Programmed death-1 (PD-1) and programmed death ligand-1 (PD-L1), the research focus in immune checkpoint regulation, play an important role in tumor immunotherapy. Inhibitors of this pathway are also the focus of tumor immunotherapy research. The PD-1/PD-L1 pathway can be blocked by selective binding to PD-L1. Clinical trials have been conducted in a variety of patients with advanced solid tumors. CS1001 is a high-affinity humanized full-length anti-PD-L1 monoclonal antibody with great clinical significance. We constructed a PD-L1-targeted radioactive molecular probe, 124/125I-labeled full-length antibody CS1001, and evaluated its binding specificity and targeting ability to PD-L1 in tumor cells and tumor models. Additionally, a comparison study with 68Ga-WL12, a PD-L1 targeting peptide, was conducted. The binding potency of 125I-CS1001 to human PD-L1 was evaluated by enzyme-linked immunosorbent assay (ELISA), and the Kd value was 52.1 ± 19.3 nM. The cellular uptake of 125I-CS1001 was examined in Chinese hamster ovary cells (CHO) and CHO expressing human PD-L1 (CHO-hPD-L1). At 2 h, the uptake values of 125I-CS1001 in CHO-hPD-L1 without blocking and in the presence of 0.1 mg non-radiolabeled CS1001 were 3.60 ± 0.08 and 0.09 ± 0.005 (%AD/2 × 105 cells, p < 0.001). Micro-PET imaging was performed between 8 to 192 h after injection of 124I-CS1001 into normal KM mice and CHO-hPD-L1 and HeLa tumor models. The standard uptake value (SUV) of relevant organs in PET images was calculated by drawing regions of interest (ROI). SUVmean of CHO-hPD-L1 tumors was significantly higher than that of HeLa tumors at 48 h (1.98 ± 0.04 vs 0.73 ± 0.14, p = 0.005). The SUVmean of 124I-CS1001 in CHO-hPD-L1 tumors at 48 h was higher than that of 68Ga-WL12 in CHO-hPD-L1 tumors at 0.5 h (1.98 ± 0.04 vs 1.09 ± 0.1 SUVmean, p = 0.007). In conclusion, this work provides a new method for monitoring and evaluating the in vivo expression of PD-L1 in tumors.
Keywords: PD-L1; PET imaging; iodine isotope; monoclonal antibody.