Dry-distillation of astatine-211 from irradiated bismuth targets: a time-saving procedure with high recovery yields

Brain intratumoural astatine-211 radiotherapy targeting syndecan-1 leads to durable glioblastoma remission and immune memory in female mice

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 (211At-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 (125I) for biodistribution and with astatine-211 (211At) for the other experiments.

FINDINGS

The 211At-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 211At 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 211At-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 211At-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.

211At on gold nanoparticles for targeted radionuclide therapy application

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.

Behaviour, use and safety aspects of astatine-211 solvated in chloroform after dry distillation recovery

Targeted alpha therapy of disseminated cancer is an emerging technique where astatine-211 is one of the most promising candidate nuclides. Astatine-211 can be produced in medium energy cyclotrons by alpha particle bombardment of natural bismuth. The produced astatine is then commonly recovered from the irradiated solid target material through dry distillation. The dry distillation process often includes elution and solvation of condensed astatine with chloroform, forming Chloroform Eluate. In this work the handling and safe use of the high activity concentration Chloroform Eluate has been investigated. Correctly performed, evaporation of Chloroform Eluate results in a dry residue with complete recovery of the astatine. The dry residue can then serve as a versatile starting material, using appropriate oxidizing or reducing conditions, for subsequent downstream chemistry. However, it has been found that when evaporating the Chloroform Eluate, astatine can be volatilized if continuing the process beyond the point of dryness. This behavior is more pronounced when the Chloroform Eluate has received a higher absorbed dose. Upon water phase contact of the Chloroform Eluate, a major part of the astatine activity becomes water soluble, leaving the organic phase. A behavior which is also dependent on dose to the solvent.

Pretargeted Alpha Therapy of Disseminated Cancer Combining Click Chemistry and Astatine-211

To enhance targeting efficacy in the radioimmunotherapy of disseminated cancer, several pretargeting strategies have been developed. In pretargeted radioimmunotherapy, the tumor is pretargeted with a modified monoclonal antibody that has an affinity for both tumor antigens and radiolabeled carriers. In this work, we aimed to synthesize and evaluate poly-L-lysine-based effector molecules for pretargeting applications based on the tetrazine and trans-cyclooctene reaction using ²¹¹At for targeted alpha therapy and ¹²⁵I as a surrogate for the imaging radionuclides ^{123, 124}I. Poly-L-lysine in two sizes was functionalized with a prosthetic group, for the attachment of both radiohalogens, and tetrazine, to allow binding to the trans-cyclooctene-modified pretargeting agent, maintaining the structural integrity of the polymer. Radiolabeling resulted in a radiochemical yield of over 80% for astatinated poly-L-lysines and a range of 66-91% for iodinated poly-L-lysines. High specific astatine activity was achieved without affecting the stability of the radiopharmaceutical or the binding between tetrazine and transcyclooctene. Two sizes of poly-L-lysine were evaluated, which displayed similar blood clearance profiles in a pilot in vivo study. This work is a first step toward creating a pretargeting system optimized for targeted alpha therapy with ²¹¹At.

Pourbaix Diagram of Astatine Revisited: Experimental Investigations

The Pourbaix diagram of an element displays its stable chemical forms with respect to the redox potential and pH of the solution, whose knowledge is fundamental for understanding and anticipating the chemistry of the element in a specified solution. Unlike most halogens, the Pourbaix diagram in the aqueous phase for astatine (At, Z = 85) is still under construction. In particular, the predominant domains of two astatine species assumed to exist under alkaline conditions, At^- and AtO(OH)₂^-, need to be refined. Through high-performance ion-exchange chromatography, electromobility measurements, and competition experiments, the existence of At^- and AtO(OH)₂^- has been confirmed and the associated standard potential has been determined for the first time (0.86 ± 0.05 V vs the standard hydrogen electrode). On the basis of these results, a revised version of astatine's Pourbaix diagram is proposed, covering the three oxidation states of astatine that exist in the thermodynamic stability range of water: At(-I), At(I), and At(III) (as At^-, At⁺, AtO⁺, AtO(OH), and AtO(OH)₂^-).

Covalent core-radiolabeling of polymeric micelles with 125I/211At for theranostic radiotherapy

Astatine-211 (²¹¹At) is one of the most promising α-emitters for targeted alpha therapy, especially of cancer metastases. However, the lack of a stable isotope, frequent in vivo deastatination, and limited radiochemical knowledge makes it challenging to apply. Here, we report a new strategy for radiolabeling the lipophilic core of polymeric micelles (PMs) with covalently bound ²¹¹At. The PMs were radiolabeled via either an indirect synthon-based method or directly on the amphipathic block copolymer. The radiochemistry was optimized with iodine-125 (¹²⁵I) and then adapted for ²¹¹At, enabling the use of both elements as a potential theranostic pair. PMs that were core-radiolabeled with both ¹²⁵I or ²¹¹At were prepared and characterized, based on a PEG(5k)-PLGA(10k) co-polymer. The stability of the radiolabeled PMs was evaluated in mouse serum for 21 h, showing radiochemical stability above 85%. After in vivo evaluation of the ²¹¹At- labeled PMs, 4-5 % ID/g of the ²¹¹At could still be detected in the blood, showing a promising in vivo stability of the PMs. Further, ²¹¹At-labeled PMs accumulated in the spleen (20-30 %ID/g) and the liver (2.5- 5.5 %ID/g), along with some detection of ²¹¹At in the thyroid (3.5-9 %ID/g). This led to the hypothesis that deastatination takes place in the liver, whereas good stability of the ²¹¹At core-radiolabel was observed in the blood.

