Anti-PD-L1 Antibody Imaging, Biodistribution, and Dosimetry

The programmed cell death ligand 1 (PD-L1) participates in an immune checkpoint system involved in preventing autoimmunity. PD-L1 is expressed on tumor cells, tumor-associated macrophages, and other cells in the tumor microenvironment. Anti–PD-L1 antibodies are active against a variety of cancers, and combined anti–PD-L1 therapy with external beam radiotherapy has been shown to increase therapeutic efficacy. PD-L1 expression status is an important indicator of prognosis and therapy responsiveness, but methods to precisely capture the dynamics of PD-L1 expression in the tumor microenvironment are still limited. In this study, we developed a murine anti–PD-L1 antibody conjugated to the radionuclide Indium-111 (111In) for imaging and biodistribution studies in an immune-intact mouse model of breast cancer. The distribution of 111In-DTPA-anti-PD-L1 in tumors as well as the spleen, liver, thymus, heart, and lungs peaked 72 hours after injection. Coinjection of labeled and 100-fold unlabeled antibody significantly reduced spleen uptake at 24 hours, indicating that an excess of unlabeled antibody effectively blocked PD-L1 sites in the spleen, thus shifting the concentration of 111In-DTPA-anti-PD-L1 into the blood stream and potentially increasing tumor uptake. Clearance of 111In-DTPA-anti-PD-L1 from all organs occurred at 144 hours. Moreover, dosimetry calculations revealed that radionuclide-labeled anti–PD-L1 antibody yielded tolerable projected marrow doses, further supporting its use for radiopharmaceutical therapy. Taken together, these studies demonstrate the feasibility of using anti–PD-L1 antibody for radionuclide imaging and radioimmunotherapy and highlight a new opportunity to optimize and monitor the efficacy of immune checkpoint inhibition therapy. Cancer Res; 76(2); 472–9. ©2015 AACR.

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