Preparation, characterization, and biological evaluation of technetium(V) and rhenium(V) complexes of novel heterocyclic tetradentate N3S ligands

Various tetradentate N3S ligands which contain pyridyl, morpholino, or imidazolyl moieties were prepared and labelled with technetium and rhenium. Metal complexation of the ligands occurred efficiently over the pH range from 2 to 11. Ligands possessing the S-THP (tetrahydropyranyl)-protected mercapto group labelled efficiently even under alkaline conditions, and among the three types of heterocyclic metal complexes, a marked difference in stability was observed; rhenium complexes decomposed to ReO4- whereas technetium complexes decomposed to TcO2/TcO4-. In general, imidazolyl complexes of both technetium and rhenium were very stable in saline; less than 10% decomposition after 24 h. The technetium histidyl complex and technetium pyridyl complex were quite stable even under cysteine challenge; less than 10% decomposition after 24 h. The rhenium and technetium morpholino complexes were very unstable; greater than 10% decomposition after only 1 h in saline and greater than 25% decomposition in 1 h under cysteine challenge. Profound pharmacokinetic differences among these metal complexes were also observed in rat biodistribution studies. The neutral pyridyl complexes exhibited high blood and liver uptake and slow clearance from these tissues. The replacement of a hydroxyl group by a carboxyl group, which resulted in an anionic complex at physiological pH, resulted in a dramatic decrease in blood and liver uptake. The neutral imidazolyl complex exhibited marked reduction in blood uptake and much faster clearance from blood and liver compared to the neutral pyridyl complex. Finally, the anionic histidyl complex, which contains both the imidazolyl and carboxyl groups, had the most favorable pharmacokinetic properties in that it exhibited very low blood, liver, and kidney uptakes and a rapid clearance from the body via the renal system. The combination of the high stability and favorable pharmacokinetic properties of the imidazolyl complexes should render them useful for targeted delivery of the medically important isotopes.