HO-1-targeting drugs synergize with BCR/ABL tyrosine kinase. (A) Imatinib-resistant K562 cells were grown in the absence of imatinib for 24 hours, and then were incubated with various concentrations of imatinib (■-■), ZnDPPIX (•-•), or a combination of imatinib and ZnDPPIX at a constant ratio of 1:200 (▴-▴) at 37°C for 48 hours. Thereafter, proliferation was measured by ³H-thymidine incorporation assay. Results are expressed as percent of medium control and represent the mean (± SD) of triplicates. (B) Combination index (CI) values were determined for each fraction affected as described in “Methods.” A CI value of 1.0 indicates an additive effect, a CI greater than 1.0 indicates antagonism, whereas a CI less than 1.0 indicates synergism. (C) Ba/F3p210^T315I cells were incubated with various concentrations of imatinib (■-■), SMA-ZnPP (•-•), or a combination of imatinib and SMA-ZnPP at a constant ratio of 1:0.1 (▴-▴). (D) Ba/F3p210^T315I cells were incubated with various concentrations of nilotinib (■-■), SMA-ZnPP (•-•), or a combination of nilotinib and SMA-ZnPP at a constant ratio of 1:0.4 (▴-▴). After incubation at 37°C for 48 hours, cell growth was determined by ³H-thymidine incorporation. Results are expressed as percent of medium control and represent the mean (± SD) of triplicates of one representative experiment for each drug combination. (E) Ba/F3p210^T315I cells were cultured in control medium, imatinib (10 μM), PEG-ZnPP (10 μM), SMA-ZnPP (1 μM), or combinations of drugs as indicated, for 48 hours. Thereafter, cell viability was determined by trypan blue exclusion test. Results show the percentage of viable (trypan blue-negative) cells and represent the mean (± SD) of 3 independent experiments. (F) Primary leukemic cells from a patient carrying the BCR/ABL T315I mutation were cultured in control medium, PEG-ZnPP (5 μM), imatinib (5 μM), or a combination of both compounds (1:1 ratio) for 48 hours. Thereafter, proliferation was determined by ³H-thymidine incorporation. Results are expressed as percent of medium control and represent the mean (± SD) of triplicates. (G) Primary leukemic cells from a patient carrying BCR/ABL T315I were cultured in control medium, SMA-ZnPP (7.5 μM), imatinib (0.75 μM), or a combination of both compounds (10:1 ratio) for 48 hours. Thereafter, proliferation was determined by ³H-thymidine incorporation. Results are expressed as percent of medium control and represent the mean (± SD) of triplicates. (H) PB MNC from 3 healthy donors were cultured in control medium, imatinib (10 μM), PEG-ZnPP (10 μM), SMA-ZnPP (1 μM), or combinations of these drugs as indicated for 48 hours. Thereafter, cell viability was determined by trypan blue exclusion test. Results show the percentage of viable (trypan blue–negative) cells and represent the mean (± SD) of 3 independent experiments (3 donors).