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Mol Cancer Ther. 2009 Apr;8(4):947-58. doi: 10.1158/1535-7163.MCT-08-0981.

The selective hypoxia inducible factor-1 inhibitor PX-478 provides in vivo radiosensitization through tumor stromal effects.

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  • 1Department of Radiation Oncology, The University of Texas M. D. Anderson Cancer Center, Unit 97, 1515 Holcombe Boulevard, Houston, TX 77030, USA.


Hypoxia inducible factor-1 (HIF-1) promotes tumor cell adaptation to microenvironmental stress. HIF-1 is up-regulated in irradiated tumors and serves as a promising target for radiosensitization. We initially confirmed that the orally bioavailable HIF-1 inhibitor PX-478 reduces HIF-1 protein levels and signaling in vitro in a dose-dependent manner and provides direct radiosensitization of hypoxic cancer cells in clonogenic survival assays using C6 glioma, HN5 and UMSCCa10 squamous cells, and Panc-1 pancreatic adenocarcinoma cell lines. However, PX-478 yields striking in vivo tumor sensitization to single-dose irradiation, which cannot be explained by incremental improvement in direct tumor cell killing. We show that PX-478 prevents postradiation HIF-1 signaling and abrogates downstream stromal adaptation in C6 and HN5 reporter xenografts as measured by serial ultrasound, vascular magnetic resonance imaging, and hypoxia response element-specific micro-positron emission tomography imaging. The primacy of indirect PX-478 in vivo effects was corroborated by our findings that (a) either concurrent or early postradiation sequencing of PX-478 provides roughly equivalent sensitization and (b) constitutive vascular endothelial growth factor expression maintains refractory tumor vessel function and progression following combined radiation and PX-478. These results confirm that disruption of postradiation adaptive HIF-1 signaling by PX-478 imparts increased therapeutic efficacy through blockade of HIF-1-dependent reconstitution of tumor stromal function. Successful translation of targeted HIF-1 radiosensitization to the clinical setting will require specific consideration of tumor microenvironmental effects and mechanisms.

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