Astatine

Physics and small-scale dosimetry of α $\alpha$ -emitters for targeted radionuclide therapy: The case of 211 At $^{211}{\rm At}$

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 keVu - 1 ${\rm u}^{-1}$ -100 MeVu - 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 of211 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.

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.

1-(N,N-Dialkylcarbamoyl)-1,1-difluoromethanesulfonyl ester as a stable and effective precursor for a neopentyl labeling group with astatine-211

Radiohalogens with a short half-life are useful radioisotopes for radiotheranostics. Astatine-211 is an α-emitting radiohalogen and is expected to be applicable to targeted α therapy. A neopentyl labeling group is an effective hydrophilic labeling unit for various radiohalogens, which includes 211At. In this study, a 1-(N,N-dialkylcarbamoyl)-1,1-difluoromethanesulfonyl (CDf) ester was developed as a stable precursor for labeling with 211At, 77Br and 125I through a neopentyl labeling group. The CDf ester remained stable in an acetonitrile solution at room temperature and enabled the successful syntheses of 211At-labeled compounds in a highly radiochemical conversion in the presence of K2CO3. 77Br- and 125I-labeled compounds can be prepared from the CDf ester without a base. The utility of the CDf ester was demonstrated in the synthesis of a benzylguanidine with a neopentyl 211At-labeling group. The developed method afforded a 32% radiochemical yield of 211At-labeled benzylguanidine. However, a partial deastatination was observed under acidic conditions during the removal of an N-Boc protecting group. Deprotecting these groups under milder acidic conditions may improve the radiochemical yield. In conclusion, the CDf ester facilitates the syntheses of 211At, 125I and 77Br-labeled compounds that use a neopentyl labeling group for radiotheranostic applications. Further optimization of protecting groups and reaction conditions should enhance the total radiochemical yield of the 211At-labeled compounds.

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.

Rationalizing the Binding Modes of PET Radiotracers Targeting the Norepinephrine Transporter

PURPOSE

A new PET radiotracer ¹⁸F-AF78 showing great potential for clinical application has been reported recently. It belongs to a new generation of phenethylguanidine-based norepinephrine transporter (NET)-targeting radiotracers. Although many efforts have been made to develop NET inhibitors as antidepressants, systemic investigations of the structure-activity relationships (SARs) of NET-targeting radiotracers have rarely been performed.

METHODS

Without changing the phenethylguanidine pharmacophore and 3-fluoropropyl moiety that is crucial for easy labeling, six new analogs of ¹⁸F-AF78 with different meta-substituents on the benzene-ring were synthesized and evaluated in a competitive cellular uptake assay and in in vivo animal experiments in rats. Computational modeling of these tracers was established to quantitatively rationalize the interaction between the radiotracers and NET.

RESULTS

Using non-radiolabeled reference compounds, a competitive cellular uptake assay showed a decrease in NET-transporting affinity from meta-fluorine to iodine (0.42 and 6.51 µM, respectively), with meta-OH being the least active (22.67 µM). Furthermore, in vivo animal studies with radioisotopes showed that heart-to-blood ratios agreed with the cellular experiments, with AF78(F) exhibiting the highest cardiac uptake. This result correlates positively with the electronegativity rather than the atomic radius of the meta-substituent. Computational modeling studies revealed a crucial influence of halogen substituents on the radiotracer-NET interaction, whereby a T-shaped π-π stacking interaction between the benzene-ring of the tracer and the amino acid residues surrounding the NET binding site made major contributions to the different affinities, in accordance with the pharmacological data.

CONCLUSION

The SARs were characterized by in vitro and in vivo evaluation, and computational modeling quantitatively rationalized the interaction between radiotracers and the NET binding site. These findings pave the way for further evaluation in different species and underline the potential of AF78(F) for clinical application, e.g., cardiac innervation imaging or molecular imaging of neuroendocrine tumors.

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.

