Synthesis and characterization of bivalent peptide ligands targeted to G-protein-coupled receptors

Background: Through the effects of avidity, multivalency can increase the apparent affinity of a ligand for its binding site. Low molecular weight, high affinity, multivalent ligands theoretically could be used to deliver a Variety of agents to specific cell subtypes. in order to target specific G-protein-coupled receptors, a series of monospecific peptide dimers were synthesized that are designed to bind to two adjacent receptor sites. Results: Three dimers, consisting of a ligand region, a short, flexible, uncharged spacer, a longer, polylysine spacer and a single cysteine residue to permit dimerization, and the corresponding monomers were synthesized by solid-phase peptide synthesis. The ligand domain was either alpha-melanocyte stimulating hormone (alpha-MSH), an alpha-MSH receptor antagonist (alpha-MSH-ANT), or bombesin. These ligands were characterized in a functional melanocyte dispersion assay. In wild-type melanophores, the alpha-MSH dimer stimulated dispersion with an EC(50) approximately seven-fold lower than that of the corresponding monomer. Similarly, in cells transfected with bombesin receptor cDNA, the bombesin dimer was approximately five-fold more potent than the monomer. The alpha-MSH-ANT monomer specifically inhibited alpha-MSH-mediated dispersion with no significant agonist activity, but the dimer acted predominantly as an agonist. Conclusions: Peptide dimers can be synthesized easily and have enhanced functional activity; monospecific dimers have greater avidity and bispecific dimers are likely to have greater selectivity. They may therefore have practical potential as specific cell-targeting agents.