Effect of SAHA and bortezomib on apoptosis in STI571-resistant K562 and patient-derived CD34⁺ cells. (A) STI571-resistant K562R cells were exposed to 7.5 nM bortezomib ± either 2.5 μM SAHA or 3.5 mM SB for 48 hours, after which the percentage of apoptotic cells was determined by examination of Wright Giemsa-stained cytospin preparations under light microscopy as described in “Materials and methods.” Values represent the means ± SD for 3 separate experiments performed in triplicate. (B) CD34⁺ cells were isolated from the peripheral blood of a patient with accelerated phase CML who experienced progressive disease while receiving STI571 and exposed to 2.0 μM SAHA ± 5.0 nM bortezomib for 48 hours. The percentage of apoptotic cells was then determined by examination of Wright Giemsa-stained cytospin preparations as shown in panel A. Values represent the means ± for 12 randomly selected fields encompassing more than 1000 cells.