99mTc-Labeled Cyclic RGDfK Dimer:  Initial Evaluation for SPECT Imaging of Glioma Integrin αvβ3 Expression

This report describes the evaluation of biodistribution properties of three radiotracers, [99mTc(SQ168)(EDDA)], [99mTc(SQ168)(tricine)(PDA)], and [99mTc(SQ168)(tricine)(TPPTS)] (SQ168 = [2-[[[5-[carboonyl]-2-pyridinyl]hydrazono]methyl]benzenesulfonic acid]-Glu(cyclo{Lys-Arg-Gly-Asp-d-Phe})-cyclo{Lys-Arg-Gly-Asp-d-Phe}; EDDA = ethylenediamine-N,N‘-diacetic acid; PDA = 2,5-pyridinedicarboxylic acid; TPPTS = trisodium triphenylphosphine-3,3‘,3‘ ‘-trisulfonate), and their potential to image the glioma integrin αvβ3 expression in BALB/c nude mice bearing the U87MG human glioma xenografts. It was found that all three radiotracers were able to localize in glioma tumors with a relatively high tumor uptake and long tumor retention time by binding to the integrin αvβ3 expressed on both tumor cells and endothelial cells of tumor neovasculature. It seems that the coligand has minimal effect on integrin αvβ3 targeting capability of the 99mTc-labeled RGDfK dimer, but it has a significant impact on their biodistribution properties. For example, the complex [99mTc(SQ168)(tricine)(TPPTS)] has the lowest liver uptake and the highest metabolic stability in normal BALB/c nude mice. Results from SPECT imaging studies show that the glioma tumors can be clearly visualized with all three radiotracers at 4 h postinjection. Among the three radiotracers evaluated in this study, [99mTc(SQ168)(tricine)(TPPTS)] has the best imaging quality and is a promising candidate for more preclinical evaluations in the future.