Discovery of prototype peptidomimetic agonists at the human melanocortin receptors MC1R and MC4R

[Nle⁴,DPhe⁷]-α-MSH (NDP-MSH), a highly potent analogne of α- melanocyte-stimulating hormone (α-MSH), possesses nanomolar efficacies at all the melanocortin receptor subtypes except the MC2R. Evaluation of the melanocortin 'message' sequence of [Nle⁴,DPhe⁷]-α-MSH was performed on the human melanocortin receptor subtypes designated hMC1, hMC3R, hMC4R, and hMC5R. Tetrapeptides and tripeptides were stereochemically modified to explore topochemical preferences at these receptors and to identify lead peptides possessing agonist activity and subtype selectivity. Four peptides were discovered to only bind to the hMC1 and hMC4 receptor subtypes. The tetrapeptide Ac-His-DPhe-Arg-Trp-NH₂ (1) possessed 0.6μM binding affinity at the hMC1R, 1.2μM binding affinity at the hMC4R, and agonist activity at both receptors. The tripeptides Ac-DPhe-Arg-Trp-NH₂ (6) and Ac-DPhe-Arg- DTrp-NH₂ (7) possessed 2.0 and 9.1 μM binding affinities, respectively, only at the hMC4R, and both compounds effected agonist activity. The tetrapeptide Ac-His-Phe-Arg-DTrp-NH₂ (4) possessed 6.3μM affinity and full agonist activity at the hMC1R, while only binding 7% at the hMC3R, 36% at the hMC4R, and 11% at the hMC5R at a maximal concentration of 10 μM. These data demonstrate that the His-Phe-Arg-Trp message sequence of the melanocortin peptides does not bind and stimulate each melanocortin receptor in a similar fashion, as previously hypothesized. Additionally, this study identified the simplest structural agonists for the hMC1R and hMC4R receptors reported to date.