Structure-activity relationships of cyclic lactam analogues of alpha-melanocyte-stimulating hormone (alpha-MSH) targeting the human melanocortin-3 receptor

Abstract

A variety of dicarboxylic acid linkers introduced between the alpha-amino group of Pro(6) and the -amino group of Lys(10) of the cyclic lactam alpha-melanocyte-stimulating hormone (alpha-MSH)-derived Pro(6)-D-Phe(7)/D-Nal(2')(7)-Arg(8)-Trp(9)-Lys(10)-NH2 pentapeptide template lead to nanomolar range and selective hMC3R agonists and antagonists. Replacement of the Pro(6) residue and the dicarboxylic acid linker with 2,3-pyrazine-dicarboxylic acid furnished a highly selective nanomolar range hMC3R partial agonist (analogue 12, c[CO-2,3-pyrazine-CO-D-Phe-Arg-Trp-Lys]-NH2, EC50 = 27 nM, 70% max cAMP) and an hMC3R antagonist (analogue 13, c[CO-2,3-pyrazine-CO-D-Nal(2')-Arg-Trp-Lys]-NH2, IC50 = 23 nM). Modeling experiments suggest that 2,3-pyrazinedicarboxylic acid stabilizes a beta-turn-like structure with the D-Phe/D-Nal(2') residues, which explains the high potency of the corresponding peptides. Placement of a Nle residue in position 6 produced a hMC3R/hMC5R antagonist (analogue 15, c[CO-(CH 2)2-CO-Nle-D-Nal(2')-Arg-Trp-Lys]-NH2, IC50 = 12 and 17 nM, respectively), similarly to the previously described cyclic gamma-melanocyte-stimulating hormone (gamma-MSH)-derived hMC3R/hMC5R antagonists. These newly developed melanotropins will serve as critical biochemical tools for elucidating the full spectrum of functions performed by the physiologically important melanocortin-3 receptor.

Figure 1

Some leading hMC3R- and hMC4R-selective melanotropin peptide ligands.

Figure 2

Design of the α-MSH-derived cyclic lactam scaffold.

Figure 3

Stereoview of the superimposed global minimum of analogue 6 (purple), obtained by MCMM/LMCS (Monte Carlo Multiple Minima-Low Frequency Mode)-OPLS 2005 simulation with the NMR-derived structure of nonselective super agonist MT-II (blue; rmsd = 1.86 Å, nonhydrogen backbone atoms of the Xaa-D-Phe-Arg-Trp pharmacophore only). Hydrogens are omitted for clarity.

Figure 4

Stereoview of the superimposed global minimum of analogue 12 (gold), obtained by MCMM/LMCS (Monte Carlo Multiple Minima-Low Frequency Mode)-OPLS 2005 simulation with the NMR-derived structure of nonselective super agonist MT-II (blue; rmsd = 1.35 Å, nonhydrogen backbone atoms of the Xaa-D-Phe-Arg-Trp pharmacophore only). Hydrogens are omitted for clarity.

Figure 5

Stereoview of the superimposed global minimum of analogue 13 (blue) with the global minimum of selective hMC4R antagonist 2 (gold), obtained by MCMM/LMCS (Monte Carlo Multiple Minima-Low Frequency Mode)-OPLS 2005 simulations (rmsd = 0.43 Å, nonhydrogen backbone atoms of the Xaa-D-Phe-Arg-Trp pharmacoph-ore only). Hydrogens are omitted for clarity.

Figure 6

Stereoview of the superimposed global minimum of analogue 15 (yellow), obtained by MCMM/LMCS (Monte Carlo Multiple Minima-Low Frequency Mode)-OPLS 2005 simulation with the NMR-derived structure of hMC3R/hMC4R antagonist SHU9119 (green; rmsd = 0.39 Å, nonhydrogen backbone atoms of the Xaa-D-Nal(2′)-Arg-Trp pharmacophore only). Hydrogens are omitted for clarity.

Figure 7

Graphical summary of the agonist activities, expressed in EC₅₀ values (Z axis), of the D-Phe⁷ analogues (X axis) at the four human melanocortin receptor subtypes (Y axis).

Figure 8

Graphical summary of the binding affinities, expressed in IC₅₀ values (Z axis), of the D-Nal(2′)⁷ analogues (X axis) at the four human melanocortin receptor subtypes (Y axis).