Preparation of 99Tcm labeled cyclic RGDfK dimer and its in vitro and in vivo evaluation

Objective: Peptides labeled with radionuclides such as 99Tcm as the tumor imaging agents induced interests among scientists with some studies published. The aim of this study was to evaluate the in vitro and in vivo characteristics of 99Tcm labeled cyclic RGDfK dimer E[c(RGDfK)]2 for imaging integrin αvβ3-positive tumors. Methods: 99Tcm-hydrazino-nicotinamide (HYNIC) E-[c(RGDfK)]2 was prepared by one step method without SnCl2, while tricine and trisodium triphenylphosphine-3,3',3-trisulfonate (TPPTS) were chosen as coligands, and HYNIC as chelator. The concentration of 50% inhibition (IC50) was determined on U87 human glioma cells. The in vitro receptor binding and disassociation kinetics, cell internalization and efflux were investigated. The biodistribution and γ imaging were performed in nude mice bearing human glioma xenografts. Results: The labeling yield of 99Tcm-HYNIC-E[c(RGDfK)]2 was over 95%, and the radiochemical purity was more than 99% after the purification with Sep-Pek C18 cartridge. Comparing to c (RGDyK) monomer, HYNIC-conjugated E[c RGDfK)]2 dimer possessed a significantly higher integrin αvβ3 binding affinity with the IC50 of 80.0 and 9.07 nmol/L, respectively. Cell experiments showed that 99Tcm-HYNIC-E[c(RGDfK)]2 rapidly bonded to integrin αvβ3 before internalized by receptor-mediated way. Biodistribution data showed that 99Tcm-HYNIC-E [c(RGDfK)]2 was excreted mainly through kidneys, and the uptake of tumors (percentage activity of injection dose per gram of tissue, % ID/g) at 0.5 and 4 h postinjection was (2.46 ± 0.66) and (3.10 ± 0.35)% ID/g, respectively, suggesting a long enough retention time. γ imaging clearly revealed xenografted tumors at 1 h postinjection, but the images at 4 h were better. Conclusion: 99Tcm-HYNIC-E[c(RGDfK)]2 is a promising radiotracer for integrin αvβ3-positive tumor imaging. (authors)