S18616, a highly potent, spiroimidazoline agonist at alpha(2)-adrenoceptors: I. Receptor profile, antinociceptive and hypothermic actions in comparison with dexmedetomidine and clonidine

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 al pha (2)-adrenoceptors (AR)s (pK(i), 9.8), native human (h) alpha (2A)-ARs ( 9.6), and cloned h alpha (2A)- (9.5), h alpha (2B) - (9.2), and h alpha (2C )- (9.0) ARs. It showed 40-fold lower affinity for h alpha (1A)-ARs (8.4) a nd greater than or equal to 100-fold lower affinity for rat alpha (1)-ARs ( 7.1), h alpha (1B)-ARs (7.7), h alpha (1D)-ARs (7.6), imidazoline(1) (7.4), and imidazoline(2) (7.4) sites and >100-fold lower affinity for all other (>50) sites. At h alpha (2A)-ARs, in guanosine-5'-O-(3-[S-35]thio) triphosp hate binding studies, S18616 was a potent (partial) agonist: log effective concentration (pEC(50)), 9.3/maximal effect, 51%. This observation was corr oborated employing a h alpha (2A)-Gi1 alpha fusion protein/ GTPase assay (9 .0/40%) in which the actions of S18616 were blocked by pertussis toxin. Emp loying guanosine-5'-O-(3[S-35] thio) triphosphate binding assays, S18616 wa s also a partial agonist at h alpha (2C)-ARs (8.2/63%) but a full agonist ( 8.4/124%) at h alpha (2B)-ARs. At h alpha (2A)-, h alpha (2B)-, and h alpha (2C)-ARs, the selective alpha (2)-AR antagonist, atipamezole, abolished th e actions of S18616: pK(b) values of 9.1, 9.1, and 9.4, respectively. As de termined by depletion of membrane-bound [H-3] phosphatidyl inositols, S1861 6 behaved as a (less potent) agonist (7.8/79%) at h alpha (1A)-ARs, an acti on abolished by prazosin (pK(b), 8.9). Reflecting alpha (2)-AR agonist prop erties, S18616 potently (greater than or equal to1 mug/kg, s.c.) and dose d ependently elicited hypothermia and antinociception (nine diverse models) i n rodents. These actions were dose dependently inhibited by chemically dive rse alpha (2)- versus alpha (1)-AR antagonists, atipamezole, idazoxan, RX82 1,002, and BRL44418 (a preferential alpha (2A)-AR ligand). In contrast, the actions of S18616 were unaffected by the alpha (1)-AR antagonists, ARC239 and prazosin (which preferentially block alpha (2B/2C)-versus alpha (2A)-AR s). Although the affinity of dexmedetomidine at alpha (2)-ARs was lower tha n S18616; it displayed a similar receptor and functional profile. Clonidine displayed lower efficacy than S18616, was substantially less potent, and h ad marked affinity for imidazoline(1) sites and alpha (1)-ARs. In conclusio n, S18616 is a novel, selective, and highly potent agonist at alpha (2)-ARs .