211At and 125I-labeling of (hetero)aryliodonium ylides: astatine wins again

Despite the growing interest in radioiodine and 211 At-labeled radiopharmaceuticals, the search for radiolabeling reactions has been somewhat neglected, resulting in a limited number of available radiosynthetic strategies. Herein we report a comparative study of nucleophilic 125 I and 211 At-labeling of aryliodonium ylides. Whereas radioiodination efficiency was low, 211 At-labeling performed efficiently on a broad scope of precursors. The most activated aryliodonium ylides led rapidly to quantitative reactions at room temperature in acetonitrile. For deactivated precursors, heating up to 90°C in glyme and addition of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) as radical scavenger appeared essential to avoid precursor degradation and to achieve high radiochemical yields and molar activity. The approach was applied successfully to the preparation of 4-[ 211 At]astatophenylalanine (4-APA), an amino acid derivative increasingly studied as radiotherapeutic drug for cancers. This validated aryliodonium ylides as a valuable tool for nucleophilic 211 At-labeling and will complement the short but now growing list of available astatination reactions.

Surface Adsorption of the Alpha-Emitter Astatine-211 to Gold Nanoparticles Is Stable In Vivo and Potentially Useful in Radionuclide Therapy

Targeted α-therapy (TAT) can eradicate tumor metastases while limiting overall toxicity. One of the most promising α-particle emitters is astatine-211 (211At). However, 211At-carbon bonds are notoriously unstable in vivo and no chelators are available. This hampers its adoption in TAT. In this study, the stability of 211At on the surface of gold nanoparticles (AuNPs) was investigated. The employed AuNPs had sizes in the 25–50 nm range. Radiolabeling by non-specific surface-adsorption in >99% radiochemical yield was achieved by mixing 211At and AuNPs both before and after polyethylene glycol (PEG) coating. The resulting 211At-AuNPs were first challenged by harsh oxidation with sodium hypochlorite, removing roughly 50% of the attached 211At. Second, incubation in mouse serum followed by a customized stability test, showed a stability of >95% after 4 h in serum. This high stability was further confirmed in an in vivo study, with comparison to a control group of free 211At. The AuNP-associated 211At showed low uptake in stomach and thyroid, which are hallmark organs of uptake of free 211At, combined with long circulation and high liver and spleen uptake, consistent with nanoparticle biodistribution. These results support that gold surface-adsorbed 211At has high biological stability and is a potentially useful delivery system in TAT.

Production, purification and availability of 211At: Near term steps towards global access

The promising characteristics of the 7.2-h radiohalogen 211At have long been recognized; including having chemical properties suitable for labeling targeting vectors ranging from small organic molecules to proteins, and the emission of only one α-particle per decay, providing greater control over off-target effects. Unfortunately, the impact of 211At within the targeted α-particle therapy domain has been constrained by its limited availability. Paradoxically, the most commonly used production method - via the 209Bi(α,2n)211At reaction - utilizes a widely available natural material (bismuth) as the target and straightforward cyclotron irradiation methodology. On the other hand, the most significant impediment to widespread 211At availability is the need for an accelerator capable of generating ≥28 MeV α-particles with sufficient beam intensities to make clinically relevant levels of 211At. In this review, current methodologies for the production and purification of 211At - both by the direct production route noted above and via a 211Rn generator system - will be discussed. The capabilities of cyclotrons that currently produce 211At will be summarized and the characteristics of other accelerators that could be utilized for this purpose will be described. Finally, the logistics of networks, both academic and commercial, for facilitating 211At distribution to locations remote from production sites will be addressed.

Simulation of the distribution of astatine-211 solution dispersion in a lab room

OBJECTIVE

This study aimed to evaluate the distribution of Astatine-211 (211At) solution dispersion in a small animal cage using autoradiography imaging to simulate the dispersion of 211At in a lab room to eventually prevent user's risk of internal exposure in terms of radiation safety.

METHODS

211At radiation sources with two chemical properties (Na211At and Free 211At) were prepared. The solutions of 211At were placed onto a dish with paper, and then, it was placed in a small animal cage for 3 h. After removing the dish, an imaging plate with attaching reference sources was placed at four walls of the cage for 15 h in a lead box. Imaging plates were read, and all pixel data were calculated using Microsoft Excel 2016 to obtain three-dimensional (3D) distribution. Calculated results were depicted using a 3D sphere model.

RESULTS

The mean activity of Free 211At was 2.3 times higher than that of Na211At on all autoradiography images. In the cage, the shape of the dispersion of Na211At was almost homogeneous, whereas that of Free 211At was more heterogeneous.

CONCLUSION

We found that the solution of 211At vaporized naturally and was distributed heterogeneously in the cage, and the chemical properties of 211At influenced their behaviors. These results must be considered to minimize the risks of radiation safety.

Overview of the Most Promising Radionuclides for Targeted Alpha Therapy: The "Hopeful Eight"

Among all existing radionuclides, only a few are of interest for therapeutic applications and more specifically for targeted alpha therapy (TAT). From this selection, actinium-225, astatine-211, bismuth-212, bismuth-213, lead-212, radium-223, terbium-149 and thorium-227 are considered as the most suitable. Despite common general features, they all have their own physical characteristics that make them singular and so promising for TAT. These radionuclides were largely studied over the last two decades, leading to a better knowledge of their production process and chemical behavior, allowing for an increasing number of biological evaluations. The aim of this review is to summarize the main properties of these eight chosen radionuclides. An overview from their availability to the resulting clinical studies, by way of chemical design and preclinical studies is discussed.

