B-cell lymphoma cells acquire resistance to ABT-737 after long-term exposure. (A) OCI-Ly1 cells were treated with increasing doses of ABT-737 for 48 hours and stained with annexin-V–FITC for flow cytometry analysis. Before treatments, OCI-Ly1 R7 cells were cultured in media containing 320nM ABT-737 and OCI-Ly1 R10 cells in media with 1μM ABT-737. ABT-737 was withdrawn 24 hours before dose response treatment. Viability is shown as a percentage of DMSO-treated control cells. SD is of quadruplicates and is indicated by error bars. (B) OCI-Ly1 R7 and OCI-Ly1 R10 cells were withdrawn (−) from or continued treatment (+) with ABT-737 for 3 weeks. Cells were then washed and treated with increasing doses of ABT-737 for 48 hours, stained with annexin-V–FITC, and assayed by flow cytometry analysis. Viability is shown as a percentage of DMSO-treated control cells. SD is of quadruplicates and is indicated by error bars. (C) SU-DHL-4 and SU-DHL-4 R2 cells were treated with increasing doses of ABT-737 for 12 hours. After treatment, cells were stained with annexin-V–FITC and PI for flow cytometry analysis. Before treatment, SU-DHL-4 R2 cells were cultured in media containing 1μM ABT-737. ABT-737 was withdrawn 3 weeks before dose response treatment. Viability is shown as a percentage of DMSO-treated control cells. SE is of triplicates and is indicated by error bars.