Synthesis and Preclinical Evaluation of Ga-68-labeled Adnectin as a PET Agent for Imaging PD-L1 Expression

Ziel/Aim Tumor cells exploit checkpoint pathways by expressing coinhibitory proteins, like PD-L1 to evade antitumor immune response. As recently demonstrated in first patients, F-18-BMS-986192 (F-18-Adnectin) provides a promising means for in vivo imaging and quantification of PD-L1 expression in tumors. The high tumor uptake of PD-L1 ligands suggests that PD-L1 may also be used as a theranostic target. As a first step for theranostic applications of radiolabeled PD-L1 ligands we evaluated biodistribution and tumor uptake of a Ga-68-Adnectin analogue.

Methodik/Methods Ga-68-labeling was carried out in NaOAc-buffer at pH 5.5 (50°C, 15min). PD-L1 binding assays were performed using the transduced PD-L1 expressing lymphoma cell line U-698M and wild-type U-698M cells as negative control. Biodistribution and small animal PET studies of Ga-68-Adnectin were carried out using PD-L1-positive and negative U-698M-bearing NSG mice.

Ergebnisse/Results Ga-68-Adnectin was obtained with quantitative RCYs (>97 %) with high RCP and in vitro stability in human serum was ≥ 95 % at 4h. High and specific binding of Ga-68-Adnectin to human PD-L1-expressing cancer cells was confirmed, which closely correlates with the respective PD-L1 expression level determined by flow cytometry and IHC staining. In vivo, Ga-68-Adnectin uptake was high in PD-L1+ tumors (9.0±2.1 %ID/g at 1hp.i.) and kidneys (56.9±9.2 % ID/g at 1hp.i.) with negligible uptake in other tissues. PD-L1 negative tumors demonstrated only background uptake of radioactivity (0.6±0.1 % ID/g). Co-injection of an excess of unlabeled Adnectin reduced tumor uptake of PD-L1 by more than 80 %.

Schlussfolgerungen/Conclusions Ga-68-Adnectin enables easy radiosynthesis and shows excellent in vitro and in vivo PD-L1 targeting characteristics. The high tumor uptake combined with low background accumulation at early imaging time points demonstrate the feasibility of Ga-68-Adnectin for imaging of PD-L1 expression in tumors and is encouraging for theranostic applications of PD-L1 ligands.