Efficient preparation of (99m)Tc(III) '4+1' mixed-ligand complexes for peptide labeling with high specific activity

An improved labeling procedure for peptides attached to organometallic (99m)Tc(III) '4+1' mixed-ligand complexes in which the radiometal is coordinated by a tripodal tetradentate chelator 2,2',2''-nitrilotriethanethiol (NS(3)) and a monodentate isocyanide ligand is presented. The labeling procedure was evaluated by the synthesis of [(99m)Tc(NS(3))(L2-RGD)]. The containing radiopharmaceutically interesting RGD-peptide cyclo[Arg-Gly-Asp-D-Tyr-Lys] was modified with 4-isocyanobutanoic acid (L2) as linker conjugated to N(6)-Lys to get the monodentate ligand L2-RGD. The structural identity of the (99m)Tc-conjugate was confirmed by comparison to a Re reference compound. The Tc- and Re-conjugates had matching retention times under identical HPLC conditions. The (99m)Tc-labeling was performed in a novel one-step procedure using the eluate of a (99)Mo/(99m)Tc generator, NS(3), the isocyanide modified peptide, SnCl(2), Na(2)EDTA, mannitol and ascorbic acid in the reaction mixture. Using optimized reagents it is possible to label 50nmol peptide with (99m)Tc within 60min at room temperature with a radiochemical yield higher than 95% and a specific activity of approximately 20GBq/micromol.