^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 α_vβ₃ 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 α_vβ₃ 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 α_vβ₃-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 α_vβ₃-positive tumors and related metastases.