Abstract 1395: Inhibition of transforming growth factor β (TGFβ) increases the therapeutic benefit of radiotherapy in a murine mammary tumor model

Abstract

Abstract Ionizing radiation triggers activation of transforming growth factor β1 (TGFβ), which is implicated in regulation of cell cycle, apoptosis, immune response. Our prior work has shown that the DNA damage response is compromised when TGFβ is inhibited prior to irradiation in mouse epithelial tissues and human mammary epithelial cells (Cancer Res 62:5627, 2002; Cancer Res 66:10861, 2006). TGFβ depletion compromises ataxia telengiectasia mutated (ATM) kinase activity and autophosphorylation in irradiated epithelial cells, leading to reduced phosphorylation of critical DNA damage transducers, abrogation of the cell cycle block and apoptosis. If cancer cells are similarly regulated, then TGFβ inhibition could improve the therapeutic effect of radiotherapy (RT). We used the highly metastatic 4T1 mouse mammary carcinoma model to evaluate the impact of TGFβ inhibition on the response to radiation in vitro and in vivo. Cultured 4T1 cells were TGFβ responsive as demonstrated by SMAD phosphorylation, but were not sensitive to TGFβ growth inhibition. 4T1 cells were radiosensitized following inhibition of TGFβ type I receptor kinase with a small molecule inhibitor as shown by clonogenic assay. For in vivo evaluation, 4T1 cells were injected s.c. in the flank of syngeneic mice and were treated when tumors became palpable 13 days later. Mice were randomly assigned to four groups receiving control isotype monoclonal antibody (mAb), 50mg/kg 1D11.16, a monoclonal pan-specific TGFβ neutralizing antibody, local RT and isotype control mAb, or RT and 1D11.16. In the first experiment, mAb were administered at 24 hr prior to the first radiation treatment and RT was delivered as three fractions of 12 Gy on three consecutive days. 1D11.16 treatment slightly reduced tumor growth rate compared to isotype mAB control-treated mice but tumor weights were not significantly different on day 28 (N=5, 0.62 ± 0.07 vs 0.54 ± 0.09 gm). When administered in combination with RT, 1D11.16 enhanced tumor growth delay and significantly reduced tumor weight at experiment termination (N=5, 0.12 ± 0.02 vs 0.24 ± 0.04 gm; p<0.02 two-tailed t-test). A second experiment compared single doses of 4, 8 and 12 Gy with and without 1D11.16 or control mAb treatment. Addition of 1D11.16 increased the response to each dose, with maximum benefit achieved with 8 Gy, which provided an 8d growth delay compared to 3d following RT alone or 2d with 1D11.16 treatment. Thus inhibition of TGFβ increases radiosensitivity of 4T1 tumor cells in vitro and in vivo, which supports the use of TGFβ inhibitors as means to increase the response to RT, in addition to potential benefit of protecting normal tissue. Supported by funding from the NYU Cancer Center. 1D11.16 provided by Genzyme, Inc. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1395.