In vivo modeling demonstrates the elevation of BCL-2 in VEN^surv cells in mice. (A) Schematic representation of the experimental strategy. wt, Bak^–/–Bax^Δcd23, and vav-huBcl-2 mice were treated daily with 100 mg/kg body weight venetoclax or vehicle (a mixture of 60% Phosal 50 propylene glycol, 30% polyethylene glycol 400, and 10% ethanol) for 1 week. (B) Absolute numbers of splenic B cells before and after venetoclax treatment in wt, Bak^–/–Bax^Δcd23, and vav-huBcl-2 mice. (C) Histograms of BCL-2, MCL-1, and BCL-XL protein in CD19⁺CD21⁺IgM⁺ B cells from wt, Bak^–/–Bax^Δcd23, or vav-huBcl-2 mice measured by flow cytometry after vehicle or venetoclax treatment. (D) Geometric mean (± SEM) of mBCL-2, huBCL-2, MCL-1, and BCL-XL protein levels in vehicle- and venetoclax-treated mice of the indicated genotypes. (E) Schematic representation of the Eμ-TCL-1 transgenic mice modeling of CLL cell response to venetoclax. Cohorts of C57BL/6 mice received 5 × 10⁵Eμ-TCL-1 transgenic CLL cells from the same donor, and once they reached 80% leukemic burden in blood, they were treated with vehicle or venetoclax daily for 7 days. (F) Total splenic cell numbers and the proportions of Ki67⁺Eμ-TCL-1 transgenic CLL cells and wt B cells before and after treatment. (G) Histograms of BCL-2, MCL-1, and BCL-XL protein in CLL cells recovered from the spleen of mice treated with vehicle or venetoclax. (H) Quantification of BCL-2, MCL-1, and BCL-XL protein expression in CLL cells recovered from the spleen of mice treated with vehicle or venetoclax. Data from panels B-D are representative of 3 independent experiments with n = 2 to 5 mice per group. Data from panels F-H are representative of 3 independent experiments with n = 5 to 6 mice per group. For all bar graphs, the mean ± SEM are shown, and each symbol represents an individual mouse; Student 2-tailed t test was used. ∗P < .05; ∗∗P < .01; ∗∗∗P < .005; ∗∗∗∗P < .001. hBCL-2, human BCL-2; mBCL-2, murine BCL-2.