Immuno-PET Monitoring of Lymphocytes Using the CD8-Specific Antibody REGN5054

Assessment of immune-cell subsets within the tumor immune microenvironment is a powerful approach to better understand cancer immunotherapy responses. However, the use of biopsies to assess the tumor immune microenvironment poses challenges, including the potential for sampling error, restricted sampling over time, and inaccessibility of some tissues/organs, as well as the fact that single biopsy analyses do not reflect discordance across mul-tiple intrapatient tumor lesions. Immuno-positron emission tomography (PET) presents a promising translational imaging approach to address the limitations and assess changes in the tumor microenvironment. We have developed 89Zr-DFO-REGN5054, a fully human CD8A-specific antibody conjugate, to assess CD8 thorn tumor-infiltrating lymphocytes (TIL) pre-and posttherapy. We used multiple assays, including in vitro T-cell activation, prolifer-ation, and cytokine production, and in vivo viral clearance and CD8 receptor occupancy, to demonstrate that REGN5054 has minimal impact on T-cell activity. Preclinical immuno-PET studies demon-strated that 89Zr-DFO-REGN5054 specifically detected CD8 thorn T cells in lymphoid tissues of CD8-genetically humanized immuno-competent mice (VelociT mice) and discerned therapy-induced changes in CD8 thorn TILs in two models of response to a CD20xCD3 T-cell activating bispecific antibody (REGN1979, odronextamab). Toxicology studies in cynomolgus monkeys showed no overt toxicity, and immuno-PET imaging in cynomolgus monkeys dem-onstrated dose-dependent clearance and specific targeting to lym-phoid tissues. This work supports the clinical investigation of 89Zr-DFO-REGN5054 to monitor T-cell responses in patients undergoing cancer immunotherapy.