Selective Arylation of Se vs. S by Gold Compounds: Consequences for GPx Inhibition and A375 Sensitivity

In this study, cyclometallated gold(III) compounds were evaluated for their capability to promote C-Se coupling reactions under biocompatible conditions, and their chemoselectivity towards selenium-containing substrates compared to sulfur-containing analogs. The compounds exhibited varying degrees of selectivity towards selenium-containing compounds, with [Au(bnopy)Cl₂] showing rapid C-Se bond formation. Competitive reactions with selenium and sulfur substrates highlighted the preference for selenium, especially for [Au(Mephpy)Cl₂], even in the presence of excess sulfur substrates. This selectivity was further confirmed in selenopeptide models mimicking the GPx active site, where arylation of selenocysteine was observed. The compounds were further investigated as inhibitors of selenoprotein glutathione peroxidase (GPx) isoenzymes, specifically GPx1 and GPx4. The six-membered metallacycles [Au(bnpy)Cl₂] and [Au(bnopy)Cl₂] exhibited potent inhibition of the GPx1 isoenzyme, particularly when co-incubated with reducing agents, while in vitro GPx4 inhibition was overall less pronounced. Notably, [Au(Mephpy)Cl₂] showed inhibition of GPx1 with no cross-inhibition of the non-selenoprotein glutathione reductase (GR). LC/MS studies identified selenocysteine (Sec51) as the primary arylation site on GPx1 by [Au(bnpy)Cl₂], highlighting its chemoselectivity. Finally, we demonstrated that the selectivity towards Sec could also be translated to an intracellular setting, using a A375 GPx4 WT cell line and an A375 GPx4 U46C mutant. Viability assays revealed a modest decrease in the IC50 for GPx4 WT cells compared to U46C mutants for both [Au(bnpy)Cl₂] and [Au(Mephpy)Cl₂]. More clearly, a proliferation assay over six days showed greater effect in GPx4 WT cells compared to the U46C mutant when treated with [Au(Mephpy)Cl₂], an effect further enhanced by co-treatment with iFSP1. This indicates a GPx4 Sec-dependent response to treatment in A375 cells and warrants further mechanistic studies.