4.4. Cell Viability Assays—MTT, Neutral Red, and LDH

LS174T and AZ-AHR cells were seeded into 96-well plates at a density of 4 × 10⁴ in 0.2 mL of the culture medium. After 24 h, the cells were treated for 24 h with the studied compounds in concentrations ranging from 1 nM to 200 µM (indole from 10 µM to 10 mM). The highest concentrations of the tested compounds were adjusted based on their solubility. The vehicle DMSO (0.1% v/v) was used as a negative control and Triton X-100 as a positive control. The MTT (3-(4,5,-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) colorimetric assay was performed as previously described [41]. The neutral red assay and LDH (Abcam, Cambridge, UK) assays were performed according to the manufacturer’s protocol. Assays were measured spectrophotometrically using a Tecan Infinite M200 plate reader (Tecan, Männedorf, Switzerland).