99mTc-Galacto-RGD2: A Novel 99mTc-Labeled Cyclic RGD Peptide Dimer Useful for Tumor Imaging

This study sought to evaluate [Tc-99m(HYNIC-Galacto-RGD(2))-(tricine)(TPPTS)] (Tc-99m-Galacto-RGD(2): HYNIC = 6-hydrazinonicotinyl; Galacto-RGD(2) = Glu[cyclo[Arg-Gly-Asp-D-Phe-Lys(SAA-PEG(2)-(1,2,3-triazole)-1-yl-4-methylamide)]](2) (SAA = 7-amino-L-glycero-L-galacto-2,6-anhydro-7-deoxyheptanamide, and PEG(2) = 3,6-dioxaoctanoic acid); and TPPTS = trisodium triphenylphosphine-3,3',3 ''-trisulfonate) as a new radiotracer for tumor imaging. Galacto-RGD(2) was prepared via the copper(I)-catalyzed 1,3 dipolar azide-alkyne Huisgen cycloaddition. HYNIC-Galacto-RGD(2) was prepared by reacting Galacto-RGD(2) with sodium succinimidyl 6-(2-(2-sulfonatobenzaldehyde)hydrazono)nicotinate (HYNIC-OSu)in the presence of diisopropylethylamine, and was evaluated for its integrin alpha(v)beta(3) binding affinity against I-125 echistatin bound to U87MG glioma cells. The IC50 value for HYNIC-Galacto-RGD(2) was determined to be 20 +/- 2 nM. Tc-99m-Galacto-RGD(2) was prepared in high specific activity (similar to 185 GBq/mu mol) and high radiochemical purity (>95%), and was evaluated in athymic nude mice bearing U87MG glioma xenografts for its tumor targeting capability and biodistribution. The tumor uptake of Tc-99m-Galacto-RGD(2) was 10.30 +/- 1.67, 8.37 +/- 2.13, 6.86 +/- 1.33, and 5.61 +/- 1.52%ID/g at 5, 30, 60, and 120 min p.i., respectively, which was in agreement with high integrin alpha(v)beta(3) expression on glioma cells and neovasculature. Its lower uptake in intestines, lungs, and spleen suggests that Tc-99m-Galacto-RGD(2) has advantages over Tc-99m-3P-RGD(2) ([Tc-99m(HYNIC-3P-RGD(2))(tricine)(TPPTS)]: 3P-RGD(2) = PEG(4)-E[PEG(4)-c(RGDfK)](2); PEG(4) = 15-amino-4,7,10,13-tetraoxapentadecanoic acid) for imaging tumors in the chest and abdominal regions. U87MG tumors were readily detected by SPECT and the tumor uptake of Tc-99m-Galacto-RGD(2) was integrin alpha(v)beta(3)-specific. Tc-99m-Galacto-RGD(2) also had very high metabolic stability. On the basis of results from this study, it was concluded that Tc-99m-Galacto-RGD(2) is an excellent radiotracer for imaging integrin alpha(v)beta(3)-positive tumors and related metastases.