68Ga-labeled multimeric RGD peptides for microPET imaging of integrin αvβ3 expression

Purpose We and others have reported that F-18- and Cu-64-labeled arginine-glycine-aspartate (RGD) peptides allow positron emission tomography (PET) quantification of integrin alpha(v)beta(3) expression in vivo. However, clinical translation of these radiotracers is partially hindered by the necessity of cyclotron facility to produce the PET isotopes. Generator-based PET isotope Ga-68, with a half-life of 68 min and 89% positron emission, deserves special attention because of its independence of an onsite cyclotron. The goal of this study was to investigate the feasibility of Ga-68-labeled RGD peptides for tumor imaging. Methods Three cyclic RGD peptides, c(RGDyK) (RGD1), E[c(RGDyK)](2) (RGD2), and E{E[c(RGDyK)](2)}(2) (RGD4), were conjugated with macrocyclic chelator 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA) and labeled with Ga-68. Integrin affinity and specificity of the peptide conjugates were assessed by cell-based receptor binding assay, and the tumor targeting efficacy of Ga-68-labeled RGD peptides was evaluated in a subcutaneous U87MG glioblastoma xenograft model. Results U87MG cell-based receptor binding assay using I-125-echistatin as radioligand showed that integrin affinity followed the order of NOTA-RGD4 > NOTA-RGD2 > NOTA-RGD1. All three NOTA conjugates allowed nearly quantitative Ga-68-labeling within 10 min (12-17 MBq/nmol). Quantitative microPET imaging studies showed that Ga-68-NOTA-RGD4 had the highest tumor uptake but also prominent activity accumulation in the kidneys. Ga-68-NOTA-RGD2 had higher tumor uptake (e.g., 2.8 +/- 0.1%ID/g at 1 h postinjection) and similar pharmacokinetics (4.4 +/- 0.4 tumor/muscle ratio, 2.0 +/- 0.1 tumor/liver ratio, and 1.1 +/- 0.1 tumor/kidney ratio) compared with Ga-68-NOTA-RGD1. Conclusions The dimeric RGD peptide tracer Ga-68-NOTA-RGD2 with good tumor uptake and favorable pharmacokinetics warrants further investigation for potential clinical translation to image integrin alpha(v)beta(3).