Protein and peptide labeling with (cyclopentadienyl)tricarbonyl rhenium and technetium

A method for labeling proteins and peptides with (methoxycarbonyl cyclopentadienyl)tricarbonyl rhenium and technetium is described. The precursors used for this labeling are conveniently produced from perrhenate and pertechnetate, respectively, using a double ligand transfer reaction. For labeling the lysine residues of the model protein bovine serum albumin, the technetium methyl ester was saponified and then transformed into its N-hydroxysuccinimidyl ester. For the labeling of the model peptides leucine enkephalin, substance P, oxytocin, and the tumor imaging/therapy candidate octreotide, the rhenium methyl ester was saponified and activated using either 1-hydoxybenzotriazole or 1-hydroxy-7-azabenzotriazole. The activation and peptide-coupling reactions were followed using reversed-phase (C18) HPLC and yields averaged approximately 70%.