calcitonin and 6-hydrazinopyridine-3-carboxylic-acid

Labeling of proteins with metallic radionuclides for use in radiopharmaceuticals involves covalently attaching a bifunctional chelator. In principle, use of smaller peptides allows this chelator to be incorporated during solid-phase peptide synthesis (SPPS) with total site specificity. To realize the advantages of this approach, a lysine-hynic conjugate Fmoc-N-epsilon-(Hynic-Boc)-Lys was synthesized for incorporating the well-known technetium-99m-binding hydrazinonicotinamide ligand into peptides during SPPS. It was used to synthesize a technetium-99m-labeled salmon calcitonin with the hynic-linked amino acid in place of lysine-18. A trifluoroacetate group protected the hynic during alkaline oxidation to the cyclic disulfide and was readily removed by mild acid treatment. The peptide was efficiently labeled (91-98% radiochemical yield) with Tc-99m in the presence of tricine and SnCl(2) with high specific activity (>100 MBq/microg). The product showed good serum stability and specific affinity for human calcitonin receptors. Fmoc-N-epsilon-(Hynic-Boc)-Lys is a highly versatile technetium-binding amino acid for incorporation into peptides during SPPS. This allows total flexibility and control in the site of attachment and is suitable for a combinatorial approach to peptide radiopharmaceuticals.

Topics: Calcitonin; Chelating Agents; Chromatography, High Pressure Liquid; Humans; Hydrazines; Isotope Labeling; Lysine; Nicotinic Acids; Radiopharmaceuticals; Receptors, Calcitonin; Spectrometry, Mass, Electrospray Ionization; Technetium; Tumor Cells, Cultured