INTRODUCTION: (68)Ga-labeled RGD peptides in combination with PET allow non-invasive determination of α(v)β(3) integrin expression which is highly increased during tumor-induced angiogenesis. The aim of this study was to synthesize and evaluate two RGD peptides containing alternative chelating systems, namely [(68)Ga]NS(3)-RGD and [(68)Ga]Oxo-DO3A-RGD and to compare their in vitro and in vivo properties with [(68)Ga]DOTA- and [(68)Ga]NODAGA-RGD.
METHODS: Syntheses of both radiotracers followed standard SPPS protocols. For in vitro characterization distribution coefficients, protein binding abilities, serum stabilities, and α(v)β(3) integrin binding affinities were determined. For in vit... More
INTRODUCTION: (68)Ga-labeled RGD peptides in combination with PET allow non-invasive determination of α(v)β(3) integrin expression which is highly increased during tumor-induced angiogenesis. The aim of this study was to synthesize and evaluate two RGD peptides containing alternative chelating systems, namely [(68)Ga]NS(3)-RGD and [(68)Ga]Oxo-DO3A-RGD and to compare their in vitro and in vivo properties with [(68)Ga]DOTA- and [(68)Ga]NODAGA-RGD.
METHODS: Syntheses of both radiotracers followed standard SPPS protocols. For in vitro characterization distribution coefficients, protein binding abilities, serum stabilities, and α(v)β(3) integrin binding affinities were determined. For in vitro tests as well as for the biodistribution assay α(v)β(3) positive human melanoma M21 and α(v)β(3) negative M21-L cells were used.
RESULTS: (68)Ga-labeling of NS(3)-RGD resulted in good radiochemical purity, whereas HPLC analysis showed two peaks with a ratio of 1:6 for [(68)Ga]Oxo-DO3A-RGD. Distribution coefficients were -3.4 for [(68)Ga]Oxo-DO3A-RGD and -2.9 for [(68)Ga]NS(3)-RGD. Both radiotracers were stable in PBS solution at 37C for 2h but lack stability in human serum. Protein binding was approximately 40% of the total activity for [(68)Ga]NS(3)-RGD and 70% for [(68)Ga]Oxo-DO3A-RGD, respectively, resulting in high blood pool activities. Biodistribution assays confirmed these findings and showed an additional high uptake in liver and kidneys, especially for [(68)Ga]NS(3)-RGD. Furthermore, [(68)Ga]Oxo-DO3A-RGD showed nearly the same activity concentrations in α(v)β(3) positive and α(v)β(3) negative tumors.
CONCLUSIONS: [(68)Ga]Oxo-DO3A-RGD and [(68)Ga]NS(3)-RGD have inferior characteristics compared to already existing (68)Ga-labeled RGD peptides and thus, both are not suited to image α(v)β(3) integrin expression. Of all our tested RGD peptides, [(68)Ga]NODAGA-RGD still possesses the most favorable imaging properties. Moreover this study shows that the use of appropriate chelators to achieve good targeting properties of (68)Ga-labeled biomolecules and careful in vitro and in vivo evaluation including comparative studies of different strategies are essential components in designing an effective imaging agent for PET.
