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Abstract 3910: Targeting urokinase plasminogen activator: evaluation of activity-based imaging probes in an orthotopic breast cancer model.

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. For the screening of uPA in breast tumors with commercially available ELISA kits that determine the total uPA content (active and inactive), fresh tumor tissue is required. Molecular imaging with activity-based uPA probes might overcome this limitation. Therefore we have developed some highly selective, non-peptidic and specific uPA activity-based probes based on the irreversible uPA inhibitor UAMC-00150. In this study we describe the evaluation of a fluorescent Cy5 labeled and a 18F labeled PET probe in the high uPA expressing MDA-MB-231 breast cancer mouse model to make the step towards clinical translation of this imaging biomarker. Orthotopic MDA-MB-231 tumors were established in nude mice and subjected to uPA imaging at a tumor volume of 150mm3. For the fluorescent imaging, three test groups were considered (n=5): mice from the first and the second experimental group received a single dose of equimolar amounts (0.23 μMol/kg) Cy5-uPA-probe or its inactivated hydrolyzed variant, respectively. The third group received an injection with unlabeled uPA inhibitor, 30 min before administration of Cy5-uPA probe, in a concentration that was 70-fold higher than the probe concentration. Mice were imaged at different time points for 48 h post injection (p.i.) and were subsequently sacrificed for ex vivo imaging and histology. PET imaging (n=15) was performed after i.v. injection of the radiotracer with a maximum of 0,2ml and/or 0,5mCi per injection, resulting in a range of 0,15-0,5mCi/injection. Animals were scanned at 15, 45, 90, 240 and 360 minutes p.i. At each time point, 3 mice were sacrificed to generate ex vivo biodistribution data. The Cy5-uPA-probe demonstrated good tumor-targeting properties in the high uPA-expressing breast tumor model, with fluorescent intensities reaching a maximum at 24 h post-probe administration in mice treated with Cy5-uPA-probe alone. The groups treated with the inactivated probe or the unlabeled inhibitor showed a significant decrease in the fluorescent tumor signal (p>0,016), which was also confirmed on histology. PET imaging with the 18F-uPA-probe showed a peak tumor uptake of 2.76 ± 0.37 %ID/g at 4 h p.i. Further in vitro PPB and ex vivo HPLC data revealed a high affinity of the tracer for blood proteins and a high metabolisation rate potentially explaining the rather moderate to low tumor uptake of the current PET uPa biomarker. In conclusion, fluorescent imaging data clearly indicate that the Cy5-uPA-probe enables non-invasive NIR-imaging of uPA expression in tumors in vivo. The first PET experiments indicate translational capabilities, for which we are now improving the pharmacokinetics of this and future uPa radiotracers. Citation Format: Johan Ides, Christel Vangestel, Jonas Messagie, Dieter Verzele, David Thomae, Sofie Thys, An Wouters, John-Paul Bogers, Sigrid Stroobants, Jurgen Joossens, Pieter Van der Veken, Marc Peeters, Filip Lardon, Steven Staelens, Koen Augustyns. Targeting urokinase plasminogen activator: evaluation of activity-based imaging probes in an orthotopic breast cancer model. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3910. doi:10.1158/1538-7445.AM2013-3910