Effect of inhibition of poly-(ADP ribose) polymerase on gemcitabine and radiation-induced cytotoxicity of pancreatic cancer cells.

Abstract

203 Background: Poly-(ADP ribose) polymerases (PARPs) are DNA-binding proteins involved in DNA repair. PARP inhibition has resulted in excellent antitumor activity when used with other cytotoxic therapies. ABT-888 is a promising PARP inhibitor with excellent potency against the PARP-1/2 enzymes and good oral bioavailability. We attempt to determine whether PARP-1/2 inhibition alone, or in combination with gemcitabine, will enhance the effects of irradiation (RT) of pancreatic cancer cells. Methods: The pancreatic carcinoma cell lines, MiaPaCa-2 and Panc02, were treated with ABT-888, gemcitabine, RT, or combinations thereof. RT was delivered with a 137-Cs Gammacell in a single fraction. Cells were pre-treated once with ABT-888 and/or gemcitabine 30 minutes prior to RT. Viability was assessed through reduction of resazurin into fluorescent resorufin. Levels of apoptosis were determined by measuring caspase-3/7 activity using a luminescent assay. PARP activity was determined using a chemiluminescent PAR elisa. Results: The half maximal inhibitory concentration (IC50) of RT was 5 Gy; IC10 for ABT-888 and gemcitabine were 10 uM and 5 nM, respectively. Treatment with ABT-888 (10 uM), gemcitabine (5 nM), or combinations of the two with RT led to increasingly higher rates of cell death 8 days after treatment (p<0.001). RT dose enhancement factors were 1.5, 1.82 and 2.36 for 1, 10 and 100 uM ABT-888, respectively. Minimal cytotoxicity was noted when cells were treated with ABT-888 alone up to 100 uM. Caspase activity was not significantly increased when treated with ABT-888 (10 uM) alone (1.28 fold, p=0.077), but became significant when RT (2 Gy) was added (2.03 fold, p=0.006). This difference was further enhanced by the addition of gemcitabine (2.95 fold, p=0.004). Conclusions: ABT-888 is a potent radiosensitizer of pancreatic cancer cells with minimal cytotoxicity when used alone. Cell death is further potentiated by cotreatment with gemcitabine. Radiation-induced apoptosis was significantly enhanced by ABT-888 and gemcitabine, suggesting a synergistic mechanism of interference with DNA repair. These data are currently being validated in an orthotopic pancreatic cancer mouse model. No significant financial relationships to disclose.