Comeback of Mercury as 197(m)Hg for Theranostics

The no-carrier-added (NCA) radionuclide 197(m)Hg is accessible in adequate quantity and quality for radiopharmaceutical research through the proton irradiation of gold using a cyclotron.1 The interest in this mercury isotope was prompted primarily by the decay characteristics of both nuclear isomers, like convenient half-life (197mHg = 23.8 h, 197Hg = 64.14 h), low energy gamma radiations useful for diagnosis (197mHg = 133.98 keV, 33.5% , 197Hg = 77.4 keV, 18.7%) and numerous Auger- and conversion electrons with high potential for cancer therapy. Additionally, the unique chemical properties of mercury allow for the development of promising radiolabeling tools and new radiopharmaceuticals. In addition to the typical metal properties to form coordination compounds with sulfur, nitrogen and oxygen containing ligands, mercury has the special ability to build water stable carbon-metal bonds. The reactivity of the mercury(II) ions towards sulfur containing ligands, solvomercuration of alkenes and electrophilic aromatic substitutions were investigated to prepare a stable labeling unit at NCA level with 197(m)Hg. While both the mercury thiolate complexes and the products of solvomercuration exhibited low stability in the presence of competing thiol ligands and are therefore unsuitable for radiopharmaceutical applications, symmetric diarylmercury compounds showed high stability against competing ligands. The development and application of a prelabeling tool based on a bis-benzoyl-mercury derivative as succinimidyl ester will be reported.