Effects of PEGylation on Imaging Contrast of 68Ga-Labeled Bicyclic Peptide PET Probes Targeting Nectin-4.

Nectin cell adhesion molecule 4 (Nectin-4) is overexpressed in various malignant tumors and has emerged as a promising target for tumor imaging. Bicyclic peptides, known for their conformational rigidity, metabolic stability, and membrane permeability, are ideal tracers for positron emission tomography (PET) imaging. In this study, we evaluated the feasibility of visualizing Nectin-4-positive tumors using radiolabeled bicyclic peptide derivatives and optimized the pharmacokinetics of radiotracers by introducing PEG chains of different lengths. Five PEGylated radiotracers radiolabeled with ⁶⁸Ga³⁺ exhibited high radiochemical purity and stability. As the chain length increased, the Log D values decreased from -2.32 ± 0.13 to -2.50 ± 0.16, indicating a gradual increase in the hydrophilicity of the radiotracers. In vitro cell-binding assay results showed that the PEGylated bicyclic peptide exhibits nanomolar affinity, and blocking experiments confirmed the specific binding of the tracers to the Nectin-4 receptor. In vivo PET imaging and biodistribution studies in SW780 and 5637 xenograft mice showed that [⁶⁸Ga]Ga-NOTA-PEG₁₂-BP demonstrated optimal pharmacokinetics, characterized by rapid and good tumor uptake, faster background clearance, and improved tumor-to-tissue contrast. Finally, compared with ¹⁸F-FDG, PET imaging, in vivo blocking assays of [⁶⁸Ga]Ga-NOTA-PEG₁₂-BP and histological staining confirmed that specific tumor uptake was mediated by Nectin-4 receptors. The results indicated that [⁶⁸Ga]Ga-NOTA-PEG₁₂-BP was a promising PET radiotracer for Nectin-4 targeting, with applications for clinical translation.