Incorporation of Indoylated Phenylalanine Yields a Sub-Micromolar Selective Melanocortin-4 Receptor Antagonist Tetrapeptide

Abstract

The melanocortin family is involved in many physiological functions, including pigmentation, steroidogenesis, and appetite. The centrally expressed melanocortin-3 and melanocortin-4 receptors (MC3R and MC4R) possess overlapping but distinct roles in energy homeostasis. Herein, the third and fourth positions of a tetrapeptide lead compound [Ac-Arg-Arg-(pI)DPhe-Tic-NH 2 ], previously reported to possess MC3R agonist and MC4R antagonist activities, were substituted with indoylated phenylalanine (Wsf/Wrf) residues in an attempt to generate receptor subtype selective compounds. At the third position, d -amino acids were required for melanocortin agonist activity, while both l - and d -residues resulted in MC4R antagonist activity. These results indicate that l -indoylated phenylalanine residues at the third position of the scaffold can generate MC4R over MC3R selective antagonist ligands, resulting in a substitution pattern that may be exploited for novel MC4R ligands that can be used to probe the in vivo activity of the MC4R without involvement of the MC3R.

Discovery of a Pan-Melanocortin Receptor Antagonist [Ac-DPhe(pI)-Arg-Nal(2′)-Orn-NH2] at the MC1R, MC3R, MC4R, and MC5R that Mediates an Increased Feeding Response in Mice and a 40-Fold Selective MC1R Antagonist [Ac-DPhe(pI)-DArg-Nal(2′)-Arg-NH2]

“…Tetrapeptides were synthesized using standard Fmoc methodologies , on a manual microwave synthesizer (Discover SPS; CEM, Matthews, NC), as previously reported. ,, The resin was swelled in DCM before iterative deprotection and amino acid coupling steps were used to assemble the tetrapeptides. The Fmoc deprotection step used a two-stage deprotection strategy with 20% piperidine in DMF (1 × 2 min at room temperature, followed by 1 × 4 min using microwave irradiation to 75 °C and 30 W).…”