Evaluation of 99mTc-Labeled Cyclic RGD Dimers: Impact of Cyclic RGD Peptides and 99mTc Chelates on Biological Properties

The main objective of this study is to explore the impact of cyclic RGD peptides and Tc-99m chelates on biological properties of Tc-99m radiotracers. Cyclic RGD peptide conjugates, HYNIC-K(NIC)-RGD(2) (HYNIC = 6-hydrazinonicotinyl; RGD(2) = E[c(RGDfK)](2) and NIC = nicotinyl), HYNIC-K(NIC)-3G-RGD(2) (3G-RGD(2) = Gly-Gly-Gly-E[Gly-Gly-Gly-c(RGDfK)](2)), and HYNIC-K(NIC)-3P-RGD(2) (3P-RGD(2) = PEG(4)-E[PEG(4)-c(RGDfK)](2)), were prepared. Macrocyclic Tc-99m complexes [Tc-99m(HYNIC-K(NIC)-RGD(2))-(tricine)] (1), [Tc-99m(HYNIC-K(NIC)-3G-RGD(2))(tricine)] (2), and [Tc-99m-(HYNIC-K(NIC)-3P-RGD(2))(tricine)] (3) were evaluated for their biodistribution and tumor-targeting capability in athymic nude mice bearing MDA-MB-435 human breast tumor xenografts. It was found that 1, 2, and 3 could be prepared with high specific activity (similar to 111 GBq/mu mol). All three Tc-99m radiotracers have two major isomers, which show almost identical uptake in tumors and normal organs. Replacing the bulky and highly charged [Tc-99m(HYNIC)(tricine)(TPPTS)] (TPPTS = trisodium triphenylphosphine-3,3',3 ''-trisulfonate) with a smaller [Tc-99m(HYNIC-K(NIC))(tricine)] resulted in less uptake in the kidneys and lungs for 3. Surprisingly, all three Tc-99m radiotracers shared a similar tumor uptake (1, 5.73 +/- 0.40%ID/g; 2, 5.24 +/- 1.09%ID/g; and 3, 4.94 +/- 1.71%ID/g) at 60 min p.i. The metabolic stability of Tc-99m radiotracers depends on cyclic RGD peptides (3P-RGD(2) > 3G-RGD(2) similar to RGD(2)) and Tc-99m chelates ([Tc-99m(HYNIC)(tricine)(TPPTS)] > [Tc-99m(HYNIC-K(NIC))(tricine)]). Immunohistochemical studies revealed a linear relationship between the alpha(v)beta(3) expression levels and tumor uptake or tumor/muscle ratios of 3, suggesting that 3 is useful for monitoring the tumor alpha(v)beta(3) expression. Complex 3 is a very attractive radiotracer for detection of integrin alpha(v)beta(3)-positive tumors.