Characterization of aliphatic, cyclic, and aromatic N-terminally "capped" His-D-Phe-Arg-Trp-NH₂ tetrapeptides at the melanocortin receptors

The melanocortin system is implicated in multiple physiological pathways including pigmentation, inflammation, erectile function, feeding behavior, energy homeostasis, weight homeostasis, and exocrine gland function, just to list a few. The endogenous agonists for the melanocortin receptors include the gene transcripts derived from the proopiomelanocortin gene and include the core tetrapeptide His-Phe-Arg-Trp sequence postulated to be important for melanocortin receptor selectivity and stimulation. Posttranslational processing of the proopiomelanocortin derived agonists results in the N-terminal acetylation and C-terminal amidation of α-melanocyte stimulation hormone (α-MSH). In this study we generated 25 N-terminally "capped" tetrapeptides containing the core sequence X-His-D-Phe-Arg-Trp-NH₂ and pharmacologically characterized them at the mouse melanocortin MC₁ receptor, melanocortin MC₃ receptor, melanocortin MC₄ receptor, and melanocortin MC₅ receptor. The N-terminal "capping" groups consisted of linear, cyclic, or aromatic moieties and all resulted in full agonist activity at the melanocortin receptors examined in this study. Increasing aliphatic chain length increased potency of the tetrapeptide derivatives, with the addition of octanoyl capping group resulting in 70- to 110-fold increased tetrapeptide potency at the melanocortin MC₁ receptor (EC₅₀=0.4 nM), melanocortin MC₃ receptor (EC₅₀=4.0 nM), and melanocortin MC₄ receptor (EC₅₀=0.4 nM) while only enhancing potency at the melanocortin MC₅ receptor (EC₅₀=0.8 nM) by 8-fold, compared to the tetrapeptide His-D-Phe-Arg-Trp-NH₂. This octanoyl derivative surprisingly resulted in a 14-fold greater potency than α-MSH (EC₅₀=5.4 nM) at the mouse melanocortin MC₄ receptor implicated in feeding behavior and obesity. The 3,3,3-triphenylpropionyl derivative resulted in greater than 14 μM agonist potencies at the melanocortin MC₁ receptor, melanocortin MC₃ receptor, and melanocortin MC₄ receptor and possessed a 140 nM agonist EC₅₀ value at the melanocortin MC₅ receptor. This 3,3,3-triphenylpropionyl-His-D-Phe-Arg-Trp-NH₂ peptide is a 100-fold selective agonist for the melanocortin MC₅ receptor, versus the other melanocortin receptors studied herein, and is the first melanocortin MC₅ receptor selective tetrapeptide derivative reported to date with nanomolar potency.