177Lu-labeled monomeric, dimeric and multimeric RGD peptides for the therapy of tumors expressing a(?)ß(3) integrins

The conjugation of peptides to gold nanoparticles (AuNPs) produces biocompatible and stable multimeric systems with target-specific molecular recognition. Peptides based on the cyclic Arg-Gly-Asp (RGD) sequence have been reported as high-affinity agents for the a()beta(3) integrin. The aim of this research was to prepare a multimeric system of 177Lu-labeled gold nanoparticles conjugated to c(RGDfK)C (cyclo(Arg-Gly-Asp-Phe-Lys)Cys) and to compare the radiation-absorbed dose with that of 177Lu-labeled monomeric and dimeric RGD peptides to a()beta(3) integrin-positive U87MG tumors in mice. DOTA-GGC (1,4,7,10-tetraazacyclododecane-N-N',N,N-tetraacetic acid-Gly-Gly-Cys) and c(RGDfK)C peptides were synthesized and conjugated to AuNPs by a spontaneous reaction of the thiol groups. Transmission electron microscopy, ultravioletvisible, X-ray photoelectron spectroscopy, Raman and far-infrared spectroscopy techniques demonstrated that AuNPs were functionalized with the peptides. For the 177Lu-AuNP-c(RGDfK)C to be obtained, the 177Lu-DOTA-GGC radiopeptide was first prepared and added to a solution of AuNPs followed by c(RGDfK)C (25 mu l, 5 mu m) at 18 degrees C for 15min. 177Lu-DOTA-GGC, 177Lu-DOTA-cRGDfK and 177Lu-DOTA-E-c(RGDfK)2 were prepared by adding 177LuCl3 (370MBq) to 5 mu l (1mg/ml) of the DOTA derivative diluted with 50 mu l of 1 m acetate buffer pH 5. The mixture was incubated at 90 degrees C in a block heater for 30min. Radiochemical purity was determined by ultrafiltration and HPLC analyses. Biokinetic studies were accomplished in athymic mice with U87MG-induced tumors. The radiochemical purity for all 177Lu-RGD derivatives was 96 +/- 2%. 177Lu-absorbed doses per injected activity delivered to U87MG tumors were 0.357 +/- 0.052Gy/MBq (multimer), 0.252 +/- 0.027Gy/MBq (dimer) and 0.102 +/- 0.018Gy/MBq (monomer). 177Lu-labeled dimeric and multimeric RGD peptides demonstrated properties suitable for targeted radionuclide therapy of tumors expressing a()beta(3) integrins. Copyright (c) 2012 John Wiley & Sons, Ltd.