. (211)At is produced by cyclotron irradiation of (209)Bi with α-particles accelerated at ~28 MeV and can be obtained in high radionuclidic purity after isolation from the target. Its chemistry resembles iodine, but there is also a tendency to behave as a metalloid. However, the chemical behavior of astatine has not yet been clearly established, primarily due to the lack of any stable isotopes of this element, which precludes the use of conventional analytical techniques for its characterization. There are also only a limited number of research centers that have been able to produce this element in sufficient amounts to carry out extensive investigations. Despite these difficulties, chemical reactions typically used with iodine can be performed, and a number of biomolecules of interest have been labeled with (211)At. However, most of these compounds exhibit unacceptable instability in vivo due to the weakness of the astatine-biomolecule bond. Nonetheless, several compounds have shown high potential for the treatment of cancers in vitro and in several animal models, thus providing a promising basis that has allowed initiation of the first two clinical studies.

Targeted alpha therapy (TAT) a promising treatment for small, residual, and micrometastatic diseases has questionable efficacy against malignant lesions larger than the α-particle range, and likely requires favorable intratumoral activity distribution. Here, we characterized and quantified the activity distribution of an alpha-particle emitter radiolabelled antibody within >100-µm micrometastases in a murine ovarian carcinoma model. Nude mice bearing ovarian micrometastases were injected intra-peritoneally with 211At-MX35 (total injected activity 6 MBq, specific activity 650 MBq/mg). Animals were sacrificed at several time points, and peritoneal samples were excised and prepared for alpha-camera imaging. Spatial and temporal activity distributions within micrometastases were derived and used for small-scale dosimetry. We observed two activity distribution patterns: uniform distribution and high stable uptake (>100% IA/g at all time points) in micrometastases with no visible stromal compartment, and radial distribution (high activity on the edge and poor uptake in the core) in tumor cell lobules surrounded by fibroblasts. Activity distributions over time were characterized by a peak (140% IA/g at 4 h) in the outer tumor layer and a sharp drop beyond a depth of 50 µm. Small-scale dosimetry was performed on a multi-cellular micrometastasis model, using time-integrated activities derived from the experimental data. With injected activity of 400 kBq, tumors exhibiting uniform activity distribution received <25 Gy (EUD=13 Gy), whereas tumors presenting radial activity distribution received mean absorbed doses of <8 Gy (EUD=5 Gy). These results provide new insight into important aspects of TAT, and may explain why micrometastases >100 µm might not be effectively treated by the examined regimen.

To reduce toxicity associated with external γ-beam radiation, we investigated radioimmunotherapy with an anti-CD45 mAb labeled with the α-emitter, astatine-211 ((211)At), as a conditioning regimen in dog leukocyte antigen-identical hematopoietic cell transplantation (HCT). Dose-finding studies in 6 dogs treated with 100 to 618 μCi/kg (211)At-labeled anti-CD45 mAb (0.5 mg/kg) without HCT rescue demonstrated dose-dependent myelosuppression with subsequent autologous recovery, and transient liver toxicity in dogs treated with (211)At doses less than or equal to 405 μCi/kg. Higher doses of (211)At induced clinical liver failure. Subsequently, 8 dogs were conditioned with 155 to 625 μCi/kg (211)At-labeled anti-CD45 mAb (0.5 mg/kg) before HCT with dog leukocyte antigen-identical bone marrow followed by a short course of cyclosporine and mycophenolate mofetil immunosuppression. Neutropenia (1-146 cells/μL), lymphopenia (0-270 cells/μL), and thrombocytopenia (1500-6560 platelets/μL) with prompt recovery was observed. Seven dogs had long-term donor mononuclear cell chimerism (19%-58%), whereas 1 dog treated with the lowest (211)At dose (155 μCi/kg) had low donor mononuclear cell chimerism (5%). At the end of follow-up (18-53 weeks), only transient liver toxicity and no renal toxicity had been observed. In conclusion, conditioning with (211)At-labeled anti-CD45 mAb is safe and efficacious and provides a platform for future clinical trials of nonmyeloablative transplantation with radioimmunotherapy-based conditioning.

Targeted radiotherapy using agents tagged with α-emitting radionuclides is gaining traction with several clinical trials already undertaken or ongoing, and others in the advanced planning stage. The most commonly used α-emitting radionuclides are 213Bi, 211At, 223Ra and 225Ac. While each one of these has pros and cons, it can be argued that 211At probably is the most versatile based on its half life, decay scheme and chemistry. On the other hand, for targeting bone metastases, 223Ra is the ideal radionuclide because simple cationic radium can be used for this purpose. In this review, we will discuss the recent developments taken place in the application of 211At-labeled radiopharmaceuticals and give an overview of the current status of 223Ra for targeted α-particle radiotherapy.

