MA9 cells are sensitive to cooperative inhibition of Bcl-2 proteins using ABT-737 and Rac proteins using NSC23766. (A) Three distinct MA9 cell lines, as well as the THP-1 cell line bearing an MLL-AF9 fusion, were exposed to serial dilutions of ABT-737, and cell proliferation was measured by WST-1 assay. All MLL lines tested were exquisitely sensitive to ABT-737, while normal human cord blood CD34⁺ cells showed minimal sensitivity. Representative plot of 3 experiments is shown. Each repeat included a different UCB control. (B-C) The MA9.3 and MA9.6 cell lines were exposed to serial dilutions of ABT-737 in the absence or presence of a subtherapeutic dose of NSC23766. (D) Combination treatment resulted in a 2.5- to 9-fold reduction in the IC50 of ABT-737. (E-F) While UCB CD34⁺ cells showed slight sensitivity to NSC23766, and HL-60 showed slight sensitivity to ABT-737, neither showed an effect from combined treatment. (G-H) Two independent AE cell lines were insensitive to all tested doses of these 2 compounds. Representative plots of 3-6 experiments are shown.