Investigation on the reactivity of nucleophilic radiohalogens with arylboronic acids in water: access to an efficient single-step method for the radioiodination and astatination of antibodies

Easy access to radioiodinated and ²¹¹At-labelled bio(macro)molecules is essential to develop new strategies in nuclear imaging and targeted radionuclide therapy of cancers. Yet, the labelling of complex molecules with heavy radiohalogens is often poorly effective due to the multiple steps and intermediate purifications needed. Herein, we investigate the potential of arylboron chemistry as an alternative approach for the late stage labelling of antibodies. The reactivity of a model precursor, 4-chlorobenzeneboronic acid (1) with nucleophilic iodine-125 and astatine-211 was at first investigated in aqueous conditions. In the presence of a copper(ii) catalyst and 1,10-phenanthroline, quantitative radiochemical yields (RCYs) were achieved within 30 minutes at room temperature. The optimum conditions were then applied to a CD138 targeting monoclonal antibody (mAb) that has previously been validated for imaging and therapy in a preclinical model of multiple myeloma. RCYs remained high (>80% for ¹²⁵I-labelling and >95% for ²¹¹At-labelling), and the whole procedure led to increased specific activities within less time in comparison with previously reported methods. Biodistribution study in mice indicated that targeting properties of the radiolabelled mAb were well preserved, leading to a high tumour uptake in a CD138 expressing tumour model. The possibility of divergent synthesis from a common modified carrier protein demonstrated herein opens facilitated perspectives in radiotheranostic applications with the radioiodine/²¹¹At pairs. Overall, the possibility to develop radiolabelling kits offered by this procedure should facilitate its translation to clinical applications.

The high reactivity of astatine and iodine in water with arylboronic acids provides access to an efficient single-step antibody radiolabelling.

Evaluation of therapeutic efficacy of 211At-labeled farletuzumab in an intraperitoneal mouse model of disseminated ovarian cancer

Introduction

Antibodies labeled with alpha-emitter astatine-211 have previously shown effective in intraperitoneal (i.p.) treatments of ovarian cancer. In the present work we explore the use of investigational farletuzumab, aimed at the folate receptor alpha. The aim was to evaluate the biodistribution and therapeutic effect of ²¹¹At-farletuzumab in in-vitro and in-vivo experiments and, using models for radiation dosimetry, to translate the findings to expected clinical result. The activity concentration used for therapy in mice (170 kBq/mL) was chosen to be in agreement with an activity concentration that is anticipated to be clinically relevant in patients (200 MBq/L).

Methods

For biodistribution, using intravenous injections and mice carrying subcutaneous (s.c.) tumors, the animals were administered either ²¹¹At-farletuzumab (n = 16); or with a combination of ¹²⁵I-farletuzumab and ²¹¹At-MX35 (n = 12). At 1, 3, 10 and 22 h, mice were euthanized and s.c.-tumors and organs weighted and measured for radioactivity. To evaluate therapeutic efficacy, mice were inoculated i.p. with 2 × 10⁶ NIH:OVCAR-3 cells. Twelve days later, the treatments were initiated by i.p.-administration. Specific treatment was given by ²¹¹At-labeled farletuzumab (group A; n = 22, 170 kBq/mL) which is specific for OVCAR-3 cells. Control treatments were given by either ²¹¹At-labeled rituximab which is unspecific for OVCAR-3 (group B; n = 22, 170 kBq/mL), non-radiolabeled farletuzumab (group C; n = 11) or PBS only (group D; n = 8).

Results

The biodistribution of ²¹¹At-farletuzumab was similar to that with ¹²⁵I as radiolabel, and also to that of ²¹¹At-labeled MX35 antibody. The tumor-free fraction (TFF) of the three control groups were all low (PBS 12%, unlabeled specific farletuzumab 9% and unspecific ²¹¹At-rituximab 14%). TFF following treatment with ²¹¹At-farletuzumab was 91%.

Conclusion

The current investigation of intraperitoneal therapy with ²¹¹At-farletuzumab, delivered at clinically relevant ²¹¹At-mAb radioactivity concentrations and specific activities, showed a 6 to 10-fold increase (treated versus controls) in antitumor efficacy. This observation warrants further clinical testing.

Human dosimetry of free 211At and meta-[211At]astatobenzylguanidine (211At-MABG) estimated using preclinical biodistribution from normal mice

BACKGROUND

211At is one of the ideal nuclides for targeted radionuclide therapies (TRTs). Meta-[211At]astatobenzylguanidine (211At-MABG) has been proposed for the treatment of pheochromocytoma. To effectively use these radiopharmaceuticals, dosimetry must be performed. It is important to determine the absorbed doses of free 211At and 211At-MABG to determine the organs that may be at risk when using TRTs. The aim of this study was to estimate human dosimetry from preclinical biodistribution of free 211At and 211At-MABG in various organs in normal mice.

METHODS

Male C57BL/6 N mice were administered 0.13 MBq of free 211At or 0.20 MBq of 211At-MABG by tail-vein injection. The mice were sacrificed at 5 min, and at 1, 3, 6, and 24 h after the injection (n = 5 for each group). The percentage of injected activity per mass in organs and blood (%IA/g) was determined. The human absorbed doses of free 211At and 211At-MABG were calculated using the Organ Level INternal Dose Assessment/EXponential Modeling (OLINDA/EXM) version 2.0 and IDAC-Dose 2.1.

RESULTS

High uptake of free 211At was observed in the lungs, spleen, salivary glands, stomach, and thyroid. The absorbed doses of free 211At in the thyroid and several tissues were higher than those of 211At-MABG. The absorbed doses of 211At-MABG in the adrenal glands, heart wall, and liver were higher than those of free 211At.

CONCLUSIONS

The absorbed doses of 211At-MABG in organs expressing the norepinephrine transporter were higher than those of free 211At. In addition, the biodistribution of free 211At was different from that of 211At-MABG. The absorbed dose of free 211At may help predict the organs potentially at risk during TRTs using 211At-MABG due to deastatination.

Realizing Clinical Trials with Astatine-211: The Chemistry Infrastructure

Despite the consensus around the clinical potential of the α-emitting radionuclide astatine-211 (211At), 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 211At, few cyclotron facilities produce the nuclide on a regular basis. (2) Lack of a chemical infrastructure, that is, isolation of 211At from irradiated targets and the subsequent synthesis of an astatinated product. At present, the research groups that work with 211At depend on custom systems for recovering 211At 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 211At. Compared with radiometals there are no well-established and generally accepted synthesis methods for forming sufficiently stable bonds between 211At and the tumor-specific vector to allow for systemic applications. Herein we present an overview of the infrastructure of producing 211At radiopharmaceuticals, from target to radiolabeled product including chemical strategies to overcome hurdles for advancement into clinical trials with 211At.

