Cyclic lactam .alpha.-melanotropin analogs of Ac-Nle4-cyclo[Asp5,D-Phe7,Lys10]-.alpha.-melanocyte-stimulating hormone-(4-10)-NH2 with bulky aromatic amino acids at position 7 show high antagonist potency and selectivity at specific melanocortin receptors

Abstract

The cloning of the melanocyte-stimulating hormone (MSH) and adrenocorticotropic hormone (ACTH) receptors (MC1-R and MC2-R, respectively) recently has led to the identification of three additional melanocortin receptors, MC3-R, MC4-R, and MC5-R. The MC2 receptor primarily recognizes only ACTH peptides, but the other four receptors all recognize alpha-melanocyte-stimulating hormone (alpha-MSH) and potent alpha-MSH agonists such as [Nle4,D-Phe7]alpha-MSH-NH2 and Ac-Nle4-c[Asp5,D-Phe7,Lys10]alpha-MSH-(4-10)-NH2 as well as ACTH. The absence of any known physiological role for these new receptors, expressed both in the brain (MC3-R and MC4-R) and throughout a number of peripheral tissues (MC5-R), has necessitated as search for potent and receptor selective agonists and antagonists. We report here that analogues of the superpotent cyclic agonist analogue Ac-Nle4-c[Asp5,D-Phe7, Lys10]alpha-MSH-(4-10)-NH2, in which a bulky aromatic amino acid is substituted in the 7-position, can produce potent and selective antagonists for melanocortin receptors. Thus, the D-p-iodophenylalanine7-containing analogue Ac-Nle4-c[Asp5,D-Phe(pI)7,Lys10]alpha-MSH-(4-10)-NH2 is a potent antagonist (pA2 = 10.3) in the classical frog skin (Rana pipiens) assay (MC1-R), as is the D-2'-naphthylalanine7 (D-Nal(2)7)-containing analogue Ac-Nle4-c[Asp5,D-Nal(2)7,Lys10]alpha-MSH-(4-10)-NH2 (pA2 > 10.3). Interestingly, the D-p-chloro- and D-p-fluorophenylalanine7-containing analogues lacked antagonist activities at all melanotropin receptors, and both exhibited full agonist potency in the frog skin assay. The activity of these analogues also was examined at four mammalian melanocortin receptors. Interestingly, Ac-Nle4-c[Asp5,(D-Nal(2)7,Lys10] alpha-MSH-(4-10)-NH2 was found to be a potent antagonist of the MC4-R (pA2 = 9.3) with minimal agonist activity, a less potent antagonist of the MC3-R (pA2 = 8.3) with minimal agonist activity, and a full agonist of the MC1 and MC5 receptors. Surprisingly, Nle4-c[Asp5,D-Phe(pI)7,Lys10]alpha-MSH was found to be a potent agonist at the cloned human MC1-R (EC50 = 0.055 nM) and mouse MC1-R (EC50 = 0.19 nM) but had potent antagonist activities at the human MC4-R (pA2 = 9.7) and human MC3-R (pA2 = 8.3) with significant partial agonist activities (EC50 = 0.57 and 0.68 nM, respectively) as well. Thus, highly potent and receptor selective antagonist analogues can arise from substitution of the D-Phe7 residue with a bulky aromatic amino acid. These analogues can be used to help determine the functional roles of these receptors.

A derivative of the melanocortin receptor antagonist SHU9119 (PG932) increases food intake when administered peripherally

“…The main finding presented in this report is that PG932, a derivative of SHU9119, a nonselective antagonist of the Mc3r and Mc4r [19], significantly increases food intake when administered by peripheral injections. In fasted mice, a single peripheral injection of PG932 increased food intake, and inhibited the anorexia and malaise associated with LPS injection.…”

“…As would be predicted, the stimulation of food intake by PG932 was found to depend on a functional Mc4r, suggesting antagonism of Mc4r as the mechanism involved in the stimulation of food intake. [19]. SHU9119 increases food intake when administered intracerebroventricularly, but not peripherally [10].…”

Structure–activity studies of new melanocortin peptides containing an aromatic amino acid at the N-terminal position

“…These compounds were prepared to explore modification of position 4 in conjunction with various substitutions at the positions 6, 9, and 10, in order to improve the selectivity of the cyclic α-MSH analogues MT-II [1,2] and SHU-9119 [26] which were previously discovered in our laboratories, and are highly potent but non-selective agonist and antagonist analogues, respectively. They were further evaluated for their binding affinities to the human melanocortin receptors 3-5 in competitive binding assays using the radiolabeled ligand [ 125 I]-NDP-α-MSH and for their agonist or antagonist (when significant) potency in cAMP assays employing the HEK293 cells expressing these receptors.…”

“…These modifications have suggested that amino acid residues at the 6 (His), 7 (Phe), 8 (Arg), and 9 (Trp) positions are important for molecular recognition and activation of the melanocortin receptors. In particular, positions 6 and 7 of cyclic and of linear derivatives of α-MSH appeared to be important for high affinity and selectivity at hMC3 and hMC4 receptors [1,17,26,36,39,41].…”

Interactions of Human Melanocortin 4 Receptor with Nonpeptide and Peptide Agonists,

“…The proposed peptide-receptor complexes were consistent with the notion that the core residues of flexible peptides reproduced the receptorbound structure of the more conformationally rigid small molecule agonists. The peptide flexibility was also restrained during calculation by packing interactions and H-bonds with the receptor, and by several intramolecular cross-linking constraints taken from related bioactive cyclic peptides.The most important pharmacophore element of all melanocortin agonists is an aromatic ring of the central D-Phe residue (18,19,49,54). It is inserted at the bottom of the binding cavity in close proximity to the conserved Trp 258 residue that triggers activation of GPCRs (31,55).…”