Comparison of in vitro and in vivo properties of ^99mTc-cRGD peptides labelled using different novel Tc-cores

The alpha_vbeta₃ integrin is involved in angiogenesis and tumor metastasis. RGD-peptides bind with high affinity to this integrin. This study compares the influence of ^99mTc-labelling applying novel Technetium-cores on imaging characteristics of the radiolabelled peptide. Different peptide conjugates based on the cyclic pentapeptide c(RGDyK) (cRGD) were prepared and characterised (HYNIC-, Cys-, L2- and Pz1-cRGD). Radiolabelling experiments using different coligands for HYNIC-cRGD, the ^99mTc(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 NS₃-coligand (4+1 concept) were performed and showed labelling yields >90% at high specific activities. A high in vitro stability was observed, plasma protein binding and lipophilicity varied considerably between different radiolabelled cRGD conjugates. Experiments on biological activity of the radiolabelled peptides using alpha_vbeta₃ positive (M21) and negative (M21L) tumour cells did show specific uptake of various conjugates. Studies in tumour bearing animals revealed significant differences between different conjugates concerning pharmacokinetic behaviour (predominant renal excretion to considerable hepatobiliary clearance) as well as tumour uptake (0.2-2.7%ID/g). Highest specific tumour uptake and tumour/background values were found for ^99mTc-EDDA/HYNIC-c(RGDyK), ^99mTc-Nitrido-PNP-Cys-c(RGDyK) and ^99mTc(CO)₃-Pz1-c(RGDyK). In conclusion, using novel Tc-cores such as the ^99mTc(CO)₃ metal fragment, Tc-nitrido- and the 4+1 concept peptides could be labelled with ^99mTc-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 labelling strategies using 99mTc have to be properly selected and optimized. Different in vitro assays are necessary to predict targeting properties in vivo.