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

This study sought to evaluate [99mTc­(HYNIC-Galacto-RGD2)­(tricine)­(TPPTS)] (99mTc-Galacto-RGD2: HYNIC = 6-hydrazinonicotinyl; Galacto-RGD2 = Glu­[cyclo­[Arg-Gly-Asp-d-Phe-Lys­(SAA-PEG2-(1,2,3-triazole)-1-yl-4-methylamide)]]2 (SAA = 7-amino-l-glycero-l-galacto-2,6-anhydro-7-deoxyheptanamide, and PEG2 = 3,6-dioxaoctanoic acid); and TPPTS = trisodium triphenylphosphine-3,3′,3″-trisulfonate) as a new radiotracer for tumor imaging. Galacto-RGD2 was prepared via the copper­(I)-catalyzed 1,3-dipolar azide–alkyne Huisgen cycloaddition. HYNIC-Galacto-RGD2 was prepared by reacting Galacto-RGD2 with sodium succinimidyl 6-(2-(2-sulfonatobenzaldehyde)­hydrazono)­nicotinate (HYNIC-OSu) in the presence of diisopropylethylamine, and was evaluated for its integrin αvβ3 binding affinity against 125I-echistatin bound to U87MG glioma cells. The IC50 value for HYNIC-Galacto-RGD2 was determined to be 20 ± 2 nM. 99mTc-Galacto-RGD2 was prepared in high specific activity (∼185 GBq/μmol) 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 99mTc-Galacto-RGD2 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 αvβ3 expression on glioma cells and neovasculature. Its lower uptake in intestines, lungs, and spleen suggests that 99mTc-Galacto-RGD2 has advantages over 99mTc-3P-RGD2 ([99mTc­(HYNIC-3P-RGD2)­(tricine)­(TPPTS)]: 3P-RGD2 = PEG4-E­[PEG4-c­(RGDfK)]2; PEG4 = 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 99mTc-Galacto-RGD2 was integrin αvβ3-specific. 99mTc-Galacto-RGD2 also had very high metabolic stability. On the basis of results from this study, it was concluded that 99mTc-Galacto-RGD2 is an excellent radiotracer for imaging integrin αvβ3-positive tumors and related metastases.