Correlation of growth inhibition (reduction of viable cell number) and apoptosis The increase of apoptotic cells is paralleled by reduction of viable cell number at the corresponding concentrations. A-B: Viability of cell lines S462 and S520 were measured by their ability to reduce XTT metabolically to a purple formazan product after 48 h of treatment with Exisulind and Sulindac Sulfide at different concentrations. The percentage of viable cells was determined setting the absorbance of cells treated with the vehicle as 100%. C-D: Percentages of apoptotic cells were determined by photometric quantification of DNA- and histone-fragmentation after 48 h of drug treatment. The absorbance mean values of treated samples (triplicates) were normalized to the corresponding control of untreated cells and the maximal absorbance from the test compound included in the assay was used as positive control (100% apoptosis).