Gold Nanostars: A Novel Platform for Developing 211At-Labeled Agents for Targeted Alpha-Particle Therapy

Aim

To develop an innovative ²¹¹At nanoplatform with high radiolabeling efficiency and low in vivo deastatination for future targeted alpha-particle therapy (TAT) to treat cancer.

Methods

Star-shaped gold nanoparticles, gold nanostars (GNS), were used as the platform for ²¹¹At radiolabeling. Radiolabeling efficiency under different reaction conditions was tested. Uptake in the thyroid and stomach after systemic administration was used to evaluate the in vivo stability of ²¹¹At-labeled GNS. A subcutaneous U87MG human glioma xenograft murine model was used to preliminarily evaluate the therapeutic efficacy of ²¹¹At-labeled GNS after intratumoral administration.

Results

The efficiency of labeling GNS with ²¹¹At was almost 100% using a simple and rapid synthesis process that was completed in only 1 min. In vitro stability test in serum showed that more than 99% of the ²¹¹At activity remained on the GNS after 24 h incubation at 37°C. In vivo biodistribution results showed low uptake in the thyroid (0.44–0.64%ID) and stomach (0.21–0.49%ID) between 0.5 and 21 h after intravenous injection, thus indicating excellent in vivo stability of ²¹¹At-labeled GNS. The preliminary therapeutic efficacy study demonstrated that ²¹¹At labeled GNS substantially reduced tumor growth (P < 0.001; two-way ANOVA) after intratumoral administration.

Conclusion

The new ²¹¹At radiolabeling strategy based on GNS has the advantages of a simple process, high labeling efficiency, and minimal in vivo dissociation, making it an attractive potential platform for developing TAT agents that warrants further evaluation in future preclinical studies directed to evaluating prospects for clinical translation.

An expanded halogen bonding scale using astatine

As a non-covalent interaction, halogen bonding is now acknowledged to be useful in all fields where the control of intermolecular recognition plays a pivotal role. Halogen-bond basicity scales allow quantification of the halogen bonding of referential donors with organic functional groups from a thermodynamic point of view. Herein we present the pK BAtI basicity scale to provide the community an overview of halogen-bond acceptor strength towards astatine, the most potent halogen-bond donor element. This experimental scale is erected on the basis of complexation constants measured between astatine monoiodide (AtI) and sixteen selected Lewis bases. It spans over 6 log units and culminates with a value of 5.69 ± 0.32 for N,N,N',N'-tetramethylthiourea. On this scale, the carbon π-bases are the weakest acceptors, the oxygen derivatives cover almost two-thirds of the scale, and sulphur bases exhibit the highest AtI basicity. Regarding the applications of 211At in targeted radionuclide therapy, stronger labelling of carrier agents could be envisaged on the basis of the pK BAtI scale.

68Ga- and 211At-Labeled RGD Peptides for Radiotheranostics with Multiradionuclides

Probes for radiotheranostics could be produced by introducing radionuclides with similar chemical characteristics into the same precursors. We recently developed an ²¹¹At-labeled RGD peptide and a corresponding radioiodine-labeled RGD peptide. Both labeled peptides accumulated in large quantities in the tumor with similar biodistribution, demonstrating their usefulness for radiotheranostics. In this study, we hypothesized that probes for radiotheranostics combined with multiradionuclides, such as ⁶⁸Ga and ²¹¹At, have useful clinical applications. New radiolabeled RGD peptide probes were synthesized via a molecular design approach, with two labeling sites for metal and halogen. These probes were evaluated in biodistribution experiments using tumor-bearing mice. [⁶⁷Ga]Ga-DOTA-c[RGDf(4-I)K] ([⁶⁷Ga]4), Ga-DOTA-[¹²⁵I]c[RGDf(4-I)K] ([¹²⁵I]4), and Ga-DOTA-[²¹¹At]c[RGDf(4-At)K] ([²¹¹At]7) showed similar biodistribution, with high and equivalent accumulation in tumors. These results indicate the usefulness of these probes in radiotheranostics with multiradionuclides, such as a radiometal and a radiohalogen, and they could contribute to a personalized medicine regimen.

