Aim: The alfa(v)beta(3) integrin is involved in angiogenesis and tumor metastasis. Arginine-glycine-aspartic acid (RGD)-peptides bind with high affinity to this integrin. This study compares the influence of (99m)Tc-labeling applying novel Technetium-cores on imaging characteristics of the radiolabeled peptide.
Methods: Different peptide conjugates based on the cyclic pentapeptide c(RGDyK) (cRGD) were prepared and characterized (HYNIC-, Cys-, L2- and Pz1-cRGD). Radiolabeling experiments using different coligands for HYNIC-cRGD, the (99m)Tc(CO)(3) metal fragment for PZ-1-cRGD (pyrazolyl-derivative), the Tc-nitrido-core using a phosphine-coligand (PNP) for Cys-cRGD and an isonitrile-conjugate (L2-cRGD) together with a NS(3)-coligand (4+1 concept) were performed and showed labeling yields >90% at high specific activities.
Results: A high in vitro stability was observed, plasma protein binding and lipophilicity varied considerably between different radiolabeled cRGD conjugates. Experiments on biological activity of the radiolabeled peptides using alfa(v)beta(3) positive (M21) and negative (M21L) tumor cells did show specific uptake of various conjugates. Studies in tumor bearing animals revealed significant differences between different conjugates concerning pharmacokinetic behavior (predominant renal excretion to considerable hepatobiliary clearance) as well as tumor uptake (0.2-2.7%ID/g). Highest specific tumor uptake and tumor/background values were found for [(99m)Tc]EDDA/HYNIC-c(RGDyK), [(99m)Tc]Nitrido-PNP-Cys-c(RGDyK) and [(99m)Tc(CO)(3)]-Pz1-c(RGDyK).
Conclusions: Using novel Tc-cores such as the (99m)Tc(CO)(3) metal fragment, Tc-nitrido- and the 4+1 concept peptides could be labeled with [(99m)Tc]technetium at high specific activities resulting in complexes with high stability, but binding moieties have to be optimized especially concerning hydrophilicity resulting in renal rather than hepatobiliary excretion. This comparative study underlines that peptide labeling strategies using (99m)Tc have to be properly selected and optimized. Different in vitro assays are necessary to predict targeting properties in vivo.