title: Biochemical and electrophysiological characterization of almorexant, a dual orexin 1 receptor (OX1)/orexin 2 receptor (OX2) antagonist: comparison with selective OX1 and OX2 antagonists.
abstract: Recent preclinical and clinical research has shown that almorexant promotes sleep in animals and humans without disrupting the sleep architecture. Here, the pharmacology and kinetics of [(3)H]almorexant binding to human orexin 1 receptor (OX(1))- and human orexin 2 receptor (OX(2))-human embryonic kidney 293 membranes were characterized and compared with those of selective OX(1) and OX(2) antagonists, including 1-(5-(2-fluoro-phenyl)-2-methyl-thiazol-4-yl)-1-((S)-2-(5-phenyl-(1,3,4)oxadiazol-2-ylmethyl)-pyrrolidin-1-yl)-methanone (SB-674042), 1-(6,8-difluoro-2-methyl-quinolin-4-yl)-3-(4-dimethylamino-phenyl)-urea (SB-408124), and N-ethyl-2-[(6-methoxy-pyridin-3-yl)-(toluene-2-sulfonyl)-amino]-N-pyridin-3-ylmethyl-acetamide (EMPA). The effect of these antagonists was also examined in vitro on the spontaneous activity of rat ventral tegmental area (VTA) dopaminergic neurons. [(3)H]Almorexant bound to a single saturable site on hOX(1) and hOX(2) with high affinity (K(d) of 1.3 and 0.17 nM, respectively). In Schild analyses using the [(3)H]inositol phosphates assay, almorexant acted as a competitive antagonist at hOX(1) and as a noncompetitive-like antagonist at hOX(2). In binding kinetic analyses, [(3)H]almorexant had fast association and dissociation rates at hOX(1), whereas it had a fast association rate and a remarkably slow dissociation rate at hOX(2). In the VTA, orexin-A potentiated the basal firing frequency to 175 +/- 17% of control in approximately half of the neurons tested. In the presence of 1 microM SB-674042 or SB-408124, the effect of orexin-A was only partially antagonized. However, in the presence of 1 microM EMPA or 1 microM almorexant, the effect of orexin-A was completely antagonized. In conclusion, almorexant exhibited a noncompetitive and long-lasting pseudo-irreversible mode of antagonism as a result of its very slow rate of dissociation from OX(2). The electrophysiology data suggest that OX(2) might be more important than OX(1) in mediating the effect of orexin-A on slow-firing of VTA dopaminergic neurons.
authors: F. Hoffmann-La Roche Ltd., Bldg. 69/333, CH-4070 Basel, Switzerland. parichehr.malherbe@roche.com
authors: Pari
authors: P
authors: Malherbe
authors: Edilio
authors: E
authors: Borroni
authors: Emmanuel
authors: E
authors: Pinard
authors: Joseph G
authors: JG
authors: Wettstein
authors: Frédéric
authors: F
authors: Knoflach
mesh_terms: D000081:Acetamides / Q000737:chemistry* / Q000494:pharmacology*; D000818:Animals; D005071:Evoked Potentials / Q000187:drug effects / Q000502:physiology; D006801:Humans; D007546:Isoquinolines / Q000737:chemistry* / Q000494:pharmacology*; D007700:Kinetics; D015394:Molecular Structure; D009474:Neurons / Q000187:drug effects / Q000502:physiology; D064446:Orexin Receptors; D010671:Phenylurea Compounds / Q000737:chemistry / Q000494:pharmacology; D011759:Pyrrolidines / Q000737:chemistry / Q000494:pharmacology; D051381:Rats; D043562:Receptors, G-Protein-Coupled / Q000037:antagonists & inhibitors*; D018013:Receptors, Neuropeptide / Q000037:antagonists & inhibitors*; D012890:Sleep / Q000187:drug effects / Q000502:physiology; D013844:Thiazoles / Q000737:chemistry / Q000494:pharmacology
chemical_list: D000081:Acetamides; D007546:Isoquinolines; D064446:Orexin Receptors; D010671:Phenylurea Compounds; D011759:Pyrrolidines; D043562:Receptors, G-Protein-Coupled; D018013:Receptors, Neuropeptide; C542397:SB 408124; C492260:SB674042; D013844:Thiazoles; C519150:almorexant