Astatine partitioning between nitric acid and conventional solvents: indication of covalency in ketone complexation of AtO. Chem Commun (Camb) 2020;56:9004-9007. [PMID: 32638758 DOI: 10.1039/d0cc03804k]

Astatine-211 has been produced at Texas A&M University on the K150 cyclotron, with a yield of 890 ± 80 MBq through the ²⁰⁹Bi(α,2n)²¹¹At reaction via an 8 h bombardment with a beam current of 4-8 μA and an α-particle beam energy of 28.8 MeV. The target was then dissolved in HNO₃ and the extraction of ²¹¹At was investigated into a variety of organic solvents in 1-3 M HNO₃. Extraction of ²¹¹At with distribution ratios as high as 11.3 ± 0.6, 12.3 ± 0.8, 42.2 ± 2.2, 69 ± 4, and 95 ± 6 were observed for diisopropyl ether, 1-decanol, 1-octanol, 3-octanone, and methyl isobutyl ketone, respectively, while the distribution ratios for ²⁰⁷Bi were ≤0.05 in all cases. The extraction of ²¹¹At into both methyl isobutyl ketone and 3-octanone showed a strong, linear dependence on the HNO₃ initial aqueous concentration and better extraction than other solvents. DFT calculations show stronger binding between the carbonyl oxygen of the ketone and the At metal center.

Possibility of cancer-stem-cell-targeted radioimmunotherapy for acute myelogenous leukemia using 211At-CXCR4 monoclonal antibody

To explore stem-cell-targeted radioimmunotherapy with α-particles in acute myelogenous leukemia (AML), pharmacokinetics and dosimetry of the 211At-labeled anti-C-X-C chemokine receptor type 4 monoclonal antibody (211At-CXCR4 mAb) were conducted using tumor xenografted mice. The biological half-life of 211At-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 211At-CXCR4 mAb for AML appears possible and requires further therapeutic studies.

A Solid-State Support for Separating Astatine-211 from Bismuth

Increasing access to the short-lived α-emitting radionuclide astatine-211 (²¹¹At) has the potential to advance targeted α-therapeutic treatment of disease and to solve challenges facing the medical community. For example, there are numerous technical needs associated with advancing the use of ²¹¹At in targeted α-therapy, e.g., improving ²¹¹At chelates, developing more effective ²¹¹At targeting, and characterizing in vivo ²¹¹At behavior. There is an insufficient understanding of astatine chemistry to support these efforts. The chemistry of astatine is one of the least developed of all elements on the periodic table, owing to its limited supply and short half-life. Increasing access to ²¹¹At could help address these issues and advance understanding of ²¹¹At chemistry in general. We contribute here an extraction chromatographic processing method that simplifies ²¹¹At production in terms of purification. It utilizes the commercially available Pre-Filter resin to rapidly (<1.5 h) isolate ²¹¹At from irradiated bismuth targets (Bi decontamination factors ≥876 000), in reasonable yield (68-55%) and in a form that is compatible for subsequent in vivo study. We are excited about the potential of this procedure to address ²¹¹At supply and processing/purification problems.

Targeted alpha therapy with astatine-211-labeled anti-PSCA A11 minibody shows antitumor efficacy in prostate cancer xenografts and bone microtumors

PURPOSE

Targeted alpha therapy (TAT) is a promising treatment for micrometastatic and minimal residual cancer. We evaluated systemic α-radioimmunotherapy (α-RIT) of metastatic castration-resistant prostate cancer (mCRPC) using the α-particle emitter ²¹¹At-labeled to the anti-PSCA A11 minibody. A11 is specific for prostate stem cell antigen (PSCA), a cell surface glycoprotein which is overexpressed in more than 90% of both localized prostate cancer and bone metastases.

METHODS

PC3-PSCA cells were implanted subcutaneously (s.c.) and intratibially (i.t) in nude mice. Efficacy of α-RIT (two fractions-14-day interval) was studied on s.c. macrotumors (0, 1.5 and 1.9 MBq) and on i.t. microtumors (~100-200 μm; 0, 0.8 or 1.5 MBq) by tumor-volume measurements. The injected activities for therapies were estimated from separate biodistribution and myelotoxicity studies.

RESULTS

Tumor targeting of ²¹¹At-A11 was efficient and the effect on s.c. macrotumors was strong and dose-dependent. At 6 weeks, the mean tumor volumes for the treated groups, compared with controls, were reduced by approximately 85%. The separate myelotoxicity study following one single fraction showed reduced white blood cells (WBC) for all treated groups on day 6 after treatment. For the 0.8 and 1.5 MBq, the WBC reductions were transient and followed by recovery at day 13. For 2.4 MBq, a clear toxicity was observed and the mice were sacrificed on day 7. In the long-term follow-up of the 0.8 and 1.5 MBq-groups, blood counts on day 252 were normal and no signs of radiotoxicity observed. Efficacy on i.t. microtumors was evaluated in two experiments. In experiment 1, the tumor-free fraction (TFF) was 95% for both treated groups and significantly different (p < 0.05) from the controls at a TFF of 66%). In experiment 2, the difference in TFF was smaller, 32% for the treated group versus 20% for the controls. However, the difference in microtumor volume in experiment 2 was highly significant, 0.010 ± 0.003 mm³ versus 3.79 ± 1.24 mm³ (treated versus controls, respectively), i.e., a 99.7% reduction (p < 0.001). The different outcome in experiment 1 and 2 is most likely due to differences in microtumor sizes at therapy, or higher tumor-take in experiment 2 (where more cells were implanted).

