Abstract 1452: mTOR inhibition radiosensitizes bladder cancer tumor cells in vitro and in vivo: A novel strategy for treatment

Abstract

Background: Radiation therapy for invasive bladder cancer allows for organ preservation but systemic toxicity and local control remain problematic. As such, there is a need to increase radiosensitization of tumor cells to improve efficacy. The aim of this study was to investigate if mTOR (mammalian target of rapamycin), a downstream kinase of the PI3K/Akt survival pathway, may be a target for combined bladder cancer therapy. Methods: Clonogenic assays were performed using 6 bladder cancer cell lines in order to address the effects of ionizing radiation (IR) on growth, when tested alone and in combination with RAD001, a potent mTOR inhibitor. Cell cycle analysis was performed using flow cytometry. In the in vivo study, nude mice were subcutaneously injected with KU7 and 253J-BV cells. Treatment with RAD001 (1.5 mg/kg, daily), fractionated IR (total 9 Gy), and the combination of RAD001 and IR was followed over 4 weeks. Tumor growth kinetics was measured and tumor weight at the experimental endpoint. Results: In vitro, a significant decrease in colony formation was observed in the combined treatment when compared to RAD001 or radiation alone (p<0.05) in all cell lines. A G0/G1 as well as a significant increase in G2 arrest was observed in the combined treatment compared to either treatment alone. Changes in the levels of Cyclin D1, p27 and p21 correlated with the observed changes in the cell cycle. Moreover, IR rapidly activated AKT whereas RAD001 effectively inhibited the mTOR downstream signaling as shown by the inhibition of the phosphorylation of S6. Furthermore, autophagy was induced following the treatment with RAD001 and in combination as indicated by the conversion of LC3-I to LC3-II, a protein marker for autophagy. Our in vivo data confirmed our in vitro data, wherein a significant decrease in tumor weight was observed in the combined treatment arm (90% decrease, p<0.001) compared to either treatment alone (60% decrease for RAD001, p<0.05; 77% decrease for IR, p<0.05). These findings point to additive and possibly synergistic beneficial effects of the combined on bladder cancer. Conclusions: The inhibition of mTOR signaling appears promising as a therapeutic modality for bladder cancer, especially in the context of combination with radiation therapy. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1452. doi:1538-7445.AM2012-1452