Table 1.

 Combination effects of bortezomib and lapatinib against SK‐BR‐3 and BT‐474 cells

Combination potency	CI values for SK‐BR‐3 cells* (mean ± SD)	CI values for BT474 cells** (mean ± SD)
IC30	0.33 ± 0.19	0.86 ± 0.88
IC50	0.43 ± 0.24	0.88 ± 0.72
IC70	0.59 ± 0.38	0.91 ± 0.69
IC90	1.04 ± 0.96	0.96 ± 0.96

CI < 1, CI = 1 and CI > 1 indicate synergism, additive effect and antagonism, respectively. In each experiment, six concentrations were used for each drug and each combination drug. Approximated IC50 equipotency ratios were used as the drug combination ratios. IC30, IC50, IC70 and IC90 = concentration required to inhibit 30%, 50%, 70% or 90%, respectively. Representative results from three experiments. The CI values were derived from the median effect plots. The one‐sample Student’s t‐test was used to determine whether the CI values at multiple effect levels (IC30∼ IC90) were statistically significantly different from 1. * and ** indicate P < 0.05. CI, combination index; IC, inhibitory concentration.