Discovery of Mixed Pharmacology Melanocortin-3 Agonists and Melanocortin-4 Receptor Tetrapeptide Antagonist Compounds (TACOs) Based on the Sequence Ac-Xaa¹-Arg-(pI)DPhe-Xaa⁴-NH₂

Abstract

The centrally expressed melanocortin-3 and -4 receptors (MC3R/MC4R) have been studied as possible targets for weight management therapies, with a preponderance of studies focusing on the MC4R. Herein, a novel tetrapeptide scaffold [Ac-Xaa1 -Arg-(pI)DPhe-Xaa4 -NH2] is reported. The scaffold was derived from results obtained from a MC3R mixture-based positional scanning campaign. From these results, a set of 48 tetrapeptides were designed and pharmacologically characterized at the mouse melanocortin-1, -3, -4, and -5 receptors. This resulted in the serendipitous discovery of nine compounds that were MC3R agonists (EC50 < 1,000 nM) and MC4R antagonists (5.7 <pA2 < 7.8). The three most potent MC3R agonists, 18 [Ac-Arg-Arg-(pI)DPhe-Tic-NH2], 1 [Ac-His-Arg-(pI)DPhe-Tic-NH2], and 41 [Ac-Arg-Arg-(pI)DPhe-DNal(2′)-NH2] were more potent (EC50 < 73 nM) than the melanocortin tetrapeptide Ac-His-DPhe-Arg-Trp-NH2. This template contains a sequentially reversed “Arg-(pI)DPhe” motif with respect to the classical “Phe-Arg” melanocortin signaling motif which results in pharmacology that is first-in-class for the central melanocortin receptors.

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

“…An antagonist pA 2 value of 7.3 was observed for KNS2-22-4 at the mMC4R (Figure 3). In prior studies, this compound was observed to possess nanomolar agonist potency at the MC3R (30–40 nM), partial agonist stimulation of the MC4R, and sub-micromolar antagonist potency at the MC4R (pA 2 of 6.6–7) [40,41], similar to the results in the present study.…”

“…Thus, a DPhe-Arg motif resulted in full agonism at the MC4R and full to partial agonism at the MC3R accompanied by antagonist activity (dependent on the DPhe para -position). Due to the MC3R agonism and MC4R antagonism observed in the Ac-His-Arg-( p I)DPhe-Tic-NH 2 ligand [40,41], it was hypothesized that different substitutions at the DPhe para -position within this scaffold (possessing an Arg-DPhe motif and a Tic residue in position 4) may modulate MC4R agonist efficacy. The results in Table 3 demonstrate that the efficacy at the mMC4R was modulated by various para -substitutions.…”

“…To identify novel scaffolds with agonist selectivity for the MC3R over the MC4R, our laboratory performed a tetrapeptide mixture-based positional scan [40]. From this study, a new scaffold tetrapeptide (Ac-His-Arg-( p I)DPhe-Tic-NH 2 ) was identified that possessed nanomolar agonist potency at the MC3R (EC 50 = 40 nM) and was an antagonist at the MC4R (pA 2 = 7.0) [40]. Compared to the endogenous tetrapeptide melanocortin sequence (His-Phe-Arg-Trp), the new scaffold switched the Phe and Arg positions and incorporated a Tic residue in place of the Trp.…”

Network dynamics of hypothalamic feeding neurons

“…6 E and F ). To selectively stimulate MC3R, we administered a previously characterized selective MC3R agonist (C18) ( 30 ) and recorded the neuronal dynamics of PVN MC4R neurons ( Fig. 6 E and F ).…”

“…Synthesis and in vitro characterization of C18 were previously described ( 30 ). C18 was administered via intraperitoneal injection (10 mg/kg, 200 L saline).…”