First 18F-labeled ligand for PET imaging of uPAR: In vivo studies in human prostate cancer xenografts

Abstract

Urokinase-type plasminogen activator receptor (uPAR) is overexpressed in human prostate cancer and uPAR has been found to be associated with metastatic disease and poor prognosis. AE105 is a small linear peptide with high binding affinity to uPAR. We synthesized an N-terminal NOTA-conjugated version (NOTA-AE105) for development of the first (18)F-labeled uPAR positron-emission-tomography PET ligand using the Al(18)F radiolabeling method. In this study, the potential of (18)F-AlF-NOTA-AE105 to specifically target uPAR-positive prostate tumors was investigated. NOTA-conjugated AE105 was synthesized and radiolabeled with (18)F-AlF according to a recently published optimized protocol. The labeled product was purified by reverse phase high performance liquid chromatography RP-HPLC. The tumor targeting properties were evaluated in mice with subcutaneously inoculated PC-3 xenografts using small animal PET and ex vivo biodistribution studies. uPAR-binding specificity was studied by coinjection of an excess of a uPAR antagonist peptide AE105 analogue (AE152). NOTA-AE105 was labeled with (18)F-AlF in high radiochemical purity (>92%) and yield (92.7%) and resulted in a specific activity of greater than 20GBq/μmol. A high and specific tumor uptake was found. At 1h post injection, the uptake of (18)F-AlF-NOTA-AE105 in PC-3 tumors was 4.22 ± 0.13%ID/g. uPAR-binding specificity was demonstrated by a reduced uptake of (18)F-AlF-NOTA-AE105 after coinjection of a blocking dose of uPAR antagonist at all three time points investigated. Good tumor-to-background ratio was observed with small animal PET and confirmed in the biodistribution analysis. Ex vivo uPAR expression analysis on extracted tumors confirmed human uPAR expression that correlated close with tumor uptake of (18)F-AlF-NOTA-AE105. The first (18)F-labeled uPAR PET ligand, (18)F-AlF-NOTA-AE105, has successfully been prepared and effectively visualized noninvasively uPAR positive prostate cancer. The favorable in vivo kinetics and easy production method facilitate its future clinical translation for identification of prostate cancer patients with an invasive phenotype and poor prognosis.