Synthesis and in vivo preclinical evaluation of an (18)F labeled uPA inhibitor as a potential PET imaging agent

The urokinase plasminogen activator (uPA) system is a proteolytic cascade involved in tumor invasion and metastasis. uPA and its inhibitor PAI-1 are described as biomarkers for breast cancer with the highest level of evidence. The present study describes the synthesis and first in vivo application of an activity based uPA PET probe.

Based on the design of a small irreversible and selective uPA inhibitor we developed an (18)F-labeled activity based probe for uPA imaging. Human uPA expressing MDA-MB-231-luc2-GFP cells were inoculated in the mammary fat pads of nude mice and treated with the probe once tumors reached a volume of 150mm(3). Scans were performed at 0.25, 0.75, 1.5, 4 and 6h post injection. To evaluate tumor uptake in vivo and ex vivo data were gathered. Biodistribution data of the organs and tissues of interest were collected at all time points. Due to a relatively low tumor uptake, probe stability was further evaluated.

The uPA targeting PET tracer was produced in high purity and with good specific radioactivity. In vivo PET data showed a maximum tumor uptake of 2,51±0,32 %ID/g at 4h p.i. A significant correlation between in vivo and ex vivo tumor uptake calculation was found (R=0.75; p<0.01). Due to a high blood signal at all time points, probe stability was further examined revealing high plasma protein binding and low plasma stability.

In vivo and ex vivo results clearly demonstrate that uPA expressing tumors can be detected with non-invasive PET imaging. Stability tests suggest that further optimization is needed to provide a better tumor-to-background contrast.