Osmocenyl-tamoxifen derivatives target the thioredoxin system leading to a redox imbalance in Jurkat cells

The synthesis and the biological effects of two ferrocifen analogs in the osmium series, namely the monophenolic complex 1, the tamoxifen-like complex 2 and their oxidized quinone methide (QM) derivatives, 1-QM and 2-QM, are reported. Inhibition of purified thioredoxin reductase (TrxR) is observed with 1 and 2 only after their enzymatic oxidation by the hydrogen peroxide/horseradish peroxidase (H₂O₂/HRP) system with IC₅₀ of 2.4 and 1.2 μM respectively. However, this inhibition is larger than that obtained with the corresponding quinone methides (IC₅₀ = 5.4 μM for 1-QM and 3.6 μM for 2-QM). The UV-Vis spectra of 1 or 2 incubated in the presence of H₂O₂/HRP show that the species generated is not a quinone methide, but probably the corresponding cation. In Jurkat cells, 2 shows high toxicity (IC₅₀ = 7.4 μM), while 1 is less effective (IC₅₀ = 42 μM). Interestingly, a significant inhibition of TrxR activity is observed in cells incubated with 2 (about 70% inhibition with 15 μM) while the inhibition induced by 1 is much weaker (about 30% inhibition with 50 μM). This strong inhibition of TrxR by 2 leads to accumulation of thioredoxin and peroxiredoxin 3 in oxidized form and to a decrease of the mitochondrial membrane potential (MMP). These results show that cytotoxicity of the osmocifens depends on their oxidation within the cell and that inhibition of thioredoxin reductase by oxidized species is a key factor in rationalizing the cytotoxicity of these complexes on Jurkat cells.