D-amino acid scan of γ-melanocyte-stimulating hormone: Importance of Trp⁸ on human MC3 receptor selectivity

In our search for potent and receptor-selective agonists and antagonists, we report here the results of D-amino acid substitution at each position of the short peptide γ-melanocyte-stimulating hormone (γ-MSH). The native γ-MSH shows weak binding at all three receptors (i.e., the human MC3, MC4, and MC5) and a selectivity of 1-2 orders of magnitude at the MC3R over the MC4R and MC5R. Sequential replacement of each residue in the γ-MSH sequence with the corresponding D-isomer results in analogues which mostly have weaker binding affinity than the native peptide, except for two analogues. For the DTrp⁸ analogue, there is an increase in binding affinity by about 1 order of magnitude (IC₅₀ = 6 nM) at the MC3R compared with that of the natural molecule and an increase in selectivity for the MC3R by 2 orders of magnitude compared with the activity at the MC4R and MC5R. The DPhe⁶ analogue is about 10-fold more potent (IC₅₀ = 8.8 nM) at the MC3R compared with the native peptide but lacks subtype selectivity. Measurement of the intracellular cAMP accumulation in human MC3R, MC4R, MC5R revealed that the native peptide shows potent activity at the MC3R (EC₅₀ = 5.9 nM) and is about 50-100-fold selective at this receptor compared with the MC4R and MC5R. The DArg¹⁰ (EC₅₀ = 35 nM) and the DPhe¹¹ (EC₅₀ = 11 nM) analogues are selective for the MC3R by 1 and 2 orders of magnitude compared with the MC4R and MC5R, respectively. The DTrp⁸ compound (EC₅₀ = 0.33 nM) shows about 300- and 250-fold increase in selectivity at the MC3R compared with the MC4R and MC5R, respectively. Finally, the DTyr¹ peptide is selective for the MC3R (EC₅₀ = 12 nM) by 40-200-fold compared with the MC4R and MC5R. In general, the trend is that D-amino acid substitutions of the aromatic residues 1, 6, 8, and 11 and the basic residue Arg¹⁰, but not Arg⁷, result in an increase in MC3R selectivity over the MC4R and MC5R and only agonist activity is observed. Thus, the key residues of γ-MSH identified in this study include the aromatic residues 1, 6, 8, and 11 and the basic residue Arg¹⁰ (but not Arg⁷), as important for MC3 selectivity over the MC4 and MC5 subtypes. Further, the study reveals the extreme importance of DTrp at position 8 in imparting potency and selectivity since this is the most selective analogue for the human MC3R reported thus far.