Preparation and Crystal Structure of a Rhenium Analogue of the Cationic Renal Agent, Tc-99m Diaminocyclohexane

We report here a chemical study on a Re analogue of one of the few cationic Tc-99m tracers previously investigated as an agent for effective renal plasma flow (ERPF) measurement. Cationic Tc-99m tracers have the potential for overcoming problems associated with common anionic Tc-99m tracers in patients who have developed a uremic state. The Tc-99m-DACH tracer, prepared from 1,2-diaminocyclohexane (1,2-DACH), is the only cationic renal agent tested in humans and has seven possible isomers. The complex isolated from the reaction of the racemic mixture, (+/-)-trans-1,2-DACH, and ReIO(2)(PPh(3))(2) after conversion to the BPh(4) (-) salt was found by X-ray crystallography to be the meso isomer, trans-[ReO(2) (trans-R,R-1,2-DACH)(trans-S,S-l,2-DACH)][BPh(4)].MeOH.2H(2)O (1). The structural parameters for 1 are normal. The complex is highly symmetrical, suggesting that the analogous meso Tc-99m-DACH agent is also symmetrical. Studies of other Tc-99m-DACH agents that were made from cis-1,2-DACH or individual trans-1,2-DACH enantiomers show that the biodistribution is not very dependent on the starting 1,2-DACH ligand stereochemistry; these agents must be less symmetrical than the meso Tc-99m-DACH agent analogue of 1. Thus, the overall charge and lipophilicity (similar for all Tc-99m-DACH isomers) exert a greater influence on biodistribution than the specific structural features of the different Tc-99m-DACH isomers.