Several strategies have recently shown convincing clinical efficacy by stimulating adaptive immunity for the treatment of cancer. Immune checkpoint inhibitors (CpIs), for example, reactivate a patient's own T cells to eliminate tumors by blocking interactions that cause tumor infiltrating lymphocytes (TILs) to become exhausted. CpIs have shown significant efficacy in metastatic melanoma, and are in testing for the treatment of numerous other tumor types, either alone or in combination. Expression of the cell surface marker PD-L1, either on TILS at the tumor margin or on the tumor itself, appears to be correlated with clinical efficacy in CpI therapy, identifying it as a potential biomarker for patiet stratification and management. The current standard for assessment of this biomarker entails biopsy of primary and metastatic tumor foci, followed by immunohistochemical characterization. Limitations of biopsy include low patient and physician acceptance due to its invasiveness and the difficulty of accessing tumors at many anatomical locations, and sampling bias related to the heterogeneous distribution of markers both within and between tumor foci, increasing the likelihood of false negative results. There is an urgent need for non-invasive imaging techniques that allow visualization of biomarker distribution on the entire surface of primary and metastatic tumor foci at diverse anatomical locations. 121 Bio is uniquely positioned to develop such a non-invasive imaging approach. We are developing a positron emission tomography (PET) agent that can non-invasively image PD-L1. Our product uses site-specific conjugation for controlled and reproducible installation of a radio-isotopic label on single domain antibodies (sdAbs). Due to its small size, the resulting imaging agents show high tissue uptake and rapid renal clearance. This design allows an exceptionally high PET signal to noise ratio, even at low levels of antigen abundance and within 2 hours of injection, enabling same day imaging at high resolution. Our mouse proof of concept experiments using radiolabelled sdAbs to specific immune cell and tumor markers shows exceptionally high resolution imaging of lymph nodes and tumors. 121 Bio has assembled a highly qualified scientific team with broad experience in protein engineering, antibody discovery and development, tumor immunology and nuclear medicine. To supplement internal capabilities, we have gained access to additional outside facilities and have recruited a panel of expert tumor immunology and PET imaging consultants. We propose here to prepare PD-L1-reactive sdAbs radiolabeled with either 89Zr or 18F. The resulting imaging data will guide our selection of the optimal radiolabel for development of our radiodiagnostic product. We will also formally demonstrate that human tumors can be visualized using our lead candidate antibody against PD-L1. Finally, we will humanize the sdAb in order to reduce potential immunogenicity. This anti-PD-L1 imaging agent offers the potential to be the first non-invasive imaging agent to be used in selection and management of patients undergoing cancer immunotherapy.