Potential labeling strategies with NCA ^197(m)Hg

Objectives: The decay properties of both nuclear isomers, like convenient half life ^197mHg (T_1/2 = 23.8 h, Eγ 133.98 keV, 33.5%) and 197Hg (T1/2 = 64.14 h, Eγ 77.4 keV, 18.7%), low energy gamma radiations for imaging and numerous Auger- and conversion electrons useful for therapy combined with unique chemical and physical properties of mercury and its compounds represent the motivation for this project. The no carrier added (NCA) radionuclide ^197(m)Hg is accessible in sufficient quantity and quality for radiopharmaceutical research by irradiation of gold with protons using a cyclotron [1]. As the following logical step after examination of the production feasibility, the search for a suitable labeling tool was intensified.
Methods: Three different approaches to prepare a stable labeling unit at NCA level with ^197(m)Hg were studied. The reactivity of the mercury(II) ions towards sulfur containing ligands (a), solvomercuration of alkenes (b) and electrophilic aromatic substitution (c) were investigated in this context. Prepared characteristic representatives of all three groups are shown in Figure 1.
Results: For all studied reactions the desired ^197(m)Hg labeled compounds were detected. The mercury thiolate complexe (a) and the product of solvomercuration (b) show low stability in the presence of competing thiol ligands and therefore the suitability for radiopharmaceutical applications is not given. In contrast, diphenylmercury (c) as the simplest representative for symmetric diarylmercury compounds shows high stability against competing
ligands.
Conclusions: As a basis for the development of a convenient labeling method different kinds of mercury compounds were prepared and characterized at NCA level. After nuclide production this was the required succeeding part of the evaluation of the cyclotron-based NCA ^197(m)Hg regarding their suitability for diagnostics and therapy of tumors. First promising results of investigations concerning the development of mercury compounds stable in vivo will be reported.

References: [1] Walther, M., Preusche, S., Bartel, S., Wunderlich, G., Freudenberg, R., Steinbach, J., Pietzsch, H.-J., Theranostic mercury: ^197(m)Hg with high specific activity for imaging and therapy (2014) Appl. Radiat. Isot. submitted