Discovery of novel melanocortin4 receptor selective MSH analogues

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Abstract
We synthesized a novel series of cyclic melanocyte stimulating hormone (MSH) analogues and tested their binding properties on cells transiently expressing the human melanocortin1 (MC1), MC3, MC4 and MC5 receptors. We discovered that compounds with 26 membered rings of [Cys4,D‐Nal7,Cys11]α‐MSH(4–11) displayed specific MC4 receptor selectivity. The preference order of the different MC receptor subtypes for the novel [Cys4D‐Nal7Cys11]α‐MSH(4–11) analogues are distinct from all other known MSH analogues, particularly as they bind the MC4 receptor with high and the MC1 receptor with low relative affinities. HS964 and HS014 have 12 and 17 fold MC4/MC3 receptor selectivity, respectively, which is much higher than for the previously described cyclic lactam and [Cys4,Cys10]α‐MSH analogues SHU9119 and HS9510. HS964 is the first substance showing higher affinity for the MC5 receptor than the MC1 receptor. HS014, which was the most potent and selective MC4 receptor ligand (Ki 3.2 nM, which is ∼300 fold higher affinity than for α‐MSH), was also demonstrated to antagonize α‐MSH stimulation of cyclic AMP in MC4 receptor transfected cells. We found that a compound with a 29 membered ring of [Cys3,Nle10,D‐Nal7,Cys11]α‐MSH(3–11) (HS010) had the highest affinity for the MC3 receptor. This is the first study to describe ligands that are truly MC4 selective and a ligand having a high affinity for the MC3 receptor. The novel compounds may be of use in clarifying the physiological roles of the MC3, MC4 and MC5 receptors.