The glutathione (GSH) levels were measured using an optimized monochlorobimane assay . The working solution of monochlorobimane (MCB, Merck, USA) was freshly prepared at the time of analysis by dilution in DPBS (pH 7; 1 mM, Merck, USA) and tempered at 37 °C for 30 min. After treating cells with the tested compounds, 20 μL of MCB solution was added to the cells in 96-well plates, and the measurement started immediately (the final concentration of MCB in a well was 40 µM). The fluorescence intensity (Ex/Em = 394/490 nm) was measured for 10 min using a SPARK microplate reader (Tecan, Austria). The fluorescence was expressed as the slope of a fluorescence change over time. The GSH levels were expressed as the percentage relative to GSH levels in untreated cells (control = 100%). In addition, we evaluated the occurrence of interference of tested compounds with the GSH assays. The maximal background fluorescence observed in blank samples containing tested compounds was always lower than 5% of the signal in untreated cells.