An alternative approach to the preparation of (188)Re radiopharmaceuticals from generator-produced [(188)ReO(4)](-): efficient synthesis of (188)Re(V)-meso-2,3-dimercaptosuccinic acid

A new efficient approach for the preparation of (188)Re radiopharmaceuticals starting from [(188)ReO(4)](-), produced at a carrier-free level through the (188)W/(188)Re generator system, is described. The reaction procedure was based on the combined action of different reagents and has been applied in detail to the preparation of the therapeutic agent (188)Re(V)-DMSA (H(2)DMSA [meso-2,3-dimercaptosuccinic acid]). The most efficient combination required the use of SnCl(2), oxalate ions, and gamma-cyclodextrin. These were reacted with [(188)ReO(4)](-) and H(2)DMSA to afford the final radiopharmaceutical in high radiochemical purity, at room temperature, and in weakly acidic solution. The role played by the various reagents in the reaction was investigated. It was found that SnCl(2) behaved as the actual reducing agent, whereas oxalate and gamma-cyclodextrin greatly enhanced the ease of reduction of [(188)ReO(4)](-) through the action of two hypothetical mechanisms. In the first step of the reaction, oxalate ions gave rise to the formation of Re(VII) complexes with the concomitant expansion of the coordination sphere of the metal. This process strongly favored the electron transfer between Sn(2+) and Re(+7) centers, giving rise to intermediate reduced rhenium complexes. These species were further stabilized by the formation of transient host-guest aggregates with gamma-cyclodextrin and finally converted into (188)Re(V)-DMSA through simple replacement of the coordinated ligands by H(2)DMSA.