Potentiation of gemcitabine and camptothecin cytotoxicity by V158411 occurs independently of fetal calf serum or oxygen concentration and under anchorage independent growth conditions. A. Potentiation of the cytotoxicity of gemcitabine or camptothecin by 400 nM V158411 was determined in HT29 cells growing in 10% FCS, 21% O2 (normoxia); 10% FCS, 0.5% O2 (hypoxia) or 0.5% FCS, 21% O2 for 72 hours. GI50 and cGI50 were calculated from the dose response curves using XLFit. The potentiation factor (Pf) was calculated as the average GI50/average cGI50 from 3 determinations. Protein biomarker changes were subsequently assessed in HT29 cells growing under the same conditions following treatment with 100 nM gemcitabine plus 0 to 1000 nM V158411 for 24 hours. B. HT29 cells growing either attached to plastic cell culture plates (anchorage dependently), anchorage independently in LMP agarose or as multicellular tumor spheroids were exposed to increasing concentrations of gemcitabine in the presence or absence of 400 nM V158411 for 72 (anchorage dependent) or 168 (anchorage independent/spheroid) hours. **, P < 0.01. Protein biomarker changes were assessed in HT29 cells growing anchorage dependently or as multi-cellular tumor spheroids following treatment with 100 nM gemcitabine plus 0 to 1000 nM V158411 for 24 hours. Protein expression was characterized by immunoblotting.