Evaluating a Single Domain Antibody Targeting Human PD-L1 as a Nuclear Imaging and Therapeutic Agent

The PD-1:PD-L1 immune checkpoint axis is central in the escape of cancer cells from anticancer immune responses. Monoclonal antibodies (mAbs) specific for PD-L1 have been approved for treatment of various cancer types. Although PD-L1 blockade has proven its merit, there are still several aspects that require further attention to fully capitalize on its potential. One of these is the development of antigen-binding moieties that enable PD-L1 diagnosis and therapy. We generated human PD-L1 binding single domain antibodies (sdAbs) and selected sdAb K2, a sdAb with a high affinity for PD-L1, as a lead compound. SPECT/CT imaging in mice following intravenous injection of Technetium-99m (99mTc)-labeled sdAb K2 revealed high signal-to-noise ratios, strong ability to specifically detect PD-L1 in melanoma and breast tumors, and relatively low kidney retention, which is a unique property for radiolabeled sdAbs. We further showed using surface plasmon resonance that sdAb K2 binds to the same epitope on PD-L1 as the mAb avelumab, and antagonizes PD-1:PD-L1 interactions. Different human cell-based assays corroborated the PD-1:PD-L1 blocking activity, showing enhanced T-cell receptor signaling and tumor cell killing when PD-1POS T cells interacted with PD-L1POS tumor cells. Taken together, we present sdAb K2, which specifically binds to human PD-L1, as a new diagnostic and therapeutic agent in cancer management.This research was performed with the financial support of the Agency of Innovation by Science and Technology (IWT), the Melanoma Research Alliance, Research Foundation-Flanders (FWO-V), the "Stichting tegen Kanker", and "Kom op tegen Kanker". K.B. (Katrijn Broos) and Q.L. are funded via IWT/FWO PhD fellowships and "Emmanuel Vanderschueren" awards. M.K. is a senior clinical investigator of the Research Foundation-Flanders. J.B. is funded via the H2020-MSCA-ITN PET3D program. This study received a free supply of the mAb avelumab from Pfizer a part of the alliance between Merck KGaA and Pfizer, Inc, New York, NY, USA