Evaluation of astatine-211-labeled benzoate derivatives for improved in vivo stability

Astatine-211 (²¹¹At) is one of the most promising α-particle-emitting radionuclides used for targeted-alpha therapy. Benzene derivatives are currently commonly used as ²¹¹At-labeling moieties for preparing ²¹¹At-labeled compounds. However, ²¹¹At-labeled compounds using benzene derivatives as the ²¹¹At-labeling moiety have known low stability against in vivo deastatination. We hypothesized that the deastatination of ²¹¹At-labeled benzene derivatives is enzyme-mediated, and that the inclusion of a bulky structure near the ²¹¹At-astatobenzene would stabilize it by inhibiting recognition by the enzymes.

In this study, we synthesized oligopeptides with D-type amino acids, such as D-glutamic acid (e), D-alanine (a), and d-lysine (k), which have high metabolic stability against peptidases, and bound them with ²¹¹At-astatobenzoate. Then, we evaluated the stability of these ²¹¹At-labeled compounds against deastatination in vivo.

²¹¹At-labeled mono- or di-D-glutamic acid ([²¹¹At]At-Bz-e or [²¹¹At]At-Bz-ee) and ²¹¹At-labeled tri-D-alanine ([²¹¹At]At-Bz-aaa) showed high to moderate accumulation of radioactivity in the stomach and thyroid, whereas ²¹¹At-labeled tri-D-glutamic acid ([²¹¹At]At-Bz-eee) and ²¹¹At-labeled tri-d-lysine ([²¹¹At]At-Bz-kkk) showed low radioactivity levels in the stomach and thyroid. Furthermore, ²¹¹At-labeled tri-L-glutamic acid ([²¹¹At]At-Bz-EEE) displayed a higher accumulation in the stomach than [²¹¹At]At-Bz-eee. These results suggested that the in vivo deastatination of ²¹¹At-astatobenzene derivatives is not attributable to the weakness of the AtC bond, but rather to metabolization to smaller molecules with less steric hindrance to the ²¹¹At-labeling moiety in vivo and subsequent enzyme-mediated deastatination.

Our findings indicated that if ²¹¹At-labeled compounds can place the steric hindrance of the tripeptide size in the vicinity of ²¹¹At-astatobenzene, in vivo stable ²¹¹At-labeled compounds can be produced, even when using the ²¹¹At-astatobenzen structure as a ²¹¹At-labeling moiety.