The MTT assay is a colorimetric assay for assessing cell survival and growth. The detection principle is that the succinate dehydrogenase in the mitochondria of active cells can reduce the exogenous MTT into the water-insoluble purple formazan crystals (Formazan) and deposit it in the cells, while dead cells have no such function. Generally, the formazan crystals are dissolved in dimethyl sulfoxide (DMSO) and its light absorption value can be measured at wavelengths of 540 or 720 nm by an ELISA (enzyme-linked immunosorbent assay) reader. The value can indirectly reflect the number of living cells. Within a certain range of cell numbers, the amount of MTT crystal formed is proportional to the number of cells. Hence, this method has been widely used in detection of biologically active factors, large-scale anti-tumor drug screening, cytotoxicity and tumor radiosensitivity testing.
The Cell Counting Kit-8 (CCK-8) kit enables assays using WST-8 (chemical name: 2-(2-Methoxy-4-nitrophenyl)-3-(4-nitrophenyl)-5-(2,4-disulfophenyl)-2H-tetrazolium sodium salt). CCK-8 is widely used in rapid and highly sensitive detection kits for cell proliferation and cytotoxicity. The detection principle is based on how the dehydrogenase in the mitochondria can reduce the WST-8 into the water-insoluble yellow formazan products in the presence of electron-coupling reagents. Generally, the color density is proportional to cell proliferation, and inversely proportional to cell cytotoxicity. The OD value measured by microplate spectrophotometer at 450 nm wavelength can indirectly reflect the number of living cells. Hence, this method has been widely used in the activity detection of high-throughput drug screening, cell proliferation and cytotoxicity, tumor radiosensitivity and for some biologically active factors.