Synergy between obatoclax and chemotherapy against primary pediatric MLL-AF4 ALL. Cells were treated with obatoclax alone, indicated chemotherapy drug alone, or increasing concentrations of chemotherapy drug combined with obatoclax at fixed concentrations. Data from MTT assays performed 72 hours after treatment of each combination are shown in surviving fraction plots (first and third rows) and response surface models (second and fourth rows). Each experimental point represents average of 2 independent experiments (3 replicates/condition per experiment). An inhibitory sigmoid E_max model was used to determine the EC₅₀ and Hill coefficient for single-agent dose responses (obatoclax: EC₅₀ = 67.2 ± 13 nM, Hill coefficient = 1.91 ± 0.60; vincristine: EC₅₀ = 150 ± 35 nM, Hill coefficient = 0.911 ± 0.160; L-aspariginase: EC₅₀ = 958 ± 123 U/L, Hill coefficient = 0.785 ± 0.067; etoposide: EC₅₀ = 1125 ± 210 nM, Hill coefficient = 0.76 ± 0.12; doxorubicin: EC₅₀ = 47.4 ± 5.99 nM, Hill coefficient = 1.13 ± 0.13; cytosine arabinodise: EC₅₀ = 693 ± 119 nM, Hill coefficient = 0.70 ± 0.10; dexamethasone: EC₅₀ = 62.6 ± 8.37 nM, Hill coefficient = 0.64 ± 0.05). Three-dimensional scatter plots show Loewe additivity zero-interaction response surfaces for each obatoclax–chemotherapy combination (gray spheres) derived from single-agent experiments. If actual experimental combination effects (black spheres) are above zero-interaction response surface, combination is synergistic; if below, antagonistic; and if on the response surface, additive. Note synergistic interactions between obatoclax and each chemotherapy agent tested. ADR, doxorubicin; Ara-C, cytosine arabinodise; DEX, dexamethasone; L-ASP, L-aspariginase; VCR, vincristine; VP16, etoposide.