Urokinase-Type Plasminogen Activator Receptor as a Potential PET Biomarker in Glioblastoma

Glieblastoma is one of the most malignant types of human cancer, and the prognosis is poor. The development and validation of novel molecular imaging biomarkers has the potential to improve tumor detection, grading, risk stratification, and treatment monitoring of gliomas. The aim of this study was to explore the potential of PET imaging of the urokinase-type plasminogen activator receptor (uPAR) in glioblastoma. Methods: The uPAR messenger RNA expression of tumors from 19 glioblastoma patients was analyzed, and a cell culture derived from one of these patients was used to establish an orthotopic xenograft model of glioblastoma. Tumor growth was monitored using bioluminescence imaging. Five to six weeks after inoculation, all mice were scanned with small-animal PET/CT using two new uPAR PET ligands (Cu-64-NOTA-AE105 and Ga-68-NOTA-AE105) and, for comparison, O-(2-F-18-fluoroethyl)-L-tyrosine (F-18-FET). One MRI scan was obtained for each mouse to confirm tumor location. The uPAR specificity of Cu-64-NOTA-AE105 was confirmed by alignment of hematoxylin- and eosin-stained and uPAR immunohistochemistry-stained slides of the brain with the activity distribution as determined using autoradiography. Results: uPAR expression was found in all 19 glioblastoma patient tumors, and high expression of uPAR correlated with decreased overall survival (P = 0.04). Radiolabeling of NOTA-AE105 with Cu-64 and Ga-68 was straightforward, resulting in a specific activity of approximately 20 GBq/mu mol and a radiochemical purity of more than 98% for Cu-64-NOTA-AE105 and more than 97% for Ga-68-NOTA-AE105. High image contrast resulting in clear tumor delineation was found for both 68Ga-NOTA-AE105 and Cu-64-NOTA-AE105. Absolute uptake in tumor was higher for F-18-FET (3.5 +/- 0.8 percentage injected dose [%ID]/g) than for Cu-64-NOTA-AE105 (1.2 +/- 0.4 %ID/g) or Ga-68-NOTA-AE105 (0.4 +/- 0.1 %ID/g). A similar pattern was observed in background brain tissue, where uptake was 1.9 +/- 0.1 %ID/g for F-18-fluorothymidine, compared with 0.05 +/- 0.01 %ID/g for Ga-68-NOTA-AE105 and 0.11 +/- 0.02 %ID/g for Cu-64-NOTA-AE105. The result was a significantly higher tumor-to-background ratio for both Ga-68-NOTA-AE105 (7.6 +/- 2.1, P < 0.05) and Cu-64-NOTA-AE105 (10.6 +/- 2.3, P < 0.01) than for 18F-FET PET (1.8 +/- 0.3). Autoradiography of brain slides confirmed that the accumulation of Cu-64-NOTA-AE105 corresponded well with uPAR-positive cancer cells. Conclusion: On the basis of our translational study, uPAR PET may be a highly promising imaging bio-marker for glioblastoma. Further clinical exploration of uPAR PET in glioblastoma is therefore justified.