BH3-mimetic drug combination therapy is highly effective in xenograft models of TP-53 defective AML. (A) Irradiated NSG mouse recipients of transplants of 10⁵ human MV-4-11 cells harboring the empty vector control (WT TP53) and TP53 KO cells at a 1:1 ratio and followed for Kaplan-Meier survival analysis. Dosing commenced on day 4 after transplant. The mice were divided into treatment groups containing 6 mice each and treated with vehicle, combined venetoclax 75 mg/kg by oral gavage daily (5 days/week for 4 weeks) and S63845 (MCL-1i) 25 mg/kg IV weekly for 4 weeks, or either drug alone (capped line shows treatment interval). Kaplan-Meier (KM) analysis (ethical end points) showed that combined treatment with venetoclax/S63845 resulted in significantly longer survival than vehicle control. *P < .05 compared with vehicle control. (B) Irradiated NSG mice received transplants of 10⁵ human OCI-AML3 cells harboring the empty vector control (WT TP53) or TP53 KO cells and were observed for KM survival analysis. Mice were divided into treatment groups containing 6 to 7 mice in each and dosing was performed as outlined in panel A. KM survival (ethical end points) showing that combined treatment with venetoclax/S63845 resulted in significantly longer survival than vehicle control independent of TP53 status (capped line shows treatment interval). *Significantly enhanced survival (P < .05) compared with vehicle control. (C) Irradiated NRG-SG3 mice received transplants of 10⁶ primary AML cells (TP53-mutant [Y126D] AML; RBWH65). Engraftment was confirmed at week 9 after transplant by detection of hCD45 leukemic cells in peripheral blood. Cohorts of 2 to 4 mice per group were then treated with vehicle (days 1–5), venetoclax 75 mg/kg days 1 to 5 by gavage, S63845 (MCL-1i) 25 mg/kg IV on day 3), or venetoclax 75 mg/kg combined with S63845 25 mg/kg for 10 days and hCD45⁺ cells in BM enumerated by flow cytometry. (D) Serial BM sampling of NRG-SG3 PDX 01-318-2015 harboring 2 TP53 variants (V173M and V143M). Patient primary sample VAF was 30% for V173M and 27% for V143M. Intrafemoral BM sampling was performed on days 34 and 48 after transplant and when mice were euthanized due to clinical deterioration. The VAF (%) for each TP53 mutation at each time point is shown. Changes in TP53 VAF percentage were quantitated by TP53-targeted NGS and normalized to hCD45⁺ cells in the BM. (E) Irradiated NRG-SG3 mice received transplants of 10⁶ primary AML cells (harboring 2 TP53 variants, V173M and V143M; 01-328-2015). Engraftment was confirmed at week 2 after transplant by detection of hCD45⁺ leukemic cells in peripheral blood. Cohorts of 5 mice per group were then treated with vehicle on days 1 to 5, venetoclax 75 mg/kg on days 1 to 5 by gavage, S63845 (MCL-1i) 25 mg/kg IV on day 3, or venetoclax 75 mg/kg combined with S63845 25 mg/kg for 4 weeks. KM survival (ethical end points) showing that combined treatment with venetoclax/S63845 resulted in significantly longer survival than vehicle-treated control mice, independent of TP53 status. *Significantly enhanced survival (P < .05) compared with vehicle control. (F) Activation of BAX (detected by antibody 1E5; x-axes) or loss of cytochrome c; y-axes) in TP53 WT or TP53 KO RS4;11 cells were determined by flow cytometry 6 hours after treatment with IC₅₀ doses of venetoclax or MCL-1i individually, or with venetoclax combined with MCL-1i at IC₂₀ or IC₅₀ doses. Data are representative of ≥3 independent experiments.