RT Journal Article
SR Electronic
T1 Pegylated Arg-Gly-Asp Peptide: <sup>64</sup>Cu Labeling and PET Imaging of Brain Tumor α<sub>v</sub>β<sub>3</sub>-Integrin Expression
JF Journal of Nuclear Medicine
JO J Nucl Med
FD Society of Nuclear Medicine
SP 1776
OP 1783
VO 45
IS 10
A1 Xiaoyuan Chen
A1 Yingping Hou
A1 Michel Tohme
A1 Ryan Park
A1 Vazgen Khankaldyyan
A1 Ignacio Gonzales-Gomez
A1 James R. Bading
A1 Walter E. Laug
A1 Peter S. Conti
YR 2004
UL http://jnm.snmjournals.org/content/45/10/1776.abstract
AB The αv-integrins, cell adhesion molecules that are highly expressed on activated endothelial cells and tumor cells but not on dormant endothelial cells or normal cells, present an attractive target for tumor imaging and therapy. We previously coupled a cyclic Arg-Gly-Asp (RGD) peptide, c(RGDyK), with 1,4,7,10-tetraazacyclododecane-N,N′,N″,N‴-tetraacetic acid (DOTA) and labeled the RGD-DOTA conjugate with 64Cu (half-life, 12.8 h; 19% β+) for solid tumor targeting, with high tumor-to-background contrast. The rapid tumor washout rate and persistent liver and kidney retention of this tracer prompted us to optimize the tracer for improved pharmacokinetic behavior. In this study, we introduced a polyethylene glycol (PEG; molecular weight, 3,400) moiety between DOTA and RGD and evaluated the 64Cu-DOTA-PEG-RGD tracer for microPET imaging in brain tumor models. Methods: DOTA was activated in situ and conjugated with RGD-PEG-NH2 under slightly basic conditions. αvβ3-Integrin-binding affinity was evaluated with a solid-phase receptor-binding assay in the presence of 125I-echistatin. Female nude mice bearing subcutaneous U87MG glioblastoma xenografts were administered 64Cu-DOTA-PEG-RGD, and the biodistributions of the radiotracer were evaluated from 30 min to 4 h after injection. microPET (20 min of static imaging at 1 h after injection) and then quantitative autoradiography were used for tumor visualization and quantification. The same tracer was also applied to an orthotopic U87MG model for tumor detection. Results: The radiotracer was synthesized with a high specific activity (14,800–29,600 GBq/mmol [400–800 Ci/mmol]). The c(RGDyK)-PEG-DOTA ligand showed intermediate binding affinity for αvβ3-integrin (50% inhibitory concentration, 67.5 ± 7.8 nmol/L [mean ± SD]). The pegylated RGD peptide demonstrated rapid blood clearance (0.57 ± 0.15 percentage injected dose [%ID]/g [mean ± SD] at 30 min after injection and 0.03 ± 0.02 %ID/g at 4 h after injection). Activity accumulation in the tumor was rapid and high at early time points (2.74 ± 0.45 %ID/g at 30 min after injection), and some activity washout was seen over time (1.62 ± 0.18 %ID/g at 4 h after injection). Compared with 64Cu-DOTA-RGD, this tracer showed improved in vivo kinetics, with significantly reduced liver uptake (0.99 ± 0.08 %ID/g vs. 1.73 ± 0.39 %ID/g at 30 min after injection and 0.58 ± 0.07 %ID/g vs. 2.57 ± 0.49 %ID/g at 4 h after injection). The pegylated RGD peptide showed higher renal accumulation at early time points (3.51 ± 0.24 %ID/g vs. 2.18 ± 0.23 %ID/g at 30 min after infection) but more rapid clearance (1.82 ± 0.29 %ID/g vs. 2.01 ± 0.25 %ID/g at 1 h after injection) than 64Cu-DOTA-RGD. The integrin receptor specificity of this radiotracer was demonstrated by blocking of tumor uptake by coinjection with nonradiolabeled c(RGDyK). The high tumor-to-organ ratios for the pegylated RGD peptide tracer (at 1 h after injection: tumor-to-blood ratio, 20; tumor-to-muscle ratio, 12; tumor-to-liver ratio, 2.7; and tumor-to-kidney ratio, 1.2) were confirmed by microPET and autoradiographic imaging in a subcutaneous U87MG tumor model. This tracer was also able to detect an orthotopic brain tumor in a model in which U87MG cells were implanted into the mouse forebrain. Although the magnitude of tumor uptake in the orthotopic xenograft was lower than that in the subcutaneous xenograft, the orthotopic tumor was still visualized with clear contrast from normal brain tissue. Conclusion: This study demonstrated the suitability of a PEG moiety for improving the in vivo kinetics of a 64Cu-RGD peptide tracer without compromising the tumor-targeting ability and specificity of the peptide. Systematic investigations of the effects of the size and geometry of PEG on tumor targeting and in vivo kinetics will lead to the development of radiotracers suitable for clinical applications such as visualizing and quantifying αv-integrin expression by PET. In addition, the same ligand labeled with therapeutic radionuclides may be applicable for integrin-targeted internal radiotherapy.