Nivolumab is a human monoclonal antibody that is specific for programmed cell death-1 (PD-1), a negative regulator of T-cell activation and response. Acting as an immune checkpoint inhibitor, nivolumab binds to PD-1 expressed on the surface of many immune cells and prevents ligation by its natural ligands. Nivolumab is only effective in a subset of patients and there is limited data supporting its use for diagnostic, monitoring, or stratification purposes.
89Zr-Df-nivolumab was synthesized to map the biodistribution of PD-1-expressing tumor infiltrating T-cells in vivo using a humanized murine model of lung cancer. The tracer was developed by radiolabeling the antibody with the positron emitter zirconium-89 (89Zr). Imaging studies were validated by ex vivobiodistribution studies, and PD-1 expression was validated by immunohistochemistry studies. Data obtained from PET imaging were used to determine human dosimetry estimations.
The tracer showed high binding to stimulated PD-1 expressing T-cells both in vitro and in vivo. PET imaging of 89Zr-Df-nivolumab allowed for clear delineation of subcutaneous tumors through targeting of localized activated T-cells expressing PD-1 in the tumors and salivary glands of humanized A549 tumor-bearing mice. In addition to the tumors, salivary and lacrimal gland infiltration of T-cells was noticeably visible and confirmed via histological analysis.
These data support our claim that PD-1-targeted agents may allow for tumor imaging in vivo, which may assist in the design and development of new immunotherapies. In the future, noninvasive imaging of immunotherapy biomarkers may assist in disease diagnostics, disease monitoring, and patient stratification.