A new high-energy and high-intensity cyclotron, ARRONAX, has been set into operation in 2010. ARRONAX can accelerate both negative ions (H- and D-) and positive ions (He++ and HH+). Protons can be accelerated from 30 MeV up to 70 MeV with a maximum beam intensity of 2 × 375 μAe whereas He++ can be accelerated at 68 MeV with a maximum beam current of 70 μAe. The main fields of application of ARRONAX are radionuclide production for nuclear medicine and irradiation of inert or living materials for radiolysis and radiobiology studies. A large part of the beam time will be used to produce radionuclides for targeted radionuclide therapy (copper-67, scandium-47 and astatine-211) as well as for PET imaging (scandium-44, copper-64, strontium-82 for rubidium-82 generators and germanium-68 for gallium-68 generators). Since the beginning of the project a particular interest has been devoted to alpha-radionuclide therapy using complex ligands like antibodies and astatine-211 has been selected as a radionuclide of choice for such type of applications. Associated with appropriate carriers, all these radionuclides will respond to a maximum of unmet clinical needs.

The 7.2-h half life radiohalogen (211)At offers many potential advantages for targeted α-particle therapy; however, its use for this purpose is constrained by its limited availability. Astatine-211 can be produced in reasonable yield from natural bismuth targets via the (209)Bi(α,2n)(211)At nuclear reaction utilizing straightforward methods. There is some debate as to the best incident α-particle energy for maximizing 211At production while minimizing production of (210)At, which is problematic because of its 138.4-day half life α-particle emitting daughter, (210)Po. The intrinsic cost for producing (211)At is reasonably modest and comparable to that of commercially available (123)I. The major impediment to (211)At availability is attributed to the need for a medium energy α-particle beam for its production. On the other hand, there are about 30 cyclotrons in the world that have the beam characteristics required for (211)At production.

Purpose: Technetium-99m (99mTc) has been widely used as an imaging agent but only recently has been considered for therapeutic applications. This study aims to analyze the potential use of 99mTc Auger electrons for targeted tumor radiotherapy by evaluating the DNA damage and its probability of correct repair and by studying the cellular kinetics, following 99mTc Auger electron irradiation in comparison to iodine-131 (131I) beta minus particles and astatine-211 (211At) alpha particle irradiation., Methods: Computational models were used to estimate the yield of DNA damage (fast Monte Carlo damage algorithm), the probability of correct repair (Monte Carlo excision repair algorithm), and cell kinetic effects (virtual cell radiobiology algorithm) after irradiation with the selected particles., Results: The results obtained with the algorithms used suggested that 99mTc CKMMX (all M-shell Coster-Kroning--CK--and super-CK transitions) electrons and Auger MXY (all M-shell Auger transitions) have a therapeutic potential comparable to high linear energy transfer 211At alpha particles and higher than 131I beta minus particles. All the other 99mTc electrons had a therapeutic potential similar to 131I beta minus particles., Conclusions: 99mTc CKMMX electrons and Auger MXY presented a higher probability to induce apoptosis than 131I beta minus particles and a probability similar to 211At alpha particles. Based on the results here, 99mTc CKMMX electrons and Auger MXY are useful electrons for targeted tumor radiotherapy.

Purpose: Monoclonal anti-CD33 antibodies conjugated with toxic calicheamicin derivative (gemtuzumab ozogamicin, GO) are a novel therapy option for acute myeloid leukaemia (AML). Key prognostic factors for patients with AML are high CD33 expression on the leukaemic cells and the ability to overcome mechanisms of resistance to cytotoxic chemotherapies, including drug efflux or other mechanisms decreasing apoptosis. Alpha particle-emitting radionuclides overwhelm such anti-apoptotic mechanisms by producing numerous DNA double-stranded breaks (DSBs) accompanied by decreased DNA repair., Methods: We labelled anti-CD33 antibodies with the alpha-emitter (211)At and compared survival of leukaemic HL-60 and K-562 cells treated with the (211)At-labelled antibodies, GO or unlabelled antibodies as controls. We also measured caspase-3/7 activity, DNA fragmentation and necrosis in HL-60 cells after treatment with the different antibodies or with free (211)At., Results: The mean labelling ratio of (211)At-labelled antibodies was 1:1,090 +/- 364 (range: 1:738-1:1,722) in comparison to 2-3:1 for GO. Tumour cell binding of (211)At-anti-CD33 was high in the presence of abundant CD33 expression and could be specifically blocked by unlabelled anti-CD33. (211)At-anti-CD33 decreased survival significantly more than did GO at comparable dilution (1:1,000). No significant differences in induction of apoptosis or necrosis or DNA DSB or in decreased survival were observed after (211)At-anti-CD33 (1:1,090) versus GO (1:1) treatment., Conclusion: Our results suggest that (211)At is a promising, highly cytotoxic radioimmunotherapy in CD33-positive leukaemia and kills tumour cells more efficiently than does calicheamicin-conjugated antibody. Labelling techniques leading to higher chemical yield and specific activities must be developed to increase (211)At-anti-CD33 therapeutic effects.

