The novel dual BCL2/BCL-XL inhibitor AZD0466 outperformed single BCL2 inhibition in T-ALL PDX models. (A) Schematic design of study. NSG mice were first sublethally irradiated (250 cGy) and inoculated with human PDX T-ALL cells (1 × 10⁶ cells per 200 μL phosphate-buffered saline) 24 hours later via tail vein injection. The engraftment status of the human cells (leukemic burden) in the recipient mice was monitored weekly from the peripheral blood (PB) by flow cytometry. Once engraftment was established, mice (n = 10/group) were randomized to receive treatment with the BCL2/BCL-XL inhibitor AZD0466, the MCL1 inhibitor AZD5991, and the BCL2 inhibitor venetoclax, along with the respective vehicles at the indicated doses and treatment schedules. (B) Tumor burden development in 2 aggressive PDX T-ALL models measured as weekly detection of circulating human CD45⁺ cells in the PB and expressed as percent of normalized human to sum of human and murine CD45⁺ cells in mice undergoing treatment with vehicle, AZD0466, AZD5991, and venetoclax; mean ± standard deviation (SD), n = 10 mice per treatment arm. The statistical differences between 2 separate treatments were evaluated by 2-way analysis of variance, ∗P = .013; ∗∗P = .0064. (C) Leukemic burden in the bone marrow after 4 weeks of the treatment (number of mice analyzed, n = 3; mean ± SD indicated in the figure). P values were determined between 2 groups using an unpaired Student t test, ∗P = .013; ∗∗P = .0064; ∗∗∗P = .0009; ∗∗∗∗P < .0001. (D) Platelet counts upon selected treatment regimens. The statistical differences between 2 separate treatments were evaluated by an unpaired Student t test. (E) Body weight monitoring during treatment period. The statistical differences between 2 separate treatments were evaluated by an unpaired Student t test, ∗P = .015. (F) Kaplan-Meier survival curves of mice transplanted with T-ALL and the PDX models PDX1 and CU76 treated with anti-apoptotic inhibitors. Treatment was initiated once the level of circulating leukemia cells in the PB reached 0.5% to 1%; the timeframe of treatment is labeled by the yellow windows on the graphs. The differences in survival between the 2 groups were compared and determined by Gehan-Breslow-Wilcoxon log-rank test (n = 4 mice per group). ∗∗∗P = .001; ∗∗∗∗P < .0001. All calculations were performed using GraphPad prism statistical software version 9. ns, not significant; PBL, peripheral blood lymphocyte.