Ψ and χ Angle Constrains at the C‑Terminus Trp Position of the Melanotropin Tetrapeptide Ac-His‑d‑Phe-Arg-Trp-NH2 Lead to Potent and Selective Agonists at hMC1R and hMC4R

Melanocortin receptors (MCRs) are a family of G protein-coupled receptors that regulate important physiological functions. Yet, drug development targeting MCRs is hindered by potential side effects due to a lack of receptor subtype-selective ligands with bioavailability. Here, we report novel synthetic pathways to introduce Ψ and χ angle constraints at the C-terminus Trp position of the nonselective prototype tetrapeptide agonist Ac-His-d-Phe-Arg-Trp-NH2. With these conformational constraints, peptide 1 (Ac-His-d-Phe-Arg-Aia) shows improved selectivity at hMC1R, with an EC50 of 11.2 nM for hMC1R and at least 15-fold selectivity compared to other MCR subtypes. Peptide 3 (Ac-His-pCF3-d-Phe-Arg-Aia) is a potent and selective hMC4R agonist with an EC50 of 4.1 nM at hMC4R and at least ninefold selectivity. Molecular docking studies reveal that the Ψ and χ angle constraints force the C-terminal Aia residue to flip and interact with TM6 and TM7, a feature that we hypothesize leads to the receptor subtype selectivity.

黑皮质素受体（Melanocortin receptors, MCRs）是一类调控重要生理功能的G蛋白偶联受体（G protein-coupled receptors）。然而，因缺乏具备生物利用度的受体亚型选择性配体，靶向MCRs的药物研发常受到潜在不良反应的制约。本研究报道了可在非选择性原型四肽激动剂Ac-His-d-Phe-Arg-Trp-NH2的色氨酸C端位点引入Ψ与χ角约束的新型合成路径。通过引入此类构象约束，肽1（Ac-His-d-Phe-Arg-Aia）对人黑皮质素受体1亚型（hMC1R）的选择性得到提升，其对hMC1R的半最大效应浓度（EC50）为11.2 nM，相较于其他MCR亚型的选择性至少达15倍。肽3（Ac-His-pCF3-d-Phe-Arg-Aia）是一种强效且具备选择性的人黑皮质素受体4亚型（hMC4R）激动剂，其对hMC4R的EC50为4.1 nM，选择性至少达9倍。分子对接（Molecular docking）研究显示，Ψ与χ角约束可迫使C端的Aia残基发生构象翻转，并与跨膜结构域6（TM6）和跨膜结构域7（TM7）产生相互作用；我们推测这一特征正是其受体亚型选择性的来源。