Effective and selective targeting of CML CP stem/progenitor cells by combined treatment with IM and SB216763. Normal and CML CD34⁺ cells were cultured for 96 hours in SFM supplemented with 5GF in the absence (−) or presence (+) of IM (1μM), dasatinib (0.15μM), and SB216763 (5μM) alone or in combination with IM or dasatinib. Cells were then assayed for primitive (A) or committed (B) progenitors in the absence of further drug exposure. The indicated mean ± SD graphed for CML and normal cells is based on replicate experiments (CML, n = 5; normal, n = 3). LTC-IC frequency was calculated in limiting-dilutions assays and normalized to the frequency of no-drug samples for each drug treatment. LTC-IC frequency (mean ± SD) of untreated CD34⁺ cells was 22 ± 13/1000 input cells for CML samples and 9 ± 5/1000 cells for normal cells. CFC frequency was plotted for each drug treatment and normalized to the colony number obtained in no-drug samples. CFC frequency (mean ± SD) of untreated cells was 345 ± 56/1000 input cells for CML samples and 122 ± 30/1000 cells for normal cells. Drug treatments at which CML progenitor frequency was significantly different from no-drug controls are indicated as follows: ***P < .001; **P < .01; and *P < .05. (C) CML CD34⁺CD38⁻ cells (n = 3) were treated as indicated and then directly assayed as LTC-ICs or expanded for 10 days in stroma-free liquid-suspension cultures enriched with high 5GF concentrations before being analyzed in LTC-IC assays. LTC-IC frequency was calculated in limiting-dilution assays and normalized to the frequency of no-drug samples for each drug treatment. LTC-IC frequency (mean ± SD) of untreated CD34⁺CD38⁻ cells was 11 ± 2/1000 input cells for CML samples before GF-mediated liquid enrichment and 21 ± 2/1000 cells afterward. **P < .01.