Structure–Activity Relationships of the Tetrapeptide Ac-His-Arg-(pI)DPhe-Tic-NH2 at the Mouse Melanocortin Receptors: Modification at the (pI)DPhe Position Leads to mMC3R Versus mMC4R Selective Ligands

Abstract

The five melanocortin receptors (MC1R–MC5R) are involved in numerous biological pathways, including steroidogenesis, pigmentation, and food intake. In particular, MC3R and MC4R knockout mice suggest that the MC3R and MC4R regulate energy homeostasis in a non-redundant manner. While MC4R-selective agonists have been utilized as appetite modulating agents, the lack of MC3R-selective agonists has impeded progress in modulating this receptor in vivo. In this study, the (pI)DPhe position of the tetrapeptide Ac-His-Arg-(pI)DPhe-Tic-NH2 (an MC3R agonist/MC4R antagonist ligand) was investigated with a library of 12 compounds. The compounds in this library were found to have higher agonist efficacy and potency at the mouse (m) MC3R compared to the MC4R, indicating that the Arg-DPhe motif preferentially activates the mMC3R over the mMC4R. This observation may be used in the design of new MC3R-selective ligands, leading to novel probe and therapeutic lead compounds that will be useful for treating metabolic disorders.

Multiresidue Tetrapeptide Substitutions Yield a 140-fold Selective Melanocortin-3 over Melanocortin-4 Receptor Agonist

“…These values are similar to a prior report where this sequence had agonist EC 50 values of 0.6, 14, and 7.6 nM at the MC1R, MC3R, and MC5R, respectively, and a pA 2 value of 5.9 at the MC4R. 18 Inverting the stereochemistry at the fourth position to DTic (yielding tetrapeptide 2) resulted in equipotent activities at the MCRs compared to 1. When reporting similar tetrapeptides, Fleming et al also observed comparable activities when Tic or DTic was substituted at the fourth position.…”

“…The parent tetrapeptide in this study, 1 (Ac-His-Arg-DBip-Tic-NH 2 ), possessed full agonist efficacy at the MC1R, MC3R, and MC5R (0.5, 22, and 9 nM potencies, respectively; Tables and ), partially stimulated the MC4R at 100 μM concentrations (35% the maximal signal of NDP-MSH), and possessed submicromolar antagonist potency at the MC4R (pA 2 = 6.5). These values are similar to a prior report where this sequence had agonist EC 50 values of 0.6, 14, and 7.6 nM at the MC1R, MC3R, and MC5R, respectively, and a pA 2 value of 5.9 at the MC4R . Inverting the stereochemistry at the fourth position to DTic (yielding tetrapeptide 2 ) resulted in equipotent activities at the MCRs compared to 1 .…”

“…A follow-up library with the most active “hits” at each position indicated that Val in position 1 and Gln at position 2 retained MC3R agonism and varying levels of MC4R agonist efficacy . Exploring the third position within the Ac-His-Arg-Xxx-Tic-NH 2 scaffold demonstrated the DBip could maintain MC3R agonist potency compared to a (pI)­DPhe residue, combined with decreased MC4R antagonist potency . These studies suggest that this tetrapeptide scaffold can be optimized to increase MC3R over MC4R selectivity.…”

“…17 Exploring the third position within the Ac-His-Arg-Xxx-Tic-NH 2 scaffold demonstrated the DBip could maintain MC3R agonist potency compared to a (pI)DPhe residue, combined with decreased MC4R antagonist potency. 18 These studies suggest that this tetrapeptide scaffold can be optimized to increase MC3R over MC4R selectivity.…”

“…In attempts to develop a more potent MC3R agonist with minimal MC4R activity from the Ac-His-Arg-(pI)­DPhe-Tic-NH 2 lead compound, a library of 12 peptides was synthesized utilizing substitutions previously found to maintain MC3R agonism and modulate MC4R activity. − In the present library, all compounds possessed a DBip in third position, replacing the (pI)­DPhe from the lead compound. Compounds were synthesized utilizing standard Fmoc-chemistry, , purified to greater than 95% purity, and characterized by mass-spectral analysis (University of Minnesota Mass Spectrometry Lab, Table ).…”