Synthesis and evaluation of ¹⁸F-labeled small-molecular radiotracers as PET agents for imaging PD-L1 expression

Programmed cell death protein 1/programmed cell death protein ligand 1 (PD-1/PD-L1) based immune checkpoint inhibitors have attracted widespread attention. Since only few patients benefit from this treatment, it is significant to screen patients by measuring PD-L1 expression levels. In this study, two small-molecule radiotracers were developed to assess PD-L1 expression in tumors.

[¹⁸F]LGA-1 and [¹⁸F]LGA-2 were designed based on the biphenyl scaffold. The stability of these tracers was detected by radioactive high performance liquid chromatography (radio-HPLC). Cellular experiments were carried out with mouse melanoma B16-F10 cells to assess their targeting specificity in vitro. The ability to detect PD-L1 expression in vivo of these tracers were investigated via positron emission tomography (PET) imaging in B16-F10 tumor models.

[¹⁸F]LGA-1 and [¹⁸F]LGA-2 were synthesized by introducing one or three glycine groups as a hydrophilic linker onto the phenoxymethyl-biphenyl scaffold. They had high radiochemical purity (>98.0%) with radiochemical yields of 15.87 ± 5.66% for [¹⁸F]LGA-1 and 12.63 ± 1.36% for [¹⁸F]LGA-2. The two tracers showed excellent stability in PBS (pH 7.4) and mouse serum after 3 h incubation. In vitro cellular experiments demonstrated that [¹⁸F]LGA-1 (5.55 ± 0.51 %AD) showed higher cellular uptake than [¹⁸F]LGA-2 (3.55 ± 0.30 %AD) at 2 h and could be blocked by corresponding non-radioactive compounds. Meanwhile, compared to [¹⁸F]LGA-2, [¹⁸F]LGA-1 displayed the better tumor imaging in vivo PET imaging studies.

Between the two radiotracers, [¹⁸F]LGA-1 showed superior PD-L1 imaging effect in tumors. The design rationale presented herein offers a reference for the future development of PD-L1 small-molecule PET probes.