In vivo antileukemic efficacy of dual inhibition of MEK and ATR in Mll-Af4/N-Ras^G12DB-ALL. (A) Kaplan-Meier survival plot of mice transplanted with 1000 secondary B-ALL cells and treated with vehicle, single-agent trametinib (0.5 mg/kg), single-agent AZ20 (50 mg/kg), or the combination. Treatment of mice began 7 days posttransplant (single red arrow) and was sustained throughout the experiment. In the combination-treatment cohort, treatment was suspended on day 32 posttransplant (double red arrows). (B) Leukemia burden, measured by double positivity of GFP and tdTomato, was assessed in several hematopoietic tissues of mice treated for 14 days after detectable engraftment of leukemic cells in the peripheral blood. (C) Apoptosis, as measured by Annexin V⁺, in double-positive leukemia cells of mice treated with trametinib, AZ20, or the combination. (D) Phospho-Erk levels in sorted leukemia cells from mice treated with a combination of MEK and ATR inhibitors. The data are representative of 3 independent treatment experiments with 2 different leukemias derived from independent donors (supplemental Figures 8 and 9). ***P < .001, **P < .01, *P < .05.