Figure 6: Combination index of the 60 trial combinations. The combination indexes (CIs) < 1, = 1, and > 1 indicate synergism, additive effect, and antagonism, respectively. The calculation of CI was accord with the theory of Chou. Sixty combinations out of 5 five iterations are shown here. The 16th and 18th combinations (black bars) showed the strongest synergism, that is, the smallest CI value.