Concurrent injection of cold antibody allows PET imaging of orthotopic pancreatic tumor with ⁶⁴Cu-labeled anti-PD-L1

Objectives: Exciting progress has been made in cancer immunotherapy with immune checkpoint blockade antibodies. Noninvasive imaging of PD-1 and PD-L1 at high sensitivity and selectivity could be useful in cancer detection and monitoring treatment response. However, detection of pancreatic ductal adenocarcinoma (PDAC) tumor in the pancreas with immunoradiotracers has been challenging due to high uptakes of antibodies in the spleen and the liver. The purpose of this study is to investigate whether it is feasible to image orthotopic PDAC tumors by PET.

Methods: ⁶⁴Cu was labeled to anti-PD-L1, anti-PD-1, and their isotype-matched IgGs through chelator 1,4,7-Triazacyclononane-1,4,7-triacetic acid (NOTA). ⁶⁴Cu-labeled antibodies were intravenously injected to C57BL/6 mice bearing orthotopic KRAS[asterisk] PDAC tumors. Mice were dissected at 24 h post-injection, and biodistribution of the antibodies in blood, tumor and other organs were quantified. MicroPET/CT images were acquired using ⁶⁴Cu-NOTA-anti-PD-L1 with or without co-injection of cold anti-PD-L1. In vivo organ uptakes of anti-PD-L1 were analyzed by quantifying PET images (Fig C).

Results: Distribution pattern of anti-PD-1 was similar to that of its corresponding IgG control. In comparison, the blood activity of anti-PD-L1 was only 5% that of its IgG control. Tumor-to-blood ratio of anti-PD-L1 was more than 40 times higher than that of anti-PD-1 and IgG controls. The uptakes of anti-PD-L1 in the liver, spleen, brown adipose tissue, and lymph nodes were significantly higher than that of its IgG control (p< 0.05). MicroPET imaging with ⁶⁴Cu-anti-PD-L1 in mice bearing orthotopic KRAS[asterisk] tumors showed strong signals in the spleen and weak signals in the tumor (Fig A). However, by co-injection of the radiotracer with cold anti-PD-L1, the tumor was clearly visualized due to markedly reduced signals in the spleen and increased radioactivity in the tumor (Fig B). Ex vivo distribution in subcutaneous KRAS[asterisk] PDAC model revealed that blocking with cold antibody elevated tumor-to-spleen ratio from 0.85 to 4.5.

Conclusions: These data suggested that co-injection with cold anti-PD-L1 prevented entrapment of ⁶⁴Cu-anti-PD-L1 in the spleen, prolonged its blood retention, and thereby successfully led to its accumulation in KRAS[asterisk] tumor.

• Download figure
• Open in new tab
• Download powerpoint