*3.4. Determination of Cytotoxicity by MTT Assay*

Cell viability was determined by MTT assay as previously described [30]. RAW 264.7 cells were seeded in 96-well culture plates at a 1 × <sup>10</sup><sup>4</sup> cells/well density in DMEM supplemented with 1% FBS and 1% P/S and incubated in a humidified 37 ◦C, 5% CO2 incubator for 24 h. After incubation, the cultured cells were treated with various concentrations of the methanol extract of *M. sirikitiae* leaves and isolated lignans (0.05–250 μg/mL, diluted with DMEM), or dimethyl sulfoxide (DMSO; vehicle) and then incubated for 36 h. The culture medium was removed and added with MTT solution (1 mg/mL in DMEM). After incubated for 4 h, the MTT solution was removed before adding DMSO to dissolve the formed purple formazan crystals. Each experiment was performed in triplicate. The absorbance was detected using an Infinite M200 microplate reader (Tecan) at a wavelength of 570 nm. The results were shown in the graph of percentage of cell viability (% cell viability = [Atreated cells/Auntreated cells] × 100) against concentrations.
