Abstract 1129: Monitoring intratumoral CD8 T cell infiltrates in human stem cell engrafted mice during single agent and combination immunotherapy with T cell bispecific antibodies using the human PET-tracer 89Zr-Df-IAB22M2C (2019)

Abstract A persisting challenge in cancer immunotherapies (CIT) is the determination of patients responding early during therapy. As major effector cells, CD8 cytotoxic T cells are a primary target of many CIT approaches. ImaginAb is developing a human CD8 specific minibody 89Zr-Df-IAB22M2C to monitor CD8 tumor infiltrates non-invasively, longitudinally and throughout the whole body using positron emission tomography (PET). We evaluated pre-clinically the capability of this tracer to quantify CD8 tumor infiltrates upon treatment with T cell activating molecules using both, single agent FOLR1-T cell bispecific (TCB) antibody and the combination of CEA-TCB with a CEA-targeted 4-1BBL (CEA-4-1BBL). In the first study, HeLa-cervical cancer xenograft bearing CD34+ human stem cell engrafted NSG mice (HSC-NSG) were treated for 2 weeks with FOLR1-TCB (2 mg/kg, once weekly), an untargeted control DP47-TCB and vehicle followed by PET imaging with the CD8 specific 89Zr-Df-IAB22M2C tracer (90-95 µCi/10 µg). In the second study, MKN-45 gastric cancer bearing HSC-NSG mice were treated with CEA-TCB (2.5 mg/kg, 2x/week), CEA 4-1BBL (3.0 mg/kg, 1x/week) or the combination of both compounds for two weeks followed by PET imaging with the CD8 tracer (50-60 µCi/10 µg). Ex vivo γ-counting of tumors and organs of interest as well as CD8 IHC was performed for both studies. Treatment with FOLR1-TCB induced the highest tumor regression and strongest CD8+ T cell infiltrates. 89Zr-Df-IAB22M2C was able to detect tumor infiltrates by PET and γ-counting (13.6±2.7 %ID/g), whereas treatment with untargeted DP47-TCB induced a low T cell infiltrate (8.7±3.2 %ID/g) as compared to the vehicle group (5.6±1.2 %ID/g). In the second study, combined treatment with CEA-TCB and CEA-4-1BBL induced the highest tumor regression accompanied by the highest intratumoral CD8 T cell infiltrates, which were detectable by PET and γ-counting (8.95±2.47 %ID/g). Single agent CEA-TCB (6.91±1.61 %ID/g) and CEA-4-1BBL (6.24±1.11 %ID/g) showed higher signals compared to vehicle (4.87±1.18 %ID/g). The differences in CD8 infiltrate upon single and combination treatment was confirmed by IHC. The 89Zr-Df-IAB22M2 PET-tracer proved to be highly sensitive for the detection of intratumoral CD8 T cell infiltrates upon single and combination treatment with T cell activating compounds. These data correlated with CD8 IHC. These results provide further evidence that the CD8 imaging probe, which is currently in Phase II clinical testing (NCT03107663), may be a promising tool for the monitoring of CD8 T cells in patients treated with immunotherapies. Citation Format: Christoph M. Griessinger, Alessandro Mascioni, Fang Jia, Michael Torgov, Tapan Nayak, Preethi Latha Bhavani Mohan, Valeria G. Nicolini, Johannes Sam, Christina Claus, Claudia Ferrara-Koller, Marina Bacac, Pablo Umana, Ian Wilson, Christian Klein, Jean J. Tessier. Monitoring intratumoral CD8 T cell infiltrates in human stem cell engrafted mice during single agent and combination immunotherapy with T cell bispecific antibodies using the human PET-tracer 89Zr-Df-IAB22M2C [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1129.