Four-drug combination cytotoxicity of ABT-737 in combination with VXL in ALL cell lines. Dose-response curves of ALL cell lines to ABT-737 (●), VXL (△), and the combinations of all 4 drugs (▲). The concentrations for VCR, DEX, and L-ASP applied were 0.5 to 5 ng/mL, 50 to 500 nM, and 0.1 to 1 IU/mL, respectively, where 1 U on the VXL axis corresponds to 1 ng/mL VCR, 100 nM DEX, and 0.2 IU/mL L-ASP. The concentrations for ABT-737 applied were 1 to 10 μM for CEM, COG-LL-317, MOLT-3, MOLT-4, and NALM-6; concentrations of 0.1 to 1 μM were used for RS4-11* and COG-LL-319*. Each condition had 12 replicates, and error bars represent SD. *Cell lines especially sensitive to ABT-737 as a single agent (Figure 1) that required lower dosing for combination drug testing.