A novel oxime derivative of radicicol, KF58333, binds to the heat shock protein 90 (Hsp90) and destabilizes its associated signaling molecules. These effects play a critical role in the growth inhibition of tumor cells. To further investigate the effects of this agent, it was administered to two human breast cancer cell lines, KPL-1 and KPL-4, both in vitro and in vivo. KF58333 dose-dependently inhibited the growth and vascular endothelial growth factor (VEGF) secretion, concomitantly with a decrease in VEGF mRNA expression, in each cell line. This agent also suppressed the increase of VEGF secretion and expression induced by hypoxia (1% O(2)). Intravenous injections of this agent into nude mice bearing either KPL-1 or KPL-4 xenografts significantly inhibited the tumor growth associated with a decrease in the Ki67 labeling index and microvascular area and an increase in apoptosis and the necrotic area. These findings indicate that the antitumor activity of this radicicol derivative may be partly mediated by decreasing VEGF secretion from tumor cells and inhibiting tumor angiogenesis. To explore the action mechanisms of the anti-angiogenic effect, the expression level of hypoxia-inducible factor (HIF)-1alpha was investigated. KF58333 provided a significant decrease in the HIF-1alpha protein expression under both normoxic and hypoxic conditions. In contrast, the mRNA expression of HIF-1alpha was not decreased by this agent. It is suggested that the post-transcriptional down-regulation of HIF-1alpha expression by this agent may result in a decrease of VEGF expression and tumor angiogenesis.