Clinical MDR1i enhance the efficacy of SM-based therapy in AML cells. (A) Approximately 5700 compounds were screened for their ability to synergize with birinapant and overcome resistance in murine HoxA9/Meis1 AML cells. Compounds that induced cell death in combination with birinapant, but not as single agents, were selected and their impact determined in a dose-response assay. (B) HoxA9/Meis1 and MLL-AF9 AML cells were treated with birinapant (Bir; 100 and 200 nM), with or without reserpine (Res; 1 µM), for 24 hours (n = 4-5). HoxA9/Meis1 (C) and MLL-AF9-NrasG12D (D) AML cells were treated with 10, 100, 500, and 1000 nM reserpine, tariquidar (Tari), or zosuquidar (Zosu), with or without Bir (200 nM), for 24 hours (n = 4). (E-F) HoxA9/Meis1 AML cells were treated with 125, 250, 500, and 1000 nM AT-406 (AT) (E) and AEG-40730 (AEG) (F), with or without Tari (1 µM), for 24 hours (n = 5). (G-H) HoxA9/Meis1 AML cells were treated with the peptide-mimetic agents ABT-199 (G) and JQ1 (H) at 100 and 1000 nM, with or without Tari or Zosu (1 μM), for 24 hours. Treatment with Bir (500 nM)+Tari and Zosu (1 µM) was used as a control (n = 4). Cell death was measured by flow cytometry analysis of PI uptake and decrease in cell volume (MLL-AF9-NrasG12D) in 3 or 4 individual tumors. Data are the mean ± SEM throughout. P values were obtained by comparison of SM alone and SM/MDR1i treatment at the indicated concentrations. *P < .05; **P < .01; ***P < .001; ****P < .0001. n.s. nonsignificant.