XL-888 is a novel and orally-bioavailable inhibitor of heat shock protein 90 (HSP90) that selectively inhibits HSP90α and HSP90β with IC50s of 22 nM and 44 nM respectively. It also exerts considerably weaker inhibition against a range of other diverse kinases with IC50 more than 3600 nM for all. In recent studies, XL-888 has exhibits strong anti-proliferative activities in a panel of tumor cells with values of IC50 ranging from 0.1 nM to 45.5 nM.