PARP Inhibition Combined With Thoracic Irradiation Exacerbates Esophageal and Skin Toxicity in C57BL6 Mice.

ABSTRACT

PURPOSE: Poly (ADP-ribose) polymerase (PARP) inhibitors have been shown to enhance the radiosensitivity of cancer cells in vitro in a replication-dependent manner. Their in vivo radiosensitizing effects have also been demonstrated in preclinical tumor models. However, whether PARP inhibition can enhance the response to radiation therapy in normal tissues has been largely neglected. We hypothesized that PARP inhibition might also potentiate the response of replicating normal tissues to radiation therapy. In this study, we examined the normal tissue response in mice treated with PARP inhibitors (BMN673 or AZD2281) in combination with thoracic irradiation. METHODS AND MATERIALS The antitumor effects of fractionated irradiation (5 Gy × 4) in combination with BMN673 were evaluated in nude mice bearing established Calu-6 human lung cancer xenografts. The normal tissue response was evaluated in C57BL6 mice that were treated with BMN673 or AZD2281 combined with fractionated irradiation, 5 Gy × 4, delivered to the whole thorax. Body weight and histology of the esophagus and skin in the field of irradiation were examined. The DNA damage response in the esophagus and skin was assessed by γH2AX immunohistochemistry. RESULTS: While PARP inhibition enhanced irradiation-induced tumor growth inhibition in nude mice, it was also associated with significant body weight loss and increased damage to the esophagus and skin within the field of irradiation in C57BL6 mice. PARP inhibition compromised the repair of irradiation-induced DNA damage in the esophagus and skin. CONCLUSIONS: Although PARP inhibition enhanced the antitumor response to fractionated irradiation, it also enhanced the irradiation response in replicating normal tissues. Therefore, our study suggests that additional caution may be warranted in the clinical development of combination therapies using PARP inhibitors and radiation therapy, in particular where the field of irradiation includes the esophagus.