Design, synthesis, and biological evaluation of new cyclic melanotropin peptide analogues selective for the human melanocortin-4 receptor.

Intensive efforts have been made to develop potent and selective ligands for certain human melanocortin receptors as possible treatments for obesity and sexual dysfunction due to the role of these receptors in feeding behavior, energy homeostasis, sexual function, etc. A number of novel alpha-MSH analogues were designed and synthesized primarily on the basis of our previous MTII NMR structure. In these peptide analogues, a disulfide or lactam bridge between residues at positions 5 and 8 was used as a conformational constraint to enhance the beta-turn spanning His6 and D-Phe7, while the pharmacophore group in Arg8 was mimicked via Nalpha-alkylation of residues 8 or 9 with the guanidinylbutyl group. Biological assays for binding affinities and adenylate cyclase activities for the hMC1R, hMC3R, hMC4R, and hMC5R showed that three analogues have good binding affinity for the hMC4R (0.7-4.1 nM), but have no binding affinity up to 10 microM at the other three melanocortin receptors. Interestingly, the three hMC4R selective analogues display only 50% binding efficiency, suggesting there is allosteric modulation of the melanocortin-4 receptor. These analogues were found to act as antagonists of the hMC4R. This result represents a discovery of very selective peptide-based antagonists for the hMC4R. The high selectivity may be due to the strong conformational constraint via ring contraction as compared to MTII, and the rigid conformation preferred by these new ligands allows them to recognize only the hMC4R, but not to activate the second messenger. The MTII NMR structure-based design thus not only examined the structural model of melanocortin ligands, but also yielded new biologically unique alpha-MSH analogues.

Solution structures of cyclic melanocortin agonists and antagonists by NMR.

The melanocortin receptors are involved in many physiological functions, including pigmentation, sexual function, feeding behavior, and energy homeostasis, making them potential targets for drugs to treat obesity, sexual dysfunction, etc. Understanding the conformational basis of the receptor-ligand interactions is crucial to the design of potent and selective ligands for these receptors. The solution structures of the cyclic melanocortin agonists, partial agonist, and antagonists MTII, VJH085, SHU9119, MK5, and MK9 were determined by two-dimensional nuclear magnetic resonance (2D NMR) spectroscopy at pH 4.5 and 25 degrees C in water (90% H(2)O/10% D(2)O). The overall backbone structures of these cyclic alpha-melanocyte-stimulating hormone (alpha-MSH) analogues around the message sequence (His(6)-D-Phe(7)/D-Nal(2')(7)-Arg(8)-Trp(9)) were similar and reasonably well defined. beta-Turns spanning His(6) and D-Phe(7)/D-Nal(2')(7) were identified in all analogues, and an amphiphilic molecular surface was obtained for the message sequence residues in most structures within the NMR ensembles. The beta-turn, which most closely resembles a type II beta-turn, leads to stacking between the aromatic rings of His(6) and D-Phe(7) in MTII and VJH085. However, no aromatic stacking between His(6) and D-Nal(2')(7) was found in structures of the D-Nal(2')(7)-containing analogues. The difference in the side-chain dispositions of His(6) and D-Nal(2')(7) may be responsible for the reduced potency or antagonist activity of the D-Nal(2')(7)-containing analogues. In addition, our results suggest that the side-chain orientations may also modulate the receptor selectivity. The information found in this study will be useful for the further design of ligands for melanocortin receptors.