Synergistic effects of romidepsin and bortezomib on cell viability and histone acetylation. (A) Isobolograms of simultaneous exposure of U266 and RPMI8226 cells to bortezomib and romidepsin are shown. The concentrations that produced 80% growth inhibition are expressed as 1.0 on the ordinate and abscissa of isobolograms. The envelope of additivity, surrounded by solid and broken lines, is constructed from dose-response curves of bortezomib and romidepsin. When the data points of the drug combination fall within the area surrounded by the envelope of additivity, the combination is regarded as additive. When the data points fall to the left of the envelope, the drug combination is regarded as supra-additive (synergism). When the data points fall to the right of the envelope, the combination is regarded as antagonistic. The isobolograms shown are representative of at least 3 independent experiments. Each point represents the mean value of at least 3 independent experiments; the SEMs were less than 25% and were omitted. (B) U266 cells were cultured in the absence or presence of either romidepsin (Romidepsin), bortezomib (Bort), or both agents for 48 hours at the indicated doses. Whole-cell lysates were subjected to immunoblotting. The membranes were reprobed with anti-GAPDH antibody to serve as a loading control. The signal intensities of each band were quantified, normalized to those of the corresponding GAPDH, and shown as relative values setting untreated controls to 1.0. Data shown are representative of multiple independent experiments.