Construction and Preclinical Evaluation of a 124/125 I-Labeled Specific Antibody Targeting PD-L2 in Lung Cancer.

Programmed cell death-ligand 2 (PD-L2) is an important emerging molecule of the immune checkpoint, which is closely related to the prognosis of patients with immune checkpoint inhibitor (ICI) therapy. The quantification of PD-L2 can provide a potential reference for patients who benefit from ICI treatment. In this study, we used iodine isotope ( nat/124/125 I)-labeled PD-L2 antibody (ATL2) to noninvasively detect PD-L2 expression in mice with human lung adenocarcinoma A549 cell lines. The radiochemical yields of 125 I-ATL2 and 124 I-ATL2 were 73.56 ± 3.72% and 69.46 ± 2.05%, respectively. The radiochemical purity (RCP) of the tracers was more than 99%. The positive cell line A549-PDL2 was constructed by lentivirus. Western blot, immunofluorescence, and flow cytometry indicated that the A549-PDL2 cells showed a higher PD-L2 protein level than the A549 cells. The dissociation constant of 125 I-ATL2 binding to the PD-L2 protein was 7.25 nM. Cellular uptake experiments confirmed that the uptake of 125 I-ATL2 in A549-PDL2 cells was higher than that in A549 cells at each time point ( P < 0.0001). Micro-PET/CT showed significant uptake in the tumor region of A549-PDL2 tumor-bearing mice 24 h postinjection of 124 I-ATL2 compared with that of other groups (SUV max = 0.75 ± 0.06, 0.19 ± 0.07, and 0.27 ± 0.05, respectively). Consistently, the biodistribution of the tracers at 24 h postinjection showed a higher tumor uptake in A549-PDL2 mice (7.11 ± 0.38 %ID/g for 124 I-ATL2 in A549-PDL2 mice vs 2.72 ± 0.15 %ID/g for 124 I-ATL2 in A549 mice vs 3.89 ± 0.65 %ID/g for 124 I-IgG in A549-PDL2 mice). The dosimetry estimation by using Olinda software showed that the effective dose of 124 I-ATL2 was 3.62 × 10 -2 mSv/MBq, which is within the range of acceptable doses. Immunohistochemical results further confirmed that the expression of PD-L2 in the tumor tissues of A549-PDL2-bearing mice was higher than that of the A549 model mice. In conclusion, the development of 124/125 I-ATL2 provides the first noninvasive quantification of PD-L2 expression in lung cancer by molecular imaging, which provides a new reference for screening potential beneficiaries of ICI therapy.