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(PP130) Suberoyl anilide hyroxamic acid (SAHA) sensitizes pancreatic cancer cells to radiation therapy.
Krishnan, Sunil*,1, Shentu, Shujun1, Schwartz, David1, Puduvalli, Vinay2, 1 MD Anderson Cancer Center, Houston, TX, United States2 MD Anderson Cancer Center, Houston, TX, United States
ABSTRACT- Purpose/Objective: Histone deacetylase (HDAC) inhibitors represent a new class of targeted anti-tumor agents that modulate gene expression and chromatin assembly thus inducing growth arrest, cell differentiation, cell cycle deregulation and apoptosis. SAHA is a potent HDAC inhibitor that has shown activity in preclinical studies and preliminary data from clinical trials have suggested promising activity and tolerability. We studied the effect of SAHA on radiation response of pancreatic cancer cells that are traditionally resistant to radiation therapy. Since repair of DNA strand breaks induced by radiation therapy and induction of specific genes by sublethal doses of radiation therapy are known mechanisms of radiation resistance, we postulated an additive or synergistic interplay between radiation therapy and SAHA. Materials/Methods: A standardized XTT assay was employed to determine cell proliferation of a panel of pancreatic cancer cell lines in response to varying concentrations of SAHA. Radiation sensitivity was assessed using a clonogenic survival assay on MiaPaCa-2 cells. Western blotting with antibody to caspases and PARP was used to document induction of apoptosis. Sublethal damage repair was studied using antibodies to DNA repair proteins. Results: SAHA inhibited cell proliferation in a dose-dependent manner in a panel of pancreatic cancer cell lines with an IC-50 of 500nM, a readily achievable serum level in humans. On a clonogenic assay, radiation alone caused dose-dependent reduction in cell survival. Treatment with SAHA enhanced the radiation-induced cell killing in a dose-dependent manner. The enhancement factors calculated at a surviving fraction of 0.1 were 1.1, 1.4 and 5.6 for 500 nM, 1 M and 3 M respectively. The 3 M SAHA concentration modified the shape of the cell survival curve by completely eliminating the shoulder region, suggesting inhibition of sublethal damage repair. This is being confirmed with split-dose radiation experiments. The effect of SAHA on levels of DNA repair proteins was performed using Western blotting for Ku70 and Ku86, neither of which was significantly altered. Other proteins in the non-homologous end-joining and homologous recombination pathways are being studied and these results will be available at the time of presentation. While radiation alone did not increase the baseline levels of apoptosis, dose- and time-dependent induction of caspase 3 and cleavage of PARP was noted with addition of SAHA. Conclusions: SAHA shows promising activity against pancreatic cancer cells by inhibiting proliferation and enhancing apoptosis in a dose-dependent manner. It also works as a radiosensitizer in vitro by reducing clonogenic survival in a dose-dependent manner. The mechanistic basis of this radiosensitization is currently being investigated but is likely related to inhibition of sublethal damage repair. HDAC inhibition with SAHA has the potential to improve radiation therapy outcomes in a clinically meaningful way in a traditionally radiation resistant tumor.
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