Interaction between tannic acid and chemotherapeutic drugs. The effect of tannic acid on chemotherapeutic effect was assessed in Mz-ChA-1 cells. A median effect analysis was performed and a combination index was derived for each two drug combination. A combination index <1, 1 or >1 indicates a synergistic, additive, or antagonistic interaction. Panel A. Individual plots of combination index against fractional effect for tannic acid in combination with mitomycin (MMC), 5-fluorouracil (5FU) or gemcitabine (GEM). Panel B. Plot of combination index for combinations of tannic acid with the indicated chemotherapeutic agents at their respective effective dose for 50% toxicity (fractional effect, Fa=0.5). The combination of MMC or 5FU with tannic acid was highly synergistic, but the combination of tannic acid and gemcitabine was not. Similar changes are noted over a broad range of fractional effect levels.