Isobolograms and combination indexes at 50% effect level of simultaneous and sequential exposure of MDA-MB-231 cells to xanthorrhizol and curcumin. The dashed line (CI = 1) indicates the alignment of theoretical values of an additive interaction between two substances. Values above the dashed line indicate an antagonistic interaction, and values below indicate synergism interaction. Simultaneous treatment involved concurrent 48 h exposure of MDA-MB-231 cells to xanthorrhizol and curcumin (A and B) whereas sequential treatment was 24 h exposure to xanthorrhizol followed by curcumin for subsequent 24 h (C and D) or vice versa (E and F). Simultaneous treatment inhibits the growth of MDA-MB-231 cells more synergistically as compared to the sequential treatment. Results are representative of three independent experiments conducted.