Christopher G. England, Emily B. Ehlerding, Reinier Hernandez, Brian T. Rekoske, Stephen A. Graves, Haiyan Sun, Glenn Liu, Douglas G. McNeel, Todd E. Barnhart, Weibo Cai

Pembrolizumab is a humanized monoclonal antibody targeting programmed cell death protein 1 (PD-1) found on T and pro-B cells. Pembrolizumab prevents PD-1 ligation by both PD-L1 and PD-L2, preventing the immune dysregulation that otherwise occurs when T-cells encounter cells expressing these ligands. Clinically, PD-1 blockade elicits potent anti-tumor immune responses and antibodies blocking PD-1 ligation, including pembrolizumab, have recently received federal drug administration approval for the treatment of advanced melanoma, renal cell cancer, and non-small cell lung cancer. In this study, we evaluated the pharmacokinetics, biodistribution, and dosimetry of pembrolizumab in vivo, accomplished through radiolabeling with positron emitter zirconium-89 ((89)Zr). Positron emission tomography (PET) imaging was utilized to evaluate the whole-body distribution of (89)Zr-Df-Pembrolizumab in two rodent models (mice and rats). Data obtained from PET scans and biodistribution studies were extrapolated to humans to estimate the dosimetry of the tracer. As a proof-of-concept, the biodistribution of (89)Zr-Df-Pembrolizumab is further investigated in a humanized murine model. The tracer remained stable in blood circulation throughout the study and accumulated the greatest in liver and spleen tissues. Both mice and rats showed similar biodistribution and pharmacokinetics of (89)Zr-Df-Pembrolizumab. In the humanized mouse model, T-cell infiltration into the salivary and lacrimal glands could be successfully visualized. These data will augment our understanding of the pharmacokinetics and biodistribution of radiolabeled pembrolizumab in vivo, while providing detailed dosimetry data that may lead to better dosing strategies in the future. These findings further demonstrate the utility of noninvasive in vivo PET imaging to dynamically track T-cell checkpoint receptor expression and localization in a humanized mouse model.