Combination of low doses of FK866 and bortezomib induces synergistic anti-MM activity. (A) RPMI-8226/S, U266, MM1S, MM1R ANBL6/WT, and ANBL6-BR cells were treated with or without increasing doses of FK866 (1–3 nM) for 48 hours, and then vehicle or bortezomib (over a range of concentrations depending on cell line) were added for a further 48 hours. Viability was assessed using PI staining and FACS analysis. Data presented are means of triplicate ± SD (n = 3; P < .05 for all cell lines). CI values refer to the highest drug concentrations used. (B) Purified patient bortezomib-sensitive and -resistant MM (CD138⁺) cells were pretreated with FK866 for 48 hours; bortezomib was then added for an additional 48 hours, followed by cell death analysis using PI staining and FACS analysis. Data are mean ± standard deviation (SD) of triplicate samples (P < .05 for all patient samples). A CI less than 1 indicates synergism. (C) PBMCs from healthy donors were treated as in panel B with indicated concentration of FK866, bortezomib, or their combination, and then analyzed for viability as described above.