Biological evaluation of an ornithine-modified 99mTc-labeled RGD peptide as an angiogenesis imaging agent

Introduction Radiolabeled RGD peptides that specifically target integrin α ν β 3 have great potential in early tumor detection through noninvasive monitoring of tumor angiogenesis. Based on previous findings of our group on radiopeptides containing positively charged aminoacids, we developed a new cyclic cRGDfK derivative, c(RGDfK)-(Orn) 3 -CGG. This new peptide availing the polar linker (Orn) 3 and the 99m Tc-chelating moiety CGG (Cys-Gly-Gly) is appropriately designed for 99m Tc-labeling, as well as consequent conjugation onto nanoparticles. Methods A tumor imaging agent, c(RGDfK)-(Orn) 3 -[CGG- 99m Tc], is evaluated with regard to its radiochemical, radiobiological and imaging characteristics. Results The complex c(RGDfK)-(Orn) 3 -[CGG- 99m Tc] was obtained in high radiochemical yield (> 98%) and was stable in vitro and ex vivo . It presented identical to the respective, fully analytically characterized 185/187 Re complex retention time in RP-HPLC. In contrary to other RGD derivatives, we showed that the new radiopeptide exhibits kidney uptake and urine excretion due to the ornithine linker. High tumor uptake (3.87 ± 0.48% ID/g at 60 min p.i.) was observed and was maintained relatively high even at 24 h p.i. (1.83 ± 0.05 % ID/g), thus providing well-defined scintigraphic imaging. Accumulation in other organs was negligible. Blocking experiments indicated target specificity for integrin receptors in U87MG glioblastoma cells. Conclusion Due to its relatively high tumor uptake, renal elimination and negligible abdominal localization, the new 99m Tc-RGD peptide is considered promising in the field of imaging α ν β 3 -positive tumors. However, the preparation of multifunctional SPECT/MRI contrast agents (RGD-conjugated nanoparticles) for dual modality imaging of integrin expressing tumors should be further investigated.