Preparation and Biological Characterization of Isomeric ¹⁸⁸Re(V) Oxocomplexes with Tetradentate S₄ Ligands Derived from DMSA for Labeling of Biomolecules

A new type of tetradentate S₄ ligand has been synthesized by bridging two molecules of meso 2,3-dimercapto succinic acid for stable binding and easy conjugation of rhenium-188 to biologically interesting structures. The stereoisomeric tetrathiolato S₄ ligands form very robust anionic five-coordinated oxorhenium(V) and oxotechnetium(V) complexes. Two routes for the preparation of the ¹⁸⁸Re(V) oxocomplexes with (iBu)₂N(O)C-C(SH)-C(SH)-C(O)NH-(CH₂)₃–NH-(CH₂)₃–NHC(O)-C(SH)-C(SH)-C(O)N(iBu)₂ (ligand 1) and its hydrophilic crown ether derivative (ligand 2) were tested and optimized. Several isomers were separated by HPLC from the preparation solutions and characterized in vitro and in vivo. The identity of the species obtained was determined by comparison with the HPLC profiles of reference ^185/187Re analogues which were characterized by ESI-MS. All of them were absolutely stable in rat and human plasma solutions. Challenge experiments with cysteine corroborated the high inertness of the isomers towards ligand exchange reactions. Various in vivo samples, taken off at different times from blood, intestine and urine of rats, confirmed the high in vivo stability of the ¹⁸⁸Re-S₄ complexes. Biodistribution studies using male Wistar rats were performed and resulted in a high uptake and fast clearance from the liver of the more lipophilic cis and trans isomers of complex I (log P_o/w between 1.5 and 1.7), whereas the isomers of the hydrophilic complex II (log P_o/w about -1.75) were rapidly excreted via the renal and the hepatobiliary pathway. The low level of radioactivity in the stomach confirms good in vivo stability. Thus, these new ¹⁸⁸Re-S₄ complexes fulfill the requirements for a stable and high specific activity labeling of biomolecules with rhenium-188.