A potent Hsp90 inhibitor (Kd = 30 nM); induces down-regulation of client proteins HER2 and Akt in vitro; inhibits proliferation in a panel of cancer cell lines (IC50s = 10-50 nM); inhibits tumor growth in a BT474 breast cancer mouse xenograft model; active metabolite of the prodrug SNX-5542 and preferentially accumulates in tumor tissue of mice treated with a single oral dose of 75 mg/kg SNX-5542