Contribut ion Hypoxia Potent iates the Radiat ion-Sensit izing Effect of Olaparib in Human Non-Small Cell Lung Cancer Xenografts by Contextual Synthet ic

We show that the PARP inhibitor olaparib enhances radiation-induced DNA damage response in hypoxic tumor cells of Calu-6 xenografts but not in nonhypoxic tumor cells of Calu-6 xenografts or in tumor cells of well-oxygenated Calu-3 xenografts. Consequently, olaparib potentiates the antitumor effect of radiation in hypoxic tumors but not in well-oxygenated tumors. This suggests that hypoxia enhances the radiationsensitizing effects of olaparib in human non-small cell Purpose: Poly(ADP-ribose) polymerase (PARP) inhibitors potentiate radiation therapy in preclinical models of human non-small cell lung cancer (NSCLC) and other types of cancer. However, the mechanisms underlying radiosensitization in vivo are incompletely understood. Herein, we investigated the impact of hypoxia on radiosensitization by the PARP inhibitor olaparib in human NSCLC xenograft models. Methodsand Materials: NSCLC Calu-6 and Calu-3 cells were irradiated in the presence of olaparib or vehicle under normoxic (21% O2) or hypoxic (1% O2) conditions. In vitro radiosensitivity was assessed by clonogenic survival assay and gH2AX foci assay. Established Calu-6 and Calu-3 subcutaneous xenografts were treated with olaparib (50 mg/kg, daily for 3 days), radiation (10 Gy), or both. Tumors (nZ 3/group) were collected 24 or 72 hours after the first treatment. Immunohistochemistry was performed to assess hypoxia (carbonic anhydrase IX [CA9]), vessels (CD31), DNA double strand breaks (DSB) (gH2AX), and apoptosis (cleaved caspase 3 [CC3]). The remaining xenografts (nZ 6/group) were monitored for tumor growth. Results: In vitro, olaparib showed a greater radiation-sensitizing effect in Calu-3 and Calu-6 cells in hypoxic conditions (1% O2). In vivo, Calu-3 tumors were well-oxygenated, whereas Calu-6 tumors had extensive regions of hypoxia associated with down-regulation of the homologous recombination protein RAD51. Olaparib treatment increased unrepaired DNA DSB (P<.001) and apoptosis (P<.001) in hypoxic cells