Positron Emission Tomography Imaging of Functional TGFβ Activity and Benefit of TGFβ Inhibition in Irradiated Intracranial Tumors.

INTRODUCTION: Transforming growth factor β (TGFβ) promotes cell survival by endorsing DNA damage repair and mediates an immunosuppressive tumor microenvironment. Thus, TGFβ activation in response to radiation therapy is potentially targetable because it opposes therapeutic control. Strategies to assess this potential in the clinic are needed.

METHODS: & Materials: We evaluated positron emission tomography (PET) to image 89 Zr -fresolimumab, a humanized TGFβ neutralizing monoclonal antibody, as means to detect TGFβ activation in intracranial tumor models. TGFβ pathway activity was validated by immunodetection of phosphorylated SMAD2 and TGFβ target, tenascin. The contribution of TGFβ to radiation response was assessed by Kaplan Meier survival analysis of mice bearing intracranial murine tumor models, GL261 and SB28 glioblastoma and brain-adapted 4T1 breast cancer (4T1-BrA) treated with TGFβ neutralizing monoclonal antibody, 1D11, and/or focal radiation (10 Gy).

RESULTS: 89 Zr-fresolimumab PET imaging detected engineered, physiological and radiation-induced TGFβ activation, which was confirmed by immunostaining of biological markers. GL261 glioblastoma tumors had more PET signal compared to similar sized SB28 glioblastoma tumors, whereas widespread PET signal of 4T1-BrA intracranial tumors is consistent with its highly dispersed histological distribution. Survival of mice bearing intracranial tumors treated with 1D11 neutralizing antibody alone was similar to that of mice treated with control antibody whereas 1D11 improved survival when given in combination with focal radiation. The extent of survival benefit of combination of radiation and 1D11 was associated with the degree of TGFβ activity detected by PET.

CONCLUSIONS: This study demonstrates that 89 Zr -fresolimumab PET imaging detects radiation-induced TGFβ activation in tumors. Functional imaging indicated a range of TGFβ activity in intracranial tumors, but TGFβ blockade provided survival benefit only in the context of radiation treatment. These studies are further evidence that radiation-induced TGFβ activity opposes therapeutic response to radiation.