Evidence for Targeting Thioredoxin Reductases with Ferrocenyl Quinone Methides. A Possible Molecular Basis for the Antiproliferative Effect of Hydroxyferrocifens on Cancer Cells

Many anticancer compounds are strong inhibitors of thioredoxin reductases (TrxRs), selenoenzymes involved in cellular redox regulation. This study examined the effect of two hydroxyferrocifens (1, FcOH; 2, FcOHTAM) and of their corresponding quinone methides (QMs), 1-QM, and 2-QM, on these enzymes. In vitro, both QMs were more potent TrxR inhibitors (IC50 ≈ 2.5 μM) than the hydroxyferrocifens (IC50 ≈ 15 μM). This inhibition was due to a Michael addition of the penultimate selenocysteine residue of TrxRs to the QMs. In Jurkat cancer cells, both 2 and 2-QM inhibited TrxRs in the same proportion, leading to accumulation of oxidized forms of thioredoxin, while 1 and 1-QM were scarcely effective. This difference of behavior was ascribed to the competitive conversion of 1-QM to an inactive indene in protic medium. This set of experiments confirms for the first time the role played by ferrocenyl quinone methides on several biological targets and gives a molecular basis for these effects. It also highlights differences in the mechanisms of action of 1 and 2 in cancer cells.