Design and pharmacology of N-[(3R)-1,2,3,4-tetrahydroisoquinolinium- 3-ylcarbonyl]-(1R)-1-(4-chlorobenzyl)- 2-[4-cyclohexyl-4-(1H-1,2,4-triazol- 1-ylmethyl)piperidin-1-yl]-2-oxoethylamine (1), a potent, selective, melanocortin subtype-4 receptor agonist

Engineering the melanocortin-4 receptor to control G(s) signaling in vivo

G-protein-coupled receptors (GPCRs) are the largest known family of cell surface receptors, and they control many important physiological events, including sensory perception, chemotaxis, neurotransmission, and energy homeostasis. However, GPCR signaling can be difficult to study in vivo because of the multitude of GPCRs, the lack of specific synthetic agonists, and the fact that some GPCRs activate multiple signaling pathways. One method to circumvent these problems is to develop an engineered receptor that is unresponsive to its endogenous agonist, yet can be fully activated by synthetic, small-molecule drugs. Such a receptor, called a receptor activated solely by a synthetic ligand (RASSL), can be rapidly and reversibly activated by a small-molecule drug and would be a powerful tool to control G-protein signaling in vivo. Here we present the development of a G(s)-coupled RASSL based on the melanocortin-4 receptor (MC4R). MC4R couples exclusively to G(s) at physiologically relevant concentrations of its endogenous ligand, alpha-melanocyte-stimulating hormone (alpha-MSH). Data from human patients and structure-activity studies have shown that several mutations in MC4R cause a decreased affinity for alpha-MSH and can be exploited for RASSL development. Synthetic, small-molecule agonists of MC4R are now available and can be used to activate mutated receptors in vivo. We are engineering a series of mutations in MC4R to remove the peptide-binding site while retaining small-molecule binding and activation. The MC4R G(s) RASSL could be used to control many physiological responses associated with G(s) signaling such as heart rate, energy homeostasis, and cell proliferation.

Discovery of tyrosine-based potent and selective melanocortin-1 receptor small-molecule agonists with anti-inflammatory properties

The melanocortin-1 receptor (MC-1R) is a G-protein-coupled receptor involved in inflammation and skin pigmentation. Compound 2 is the first highly potent and selective MC-1R small-molecule agonist reported. Compound 2 showed efficacy in an acute model of inflammation, which has demonstrated the role of MC-1R in modulation of inflammation.

Activation of melanocortin MC(4) receptors increases erectile activity in rats ex copula

Melanocortin peptide agonists, alpha-melanocyte stimulating hormone (alpha-MSH) and melanotan-II, stimulate erectile activity in a variety of species, including man. Since neither peptide discriminates amongst melanocortin receptors, it is not clear which subtype mediates these pro-erectile effects. Here, we present data that melanocortin-induced erectogenesis is mediated by melanocortin MC(4) receptors. Systemic administration of a melanocortin MC(4) receptor agonist (N-[(3R)-1,2,3,4-tetrahydroisoquinolinium-3-ylcarbonyl]-(1R)-1-(4-chlorobenzyl)-2-[4-cyclohexyl-4-(1H-1,2,4-triazol-1ylmethyl)piperidin-1-yl]-2-oxoethylamine; THIQ) with high selectivity over other melanocortin receptors enhanced intracavernosal pressure and stimulated erectile activity in rats ex copula. THIQ dose-dependently (1-5 mg/kg, i.v.) increased the total number of erections, to an extent comparable or greater than that produced by apomorphine (0.025 mg/kg, s.c.). Central administration of THIQ (20 microg, intracerebroventricular (i.c.v.)) increased the number of reflexive penile erections; whereas administration of both a nonselective endogenous melanocortin MC(4) receptor antagonist (agouti-related protein (AgRP), 5.5. microg, i.c.v.) and a melanocortin MC(4) receptor preferring antagonist (MPB10, 1 mg/kg, i.v.) blocked THIQ-induced erectogenesis. These pro-erectile effects were also attenuated by systemic or central administration of an oxytocin antagonist (L-368899, 1 mg/kg, i.v.). Thus, melanocortin MC(4) receptor activation is sufficient for erectogenesis and these effects may involve oxytocinergic pathways.