Combined treatment with ARV-771 and BC2059 exhibits superior antileukemia activity compared with treatment with either agent alone in NSG mice engrafted with OTX-P/R sAML cells or innately resistant patient-derived AML xenografts. (A) SET-2, SET-2-OTX P/R, HEL, HEL-OTX P/R, THP1, and THP1 OTX P/R cells were treated with ARV-771 (dose range, 20-250 nM) and/or BC2059 (10-100 nM) for 48 hours. Following this, the percentage of annexin V–positive, apoptotic cells were determined by flow cytometry. CI values were calculated utilizing Compuyn and graphed with GraphPad V7. The box plot shows the range of CI values in each cell line treated with the combination. CI values <1.0 indicate a synergistic interaction of the drugs. (B) HEL-OTX P/R-GFP-Luc cells were engrafted into preirradiated NSG mice. Once engraftment was confirmed by bioluminescent imaging (Xenogen IVIS), mice were divided into groups and treated with vehicle, ARV-771, and/or BC2059 as indicated for 3 weeks. Mice were imaged weekly to document treatment efficacy. (C) Kaplan-Meier survival plot of NSG mice engrafted with HEL-OTX P/R-GFP-Luc cells and treated for 3 weeks with ARV-771 and/or BC2059. (D) Immunoblot analysis of protein expression in AML PDX blast cells. The expression levels of GAPDH served as the loading control. (E) Innately BETi-resistant AML PDX-GFP-Luc cells were engrafted into preirradiated NSG mice. Once engraftment was confirmed by bioluminescent imaging (Xenogen IVIS), mice were divided into groups and treated with vehicle, ARV-771, and/or BC2059 as indicated for 3 weeks. Mice were imaged weekly to document treatment efficacy. (F) Kaplan-Meier survival plot of NSG mice engrafted with BETi-resistant AML PDX-GFP-Luc cells and treated for 3 weeks with ARV-771 and/or BC2059.