4-HC and combinations of 4-HC and F-ara-A induce high levels of cytotoxicity, even in the presence of MSCs. CLL cells were cultured with or without MSCs and with 10 μM F-ara-A and/or 60 μM 4-HC. (A) Bar diagram depicts the mean (± SEM) relative viabilities of CLL cells treated with F-ara-A, 4-HC, or a combination of F-ara-A and 4-HC after 24 hours. Results are presented as mean relative viability compared with untreated controls (100%) and are the mean (± SEM) viabilities of CLL samples from 8 different patients. * and ** indicate significant increases in cytotoxicity of the combination of 4-HC plus F-ara-A compared with 4-HC alone (P < .05 or P < .01). (B) Contour plots show the viability of CLL cells after 24 hours of incubation for 1 representative patient. The percentage of viable cells (DiOC₆^bright PI^exclusion) is shown above each of the gates.