Astatine-211-labeled radiotherapeutics: An emerging approach to targeted alpha-particle radiotherapy

Targeted radiotherapy or endoradiotherapy is an appealing approach to cancer treatment because of the potential for delivering curative doses of radiation to tumor while sparing normal tissues. Radionuclides that decay by the emission of a-particles such as the heavy halogen astatine-211 ((211)At) offer the exciting prospect of combining cell-specific molecular targets with radiation having a range in tissue of only a few cell diameters. Herein, the radiobiological advantages of alpha-particle targeted radiotherapy will be reviewed, and the rationale for using (211)At for this purpose will be described. The chemistry of astatine is similar to that of iodine; however, there are important differences which make the synthesis and evaluation of (211)At-labeled compounds more challenging. Perhaps the most successful approach that has been developed involves the astatodemetallation of tin, silicon or mercury precursors. Astatine-211 labeled agents that have been investigated for targeted radiotherapy include [(211)At]astatide, (211)At- labeled particulates, (211)At-labeled naphthoquinone derivatives, (211)At-labeled methylene blue, (211)At-labeled DNA precursors, meta-[(211)At]astatobenzylguanidine, (211)At-labeled biotin conjugates, (211)At-labeled bisphosphonates, and (211)At-labeled antibodies and antibody fragments. The status of these (211)At-labeled compounds will be discussed in terms of their labeling chemistry, cytotoxicity in cell culture, as well as their tissue distribution and therapeutic efficacy in animal models of human cancers. Finally, an update on the status of the first clinical trial with an (211)At-labeled targeted therapeutic, (211)At-labeled chimeric antitenascin antibody 81C6, will be provided.