Synergy analyses of the interaction between oleuropein aglycone and Tzb against in Tzb-sensitive SKBR3 cells. The nature of the interaction between oleuropein aglycone and trastuzumab was evaluated by the isobologram technique, a dose-oriented geometric method of assessing drug interactions. Upon this approach, the concentration of oleuropein aglycone producing a desired (e.g., 30% inhibitory) effect was plotted on the horizontal axis, and the concentration of trastuzumab producing the same cytostatic degree was plotted on the vertical axis; a straight line joining these two points represents zero interaction between the two agents (CI30 = 1.0, additivity). An experimental isoeffect point is the concentration (expressed relative to oleuropein aglycone and trastuzumab IC30 concentrations) of the two agents which when combined reduced cell viability by 30%. Within the designed assay range, a set of isoeffect points was generated because there were multiple oleuropein aglycone and trastuzumab concentrations that achieved the same isoeffect. In our present study, the mean values of the survival fractions were used to generate the set of experimental isoeffect points and construct the isobole for the oleuropein aglycone-trastuzumab combination. Data points above the diagonal line of the additive effects in the isobole suggest antagonism and those below the diagonal suggest synergism. The value of the mean CI30 value is also labeled. Student t tests were applied to formally evaluate whether synergism or antagonism was evident as compared to a null-hypothesized CI30 of 1,0; * P < .01, ** P < .001).