Covalent core-radiolabeling of polymeric micelles with¹²⁵I/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 bound²¹¹At. 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 for²¹¹At, enabling the use of both elements as a potential theranostic pair. PMs that were core-radiolabeled with both¹²⁵I or²¹¹At 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 the²¹¹At-labeled PMs, 4-5 % ID/g of the²¹¹At 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 of²¹¹At in the thyroid (3.5-9 %ID/g). This led to the hypothesis that deastatination takes place in the liver, whereas good stability of the²¹¹At core-radiolabel was observed in the blood.