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

Synthesis Principle and Practice with Radioactive Iodines and Astatine: Advances Made So Far

Radioactive iodines and astatine, possessing distinct exploitable nuclear properties, play indispensable roles in the realms of nuclear imaging and therapy. Their analogous chemical characteristics shape the design, preparation, and substrate range for tracers labeled with these radiohalogens through interconnected radiosynthetic chemistry. This perspective systematically explores the labeling methods by types of halogenating reagents─nucleophilic and electrophilic─underpinning the rational design of such compounds. It delves into the rapidly evolving synthetic strategies and reactions in radioiodination and radioastatination over the past decade, comparing their intrinsic relationships and highlighting variations. This comparative analysis illuminates potential radiosynthetic methods for exploration. Moreover, stability concerns related to compounds labeled with radioactive iodines and astatine are addressed, offering valuable insights for radiochemists and physicians alike.

Development of a Neopentyl 211At-Labeled Activated Ester Providing In Vivo Stable 211At-Labeled Antibodies for Targeted Alpha Therapy

In this study we developed a neopentyl 211At-labeled activated ester that incorporates a triazole spacer and applied it to the synthesis of an 211At-labeled cetuximab. The activated ester was synthesized via the nucleophilic 211At-astatination of a neopentyl sulfonate carrying two long alkyl chains that serve as a lipid tag, which was followed by the hydrolysis of an acetal. Additionally, we developed a novel Resin-Assisted Purification and Deprotection (RAPD) protocol involving a solid-phase extraction of the protected 211At-labeled compound from the mixture of the labeling reaction, hydrolysis of the acetal on the resin, and finally an elution of the 211At-labeled activator from the resin. This method allows the synthesis of an 211At-labeled activated ester with high purity through a simplified procedure that circumvents the need for HPLC purification. Using this 211At-labeled activated ester, we efficiently synthesized 211At-labeled cetuximab in 27±1 % radiochemical yield with 95 % radiochemical purity. This 211At-activated ester demonstrated high reactivity, and enabled the completion of the reaction with the antibody within 10 min. In comparative biodistribution studies between 211At-labeled cetuximab and the corresponding 125I-labeled cetuximab in normal mice, both the thyroid and stomach showed radioactivity levels that were less than 1.0 % of the injected dose.

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.

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.

Development of radiopharmaceuticals for targeted alpha therapy: Where do we stand?

Targeted alpha therapy is an oncological treatment, where cytotoxic doses of alpha radiation are locally delivered to tumor cells, while the surrounding healthy tissue is minimally affected. This therapeutic strategy relies on radiopharmaceuticals made of medically relevant radionuclides chelated by ligands, and conjugated to targeting vectors, which promote the drug accumulation in tumor sites. This review discusses the state-of-the-art in the development of radiopharmaceuticals for targeted alpha therapy, breaking down their key structural components, such as radioisotope, targeting vector, and delivery formulation, and analyzing their pros and cons. Moreover, we discuss current drawbacks that are holding back targeted alpha therapy in the clinic, and identify ongoing strategies in field to overcome those issues, including radioisotope encapsulation in nanoformulations to prevent the release of the daughters. Lastly, we critically discuss potential opportunities the field holds, which may contribute to targeted alpha therapy becoming a gold standard treatment in oncology in the future.