4.2. MTT Assay

The human osteosarcoma HOS and MG-63 cells were seeded in 24-well plates for 24 h. The cells were exposed to different concentrations of carnosol for 24 h. At the end of the assay time, 20 µL of MTT solution (5 mg/mL) (Invitrogen, Carlsbad, CA, USA) was added to each well, and then incubated for 2 h at 37 °C. After removing the cultured medium, 200 µL of dimethyl sulfoxide (DMSO) was added to each well. Absorbance at 590 nm of the dissolved formazan product was read using a spectrophotometric plate reader (Thermo Multiskan SPECTRUM, Thermo Fisher Scientific, Waltham, MA, USA). The half maximal inhibitory concentration (IC₅₀) for HOS and MG-63 cells were calculated by the “Forecast” function in Microsoft Excel.