89Zr-labeled CD8 cys-diabody for imaging immunotherapy response in GBM

276 Background: Clinical evaluation of immunotherapy treatment in glioblastoma multiforme (GBM) is impeded by the presence of apparent pseudoprogression as immune cells are recruited to the tumor area. This can lead to unnecessary cessation of treatment in patients who are responding, as well as unwarranted continuation of treatment in patients who are not responding. Evaluation of CD8+ T cell recruitment with a noninvasive imaging modality may allow for the discrimination of immune cell recruitment from inflammation and increased metabolic activity from tumor progression. Subsequently, a diabody developed to bind to CD8+ T cells was labeled with 89Zr for PET imaging of immune response to immunotherapy in murine models of glioblastoma multiforme (GBM). We utilized 89Zr-labeled CD8 diabody to detect immune response to oncolytic herpes simplex virus (oHSV) M002 immunotherapy in a syngeneic GBM model. Methods: Immunocompetent mice were implanted intracranially with syngeneic GBM tumors. Immune response was stimulated with intratumoral injection of oHSV M002. Mice were imaged with PET using 89Zr-labeled CD8 diabody at baseline and seven days post oHSV administration. Ex vivo tissue analyses including autoradiography and immunohistochemistry were used to correlate imaging and immune cell recruitment patterns. Results: PET imaging showed specific localization of the radiolabeled diabody in GBM tumors and lymphoid tissues. Accumulation in target areas, especially in lymph nodes, was significantly reduced by blocking with the non-labeled diabody (p<0.05). oHSV treatment resulted in increased PET signal and CD8+ cell recruitment in tumors and cervical lymph nodes compared to non-treated controls. Radioactive probe accumulation in brains correlated with GBM growth and CD8+ cell trafficking patterns. Conclusions: PET imaging allowed non-invasive assessment of CD8+ cell response to oHSV in syngeneic GBM models. This strategy could aid in distinguishing responders from non-responders during immunotherapy of GBM.