In vivo positron emission tomography imaging for PD-L1 expression in cancer using aptamer.

The programmed death-1 (PD-1) receptor is an immunosuppressive receptor expressed on activated T-cells that elicits an inhibitory signal upon the engagement of its ligand, which is the programmed death ligand 1 (PD-L1). Recent studies have shown that PD-1/PD-L1 blockade can enhance endogenous antitumor immunity. Thus, the PD-1/PD-L1 axis may be a potential therapeutic target for cancer immunotherapy. Aptamers are oligonucleotides with high specificity and affinity for target molecules and promising candidates for molecular imaging and targeted therapy. ⁶⁸ Ga is an attractive radionuclide that serves as a low-cost alternative to cyclotron-produced positron emission tomography (PET) radionuclides. In this study, we developed a ⁶⁸ Ga-labeled PD-L1 aptamer and investigated its target specificity and utility for in vivo PET scanning. In the first part of our study, we evaluated the binding affinity of three PD-L1 aptamers in PD-L1-positive (H1975 and B16F10) and negative (A549 and HT-29) tumor cells by flow cytometry and confocal microscopy. Optical imaging studies of PD-L1 aptamers were performed in H1975 tumor-bearing mice, and the aptamer with the highest binding affinity to PD-L1 positive tumors was selected. PD-L1 aptamers were radiolabeled with ⁶⁸ Ga. PET was performed for in vivo imaging of the ⁶⁸ Ga-NOTA-PD-L1 aptamer in H1975 tumor-bearing mice (PD-L1-positive cells) and A549 tumor-bearing mice (PD-L1-negative cells). Flow cytometry and confocal microscopy showed that PD-L1 aptamers had strong binding to PD-L1-positive H1975 and B16F10 cells. In contrast, PD-L1-negative A549 and HT-29 cells showed low binding to PD-L1 aptamers. Optical imaging studies of H1975 tumor-bearing mice showed the highest uptake of the 2198-06-07 PD-L1 aptamer. PET of ⁶⁸ Ga-NOTA-PD-L1 aptamers demonstrated increased uptake into PD-L1-positive H1975 tumors compared with PD-L1-negative A549 tumors. We confirmed that ⁶⁸ Ga-NOTA-PD-L1 aptamers facilitated the visualization of PD-L1 expression by in vivo PET scanning. These data suggest that ⁶⁸ Ga-NOTA-PD-L1 aptamers could potentially act as tracers for imaging for PD-L1-positive cancers.
Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Won Jun Kang reports financial support was provided by Korea Ministry of Science and ICT.
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