The ATP-competitive mTOR inhibitor INK128 enhances in vitro and in vivo radiosensitivity of pancreatic carcinoma cells.

PURPOSE Radiotherapy remains a primary treatment modality for pancreatic carcinoma, a tumor characterized by aberrant mTOR activity. Given the regulatory role of mTOR in gene translation, in this study, we defined the effects of the clinically relevant, ATP-competitive mTOR inhibitor, INK128 on the radiosensitivity of pancreatic carcinoma cell lines. EXPERIMENTAL DESIGN Clonogenic survival was used to determine the effects of INK128 on in vitro radiosensitivity of three pancreatic carcinoma cell lines and a normal fibroblast cell line with mTOR activity defined using immunoblots. DNA double-strand breaks were evaluated according to γH2AX foci. The influence of INK128 on radiation-induced gene translation was determined by microarray analysis of polysome-bound mRNA. Leg tumor xenografts grown from pancreatic carcinoma cells were evaluated for mTOR activity, eIF4F cap complex formation, and tumor growth delay. RESULTS INK128, while inhibiting mTOR activity in each of the cell lines, enhanced the in vitro radiosensitivity of the pancreatic carcinoma cells but had no effect on normal fibroblasts. The dispersal of radiation-induced γH2AX foci was inhibited in pancreatic carcinoma cells by INK128 as were radiation-induced changes in gene translation. Treatment of mice with INK128 resulted in an inhibition of mTOR activity as well as cap complex formation in tumor xenografts. Whereas INK128 alone had no effect of tumor growth rate, it enhanced the tumor growth delay induced by single and fractionated doses of radiation. CONCLUSION These results indicate that mTOR inhibition induced by INK128 enhances the radiosensitivity of pancreatic carcinoma cells and suggest that this effect involves the inhibition of DNA repair.