Milameline (CI-979/RU35926): a muscarinic receptor agonist with cognition-activating properties: biochemical and in vivo characterization.

Milameline (E-1,2,5,6-tetrahydro-1-methyl-3-pyridinecarboxaldehyde, O-methyloxime monohydrochloride, CI-979, PD129409, RU35926) was characterized in vitro and evaluated for effects on central and peripheral cholinergic activity in rats and rhesus monkeys. In muscarinic binding studies, milameline displayed nanomolar affinity with an agonist ligand and micromolar affinity with antagonist ligands, with approximately equal affinities determined at the five subtypes of human muscarinic receptors (hM(1)-hM(5)) with whole cells or membranes from stably transfected Chinese hamster ovary (CHO) cells. On binding, milameline stimulated phosphatidylinositol hydrolysis in hM(1) and hM(3) CHO cells and inhibited forskolin-activated cAMP accumulation in hM(2) and hM(4) CHO cells. Additionally, it decreased K(+)-stimulated release of [(3)H]acetylcholine from rat cortical slices. Responses were not caused by the inhibition of acetylcholinesterase, and there was no significant binding to approximately 30 other neurotransmitter binding sites. In rats, milameline decreased spontaneous and scopolamine-induced swimming activity, improved water-maze performance of animals impaired by basal forebrain lesions, increased cortical blood flow, decreased core body temperature, and increased gastrointestinal motility. Electroencephalogram activity in both rats and monkeys was characterized by a predominance of low-voltage desynchronized activity consistent with an increase in arousal. Milameline also reversed a scopolamine-induced impairment of attention on a continuous-performance task in monkeys. Thus, milameline possesses a pharmacological profile consistent with that of a partial muscarinic agonist, with central cholinergic actions being produced in rats and monkeys at doses slightly lower than those stimulating peripheral cholinergic receptors.

Structure-activity relationships for enhancement of adenosine A1 receptor binding by 2-amino-3-benzoylthiophenes.

The structural requirements for stimulation of adenosine A1 agonist binding by 2-amino-3-benzoylthiophenes and related compounds were investigated. Slowing of the dissociation of [3H]N6cyclohexyladenosine binding was used as a specific measure of the allosteric effects of these compounds. The thiophene ring could be replaced with benzene but not with several nitrogen-containing heterocycles. The 2-amino group was required, and at least one hydrogen on the amino group appeared to be necessary for activity. The keto carbonyl was also essential. Alkyl substitution at the 4-position of the thiophene ring increased activity, whereas 5-position substitution appeared to have little effect. Activity was also increased by various substitutions on the phenyl ring, with 3-(trifluoromethyl) showing optimal activity. The phenyl ring could be replaced with cyclohexyl without major loss of activity. 1-Aminofluoren-9-one, a conformationally locked derivative, was active. Based in part in the latter observation, the active conformation is proposed to have an intramolecular hydrogen bond between the amino nitrogen and the carbonyl oxygen. Because the 2-amino-3-benzoylthiophenes showed competitive adenosine antagonism as well as allosteric enhancement, their affinities as competitive inhibitors of [3H]8-cyclopentyl-1,3-dipropylxanthine binding to A1 receptors were also assessed. Structure-activity relationships for competitive antagonism were distinct from those for allosteric enhancement, with ratios between the two activities varying by more than 1000-fold. Of the analogs tested, (2-amino-4,5-dimethyl-3-thienyl)-[3-(trifluoromethyl)phenyl]methanone (PD 81,723) had the most favorable ratio of enhancement to antagonism.

Comparison of the behavioral effects of adenosine agonists and dopamine antagonists in mice.

The adenosine agonists 5'-N-ethylcarboxamideadenosine (NECA), 2-chloroadenosine (2-CLA), N6-cyclohexyladenosine (CHA), N6-cyclopentyladenosine (CPA), 2-(phenylamino)adenosine (CV-1808) and R and S isomers of N6-phenylisopropyladenosine (R-PIA and S-PIA) decreased spontaneous locomotor activity in mice and, except for CPA, did so at doses that did not impair motor coordination, a profile shared by dopamine antagonists. CV-1808, the only agent with higher affinity for A2 as compared with A1 adenosine receptors, displayed the largest separation between locomotor inhibitory and ataxic potency. Like dopamine antagonists, NECA and CV-1808 also decreased hyperactivity caused by d--amphetamine at doses that did not cause ataxia whereas A1-selective adenosine agonists reduced amphetamine's effects only at ataxic doses. Unlike dopamine antagonists, adenosine agonists inhibited apomorphine-induced cage climbing only at doses that caused ataxia. Involvement of central adenosine receptors in these effects was suggested by the significant correlation obtained between potency for locomotor inhibition after IP and ICV administration. Affinity for A1 but not A2 adenosine receptors was significantly correlated with potency for inducing ataxia. These results suggest that the behavioral profile of adenosine agonists in mice is related to their affinity for A1 and A2 adenosine receptors and indicate that adenosine agonists produce certain behavioral effects that are similar to those seen with dopamine antagonists.