Steps toward High Specific Activity Labeling of Biomolecules for Therapeutic Application:  Preparation of Precursor [¹⁸⁸Re(H₂O)₃(CO)₃]⁺ and Synthesis of Tailor-Made Bifunctional Ligand Systems

Two kit preparations of the organometallic precursor [¹⁸⁸Re(H₂O)₃(CO)₃]⁺ in aqueous media are presented. Method A uses gaseous carbon monoxide and amine borane (BH₃·NH₃) as the reducing agent. In method B CO(g) is replaced by K₂[H₃BCO₂] that releases carbon monoxide during hydrolysis. Both procedures afford the desired precursor in yields >85% after 10 min at 60 °C. HPLC and TLC analyses revealed 7 ± 3% of unreacted ¹⁸⁸ReO₄^- and <5% of colloidal ¹⁸⁸ReO₂. Solutions of up to 14 GBq/mL Re-188 have been successfully carbonylated with these two methods. The syntheses of two tailor-made bifunctional ligand systems for the precursor [¹⁸⁸Re(H₂O)₃(CO)₃]⁺ are presented. The tridentate chelates consist of a bis[imidazol-2-yl]methylamine or an iminodiacetic acid moiety, respectively. Both types of ligand systems have been prepared with alkyl spacers of different length and a pendent primary amino or carboxylic acid functionality, enabling the amidic linkage to biomolecules. The tridentate coordination of the ligands to the rhenium-tricarbonyl core could be elucidated on the macroscopic level by X-ray structure analyses and 1D and 2D NMR experiments of two representative model complexes. On the nca level, the ligands allow labeling yields >95% with [¹⁸⁸Re(H₂O)₃(CO)₃]⁺ under mild reaction conditions (PBS buffer, 60 °C, 60 min) at ligand concentrations between 5 × 10^-4 M and 5 × 10^-5 M. Thus, specific activities of 22−220 GBq per μmol of ligand could be achieved. Incubation of the corresponding Re-188 complexes in human serum at 37 °C revealed stabilities between 80 ± 4% and 45 ± 10% at 24 h, respectively, and 63 ± 3% and 34 ± 3% at 48 h postincubation in human serum depending on the chelating system. Decomposition product was mainly ¹⁸⁸ReO₄^-. The routine kit-preparation of the precursor [¹⁸⁸Re(H₂O)₃(CO)₃]⁺ in combination with tailor-made ligand systems enables the organometallic labeling of biomolecules with unprecedented high specific activities.