PD166326 is a more potent inhibitor of Bcr/Abl- and stem cell factor–dependent proliferation than imatinib mesylate. (A) The P210-expressing MO7e cell line R10(-) growing in the absence of exogenous cytokine, and (B) parental MO7e cells growing in the presence of stem cell factor (MO7e/KL) were treated with the indicated concentrations of PD166326 (•) and imatinib mesylate (▴) for 48 hours and then analyzed by [³H]thymidine incorporation as described in “Materials and methods.” Results are expressed as the counts per minute (CPM) relative to cells treated with diluent alone (DMSO) versus the concentration (log₁₀ scale) of PD166326 or imatinib mesylate. The calculated IC₅₀ for each drug is shown. The results depicted are representative of multiple independent experiments.