Noninvasive visualization and quantification of tumor αVβ3 integrin expression using a novel positron emission tomography probe, 64Cu-cyclam-RAFT-c(-RGDfK-)4

Introduction: The alpha(V)beta(3) integrin is a well-known transmembrane receptor involved in tumor invasion, angiogenesis and metastasis. Our aim was to evaluate a novel positron emission tomography (PET) probe, Cu-64-cyclam-RAFT-c(-RGDfK-)(4), for noninvasive visualization and quantification of alpha(V)beta(3) integrin expression. Methods: RAFT-c(-RGDfK-)(4), a tetrameric cyclic Arg-Gly-Asp (RGD)-based peptide, was conjugated with a bifunctional chelator, 1,4,8,11-tetraazacyclotetradecane (cyclam), radiolabeled with the positron emitter Cu-64 and evaluated in vitro by cell binding and competitive inhibition assays and in vivo by biodistribution and receptor blocking studies, and PET imaging. The following cell lines, human embryonic kidney HEK293(beta(1)) [alpha(V)beta(3)-negative] and HEK293(beta(3)) [alpha(V)beta(3)-overexpressing] and human glioblastoma U87MG [naturally expressing alpha(V)beta(3)], together with their subcutaneous xenografts in athymic nude mice, were used for the present study. The expression levels of alpha(V)beta(3) on these cell lines and tumor xenografts were analyzed by flow cytometry and sodium dodecyl sulfate polyacrylamide gel electrophoresis/autoradiography, respectively. Results: Cu-64-cyclam-RAFT-c(-RGDfK-)4 demonstrated the in vitro and in vivo specificity for the alpha(V)beta(3) integrin and displayed rapid blood clearance, predominantly renal excretion and low uptake in nontumor tissues. Tumor uptake of Cu-64-cyclam-RAFT-c(-RGDfK-)(4) (3 h postinjection) in HEK293(beta(3)) (high levels of alpha(V)beta(3)), U87MG (moderate levels of alpha(V)beta(3)) and HEK293(beta(1)) (undetectable levels of alpha(V)beta(3)) tumors was 9.35%+/-1.19%, 3.46%+/-0.45% and 1.18%+/-0.30% injected dose per gram, respectively, with a strong and positive correlation with the tumor alpha(V)beta(3) expression levels (correlation coefficient=0.967; P<.0001). Positron emission tomographic images showed that alpha(V)beta(3)-positive tumors were clearly visualized with high tumor-to-background contrast, and agreed well with the biodistribution results. Conclusion: Cu-64-cyclam-RAFT-c(-RGDfK-)(4) exhibits potential for noninvasively quantifying alpha(V)beta(3) expression. (C) 2011 Elsevier Inc. All rights reserved.