Astatine Facing Janus: Halogen Bonding vs. Charge-Shift Bonding

The nature of halogen-bond interactions was scrutinized from the perspective of astatine, potentially the strongest halogen-bond donor atom. In addition to its remarkable electronic properties (e.g., its higher aromaticity compared to benzene), C₆At₆ can be involved as a halogen-bond donor and acceptor. Two-component relativistic calculations and quantum chemical topology analyses were performed on C₆At₆ and its complexes as well as on their iodinated analogues for comparative purposes. The relativistic spin–orbit interaction was used as a tool to disclose the bonding patterns and the mechanisms that contribute to halogen-bond interactions. Despite the stronger polarizability of astatine, halogen bonds formed by C₆At₆ can be comparable or weaker than those of C₆I₆. This unexpected finding comes from the charge-shift bonding character of the C–At bonds. Because charge-shift bonding is connected to the Pauli repulsion between the bonding σ electrons and the σ lone-pair of astatine, it weakens the astatine electrophilicity at its σ-hole (reducing the charge transfer contribution to halogen bonding). These two antinomic characters, charge-shift bonding and halogen bonding, can result in weaker At-mediated interactions than their iodinated counterparts.

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.

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.

An improved 211At-labeled agent for PSMA-targeted alpha therapy

α-Particle emitters targeting the prostate-specific membrane antigen (PSMA) proved effective in treating patients with prostate cancer who were unresponsive to the corresponding β-particle therapy. Astatine-211 is an α-emitter that may engender less toxicity than other α-emitting agents. We synthesized a new ²¹¹At-labeled radiotracer targeting PSMA that resulted from the search for a pharmacokinetically optimized agent. Methods: A small series of ¹²⁵I-labeled compounds were synthesized from their tin precursors to evaluate the effect of location of radiohalogen within the molecule and the presence of lutetium in the chelate on biodistribution. On that basis, ²¹¹At-VK-02-90-Lu was selected and evaluated in cell uptake and internalization studies, biodistribution and PSMA+ PC3 PIP tumor growth control in experimental flank and metastatic (PC3-ML-Luc) models. A long-term (13-month) toxicity study was performed for ²¹¹At-VK-02-90-Lu, including tissue chemistries and histopathology. Results: The radiochemical yield of ²¹¹At-VK-02-90-Lu was 17.8 ± 8.2%. Lead compound ²¹¹At-VK-02-90-Lu demonstrated total uptake within PSMA+ PC3 PIP cells of 13.4 ± 0.5% of the input dose after 4 h of incubation with little uptake in control cells. In SCID mice, ²¹¹At-VK-02-90-Lu provided 30.6 ± 4.8 percentage of injected dose per gram (%ID/g) of uptake in PSMA+ PC3 PIP tumor at 1 h post-injection that decreased to 9.46 ± 0.96 %ID/g by 24 h. Tumor-to-salivary gland and tumor-to-kidney ratios were 129 ± 99 at 4 h and 130 ± 113 at 24 h, respectively. De-astatination was not significant (stomach 0.34 ± 0.20%ID/g at 4 h). Dose-dependent survival was demonstrated at higher doses (>1.48 MBq) in both flank and metastatic models. There was little off-target toxicity as demonstrated by hematopoietic stability, unchanged tissue chemistries, weight gain rather than loss throughout treatment, and favorable histopathology. Conclusion: Compound ²¹¹At-VK-02-90-Lu or close analogs may provide limited and acceptable toxicity while retaining efficacy in management of prostate cancer.

