An unusual conformation of γ-melanocyte-stimulating hormone analogues leads to a selective human melanocortin 1 receptor antagonist for targeting melanoma cells.

γ-MSH (γ-melanocyte-stimulating hormone, H-Tyr-Val-Met-Gly-His-Phe-Arg-Trp-Asp-Arg-Phe-Gly-OH), with its exquisite specificity and potency, has recently created much excitement as a drug lead. However, this peptide is like most peptides susceptible to proteolysis in vivo, which potentially decreases its beneficial activities. In our continued effort to design a proteolytically stable ligand with specific receptor binding, we have engineered peptides by cyclizing γ-MSH using a thioether bridge. A number of novel cyclic truncated γ-MSH analogues were designed and synthesized, in which a thioether bridge was incorporated between a cysteine side chain and an N-terminal bromoacyl group. One of these peptides, cyclo-[(CH(2))(3)CO-Gly(1)-His(2)-D-Phe(3)-Arg(4)-D-Trp(5)-Cys(S-)(6)]-Asp(7)-Arg(8)-Phe(9)-Gly(10)-NH(2), demonstrated potent antagonist activity and receptor selectivity for the human melanocortin 1 receptor (hMC1R) (IC(50) = 17 nM). This novel peptide is the most selective antagonist for the hMC1R to date. Further pharmacological studies have shown that this peptide can specifically target melanoma cells. The nuclear magnetic resonance analysis of this peptide in a membrane-like environment revealed a new turn structure, specific to the hMC1R antagonist, at the C-terminus, where the side chain and backbone conformation of D-Trp(5) and Phe(9) of the peptide contribute to hMC1R selectivity. Cyclization strategies represent an approach for stabilizing bioactive peptides while keeping their full potencies and should boost applications of peptide-based drugs in human medicine.

Design, synthesis and biological evaluation of ligands selective for the melanocortin-3 receptor.

The processed products of the proopiomelanocortin gene (ACTH, alpha-MSH, beta-MSH, gamma-MSH, etc.) interact with five melanocortin receptors, the MC1R, MC2R, MC3R, MC4R, and MC5R to modulate and control many important biological functions crucial for good health both peripherally (as hormones) and centrally (as neurotransmitters). Pivotal biological functions include pigmentation, adrenal function, response to stress, fear/flight, energy homeostasis, feeding behavior, sexual function and motivation, pain, immune response, and many others, and are believed to be involved in many disease states including pigmentary disorders, adrenal disorders, obesity, anorexia, prolonged and neuropathic pain, inflammatory response, etc. The melanocortin-3 receptor (MC3R) is found primarily in the brain and spinal cord and also in the periphery, and its biological functions are still not well understood. Here we review some of the biological functions attributed to the MC3R, and then examine in more detail efforts to design and synthesize ligands that are potent and selective for the MC3R, which might help resolve the many questions still remaining about its function. Though some progress has been made, there is still much to be done in this critical area.

Structure-activity studies of the melanocortin peptides: discovery of potent and selective affinity antagonists for the hMC3 and hMC4 receptors.

We have designed and synthesized several novel cyclic SHU9119 analogues (Ac-Nle4-[Asp5-His6-DNal(2')7-Arg8-Trp9-Lys10]-NH2) modified in position 6 with nonconventional amino acids. SHU9119 is a high affinity nonselective antagonist at hMC3R and hMC4R with potent agonist activity at hMC1R and hMC5R. We measured the binding affinity and agonist potency of the novel analogues at cloned hMC3R, hMC4R, and hMC5R receptors and identified several selective, high affinity hMC3R and hMC4R antagonists. Compound 4 containing Che substitution in position 6 is a high affinity hMC4R antagonist (IC50 = 0.48 nM) with 100-fold selectivity over hMC3R antagonist. Analogue 7 with a Cpe substitution in position 6 is a high affinity hMC4R antagonist (IC50 = 0.51 nM) with a 200-fold selectivity vs the hMC3R. Interestingly, analogue 9 with an Acpc residue in position 6 is a high affinity hMC3R antagonist (IC50 = 2.5 nM) with 100-fold selectivity vs the hMC4R antagonist based on its binding affinities. This compound represents the first cyclic lactam antagonist with high selectivity for the hMC3R vs hMC4R. To understand the possible structural basis responsible for selectivity of these peptides at hMCR3 and hMCR4, we have carried out a molecular modeling study in order to examine the conformational properties of the cyclic peptides modified in position 6 with conformationally restricted amino acids.