After 24 h incubation, the medium containing serially diluted substances was aspirated from each well and replaced by 100 µL of fresh medium containing MTT (0.5 mg/mL). Plates were subsequently incubated at 37 °C in a CO2 incubator for 1 h. Thereafter, the medium was aspirated, and purple crystals of MTT formazan were dissolved in 100 μL DMSO under shaking. The optical density of each well was measured using Spark® multimode microplate reader (Tecan Group Ltd., Männedorf, Switzerland) at 570 nm.