MCL-1i has single-drug activity and synergizes with BCL-2i and BCL-XLi in HMCLs. (A) IC₅₀ (µM) of BH3 mimetics in HMCLs, single and combined (ratios of combined BH3 mimetics are 1:1 or 1:1:1) after treatment for 6 hours and determined by Celltiter Glo assay. Black lines indicate median values. (B) For all 31 HMCLs, combination indices (CIs) were calculated for the indicated inhibitor combinations. CI <1 indicates synergy (dotted line). Cell lines insensitive to MCL-1i (ie, IC₅₀ > 1 μM) are shown in blue. Solid lines indicate median values. (C) Synergy between MCL-1i and BCL-XLi was studied by combining concentration series of the 2 inhibitors in a matrix after treatment for 24 hours using DiOC6/TO-PRO-3 flow cytometric staining as readout. The isobolograms show concentrations at which the combined inhibitors induced 50% specific apoptosis (IC₅₀; blue points). Red diamonds indicate the IC₅₀s of the single drugs. Black lines illustrate where CI = 1. For MM1.S, CI < 1, indicating synergy of MCL-1i and BCL-XLi. In OPM-2, the IC₅₀ of BCL-XLi was >1000, but it was set to 1000 to be able to calculate CI values. The resulting CI (<1) therefore underestimates the synergistic effect in OPM-2.