Pentathiepins: a novel class of glutathione peroxidase 1 inhibitors that induce oxidative stress, loss of mitochondrial membrane potential and apoptosis in human cancer cells.

A novel class of glutathione peroxidase 1 (GPx1) inhibitors, namely tri tetracyclic- pentathiepins, has been identified that is ca. 15 times more potent than the most active known GPx1 inhibitor, mercaptosuccinic acid. Enzyme kinetic studies with bovine erythrocyte GPx1 indicate that pentathiepins reversibly inhibit oxidation of the substrate glutathione (GSH). Moreover, no inhibition of superoxide dismutase, catalase, thioredoxin reductase or glutathione reductase was observed at concentrations that effectively inhibit GPx1. As well as potent enzyme inhibitory activity, the pentathiepins also show strong anticancer activity in various human cancer cell lines, with IC 50 values in a low micro molar range. A representative tetracyclic pentathiepin causes formation of reactive oxygen species in these cells, fragmentation of nuclear DNA and induces apoptosis via the intrinsic pathway. Moreover, this pentathiepin leads to a rapid and strong loss of mitochondrial membrane potential in treated cancer cells. On the other hand, evidence for the induction of ferroptosis as a form of cell death was negative. These new findings show that pentathiepins possess interesting biological activities beyond those originally ascribed to these compounds.