Cell viability assay

Cell viability assays were performed using the MTT method according to the manufacturer's instructions (Sigma-Aldrich; Merck KGaA). Cells were seeded into 96-well plates (Costar; Corning, Inc.) at a density of 4×10³ cells/well. After a 24-h incubation period, triplicate wells were treated with various concentrations of TetC (0, 5, 10, 20 and 40 µmol/l) for 48 h. Next, 20 µl MTT solution (5 mg/ml in PBS) was added to each well and incubated at 37°C for 4 h. The formazan crystals were dissolved by adding 150 µl DMSO to each well, and the absorbance was measured with a microplate reader (Multiskan™ MK3; Thermo Fisher Scientific, Inc.) at a wavelength of 570 nm. IC₅₀ values were calculated from cytotoxicity curves using the Bliss independence method (29).