Anti‐human pd‐l1 nanobody for immuno‐pet imaging: Validation of a conjugation strategy for clinical translation

Abstract

Immune checkpoints, such as programmed death‐ligand 1 (PD‐L1), limit T‐cell function and tumor cells use this ligand to escape the anti‐tumor immune response. Treatments with monoclonal antibodies blocking these checkpoints have shown long‐lasting responses, but only in a subset of patients. This study aims to develop a Nanobody (Nb)‐based probe in order to assess human PD‐L1 (hPD‐L1) expression using positron emission tomography imaging, and to compare the influence of two different radiolabeling strategies, since the Nb has a lysine in its complementarity determining region (CDR), which may impact its affinity upon functionalization. The Nb has been conjugated with the NOTA chelator site‐specifically via the Sortase‐A enzyme or randomly on its lysines. [68Ga]Ga‐NOTA‐(hPD‐L1) Nbs were obtained in >95% radiochemical purity. In vivo tumor targeting studies at 1 h 20 post‐injection revealed specific tumor uptake of 1.89 ± 0.40%IA/g for the site‐specific conjugate, 1.77 ± 0.29%IA/g for the random conjugate, no nonspecific organ targeting, and excretion via the kidneys and bladder. Both strategies allowed for easily obtaining68Ga‐labeled hPD‐L1 Nbs in high yields. The two conjugates were stable and showed excellent in vivo targeting. Moreover, we proved that the random lysine‐conjugation is a valid strategy for clinical translation of the hPD‐L1 Nb, despite the lysine present in the CDR.