Vol. 295, Issue 3, 1192-1205, December 2000

S18616, a Highly Potent, Spiroimidazoline Agonist at ₂-Adrenoceptors: I. Receptor Profile, Antinociceptive and Hypothermic Actions in Comparison with Dexmedetomidine and Clonidine

Mark J. Millan, Anne Dekeyne, Adrian Newman-Tancredi, Didier Cussac, Valérie Audinot, Graeme Milligan, Delphine Duqueyroix, Sylvie Girardon, Jimmy Mullot, Jean A. Boutin, Jean-Paul Nicolas, Anne Renouard-Try, Jean-Michel Lacoste and Alex Cordi

Psychopharmacology Department (M.J.M., A.D., A.N.-T., D.C., D.D., S.G., J.M.) and Cellular and Molecular Pharmacology Department (V.A., J.A.B., J.-P.N., A.R.-T.), Institut de Recherches Servier, Centre de Recherches de Croissy, Croissy/Seine, Paris, France; Chemistry C Department (J.-M.L., A.C.), Institut de Recherches Servier, Centre de Recherches de Suresnes, Suresnes, Paris, France; and Division of Biochemistry and Molecular Biology (G.M.), Davidson Building, Institute of Biomedical and Life Sciences, University of Glasgow, Glasgow, Scotland, United Kingdom

S18616 {(S)-spiro[(1-oxa-2-amino-3-azacyclopent-2-ene)-4,2'-(8'-chloro-1',2',3',4'-tetrahydronaphthalene)]} displayed high affinity at native rat ₂-adrenoceptors (AR)s (pK_i, 9.8), native human (h)_2A-ARs (9.6), and cloned h_2A- (9.5), h_2B- (9.2), and h_2C- (9.0) ARs. It showed 40-fold lower affinity for h_1A-ARs (8.4) and 100-fold lower affinity for rat ₁-ARs (7.1), h_1B-ARs (7.7), h_1D-ARs (7.6), imidazoline₁ (7.4), and imidazoline₂ (7.4) sites and >100-fold lower affinity for all other (>50) sites. At h_2A-ARs, in guanosine-5'-O-(3-[³⁵S]thio)triphosphate binding studies, S18616 was a potent (partial) agonist: log effective concentration (pEC₅₀), 9.3/maximal effect, 51%. This observation was corroborated employing a h_2A-Gi1 fusion protein/GTPase assay (9.0/40%) in which the actions of S18616 were blocked by pertussis toxin. Employing guanosine-5'-O-(3-[³⁵S]thio)triphosphate binding assays, S18616 was also a partial agonist at h_2C-ARs (8.2/63%) but a full agonist (8.4/124%) at h_2B-ARs. At h_2A-, h_2B-, and h_2C-ARs, the selective ₂-AR antagonist, atipamezole, abolished the actions of S18616: pK_b values of 9.1, 9.1, and 9.4, respectively. As determined by depletion of membrane-bound [³H]phosphatidyl inositols, S18616 behaved as a (less potent) agonist (7.8/79%) at h_1A-ARs, an action abolished by prazosin (pK_b, 8.9). Reflecting ₂-AR agonist properties, S18616 potently (1 µg/kg, s.c.) and dose dependently elicited hypothermia and antinociception (nine diverse models) in rodents. These actions were dose dependently inhibited by chemically diverse ₂- versus ₁-AR antagonists, atipamezole, idazoxan, RX821,002, and BRL44418 (a preferential _2A-AR ligand). In contrast, the actions of S18616 were unaffected by the ₁-AR antagonists, ARC239 and prazosin (which preferentially block _2B/2C- versus _2A-ARs). Although the affinity of dexmedetomidine at ₂-ARs was lower than S18616; it displayed a similar receptor and functional profile. Clonidine displayed lower efficacy than S18616, was substantially less potent, and had marked affinity for imidazoline₁ sites and ₁-ARs. In conclusion, S18616 is a novel, selective, and highly potent agonist at ₂-ARs.