What lies beyond thioredoxin reductase? Cyclometallated gold compounds reveal Sec selectivity in glutathione peroxidases.

Cyclometallated gold(III) compounds were evaluated for their chemoselective capability to promote C-Se coupling reactions under biocompatible conditions. Competitive reactions with selenium and sulfur substrates highlighted the preference for selenium, and this selectivity was further confirmed in selenopeptide models mimicking the GPx active site. Given that thioredoxin reductase (TXNRD1) is a canonical target for gold compounds, we confirmed that our complexes also inhibit this enzyme, with the two six-membered metallacycles exhibiting a higher potency than auranofin. Expanding beyond TXNRD1, the compounds were further investigated as inhibitors of other selenoenzymes, specifically glutathione peroxidase isoenzymes (GPx1, GPx4). The metallacycles were potent inhibitors of GPx1, while in vitro GPx4 inhibition was overall less pronounced, with LC/MS studies identifying selenocysteine (Sec51) as the primary arylation site on GPx1. We demonstrated that this chemoselectivity could be translated to an intracellular setting. The selectivity towards Sec over Cys was further explored using A375 GPx4 WT and A375 GPx4 U46C mutant cell lines, where proliferation assays showed a greater effect in the GPx4 WT cells. By integrating structural and functional insights across selenoenzyme families, this study reveals glutathione peroxidases as pivotal molecular targets of cyclometallated gold(III) compounds and lays the groundwork for designing selective Sec-targeting metallodrugs, an approach with untapped potential in anticancer therapy.