Isobolograms of in vitro drug combinations. Isobologram analysis of the combination of gemcitabine and troxacitabine in Panc-1, MIA PaCa-2, AsPc-1 and Capan-2 cells. The individual doses of gemcitabine and troxacitabine to achieve 90% (straight line) growth inhibition (Fa = 0.90), 75% (hyphenated line) growth inhibition (Fa = 0.75), and 50% (dotted line) growth inhibition (Fa = 0.50) were plotted on the x- and y-axes. Combination index (CI) values calculated using Calcusyn software is represented by points above (indicate antagonism between drugs) or below the lines (indicate synergy). (X symbol) ED50, (plus sign) ED75 and (open dotted circle ) ED90.