The PET-Tracer 89Zr-Df-IAB22M2C Enables Monitoring of Intratumoral CD8 T-cell Infiltrates in Tumor-Bearing Humanized Mice after T-cell Bispecific Antibody Treatment

Abstract

CD8-expressing T cells are the main effector cells in cancer immunotherapy (CIT). Treatment-induced changes in intratumoral CD8+ T cells may represent a biomarker to identify patients responding to CIT. Here we have used a 89Zr-radiolabeled human CD8-specific minibody (89Zr-Df-IAB22M2C) to monitor CD8+ T cell tumor infiltrates by positron emission tomography (PET). The ability of this tracer to quantify CD8+ T cell tumor infiltrates was evaluated in preclinical studies following single agent treatment with FOLR1-T cell bispecific antibody and combination therapy of CEA-TCB (RG7802) and CEA-targeted 4-1BB agonist CEA-4-1BBL. In vitro cytotoxicity assays with PBMC and CEA-expressing MKN-45 gastric or FOLR1-expressing HeLa cervical cancer cells confirmed non-interference of the anti-CD8-PET-tracer with the mode of action of CEA-TCB/CEA-4-1BBL and FOLR1-TCB at relevant doses. In vivo, the extent of tumor regression induced by combination treatment with CEA-TCB/CEA-4-1BBL in MKN-45 tumor-bearing humanized mice correlated with intratumoral CD8+ T cell infiltration. This was detectable by 89Zr-IAB22M2C-PET and γ-counting. Similarly, single agent treatment with FOLR1-TCB induced strong CD8+ T-cell infiltration in HeLa tumors, where 89Zr-Df-IAB22M2C again was able to detect CD8 tumor infiltrates. CD8-immunohistochemistry confirmed the PET imaging results. Taken together, the anti-CD8-minibody 89Zr-Df-IAB22M2C revealed a high sensitivity for the detection of intratumoral CD8+ T cell infiltrates upon either single or combination treatment with T cell bispecific antibody-based fusion proteins. These results provide further evidence that the anti-CD8 tracer, which is currently in Clinical Phase II, is a promising monitoring tool for intratumoral CD8+ T cells in patients treated with CIT.