Exploration of Bicyclic Peptide Ligands for Immune-Specific PET Imaging: Targeting Tumor PD-L1 with [¹⁸F]AlF-BCY10959

As a new modality of ligands, bicyclic peptides hold great promise in the discovery of novel programmed death ligand 1 (PD-L1) targeted radiotracers, which have not yet been reported. In this study, first-in-class bicyclic peptide-based radiotracers [¹⁸F]AlF-BCY509 and [¹⁸F]AlF-BCY10959 were developed and evaluated for PET imaging of tumor PD-L1 expression. The automatic radiosynthesis was achieved with robust radiochemical yields (55.1-90.2%) and high molar activity (42.5-90.8 GBq/μmol). Cell-based assays demonstrated high specificity and affinity of [¹⁸F]AlF-BCY509 and [¹⁸F]AlF-BCY10959 with IC₅₀ values of 9.36 ± 1.35 and 7.12 ± 1.24 nM and <i>K</i>_D values of 11.41 ± 1.04 and 8.09 ± 0.85 nM. In PET imaging, the accumulation of [¹⁸F]AlF-BCY10959 in PD-L1-positive tumors with moderate retention over 120 min was discovered, with the tumor uptake of 14.74 ± 1.67%ID/cc and tumor-to-muscle ratio of 12.41 ± 1.07 at 30 min. The <i>in vivo</i> specificity was strictly verified by PD-L1-knockout and PD-L1-positive tumors with blocking. The biodistribution manifested a rapid distribution and fast clearance from the body, supporting the favorable pharmacokinetics of [¹⁸F]AlF-BCY10959. [¹⁸F]AlF-BCY10959 was excreted through the urinary and hepatobiliary systems, indicating the doomed radiation exposure organs. The effective doses of [¹⁸F]AlF-BCY10959 and [¹⁸F]-FDG were comparable, highlighting its safety for human use. In conclusion, [¹⁸F]AlF-BCY10959 provides an attractive option to detect PD-L1 expression and lays the groundwork to further develop promising bicyclic peptide tracers for clinical use.