We have recently shown that small molecule inhibitors of Chk1 abrogate the G2 checkpoint and inhibit HRR (homologous recombination repair) in pancreatic cancer cells. Importantly, Chk1 inhibition preferentially sensitizes p53-mutant tumor cells and K-Ras mutation may also confer sensitivity to Chk1 inhibition. PARP inhibitors have demonstrated preferential efficacy as single agents and as radiosensitizers in cells with other DNA damage repair defects. Thus, we hypothesize that inhibition of HRR (by the Chk1 inhibitor MK-8776) and PARP1 (by olaparib) will selectively sensitize pancreatic cancer cells to radiation. The long-term goal of our work is to improve the outcome of patients with pancreatic cancer (and other p53/K- Ras -mutant tumors) by selectively targeting DNA repair pathways (in combination with chemoradiation). The goals of this application are to combine inhibitors of DNA damage response pathways in order to improve gemcitabine-based chemoradiotherapy for pancreatic cancer as well as to understand the mechanisms of sensitization and tumor cell selectivity by Chk1 and PARP1 inhibition. In Specific Aim 1 we will determine the mechanisms of tumor cell selectivity and radiosensitization by Chk1 and PARP1 inhibition. We will test the hypothesis that combined inhibition of Chk1 and PARP1 produce synergistic sensitization to DNA damage (radiation and gemcitabine-radiation) by increasing unrepaired double strand breaks selectively in p53- and K-Ras mutant pancreatic cancer cells. We will utilize normal cells as well as isogenic p53 and K-Ras cancer cells to determine the roles of p53 and K-Ras in selectivity. To address the mechanisms of radiosensitization in response to the combination of Chk1 and PARP1 inhibition in cancer cells, we will assess DNA damage, DNA damage responses, and HRR. To determine how Chk1 and PARP1 inhibitors interact to produce radiosensitization we will selectively manipulate the inhibitory effects of MK-8776 on HRR versus the G2 checkpoint. In Specific Aim 2 we will test the hypothesis that combined Chk1 and PARP1 inhibition selectively sensitize p53 and K-Ras mutant pancreatic tumor xenografts to radiation and gemcitabine-radiation with minimal normal tissue sensitization. We will identify p53 and K- Ras -mutant patient-derived pancreatic tumors and determine the efficacy of combined Chk1 and PARP1 inhibition on sensitization to gemcitabine and radiation. To understand the mechanisms associated with sensitization, we will analyze key DNA damage response proteins identified in Aim 1. Duodenal toxicity, the dose limiting toxicity for irradiation of the pancreas, will be assessed in response to Chk1 and PARP1 inhibition in combination with radiation and gemcitabine-radiation. Successful completion of these aims will IMPACT therapy for patients with pancreatic cancer by providing new therapy options as well as biomarkers for patient selection. Our track record and team of investigators make it highly likely that we will succeed in the completion of these aims and their translation to the clinic.