Iminodiacetic acid as bifunctional linker for dimerization of cyclic RGD peptides

IntroductionIn 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.MethodsHYNIC-I2P-RGD2 was prepared by reacting I2P-RGD2 with HYNIC-OSu in the presence of diisopropylethylamine, and was evaluated for its αvβ3 binding affinity against 125I-echistatin bound to U87MG glioma cells. 99mTc-I2P-RGD2 was prepared with high specific activity (~185 GBq/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.ResultsThe IC50 value for HYNIC-I2P-RGD2 was determined to be 39 ± 6 nM, which was very close to that (IC50 = 33 ± 5 nM) of HYNIC-3P-RGD2. Replacing glutamic acid with iminodiacetic acid had little impact on αvβ3 binding affinity of cyclic RGD peptides. 99mTc-I2P-RGD2 and 99mTc-3P-RGD2 shared similar tumor uptake values over the 2 h period, and its αvβ3-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.ConclusionIminodiacetic 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.

This report presents novel approach for dimerization of cyclic RGD peptides using iminodiacetic acid as a bifunctional linker. The results from this study clearly show that the bifunctional linker (glutamic acid vs. iminodiacetic acid) have significant impact on pharmacokinetics of their 99mTc radiotracers, and 99mTc-I2P-RGD2 is good as 99mTc-3P-RGD2 for imaging tumors of different origin.123