4.4. Cell Viability Assay (MTT) and Determination of IC50 Value of FKB

MTT was used to determine the cell viability [47]. Briefly, 5 × 10⁴ cells/well were grown in a 24-well plate and were exposed to various concentrations of FKB (0–20 µg/mL) for 24 h. After treatments, the supernatant was removed and replaced with 400 μL of 0.5 mg/mL MTT and then incubated at 37 °C for 2 h. MTT formazan crystals were dissolved in equal volumes in 90% isopropanol and 0.5% SDS mixture and at 570 nm (A₅₇₀), absorbance was recorded with an ELISA microplate reader (µ-Quant, Winooski, VT, USA). The following formula was used to calculate the percentage (%) of cell viability: (A₅₇₀ of treated cells/A₅₇₀ of untreated cells) × 100. The cytotoxic effects of FKB were expressed as IC₅₀.