has mode of action has percent response value percent cytotoxicity 1 % cytotoxicity It is a measure of the number of dead cells in a culture well. We consider it as a subclass of 'percent inhibition', because cell death is induced by inhibition of vital cellular processes and percent cytotoxicity increases as the number of dead cells increases. It can be estimated in the compound treated wells in comparison to vehicle-treated control wells and is expressed as a percent. The compound treatment is preferably performed in a short window of time to avoid masking the effect due to cell proliferation. Compound cytotoxicity is an important parameter to measure when developing potential human therapeutics. Cytotoxicity is determined as a measure of radioisotope (3H thymidine or 51Cr) release, lactate dehydrogenase release from damaged cells, tetrazolium salt and alamar blue reduction, fluorescent dyes that selectively stain live or dead cells, and decrease in ATP content. ATP levels are detected using a luminescence based assay kit such as CellTiter-Glo (Promega). ATP values higher than controls indicate proliferation and cultures with ATP concentrations lower than controls indicate cytotoxicity. Percent cytotoxicity = 100-percent viability cytotoxicity percent inhibition