In vivo efficacy of CCT241736 in FLT3 inhibitor–resistant xenografts. (A) Athymic nude mice (6 per cohort) were injected subcutaneously with 2 × 10⁶ MOLM-13-RES cells. Thirteen days after implantation, mean tumor diameter was 6 mm (day 0 on graphs), and dosing began orally twice per day for 5 days with solvent vehicle, CCT241736 at 100 mg/kg twice per day or MLN518 at 160 mg/kg twice per day. Tumor volumes were measured at days 0, 1, 2, and 5. Mean tumor volumes ± SEM are shown. Mice were culled at 1 and 6 hours after the final dose. Unpaired Student t test of 95% confidence intervals on day 5. ns, P = .13 (control: MLN518) or P = .4 (control: CCT241736). Significantly different, *P = .02 (MLN518:CCT241736). (B) Tumor lysates were analyzed by immunoblotting for expression of the indicated proteins. GAPDH, Aurora A, and STAT5 were used as controls. The bar graphs (bottom panel) show the in vivo potency of compounds after normalization to the control total proteins. (C) Systemic model of BaF3FLT3-ITD F691L luciferase–expressing tumors in NOD SCID mice. On day 7, the flux in the treated groups (mean total flux ± SEM) is indicated. Mice were dosed (starting 4 days after tumor cell implantation and randomization) with AC220 at 5 mg/kg orally once per day or CCT241736 at 100 mg/kg orally twice per day, and tumor burden was assessed by whole-body bioluminescent imaging. Animals were culled when they showed signs of deterioration as a result of tumor burden (body weight loss, rapid breathing). Unpaired Student t test of 95% confidence intervals on day 7. ns, P = .6 (control: AC220) or P = .1 (control: CCT241736). Significantly different, *P = .01 (AC220:CCT241736).