Generalization of Intrinsic Orbitals to Kramers-Paired Quaternion Spinors, Molecular Fragments, and Valence Virtual Spinors

Localization of molecular orbitals finds its importance in the representation of chemical bonding (and antibonding) and in the local correlation treatments beyond mean-field approximation. In this paper, we generalize the intrinsic atomic and bonding orbitals [G. Knizia, J. Chem. Theory Comput. 2013, 9, 11, 4834-4843] to relativistic applications using complex and quaternion spinors, as well as to molecular fragments instead of atomic fragments only. By performing a singular value decomposition, we show how localized valence virtual orbitals can be expressed on this intrinsic minimal basis. We demonstrate our method on systems of increasing complexity, starting from simple cases such as benzene, acrylic acid, and ferrocene molecules, and then demonstrate the use of molecular fragments and inclusion of relativistic effects for complexes containing heavy elements such as tellurium, iridium, and astatine. The aforementioned scheme is implemented into a standalone program interfaced with several different quantum chemistry packages.

α-Emitting cancer therapy using 211 At-AAMT targeting LAT1

α-Methyl-l-tyrosine (AMT) has a high affinity for the cancer-specific l-type amino acid transporter 1 (LAT1). Therefore, we established an anti-cancer therapy, with ²¹¹ At-labeled α-methyl-l-tyrosine (²¹¹ At-AAMT) as a carrier of ²¹¹ At into tumors. ²¹¹ At-AAMT had high affinity for LAT1, inhibited tumor cell growth, and induced DNA double-stranded breaks in vitro. We evaluated the accumulation of ²¹¹ At-AAMT in vivo and the role of LAT1. Treatment with 0.4 MBq/mouse ²¹¹ At-AAMT inhibited tumor growth in the PANC-1 tumor model and 1 MBq/mouse ²¹¹ At-AAMT inhibited metastasis in the lung of the B16F10 metastasis model. Our results suggested that ²¹¹ At would be useful for anti-cancer therapy and that LAT1 is suitable as a target for radionuclide therapy.

Relativistic four-component potential energy curves for the lowest 23 covalent states of molecular astatine (At2)

The potential energy curves (PECs) of all covalent states of Molecular Astatine (At₂) have been investigated in this work within a four-component relativistic framework using the MOLFDIR program package. The ground state was determined using multireference configuration interaction with all single and double excitations including Davidson size-extensivity correction (MRCISD+Q) whereas the 22 excited states were treated by complete open shell configuration interaction (COSCI). Spectroscopic constants (R_e,ω_e,ω_ex_e,ω_ey_e, D_e,B_e,α_e,β_e,T_e) are presented for all states as well as vertical excitations obtained at COSCI, MRCISD and MRCISD+Q levels. In addition, it is also presented accurate extended Rydberg analytical form for the ground state X: (1)0_g⁺.

Advances in Carbon–Element Bond Construction under Chan–Lam Cross-Coupling Conditions: A Second Decade

Copper-mediated carbon–heteroatom bond-forming reactions involving a wide range of substrates have been in the spotlight for many organic chemists. This review highlights developments between 2010 and 2019 in both stoichiometric and catalytic copper-mediated reactions, and also examples of nickel-mediated reactions, under modified Chan–Lam cross-coupling conditions using various nucleophiles; examples include chemo- and regioselective N-arylations or O-arylations. The utilization of various nucleophiles as coupling partners together with reaction optimization (including the choice of copper source, ligands, base, and other additives), limitations, scope, and mechanisms are examined; these have benefitted the development of efficient and milder methods. The synthesis of medicinally valuable or pharmaceutically important nitrogen heterocycles, including isotope-labeled compounds, is also included. Chan–Lam coupling reaction can now form twelve different C–element bonds, making it one of the most diverse and mild reactions known in organic chemistry.

