Ligand development for the Radiometal Hg-197(m)


Ligand development for the Radiometal Hg-197(m)

Gilpin, M.; Walther, M.; Pietzsch, H.-J.; Steinbach, J.

Reactor-produced Hg-197 had previous medical use for imaging[1] but was discontinued due to low stability and low specific-activity. Cyclotron-produced Hg-197(m) can overcome the toxicity problem, due to much higher molar activity[2], allowing access to the radiometal’s useful decay modes (γ for SPECT-imaging. Conversion & auger electrons for therapy) at sub-toxic Hg-concentrations.
Development in Hg ligands for medicinal applications necessitates stability in vivo but the poor long-term stability of Mercury compounds in solution is an ongoing issue[3]. Hg-organometallics show good water-stability and bypass the issue of Hg-S bonds suffering from competition by common biomolecules, e.g. cysteine. Therefore, our focus is on Hg-C chemistry, specifically the strongest bond kind: the mercury-phenyl bond[4].
As prior research has shown that the synthetically simpler route of monodentate ligands (κ1-L2Hg) suffers from significant cleavage[5], this research is centred on the syntheses of bidentate chelators benefitting from entropic stability. Purification of the Hg-197(m) leaves it in an acidic aqueous medium as the chloride salt, thus transmetallation, via stannyl or boronic acid derivatives, was chosen as a viable option for mercury attachment. Chelator designs began with a dibenzylisophthalamide template but low selectivity for the 1:1-compound encouraged a better fitting structure. Recent radio-labelling experiments show promise for specific binding with a design based on the bispidine backbone (attractive for being known in co-ordination chemistry for a variety of metals[6] and possessing bridge linking-functionalisation).
Analyses are performed through radio-TLC and HPLC, whilst stable mercury compound analysis includes Hg-199 NMR.

References :

(1) Sodee. J. Nucl. Med. 1968; 9: 645.
(2) Walther et al. Appl. Radiat. Isot. 2015; 97: 177–181.
(3) Henke et al. Wat. Res. 2000; 34: 3005-3013.
(4) Dean. Lange’s Handbook of Chemistry, 15th ed.; McGraw-Hill Inc, 1998; 606.
(5) Wilhelm. et al. Z. Naturforsch. 2000; 55b: 35–38.
(6) Comba et al. Inorg. Chem. 2009; 48: 6604–6614.

Keywords: Mercury 197; Chelator; Ligand; Radiometal; Bispidine; Cancer; Theragnostics

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