Contribution of Mcl-1 to drug resistance probed by ABT-737. (A) CLL cells were treated immediately after thawing with the indicated concentrations of ABT-737 or the inactive enantiomer. After 24 hours, apoptosis was measured by Mitotracker staining. (B) CLL cells were cultured for 2 days in medium, with 3T3 control cells or with 3T40L cells before treatment with ABT-737 as above. Data in panels A and B represent averages plus or minus SD from 3 different CLL samples. (C) Sublethal doses of ABT-737 after CD40 stimulation as determined in panel B (0.1 μM and 1.0 μM) were combined with various other drugs as indicated to test synergy in reversal of drug resistance. Data are averages plus or minus SD from 5 (0.1 μM) or 4 (1 μM) patient samples, tested in 3 independent experiments