CONCLUSION

Evaluating fractionated α-RIT with ²¹¹At-labeled anti-PSCA A11 minibody, we found clear growth inhibition on both macrotumors and intratibial microtumors. For mice treated with multiple fractions, we also observed radiotoxicity manifested by progressive loss in body weight at 30 to 90 days after treatment. Our findings are conceptually promising for a systemic TAT of mCRPC and warrant further investigations of ²¹¹At-labeled PSCA-directed vectors. Such studies should include methods to improve the therapeutic window, e.g., by implementing a pretargeted regimen of α-RIT or by altering the size of the targeting vector.

Development of an autonomous solvent extraction system to isolate astatine-211 from dissolved cyclotron bombarded bismuth targets

Cyclotron-produced astatine-211 (²¹¹At) shows tremendous promise in targeted alpha therapy (TAT) applications due to its attractive half-life and its 100% α-emission from nearly simultaneous branched alpha decay. Astatine-211 is produced by alpha beam bombardment of naturally monoisotopic bismuth metal (²⁰⁹Bi) via the (α, 2n) reaction. In order to isolate the small mass of ²¹¹At (specific activity = 76 GBq·µg⁻¹) from several grams of acid-dissolved Bi metal, a manual milliliter-scale solvent extraction process using diisopropyl ether (DIPE) is routinely performed at the University of Washington. As this process is complex and time consuming, we have developed a fluidic workstation that can perform the method autonomously. The workstation employs two pumps to concurrently deliver the aqueous and organic phases to a mixing tee and in-line phase mixer. The mixed phases are routed to a phase settling reservoir, where they gravity settle. Finally, each respective phase is withdrawn into its respective pump. However, development of a phase boundary sensor, placed in tandem with the phase settling reservoir, was necessary to communicate to the system when withdrawal of the denser aqueous phase was complete (i.e., the intersection of the two phases was located). The development and optimization of the autonomous solvent extraction system is described, and the ²¹¹At yields from several ~1.1 GBq-level ²¹¹At processing runs are reported.

Investigation of a tellurium-packed column for isolation of astatine-211 from irradiated bismuth targets and demonstration of a semi-automated system

Astatine-211 is an attractive radionuclide for use in targeted alpha therapy of blood-borne diseases and micrometastatic diseases. Efficient isolation methods that can be adapted to robust automated ²¹¹At isolation systems are of high interest for improving the availability of ²¹¹At. Based on the early studies of Bochvarova and co-workers involving isolation of ²¹¹At from irradiated thorium targets, we developed a method for ²¹¹At isolation from bismuth targets using tellurium-packed columns. Dissolution of irradiated bismuth targets is accomplished using HNO₃; however, ²¹¹At is not captured on the Te column material in this matrix. Our method involves slow addition of aqueous NH₂OH·HCl to the Bi target dissolved in HNO₃ to convert to a HCl matrix. The amount of NH₂OH·HCl was optimized because (1) the quantity of NH₂OH·HCl used appears to affect the radiolabeling yield of phenethyl-closo-decaborate(2-) (B10)-conjugated antibodies and (2) reducing the volume of NH₂OH·HCl solution can effectively shorten the overall isolation time. A proof-of-concept semi-automated process has been demonstrated using targets containing ~0.96 GBq (~26 mCi) of ²¹¹At. High isolation yields (88–95%) were obtained. Radiochemical purity of the isolated ²¹¹At was assessed by radio-HPLC. Concentrations of Bi and Te contaminants in the ²¹¹At and the astatinated antibodies were evaluated using ICP-MS.

Towards elucidating the radiochemistry of astatine - Behavior in chloroform

Targeted alpha therapy of disseminated cancer is an emerging technique where astatine-211 is one of the most promising candidate nuclides. Although astatine has been known for over 70 years, its chemistry is still largely unexplored, mainly due to the lack of stable or long-lived isotopes. However, substantial amounts of astatine-211 can be produced in cyclotrons by the bombardment of natural bismuth. The astatine can be recovered from the resulting irradiated target material through either wet extraction or dry-distillation. Chloroform has become an important intermediate solvent for the recovery of astatine after production, especially following dry distillation. In this work, the radiochemistry of astatine in chloroform was investigated using evaporation, solvent extraction, chromatographic methods and molecular modeling. The extraction of astatine in chloroform led to the formation of multiple astatine species, allowing for evaporation of the solvent to dryness without any loss of activity. Radiolysis products of chloroform were shown to play an important role in the speciation of astatine forming both reactive and kinetically stable compounds. It was hypothesized that reactions with chlorine, as well as trichloromethyl hydroperoxide, forming polar astatine compounds are important reactions under the current experimental conditions.

Alpha-Emitters and Targeted Alpha Therapy in Oncology: from Basic Science to Clinical Investigations

Alpha-emitters are radionuclides that decay through the emission of high linear energy transfer α-particles and possess favorable pharmacologic profiles for cancer treatment. When coupled with monoclonal antibodies, peptides, small molecules, or nanoparticles, the excellent cytotoxic capability of α-particle emissions has generated a strong interest in exploring targeted α-therapy in the pre-clinical setting and more recently in clinical trials in oncology. Multiple obstacles have been overcome by researchers and clinicians to accelerate the development of targeted α-therapies, especially with the recent improvement in isotope production and purification, but also with the development of innovative strategies for optimized targeting. Numerous studies have demonstrated the in vitro and in vivo efficacy of the targeted α-therapy. Radium-223 (²²³Ra) dichloride (Xofigo®) is the first α-emitter to have received FDA approval for the treatment of prostate cancer with metastatic bone lesions. There is a significant increase in the number of clinical trials in oncology using several radionuclides such as Actinium-225 (²²⁵Ac), Bismuth-213 (²¹³Bi), Lead-212 (²¹²Pb), Astatine (²¹¹At) or Radium-223 (²²³Ra) assessing their safety and preliminary activity. This review will cover their therapeutic application as well as summarize the investigations that provide the foundation for further clinical development.

