Iminodiacetic acid as bifunctional linker for dimerization of cyclic RGD peptides

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 the ^99mTc-labeled dimeric cyclic RGD peptides.

HYNIC-I2P-RGD₂ was prepared by reacting I2P-RGD₂ with HYNIC-OSu in the presence of diisopropylethylamine, and was evaluated for its α_vβ₃ binding affinity against ¹²⁵I-echistatin bound to U87MG glioma cells. ^99mTc-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 of ^99mTc-3P-RGD₂.

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 α_vβ₃ binding affinity of cyclic RGD peptides. ^99mTc-I2P-RGD₂ and ^99mTc-3P-RGD₂ shared similar tumor uptake values over the 2h period, and its α_vβ₃-specificity was demonstrated by a blocking experiment. The uptake of ^99mTc-I2P-RGD₂ was significantly lower than ^99mTc-3P-RGD₂ in the liver and kidneys. The U87MG glioma tumors were visualized by SPECT with excellent contrast using both ^99mTc-I2P-RGD₂ and ^99mTc-3P-RGD₂.

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