1 Introduction

2 Construction of C–N and C–O Bonds

2.1 C–N Bond Formation

2.1.1 Original Discovery via Stoichiometric Copper-Mediated C–N Bond Formation

2.1.2 Copper-Catalyzed C–N Bond Formation

2.1.3 Coupling with Azides, Sulfoximines, and Sulfonediimines as Nitrogen­ Nucleophiles

2.1.4 Coupling with N,N-Dialkylhydroxylamines

2.1.5 Enolate Coupling with sp3-Carbon Nucleophiles

2.1.6 Nickel-Catalyzed Chan–Lam Coupling

2.1.7 Coupling with Amino Acids

2.1.8 Coupling with Alkylboron Reagents

2.1.9 Coupling with Electron-Deficient Heteroarylamines

2.1.10 Selective C–N Bond Formation for the Synthesis of Heterocycle-Containing Compounds

2.1.11 Using Sulfonato-imino Copper(II) Complexes

2.2 C–O Bond Formation

2.2.1 Coupling with (Hetero)arylboron Reagents

2.2.2 Coupling with Alkyl- and Alkenylboron Reagents

3 C–Element (Element = S, P, C, F, Cl, Br, I, Se, Te, At) Bond Forma tion under Modified Chan–Lam Conditions

4 Conclusions

Absorbed dose simulation of meta-211At-astato-benzylguanidine using pharmacokinetics of 131I-MIBG and a novel dose conversion method, RAP

OBJECTIVE

We aimed to estimate in vivo ²¹¹At-labeled meta-benzylguanidine (²¹¹At-MABG) absorbed doses by the two dose conversion methods, using ¹³¹I-MIBG biodistribution data from a previously reported neuroblastoma xenograft model. In addition, we examined the effects of different cell lines and time limitations using data from two other works.

METHODS

We used the framework of the Monte Carlo method to create 3200 virtual experimental data sets of activity concentrations (kBq/g) to get the statistical information. Time activity concentration curves were produced using the fitting method of a genetic algorithm. The basic method was that absorbed doses of ²¹¹At-MABG were calculated based on the medical internal radiation dose formalism with the conversion of the physical half-life time of ¹³¹I to that of ²¹¹At. We have further improved the basic method; that is, a novel dose conversion method, RAP (Ratio of Pharmacokinetics), using percent injected dose/g.

RESULTS

Virtual experiments showed that ²¹¹At-MABG and ¹³¹I-MIBG had similar properties of initial activity concentrations and biological components, but the basic method did not simulate the ²¹¹At-MABG dose. Simulated ²¹¹At-MABG doses from ¹³¹I-MIBG using the RAP method were in agreement with those from ²¹¹At-MABG, so that their boxes overlapped in the box plots. The RAP method showed applicability to the different cell lines, but it was difficult to predict long-term doses from short-term experimental data.

CONCLUSIONS

The present RAP dose conversion method could estimate ²¹¹At-MABG absorbed doses from the pharmacokinetics of ¹³¹I-MIBG with some limitations. The RAP method would be applicable to a large number of subjects for targeted nuclide therapy.

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.

Spin-orbit coupling is the key to unraveling intriguing features of the halogen bond involving astatine

The effect of spin-orbit coupling (SOC) on the halogen bond involving astatine has been investigated using state-of-the-art two- and four-component relativistic calculations. Adducts between Cl-X (X = Cl, Br, I and At) and ammonia have been selected to establish a trend on going down the periodic table. The SOC influence has been explored not only on the geometric and energetic features that can be used to characterize the halogen bond strength but also on the three main contributions to it that are the charge transfer, the "σ-hole" (i.e. the localized region with a net positive electrostatic potential at the halogen site) and the "polar flattening" (which is related to the effective shape of the halogen site). A surprisingly large increase of the Cl-At dipole moment, due to the inclusion of SOC, has been worked out using four-component CCSD(T) reference calculations, indicating that this bond is significantly more ionic than one may predict. Due to the SOC effect, which induces a peculiar charge accumulation on the At side in the Cl-At dimer, a weakening of the astatine-mediated halogen bond occurs arising from the (i) reduced amount of charge transfer, (ii) decrease of the polar flattening and (iii) lowering of the short-range Coulomb potential. The analysis of the electronic structure of the Cl-At moiety allows for a rationalization of the SOC effects on all the considered features of the halogen bond, including an unprecedented unsymmetrical charge back-donation from Cl-At to ammonia.

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.

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.