Labeling of Anti-HER2 Nanobodies with Astatine-211: Optimization and the Effect of Different Coupling Reagents on Their in Vivo Behavior

The use of nanobodies (Nbs) as vehicles in targeted alpha therapy (TAT) has gained great interest because of their excellent properties. They combine high in vivo affinity and specificity of binding with fast kinetics. This research investigates a novel targeted therapy that combines the α-particle emitter astatine-211 (²¹¹At) and the anti-HER2 Nb 2Rs15d to selectively target HER2+ cancer cells. Two distinctive radiochemical methodologies are investigated using three different coupling reagents. The first method uses the coupling reagents, N-succinimidyl 4-(1,2-bis-tert-butoxycarbonyl)guanidinomethyl-3-(trimethylstannyl)benzoate (Boc₂-SGMTB) and N-succinimidyl-3-(trimethylstannyl)benzoate (m-MeATE), which are both directed to amino groups on the Nb, resulting in random conjugation. The second method aims at obtaining a homogeneous tracer population, via a site-specific conjugation of the N-[2-(maleimido)ethyl]-3-(trimethylstannyl)benzamide (MSB) reagent onto the carboxyl-terminal cysteine of the Nb. The resulting radioconjugates are evaluated in vitro and in vivo. 2Rs15d is labeled with ²¹¹At using Boc₂-SGMTB, m-MeATE, and MSB. After astatination and purification, the binding specificity of the radioconjugates is validated on HER2+ cells, followed by an in vivo biodistribution assessment in SKOV-3 xenografted mice. α-camera imaging is performed to determine uptake and activity distribution in kidneys/tumors. 2Rs15d astatination resulted in a high radiochemical purity >95% for all radioconjugates. The biodistribution studies of all radioconjugates revealed comparable tumor uptake (higher than 8% ID/g at 1 h). [²¹¹At]SAGMB-2Rs15d showed minor uptake in normal tissues. Only in the kidneys, a higher uptake was measured after 1 h, but decreased rapidly after 3 h. Astatinated Nbs consisting of m-MeATE or MSB reagents revealed elevated uptake in lungs and stomach, indicating the presence of released ²¹¹At. α-Camera imaging of tumors revealed a homogeneous activity distribution. The radioactivity in the kidneys was initially concentrated in the renal cortex, while after 3 h most radioactivity was measured in the medulla, confirming the fast washout into urine. Changing the reagents for Nb astatination resulted in different in vivo biodistribution profiles, while keeping the targeting moiety identical. Boc₂-SGMTB is the preferred reagent for Nb astatination because of its high tumor uptake, its low background signals, and its fast renal excretion. We envision [²¹¹At]SAGMB-2Rs15d to be a promising therapeutic agent for TAT and aim toward efficacy evaluation.

Radiochemical aspects of alpha emitting radionuclides for medical application

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

Prosthetic groups for radioiodination and astatination of peptides and proteins: A comparative study of five potential bioorthogonal labeling strategies

¹²⁵I- and ²¹¹At-labeled azide and tetrazine based prosthetic groups for bioorthogonal conjugation were designed and tested in a comparative study of five bioorthogonal systems. All five bioconjugation reactions conducted on a model clickable peptide led to quantitative yields within less than a minute to several hours depending on the system used. Transferability to the labeling of an IgG was demonstrated with one of the bioorthogonal system. This study provides several new alternatives to the conventional and suboptimal approach currently in use for radioiodination and astatination of biomolecules and should accelerate the development of new probes with these radionuclides for applications in nuclear imaging and targeted alpha-therapy.

Experimental and computational evidence of halogen bonds involving astatine

The importance of halogen bonds-highly directional interactions between an electron-deficient σ-hole moiety in a halogenated compound and an acceptor such as a Lewis base-is being increasingly recognized in a wide variety of fields from biomedicinal chemistry to materials science. The heaviest halogens are known to form stronger halogen bonds, implying that if this trend continues down the periodic table, astatine should exhibit the highest halogen-bond donating ability. This may be mitigated, however, by the relativistic effects undergone by heavy elements, as illustrated by the metallic character of astatine. Here, the occurrence of halogen-bonding interactions involving astatine is experimentally evidenced. The complexation constants of astatine monoiodide with a series of organic ligands in cyclohexane solution were derived from distribution coefficient measurements and supported by relativistic quantum mechanical calculations. Taken together, the results show that astatine indeed behaves as a halogen-bond donor-a stronger one than iodine-owing to its much more electrophilic σ-hole.

Bifunctional aryliodonium salts for highly efficient radioiodination and astatination of antibodies

In this report we describe the development of an alternative approach to arylstannane chemistry for radiolabeling antibodies with radioiodine or astatine based on aryliodonium salts precursors. Bifunctional aryliodonium salts were designed and tested for the synthesis of ¹²⁵I and ²¹¹At labeled prosthetic groups for bioconjugation. The nature of the electron rich aryl group was varied and its impact on the regioselectivity of radiohalogenation was evaluated. Unexpectedly, whereas the 2-thienyl group provided the best regioselectivity towards the radioiodination of the aryl moiety of interest (98:2), it was less selective for astatination (87:13); the anisyl group providing the best regioselectivity of astatination (94:6). Under optimized conditions, both radioiodination and astatination could be performed very efficiently in mild conditions (radiochemical yields>85%). The ionic nature of the precursors was exploited to develop an efficient purification approach: the HPLC step that is usually necessary in conventionnal approaches to optimize removal of organotin toxic precursors and side products was replaced by a filtration through a silica cartridge with a significantly reduced loss of radiolabeled product. The purified radioiodinated and astatinated prosthetic groups were then conjugated efficiently to an anti-CD138 monoclonal antibody (75-80% conjugation yield). By using this novel and simple radiohalogenation procedure, higher overall radiochemical yields of astatination were obtained in comparison with the use of an arylstannane precursor and procedures of the litterature for labeling the same antibody. Overall, due to their simplicity of use and high robustness, these new precursors should simplify the labeling of proteins of interest with iodine and astatine radioisotopes for imaging and therapeutic applications.

