Replacement of Arg with Nle and modified D-Phe in the core sequence of MSHs, Ac-His-D-Phe-Arg-Trp-NH2, leads to hMC1R selectivity and pigmentation

Abstract

Melanoma skin cancer is the fastest growing cancer in the US [1]. A great need exists for improved formulations and mechanisms to prevent and protect human skin from cancers and other skin damage caused by sunlight exposure. Current efforts to prevent UV damage to human skin, which in many cases leads to melanoma and other skin cancers. The primordial melanocortin-1 receptor (MC1R) is involved in regulating skin pigmentation and hair color, which is a natural prevention from UV damage. The endogenous melanocortin agonists induce pigmentation and share a core pharmacophore sequence "His-Phe-Arg-Trp", and it was found that substitution of the Phe by D-Phe results in increasing melanocortin receptor potency. To improve the melanocortin 1 receptor (MC1R) selectivity a series of tetra-peptides with the moiety of Ac-Xaa-Yaa-Nle-Trp-NH_2, and structural modifications to reduce electrostatic ligand-receptor interactions have been designed and synthesized. It is discovered that the tetrapeptide Ac-His-D-Phe(4-CF₃)-Nle-Trp-NH₂ resulted in a potent and selective hMC1R agonist at the hMC1R (EC₅₀: 10 nM). Lizard anolis carolinensis pigmentation study shows very high potency in vivo. NMR studies revealed a reversed β turn structure which led to the potency and selectivity towards the hMC1R.

Keywords: Melanocortin peptide; Selectivity; Skin pigmentation; Tetra-peptide; hMC1R; α-MSH.

Fig. 1

Ac-His-D-Phe(4-CF₃)-Nle-Trp-NH₂ induced pigmentation on the lizard. Left, before injection; Right, after injection. Peptide samples were dissolved in saline at the concentration of 1 mM. The total amount of peptide was through i.p. injection with 3 μg/g of each lizard. The strategy follows previous publications [,–41].

Fig. 2

The natural abundance ¹⁵N-HSQC spectrum of peptide 5, sample concentration: 4.7 mM, number of scans: 128, F1 dimension: 128 data points, 40 ppm spectral width centered at 120 ppm, F2 dimension: 2048 data points, 14 ppm spectrum width centered at 120 ppm, relaxation delay:1.5s.

Fig. 3

The ensemble of the 10 representative NMR derived structures of peptide 5, AcHis-D-Phe(4-CF₃)-Nle-Trp-NH₂. The hydrogen bond between the His1 CO and the Trp⁴NH is shown.

Fig. 4

Best docking pose for hMC1R selective ligand: Ac-His-D-Phe(4-CF₃)-Nle-Trp-NH₂ into hMC1R. (Docking Score −11). The hMC1R-MCL interactions distance cut off 3 Å. Up Left: the selective molecule is highlight as tubing structure. The D-Phe(4-CF₃) and Trp of the tetra peptide form π-π stacking interactions with the hMC1R transmembrane domains. Up Right: 3D view of selective molecule docking into hMC1R. Down: 2D structure of hMC1R-MCL interactions.