Cytotoxicity of vorinostat, PEITC, and their combination in fresh myeloid leukemia cells. (A) Fresh primary leukemia cells isolated from 7 patients with AML were each treated with various concentrations of vorinostat (1μM, 1.5μM, and 2μM) and 2.5μM PEITC alone or their combination for 48 hours. Cell viability was determined by annexin V/PI assay. The expected viable cell fraction (percentage) was calculated by multiplying percentage viable cells in the vorinostat-treated sample with the percentage of viable cells in the PEITC-treated sample. The combination effect is considered additive when the observed viable cell fraction is equal to the expected value. When observed value is less than the expected value, the combination effect is considered more than additive. (B) Total glutathione levels in primary AML cells treated with 2μM vorinostat or/and 2.5μM for 16 hours. Values are normalized to the levels in untreated cells. Bars represent mean ± SD (n = 4 AML patient samples).