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 [Re-188(H2O)(3)(CO)(3)](+) in aqueous media are presented. Method A uses gaseous carbon monoxide and amine borane ((BH3NH3)-N-.) as the reducing agent. In method B CO(g) is replaced by K-2[H3BCO2] that releases carbon monoxide during hydrolysis. Both procedures afford the desired precursor in yields >85% after 10 min at 60 degreesC. HPLC and TLC analyses revealed 7 +/- 3% Of unreacted (ReO4-)-Re-188 and <5% of colloidal (ReO2)-Re-188. 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 [Re-188(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 [Re-188(H2O)(3)(CO)(3)](+) under mild reaction conditions (PBS buffer, 60 degreesC, 60 min) at ligand concentrations between 5 x 10(-4) M and 5 x 10(-5) M. Thus, specific activities of 22-220 GBq per, mumol of ligand could be achieved. Incubation of the corresponding Re-188 complexes in human serum at 37 degreesC 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 (ReO4-)-Re-188. The routine kit-preparation of the precursor [Re-188(H2O)(3)(CO)(3)](+) in combination with tailor-made ligand systems enables the organometallic labeling of biomolecules with unprecedented high specific activities.