PD-1 is expressed on the suface of T, B and NK cells and is responsible for T cell suppression. PD-1, may interact with one of its two ligands PD-L 1 and PD-L 2 which are found on the surface of antigen-presenting cells and tumors. There are currently two FDA-approved monoclonal antibodies targeting PD-1 – pembrolizumab and nivolumab.

PD-1 expression on tumor cells may predict response to checkpoint blockade therapy, but tissue samples are not always on hand to guide therapy. Imaging specialists have addressed this issue by developing and evaluating techniques for non-invasive imaging of PD-1 in tumor.

This method, by using radiotracers, provides an elegant opportunity to obtain quantitative and kinetic information on the whole-body biodistribution of these antibodies, including parameters such as tumor accumulation and blood half-life.

In another study scientists used quantitative FDG-PET at baseline and again after 6 months of treatment with atezolizumab (antibody against PD-L1) in NSCLS patients. The researchers noted that the utility of FDG-PET to distinguish pseudo from true progression of tumor could not be determined. But the research showed that higher baseline tumor volumes of FDG and a further increase in tumor volume at 6 weeks were predictive of reduced patient survival.