68Ga-FAPI PET imaging assesses tumor immunity and guides TGF-ß inhibition to sensitize metastatic colorectal cancer to immunotherapy
BACKGROUND. Improving and predicting tumor response to immunotherapy remains challenging. Combination therapy with a transforming growth factor ß (TGF-ß) inhibitor that targets cancer associated fibroblasts (CAFs) is promising to enhance efficacy of cancer immunotherapies. However, the effect of this approach in clinical trials is limited, requiring in vivo methods to better assess tumor responses to combination therapy. METHODS. We measure CAFs in vivo using gallium 68-labeled fibroblast activation protein inhibitor (68Ga-FAPI) for PET/CT imaging to guide TGF-ß inhibition and sensitize metastatic colorectal cancer (CRC) to immunotherapy. A total of 131 patients with metastatic CRC underwent 68Ga-FAPI and 18F-fludeoxyglucose (18F-FDG) PET/CT imaging. Fourteen patients underwent surgery after the imaging. Relationship between uptake of 68Ga-FAPI and tumor immunity was analyzed. Mouse cohorts of metastatic CRC were treated with TGF-ß receptor (TGF-ßR) inhibitor combined with KN046 which blocks PD-L1 and CTLA4, followed with 68Ga-FAPI and 18F-FDG micro-PET/CT imaging to assess tumor responses. RESULTS. Patients with metastatic CRC demonstrated high uptakes of 68Ga-FAPI, along with suppressive tumor immunity and poor prognosis. TGF-ßR inhibitor enhanced tumor infiltrating T cells and significantly sensitized metastatic CRC to KN046. 68Ga-FAPI PET/CT imaging accurately monitored the dynamical changes of CAFs and tumor response to combined TGF-ßR inhibitor with immunotherapy. CONCLUSION. 68Ga-FAPI PET/CT imaging is powerful in assessing tumor immunity and response to immunotherapy in metastatic CRC. This study supports future clinical application of 68Ga-FAPI PET/CT to stratify and guide patients with CRC for precise TGF-ß inhibition plus immunotherapy, recommending 68Ga-FAPI and 18F-FDG dual PET/CT for CRC management. Overall design: RNA-seq on tumor tissues isolated from mice bearing MC38 liver metastasis treated with PBS control, TGF-ßR1 inhibitor SB525334, PD-L1/CTLA4 inhibitor KN046, combination of SB525334 and KN046.