The melanocortin system has five receptors, and antagonists of the central melanocortin receptors (MC3R, MC4R) are postulated to be viable therapeutics for disorders of negative energy balance such as anorexia, cachexia, and failure to thrive. Agouti-related protein (AGRP) is an antagonist of the MC3R and an antagonist/inverse agonist of the MC4R. Biophysical NMR-based structural studies have demonstrated that the active sequence of this hormone, Arg-Phe-Phe, is located on an exposed β-hairpin loop. It has previously been demonstrated that the macrocyclic octapeptide scaffold c[Pro¹-Arg²-Phe³-Phe⁴-Asn⁵-Ala⁶-Phe⁷-DPro⁸] is 16-fold less potent than AGRP at the mouse MC4R (mMC4R). Herein it was hypothesized that the Phe⁷ position may be substituted to produce more potent and/or selective melanocortin receptor antagonist ligands based on this template. A 10-membered library was synthesized that substituted small (Gly), polar (Ser), acidic (Asp), basic (Lys), aliphatic (Leu, Nle, and Cha), and aromatic (Trp, Tyr, hPhe) amino acids to explore potential modifications at the Phe⁷ position. The most potent mMC4R antagonist contained a Nle⁷ substitution, was equipotent to the lead ligand 200-fold selective for the mMC4R over the mMC3R, and caused a significant increase in food intake when injected intrathecally into male mice. Three compounds possessed sigmoidal dose–response inverse agonist curves at the mMC5R, while the remaining seven decreased cAMP production from basal levels at a concentration of 100 μM. These findings will add to the knowledge base toward the development of potent and selective probes to study the role of the melanocortin system in diseases of negative energy balance and can be useful in the design of molecular probes to examine the physiological functions of the mMC5R.

中文翻译：

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大环AGRP模仿支架c [Pro-Arg-Phe-Phe-Asn-Ala-Phe-DPro]的结构-活性关系研究提高了选择性和有效地增加食物摄入量的Melanocortin-4受体拮抗剂和Melanocortin-5受体反向激动剂的活性。在老鼠

黑皮质素系统具有五个受体，并且假定中枢黑皮质素受体的拮抗剂（MC3R，MC4R）是治疗负能量平衡失调（例如厌食症，恶病质和xia壮成长）的可行疗法。刺痛相关蛋白（AGRP）是MC3R的拮抗剂和MC4R的拮抗剂/反向激动剂。基于生物物理NMR的结构研究表明，该激素的活性序列Arg-Phe-Phe位于暴露的β-发夹环上。先前已证明大环八肽支架c [Pro ¹ -Arg ² -Phe ³ -Phe ⁴ -Asn ⁵ -Ala ⁶ -Phe ⁷ -DPro ⁸]在小鼠MC4R（mMC4R）上的效力比AGRP低16倍。在此假设基于该模板，可以取代Phe ⁷位以产生更有效和/或选择性的黑皮质素受体拮抗剂配体。合成了一个由10个成员组成的文库，该文库取代了小（Gly），极性（Ser），酸性（Asp），碱性（Lys），脂族（Leu，Nle和Cha）和芳香族（Trp，Tyr，hPhe）氨基酸探索在Phe ⁷位置的潜在修饰。最有效的mMC4R拮抗剂包含Nle ⁷取代，相当于铅配体对mMC4R的选择性是mMC3R的200倍，并在鞘内注射到雄性小鼠中时引起食物摄入的显着增加。三种化合物在mMC5R上具有S型剂量响应反激动剂曲线，而其余七个在浓度为100μM时从基础水平降低了cAMP的产生。这些发现将为开发有效和选择性探针以研究黑皮质素系统在负能量平衡疾病中的作用增加知识基础，并且可用于设计分子探针以检查mMC5R的生理功能。