A convenient solid-phase synthesis methodology for preparing peptide-derived molecular imaging agents — Synthesis, characterization, and in vitro screening of Tc(I) – chemotactic peptide conjugates

A versatile solid-phase synthesis strategy for preparing peptide–chelate conjugates was developed. The methodology was optimized using a series of ligands, designed to bind Tc(I)/Re(I), and a chemotactic peptide fMFL, which was exploited as a model targeting vector. The peptide derivatives were prepared in parallel using a conventional automated peptide synthesizer in multi-milligram quantities, which provided sufficient material to perform complete characterization, radiolabelling, and in vitro screening studies. Because of the robust nature of the metal–chelate complexes, the Re complex of a chelate–peptide conjugate was prepared on the resin using the same methodology employed to prepare the free ligand conjugates. As such, the reported methodology is amenable to the preparation of libraries of novel Tc radiopharmaceutical ligands and their corresponding Re reference standards in which several factors, including peptide sequence, site of derivatization, and both the type and length of the spacer, can be easily varied.Key words: radiopharmaceuticals, technetium, rhenium, peptides, solid-phase synthesis.