211At labeled substance P (5-11) as potential radiopharmaceutical for glioma treatment

INTRODUCTION

The purposes of the present work were to label substance P (5-11) with ²¹¹At using a rhodium(III) complex with a bifunctional ligand-2-(1,5,9,13-tetrathiacyclohexadecan-3-yloxy)acetic acid ([16aneS₄]-COOH) and to assess the in vitro stability and toxicity of the obtained radiobioconjugate.

METHODS

Two approaches were evaluated to obtain ¹³¹I/²¹¹At-Rh[16aneS₄]-SP_5-11 radiobioconjugates, based on 2-step and 1-step syntheses. In the first method ¹³¹I/²¹¹At-Rh[16aneS₄]-COOH complexes were obtained that required further coupling to a biomolecule. In the second approach, the bioconjugate [16aneS₄]-SP_5-11 was synthesized and further labeled with ¹³¹I and ²¹¹At through the utilization of a Rh(III) metal cation bridge. The synthesized compounds were analyzed by HPLC, TLC and paper electrophoresis.

RESULTS

The ¹³¹I/²¹¹At-Rh[16aneS₄]-COOH complexes were obtained in high yield and possessed good stability in PBS and CSF. Preliminary studies on coupling of ¹³¹I-Rh[16aneS₄]-COOH to substance P (5-11) in 2-step synthesis showed that this procedure was too long with respect to ²¹¹At 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 ¹³¹I/²¹¹At-Rh[16aneS₄]-SP_5-11 radiobioconjugates. The stability of both compounds in PBS and CSF was high. Toxicity studies with the ²¹¹At-Rh[16aneS₄]-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 ²¹¹At-Rh[16aneS₄]-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.

Microarray Studies on 211At Administration in BALB/c Nude Mice Indicate Systemic Effects on Transcriptional Regulation in Nonthyroid Tissues

Targeted α-therapy is a promising treatment option for various types of malignant tumors. Radiolabeled cancer-seeking agents, however, undergo degradation, resulting in a certain percentage of free radionuclide in the body. The radiohalogen ²¹¹At accumulates in various tissues, with specifically high uptake in the thyroid. When normal thyroid function is disturbed because of ionizing radiation (IR) exposure, deleterious effects can occur in tissues that depend on thyroid hormone (TH) regulation for normal physiologic function. However, knowledge of systemic effects is still rudimentary. We previously reported similarities in transcriptomic regulation between the thyroid and other tissues despite large differences in absorbed dose from ²¹¹At. Here, we present supportive evidence on systemic effects after ²¹¹At administration.

METHODS

Expression microarray data from the kidney cortex and medulla, liver, lungs, and spleen were used from previous studies in which mice were intravenously injected with 0.064-42 kBq of ²¹¹At and killed after 24 h or injected with 1.7 kBq of ²¹¹At and killed after 1, 6, or 168 h. Controls were mock-treated and killed after 24 h. Literature-based gene signatures were used to evaluate the relative impact from IR- or TH-induced regulation. Thyroid- and TH-associated upstream regulators as well as thyroid-related diseases and functions were generated using functional analysis software.

RESULTS

Responses in IR- or TH-associated gene signatures were tissue-specific and varied over time, and the relative impact of each gene signature differed between the investigated tissues. The liver showed a clear dominance of TH-responding genes. In the kidney cortex, kidney medulla, and lungs, the TH-associated signature was detected to at least an extent similar to the IR-associated signature. The spleen was the single tissue showing regulation of only IR-associated signature genes. Various thyroid-associated diseases and functions were inferred from the data: L-triiodothyronine, TH, TH receptor, and triiodothyronine (reverse) were inferred as upstream regulators with differences in incidence and strength of regulation depending on tissue type.

CONCLUSION

These findings indicate that transcriptional regulation in various nonthyroid tissues was-in part-induced by thyroid (hormone)-dependent signaling. Consideration of the systemic context between tissues could contribute to normal tissue risk assessment and planning of remedial measures.

Cure of Human Ovarian Carcinoma Solid Xenografts by Fractionated α-Radioimmunotherapy with 211At-MX35-F(ab')2: Influence of Absorbed Tumor Dose and Effect on Long-Term Survival

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 mm³, NIH:OVCAR-3) were injected repeatedly (1-3 intravenous injections 7-10 d apart, allowing bone marrow recovery) with ²¹¹At-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 ²¹¹At-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.

Unexpected Behavior of the Heaviest Halogen Astatine in the Nucleophilic Substitution of Aryliodonium Salts

Aryliodonium salts have become precursors of choice for the synthesis of (18) F-labeled tracers for nuclear imaging. However, little is known on the reactivity of these compounds with heavy halides, that is, radioiodide and astatide, at the radiotracer scale. In the first comparative study of radiohalogenation of aryliodonium salts with (125) I(-) and (211) At(-) , initial experiments on a model compound highlight the higher reactivity of astatide compared to iodide, which could not be anticipated from the trends previously observed within the halogen series. Kinetic studies indicate a significant difference in activation energy (Ea =23.5 and 17.1 kcal mol(-1) with (125) I(-) and (211) At(-) , respectively). Quantum chemical calculations suggest that astatination occurs via the monomeric form of an iodonium complex whereas iodination occurs via a heterodimeric iodonium intermediate. The good to excellent regioselectivity of halogenation and high yields achieved with diversely substituted aryliodonium salts indicate that this class of compounds is a promising alternative to the stannane chemistry currently used for heavy radiohalogen labeling of tracers in nuclear medicine.

