Targeting Hsp90 by IPI-504 inhibits survival of leukemia stem cells. (A) Bone marrow cells isolated from C57BL/6 (B6) mice with BCR-ABL-T315I–induced CML were cultured in vitro (5 × 10⁶ cells/6 cm tissue culture plate) under the stem cell condition (“Materials and methods”) in the presence or absence of IPI-504 (0.1 μM) or imatinib (2 μM) for 6 days (changing the stem cell medium containing placebo or IPI-504 at day 3) followed by FACS analysis of leukemia stem cells (GFP⁺Lin⁻c-Kit⁺Sca-1⁺). (B) Mice with BCR-ABL-T315I–induced CML were treated with a placebo (n = 5), imatinib (100 mg/kg, twice a day by gavage) (n = 5), and IPI-504 (50 mg/kg, once every 2 days by gavage) (n = 5), respectively, for 6 days beginning at day 8 after transplantation. Bone marrow cells were isolated from the treated CML mice, and leukemia stem cells were analyzed by FACS. The numbers of cells represent total leukemia stem cells in average from femur and tibia of each treated CML mouse. (C) IPI-504 had no inhibitory effect on survival of normal HSCs in mice. B6 mice were treated with a placebo (n = 5), imatinib (100 mg/kg, twice a day by gavage) (n = 5), and IPI-504 (50 mg/kg, once every 2 days by gavage) (n = 5), respectively, for 2 weeks. Bone marrow cells were isolated from the treated mice, and were analyzed by FACS.