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

Abstract

Glioblastoma 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 ( 64 Cu-NOTA-AE105 and 68 Ga-NOTA-AE105) and, for comparison, O-(2-18 F-fluoroethyl)-L-tyrosine ( 18 F-FET). One MRI scan was obtained for each mouse to confirm tumor location. The uPAR specificity of 64 Cu-NOTA-AE105 was confirmed by alignment of hematoxylin-and eosin-stained and uPAR immunohistochemistrystained 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 5 0.04). Radiolabeling of NOTA-AE105 with 64 Cu and 68 Ga was straightforward, resulting in a specific activity of approximately 20 GBq/μmol and a radiochemical purity of more than 98% for 64 Cu-NOTA-AE105 and more than 97% for 68 Ga-NOTA-AE105. High image contrast resulting in clear tumor delineation was found for both 68 Ga-NOTA-AE105 and 64 Cu-NOTA-AE105. Absolute uptake in tumor was higher for 18 F-FET (3.5 ± 0.8 percentage injected dose [%ID]/g) than for 64 Cu-NOTA-AE105 (1.2 ± 0.4 %ID/g) or 68 Ga-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 18 F-fluorothymidine, compared with 0.05 ± 0.01 %ID/g for 68 Ga-NOTA-AE105 and 0.11 ± 0.02 %ID/g for 64 Cu-NOTA-AE105. The result was a significantly higher tumor-to-background ratio for both 68 Ga-NOTA-AE105 (7.6 ± 2.1, P , 0.05) and 64 Cu-NOTA-AE105 (10.6 ± 2.3, P , 0.01) than for 18 F-FET PET (1.8 ± 0.3). Autoradiography of brain slides confirmed that the accumulation of 64 Cu-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 biomarker for glioblastoma. Further clinical exploration of uPAR PET in glioblastoma is therefore justified.

uPAR as a Glioma Imaging Target

“…Although this model is still somewhat different from naturally occurring tumors, we consider the present findings to provide more reliable evidence justifying clinical studies of the new tracers. Attention should be paid to the radionuclides used by Persson et al Whereas the production of 18 F, 11 C, and 64 Cu (half-life, 12.7 h) requires a cyclotron, 68 Ga (half-life, 68 min) is produced by a 68 Ge/ 68 Ga generator, and thus the preparation of 68 Ga-DOTA-AE105 does not require an in-house cyclotron. Conversely, the relatively short half-life of 68 Ga may restrict imaging at a late phase.…”

“…This may be the significant factor causing the high tumor-to-background ratio in the study (i.e., the tumor is of human origin but the background is of mouse origin). uPAR ligands are also different from analogs of more ubiquitous substrates such as glucose (e.g., 18 F-FDG) and amino acids (e.g., 11 C-methionine and 18 F-fluoroethyltyrosine). Thus, clinical studies using uPAR ligands are expected to result in lower contrast than in the present study.…”

“…After several pilot studies are performed, it will be necessary to conduct studies of large populations with multivariate analyses that include known prognosis factors such as age, glioma grade, and surgical procedure. Comparisons with established tracers such as 18 F-FDG, 11 C-methionine, 18 F-fluoroethyltyrosine, and 18 F-fluoromisonidazole are also important. The question of whether the imaging technique can be used for monitoring treatment response is also of interest.…”

“…More recently, the Kjaer group performed a series of important steps to modify and characterize radioactive uPAR ligands. They developed and examined the radioactive uPAR ligands 68 Ga-DOTA-AE105-NH 2 (14), 68 Ga-NODAGA-AE105-NH 2 (14), 18 F-AlF-NOTA-AE105 (15), 64 Cu-CB-TE2A-PA-AE105 (16), and 64 Cu-DOTA-AE105 (17). In this issue of The Journal of Nuclear Medicine, Persson et al from the Kjaer group present evidence of the usefulness of two new uPAR PET tracers, 64 Cu-NOTA-AE105 and 68 Ga-NOTA-AE105, in glioblastoma imaging (18).…”

“…They developed and examined the radioactive uPAR ligands 68 Ga-DOTA-AE105-NH 2 (14), 68 Ga-NODAGA-AE105-NH 2 (14), 18 F-AlF-NOTA-AE105 (15), 64 Cu-CB-TE2A-PA-AE105 (16), and 64 Cu-DOTA-AE105 (17). In this issue of The Journal of Nuclear Medicine, Persson et al from the Kjaer group present evidence of the usefulness of two new uPAR PET tracers, 64 Cu-NOTA-AE105 and 68 Ga-NOTA-AE105, in glioblastoma imaging (18). Three highlights of their study are, first, that an orthotopic glioblastoma model was used to demonstrate strong accumulation of both tracers in the tumor; second, that compared with 18 F-fluoroethyltyrosine, the new tracers showed lower absolute uptake values but higher tumor-to-background ratios; and third, that autoradiography revealed the intratumoral tracer distribution, which resembled the immunohistochemical staining of uPAR.…”