Abstract 69: In vitro and in vivo tumor cell radiosensitization by the competitive mTOR inhibitor INK128.
Authors: Hayman Thomas J. , Kramp Tamalee , Camphausen Kevin , Tofilon Philip J. 
Published: August 2014

Abstract
Radiation-induced translational control of gene expression has been shown to be a component of the cellular radioresponse. Furthermore, recent data suggest strategies targeting eIF4E may be effective for tumor cell radiosensitization. The mechanistic target of rapamycin (mTOR) is a critical kinase in the regulation of eIF4E and mRNA translation. The newly developed ATP-competitive mTOR inhibitor, INK128, inhibits both mTORC1 and mTORC2 activity, and is in currently undergoing evaluation in Phase 1 clinical trials. In the study described here we evaluated the effects of INK128 treatment on tumor cell radiosensitivity in vitro using a clonogenic survival assay and in vivo according to tumor growth delay. For tumor cell lines initiated from different histologic origins in vitro exposure to INK128 resulted in the complete inhibition of both mTORC1 and 2. Treatment with the same concentration of INK128 immediately after irradiation increased tumor cell radiosensitivity with DEFs greater than 1.3. To determine whether the INK128 induced radiosensitization could be extended to an in vivo model we used subcutaneous xenografts initiated from U251 or PSN1 tumor cells. INK128 treatment (oral gavage) inhibited mTOR activity in tumors established from both cell lines as defined by loss of 4E-BP1 phosphorylation. Furthermore, m7-GTP batch chromatography showed that INK128 treatment decreased cap-binding activity in xenografts. To evaluate the effects of INK128 on in vivo tumor radiosensitivity a tumor growth delay assay was performed using mice bearing U251 tumor xenografts. When tumors reached approximately 210mm3 mice were randomized into 4 groups: vehicle, radiation, INK128, and INK128 plus radiation. INK128 was delivered once a day (3 mg/kg, oral gavage) for four days with the tumor locally irradiated (2 Gy) 2h after each of the four drug treatments. Absolute growth delays were calculated as the time in days for tumors in treated mice to grow from 210 to 1000mm3 minus the time in days for tumors to reach the same size in vehicle treated mice. The absolute growth delays for INK128 alone and radiation alone groups were 1.7 and 12.7 days respectively. The growth delay in mice treated with the combination of INK128 and radiation was 22.2 days, which is greater than the sum of the growth delays caused by INK128 and radiation alone. The DEF, obtained by dividing the normalized tumor growth delay in mice treated with the radiation/PP242 combination (20.5) by the absolute growth delay in mice treated with radiation only (12.7), was 1.6. These results suggest that mTOR inhibition by the clinically relevant ATP-competitive mTOR inhibitor, INK128, enhances tumor cell radiosensitivity both in vitro and in vivo.
Citation Format: Thomas J. Hayman, Tamalee Kramp, Kevin Camphausen, Philip J. Tofilon. In vitro and in vivo tumor cell radiosensitization by the competitive mTOR inhibitor INK128. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 69. doi:10.1158/1538-7445.AM2013-69