Synthesis and biological evaluation of a Tc-99m-labelled sulfonamide conjugate for in vivo visualization of carbonic anhydrase IX expression in tumor hypoxia

Introduction: Carbonic anhydrase (CA) IX is a transmembrane protein overexpressed in many frequently occurring tumors associated with tumor hypoxia. Sulfonamides and their bioisosteres are known to inhibit CA IX activity. In this study, 4-(2-aminoethyl)benzenesulfonamide was conjugated to a tridentate ligand, N-2-picolyl-N-acetic acid and labeled with a Tc-99m(I)-tricarbonyl moiety resulting in [Tc-99m(CO)(3) (L)] (L=N-(pyridin-2-yl-methyl)-N[2-(4-sulfamoylphenyl)-ethyl]aminoethyl acetate) complex, [Tc-99m]-5. Similarly the corresponding rhenium congener (Re-4) was synthesized. The in vitro CA IX affinity and inhibitory activity of Re-4 were determined and [Tc-99m]-5 was evaluated as a tracer for in vivo visualisation of CA IX expression. Methods: Evaluation of the in vitro affinity (inhibition constant, K-i) of Re-4 for CA isozymes I, II, IX and XII was carried out by assaying the CA catalyzed CO2, hydration activity and efficacy studies were performed in HT 29 cell lines expressing CA IX under normoxia or hypoxia. Biodistribution studies of [Tc-99m]-5 were performed in xenograft mice bearing CA IX expressing tumors. Results: The in vitro affinity of Re-4 for CA IX was 58 nM and CA IX induced acidification of extracellular medium was efficiently reduced (P