Background: Pretargeting is an approach for enhancing the therapeutic index of radioimmunotherapy by separating the administrations of tumor-targeting substance and radiolabel. In this study, a pretargeting model system of avidin-conjugated monoclonal antibody trastuzumab and biotinylated, (211)At-labeled poly-L-lysine was constructed and analyzed in vitro., Methods: Avidin activated by 4-(N-maleimidomethyl)cyclohexane-1-carboxylic acid 3-sulfo-N-hydroxysuccinimide ester sodium salt (sulfo-SMCC) and thiolated trastuzumab were incubated overnight at 4 degrees C. The monomeric fraction was extracted using size exclusion fast protein liquid chromatography (FPLC) and further purified on an iminobiotin affinity column. Poly-L-lysine was biotinylated with succinimidyl-6-(biotinamido)hexanoate (NHS-LC-biotin), followed by direct (211)At-labeling with N-succinimidyl-3-(trimethylstannyl)benzoate (m-MeATE), and succinylation with succinic anhydride. The avidin-trastuzumab conjugate was characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and FPLC, together with cell-binding and biotin-binding analyses. The labeled poly-L-lysine conjugate was assessed in terms of radiochemical purity and avidin binding. Furthermore, the full pretargeting system was evaluated in a tumor cell binding assay., Results: The estimated size of the pretargeting molecule was 220 kDa, which corresponds to that of the expected avidin-trastuzumab monomer. Neither cell-binding ability (64%) nor biotin-binding ability (85%-95%) indicated any severe adverse effects from the chemical modifications. The radiochemical purity of the effector molecule was 92%-97%, and the avidin binding capacity was 91%-93%. The complete pretargeting assay resulted in a binding of 75.3 +/- 6.2% of added effector molecules to cells., Conclusions: The high binding of effector molecules to cells demonstrates a proof of concept for the synthesized molecules and pretargeting system, which will be further evaluated in vivo in future studies., ((c) 2010 American Cancer Society.)

Besides bone marrow, the kidneys are often dose-limiting organs in internal radiotherapy. The effects of high-linear energy transfer (LET) radiation on the kidneys after alpha-radioimmunotherapy (alpha-RIT) with the alpha-particle emitter, (211)At, were studied in nude mice by serial measurements of the glomerular filtration rate (GFR). The renal toxicity was evaluated at levels close to the dose limit for the bone marrow and well within the range for therapeutic efficacy on tumors. Astatinated MX35-F(ab')(2) monoclonal antibodies were administered intravenously to nude mice. Both non-tumor-bearing animals and animals bearing subcutaneous xenografts of the human ovarian cancer cell line, OVCAR-3, were used. The animals received approximately 0.4, 0.8, or 1.2 MBq in one, two, or three fractions. The mean absorbed doses to the kidneys ranged from 1.5 to 15 Gy. The renal function was studied by serial GFR measurements, using plasma clearance of (51)Cr-EDTA, up to 67 weeks after the first astatine injection. A dose-dependent effect on GFR was found and at the time interval 8-30 weeks after the first administration of astatine, the absorbed doses causing a 50% decrease in GFR were 16.4 +/- 3.3 and 14.0 +/- 4.1 Gy (mean +/- SEM), tumor- and non-tumor-bearing animals, respectively. The reduction in GFR progressed with time, and at the later time interval, (31-67 weeks) the corresponding absorbed doses were 7.5 +/- 2.4 and 11.3 +/- 2.3 Gy, respectively, suggesting that the effects of radiation on the kidneys were manifested late. Examination of the kidney sections showed histologic changes that were overall subdued. Following alpha-RIT with (211)At-MX35-F(ab')(2) at levels close to the dose limit of severe myelotoxicity, the effects found on renal function were relatively small, with only minor to moderate reductions in GFR. These results suggest that a mean absorbed dose to the kidneys of approximately 10 Gy is acceptable, and that the kidneys would not be the primary dose-limiting organ in systemic alpha-RIT when using (211)At-MX35-F(ab')(2).