LBH589 and bortezomib trigger synergistic cytotoxicity in RPMI8226 MM cell lines. (A) RPMI8226 MM cells were incubated with LBH589 (0-20 nM), bortezomib (0-20 nM), or both for 48 hours. Synergistic cytotoxicity was demonstrated by MTS assay. (B) Cytotoxicity of LBH589 and bortezomib was partly inhibited by pan-caspase inhibitor ZVAD-FMK. Data represent mean ± SD of quadruplicate cultures. The viability, as determined by MTS assay and shown in panel A, was converted into effect (1-viability), and shown as an isobologram graph (C), as well as converted to combination index (CI) (D) according to the Chou-Talalay equation (“Materials and methods”). Synergy is present when the CI is less than 1.0. The combination is additive when CI equals 1.0, and antagonistic when it is more than 1.0. Synergy was observed with a CI of 0.404, 0.054, and 0.067 at 12, 16, and 20 nM of LBH589 and bortezomib. (E) Healthy donor peripheral-blood mononuclear cells (PBMCs) were incubated with LBH589 (0-100 nM), bortezomib (0-100 nM), or both for 48 hours. No synergy was observed against PBMCs. Data represent mean ± SD of quadruplicate cultures.