Figure 2. Synergism effect of combination drug (NG+VPA). a: Effect of combination drug on K562 cell viability. In all administered concentrations of combination drug the cell viability decreases and the most reduction in cell viability has been occurred in 150 μM at 72 hours. b: Synergism curve of combination drug. A combination index (CI) is estimated from dose-effect data of single and combined drug treatments. According to the combination index (CI), CI <1, CI = 1, and CI > 1 represent synergism, additive effect, and antagonism, respectively. Fa is fraction of cells that affected by treatment. Except for concentration with 150 µΜ (CI<1) in other used doses antagonism has been occurred (CI>1). Different letters indicate significant differences between groups (p<0.05).