Electroless semi-automatic synthesis of ²²⁵Ac-labeled PSMA-derivatives for clinical application and evaluation of rechelation capability

Sustainable green radiochemistry is the result of current research, in which radiolabeling strategies are increasingly being developed in the direction of kit labeling. This includes the use of sunlight for reactions, but also tailor-made chelators allowing a to quantitative chelation of the corresponding radionuclide even at temperatures around 20 °C. The ultimate goal is to use these reaction conditions for labeling kit development in which only the radionuclide needs to be added and the reaction then occurs quantitatively in a sterile vial. Chelators such as HBED are already included in approved kits for the production of [⁶⁸Ga]Ga-PSMA-11 (Locametz®, IsoPROtrace11®, and Illuccix®) and allow radiolabeling at 20 °C. This paper reports on the semi-automatic labeling method of macropa(mcp)-based PSMA derivatives with ²²⁵Ac, which was performed at 20 °C without heating and, according to subsequent quality control, is technically ready-to-use for patient applications. To further strengthen the green aspect, critical chemicals like environmental persistent trifluoroacetic acid (TFA) were considered to be replace against more eco-friendly chemicals. Furthermore, additives like ascorbic acid or DTPA, which are usually used for protection against radiolysis or scavenge free progenies were considered to be eliminated from the process due to the reason, that the mcp-based precursor might be able to rechelate free progenies, as soon as they are generated from their parent nuclide ²²⁵Ac.

An iQS module was equipped with sterile single-use components and an already automated radiolabeling process was adopted to the semi-automatic system. Three validation batches for two different mcp-based PSMA derivatives were produced including quality control with criteria fulfilling clinical standards.

Three consecutive validation batches were successfully produced for each mcp-M-PSMA and mcp-D-PSMA derivatives to evaluate the safety and robustness of the labeling method. Overall, the six batches reached radiochemical yields >80% and hit the quality parameters such as radiochemical purity of >95% for clinical application. Additionally, the replacement of environmental persistent TFA of the HPLC eluents with phosphoric acid (H₃PO₄) led to similar retention times combined with a higher resolution in the radio-HPLC chromatograms. The effect of rechelation was carefully evaluated using an excess amount of precursor. It was found that, with increasing concentration, a rechelation of free progenies occurred from 22 to 144 h. Either the reaction solution or the diluted product solution was analyzed by TLC and HPLC.

The electroless synthesis of ²²⁵Ac-mcp-PSMA derivatives was successfully implemented to the iQS module, and the products fulfilled all criteria for patient application. H₃PO₄ has successfully replaced environmental persistent TFA within the HPLC eluents. With the use of mcp-based precursors, the addition of radiolysis protection agents was no longer mandatory. Rechelation of free progenies was observed at higher precursor concentration from 22 up to 144 h in the diluted product solutions. In future, this labeling method could be used for safe production of ²²⁵Ac-labeled macropa-containing peptides without electricity.

Mcp-based precursors enable for electroless reactions at 20 °C yielding ²²⁵Ac-radiopharmaceuticals with 10× increased molar activity compared to DOTA- or DOTAGA-based precursors and diluted product solution leads to higher stability and lesser radiolysis due to the rechelation effect compared to the reaction solutions.