Biologic effects of depsipeptide in CLL patient cells. (A) CLL patient cells were isolated prior to therapy with depsipeptide, as well as immediately before treatment and 4 and 24 hours after the start of infusion. Lysates were prepared and analyzed for specific HDAC activity. Activity is shown as a percentage relative to baseline (pretreatment). Significant HDAC inhibition is noted at completion of infusion (4 hours) and diminishes by 24 hours. (B) CLL patient cells isolated before therapy and at 4 hours following the start of the infusion were analyzed by fluorescent immunocytochemistry using an antibody to acetylated H3. Acetylation of H3 is minimal at baseline but is evident in the majority of the cells at the 4-hour time point.