Cells were incubated at 37 C for 3 h, the untransformed MTT was removed and 50 ll of acid–isopropanol (0.04 N HCl in isopropanol) was added to each well. After a few minutes at room temperature for dissolving the crystals, the plates were read on a microplate reader using a test wavelength of 570 nm and a reference wavelength of 620 nm.
The concentration of oxyresveratrol reducing cell viability by 50% (CC50) was determined. The cream base was diluted in the same manner as oxyresveratrol in cream vehicle and was subjected to the MTT reduction assay as described above.