Resorufin conversion: U-87 MG cells were additionally incubated with a resazurin incubation mixture to convert resazurin to resorufin by metabolism in the cells. For this purpose, a resazurin stock solution with a concentration of 0.011 mg/ml in PBS was diluted 1:10 in complete cultivation medium and the final resazurin incubation mixture was added to the cells overnight at 37 °C and 5 % CO2 atmosphere. During this period, resazurin was quantitively metabolized to resorufin, which was mainly released into the cultivation medium by the cells. Resorufin conversion was completed after the incubation mixture turned from blue (resazurin) to pink (resorufin). No further exchange of cultivation medium was performed, since the released resorufin was needed for FLIM experiments. Hypericinincubated-samples and controls were treated similarly according to this procedure. All preparation steps and incubation experiments were performed in the dark. Cells were kept at 37 °C and 5 % CO2 atmosphere until PI supplementation or FLIM measurements. PI supplementation: Aliquots of PI stock with a concentration of 500 µg/ml in PBS were freshly thawed for each cell treatment. To obtain a final PI concentration of 33 µg/ml in the cell culture dish, 100 µl PI stock was added to the cultivation medium after 100 µl cultivation medium was removed. This approach was carried out identically for hypericin-incubated cells and control cells. All preparation steps and incubation experiments were performed in the dark. Cells were kept at 37 °C and 5 % CO2 atmosphere prior to the optical measurements.