Overexpression of hypoxia-inducible factor-1α (HIF-1α) in human tumors is associated with poor prognosis and poor outcome to radiation therapy. Inhibition of HIF-1α is considered as a promising approach in cancer therapy. The purpose of this study was to test the efficacy of a novel HIF-1α inhibitor PX-478 as a radiosensitizer under normoxic and hypoxic conditions in vitro. PC3 and DU 145 prostate carcinoma cells were treated with PX-478 for 20 hr, and HIF-1α protein level and clonogenic cell survival were determined under normoxia and hypoxia. Effects of PX-478 on cell cycle distribution and phosphorylation of H2AX histone were evaluated. PX-478 decreased HIF-1α protein in PC3 and DU 145 cells. PX-478 produced cytotoxicity in both cell lines with enhanced toxicity under hypoxia for DU-145. PX-478 (20 μmol/L) enhanced the radiosensitivity of PC3 cells irradiated under normoxic and hypoxic condition with enhancement factor (EF) 1.4 and 1.56, respectively. The drug was less effective in inhibiting HIF-1α and enhancing radiosensitivity of DU 145 cells compared to PC3 cells with EF 1.13 (normoxia) and 1.25 (hypoxia) at 50 μmol/L concentration. PX-478 induced S/G2M arrest in PC3 but not in DU 145 cells. Treatment of PC3 and DU 145 cells with the drug resulted in phosphorylation of H2AX histone and prolongation of γH2AX expression in the irradiated cells. PX-478 is now undergoing Phase I clinical trials as an oral agent. Although the precise mechanism of enhancement of radiosensitivity remains to be identified, this study suggests a potential role for PX-478 as a clinical radiation enhancer. Published 2008 Wiley-Liss, Inc.