Use of XTT for quantitating clonogenic growth in soft agar

The purpose of this study was to investigate the use of the tetrazolium salt. XTT, in a microassay for quantitation of anchorage-independent growth of cells in soft agar, using two human ovarian tumour cell lines (OAW 42 and OAW 28). The response of OAW 42 to Cis-platinum, EGF, TGF β and insulin, and of OAW 28 to EGF, TGF β and insulin were determined both by visual colony counting and use of the XTT assay. Drug inhibition and growth factor inhibition and stimulation were clearly demonstrated, with increases or decreases in visually counted colony numbers being paralleled by increases or decreases in optical density values obtained by using XTT. The XTT colorimetric assay was simple to perform and reproducible with low variability, and was found to be an acceptable alternative to visual colony counting for the quantitation of cell line response to drugs and growth factors under anchorage-independent growth conditions.