Effect of combination treatment on imatinib-resistant cells. K562 and K562R cells were treated with imatinib at the indicated doses for 72 hours with or without MPA. Apoptosis was measured by annexin-V and PI staining. Imatinib treatment alone is indicated by the closed symbols. The controls in the absence of drug have 10.2% ± 2.2% and 10.9% ± 2.7% annexin V/PI-positive cells for K562 and K562R, respectively. Imatinib plus MPA (2 μM) is indicated by the open symbols. MPA alone induced 22.5% ± 0.6% and 26.3% ± 2.3% apoptosis for K562 and K562R, respectively. Data represent the mean ± SD of 3 independent experiments.