IFN-γ PET imaging stratifies and predicts response to CD137 agonism in a syngeneic colorectal tumor model

The secretion of interferon-γ (IFN-γ) by activated immune cells is one of the hallmarks of response to cancer immunotherapy. Hence, detection of this cytokine via imaging can help determine an activated antitumor immune response. We have previously shown that positron emission tomography (PET) imaging of IFN-γ using [⁸⁹Zr]Zr-Deferoximane (DFO)-anti-IFN-γ measured changes in its intratumoral expression after immune modulation. However, assessment of its potential to predict and identify treated responders (TR) versus treated non-responders (TNR) at the early stages of treatment, before any morphological effects occur, are key clinical needs. Thus, we evaluated whether IFN-γ PET can predict response to immunotherapy in a preclinical mouse model.

Biolayer interferometry and neutralization assays were performed. [⁸⁹Zr]Zr-DFO-anti-IFN-γ was administered to syngeneic CT26 murine colorectal tumors grown on immunocompetent mice that were either untreated (control) or treated with a CD137 agonist. Tumor uptake of [⁸⁹Zr]Zr-DFO-anti-IFN-γ was compared between groups. The uptake of a ⁸⁹Zr-labeled irrelevant immunoglobulin was measured in separately treated CT26-bearing mice. Expression of IFN-γ was analyzed via ELISA and in situ hybridization assays after PET imaging. Imaging-derived IFN-γ uptake was evaluated for its ability to predict treatment response and disease outcomes.

The affinity of the DFO-anti-IFN-γ for IFN-γ remained similar to the unconjugated anti-IFN-γ. Binding did not perturb the signaling function of the cytokine. The uptake of [⁸⁹Zr]Zr-DFO-anti-IFN-γ strongly correlated with detected tumor levels of IFN-γ. The tracer stratified TR from TNR with high accumulation associated with tumor regression. A strong inverse correlation was further demonstrated between the tracer uptake and tumor growth slopes, where the higher the uptake, the slower the tumor growth. Accumulation of the ⁸⁹Zr-labeled IgG in treated tumors was lower compared with the uptake of the IFN-γ tracer across all groups. Cox regression analysis revealed that increased uptake (>13.9 %ID/mL (percent injected dose per milliliter)) was significantly associated with improved overall survival.

Delineating IFN-γ through PET imaging stratified tumor response at the early stages of CD137 agonism prior to tumor morphological changes. Thus, IFN-γ PET holds promise as a predictive biomarker of response to immunotherapy.

IFN-γ PET imaging is predictive of response to immunotherapy, potentially bridging an unmet clinical need for early detection of antitumor immunity.