CYCLIC LACTAM ALPHA-MELANOTROPIN ANALOGS OF AC-NLE(4)-CYCLO[ASP(5),D-PHE(7),LYS(10)] 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

The cloning of the melanocyte-stimulating hormone (MSH) and adrenocort icotropic hormone (ACTH) receptors (MC1-R and MC2-R, respectively) rec ently has led to the identification of three additional melanocortin r eceptors, MC3-R, MC4-R, and MC5-R. The MC2 receptor primarily recogniz es only ACTH peptides, but the other four receptors all recognize alph a-melanocyte-stimulating hormone (alpha-MSH) and potent alpha-MSH agon ists such as [Nle(4),D-Phe(7)]alpha-MSH-NH2 and (4)-c[Asp(5),D-Phe(7), Lys(10)]alpha-MSH-(4-10)-NH2 as well as ACTH. The absence of any known physiological role for these new receptors, expressed both in the bra in (MC3-R and MC4-R) and throughout a number of peripheral tissues (MC 5-R), has necessitated a search for potent and receptor selective agon ists and antagonists. We report here that analogues of the superpotent cyclic agonist analogue (4)-c[Asp(5),D-Phe(7),Lys(10)]alpha-MSH-(4-10 )NH2, in which a bulky aromatic amino acid is substituted in the 7-pos ition, can produce potent and selective antagonists for melanocortin r eceptors. Thus, the D-p-iodophenylalanine(7)-containing analogue -c[As p(5),D-Phe(pI)(7),Lys(10)]alpha-MSH-(4-10-NH2 is a potent antagonist ( pA(2) = 10.3) in the classical frog skin (Rana pipiens) assay (MC1-R), as is the D-2'-naphthylalanine(7) (D-Nal(2)(7))-containing analogue A c-Nle(4)-c[Asp(5),D-Nal(2)(7), Lys(10)]alpha-MSH-(4-10)-NH2 (pA(2) > 1 0.3). Interestingly, the D-p-chloro- and D-p-fluorophenylalanine(7)-co ntaining analogues lacked antagonist activities at all melanotropin re ceptors, and both exhibited full agonist potency in the frog skin assa y. The activity of these analogues also was examined at four mammalian melanocortin receptors. Interestingly, Ac-Nle(4)-c[Asp(5),(D-Nal(2)(7 ), Lys(10)]alpha-MSH-(4-10)-NH2 was found to be a potent antagonist of the MC4-R (pA(2) 9.3) with minimal agonist activity, a less potent an tagonist of the MC3-R (pA(2) = 8.3) with minimal agonist activity, and a full agonist of the MC1 and MC5 receptors. Surprisingly, Nle(4)-c[A sp(5),D-Phe(pI)(7),Lys(10)]alpha-MSH was found to be a potent agonist at the cloned human MC1-R (EC(50) = 0.055 nM) and mouse MC1-R (EC(50) = 0.19 nM) but had potent antagonist activities at the human MC4-R (pA (2) = 9.7) and human MC3-R (pA(2) = 8.3) with significant partial agon ist activities (EC(50) = 0.57 and 0.68 nM, respectively) as well. Thus , highly potent and receptor selective antagonist analogues can arise from substitution of the D-Phe(7) residue with a bulky aromatic amino acid. These analogues can be used to help determine the functional rol es of these receptors.