Figure 6. MCL-1-dependent Anti-leukemia Activity of MIM1 and Synergy with ABT-737.

(A) ABT-737 selectively impaired the viability of p185⁺Arf^−/−Mcl-1-deleted B-ALL cells rescued by BCL-X_L, but not MCL-1, as measured by CellTiter-Glo assay at 24 h. The selective cytotoxic effect of ABT-737 was accompanied by dose-responsive caspase 3/7 activation in the BCL-X_L-rescued leukemia cell line, as measured at 8 h post-treatment.

(B) MIM1 exhibited the opposite cellular activity profile of ABT-737, selectively impairing the viability of p185⁺Arf^−/−Mcl-1-deleted B-ALL cells rescued by MCL-1, but not BCL-X_L. Correspondingly, the selective cytotoxic effect of MIM1 was accompanied by dose-responsive caspase 3/7 activation only in the MCL-1-rescued leukemia cell line.

(C) Combination treatment with MIM1 and ABT-737 induced synergistic killing of parental p185⁺Arf^−/− B-ALL cells that express endogenous MCL-1 and BCL-X_L, as reflected by a leftward shift of the viability isotherm (left) and the CalcuSyn dose effect curve (right), with calculated CI values of <1 at ED₅₀, ED₇₅, and ED₉₀. CI, combination index; ED, effective dose.

(D) The addition of ABT-737 to MIM1 treatment of MCL-1-rescued p185⁺Arf^−/−Mcl-1-deleted B-ALL cells had little to no additional cytotoxic effect, consistent with the relative inactivity of ABT-737 in the context of MCL-1-dependence.

(E) Correspondingly, the addition of MIM1 to ABT-737 treatment of BCL-X_L-rescued p185⁺Arf^−/−Mcl-1-deleted B-ALL cells provided no additional cytotoxic effect, consistent with the relative inactivity of MIM1 in the context of BCL-X_L-dependence.

Data are mean ± SEM for experiments performed in at least duplicate, normalized to vehicle control, and repeated twice with independent cell cultures with similar results. See also Figure S3.