An inhibitor of Hsp90 (IC,sub>50s = 5 and 943 nM for BT474 tumor cell and fibroblast Hsp90, respectively); inhibits the growth of prostate cancer cell lines (IC50s = 25-45 nM); promotes the degradation of HER2 and induces growth arrest and apoptosis in breast cancer cells overexpressing HER2 (IC50s = 4-72 nM); reduces tumor size in a G-415 gallbladder cancer mouse xenograft model at 25 mg/kg ,