Abstract LB-188: Sensitivity of89Zr-labeled anti-CD8 minibody for PET imaging of infiltrating CD8+ T cells

Background- The ability to monitor CD8 positive tumor infiltrating lymphocytes (TILs) in vivo is important for evaluating response to immunotherapies and assisting in the development of more effective immune cell targeted single and combination therapies. “ImmunoPET” imaging of tumor infiltrating T cells can provide a specific and sensitive modality to aid selection of patients for specific immunotherapy regimens and determine whether the therapy is working. Here, we report initial results to define the number of CD8+ T cells that can be detected with 89 Zr-Df-IAB22M2C, an anti-CD8 immunoPET probe, using different animal models Methods- IAB22M2C, a humanized anti-CD8 minibody, was conjugated with desferrioxamine (Df) and radiolabeled with 89 Zr. NOD scid mice were implanted with varying ratios of CD8+ T and tumor cell admixtures either intramuscularly (IM) without Matrigel or subcutaneously (SC) with Matrigel. One or six days later, CD8+ T-cells were visualized with 89 Zr-Df-IAB22M2C. The same probe was used to detect CD8+ T cells in NSG TM mice engrafted with human PBMCs for 1 and 4 weeks to monitor the temporal progression of Graft versus Host Disease (GvHD). Results - CD8+ T cells implanted in the muscles of mice were imaged one day later and SC implanted Matrigel plugs imaged 6 days later. Both approaches yielded similar results and indicated that the lower limit of detection was between 1.6 and 4 million CD8+ T cells in a volume of ∼480 mm 3 in the presence of normal tissue background activity. The sensitivity of detection increased 10-fold when ex vivo radiolabeled CD8+ T cells were implanted SC with Matrigel. NSG TM mice engrafted with human PBMCs provide a reliable model for xenogeneic T cell driven Graft versus Host Disease (GvHD). Human CD8 + T cells were readily detectable in the spleens of mice 1 week post PBMC engraftment using 89 Zr-Df-IAB22M2C. As GvHD progressed 4 weeks later, expansion and trafficking of the engrafted T cells to extra-lymphoid tissues including lungs could be followed. Terminal biodistribution showed a 2-3 fold increase in radioactivity uptake in lungs by week 4 post-engraftment; a result that was confirmed by IHC analysis. T cell enumeration and IHC analyses are in progress to further define the sensitivity range using an optimal dose and specific activity of 89 Zr-Df-IAB22M2C. Conclusion- These studies show that the lower limit of CD8+ T cell detection by 89 Zr-Df-IAB22M2C is between 1.6-4.0 million cells in the presence of normal tissue background activity and that the probe can be used to monitor CD8+ T cell trafficking in a GvHD model in vivo. 89 Zr-Df-IAB22M2C has sensitivity properties that may enable the detection of CD8+ T cells in human tumors. Clinical trials with 89 Zr-Df-IAB22M2C in melanoma patients will commence later this year. Citation Format: Tove Olafsen, Ziyue Karen Jiang, Jason Romero, Charles Zamilpa, Filippo Marchioni, Green Zhang, Michael Torgov, Daulet Satpayev, Jean M. Gudas. Sensitivity of 89 Zr-labeled anti-CD8 minibody for PET imaging of infiltrating CD8+ T cells. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr LB-188.