68Ga-labeling and in vivo evaluation of a uPAR binding DOTA- and NODAGA-conjugated peptide for PET imaging of invasive cancers

Introduction: The urokinase-type plasminogen activator receptor (uPAR) is a well-established biomarker for tumor aggressiveness and metastatic potential. DOTA-AE105 and DOTA-AE105-NH2 labeled with Cu-64 have previously been demonstrated to be able to noninvasively monitor uPAR expression using positron emission tomography (PET) in human cancer xenograft mice models. Here we introduce Ga-68-DOTA-AE105-NH2 and Ga-68-NODAGA-AE105-NH2 and evaluate their imaging properties using small-animal PET. Methods: Synthesis of DOTA-AE105-NH2, and NODAGA-AE105-NH2, was based on solid-phase peptide synthesis protocols using the Fmoc strategy. (GaCl3)-Ga-68 was eluted from a Ge-68/Ga-68 generator. The eluate was either concentrated on a cation-exchange column or fractionated and used directly for labeling. For in vitro characterization of both tracers, partition coefficient, buffer and plasma stability, uPAR binding affinity and cell uptake were determined. To characterize the ill vivo properties, dynamic microPET imaging was carried out in nude mice bearing human glioma U87MG tumor xenograft. Results: In vitro experiments revealed uPAR binding affinities in the lower nM range for both conjugated peptides and identical to AE105. Labeling of DOTA-AE105-NH2 and NODAGA-AE105-NH2 with Ga-68 was done at 95 degrees C and room temperature, respectively. The highest radiochemical yield and purity were obtained using fractionated elution, whereas a negative effect of acetone on labeling efficiency for NODAGA-AE105-NH2 was observed. Good stability in phosphate-buffered saline and mouse plasma was observed. High cell uptake was found for both tracers in U87MG tumor cells. Dynamic microPET imaging demonstrated good tumor-to-background ratio for both tracers. Tumor uptake was 2.1% ID/g and 1.3% ID/g 30 min postinjection and 2.0% ID/g and 1.1% ID/g 60 min postinjection for Ga-68-NODAGA-AE105-NH2 and Ga-68-DOTA-AE105-NH2, respectively. A significantly higher tumor-to-muscle ratio (P<.05) was found for Ga-68-NODAGA-AE105-NH2 60 min postinjection. Conclusions: The use of Ga-68-DOTA-AE105-NH2 and Ga-68-NODAGA-AE105-NH2 as the first gallium-68 labeled uPAR radiotracers for noninvasive PET imaging is reported, which combine versatility with good imaging properties. These new tracers thus constitute an interesting alternative to the Cu-64-labeled version (Cu-64-DOTA-AE105 and 64Cu-DOTA-AE105-NH2) for detecting uPAR expression in tumor tissue. In our hands, the fractionated elution approach was superior for labeling of peptides, and Ga-68-NODAGA-AE105-NH2 is the favored tracer as it provides the highest tumor-to-background ratio. (C) 2012 Elsevier Inc. All rights reserved.