Preparation and Bioevaluation of ¹⁸F-Labeled Small-Molecular Radiotracers via Sulfur(VI) Fluoride Exchange Chemistry for Imaging of Programmed Cell Death Protein Ligand 1 Expression in Tumors

Nowadays,one of the most effective methods of tumor immunotherapyis blocking programmed cell death protein 1/programmed cell deathprotein ligand 1 (PD-1/PD-L1) immune checkpoints. However, there isstill a significant challenge in selecting patients to benefit fromimmune checkpoint therapies. Positron emission tomography (PET), anoninvasive molecular imaging technique, offers a new approach toaccurately detect PD-L1 expression and allows for a better predictionof response to PD-1/PD-L1 target immunotherapy. Here, we designedand synthesized a novel group of aryl fluorosulfate-containing small-moleculecompounds (LGSu-1, LGSu-2, LGSu-3, and LGSu-4) based on the phenoxymethyl-biphenyl scaffold. Afterscreening by the time-resolved fluorescence resonance energy transfer(TR-FRET) assay, the most potent compound LGSu-1 (half maximal inhibitory concentration (IC50):15.53 nM) and the low-affinity compound LGSu-2 (IC50: 189.70 nM) as a control were selected for F-18-radiolabeling by sulfur(VI) fluoride exchange chemistry(SuFEx) to use for PET imaging. [F-18]LGSu-1 and [F-18]LGSu-2 wereprepared by a one-step radiofluorination reaction in over 85% radioconversionand nearly 30% radiochemical yield. In B16-F10 melanoma cellassays, [F-18]LGSu-1 (5.00 +/- 0.06%AD) showed higher cellular uptake than [F-18]LGSu-2 (2.55 +/- 0.04%AD), in which cell uptakecould be significantly blocked by the nonradioactivity LGSu-1. In vivo experiments, micro-PETimaging of B16-F10 tumor-bearing mice and radiographic autoradiographyof tumor sections showed that [F-18]LGSu-1 was more effectively accumulated in the tumor due to thehigher binding affinity with PD-L1. The above experimental resultsconfirmed the potential of the small-molecule probe LGSu-1 as a targeting PD-L1 imaging tracer in tumor tissues.