Reaction kinetics and targeting to cellular GST of the glutathione peroxidase mimetic PhSeZnCl and its D, L-polylactide microparticle formulation

Catalytic properties and cellular effects of the GPx mimetic compound PhSeZnCl or its D,L-lactide polymer microencapsulation form (M-PhSeZnCl), were investigated and compared with the prototypical Se-organic compounds Ebselen and with diselenide (PhSe)2. PhSeZnCl was confirmed to catalyze the ping-pong reaction of GPx with higher Vmax than Ebselen and (PhSe)2, but the catalytic efficiency calculated for the cosubstrates GSH and H2O2, and particularly the high reactivity against thiols (lowest KM for GSH in the series of test molecules), suggested poor biological applicability of PhSeZnCl as GPx mimetic. Cytotoxicity of PhSeZnCl was demonstrated in various cancer cell lines via increased ROS generation, depletion of intracellular thiols and induction of apoptosis. Experiments carried out in GSH-S-transferase P (GSTP)-overexpressing K562 human erythroleukemia cells and in GSTP1-1 knockout murine embryonic fibroblasts (MEFs) demonstrated that this cytosolic enzyme protein represents a preferential target of the redox disturbances produced by this Se-compound with a key role in preventing the stimulation of H2O2 generation and the perturbation of stress/survival kinase signaling. In order to test applications alternative to anticancer, microencapsulation was adopted as a strategy to control thiol reactivity and oxidative stress effects of PhSeZnCl. In fact, the uptake of this “depowered” GPx mimetic formulation which occurred through an endocytosis-like mechanism in MCF-7 and MEFs, resulted in a marked reduction of toxicity effects. In MCF-7 cells transfected with different allelic variants of GSTP, M-PhSeZnCl lowered the burst of cellular ROS induced by the exposure to extracellular H2O2 and the extent of this effect changed between the GSTP variants. Microencapsulation of thiol reactive Se-organic drugs such as PhSeZnCl, is a straightforward strategy to mitigate toxicity that disclosed antioxidant and cellular protection effects of this compound. The mechanistic linkage of these effects with the expression pattern and redox signaling of GSTP overcomes the GPx mimetic paradigm proposed so far for Se-organic drugs to open up the path to the more pragmatic concept of GSTP signaling modulators.