S18616, a Highly Potent, Spiroimidazoline Agonist at α2-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 α2-adrenoceptors (AR)s (p K i, 9.8), native human (h)α2A-ARs (9.6), and cloned hα2A- (9.5), hα2B- (9.2), and hα2C- (9.0) ARs. It showed 40-fold lower affinity for hα1A-ARs (8.4) and ≥100-fold lower affinity for rat α1-ARs (7.1), hα1B-ARs (7.7), hα1D-ARs (7.6), imidazoline1 (7.4), and imidazoline2 (7.4) sites and >100-fold lower affinity for all other (>50) sites. At hα2A-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 hα2A-Gi1α 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 hα2C-ARs (8.2/63%) but a full agonist (8.4/124%) at hα2B-ARs. At hα2A-, hα2B-, and hα2C-ARs, the selective α2-AR antagonist, atipamezole, abolished the actions of S18616: p K 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 hα1A-ARs, an action abolished by prazosin (p K b, 8.9). Reflecting α2-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 α2- versus α1-AR antagonists, atipamezole, idazoxan, RX821,002, and BRL44418 (a preferential α2A-AR ligand). In contrast, the actions of S18616 were unaffected by the α1-AR antagonists, ARC239 and prazosin (which preferentially block α2B/2C- versus α2A-ARs). Although the affinity of dexmedetomidine at α2-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 α1-ARs. In conclusion, S18616 is a novel, selective, and highly potent agonist at α2-ARs.