Uses of alpha particles, especially in nuclear reaction studies and medical radionuclide production

Alpha particles exhibit three important characteristics: scattering, ionisation and activation. This article briefly discusses those properties and outlines their major applications. Among others, α-particles are used in elemental analysis, investigation and improvement of materials properties, nuclear reaction studies and medical radionuclide production. The latter two topics, dealing with activation of target materials, are treated in some detail in this paper. Measurements of excitation functions of α-particle induced reactions shed some light on their reaction mechanisms, and studies of isomeric cross sections reveal the probability of population of high-spin nuclear levels. Regarding medical radionuclides, an overview is presented of the isotopes commonly produced using α-particle beams. Consideration is also given to some routes which could be potentially useful for production of a few other radionuclides. The significance of α-particle induced reactions to produce a few high-spin isomeric states, decaying by emission of low-energy conversion or Auger electrons, which are of interest in localized internal radiotherapy, is outlined. The α-particle beam, thus broadens the scope of nuclear chemistry research related to development of non-standard positron emitters and therapeutic radionuclides.

The pre-clinical characterization of an alpha-emitting sigma-2 receptor targeted radiotherapeutic

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.

Transcriptional Response in Mouse Thyroid Tissue after 211At Administration: Effects of Absorbed Dose, Initial Dose-Rate and Time after Administration

BACKGROUND

211At-labeled radiopharmaceuticals are potentially useful for tumor therapy. However, a limitation has been the preferential accumulation of released 211At in the thyroid gland, which is a critical organ for such therapy. The aim of this study was to determine the effect of absorbed dose, dose-rate, and time after 211At exposure on genome-wide transcriptional expression in mouse thyroid gland.

METHODS

BALB/c mice were i.v. injected with 1.7, 7.5 or 100 kBq 211At. Animals injected with 1.7 kBq were killed after 1, 6, or 168 h with mean thyroid absorbed doses of 0.023, 0.32, and 1.8 Gy, respectively. Animals injected with 7.5 and 100 kBq were killed after 6 and 1 h, respectively; mean thyroid absorbed dose was 1.4 Gy. Total RNA was extracted from pooled thyroids and the Illumina RNA microarray platform was used to determine mRNA levels. Differentially expressed transcripts and enriched GO terms were determined with adjusted p-value <0.01 and fold change >1.5, and p-value <0.05, respectively.

RESULTS

In total, 1232 differentially expressed transcripts were detected after 211At administration, demonstrating a profound effect on gene regulation. The number of regulated transcripts increased with higher initial dose-rate/absorbed dose at 1 or 6 h. However, the number of regulated transcripts decreased with mean absorbed dose/time after 1.7 kBq 211At administration. Furthermore, similar regulation profiles were seen for groups administered 1.7 kBq. Interestingly, few previously proposed radiation responsive genes were detected in the present study. Regulation of immunological processes were prevalent at 1, 6, and 168 h after 1.7 kBq administration (0.023, 0.32, 1.8 Gy).

Automated astatination of biomolecules--a stepping stone towards multicenter clinical trials

To facilitate multicentre clinical studies on targeted alpha therapy, it is necessary to develop an automated, on-site procedure for conjugating rare, short-lived, alpha-emitting radionuclides to biomolecules. Astatine-211 is one of the few alpha-emitting nuclides with appropriate chemical and physical properties for use in targeted therapies for cancer. Due to the very short range of the emitted α-particles, this therapy is particularly suited to treating occult, disseminated cancers. Astatine is not intrinsically tumour-specific; therefore, it requires an appropriate tumour-specific targeting vector, which can guide the radiation to the cancer cells. Consequently, an appropriate method is required for coupling the nuclide to the vector. To increase the availability of astatine-211 radiopharmaceuticals for targeted alpha therapy, their production should be automated. Here, we present a method that combines dry distillation of astatine-211 and a synthesis module for producing radiopharmaceuticals into a process platform. This platform will standardize production of astatinated radiopharmaceuticals, and hence, it will facilitate large clinical studies focused on this promising, but chemically challenging, alpha-emitting radionuclide. In this work, we describe the process platform, and we demonstrate the production of both astaine-211, for preclinical use, and astatine-211 labelled antibodies.

Absorbed Doses and Risk Estimates of (211)At-MX35 F(ab')2 in Intraperitoneal Therapy of Ovarian Cancer Patients

PURPOSE

Ovarian cancer is often diagnosed at an advanced stage with dissemination in the peritoneal cavity. Most patients achieve clinical remission after surgery and chemotherapy, but approximately 70% eventually experience recurrence, usually in the peritoneal cavity. To prevent recurrence, intraperitoneal (i.p.) targeted α therapy has been proposed as an adjuvant treatment for minimal residual disease after successful primary treatment. In the present study, we calculated absorbed and relative biological effect (RBE)-weighted (equivalent) doses in relevant normal tissues and estimated the effective dose associated with i.p. administration of (211)At-MX35 F(ab')2.

METHODS AND MATERIALS

Patients in clinical remission after salvage chemotherapy for peritoneal recurrence of ovarian cancer underwent i.p. infusion of (211)At-MX35 F(ab')2. Potassium perchlorate was given to block unwanted accumulation of (211)At in thyroid and other NIS-containing tissues. Mean absorbed doses to normal tissues were calculated from clinical data, including blood and i.p. fluid samples, urine, γ-camera images, and single-photon emission computed tomography/computed tomography images. Extrapolation of preclinical biodistribution data combined with clinical blood activity data allowed us to estimate absorbed doses in additional tissues. The equivalent dose was calculated using an RBE of 5 and the effective dose using the recommended weight factor of 20. All doses were normalized to the initial activity concentration of the infused therapy solution.

RESULTS

The urinary bladder, thyroid, and kidneys (1.9, 1.8, and 1.7 mGy per MBq/L) received the 3 highest estimated absorbed doses. When the tissue-weighting factors were applied, the largest contributors to the effective dose were the lungs, stomach, and urinary bladder. Using 100 MBq/L, organ equivalent doses were less than 10% of the estimated tolerance dose.

CONCLUSION

Intraperitoneal (211)At-MX35 F(ab')2 treatment is potentially a well-tolerated therapy for locally confined microscopic ovarian cancer. Absorbed doses to normal organs are low, but because the effective dose potentially corresponds to a risk of treatment-induced carcinogenesis, optimization may still be valuable.