Questioning the Affinity of Electrophilic Astatine for Sulfur-containing Compounds: Unexpected Bindings Revealed

In vivo stability of 211At-radiopharmaceuticals: on the impact of halogen bond formation

211At, when coupled to a targeting agent, is one of the most promising radionuclides for therapeutic applications. The main labelling approach consists in the formation of astatoaryl compounds, which often show a lack of in vivo stability. The hypothesis that halogen bond (XB) interactions with protein functional groups initiate a deastatination mechanism is investigated through radiochemical experiments and DFT modelling. Several descriptors agree on the known mechanism of iodoaryl substrates dehalogenation by iodothyronine deiodinases, supporting the higher in vivo dehalogenation of N-succinimidyl 3-[211At]astatobenzoate (SAB) conjugates in comparison with their iodinated counterparts. The guanidinium group in 3-[211At]astato-4-guanidinomethylbenzoate (SAGMB) prevents the formation of At-mediated XBs with the selenocysteine active site in iodothyronine deiodinases. The initial step of At-aryl bond dissociation is inhibited, elucidating the better in vivo stability of SAGMB conjugates compared with those of SAB. The impact of astatine's ability to form XB interactions on radiopharmaceutical degradation may not be limited to the case of aryl radiolabeling.

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

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

PURPOSE

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

PROCEDURES

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

MAIN FINDINGS

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

PRINCIPAL CONCLUSIONS

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

Site-Specific Radiohalogenation of a HER2-Targeted Single-Domain Antibody Fragment Using a Novel Residualizing Prosthetic Agent

Because of their rapid tumor accumulation and normal tissue clearance, single-domain antibody fragments (sdAbs) are an attractive vehicle for developing radiotherapeutics labeled with the α-emitter ²¹¹At. Herein, we have evaluated iso-[²¹¹At]AGMB-PODS, a prosthetic agent that combines a functionality for residualizing radiohalogens with a phenyloxadiazolyl methylsulfone (PODS) moiety for site-specific sdAb conjugation. Iso-[²¹¹At]AGMB-PODS and its radioiodinated analogue were evaluated for thiol-selective conjugation to anti-HER2 5F7 sdAb bearing a C-terminus GGC tail. Both radiohalogenated PODS-5F7GGC conjugates were synthesized in good radiochemical yields and retained high binding affinity on HER2-positive BT474 breast carcinoma cells. Iso-[²¹¹At]AGMB-PODS-5F7GGC was considerably more stable in vitro than its maleimide analogue in the presence of cysteine and human serum albumin (HSA) and exhibited excellent tumor uptake and high in vivo stability. Superior tumor-to-kidney activity ratios were seen for both radiohalogenated PODS-5F7GGC conjugates compared with [¹⁷⁷Lu]Lu-DOTA-PODS-5F7GGC. These results suggest that iso-[²¹¹At]AGMB-PODS-5F7GGC warrants further evaluation for the treatment of HER2-expressing malignancies.

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^-).

Advances in the Chemistry of Astatine and Implications for the Development of Radiopharmaceuticals

ConspectusAstatine (At) is the rarest on Earth of all naturally occurring elements, situated below iodine in the periodic table. While only short-lived isotopes (t1/2 ≤ 8.1 h) are known, 211At is the object of growing attention due to its emission of high-energy alpha particles. Such radiation is highly efficient to eradicate disseminated tumors, provided that the radionuclide is attached to a cancer-targeting molecule. The interest in applications of 211At in nuclear medicine translates into the increasing number of cyclotrons able to produce it. Yet, many challenges related to the minute amounts of available astatine are to be overcome in order to characterize its physical and chemical properties. This point is of paramount importance to develop synthetic strategies and solve the labeling instability in current approaches that limits the use of 211At-labeled radiopharmaceuticals. Despite its discovery in the 1940s, only the past decade has seen a significant rise in the understanding of astatine's basic chemical and radiochemical properties, thanks to the development of new analytical and computational tools.In this Account, we give a concise summary of recent advances in the determination of the physicochemical properties of astatine, putting in perspective the duality of this element which exhibits the characteristics both of a halogen and of a metal. Striking features were evidenced in the recent determination of its Pourbaix diagram such as the identification of stable cationic species, At+ and AtO+, contrasting with other halogens. Like metals, these species were shown to form complexes with anionic ligands and to exhibit a particular affinity for organic species bearing soft donor atoms. On the other hand, astatine shares many characteristics with other halogen elements. For instance, the At- species exists in water, but with the least range of EH-pH stability in the halogen series. Astatine can form molecular interactions through halogen bonding, and it was only recently identified as the strongest halogen-bond donor. This ability is nonetheless affected by relativistic effects, which translate to other peculiarities for this heavy element. For instance, the spin-orbit coupling boosts astatine's propensity to form charge-shift bonds, catching up with the behavior of the lightest halogens (fluorine, chlorine).All these new data have an impact on the development of radiolabeling strategies to turn 211At into radiopharmaceuticals. Inspired by the chemistry of iodine, the chemical approaches have sparsely evolved over the past decades and have long been limited to electrophilic halodemetalation reactions to form astatoaryl compounds. Conversely, recent developments have favored the use of the more stable At- species including the aromatic nucleophilic substitution with diaryliodonium salts or the copper-catalyzed halodeboronation of arylboron precursors. However, it is clear that new bonding modalities are necessary to improve the in vivo stability of 211At-labeled aryl compounds. The tools and data gathered over the past decade will contribute to instigate original strategies for overcoming the challenges offered by this enigmatic element. Alternatives to the C-At bond such as the B-At and the metal-At bonds are typical examples of exciting new axes of research.

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.