a Fa denotes fraction affected (e.g., Fa of 0.5 is equivalent to a 50% reduction in cell growth). CI >1 denotes antagonism, CI = 1 denotes additivity, and CI < 1 denotes synergism.
Cells were exposed to graded concentrations of troxacitabine or gemcitabine either alone or in combination at a ratio of 1:100 gemcitabine vs. troxacitabine for 72 h as shown in Fig. 1. Values for GI50 (± SEM) (the drug concentration that inhibited cell growth by 50% as determined from concentration-effect relationships using GraphPad Prism version 3.0 software) from three experiments, each conducted with three replicates, were calculated from non-linear regression analysis of data plotted as percentages of control values against the logarithm of drug concentrations. CI (combination index) values at a Fa value of 0.5 were generated from the data of Fig. 2 with the CalcuSyn, version 2.0 software.