Iminodiacetic acid as bifunctional linker for dimerization of cyclic RGD peptides.

INTRODUCTION: In this study, I2P-RGD2 was used as the example to illustrate a novel approach for dimerization of cyclic RGD peptides. The main objective of this study was to explore the impact of bifunctional linkers (glutamic acid vs. iminodiacetic acid) on tumor-targeting capability and excretion kinetics of the99m Tc-labeled dimeric cyclic RGD peptides.

METHODS: HYNIC-I2P-RGD2 was prepared by reacting I2P-RGD2 with HYNIC-OSu in the presence of diisopropylethylamine, and was evaluated for its αv β3 binding affinity against125 I-echistatin bound to U87MG glioma cells.99m Tc-I2P-RGD2 was prepared with high specific activity (~185GBq/μmol). The athymic nude mice bearing U87MG glioma xenografts were used to evaluate its biodistribution properties and image quality in comparison with those of99m Tc-3P-RGD2 .

RESULTS: The IC50 value for HYNIC-I2P-RGD2 was determined to be 39±6nM, which was very close to that (IC50 =33±5nM) of HYNIC-3P-RGD2 . Replacing glutamic acid with iminodiacetic acid had little impact on αv β3 binding affinity of cyclic RGD peptides.99m Tc-I2P-RGD2 and99m Tc-3P-RGD2 shared similar tumor uptake values over the 2h period, and its αv β3 -specificity was demonstrated by a blocking experiment. The uptake of99m Tc-I2P-RGD2 was significantly lower than99m Tc-3P-RGD2 in the liver and kidneys. The U87MG glioma tumors were visualized by SPECT with excellent contrast using both99m Tc-I2P-RGD2 and99m Tc-3P-RGD2 .

CONCLUSION: Iminodiacetic acid is an excellent bifunctional linker for dimerization of cyclic RGD peptides. Bifunctional linkers have significant impact on the excretion kinetics of99m Tc radiotracers. Because of its lower liver uptake and better tumor/liver ratios,99m Tc-I2P-RGD2 may have advantages over99m Tc-3P-RGD2 for diagnosis of tumors in chest region.