There is increasing evidence that antibodies blocking the PD-1/PD-L1 checkpoint (either anti-PD-1 or anti-PD-L1) increase in-field anti-tumor responses to ionizing radiation and enhance abscopal effects on non-irradiated metastases. Here, we developed PET tracers based on therapeutic antibodies to visualize whole-body expression of PD-1 and PD-L1 in mice and the biodistribution of the surrogate checkpoint-blocking antibodies.
Two novel PET tracers were developed based on anti-PD-1 and anti-PD-L1 checkpoint-blocking antibodies. Non-invasive PET imaging was performed on naïve and tumor-bearing mice. Mice bearing s.c. B16 melanomas were treated with hypofractionated radiation therapy (hRT) in combination with CTLA-4 checkpoint blockade before PET imaging. PD-1 or PD-L1 knockout mice and PD-L1-deficient B16 cells generated using the CRISPR/Cas technology served as specificity controls.
The newly developed PET tracers allowed the highly specific and high-resolution imaging of PD-1 and PD-L1 expression. In addition, they permitted the noninvasive imaging of the biodistribution of the two therapeutic antibodies in both naïve and tumor-bearing mice treated with hRT and CTLA-4 checkpoint blockade. Imaging of the respective knockout mice, blocking experiments with an excess amount of unlabeled antibodies, and the analysis of animals bearing both wild-type B16 melanomas and PD-L1-CRISPR knockout melanomas demonstrated the high specificity of the two newly developed PET tracers. The in vivo imaging data were confirmed by ex vivo biodistribution analyses. The targets of the PET tracer antibodies were verified by ex vivo flow cytometric analyses. Visualization of immune-related adverse events was also possible.
In conclusion, we have developed two innovative PET tracers that allow imaging the expression of the receptor/ligand pair of the important PD-1/PD-L1 checkpoint and the biodistribution of surrogate checkpoint-blocking antibodies in fully immunocompetent mice. This technology also enables whole-body pictures of combination radio/immunotherapies.
Legal entity responsible for the study
Animal care committee Freiburg
All authors have declared no conflicts of interest.