Effects of combination of CDK5 inhibitor roscovitine, SCH727965, and bortezomib on myeloma cells. (A) KMS11 and OPM1 cells were seeded in 96-well plates and incubated with roscovitine (Ros), SCH727965, and bortezomib alone and either of both drugs in combination with bortezomib at the indicated concentrations. After 72-hour incubation, cell viability was determined using MTT assay and the dose-response curves were generated. (B) Bliss independence score for each combination of 2 compounds on both cell lines was calculated to verify the synergistic, independent, and antagonistic cytotoxic effects at a broad range of concentrations. Negative value suggests synergism between 2 compounds. (C) The bone marrow from myeloma patients was collected, and CD138⁺ cells were isolated and cultured with vehicle, bortezomib (Bor), SCH727965 (Sch) alone, and 2 drugs (Sch + Bor) together for 72 hours. The cells were then harvested and stained with annexin V-fluorescein isothiocyanate and propidium iodide, followed by FACScan analysis.