Abstract 2659: Radiosensitization of pancreatic cancer cells in vitro and in vivo through poly (ADP ribose) polymerase-1/2 inhibition
Authors: Tuli Richard , Surmak Andrew J. , Hacker-Prietz Amy , Herman Joseph M. 
Published: June 2012

Background: Poly (ADP ribose) polymerases (PARPs) are DNA-binding proteins involved in DNA repair. PARP inhibition with ABT-888 has resulted in excellent antitumor activity when used with other cytotoxic therapies, including radiation (RT). Herein, we attempt to determine whether PARP-1/2 inhibition enhances RT-induced cytotoxicity of pancreatic cancer in vitro and in vivo, and the mechanism by which this occurs.

Methods: Luciferase-expressing pancreatic carcinoma cells (MiaPaCa-2) were treated with ABT-888, RT, combinations thereof, or left untreated. Cell viability was assessed through reduction of resazurin into fluorescent resorufin. Levels of apoptosis were determined by measuring caspase-3/7 activities. PARP activity was determined by measuring PAR protein levels using an elisa. Orthotopic xenografts were generated by injecting 5×105 cells into the pancreatic tail of athymic nude female mice per institutional protocol. Anesthetized mice were imaged for bioluminescence and cone-beam CT was obtained with our small animal linear accelerator. Animals were randomized to receive ABT-888 (25 mg/kg), RT (5 Gy), ABT-888 + RT, or no treatment (saline injection). RT was delivered using bioluminescence image-guided focal treatment. Animals were longitudinally monitored with bi-weekly bioluminescence imaging. All animal experiments were conducted per institutional protocol.

Results: Half maximal inhibitory concentration (IC50) of RT was 2 Gy; IC10 for ABT-888 was 10 uM. Treatment with ABT-888 (10 uM) and RT (2 Gy) led to higher rates of cell death 8 days after treatment (P&lt;.01). Co-treatment with 5 Gy and 1, 10 or 100 uM of ABT-888 led to dose enhancement factors of 1.29, 1.41 and 2.36, 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=.077), but became significant when RT (2 Gy) was added (2.03 fold, P&lt;.01). PARP activity significantly increased 6 hours post-RT; this activity was abrogated following co-treatment with ABT-888. Preliminary in vivo results suggest that treatment with ABT-888 alone does not result in significant tumor growth inhibition (TGI) relative to untreated controls. Additionally, TGI seen with RT is further enhanced following co-treatment with a single dose of ABT-888.

Conclusions: ABT-888 appears to be a potent in vitro and in vivo radiosensitizer of MiaPaCa-2 with minimal intrinsic cytotoxicity when used alone. Radiation-induced apoptosis was significantly enhanced by ABT-888, suggesting a synergistic mechanism of interference with DNA repair. In vitro PAR levels correlated with levels of tumor apoptosis suggesting its potential as a predictive clinical biomarker. These data are currently being translated into a phase I clinical trial for patients with locally advanced pancreatic cancer.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2659. doi:10.1158/1538-7445.AM2011-2659